aci 332-20 residential concrete foundation industry basics

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ACI 332-20 Residential Concrete

Foundation Industry Basics

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James BatyExecutive [email protected]

www.cfawalls.org319-895-6940

ACI 332 - Past Chair(‘04, ‘08, ‘10)

ACI 332-D ChairACI 380 ChairACI 551 SecretaryACI C-655 Chair 2

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Learning ObjectivesUpon completing this program, you should be able to:

q Assess the application differences of ACI 332 when compared to IRC

q Identify the major fundamentals of materials, design, and construction prescribed by ACI 332

q Evaluate variability in design of foundations through prescriptive method

q Identify safety issues and code compliance

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Industry ResourcesThe major documents affecting this industry.

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CERTIFICATION RESOURCES5

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International Residential CodeNational StandardChapter 4, 2021 Edition Basic prescriptive standardsRequirements standardized for a broad

marketLimited Design Variety

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ACI 332 StandardReferenced by the International Residential Code (IRC)

“…or comply with the applicable standards of ACI 318 or ACI 332…”

Section R402.2 (Materials)

Section R403.1 (Footings)

Section R404 (Walls)7

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• Statewide building code for one-and two-family dwellings

• Structures built since June 1, 1980.

• The Industry Services Division provides consultation and education concerning UDC construction standards and inspection procedures.

• Building materials are evaluated for conformance with standards.

• UDC inspection and contractor credentials are administered.

• The UDC is enforced in all Wisconsin municipalities.

• 2018 version > ACI 332-14 for concrete prescription

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IRC 2003: Foundation Wall Prescriptive DesignEvolution of Code Provisions

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• Combined table resulting from ACI 332-04

• Today one table for each wall thickness

• Single Concrete Strength (2500 psi)

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Simplified comparison of primary differences

Practical guidance for selecting either IRC or ACI 332

Condensed evidence to use in discussions with Code Officials and Building Inspectors

Tech Note TN-001

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ACI 332 GuideRecommended PracticeCompanion to the “Code”Non-mandatory languageExplains your industryGoes beyond the code

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Tech Note TN-002Consolidated reference to all issues

pertaining to backfillDiscusses timing, depth, compaction,

soil conditions and restraintProvides guidance as to why or why notShould be used as an informational tool

to builders and clients 12

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Lab and Field ResearchDiscussion of MaturityDescription of possible mix designsRecommended best practicesReferenced by ACI 332 Code and ACI 306

CFA’s Cold Weather Research Report

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Tech Note TN-003Condenses the 70-page CFA Cold

Weather Research into most usable characteristics

Intended as a primer to explain the concepts derived from research

Not intended to be the sole reference

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Tech Note TN-004Why walls crack and should I be concerned?Provides practical guidance for cracking

conditions and the related cause/effectShould be used as a primary reference for

customers and builders

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Tech Note TN-010Primary drainage system componentsWaterproofing vs DampproofingCoatings and membranesTroubleshooting

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Tech Note TN-011Primary elements of soil classification

impacting excavationsSoil load determinationPermeabilitySettlementBearing capacity (affects footing size)FrostExcavation Safety

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Tech Note TN-012Primary constituents for concreteSustainability influencesAdmixturesMix ProportioningCuring PracticesAffecting Concrete StrengthSafety Issues for Chemical Nature 18

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ACI 332 AdvantagesReferences for:

Cold weather placementFooting excavationsWall jumps – footing discontinuityFooting spansIntegrated footingsConsolidation of wall requirementsIncreased design flexibility

and more… 19

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Committee 332 –Residential Concrete Work• Who we are

• 50 members: � 43 voting� 7 consulting

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• Code

• Mandatory language

• 2004 – 26 pages

• 2008 – 31 pages

• 2010 – 34 pages

• 2014 – 58 pages

• 2020 – 74 pages

• Guide

• Not mandatory language

• 1984 – 38 pages

• 2006 – 52 pages

• 2018 – 68 pages

We’ve been doing this for a while

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Intent of CodesR101.3 Intent. …to establish minimum requirements to safeguard the public safety, health and general welfare through affordability, structural strength, means of egress facilities, stability, sanitation, light and ventilation, energy conservation and safety to life and property from fire and other hazards attributed to the built environment, and to provide safety to fire fighters and emergency responders during emergency operations.

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2020 Code Scope• Same as previous

• Improvements � Exposure categories, classes� Additional freeze-thaw requirements� Cementitious materials for sulfate exposure� Vapor retarders

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2020 Code Scope (cont.)• Most Significant Changes

� Concrete requirements� Design of structural concrete

� Footings� Foundation walls� Above-grade walls� Lintels

• Complements the prescriptive design

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Materials

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Code Influence§ Chapter 4 – Materials§ Concrete§ Reinforcement§ Formwork

§ Chapter 5 – Concrete Requirements§ Exposure Categories§ Concrete Cover

§ Chapter 7 – Concrete Production, Delivery & Placement§ Placement § Curing § Extreme weather§ Form removal

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Admixtures: Improving Concrete§ Retarder - slows cure

(good in hot weather)

§ Air entrainment - durability & workability

§ Water-reducers (MRWR) - reduce w/cm & increase slump

§ Superplasticizers (HRWR) - creates increased flowability

§ Accelerators: early-age strength gain

§ Coloring – aesthetics

§ Others - corrosion inhibitors, shrinkage reducers

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Code InfluenceACI 332 – 20Reinforcement Provisions(Since 2004) 28

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Code Influence:

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Material Interaction:Rebar Contaminants• 4.2.4 Surface conditions of

reinforcement—At the time concrete is placed, deformed bar and welded wire reinforcement shall be free of materials deleterious to development of bond strength between the reinforcement and the concrete.

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R4.2.4 Common surface contaminants such as concrete splatter, rust, form oil, or other

release agents have been found not to be deleterious to bond

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Exposure Categories / Classes

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R5.1—Exposure Categories / Classes

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RF Category Applications

RF0 Basement and foundation walls that extend above grade less than 12 in. are unlikely to be saturated

RF1 Vertical concrete members where excessive accumulation of ice and snow is not anticipated (i.e., above-grade walls, and columns)

RF2 Vertical concrete members and elevated or on-grade horizontal structurally reinforced concrete members that have a likelihood for prolonged contact with water to achieve a saturation state

RF3 Non-structurally reinforced soil-supported exterior slabs that are subject to freezing-and-thawing cycling and deicing chemicals

RF4 Same members as RF2 when exposed to deicing chemicals (i.e., above-grade columns and structural slabs exposed to salts in a marine environment)

Depending on

exposure conditions,

garage floors can

be classified as RF0

to RF3, but not RF4.

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Minimum Concrete Provisions

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AC1 332-14

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Minimum Concrete ProvisionsAir Content Cementitious Materials

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ACI 332:Minimum Cover

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ACI 332:Footing Connections

377.3.4.1 (cont.) To facilitate positioning before concrete placement, vertical dowels are permitted to be driven into the grade in the bottom of the footing.

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Code Influence:Material PerformanceCold weather means the need for protection and the opportunity for better quality concrete. 39

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Code Influence:Material Performance• 6.6.1 During anticipated ambient

temperature conditions of 35 °F or less, concrete temperature shall be maintained above a frozen state until a concrete compressive strength of 500 psi has been reached.

• Frozen concrete wall (MN)

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Code Influence:Material Performance• 6.6.2 – Concrete materials,

reinforcement, forms, and any earth with which concrete is to come in contact shall be free from ice, snow and frost.

• 6.6.3 Frozen materials or materials containing ice shall not be used.

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Snow Insulates?Contractor felt the snow would be melted from the forms…instead, it was pushed to one mound.

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Cold Weather Research§ Referenced by ACI 332

§ Referenced by ACI 306

§ Only research specifically conducted on residential concrete foundation walls

§ Validates mix design options

§ Encourages use of Maturity

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CFA’s TN-003 Cold Weather44Reference Technical Paper Summarizing Key Findings = Conversation Starter

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Code Influence: Material Performance

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• Anchorage, Alaska� “…we now have a new policy in

Anchorage.� Mix # 29 and # 34 down to 20˚F� No tenting or heat.� Replacing calcium with NCAs.

• “Last tent before change (April 2004) 35˚F, cost about $2,700.”

• “First foundation with new policy…”� Suddenly dropped to 0˚F� Unprepared� Had to spend $1,500 on a tent

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ACI 332-20 from ACI 332-14

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New

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ACI318 VS ACI332

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Code InfluenceCHAPTER 6—DESIGN OF STRUCTURAL. CONCRETE

§ 6.2—Materials

§ 6.3—Load factors and combinations

§ 6.4—Strength reduction factors

§ 6.5—Plain concrete

§ 6.6—Sectional strength of reinforced sections

§ 6.7—Design limits§ 6.8—Reinforcement detailing

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Basement wall design in ACI 332 -20

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Foundation’s Purpose

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Stable, durable platform

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Settlement (soil)

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Heaving (depth)

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Transfer loads to soil

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Resist lateral loads (below grade)

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Foundation SystemsSeveral components and products must work together to have an effective system.

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Site/Soil

FoundationMaterial

Footing

Vertical Wall System

Wall

Top Restraint

Bottom Restraint

Drainage System

Water Protection

Water-proofing/Damp-

proofing

Backfill/Finish

Grading

The Foundation

System

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ACI332 Tables

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ACI 332Prescriptive Power

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Chapter 9—Foundation Walls

9.1 – Scope Defines the provisions of this chapter that shall apply, such as Lateral support is required at the top and bottom of the wall.

9.2 – General Design properties for concrete rebar, dowels, Additional wall reinforcement where necessary such as reentrant corners—Where a wall opening exists.

9.3 – Design Limits Minimum wall thickness.

9.4 – Design Strength Required strength shall be calculated in accordance with the factored load combination in. Required strength shall be calculated in accordance with the analysis procedures in Chapter 6.

9.5 – Construction Requirements Forms—Foundation wall forms shall be stable during placement of concrete and shall result in a structure, Construction joints, Lateral restraint.

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ACI 332: Chapter 9

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ACI 332: Chapter 9

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ACI 332 – Lateral Soil Load

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• *The definition and classification of soil materials shall be in accordance with ASTM D2487.

• †Design lateral soil loads are given for moist conditions for the specified soils at their optimum densities. Actual field conditions shall govern. Submerged or saturated soil pressures shall include the weight of the buoyant soil plus the hydrostatic loads.

• ‡For relatively rigid walls, as when braced by floors, the design lateral soil load shall be increased for sand and gravel type soils to 60 psf per foot of depth. Basement walls extending not more than 10 ft below grade and supporting light floor systems are not considered as being relatively rigid walls.

• §Unsuitable as backfill material.

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Chap

ter 6

—D

esig

n of

Str

uctu

ral

Conc

rete • 6.1—Scope, p. 22

• 6.2—Materials, p. 22

• 6.3—Load factors and combinations, p. 23• 6.4—Strength reduction factors, p. 24• 6.5—Plain concrete, p. 24

• 6.6—Sectional strength of reinforced sections, p. 26

• 6.7—Design limits, p. 30• 6.8—Reinforcement detailing, p. 30

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ACI 332Load Factor Combinations

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Forces at Maximum Moment Location

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Plain concrete capacity

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Modulus of rupture of plain concrete,

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Reinforced concrete capacity

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Plain/Reinforced concrete capacity- Table

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Footing Design & Construction

Isolated pads, continuous strip, trench, integrated slab, piers and pilings…a complicated system. 65

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Footings & WallsFoundation Wall Definitions Simply supported vertical flexural member

Top and bottom laterally supported.

9.2.2 Lateral support is required at the top and bottom of the wall. Wall-to-footing joints that comply with 8.2.2.1 are deemed to have satisfied the bottom lateral support requirement.

The top and bottom restraint for the foundation wall shall be in place before the introduction of backfill against the foundation wall. Temporary lateral restraint shall be permitted. 661

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ACI 332-20:Tables 8.6.1.a – f

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Load Analysis68

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ACI 332-20:Tables 8.6.1.g – h 112

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Detailing Footings:

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Drainage Tile:

• IRC – No mention of footing thickness reduction, detailing, restrictions

• ACI 332 – 8.2.4

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ACI 332 8.2.4 –Unsupported Footings• 8.2.4.1 Trenches under footings shall be

backfilled to prevent movement of the adjacent soil and compacted to match the adjacent soil conditions

• 8.2.4.2 Where an unsupported wall footing section does not exceed a 3 ft span, at least two No. 4 reinforcing bars shall be securely positioned in the bottom of the footing and extend at least 18 in. into the supported sections on both sides. Reinforcing bars shall have a cover as specified in 5.6.4.

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Continuous Footings:

• IRC – R403.1 – All exterior walls shall be supported on continuous solid…concrete footings.

• ACI 332 – 8.2.5

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IRCContinuous Footings:Not always effective in their application.

Footing dimensions don’t match available form widths

Walls are not designed for continuous supported by a “concrete column”

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ACI 332-20Footing Requirements8.2.5 Maximum length of discontinuity 4 feet

The walls must be designed for the span

The footing around the discontinuity shall bear on at least 4 feet of undistributed soil

Typical additional discontinuity wall reinforcement:

2 - #4 reinforcement bars across the bottom

1 - #4 reinforcement bars across the top

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ACI 332-20 Footing Discontinuity:123

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Footing Connections

8.2.2.1.1 A continuous keyway shall be formed in the middle third of the wall…shall be a minimum of 1-1/2 in. deep and 1-1/2 in. wide at the top.

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Footing Connections

8.2.2.1.2 A No. 4 dowel shall extend at least 36dbinto the wall and 6 in. into the footing at a maximum of 24 in. on-center along the footing.

And…

1785

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ACI 332-20: Footing Connections

798.2.2.1.2 (cont.) To facilitate positioning before concrete placement, vertical dowels are permitted to be driven into the grade in the bottom of the footing.

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Excavation & Placement

7.2.4 Areas prepared for the placement of concrete shall be free of debris and contaminants. Confined footing areas shall also be free of waterR7.2.4 Refer to 7.6 for the placement of concrete on frozen material. If the footing form permits water to exit, the hydraulic pressure of the concrete placement is sufficient to displace the water from the formed areas and prevent segregation. 131

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Code Influence:Footing Excavations

There are practical and reasonable limitations to the interpretation…

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Site/Soil

FoundationMaterial

Footing

Vertical Wall

System

Wall

Top Restraint

Bottom Restraint

Drainage System

Water Protection


proofing

Backfill/Finish

Grading

TheFoundation

System

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Footings & Walls• Foundation Wall Definitions

• Simply supported vertical flexural member

• Top and bottom laterally supported.

• 8.2.4 Lateral restraint—…The foundation walls shall be restrained top and bottom against lateral movement. The top and bottom restraint for the foundation wall shall be in place before the introduction of backfill against the foundation wall. Temporary lateral restraint shall be permitted.

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Wall Design & Construction

Complicated and creative industry requires codes to respond to provide flexibility. 84

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ACI 332 – Wall Design Tables• Combined tables for plain and reinforced

originated in 2004 edition

• Ten (10) tables for concrete strengths from 2,500 psi to 4,500 psi at steel strengths of 40 and 60 ksi

• Significant departure from IRC design tables

• Wall heights up to 10-ft.

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ACI 332 CodeFoundation Wall Structure• Extensive history of plain concrete

resulted in:� ACI 318 14.3 and 22.6.6 excluded� Horizontal reinforcement required in all

concrete walls � Minimum area of vertical wall

reinforcement shall be 0.067 sq.in./ft of wall

� Maximum vertical wall reinforcement spacing shall be 48 in on center, whenrequired.

� Minimum vertical wall reinforcement spacing shall be 0.5 times the wall thickness

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ACI 332-14Foundation Wall Structure:9.2.7 - Horizontal Reinforcement

• All walls must have horizontal reinforcement.

• Vertical rebar is placed on the inside.

• 3 horizontal bars are needed if the wall 8 feet or less in height

• 4 horizontal bars if the wall is more than 8 feet high

• 1 bar in the upper 24 in. and 1 bar in lower 24 in.,except SDC D, E and F (2 in upper 24 in.)

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Wall Design Tables:

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Wall Design Tables:2,500 psi 3,500 psi 4,500 psi

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Footings & Walls• Foundation Wall Definitions

• Simply supported vertical flexural member

• Top and bottom laterally supported.

• 9.2.2 Lateral restraint (cont.) — …The connection of the lateral support system to the top of the wall shall be in accordance with 9.6.1. The design of top lateral support is beyond the scope of this code.

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Connection to floor framing system at top of wall9.6.1 - Lateral Restraint

§ Minimum anchors dia. 0.5 in.

§ Minimum embedment depth 7 in.

§ Maximum anchor spacing 6 ft.

§ Minimum distance of anchor from each change of wall direction, height or termination 12 in.

§ Minimum distance of anchor from each side of each door or window opening 12 in.

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Why Not Tables?

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Reduced Wall Thickness:Getting brick to grade for the homeowner. 93

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Reduced Wall Thickness:9.2.5 - Reduced Wall Thickness

• The thickness of the top of a foundation wall shall be permitted to be reduced.

• Maximum height of 24-in unless meeting minimum wall thickness

• Minimum thickness of 3.5-in

• Reinforced with No. 4’s at 24-in. o.c. ≤ 4-in

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Lintels• IRC: No lintel design in foundation chapter.

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Reduced Wall Thickness:

8.2.6 - Lintel Beams - the following are permitted...§ beam depth not less than 8-

in.

§ beam span shall not exceed 40-in.

§ minimum of two (2) No. 4 longitudinal bars at the bottom, extending 24-in. into the wall each way. 96

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Site/Soil

FoundationMaterial

Footing

Vertical Wall

System

Wall

Top Restraint

Bottom Restraint

Drainage System

Water Protection


proofing

Backfill/Finish

Grading

TheFoundation

System

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Backfill:

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R404.1.7 – Shall not be placed until:• Sufficient strength• Anchored to floor, or• Sufficiently

braced against damage

• Except ≤ 4 feet unbalanced

9.7.4.1 – Before backfill• The top and

bottom restraint shall be in place before backfill

• Temporary lateral restraint shall be permitted.

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Surface Irregularities:9.7.3 – fins or projections greater than 0.5 in. shall be removed to prevent interference with waterproofing systems or interior finishes.

R9.7.3 Remove fins or other projections

• to prevent interference with dampproofing and waterproofing systems

• to prevent interference with interior finish systems where the wall surface encloses occupied space.

IRC does not address irregularities.

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Code Influence:

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ACI codes only detail concrete elements. Provisions for non-structural foundation performance specified by the IRC only.

• Drainage

• Waterproofing/Dampproofing

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James BatyExecutive [email protected]

www.cfawalls.org319-895-6940

FinalQuestions?

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TECH NOTES

Concrete Foundations Associationof North America

Using ACI 332 with the IRC

USING ACI 332 WITH THE IRC

A product of

CFA-TN-001


GOAL AND PURPOSEThis edition of Tech Notes explains the two major consensus standards that cover residential concrete construction and the potential benefi ts of using them together.

A standard, while written in mandatory code language, does not become a code until it is adopted by a government jurisdiction responsible for enacting and adopting codes. Most city and county governments adopt one or more standard documents developed by the ICC (International Codes Council), ACI, NEC and other standards developing organizations.

The two most relevant residential standards currently are:

• ACI 332 Standard

The ACI 332 Committee—Residential Concrete, has developed a standard for concrete construction in residential applications. The complete title is Requirements for Residential Concrete Construction (ACI 332-04). The ’07 edition has expanded content for a broad range of concrete construction applications (available in 2008).

• International Residential Code (IRC)

Instead of totally rewriting the IRC to include the provisions of ACI 332, the proponents of the document (CFA, NAHB, ACI, and NRMCA) have taken the approach of having several chapters or sections of the ACI 332 Standard referenced in the IRC. The 2006 Edition of the IRC was the fi rst to reference ACI 332-04. The 2006 Edition references the use of ACI 332 in the design of foundation walls, (Section R404) as follows:

R404.1 Concrete and masonry foundation walls. Concrete and masonry foundation walls shall be selected and constructed in accordance with the provisions of Section 404 or in accordance with the ACI 318, ACI 332, NCMA TR68-A or ACI 530/ASCE 5/TMS 402 or other approved structural standards. When ACI 318, ACI 332 or ACI 530/ASCE 5/

Using ACI 332 with the IRC


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TECH NOTES


TMS 402 or the provisions of Section R404 are used to design concrete or masonry foundation walls, project drawings, typical details and specifi cations are not required to bear the seal of the architect or engineer responsible for design, unless otherwise required by the state law of the jurisdiction having authority.

This simple reference enables a concrete foundation contractor in a jurisdiction that has adopted the IRC 2006 to use the provisions of Chapter 7 and Appendix A of ACI 332-04 for the design of residential foundation walls. A primary advantage is the Prescriptive Tables that offer greater fl exibility and broader recognition for plain structural concrete. The tables offer concrete strengths ranging from 2500 psi to 4500 psi and wall heights up to 10 feet in height – a considerably wider range than IRC tables allow.

Below is a portion from one of the ten tables that are provided by 332. Each table combines both plain structural concrete requirements as well as reinforced concrete requirements to simplify the decision of wall design for the user.

ACI 332 results in an increased application of plain structural concrete. However, it should be noted that if reinforcement is required you may be better served by remaining in the 2006 IRC and using those combined tables. The IRC provides for what is termed “Moderately Reinforced Design” which prescribed less steel than the minimum amount permitted by ACI 318. The 332 committee plans to address this disconnect in future editions of the standard.

continued on back

NOTES:

CFA-TN-001

www.cfawalls.org PO Box 204, Mount Vernon, IA 52314 Phone 319-895-6940 Fax: 320-213-5556 Toll Free 866-232-9255



2009 IRC The 2009 Edition of the IRC will reference the ’07 edition of the 332 Standard in the existing Section R404 as well as R402 and R403. This broadens the application to address more of the advantageous information found in the updated 332 Standard. The IRC Section R402.2—Concrete will reference the 332 Materials chapter (Chapter 3) as:

Materials used to produce concrete and testing thereof shall comply with the applicable standards listed in Chapter 3 of ‘ACI 318’ or ‘ACI 332’.

The most signifi cant advantage to this reference is the fi rst mandatory reference to accepting contaminants on reinforcement…

3.2.4 Surface conditions of reinforcement—At the time concrete is placed, deformed bar and welded wire reinforcement shall be free of materials deleterious to development.

R3.2.4 Surface contaminants such as concrete splatter, form oil or other release agents, will not prevent the reinforcing bars from achieving design values cited in the code requirements.

The IRC Section 403.1 will reference the footings chapter (Chapter 6) as:

Concrete footings shall be selected and constructed in accordance with the provisions of Section R403 or in accordance with ‘ACI 332’ or other approved structural standards.

Some of the relevant and important provisions that the inclusion of this chapter brings into the code include the acceptance of discontinuous footings, and the placement of footing dowels.

Discontinuous Wall Footings

ACI 332 states:

6.2.5 Discontinuous wall footings—Where a wall footing is discontinuous due to an abrupt elevation change, the maximum horizontal discontinuity of the wall footing shall be 4 ft. In addition, the reinforcement in the foundation wall at such a location shall conform to the requirements of 7.2.9.

R6.2.5 Abrupt elevation changes, commonly referred to as steps, usually occur in locations such as walk-out basements, grade changes, and transitions to garage foundations. At such locations, the wall spans the horizontal discontinuity of the footing. Refer to Fig. R6.2.

Footing Dowels

ACI 332 states:

6.3.4.1 A No. 4 dowel shall extend at least 12 in. into the wall and 6 in. into the footing at a maximum of 24 in. on-center along the footing. To facilitate positioning before concrete placement, vertical dowels are permitted to be driven into the grade in the bottom of the footing.

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TECH NOTES


Backfi llingFoundation

Walls

BACKFILLING FOUNDATION WALLS

A product of

CFA-TN-002


Recommendations and code requirements regarding backfi ll of newly poured basements are one of the most ignored aspects of foundation construction. The International Residential Code, American Concrete Institute (ACI) 332 Standard and the CFA Standard all state that foundation walls must be supported at the top and bottom before backfi ll is placed. Empirical tables presented in each are based on that premise.

The 2006 IRC States:

R404.1.7 Backfi ll placement. Backfi ll shall not be placed against the wall until the wall has suffi cient strength and has been anchored to the fl oor above, or has been suffi ciently braced to prevent damage by the backfi ll.

Exception: Bracing is not required for walls supporting less than 4 feet of unbalanced backfi ll.

A foundation wall is designed as a simply supported beam with restraint at the top and bottom (Fig 1). If there is no support at the top, the wall becomes

Backfi lling Foundation Walls


NOTES:

1: This plate-to-deck connection is the weakest part of the foundation assembly. Fig 1a below is enlarged from circled area of Fig. 1.

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TECH NOTES

Fig 1: Typical simply-supported foundation wall

Fig 1a

1a

a cantilevered element requiring a signifi cantly different design, as well as reinforcement requirements for both the wall and footing.

The reality, however, is that most walls are backfi lled without the stipulated support. The fact that the walls are much stronger than they need to be to resist designed lateral loads helps keep problems to a minimum but in many cases, backfi lling without suffi cient support is a problem waiting to happen.


This means that either temporary bracing or a properly constructed and connected deck (Fig 2) must be present at the top of the wall and either a keyway (Fig 3) or dowels (Fig 4) are in-place at the bottom of the wall.

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CFA-TN-002

There are several methods that foundation contractors can use to reduce the likelihood of a problem. First, keep the height of the backfi ll to no more than 4’ except at the corners or offsets until the deck is in place. Four feet of unbalanced backfi ll will usually not exert suffi cient pressure to damage the wall.

You can generally backfi ll to full height at corners and offsets. A wall extending at right angles to the backfi lling is the best support you can have for a wall. In most cased you can backfi ll short segments of walls (up to 12’ in length) full height. When you have closely spaced supports at right angles to the wall (such as corners or offsets) the wall actually can span horizontally as well as vertically. This recommendation should be used with caution unless an engineer has given specifi c design requirements for the method. (Fig 5)

Closely related to offsets is the use of counterforts (Fig 6)—thickened areas of walls or buttress walls—that, in effect, act the same way as offsets or corners. continued on back

Fig 2, right: Typical completed deck connection

Fig 3, below left: Typical keyway footing connection

Fig 4, below right: Typical rebar dowel footing connection

Fig 2

Fig 3

Fig 4

Fig 5: Common allowable areas for full-height backfi ll

The type and consistency of soils greatly impacts the design lateral load on foundations. Consequently, the backfi ll condition should be considered during the design of the foundation wall. If the original soil excavated from the over dig is to be used, the wall must be designed for the resulting soil pressures. The pressure on a foundation can be reduced if well-draining soils or granular fi lls replace the excavated native soils.

When it is time to backfi ll, the process used is as important or perhaps even more important than the material. It is not acceptable to compact the soils by driving heavy equipment next to the walls - the force exerted will exceed even the largest of assumed soil pressures. Neither is it suitable to saturate the fi ll with a garden hose to accelerate the settlement. If the soil used for the excavation is not well-drained, the excess water may cause a wall failure. The recommended backfi ll procedure involves light equipment, preferably a tamper. The fi ll is set into the hole in two-foot lifts (Fig 7) and then compacted prior to the next layer. This process achieves full height compaction rather than a surface compaction that will maintain the fi nal grade.

Finally, always leave the fi nal grade with a positive slope away from the foundation. The top of grade must not be higher than 4-in. from the top of a foundation wall with masonry veneer or 6-in. in all other cases. The grade must then slope away (positive) from the foundation a minimum of six inches (6-in.) in the fi rst ten feet (10-ft.). The greater the positive slope, the better maintenance of slope considering settlement.

Treating the foundation properly during the initial stages of construction, will pay dividends for the life of the house. For more information on residential foundations, visit the Concrete Foundations Association web site at www.cfawalls.org.




NOTES:

CFA-TN-002

These can be on either the inside or outside of the wall and should be cast integrally with the wall. This concept is similar to the use of piers in masonry construction.

Fig 6: A counterfort monolithic with the foundation wall will

support longer wall lengths like an offset or corner

Fig 7: Backfi ll in tamped lifts and fi nish with proper slope

TECH NOTES


Casting Residential Foundation Walls In Cold Weather

CASTING RESIDENTIAL FOUNDATION WALLS IN COLD WEATHER

A product of

CFA-TN-003


Casting Residential Foundation Walls In Cold Weather

CASTING RESIDENTIAL FOUNDATION WALLS I N COLD WEATHER

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A product of

CFA-TN-003

TECH NOTES

GOAL AND PURPOSEThis edition of Tech Notes explains the practical implementation of the CFA Cold Weather Research completed in 2004. This research was undertaken with the goal of providing evidence for the performance of “real world” mixes that contractors regularly use and establishing the validation process by which variations in mix design can be evaluated and applied to foundation wall construction during cold to frozen conditions.

Codes and Perceptions:

Code offi cials and builders often fear that concrete cast in ambient tempera-tures 40 F or less will be substandard, or even worse a failure if it freezes prematurely. Most national codes and standards, however, are not as alarmist on the subject. Requirements for Residential Concrete Construction (ACI-332-08) states “…concrete temperature shall be maintained above a frozen state until concrete compressive strength of 500 psi has been reached.” The CFA Standard makes a similar statement. However, some building offi cials often choose the more conservative directives of ACI’s Standard Specifi cation for Cold Weather Concreting (ACI-306.1-90), which prescribes stringent protec-tion procedures and concrete temperature monitoring “…when for more that three successive days the average daily outdoor temperature drops below 40 F.” Following ACI-306 causes extra costs to the builder and consumer. Are such extra costs in residential construction worth it?

Empirical Experience and Evidence:

Based on the experience of many CFA contractors, the cold weather mandates for protection procedures of blankets, tenting, and heating are often excessive in terms of both time and money. Using concrete mixes designed for freezing conditions, CFA concrete contractors have successfully poured thousands of concrete footings and walls, far exceeding design specifi cations, in freezing weather down to 10-degrees F without any protection procedures.

Figure 1: The refrigerated test chamber at Master Builder’s laboratory in Cleveland, OH with all cylinders from Phase I ready for testing. Note the maturity meters in cylinder 1 of each set.

CASTING RESIDENTIAL FOUNDATION WALLS I N COLD WEATHER

continued on back

NOTES:

CFA-TN-003

The Research Project:

In order to resolve the confl ict between theory and practice, the CFA funded a cold weather concreting study to understand the thermodynamic processes at work in real world conditions as concrete hydrates and cures, particularly as it relates to concrete foundation walls. The research was conducted in three phases at the Master Builders Test Facility in Cleveland, Ohio in January 2003.Phase 1: Thirty-six different mix designs were cast in cylinders and cured in

controlled conditions of 30 F for 28 days. Cylinders were tested for com-pressive strength at 1,2,3,7,14, and 28 days in order to develop maturity curves for each mix.

Phases 2 & 3: Twelve six-foot by eight-foot by eight-inch thick walls were cast in mid-January in Cleveland, using six common cold weather mix designs. Conditions were very cold, 21 F and falling at the time of casting, and temperatures remained below freezing for 21 days following. Half the panels were poured without cover, the other half with a six-foot blanket covering the top three-feet of the wall. All forms and cover were removed the day after casting. Temperature sensors to monitor internal concrete temperatures, cylinders, and cores were used to access curing history, strength, and durability properties.

Research Results:

Phase 1 - all mixes achieved a minimum of 3000 psi in 28 days, despite the 30 F curing conditions.

Phase 2 - despite the extreme weather conditions, all cylinders (cured next to the walls), and cores achieved strengths in excess of 3000 psi; most achieved strengths from 4000 to 6000 psi.

Phase 3 - accelerated freeze-thaw testing and petrographic analysis concluded that all the mixes “…were…freeze-thaw durable.”

Conclusions:

1) Concrete foundation walls can be cast in extreme cold weather condi-tions, without protection, using standard winter mixes, without jeopardiz-ing structural integrity of the wall.

2) Concrete in today’s world of heated materials, high-early cements, water-reducing admixes, and accelerators is capable of continued strength gain at internal temperatures well below 32 F. Rules and regulations based on ambient temperatures have little or no validity.

3) Maturity curves developed in the lab are remarkable indicators of con-crete performance in actual conditions.

Recommendations:1) Contractors should work with their ready-mixed concrete producers to

design mixes suitable for cold weather use in their area.2) Contact the CFA to obtain information for mixes CFA contractors success-

fully have used, with little or no protection, in extreme conditions down to 0 F.

3) Contact the CFA if you would like to purchase a copy of the CFA’s Cold Weather Research Report 2004. This report will be provided free of charge to building and code enforcement offi cials upon request.

Figure 2: Placement conditions for the full scale walls in Phase 2 included temperatures of 26°F and falling along with active frozen precipitation.

Figure 3: The remaining wall segments following core sampling to compare to the cylinders taken during placement.




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CFA-TN-001

Applying the Research:

There are practical procedures and methods that can be absorbed from this research and applied to your specifi c projects and markets.

1. The heat of hydration creates a signifi cant lag in internal concrete temper-ature in relation to ambient temperatures—staying above freezing much longer than the ambient temperature.

2. Cylinders demonstrated strength gains at each break including successive early-age breaks. Recorded strengths from Days 1, 2, 3, 7, 14 and 28 increased at each test for each mix. This information supports a revised theory that cement hydration doesn’t stop at 40°F, in fact, strength gain continues well below this temperature.

3. Maturity curves created with prediction software very closely resembled the strengths tested from cylinders and cores in both research phases. This relationship gives further support to the theory that in-place strength can be accurately and adequately determined using simple maturity meters for prediction purposes—making it easier to adjust mix designs to suit indi-vidual and regional differences and requirements.

4. Admixtures that reduce water content enhance strength gain at lower tem-peratures.

5. The amount of “free” water in the mix has a direct relationship to the affects of freezing temperatures. Concrete produced with modern tech-nologies can continue to gain strength, even as the internal temperature approaches a frozen state. Attaining an early-age strength of 500psi prior to the fi rst freeze is suffi cient to prevent damage. Ambient tempera-tures at or below 32°F did not negatively affect the fi nal performance of the concrete. All samples reached or exceeded their designed ultimate strength. Mix designs did cause variable time intervals for gaining target strengths. This provides contractors with information to use with their own mixes to achieve specifi c performance requirements.

6. In severe cold conditions, although wall strengths may be similar between covered and uncovered walls, covering the concrete for the initial 24-hr period reduces micro-fracturing and therefore improves long-term behav-ior.

7. It should be no surprise that Type III cement mix designs gain strength faster than Type I cement.

8. Similarly, accelerators such as calcium chloride and non-chloride accel-erators speed up the production of heat and therefore produce strengths faster.

TECH NOTES


Cracking InConcrete Walls

CRACKING IN CONCRETE WALLS

A product of

CFA-TN-004


Cracking In Concrete Walls


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A product of

CFA-TN-004

TECH NOTES

GOAL AND PURPOSEThis edition of Tech Notes answers common questions about cracking in con-crete walls: What Causes Them? How Can They Be Reduced? When Should You Be Concerned?.

Cracks in concrete walls and slabs are a common occurrence. They appear in fl oors, driveways, walks, structural beams, and walls. Cracking can not be prevented but it can be signifi cantly reduced or controlled when the causes are taken into account and preven-tative steps are taken. Most cracks should not be a cause for alarm.

• Causes of Cracks

Cracking can be the result of one or a combination of factors, all of which involve some form of restraint. Some examples include: - Drying Shrinkage—This occurs as water used in the mix design evaporates. - Thermal Contraction/Expansion—Due to temperature changes. - Subgrade Settlement (or Expansion) - Resulting from poor soil conditions or

changes in soil moisture content. - Differential Bearing Capacity— Harder soils under part of the foundation can

cause stresses as the building “settles in.” - Applied Stresses—Forces such as building load, earth load, hydrostatic pres-

sure, or heavy equipment operated too close to the wall.

• Types of Cracks

Tremendous forces can build up inside the wall due to any of these causes. When the forces ex-ceed the strength of the material, cracks will develop. Each of these causes normally leave a “signature” in the type of crack it cre-ates. The vast majority of cracks are of little concern by themselves.


continued on back

NOTES:

CFA-TN-004

Shrinkage and Temperature cracks are most often vertical to diagonal. They typically emanate from a corner of a window, beam pocket, or other opening. Cracks of this type are called reentrant cracks. These are very common and, unless they leak or show signifi cant lateral displacement, are of no structural concern.

Cracks which are horizontal are most likely caused by an applied load. Verti-cal cracks which are sig-nifi cantly wider at the top or bottom could indicate heaving or settlement. With these cracks it is very likely that the crack itself is not the problem, but rather the result of an external problem such as poor drainage, overloading, etc.

• Minimizing the Problem

Contractors can employ several methods of reducing the occurrence and width of cracks. - The fi rst is the use of proper concrete mix designs. A mix with suffi cient

strength using the minimum amount of water necessary to distribute the concrete throughout the wall without voids should be used. The type and amount of cement, as well as course and fi ne aggregates, can also have a large effect on the amount of shrinkage.




NOTES:

CFA-TN-004

- A small amount of temperature steel reinforcement will reduce the width of cracks that do occur.

- Control joints are intentional weak spots designed to induce shrinkage or thermal cracks in pre-determined locations. These can be very effective if waterproofed carefully.

- Rapid water loss and extreme temperature swings while the concrete is in the early stages of curing should be avoided where possible.

- Careful backfi lling is mandatory. Typical basement walls are not designed to act as retaining walls. They must be secured with the basement fl oor at the bottom and the fl oor deck at the top, or be braced adequately, before being backfi lled. The use of heavy equipment near the wall should be restricted and carefully considered.

- Anchoring the deck in accordance with local building codes, including the use of anchor bolts/straps and blocking, is very important. Improper an-choring has been the cause of a number of failures.

• When Should You Be Concerned

Temperature and shrinkage cracks in walls or slabs are likely to occur in nearly all structures. When the width of a crack exceeds 1/4” in width; when they show 1/4” in lateral dis-placement; when water leaks through the cracks; or you fi nd long horizontal cracks, it is probably time to seek professional assis-tance. The contractor that built the wall, or your local CFA member should be able to help you.

TECH NOTES


WATERPROOFING, BACKFILL, AND MAINTENANCE

A product of

CFA-TN-010

Waterproofing,Backfill,

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Practices for the treatment of water around the foundation including drainage, waterproofing and dampproofing.


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CFA-TN-010 page 2

TECH NOTESDRAINAGEIn residential construction the foundation walls are designed for specificpressure, which depends on the properties of the soil and its water content. Changes in the water content due to weather conditions, changes to the building systems” or rising water table can result in significant increases in the pressure and thus the loading on the wall. Proper drainage can ensure that the design pressure is consistent with the actual pressure on the walls.

The goal or design theory behind proper drainage is the placement of materials with a high coefficient of permeability near the bottom part of the foundation. A medium size aggregate and sand is most commonly used, permitting water to easily collect from the soil above. Then by using drain tiles the water can be taken either away from the walls or stored in a tank. It can also be drawn away with a pump. Figure 1 below shows drainage configurations with outside and inside drain tiles.


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4" Drain Tile

to Sump Basket4" Pourous Layer

Water Table

6 mil Vapor Barrier

Basement Slab

Sump Basket

Aggregate with

Filter Material

Drainage Sleeve

4 in. Drain Tile

Drainage Relief @ 4 ft. o.c.

(Optional)

FIGURE 1


continued on next page

NOTES:

CFA-TN-010 page 3

Proper drainage can be provided by using gravel drains, drainage tiles, perforated tiles or other proprietary commercially available systems. If a proprietary system is used it should be installed as per manufacturers recommendations. When using gravel or crushed stone for filler material it should be placed at least 1 foot beyond the outside edge of the footing and should be at least 6 inches above it. The gravel around the drainage system might have to be covered with a filter fabric depending on soil type.

In cases where a high water table is present both interior and exterior drain tiles should be installed. A minimum of four inches of gravel or crushed stone are typically placed underneath the floor slab to drive water to the drain tiles. It is also recommended to install a vapor barrier between the porous layer and the slab, shown in Figure 2.

COATINGS & MEMBRANESThe treatment of the foundation wall prior to backfill must include a coating or membrane applied to the surface of the concrete wall. Depending on the applicable code, project requirements or homeowner preference, systems are selected as either damppprofing or waterproofing.

The International Residential Code (IRC) in section R406.1 (2012) prescribes that dampproofing is always required when the foundation walls enclose a habitable or other usable space except in cases where waterproofing is required. The application must extend from the top of the footing to the finished grade.

A common material for dampproofing is bituminous coating, which is applied to the surface of the concrete wall. As a general rule, dampproofing coatings have poor elongation characteristics. This means they are not generally capable of stretching due to movement in the wall, especially when cracks

FIGURE 2

4" Porous Layer

Basement Slab

6 mil Vapor Barrier

Water Table 4" Drain Tile to

Sump Basket

4" Perimeter Drain Tile

Aggregate with

Filter Material


NOTES:

CFA-TN-010 page 4

develop from shrinkage taking place after the coating is applied or from poor maintenance to the structure.

Waterproofing Systems that can expand or stretch as the concrete surface changes are classified as waterproofing. They are capable of protecting the interior space over greater lengths of time and under more significant pressures.

Waterproofing is required when the foundation walls enclose a habitable or other usable space and the area is subject to a high water table or other ground-water conditions. It should completely cover the walls that are below grade and extend from the top of the footing to the finished grade.

Materials that can be used for waterproofing include: 3 ply hot mopped systems, bitumen, asphalt, and rubber sheets, or other commercially available waterproofing systems. When using propriety systems the Concrete

FIGURE4

FIGURE 3


NOTES:

CFA-TN-010 page 5

Foundations Association recommends the use of ones with more than 5 years of labor and material replacement warranties.

TROUBLE SHOOTING

Water can appear on the foundation walls due to several reasons (by the IRC in section R406.2):

• Free flowing surface or ground water penetrating through cracks or openings both on the walls and the slab.

• Moisture traveling through the soil and concrete capillaries.• Condensation of water on cold foundation walls.• Vapor migration from cold damp soil to a warmer basement.

Water appearing on the inside surface of the foundation, whether on the wall or on the slab should be investigated. While concrete foundation walls do not have organic material to support the growth of mold or mildew, the persistent presence of moisture coupled with the natural dusts and finish materials of the interior space can encourage mold to grow, using the wall as a supporting structure. More on the topic of mold and foundations from can be obtained from the CFA website, www.cfawalls.org.

Troubleshooting these conditions is based on the type of drainage and coating system installed as well as the deficiencies in the maintenance of the structure. The first step should be to determine why there is water building up and passing through the wall. As seen in Figures 1 and 2, proper drainage and aggregate fill will keep the natural amounts of water in the soils from becoming problematic meaning that either more water exists or a failure has occurred.

• Inspect the gutters and downspouts. Homeowners frequently knock downspouts off or remove them to mow and fail to restore them to their installed position. Gutters become clogged with plant material forcing water to build up and flow over a low point in the run rather than dropping through the downspout.

• Look for water ponds along the foundation. This is an indication that the grade has settled over time or changes to the landscaping have resulted in a negative slope rather than the IRC requirement for a 10% slope (see CFA-TN-002 for more information).

• Determine if water is draining from the foundation. By looking into the sump pit (if it exists) or observing water actively draining to daylight from the foundation (common on sites with steep slopes), this is likely to be determined. If water cannot be seen draining from the system installed for the home, it is likely drainage tiles have been crushed or become clogged with roots or silt.

Once this determination is made, the remedies for the water condition range from simple to extensive. Putting back downspouts and cleaning out gutters, as well as perhaps installing gutter screens are quick an relatively inexpensive solutions. Restoring a minimum grade aware from the foundation of 10% may be much harder or impossible, depending on lot and access. Repairing drainage tile is the most invasive requiring the soil around the foundation to be dug away to expose the point where the drainage tile exists. This may not be warranted.

continued on next page


NOTES:

CFA-TN-010 page 6

If water has been determined or confirmed to be actively passing through the wall, it is likely that a dampprooing coating was used or no coating at all. There are two primary options for controlling water through the wall without digging away the soil from the foundation; filling the cracks and installing an interior water remediation barrier.

Crack remediation is a common solution for leaking cracks. The use of an epoxy-injection system will seal the crack, preventing moisture from leaking through to the interior, and it will strengthen the wall by bridging across the crack. It is recommended that a wall repair specialist be contracted with to provide this solution.

Water remediation barriers are systems designed to continue permitting water to move through the wall, capturing it and diverting it to a method of removal, such as a sump pit. These systems are often advantageous where interior finishes on the exterior of the wall exist and have to be removed to repair the condition.

The most aggressive method for fixing a water problem remains the excavation of the foundation perimeter and applying a waterproofing membrane or sheet, such as is recommended during new construction.

FIGURE 5

Excavation

Anchor Bolts @

6'-0" o.c. max.

Waterproofing Membrane

Designed Grade Line

6" in 10'-0" min.

Settlement Line

from Backfill

4" Perimeter Drain Tile

Aggregate with

Filter Material

Top Connection and Floor

Joists Req'd Before

Backfill or Bracing

Downspout


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TECH NOTES


SOILS & EXCAVATION SAFETY

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CFA-TN-011

Soils&

ExcavationSafety

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What the concrete contractor needs to know about soils, OSHA excavation regulations and the safe practices of excavating for concrete foundations.

SOILS & EXCAVATION SAFETYCFA-TN-011 page 2

TECH NOTESFoundations walls are designed based on a constant soil condition that assumes the site is maintained to ensure their long term performance. The properties of the soil, its water content and the conditions that can affect those factors must be identified to successfully design the footings and walls

Once the foundation is designed, excavation is the first step of actual on site work. It sets the stage for the construction of the walls and must be done according to common regulations to ensure the safety of the workers and the integrity of the structure.

SOILS A complex matrix of inorganic and organic particles, the physical properties of soils vary by region as well as within localities. The most important properties influencing construction are texture, density, porosity and consistency. Soil texture is determined by the relative proportion of sand, silt, and clay. These three particles adhere in varying ways becoming larger, relatively stable secondary structures. Soil density is a measure of its compaction. Soil porosity is the discussion of the voids between the particles and secondary structures that become occupied by or permitting the travel of gases or water. Soil consistency, sometimes also referred to as cohesion, is the ability of soil to stick together.

The structural behavior of the soil is governed by these characteristics and the resulting water content. The most commonly used measures of soil conditions are: • Maximum particle size. • Density. • Porosity ratio.

Maximum Particle SizeThe Standard Test Method for Particle-Size Analysis of Soils of the American Society of Testing and Materials (ASTM D422-63) is the most widely recognized standard for particle size. Based on the particles passing through the decreasing screen hole diameters in sieves, soils are given designation within the following general definitions: • Gravel - from 3 in. to 0.08 in. • Sand – from 0.08 in. to 0.003 in. • Silt – from 0.003 in. to 0.0002 in. • Clay - less than 0.0002 in.

Figure 1 below demonstrates the relationship of these common general categories to the mix of soil types as defined by the United States Department of Agriculture.

FIGURE 1: SOIL CLASSIFICATION RELATIONSHIP PYRAMID (U.S.D.A.)


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CFA-TN-011 page 3

Density

Density, defining the level of compaction for the soil, is the weight per unit volume of an object. Soil particle density is usually unchanging for a given soil, however, the resulting compaction desired or assumed for the project design, can be altered if the soil type used in the backfill is changed or if the process for compaction is not completed properly (see CFA TN-002). Soil particle density is lower for soils with high organic matter content, and is higher for soils with high iron-oxide content. The higher the organic matter content, the less compacted the soil can become.

Soil bulk density is equal to the dry mass of the soil divided by the entire volume of the soil including air space and organic materials. A high bulk density means either the soil compaction was great or the sand content was high whereas lower bulk density means the amount of cultivated loam is greater. The soil bulk density for a given soil is highly variable and is always less than the actual soil particle density.

PorosityPorosity is the opposite ratio to density as it divides the volume of the voids by the total volume of the soil sample. The voids, or pore spaces are not occupied by either mineral or organic matter and therefore are open space to be occupied by either gases or water. On average, the total pore space is 50% of the soil volume.

Soils that exhibit high water content are generally those with lower porosity but having forces of attraction that hold water in place greater than the gravitational force acting to drain the water. Having large pore spaces that allow rapid gas and water movement is superior to smaller pore space soil that has a greater percentage pore space. Soil texture determines the pore space at the smallest scale, but at a larger scale, soil structure has a strong influence on soil aeration, water infiltration and drainage. Clay soils have smaller pores, but more total pore space than sand. The Affects of Particle Size, Density and PorosityMaximum particle size, density and the porosity ratio are all loosely related. Typically the larger the aggregate size the larger the porosity ratio. However soils with large aggregate sizes can be very compact and thus have low porosity ratio.

Three important soil design parameters are based on the impact of those properties and generally govern foundation wall designs. They are: • Coefficient of permeability. • Settlement. • Bearing capacity

Coefficient of PermeabilityAllowing for proper drainage is the single most important thing that can be done to ensure the integrity of the foundation walls. The coefficient of permeability measures the rate at which water can travel through the soil. It shows how fast water can fill and leave the voids in the soil.


It depends on several factors: the size of the soil grains or the maximum aggregate size as described above, the shapes and the arrangement of the soils, the properties of the water, and the porosity of the soil.

There are standard tests developed to determine this coefficient. It is measured in inches per hour and usually soils with permeability of less than 0.8 in/hr are considered moderate or slow permeability. Soils with low permeability like clay drain slower and are more likely to retain water for long periods and thus become unstable.

Permeability, or rather proper drainage is very influential on the long-term performance of foundation walls. See ‘CFA TN-010’ for more information on the design and construction solutions to ensure proper perimeter drainage.

SettlementSettlement is another soil property that affects both the short- and long- term life of the foundation. Its effects can become noticeable soon after the foundation is completed or after several years, particularly due to poor maintenance of the water drainage systems.

The amount of settlement is directly attributed to the porosity of the soil, the particle size and the relationship of particle mix (Figure 1). Settlement of the foundation occurs when the weight of the building compresses the soil and either air or water has been squeezed out. It is known that all foundations settle with time. Settlement of the surrounding soils occurs when the pore spaces of the soil are filled repeatedly with water, compressing the soil under greater weight until the water is pushed out from the resulting densification. Depending on the soil type settlement can vary both in the duration over which settlement occurs and the amount as seen in Table 1. Figure 2 demonstrates the process of settlement.

FIGURE 2: SETTLEMENT PROCESS FOR FOUNDATIONS

TABLE 1: TYPICAL SETTLEMENT PATTERNS FOR COMMON SOIL TYPES

CFA Certification Study Guide – Soils & Excavation

TABLE 1-1: TYPICAL SETTLEMENT PATTERNS FOR COMMON SOIL TYPES Type of Soil Timeframe Amount (inches) Sands Weeks to months 1/8 – 3/8 Silts Months to years 1/8 – 1/2 Clays Years to decades 3/8 – 3/4 BEARING CAPACITY The bearing capacity of the soil represents its ability to support, distribute, and transfer loads to the ground. The allowable bearing capacity shows the maximum pressure that the soil can be subject to before failure or excessive settlement occurs. It is measured in pounds per square foot. The following rating is commonly accepted for soil bearing capacities:

TABLE 1-2: SOIL BEARING CLASIFFICATION

From (pounds per square foot)

To (pounds per square

foot)

Bearing Capacity

500 1,000 Poor

1,000 1,500 Marginal 1,500 2,000 Low 2,000 3,000 Medium 3,000 4,000 Good

More than 4000 Excellent Soils with bearing capacity of above 2,500 psf are considered adequate for foundation construction. The Concrete Foundations Association has provided a specification for the most common soil types and design parameters:

TABLE 1-3: PRESUMPTIVE LOAD BEARING VALUES OF FOUNDATION MATERIALS

CLASS OF MATERIAL L0AD-BEARING PRESSURE (pounds per square foot)

Intact Crystalline Bedrock 30,000 Fractured Crystalline Bedrock 12,000

Intact Sedimentary Rock 15,000 Fractured Sedimentary Rock 6,000

Sandy Gravel or Gravel 5,000 Sand, Silty Sand, Clayey Sand, Silty Gravel

and Clayey Gravel 3,000

Clay, Sandy Clay, Silty Clay and Clayey Silt 2,000

James Baty 6/18/15 11:54 AMComment: Sub-‐heading


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CFA-TN-011 page 5

Bearing Capacity

Without getting to deep in the process of geotechnical engineering, the bearing capacity of the soil represents its ability to support, distribute, and transfer loads to the ground. The size of a footing in a foundation design accounts for the allowable bearing capacity or the maximum pressure the soil can be subjected to before the soil structure fails in either shear or excessive settlement. Allowable bearing capacity is generally expressed in pounds per square foot (psf) and is a criteria in most prescriptive footing design tables. Table 2 below shows soil bearing capacity ranges and the respective commonly-adopted ratings:

TABLE 2: SOIL BEARING CLASSIFICATION

Soils with bearing capacity of above 2,500 psf are considered adequate for foundation construction. The most common soil types and average allowable bearing capacities can be found in Table 3.

TABLE 3: PRESUMPTIVE LOAD BEARING VALUES OF FOUNDATION MATERIALS






foot)

Bearing Capacity

500 1,000 Poor
















foot)

Bearing Capacity

500 1,000 Poor












Table 4 presents the properties of the common soil types that impact the design and performance of foundations.

TABLE 4 – SOIL PROPERTIES ACCORDING TO THE UNIFIED CLASSIFICATION SYSTEM


TABLE – SOIL PROPERTIES ACCORDING TO THE UNIFIED CLASSIFICATION SYSTEM

James Baty 6/18/15 11:55 AMComment: Will need to recreate this in text or get a better graphic. Is it just as easy for you to insert a table and format it appropriately?

NOTES:


FROST One of the factors affecting the long-term performance of sub-grade and on-grade concrete is frost. Frost is the crystallization of water vapor within the poor spaces or on the surface of soils. It is usually formed when there are long periods of temperature of less than 32 o F, there is water present, and the soil has fine grains, which allow capillary action. Since frost is a product of the phase change of water from liquid to solid, the volume of the soils must change and expansion can occur if there is not sufficient pore space within the particles to accommodate the change. EffectIn its solid state water occupies about 10% more volume than in liquid so it forces the soil to expand. Since bearing capacity is based on the soil having a consistent volume and cohesion, the presence of frost can alter the bearing capacity and influence settlement.

Therefore, if a footing or a slab is poured on ground that is frozen or has enough frost in it, a non-uniform settling might occur as the soil defrosts or thaws naturally that might cause the concrete to crack. See ‘CFA TN-003’ for more information on the effects of cold weather on foundation construction and ACI 332 for the minimum requirements for placing concrete during cold weather.

EXCAVATION SAFETY The cohesion of the soils and allowable bearing capacity is affected when loads are applied adjacent to an open excavation without sufficient design. When a hole is dug to prepare for a foundation, the soil capacity is altered by exposure to the elements. Additionally, the wall resulting from the removal of soils no longer has lateral support for the transfer of vertical forces including gravity. The results of not attending to proper excavation design can be catastrophic.

RequirementsContractors must comply with the Occupational Safety and Health Administration (OSHA 1926.652) requirements. This standard provides regulations on excavation and backfilling safety.

Some of the basic requirements are: • The minimum distance between the excavation and the foundation wall

forming system should be 2 ft (see Fig. 3).• If the wall is more than 7-1/2 ft deep the excavation should be benched 2

ft horizontally for every 5 ft of vertical depth (see Fig. 3). • All heavy equipment should not operate at a distance to the top edge

of the excavation, (see Fig. 4). Organizing the site to permit the perpendicular approach of heavy equipment will minimize the potential for collapse of the excavation walls (see Fig. 5) or the use of placement equipment such as concrete pumps and conveyors is recommended.

• All excavations must provide workers a method of egress within 20 ft.• The work in the excavation should be planned such that it requires a

minimum number of workers and minimum amount of time.


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FIGURE – BENCHING REQUIREMENT

FIGURE – MINIMUM LOAD DISTANCE REQUIREMENT

FIGURE 4 - MINIMUM DISTANCE FROM EXCAVATION FOR LOAD SURCHARGE

2'-0" Min. 2'-0" Min.

5'-0"

9'-4"

FIGURE 3 - BENCHING REQUIREMENTS PER OSHA 1926.652


FIGURE – EQUIPMENT LOCATION

FIGURE 5 - TYPICAL DELIVERY LOCATIONS FOR FOUNDATIONS AT EXCAVATIONS

TECH NOTES


CONCRETE FOR THE RESIDENTIAL CONTRACTOR

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CFA-TN-012

Concrete for the Residential Contractor

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What the residential concrete contractor needs to know about concrete.

CONCRETE FOR THE RESIDENTIAL CONTRACTORCFA-TN-012 page 2

TECH NOTESConcrete is the most widely used man made material on earth, but it is also a misunderstood product. The most common misconception is perhaps the interchangeable use of the words cement and concrete. Another is the assumption that a slab or wall crack is a failure. Our challenges are to educate people about the product that provides our livelihood, and to deliver the best possible concrete installation we can produce. There are two infallible characteristics of concrete: it gets hard; and, it cracks. The procedures and methods by which we manufacture, mix, handle, place and cure concrete however, can make a tremendous difference in how hard the concrete gets, how it weathers and performs, and how many or how wide the inevitable cracks become. Having a better understanding of this resilient and versatile material will enable you to be better at your job and deliver a better product to the customer.

CONCRETE MATERIALS Concrete can be described as a basic mix of four main constituents: cement, large aggregate, small aggregate (sand) and water. Each of these materials plays an important role in delivering an economical, yet strong and serviceable concrete building component. Concrete is relatively heavy, weighing between 140-150 pounds per cubic foot and it is very strong when placed in compression. Special concrete mixes can produce much heavier or lighter weights if conditions warrant but the properties of these special mixes will also vary. Let’s take a look at each of these four basic constituents in greater detail.

CementConcrete is able to achieve its first infallible characteristic because of the presence of a fine powder-like substance commonly referred to as cement. Today, the complexity of mix design and the goal for sustainability in the environment mean we should actually use the phrase cementitious material. The most common form of this material in today’s concrete remains portland cement. Portland cement is a hydraulic cement which means that it sets and hardens by reacting chemically with water. Portland cement is manufactured from limestone, which is ground and mixed with a small amount of iron, silica, alumina, fly ash and a few additional additives. The limestone is first exposed to very high temperatures, typically between 2600 - 3000 degrees F, in a long (or tall) rotary kiln. The heat breaks the limestone down in a process called calcination, which drives off CO2 from the limestone.

FIGURE 1 Pantheon, Rome circa 130 ad

FIGURE 2Large Aggregate, Small Aggregate (sand),

Portland Cement, Water

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Concrete is the most widely used man made material on earth, but it is also a misunderstood product. The most common misconception is perhaps the interchangeable use of the words cement and concrete. Another is the assumption that a slab or wall crack is a failure. Our challenges are to educate people about the product that provides our livelihood, and to deliver the best possible concrete installation we can produce. There are two infallible characteristics of concrete: it gets hard; and, it cracks. The procedures and methods by which we manufacture, mix, handle, place and cure concrete however, can make a tremendous difference in how hard the concrete gets, how it weathers and performs, and how many or how wide the inevitable cracks become. Having a better understanding of this resilient and versatile material will enable you to be better at your job and deliver a better product to the customer.

Fig 1: Pantheon, Rome circa 130 ad

Concrete Materials Concrete can be described as a basic mix of four main constituents: cement, large aggregate, small aggregate (sand) and water. Each of these materials plays an important role in delivering an economical, yet strong and serviceable concrete building component. Concrete is relatively heavy, weighing between 140-‐150 pounds per cubic foot and it is very strong when placed in compression. Special concrete mixes can produce much heavier or lighter weights if conditions warrant but the properties of these special mixes will also vary. Let’s take a look at each of these four basic constituents in greater detail.

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Fig 2: Large Aggregate, Small Aggregate (sand), Portland Cement, Water

Cement

Concrete is able to achieve its first infallible characteristic because of the presence of a fine powder-‐like substance commonly referred to as cement. Today, the complexity of mix design and the goal for sustainability in the environment mean we should actually use the phrase cementitious material. The most common form of this material in today’s concrete remains portland cement. Portland cement is a hydraulic cement which means that it sets and hardens by reacting chemically with water. Portland cement is manufactured from limestone, which is ground and mixed with a small amount of iron, silica, alumina, fly ash and a few additional additives. The limestone is first exposed to very high temperatures, typically between 2600 -‐ 3000 degrees F, in a long (or tall) rotary kiln. The heat breaks the limestone down in a process called calcination, which drives off CO2 from the limestone. The resultant lime “clinkers” are then mixed with gypsum and limestone, which regulates setting time, and ground to a fine powder to give us the cement product we use today.

The American Society of Testing and Materials (ASTM) defines standards for many of the materials used in construction, including cement. ASTM C 150 is the standard used to define the five primary types of Portland Cement. The categories are based on the properties or behaviors exhibited during the hydration process or in-‐place durability. The most common is Type I portland cement. Type III portland cement is the second most common type as it obtains higher early strengths from a finer powder that reacts more quickly with water. The finer consistency provides more surface area to speed the hydration process resulting in higher early strengths, and elevated temperatures. ASTM also defines types of Portland cement for other applications that are less common throughout most markets. Type II and Type V


NOTES:

CFA-TN-012 page 3

The resultant lime “clinkers” are then mixed with gypsum and limestone, which regulates setting time, and ground to a fine powder to give us the cement product we use today.

The American Society of Testing and Materials (ASTM) defines standards for many of the materials used in construction, including cement. ASTM C 150 is the standard used to define the five primary types of Portland Cement. The categories are

based on the properties or behaviors exhibited during the hydration process or in-place durability. The most common is Type I portland cement. Type III portland cement is the second most common type as it obtains higher early strengths from a finer powder that reacts more quickly with water. The finer consistency provides more surface area to speed the hydration process resulting in higher early strengths, and elevated temperatures. ASTM also defines types of Portland cement for other applications that are less common throughout most markets. Type II and Type V cements are for moderate and high sulfate resistance respectively and Type IV is designed to produce a lower heat of hyration and slower set time.

SustainabilityThe manufacturing process just described for cement results in the release of CO2 from the limestone as well as the combustion of the fuel to produce the high temperatures for the kiln. Cement contributes approximately 96% of the carbon footprint of concrete during a very short duration and before it ever becomes part of the concrete. The emission during calcination accounts for about 65% of the CO2 and burning of fuels to heat the kilns accounts for the remaining balance. The industry is making great strides to reduce its carbon footprint by utilizing waste materials such as old tires in the fueling of kilns, and modernizing manufacturing plants.

Cement, however, is only one component of concrete comprising between 10-15% of the total content and the remaining 85-90% offers very low embodied energy and produces virtually insignificant emission of greenhouse gasses. Figure 4 demonstrates the relationship of some common mix designs and the comparison of cement content to the remaining three primary constituents. When water,

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cements are for moderate and high sulfate resistance respectively and Type IV is designed to produce a lower heat of hyration and slower set time.

Fig 3: Rotary Cement Kiln

Sustainability

The manufacturing process just described for cement results in the release of CO2 from the limestone as well as the combustion of the fuel to produce the high temperatures for the kiln. Cement contributes approximately 96% of the carbon footprint of concrete during a very short duration and before it ever becomes part of the concrete. The emission during calcination accounts for about 65% of the CO2 and burning of fuels to heat the kilns accounts for the remaining balance. The industry is making great strides to reduce its carbon footprint by utilizing waste materials such as old tires in the fueling of kilns, and modernizing manufacturing plants.

Cement, however, is only one component of concrete comprising between 10-‐15% of the total content and the remaining 85-‐90% offers very low embodied energy and produces virtually insignificant emission of greenhouse gasses. Figure 4 demonstrates the relationship of some common mix designs and the comparison of cement content to the remaining three primary constituents. When water, locally quarried sand and large aggregate, as well as the local production of and delivery of the mix, are factored into the process, concrete compares very favorably to other building materials.

FIGURE 3Rotary Cement Kiln

FIGURE 4Proportions of Basic Concrete Materials

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Fig 4: Proportions of Basic Concrete Materials

Another factor reducing the carbon footprint of concrete is the use of supplementary cementitious materials (SCMs) to replace a portion of the portland cement. These products, called blended cements, add waste materials such as fly ash, silica fume, and slag, have similar properties to Portland cement and are materials that otherwise would be disposed of in land fills. Up to 50% of the portland cement can be replaced with SCM’s if proportioned properly. The workers and specifiers must understand that some concrete properties, such set and early-‐age times affecting finishing and curing might be altered. It is best to have working knowledge of these impacts if you are using large percentages of SCMs.

Fig 5: Supplementary Cementitious Materials

Water


locally quarried sand and large aggregate, as well as the local production of and delivery of the mix, are factored into the process, concrete compares very favorably to other building materials.

Another factor reducing the carbon footprint of concrete is the use of supplementary cementitious materials (SCMs) to replace a portion of the portland cement. These products, called blended cements, add waste materials such as fly ash, silica fume, and slag, have similar properties to Portland cement and are materials that otherwise would be disposed of in land fills. Up to 50% of the portland cement can be replaced with SCM’s if proportioned properly. The workers and specifiers must understand that some concrete properties, such set and early-age times affecting finishing and curing might be altered. It is best to have working knowledge of these impacts if you are using large percentages of SCMs.

Water It was mentioned previously that cement is a hydraulic material, that is activated when introduced to water. Water then, is necessary to support the process of concrete hardening. The water should not have impurities or other chemicals. Potable (drinking) water is considered adequate for concrete manufacturing. You must also have a sufficient amount of water to insure that hydration occurs and that the concrete can be placed and worked, but too much water reduces the strength of concrete and causes excessive shrinkage which results in wider and more frequent cracks. Too much water also increases the porosity of concrete, making it more vulnerable to problems related to freeze/thaw conditions. A ratio called the water/cementitious material ratio (w/cm) is used to express the amount of water relative to the amount of total cementitious product (both portland cement and SCMs). This is determined by dividing the weight of the water by the weight of the cementitious material in a given batch. Generally, in terms of strength development then, a lower w/cm ratio (stiffer mix) is best, assuming enough water is present to fully activate the amount of cementitious material.

AggregatesAggregates are an essential component of concrete. Size, hardness, shape, and surface absorption are characteristics of the large aggregates that impact the quality and strength properties of the concrete. Crushed rock, or gravel, is the most common type of aggregate used. The angularity of the crushed rock adds to the strength of the mix. Round stones, such as river gravel, can be used to produce special concrete mixes such as exposed aggregate concrete but allowances must be made for potential changes in properties, like reduction of in-place strength. The strength of the aggregates will also directly impact the strength of the concrete since strength is largely based on compression. This is particularly evident when light weight aggregates, that are less dense and often softer, are used.

Another requirement for aggregates is that they be graded. The small

FIGURE 5Supplementary Cementitious Materials

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Fig 4: Proportions of Basic Concrete Materials

Another factor reducing the carbon footprint of concrete is the use of supplementary cementitious materials (SCMs) to replace a portion of the portland cement. These products, called blended cements, add waste materials such as fly ash, silica fume, and slag, have similar properties to Portland cement and are materials that otherwise would be disposed of in land fills. Up to 50% of the portland cement can be replaced with SCM’s if proportioned properly. The workers and specifiers must understand that some concrete properties, such set and early-‐age times affecting finishing and curing might be altered. It is best to have working knowledge of these impacts if you are using large percentages of SCMs.

Fig 5: Supplementary Cementitious Materials

Water

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It was mentioned previously that cement is a hydraulic material, that is activated when introduced to water. Water then, is necessary to support the process of concrete hardening. The water should not have impurities or other chemicals. Potable (drinking) water is considered adequate for concrete manufacturing. You must also have a sufficient amount of water to insure that hydration occurs and that the concrete can be placed and worked, but too much water reduces the strength of concrete and causes excessive shrinkage which results in wider and more frequent cracks. Too much water also increases the porosity of concrete, making it more vulnerable to problems related to freeze/thaw conditions. A ratio called the water/cementitious material ratio (w/cm) is used to express the amount of water relative to the amount of total cementitious product (both portland cement and SCMs). This is determined by dividing the weight of the water by the weight of the cementitious material in a given batch. Generally, in terms of strength development then, a lower w/cm ratio (stiffer mix) is best, assuming enough water is present to fully activate the amount of cementitious material.

Fig 6: Regular slump shown in a cone and SCC concrete shown in a puddle or flow.

The water/cementitious material ratio can best be seen in the amount of flow of the mix. The standard of measure for this flowability of concrete resulting from the addition of water is called slump. Slump is measured using an open-‐ended, truncated metal cone which is filled with concrete in accordance with ACI procedures. The cone is then lifted off the concrete and place adjacent to the remaining mass. The distance the concrete mass slumps from the height of the cone is called its slump. Slump ranges are specified depending on use and commonly range between 3-‐5 in. (75-‐125 mm) for most applications. A slump of this amount is typically required to place and finish most concrete. Since the slump is a result of the targeted water/cementitious ratio and the properties of the concrete hardening are based on this w/cm, obtaining higher slumps by adding more water may

FIGURE 6Regular slump shown in a cone

and SCC concrete shown in a puddle or flow

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seriously impact the quality of the concrete. Higher slumps or greater flow can still be obtained (see admixtures) without negatively altering the physical properties of the cured concrete.

During the mix production process, the portland cement combines with the water to make a paste. The paste naturally coats the large and small aggregate during the revolutions of the mixing drum and in turn the reinforcement during placement to comprise the matrix that will become hardened concrete.

Fig 7: (Photo of cross section cut through concrete)

Electron Microscope Photo – Hydroxide Crystals.

Aggregates

Aggregates are an essential component of concrete. Size, hardness, shape, and surface absorption are characteristics of the large aggregates that impact the quality and strength properties of the concrete. Crushed rock, or gravel, is the most common type of aggregate used. The angularity of the crushed rock adds to the strength of the mix. Round stones, such as river gravel, can be used to produce special concrete mixes such as exposed aggregate concrete but allowances must be made for potential changes in properties, like reduction of in-‐place strength. The strength of the aggregates will also directly impact the strength of the concrete since strength is largely based on compression. This is particularly evident when light weight aggregates, that are less dense and often softer, are used.

Another requirement for aggregates is that they be graded. The small aggregate size of sand is important for filling much smaller holes but large aggregates must contain a variety of sizes to ensure a more cohesive cross section. Grading is the process where aggregate is run across screens of different size holes. The actual aggregate used in the mix is then assembled from an even distribution of

FIGURE 7Electron Microscope Photo – Hydroxide Crystals


NOTES:

CFA-TN-012 page 5

aggregate size of sand is important for filling much smaller holes but large aggregates must contain a variety of sizes to ensure a more cohesive cross section. Grading is the process where aggregate is run across screens of different size holes. The actual aggregate used in the mix is then assembled from an even distribution of rock pieces or sizes fitting through many different hole sizes. This is also called continuous-grading.

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rock pieces or sizes fitting through many different hole sizes. This is also called continuous-‐grading.

Typical Concrete with Graded Aggregates.

Air, entrained and entrapped

The process of combining cementitious material and water into a paste naturally results in the presence of air bubbles. These may increase throughout the mixing process. This type of air is called entrapped air. The entrapped air is in the form of tiny bubbles approximately the same size as the sand or fine aggregate. The amount is typically less than 2 percent by volume. These air bubbles can allow moisture to enter into the concrete, which can decrease resistance to freeze/thaw in some climates and greater porosity. Therefore, a vibration process is important during the concrete placement process in an effort to drive these air bubbles to the surface and consolidate the paste more thoroughly throughout the volume.

Different than entrapping air through the natural mixing process, entrained air is an intentional mix design specifically desired for the long-‐term performance of some types of concrete. Air entrainment is achieved with a chemical that can be added to the concrete to increase weatherability. While the larger entrapped air bubbles will allow water to penetrate the concrete resulting in increased porosity as well as greater degradation in freeze/thaw cycles, entrained air is the introduction of microscopic air bubbles. Millions of microscopic air bubbles (as small as 0.003 inch diameters), that are too small to allow moisture to penetrate, provide millions of voids for the concrete to fill as it shrinks. Air entrainment can also improve the workability of the concrete, acting like tiny ball bearings to help the concrete flow more easily across or through surfaces.

.

Admixtures

Modern chemistry has come to the aid of concrete with a wide variety of chemicals that can be added during the mixing operation. These are called admixtures. These chemical admixtures are most often used to alter the

FIGURE 8Typical Concrete with Graded Aggregates.

Air, entrained and entrappedThe process of combining cementitious material and water into a paste naturally results in the presence of air bubbles. These may increase throughout the mixing process. This type of air is called entrapped air. The entrapped air is in the form of tiny bubbles approximately the same size as the sand or fine aggregate. The amount is typically less than 2 percent by volume. These air bubbles can allow moisture to enter into the concrete, which can decrease resistance to freeze/thaw in some climates and greater porosity. Therefore, a vibration process is important during the concrete placement process in an effort to drive these air bubbles to the surface and consolidate the paste more thoroughly throughout the volume.

Different than entrapping air through the natural mixing process, entrained air is an intentional mix design specifically desired for the long-term performance of some types of concrete. Air entrainment is achieved with a chemical that can be added to the concrete to increase weatherability. While the larger entrapped air bubbles will allow water to penetrate the concrete resulting in increased porosity as well as greater degradation in freeze/thaw cycles, entrained air is the introduction of microscopic air bubbles. Millions of microscopic air bubbles (as small as 0.003 inch diameters), that are too small to allow moisture to penetrate, provide millions of voids for the concrete to fill as it shrinks. Air entrainment can also improve the workability of the concrete, acting like tiny ball bearings to help the concrete flow more easily across or through surfaces.

AdmixturesModern chemistry has come to the aid of concrete with a wide variety of chemicals that can be added during the mixing operation. These are called admixtures. These chemical admixtures are most often used to alter the performance or simplify the placement of concrete. Admixtures known as retarders slow down the curing rate of concrete, which can be helpful in hot weather or extremely intricate configurations. Accelerators are admixtures that can speed up the hydration process which can be advantageous when placing


concrete in cold weather or when a higher early strength is required. Accelerators are an example of an admixture class that can accomplish the same goal using different forms. Accelerators are most often divided into two categories, calcium-chloride and non-chloride. The type of concrete application and the applicable code will instruct whether one or the other can be used. Calcium-chloride can increase the potential for corrosion of the reinforcement and other metals under certain exposure conditions.

Water reducers (also known as plasticizers) include both mid-range (MRWR) and high-range (HRWR). They can be used to increase the flowability of concrete without the negative impact that adding water can produce. This aids in the pumping of concrete as well as placement in tight forming conditions or where a large amount of reinforcement is present. Flowability is increased without sacrificing the strength since the w/cm can remain as designed or even lowered. Where water runs the risk of inducing separation of the aggregate from the paste, water reducers allow the aggregates to remain suspended in the matrix instead of settling to the bottom of the element.

Advancements in mix design have also resulted in a liquefying chemical admixture to produce a concrete mix called self-consolidating concrete (SCC). Concrete mix designs as SCC are very flowable and give great definition of forms while reducing placement time, labor and vibration.

Other admixtures include air entrainment, coloring agents, finely ground minerals, corrosion inhibitors, pumping aids, and latex modifiers to name a few.

Mix ProportioningDetermining the amount of each material, including admixtures, that comprise a given concrete mix is called mix design or mix proportioning. Proportions and materials for a mix can be varied to meet specific design requirements such as strength or durability; use and placement considerations such as cold or hot weather; differences in available materials such as aggregate types and cements; set time and finishing characteristics; environmental goals including sustainability; and, aesthetic concerns such as form definition, exposing of aggregates, and colored concrete.

The ready-mix supplier, admixture supplier as well as the engineer and architect may all have input regarding their special requirements. Typically, these requirements are part of the specifications for a project. Ideally, a ready-mix supplier will have standard mix designs previously tested to fit the most common requests or criteria in a given area. In addition to the specific job criteria, the mix designer also tries to meet the following job specifications:

1) The hardened concrete will have the strength, durability and wear resistance to meet the job requirements;2) The concrete will be workable enough for the intended application;3) It will be economical, and;4) Shrinkage is minimized.

HydrationThe hydraulic nature of the cementitious material means that concrete hardens due to a chemical reaction called hydration. Since chemical reactions always have one or more byproducts, that of the hydration process in concrete is heat. This heat of hydration can be helpful in cold weather applications and it is a good indicator that concrete setting is occurring. Conversely, during the hotter months of summer or in warmer climates, this heat of hydration can be amplified by the climate resulting in curing taking place at to high of a rate. Control of the working concrete temperature is important in both cold and hot weather applications.

Hydration can be likened to a crystallization growing process where the crystals grow to surround the aggregate particles, reinforcement and any other embedded object creating an intertwined mass. When moisture is no longer present, hydration ceases and the concrete no longer gains strength. Concrete that is submerged under water can

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performance or simplify the placement of concrete. Admixtures known as retarders slow down the curing rate of concrete, which can be helpful in hot weather or extremely intricate configurations. Accelerators are admixtures that can speed up the hydration process which can be advantageous when placing concrete in cold weather or when a higher early strength is required. Accelerators are an example of an admixture class that can accomplish the same goal using different forms. Accelerators are most often divided into two categories, calcium-‐chloride and non-‐chloride. The type of concrete application and the applicable code will instruct whether one or the other can be used. Calcium-‐chloride can increase the potential for corrosion of the reinforcement and other metals under certain exposure conditions.

Water reducers (also known as plasticizers) include both mid-‐range (MRWR) and high-‐range (HRWR). They can be used to increase the flowability of concrete without the negative impact that adding water can produce. This aids in the pumping of concrete as well as placement in tight forming conditions or where a large amount of reinforcement is present. Flowability is increased without sacrificing the strength since the w/cm can remain as designed or even lowered. Where water runs the risk of inducing separation of the aggregate from the paste, water reducers allow the aggregates to remain suspended in the matrix instead of settling to the bottom of the element.

Advancements in mix design have also resulted in a liquefying chemical admixture to produce a concrete mix called self-‐consolidating concrete (SCC). Concrete mix designs as SCC are very flowable and give great definition of forms while reducing placement time, labor and vibration.

Other admixtures include air entrainment, coloring agents, finely ground minerals, corrosion inhibitors, pumping aids, and latex modifiers to name a few.

Chemical Admixtures

Mix Proportioning

FIGURE 8Chemical Admixtures

NOTES:


continue to gain strength for a long period of time.

Curing ConcreteCuring is the process by which we try to optimize the conditions allowing the concrete to harden. Three factors effect the curing of concrete and the rate at which strength gain occurs: moisture, temperature, and time. Moisture is a necessity to support hydration, the process by which concrete gets its strength. The use of chemicals to seal the surface and slow evaporation are common as are plastic sheeting and burlap. Misting the surface may also be used if conditions are conducive. The purpose of all of these efforts is to retain the optimum amount of moisture so that the hydration process can continue.

Temperature is also important. The temperature must be at a sufficient level so that the water can hydrate the cement paste. If the temperature is too hot, the moisture will evaporate too quickly and not enough water will be present for hydration to occur. This is of particular concern on hot, windy days, especially with slabs. Time is the third necessity. The longer ideal conditions for hydration are present, the stronger the concrete will get.

Temperature is more difficult to control and we must protect the concrete from both extremes. If concrete becomes too hot, the water will evaporate on the resulting in a weakened surface that impacts the durability of concrete. Concrete can also become too cold. When the water in the concrete drops below freezing, the hydration process stops. It will continue once it thaws as long as moisture is present but continued freezing and thawing will again produce weaker concrete. One thing worth noting however is that hydration produces heat. Concrete can withstand a temperature at freezing or slightly below since it is producing heat internally. If the concrete is covered with thermal blankets then much of this heat of hydration will be held in the confines of the element.

Cold weather concrete is an area that has received considerable attention in the past several years. Independent testing conducted by the Concrete Foundations Association has resulted in the conclusion that as long as concrete is protected from freezing until it reaches 500 psi strength, it will continue to gain strength when the temperature rises above freezing. The CFA ‘Cold Weather Concrete’ report has tested a variety of mix designs and protection measures to determine the best practices in cold weather conditions.

FIGURE 9Burlap and Plastic Curing

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Burlap and Plastic Curing

Time is the third component of concrete curing. Concrete cures rapidly during the first few days, often reaching nearly 75% of its 28-‐day design strength in the period. The curve then flattens somewhat but the concrete will continue to gain strength indefinitely as longs as there is moisture present for hydration. Temperature, chemical admixtures and other factors can speed or slow the curing process.

Concrete Strength

There are two basic measures of concrete strength we need to understand: compressive strength and tensile strength (often called modulus of rupture). A compressive load is the force that tend to compress the concrete element. Walls are typically very good a resisting compressive forces. Walls, however, may also be subjected to horizontal loads which introduce bending force. A bending force produces tension on the side opposite the applied load (such as soil or wind) and compressive force on the load side. Since the compressive forces on a residential foundation wall are so low relative to its capacity, the load on the tension side is the most critical force that must be analyzed. The tensile strength of standard concrete is roughly 10% of the compressive strength. Fortunately, most codes have empirical tables that identify wall configurations to meet most loading conditions.

Compressive strength (ƒ’c) is measured in pounds per square inch (psi). Strength at 28 days from placement is typically the specified measure by we judge concrete. The 28-‐day compressive strength is the typical design parameter for engineers for structural elements. The compressive strength can be varied depending on the amount of cement, water, and the nature of aggregates used but it


Concrete StrengthThere are two basic measures of concrete strength we need to understand: compressive strength and tensile strength (often called modulus of rupture). A compressive load is the force that tend to compress the concrete element. Walls are typically very good a resisting compressive forces. Walls, however, may also be subjected to horizontal loads which introduce bending force. A bending force produces tension on the side opposite the applied load (such as soil or wind) and compressive force on the load side. Since the compressive forces on a residential foundation wall are so low relative to its capacity, the load on the tension side is the most critical force that must be analyzed. The tensile strength of standard concrete is roughly 10% of the compressive strength. Fortunately, most codes have empirical tables that identify wall configurations to meet most loading conditions.

Compressive strength (ƒ’c) is measured in pounds per square inch (psi). Strength at 28 days from placement is typically the specified measure by we judge concrete. The 28-day compressive strength is the typical design parameter for engineers for structural elements. The compressive strength can be varied depending on the amount of cement, water, and the nature of aggregates used but it typically varies from 2,500 psi to 4,500 psi for standard concrete elements. The tensile strength of standard concrete is roughly 10% of the compressive strength. These mixes perform well and are economical for most concrete structures. Strengths of up to 10,000 psi are attainable and are sometimes used in structural frames and other high performance structures.

Cylinders are generally used to determine whether or not concrete has attained the specified strength. Multiple cylinders should be taken and prepared in accordance with ACI recommended practices. The standard size is 6” in diameter by 12” long. At least three cylinders should be prepared for each batch. Multiple cylinders can be of considerable value if, for example, the first one tested does not meet specifications. Testing multiple cylinders at different time periods can predict the strength you should attain as time progresses. Cylinders should be stored and cured in the same conditions as the structural element to give a better relationship between cylinder break strength and the strength of the concrete in the structure. Required strength for concrete is dictated by project specifications, building codes or owner requirements. Most concrete reaches the specified strength before 28 days but even if it hasn’t, it is likely to reach design strength as long as it continues to cure so as to be serviceable for the intended use.

If flexural strength is a design consideration, the use of beam test are best for determining if the correct strength has been obtained.

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typically varies from 2,500 psi to 4,500 psi for standard concrete elements. The tensile strength of standard concrete is roughly 10% of the compressive strength. These mixes perform well and are economical for most concrete structures. Strengths of up to 10,000 psi are attainable and are sometimes used in structural frames and other high performance structures.

Concrete Strength with Age

Cylinders are generally used to determine whether or not concrete has attained the specified strength. Multiple cylinders should be taken and prepared in accordance with ACI recommended practices. The standard size is 6” in diameter by 12” long. At least three cylinders should be prepared for each batch. Multiple cylinders can be of considerable value if, for example, the first one tested does not meet specifications. Testing multiple cylinders at different time periods can predict the strength you should attain as time progresses. Cylinders should be stored and cured in the same conditions as the structural element to give a better relationship between cylinder break strength and the strength of the concrete in the structure.

FIGURE 10Concrete Strength with Age

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typically varies from 2,500 psi to 4,500 psi for standard concrete elements. The tensile strength of standard concrete is roughly 10% of the compressive strength. These mixes perform well and are economical for most concrete structures. Strengths of up to 10,000 psi are attainable and are sometimes used in structural frames and other high performance structures.

Concrete Strength with Age

Cylinders are generally used to determine whether or not concrete has attained the specified strength. Multiple cylinders should be taken and prepared in accordance with ACI recommended practices. The standard size is 6” in diameter by 12” long. At least three cylinders should be prepared for each batch. Multiple cylinders can be of considerable value if, for example, the first one tested does not meet specifications. Testing multiple cylinders at different time periods can predict the strength you should attain as time progresses. Cylinders should be stored and cured in the same conditions as the structural element to give a better relationship between cylinder break strength and the strength of the concrete in the structure.

FIGURE 11

Concrete Cylinders

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Reinforced ConcreteConcrete is much weaker in tensile strength. Tension is the force that tends to pull things apart and we express that force as tensile strength. The side of a beam or wall opposite the applied load is generally placed in a tensile mode. In general, the tensile strength of concrete is about 10% of its compressive strength. When the tensile strength of concrete is exceeded, it will crack. Another cause of tensile cracks is shrinkage. If the ends or portions of the concrete are retrained such as corners or the ground in a slab, the concrete will crack as excess moisture evaporates from the element, thus causing shrinkage or tensile load.

Steel reinforcement is used to withstand or take the tensile forces resulting from applied loads or shrinkage. Steel has a much greater tensile capacity than concrete. The steel that is used in most applications such as walls and footings, has tensile strength of either 40,000 pounds per square inch (40 ksi) or 60 ksi. Higher strengths are used in special applications. The steel is classified as cold rolled deformed bar reinforcement. The deformations are critical as they are what allows the concrete and steel to interact. Another important aspect of steel is that the rate at which steel expands and contracts in response to temperature change (coefficient of expansion) is very similar to that of concrete. If it wasn’t, putting reinforcement in concrete would cause it to break apart when exposed to large temperature swings. Steel should be mill-certified to ensure that what is being provided meets specification.The steel must be placed in the correct location to work according to design. In the case of simply supported structural elements such as beams or reinforced walls, this location is generally on the side opposite the applied load. ACI standard prescribe distances that must be maintained from the outside of the concrete element to the steel based on exposure. Cover requirements are indicated in the respective reference governing the elements or type of construction where the concrete is placed.

Temperature and shrinkage steel is placed to minimize cracking that occurs when concrete shrinks as unused water evaporates or as concrete dimensions change in response to temperature change. Steel placed for this purpose should be closer to the center of the concrete element. A wall that has only temperature and shrinkage steel is classified as plain structural concrete. The reinforcement is not included in the design to resist applied loading although it no doubt will increase the load carrying capacity of the element.

SafetyThere are many hazards workers must be aware of when working on a job site where concrete is being poured. One thing that is often ignored, however, is the wet concrete itself. Fresh concrete is highly alkaline (caustic) and can cause significant irritation or burning of unprotected skin or eyes. Proper equipment must be worn to protect the skin and eyes. Safety glasses, gloves, proper shoes, pants and shirts should be worn whenever placing concrete. If your skin comes in contact with the fresh concrete, wash the affected areas as soon as possible with water. It can take as little as 45 minutes to several hours for the impact to be felt.

The information presented in this section should provide you with the basic knowledge you need to converse intelligently with the home owner, building official or other lay person regarding the concrete you are providing. For


more information, consult documents published by the American Concrete Institute (ACI), the National Ready-Mixed Concrete Association (NRMCA), the Portland Cement Association (PCA) or the Concrete Foundations Association (CFA).

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aci 332-20 residential concrete foundation industry basics

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