Structural Fibers for Concrete Reinforcement

A presentation to Engineers, Owners, Contractors, Producers, Architects and Specifiers interested in macro-synthetic fibers and

the use of Fiber Reinforced Concrete

by: Michael MahoneyThe Euclid Chemical Company

TUF-STRAND SF - Structural Synthetic Fibers 1/3

Introduction

Fiber reinforced concrete and shotcrete are now becoming a viable alternative (economic and technical) to conventional reinforcement in concrete applications

However, national and local specifications do not provide guidance (and sometimes, even a reference) to their use as a replacement for structural steel

Fiber Reinforced ConcreteNew Synthetic FibersPre-Cast ConcreteSlab on Grade ConcreteShotcreteSpecial Notes

Presentation Topics

What is Fiber Reinforced Concrete (FRC)?

FRC is a mixture of:

• Conventional Concrete (cement, water, rock and sand) + Fibers

Fibers can be made available in many different material types:• Steel• Synthetic - polypropylene, nylon, carbon, polyester, aramid, etc.• Natural (cellulose)• Glass (alkali resistant)

what type of fibers should I use?

Why Use Fibers?

Before knowing which fibers to use, it is necessary to understand what they do and where to use them. Over the past 20 years, R&D in the field of Fiber Reinforced Concrete (FRC) has shown that steel and new synthetic fibers can provide a better reinforcing alternative than welded wire mesh and light bar reinforcing (sometimes at lower cost as well).

All proven from standards tests and applied research:ASTM C 1018 – Flexural Toughness Test MethodASTM C 1399 – Residual Strength FactorsASTM C 1550 – Round Determinate Panelsetc..

Additional benefits

toughness, impact resistance, fatigue endurance, freeze-thaw resistance, abrasion resistance, etc.

Why Use Fibers cont.

Problems encountered with WWM:

• Fabricating cages for complicated shapes, thin walls

• Cage placement in forms

• Labor costs

• Corrosion and durability issues

• Warehousing of rebar and mesh

• Safety

• Compacting/consolidating heavily reinforced pieces

• and of course - the price of steel!

The Industry Today

By today’s standards, the macro fiber industry (steel and synthetic) is estimated to be between $100 to $160 Million dollars per year.

Approvals have been granted in the following areas for complete replacement of steel for the following products:

Florida, Georgia, Alabama, Virginia - septic tanksGeorgia - manhole risers and flared end sectionsCanada - septic tanksQuebec, Virginia - poured foundation walls

plus: engineered approvals for industrial floors, elevated decks, shotcrete applications, tilt-up projects, etc.

How Do Fibers Work in Concrete?

How can a whole bunch of short little “toothpicks” do the same job as rebar?

big, strong barstiny little hairs

Fibers actually provide the same function as conventional reinforcing - prevent cracks that form in concrete from opening - the key is how much.

How Fibers Work cont.

fiberfailure

fiber pull-out

fiber bridging

fiber / matrixdebonding

matrix cracking

All fibers behave differently in concrete. It therefore is important to understand the properties and benefits of each and how they work.

Always remember:

You get what you pay for!

Micro Fibers

Fibers for Plastic Shrinkage Only

Mostly the Domain of Synthetic Fibers at Low Fiber Addition Rates (0.1 to 0.3% Vol.) 1 to 3 lb/yd

Fibers have been shown to reduce plastic shrinkage cracking by up to 80-90% by providing concrete with early tensile strength and by intercepting and arresting cracks as they travel through concrete.

Beyond Plastic Shrinkage

Protection against plastic shrinkage cracking in concrete can be properly managed with the use of commercially available products but what happens beyond the initial set of concrete when these low volume fiber products or improperly placed WWM reinforcing alternatives are challenged by other loading conditions?

• Drying Shrinkage• Thermal Induced Shrinkage (thermal shock, seasonal)• Structural (static and live loads, reflective, creep)• Chemical (corrosion, ASR, DEF)

Resistance is characterized by

Toughness

Macro fibers

Large fibers are too far apart to arrest, deflect or modify the behavior of

micro-cracks in any significant way.

No good for shrinkage control

Will affect only post-cracking behavior steel or synthetic

fiber lengths usually greater than 1” (25 mm)

Bridging the Gap

For most applications, what is truly desired is a combination of plastic shrinkage resistance and good toughness that could provide some structural component over time.

Products can be combined (WWM and low denier and/or blended fibers) in concrete to achieve this but it is ultimately the responsibility of the owner / specifier to decide what is required.

Over the past 10 years, there has been a significant effort to develop such synthetic products that combine plastic shrinkage protection, toughness and load carrying capabilities while eliminating the need for WWM and its costly side effects.

“Structural” FibersNote: new fibers must comply to standards such as ASTM C1116

Synthetic Fiber Typical Comments

“I’ve seen these fibers before – get out the lawnmower!”

“I used these twenty years ago and I still got cracks!”

“You want me to put how much of that stuff in my concrete?!”

In reality, the new synthetic fibers available on the market today are capable of competing directly with steel fibers and welded wire mesh in a wide variety of applications for primary reinforcing requirements.

Issues: Higher reinforcing volumes than conventional synthetic fibersWorkability and slump of concrete – do not add water!

Synthetic Fiber Performance

Testing of many Synthetic Fibers is currently being done at Universities and private research companies.

ASTM C1116C1018C1399C1550

other tests:impactfatiguecreepscaling resistanceplastic shrinkagetoughnessetc.

Fiber Performance cont.

Project: Synthetic Fiber Comparisons

Date: March 19,2002

Beam Type: Synthetic Fibers @ 4.6 & 6.9 kg/m^3

Subject: ASTM C 78 and ASTM C 1018 with combined Toughness Levels (Morgan) at 28 Days

Sample Defl. @ Specimen Cross-Section Max. Flexural ASTM C - 1018 ASTM C - 1018 Toughness

No. 1st crack Base Height Load Strength Residual Strength Factors JSCEt Performance

(mm) (mm) (mm) (kN) (MPa) I5 I10 I20 I30 I60 R5,10 R10,20 R20,30 R30,60 (MPa) Levelsx

SnFRC 1 @ 4.6 kg/m 0.047 102.3 100.4 24.15 7.03 3.71 6.52 9.90 13.88 26.68 56.19 33.78 39.85 42.67 3.22 III-IV

SnFRC 1 @ 6.9 kg/m 0.044 104.2 100.0 22.39 6.45 4.12 6.96 12.69 18.77 38.29 56.94 57.24 60.84 65.07 4.40 V

* Value measured in the unstable zonet Japanese Society for Civil Engineeringx University of British Columbia Toughness Performance Level Approach ( Dr. D. R. Morgan )

Toughness Indices

0

1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Deflection (mm)

Str

ess

(MP

a)

SnFRC1 @ 4.6 kg/m

SnFRC1 @ 6.9 kg/m

Level I

Level III

Level II

Level V

Level IV

Fiber Performance cont. (2)

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

0 15 30 45 60 75 90 105 120 135 150

Deflection (mm)

Loa

d (k

N)

5 kg/m3 synthetic fiber

7 kg/m3 synthetic fiber with WWM reinforcing along potential cold joint

7 kg/m3 synthetic fiber

4x4 8/8 WWM

40 kg/m3 steel fiber type #1

Fiber Performance cont. (3)

Steel vs. Synthetic Fibers

• Corrosion

Steel fibers will lose crack bridging properties over time where exposed to moisture and/or chloride. Even on uncracked concrete rust staining from steel fibers is a major impediment to use - can be an aesthetics issue.

• Safety

Steel fibers are stiff and sharp and can become be a safety hazard

• Wear and Tear

Steel fibers cause wear and tear on certain pieces of equipment

-Pumps, hoses, mixing units.

• Pumping & Mixing

Steel fiber length is limited to portion of hose diameter – affects performance

Although steel fibers provide excellent toughness and structural consideration, there are some issues with these materials that will may cause concern.

Replacement of Specified Steel Reinforcing

Traditional engineering design must provide for reinforcement in most concrete products

Specified reinforcing levels have been based on moment, stress and/or minimum temperature and shrinkage requirements – the most of which have been pre-defined in many Codes and publications.

If documented evidence for FRC can be shown that matches required performance with steel, can fibers be used?

Note: In the past, micro fibers have been specified as replacement to WWM but it has now been proven that these fibers have no structural capacity or resistance to crack width opening.

Pre-Cast Concrete

steps

tanks

manholescovers

curbs

pipes

Advantages of Fibers in Pre-Cast

EconomicAdvantages

• Significant decrease in production cycle time• Reduced labor costs• Reduction in breakage and repair costs• Elimination of the potential for corrosion

TechnicalAdvantages

• Crack - prevention• Increased ductility and flexural toughness• Good impact resistance• Prevention of concrete spalling

but how do you design with fibers for “structural” reinforcement ?

Pre-Cast Design Assumptions

Using data from ASTM C1399, a test method for fiber reinforced concrete, the “moment” capacity of FRC can be determined.

= Mf y / I

requirement fiber dosage

By equating the moment capacity of the steel section to the capacity of the fiber reinforced concrete, we can establish dosages for equal performance.

Mr = s As Fy (d-a/2)

For pre-cast concrete, the relationship of bending moment capacities is used.

b

dAs

C

T

a

Pre-Cast - Design Example

Thin wall section of element contains 6x6 6/6 reinforcing placed at mid-depth of a 3” (75 mm) wall section. Concrete strength is 4000 psi (28 MPa), Steel reinforcing strength is Imperial Grade 60 – 60,000 psi.

The contractor would like to replace this steel with synthetic fibers. What fiber dosage and type would be applicable? – lbs/yd3

Pre-Cast – Solution

Mr = (0.9) (0.058 in2/ft) (60,000 psi) 1.5 in -0.134 in

2= 4487 in lbs

set this value equal to the required synthetic fiber performance

Fiber dosage must provide an Average Residual Strength test result (from ASTM C1399) of 249 psi.

(4487 in lbs) (1.5 in)

(1/12) (12 in) (3 in)3

therefore;

ARS value = 249 psi

= = 249 psi

Fiber Dosage Solution

Average Residual Strength vs. Deflection

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0Deflection (mm)

Stre

ss (

MPa

)

sample curve

Plot of Volume of Synthetic Fiber "X" vs. Average Residual Strength

y = 1.75x

0

1

2

3

4

5

6

7

8

9

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

MPa

kg/m

3250 psi (1.7 MPa)

5-8 lbs/yd3

(synthetic fibers)

30-50 lbs/yd3

(steel fibers)

Testing Verification

Fiber dosages should be verified by using vacuum testing on full-size septic tanks

FRC tanks exhibited similar failure loads

Pre-Cast Notes

ACI 318-02, 16.4.2 – For pre-cast non-pre-stressed walls, the minimum allowable steel reinforcing ratio must be 0.1%. Testing on fibers using ASTM C 1399 must be able to meet or exceed this requirement.

It is ultimately the responsibility of the engineer / owner / specifier to recommended a particular product. Fiber dosages should be prepared in accordance with ACI Design requirements. Lower fiber volumes than those specified by calculation will not have the intended effect on overall product performance and perhaps another product (micro fiber / fibrillated fiber) may be warranted.

Remember:The cheapest fiber isn’t always the best fiber - Demand a performance solution!

Pavements & Slabs on Grade

Can fibers be used to resist dynamic wheel loads, static rack loads and uniformly distributed loads? What about fork truck traffic and impact from falling loads or equipment? Fiber reinforced concrete, which is designed as a homogeneous material, can provide a solution.

Slab Applications

• Factories• Warehouses• Hangers• Concrete Overlays

Pavement Applications

• Airport pavements: runways, aprons and taxiways• Highways and roads• Parking areas• Bridge decks• Pavement repairs• Overlays• Canal and reservoir linings

Advantages of Fibers in Slabs

EconomicAdvantages

TechnicalAdvantages

• Suppresses propagation of cracks and spalling• Ensures a homogeneously reinforced product• Increased toughness provides a high resistance to impact loading and abrasion• Increase in fatigue resistance

• Elimination of traditional mesh or rebar with reduced labor and material costs• Reduction in slab thickness• Increased joint spacing possibilities• Less complicated construction leading to earlier completion• Lower maintenance costs

Same design question:How much fiber do I need?

Slab on Grade Design Assumptions

fFRC = 0.667 fr Re3

100

fWWM = As Fy

1.15 x b d

Equivalent tensile resistance

From ASTM C 1018 testing (another fiber reinforced concrete test), the capacity of fibers in tension can be approximated. This capacity can be converted to an equation that takes into account the flexural strength of the concrete. Re3 is the percentage of the post-crack tensile strength with respect to the concrete flexural strength.

Shrinkage

Subgrade drag

L

Shrinkage

Subgrade drag

L

Slab on Grade - Design Example

Slab on Grade project is currently using a single layer of WWM 4x4 4/4 reinforcing placed near the top of an 6” thick slab. Concrete strength is 5000 psi (35 MPa), with an approximate tensile strength of 575 psi. Steel reinforcing strength is Imperial Grade 75 – 75,000 psi.

The contractor would like to replace this steel with fibers. What fiber dosage would be applicable? – lbs/yd3

Slab on Grade – Solution

fWWM =(0.12 in2/ft) (75,000 psi)

1.15 x (12”) (6”)= 109 psi

set this value equal to the required fiber performance

fFRC =109 psi = (0.667) (575 psi)Re3

100

therefore;

Re3 = 28.4

Fiber dosage must provide a toughness value to provide an Re3 = 28.4- approximately 4 – 5 lbs/yd3 for synthetic fiber.- approximately 30 - 60 lbs/yd3 for steel fiber.

Slab on Grade Notes

Note: Exercise caution when recommending very low synthetic fiber dosages (< 5 lbs/yd3) as the benefits of increased toughness and shrinkage reduction are reduced with low fiber volumes. Check for economy as well.

Also; when recommending higher fiber dosages (> 8-10 lbs/yd), adjustments to the mix design may be required which will in turn affect the overall cost of the product.

Local Code Requirements:ACI vs PCA, etc..

Please refer to Presentation 3/3 for successful applications of TUF-STRAND SF slabs

The Contractor vs. The Engineer

The Engineer wants to know:

• who else has used it?• what Code is it in?• do you have independent test data?• will you stand behind your work?• we will need to write a spec for it.

• Well - you’ll need to get the contractor, owner, architect and ready-mix supplier to want to use it too.

The Contractor wants to know:

• who else has used it?• what Code is it in?

• how much does it cost?• how do I finish it?• where do I buy it from?• will my engineer let me use it?

When the Fiber Manufacturer / Supplier shows up…..

Application Potential

Pre-Cast and Slab on Grade applications where WWM and light bar reinforcing is used as secondary reinforcing and light structural reinforcing are prime candidates for structural synthetic fiber use.

• Slabs on Grade with WWM or small bars (#’s 3, 4, 5) at spacings greater than 12” c.c.• Thin wall and pre-cast products with required residual strengths less than 500 psi.• Shotcrete projects where reinforcing is provided by single layer WWM.• Odd shapes and areas where shrinkage protection is important.

What types of projects should be considered for application using “structural” or macro-fibers?

Remember - micro-fibers are for plastic shrinkage only.

More than just Pavements

warehouse floors

designed for wheel loads, rack and post and distributed loads

Shotcrete

The Concrete Job

Every project has different requirements. Fiber dosages should always be selected on the basis of the required performance first.

Know the competition weaknesses and strengths

Know the requirements, specificationsand economics of the job.

Concrete Mixture Notes

For higher fiber dosages (> 5 lbs/yd macro-synthetic, >40 lbs/yd steel)Concrete mixture proportions can be slightly modified with approval

- Easy fiber mixing- Good fiber distribution- Proper workability- Ease of pumping if required- Good finishability- Good consolidation

Typically, FRC mixtures require:higher cementitious contenthigher fine aggregate contenthigh range water reducers

Ensure that the hardened properties are not adversely affected.• compressive strength• hardened air void-characteristics• flexural strength, etc.

Documents and Publications

ASTM, CSA relevant standardsASTM C 1018ASTM C 1550ASTM C 1399, etc..

ACI 544 State of the Art Report on FRC

ACI 506 State of the Art Report on Shotcrete

ACI 318 Manual of Concrete Practice

To date, there are over 8000 publications of fiber reinforced concrete from University and other Research groups but the concept of FRC is still “foreign” to many in industry.

Useful Documents

Contact your fiber manufacturer / supplier

for information

Path Forward

Referenced jobs are now available to demonstrate macro synthetic FRC benefits

ACI Spring ConventionCharlotte, NC; March 26-29, 2006

Symposium on “Applications of Fiber Reinforced Concrete -

Past Present and Future”

Conclusions

Demonstration projects using steel and new macro-synthetic fibers are now being recognized as viable alternatives to conventional reinforcing materials with follow up work now being scheduled and completed successfully.

The market is no longer a “wait and see” industry

Fiber Reinforced Concrete works!

Start saving today.

Thank you for your attention

If you have any additional questions on TUF-STRAND SF or fiber reinforced concrete, please contact your local Euclid Chemical Sales Representative.