1CIVL 111 Construction Materials

Prof. Zongjin LiDepartment of Civil Engineering

25.4 Admixtures--Definition and Classifications

Material other than water, aggregates, cement and reinforcing fibers that is used in concrete as an ingredient and added to the batch immediately before or during mixing.i. Air-entraining agents (ASTM C260)ii. Chemical admixtures (ASTM C494 and BS5075)iii. Mineral admixturesiv. Miscellaneous admixtures include:

LatexesCorrosion inhibitorsExpansive admixtures

3Beneficial effects of admixtures on concrete properties

5.4 Admixtures --Definition and Classifications (2)

Concrete property Admixture

Workability SuperplasticizerAir-entraining agents

Setting AcceleratorsRetarders

Strength Silica fumePolymers

Durability Air-entraining agentSilica fumeCorrosion inhibitors

45.4 Admixtures for concrete

Admixture in liquid form

Admixture in solid form

55.4 Admixtures-- Water reducing admixtures (1)

Water-reducing admixture lowers the water required to attain a given workability.

Compressive strength (Mpa) Test series

Cement content (kg/m3)

Water / cement ratio

Slump (mm) 7 days 28 days

1Reference concrete (no admixture) 300 0.62 50 25 37

A given dosage of a water-reducing admixture is added with the purpose of:

2Consistency increase 300 0.62 100 26 38

3Strength increase 300 0.56 50 34 46 4Cement saving 270 0.62 50 25.5 37.5

65.4 Admixtures-- Water reducing admixtures (2)

a. Mechanism:separate the cement particlesRelease the entrapped water

Cement ParticleWater is trapped

7b. Two kinds of water-reducing admixture:The normal range (WR):

i. Reduce 5 10% of water

The high range water reducing admixture (HRWR):i. Superplasticizerii. Reduce water in a range of 15-30%

5.4.2.1 Water reducing admixtures (3)

8c. SuperplasticizerSuperplasticizers are used for two main purposes:

i. To produce high strength concrete at w/c ratio in a range of 0.23 0.3 (60 150MPa)

ii. To create flowing concrete with high slumps in the range of 175 to 225mm. Self compacting concrete: for beam-column joint and footing (heavy reinforced)

Two formsi. Solid powerii. Liquid --- 40% - 60% of water

5.4.2.1 Water reducing admixtures (4)

9c. SuperplasticizerDosage:Normal dosage of superplasticizer for concrete is 1%-2% by weight of cement.Other benefits on hardened concrete may be stated as: A lower w/c ratio would lead to better durability and lower creep and shrinkage.The major drawbacks of superplasticizer are:

i. retarding of setting (especially at large amount addition)

ii. causing more bleedingiii. entraining too much air.

5.4.2.1 Water reducing admixtures (5)

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5.4.2.2 Setting control admixtures (1)

a. MechanismsChange the rate of the crystallization of portland cement by adding certain soluble chemicals to influence the ion dissolution rate.

Anions (silicate and aluminate)Cations (calcium)

The setting will be speeded up when dissolution rates of cations and anions are higher. On the other hand, the setting will be slow down when dissolution rates of cations and anions are lower.

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a. Mechanisms (Contd)Accelerating admixture:must promote the dissolution of cations and anions.Retarding admixture must impeded the dissolution of cations and anionsTwo different setting control admixture:

5.4.2.2 Setting control admixtures (2)

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a. Mechanisms (Contd)i. Dual role setting control admixture

A lot of chemical admixtures has both complementary and opposing effects.The dominate effect usually depends on the concentration.

ii. Monotonic retarding agentForming insoluble and impermeable products or delaying bond formation.

5.4.2.2 Setting control admixtures (3)

C3SCoating

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a. Mechanisms (Contd)ii. Once insoluble and dense coatings are formed

around the cement grains, further hydration slows down considerably. Surger and carbonated beverage belongs to this category.

5.4.2.2 Setting control admixtures (4)

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b. Applicationsi. Retarding admixtures:

1. Offset fast setting caused by hot weather2. Setting control of large structural units3. Setting control of long distance transport

ii. Accelerators:1. Plugging leaks:2. Emergency repair: High way; Bridge3. Winter construction in cold region

E.g. use calcium chloride (CaCl2)

5.4.2.2 Setting control admixtures (5)

Swimming pool

Use cement to mend the leak

Side effects: corrosion

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5.4.2.2 Setting control admixtures (6)

Influence of calcium chloride in relation to setting period

Effect of calcium chloride on strength development at different temperatures

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5.4.3 Air-entraining admixtures entrain air in the concrete

Small size air bubbles is introduced into concrete uniformly

The air bubbles are enclosed, not connected

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5.4.3.2 Two types of air bubble introduced in concrete (1)

Entrained air: On purpose Size: 50 to 200 m

Entrapped air: By chance As large as 3mm

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5.4.3.2 Two types of air bubble introducedin concrete (2)

Dimensional range of solids and pores in a hydrated cement paste

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5.4.3.3 Mechanism of air-entraining admixtures

Schematic representation of air entrainment by surface active molecule

Hydrophilic group

Hydrophobic component

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5.4.3.4 Advantages of adding air entraining admixtures

Improved workability --- air bubble as lubricant Improved ductility --- more deformation from small hole Reduced permeability --- isolated air bubble Improved impact resistance --- more deformation Improved durability --- freezing and thawing(release ice

forming pressure)

water water

air bubbles air bubbles

Connected air bubbles Isolated air bubbles

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5.4.3.5 Disadvantage of adding air entraining admixtures

Strength loss of 10-20%

Effect of entrained air on durability

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The smaller the spacing factor, the more durable the concrete.

Critical spacing: 0.3mm

5.4.3.6 Role of bubble spacing (1)

Relationship between durability and bubble spacing factor of entrained air

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5.4.3.7 Gel spacing ratio

The formula used to calculate the gel space ratio (X) has to be modified if entrained air is added into cement paste as follows:

( )

airentrainedcw

airentrainedporescapillaryofvolumegelofvolumeporesgelincludinggelofvolume

X

++=++=

/32.068.0

Where = degree of hydration

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5.4.4 Mineral admixtures

Finely divided siliceous materials which are added into concrete in relatively large amount.

5.4.4.1 Silica fume5.4.4.2 Fly ash

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5.4.4.1 Silica fume (1)

Silica fume is a by-product of the induction arc furnaces in the silicon metal and ferrosilicon alloy industries.

Silica fume has very fine particles

20% below 0.05 micron70% below 0.10 micron95% below 0.20 micron99% below 0.50 micron

%

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80

100

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5.4.4.1 Silica fume (2)

The typical chemical composition of silica fume

LOI --- Loss of ignition

Typical St.dev. Min Max Moisture % 0.30 0.09 0.09 0.50 LOI % 1.18 0.26 0.79 0.73 SiO2 % 92.9 0.60 92.0 94.0 Al2O3 % 0.69 0.10 0.52 0.86 Fe2O3 % 1.25 0.46 0.74 2.39 CaO % 0.40 0.09 0.28 0.74 MgO 1.73 0.31 1.23 2.24 K2O % 1.19 0.15 1.00 1.53 Na2O % 0.43 0.03 0.37 0.49 C % 0.88 0.19 0.62 1.30 Cl % 0.02 0.01 0.01 0.03 S % 0.20 0.10 0.30 P % 0.07 0.03 0.12

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5.4.4.1 Silica fume (3)

Silica fume particle

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5.4.4.1 Silica fume (4)

A comparison of size distribution of silica fume to other materials

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Effect of silica fume Physical: Packing Chemical: Pozzolanic reaction

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Packing effect

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Pozzolanic reaction

pozzolan + calcium hydroxide+water=

calcium silicate hydrate (secondary)

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Classification of coal ash (1)

Fly ashThe fine particulate matter rises with flue gasses collected by electrostatic precipitators

Bottom ashLarger particles and fused particles fall to bottom

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5.4.4.2 Fly ash

Fly ash particles

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Classification of coal ash (2)- Fly ash classification (ASTM)

Classified according the coal burned- Class F: Anthracite or bituminous coal- Class C: Lignite or subbituminouscoalClass C is more active than Class F

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Various Usage of Coal Ash- Construction

Concrete production

- 25% to 30% replacement- can improve durability - Good for long term strength- better to used with other mineral admixtures

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Advantages of incorporation of fly ash Low cost --- $200-300/Ton (cement $500/Ton) Low energy demand --- Industry by product Low hydration hear --- pozzolanic reaction

Disadvantages of incorporation of fly ash Low early age strength Longer initial setting time

Solution: Alkali activator (1% NaOH)

5.4.4.2 Fly ash

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Chapter 5 Concrete --Hardened concrete

Harden concrete is the concrete have gain sufficient strength and ready to carry out the external load. Its properties have to meet the requirement of end users.

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Chapter 5 Concrete -5.5 Hardened concrete Strength Definition (1)

Definition The ability of concrete to resist stress without

failure Failure identification Appearance of cracks

Why is strength specified in construction design and quality control? It is relatively easy to be measured Other properties are related to the strength

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5.5 Hardened concrete --Strength Definition (2)

General index of concrete strength28 day compressive strength of concrete

determined by a standard uniaxialcompression test

More realistic criteria of failure of concreteLimiting strain

Tension: 100 X 10-6 to 200 X 10-6Compression: 70 MPa 2000 X 10-6

14 MPa 4000 X 10-6

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Examples of transducers for strength test

Transducer

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Compressive strength test-- Failure mechanism

a. Random microcrackb. Stably growth of microcracksc. Microcrack localizationd. Failure (major crack in vertical direction)

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Compressive strength test-- Specimen preparation (1)

a. Cube specimenBritish Standard (BS) 1881: Part 108: 1983. Filling in 3 layers with 50mm for each layer. Stokes 35 times (150mm cube) and 25 times (100mm cube). Part 11 is for curing. 20 5oC. Relative humility: 90%

L

B

L/B = 1

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b. Cylinder specimenAmerican Society of Testing & Materials (ASTM) C470-81. Standard cylinder size is 150 x 300mm. Curing condition is temperature of 23 1.7oC and moist condition. Grinding or capping are needed for level and smooth compression surface.

Compressive strength test-- Specimen preparation (2)

L

D

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Compressive strength test--Factors influencing experiment results (1)a. Loading rate

The faster the load rate, the higher the ultimate load obtained. The standard load rate is 0.15 0.34 MPa/s for ASTM and 0.2 - 0.4 MPa/s for BS.

Loading rate

Max. Load

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b. End conditionInfluence of platen restraint. Cubes apparent strength is 1.15 time of cylinders.

Compressive strength test--Factors influencing experiment results(2)

specimen

shear force frictiondeformation on L/R side

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c. Size effectProbability having large deficiencies (such as void and crack) increases with size increases.

Compressive strength test--Factors influencing experiment results(3)

Standard size: cube 150 x 150 x 150mmcylinder 150 x 300mm

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Tensile strength and corresponding tests --Failure mechanism

a. Random crack developmentb. Localization of micocracksc. Major crack propagation

80%

30%max/

/ max

Detection of microcrack: use of acoustic emission

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Stress concentration factor

Stress concentration is the stress at the edge of the hole, smax , is large than the normal stress sN

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Direct tension test methods (1)

Direct tension tests of concrete are seldom to carry out because it is difficult to control and because the specimen holding devices introduce secondary stress that cannot be ignored. Two frequently used methods are described below.

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a. Grips dog bone test

Direct tension test methods (2)

b. End plate loading methodtension force

grip

specimen

steel plate

pin

LBoundary stresses are complicated

L-Uniform stress distribution zone

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Indirect tension test --(split cylinder test or Brazilian test) (1)

BS 1881: Part 117:1983150 x 300mm cylinder. Loading rate 0.02 to 0.04 MPa/s

ASTM C496-71:150 x 300mm cylinder. Loading rate 0.011 to 0.023 MPa/s

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Indirect tension test --(split cylinder test or Brazilian test) (2)

(a) Set-up for indirect tension test

(b) The stress distribution along diameter

The splitting tensile strength:

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Flexural strength and corresponding test (1)

BS 1881: Part 118: 1983Flexural test. 150 x 150 x 750mm or 100 x 100 x 500mm (Max. size of aggregate is less than 25mm)The arrangement for modulus of rupture is shown in the following figure, in which, the moment diagram and distribution of strain and stress along the height of specimen are also shown.

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Flexural strength and corresponding test (2)

Flexural strength test

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Modulus of rapture:For the case of fracture takes place within the middle one thirdof the beam,

Flexural strength and corresponding test (3)

b

d Mmax = =.p2

l3

pl6

Ymax = d2

I = bd3

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2bdPLf bt =

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For fracture occurs outside of the middle one-third,

Flexural strength and corresponding test (4)

Mmax = p

2a

Ymax = d2

I = bd312

Valid for ASTM standard only