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Topic 1 - Properties of Concrete

1. Quick Revision 1.1 Constituent Materials of concrete

Concrete is composed mainly of three materials, namely, cement, water and

aggregate, and sometimes additional material, known as admixture, is added to

modify certain of its properties.

1.2 Cement

Concrete sets to a rock-like mass due to a chemical reaction (hydration) which

takes places between cement and water, resulting in a paste of matrix (Calcium

Silicate Hydrate (C-S-H)) which binds the other constituents together.

2(2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2 2(3CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2 Calcium Calcium

silicate hydrate hydroxide

There are several different types of cement, among which Ordinary Portland

Cement is most commonly used in Hong Kong.

Setting – This is the term used to describe the stiffening of the cement paste.

Broadly speaking, setting refers to the change from a fluid to a rigid state.

1.3 Aggregate

Aggregates are inert, inexpensive materials dispersed throughout the cement

paste so as to produce a large volume of concrete.

Coarse aggregate – aggregate mainly retained on a 5 mm test sieve.

Fine aggregate – aggregate mainly passing a 5 mm test sieve.

1.4 Workability of Concrete

Workability can be best defined as the amount of useful work necessary to

produce full compaction.

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The workability of the mix shall be such that the concrete can be transported,

placed and compacted sufficiently easily and without segregation.

Measurement of Workability

Tests devised for workability test:

Slump Test (most commonly used in HK) (CS1:1990 Section 2)

Compacting Factor Test (CS1:1990 Section 3)

Vebe Consistometer Test (V-B Time Test) (CS1:1990 Section 4)

2. Strength of Concrete Concrete is naturally strong in compression, i.e. it can resist quite high crushing loads.

On the other hand, it is relatively weak in tension, i.e. it cracks fairly readily if

stretched or bent.

The compressive strength of concrete is assessed by measuring the maximum

resistant to crushing offered by a standard test cube. Because it is readily

determined, and because most of the desirable hardened-state properties of concrete

improve with its compressive strength, this parameter is commonly used as a measure

of the quality of the material. For example, Grade 40 concrete means that the

concrete has a characteristic compressive strength of 40 N/mm2.

Factors affecting the compressive strength of concrete

Porosity is primary factor influencing strength. However, it is a property difficult to

measure in engineering practice. For this reason the main influencing factors on

strength are taken in practice as:

Water / Cement ratio

Degree of compaction

Conditions for strength development (temperature, moisture condition)

Type and quality of cement

Quality of aggregate (grading, surface texture, strength, and stiffness)

Age

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2.1 Water-cement ratio

Porosity is a primary factor influencing strength of materials. This applies to

most materials as well as concrete.

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For fully compacted concrete, the strength of concrete decreases with the

water/cement ratio increases, since excessive water results in voids and

capillaries.

Therefore, the water-cement ratio of a concrete mix is one of the most

important influences on concrete strength. It is calculated by

Water cement ratio = mass of free water ÷ mass of cement

The effect of extent of air voids on potential concrete strength

(source: C&CAA T41)

Relation between strength and water/cement ratio of concrete

(source: Neville, Brooks)

Influence of porosity on relative strength of various materials (Source: Neville)

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2.2 Degree of compaction (air void content) For a given concrete mix, the maximum potential strength will be achieved

only with full compaction, i.e. if all voids or spaces between the particles of

aggregate are filled with cement paste and all air is expelled from the system.

Otherwise, the strength of concrete decreases with the air void content

increases.

2.3 Age

The hydration of reaction, and therefore strength development, will continue

for long periods of time.

The reaction is most vigorous in the first week, but then slows progressively to

an almost imperceptible rate which may continue for many years.

Strength development of concrete containing 335 kg OPC per cubic meter

(Source: Neville, Brookd)

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2.4 Degree of Hydration (curing) Since the hydration of reaction will continue for long periods of time, it is

essential that moisture is present for a sufficient time to allow the reaction to

proceed.

Substantial reduction in potential strength will result from inadequate curing

(as shown in the figure below).

2.5 Type of cement The rate of strength development will depend also on the type of cement.

Type I – Ordinary Portland Cement (Class 42.5)

Type II – Modified Portland Cement

Type III – Low-heat Portland Cement

Type IV – Rapid-hardening Portland Cement (Class 52.5)

Type V - Sulphate-resisting Portland Cement

The influence of moist curing on the strength of concrete with a water/cement ratio 0.5

(source: Neville & Brooks)

Strength development of concrete

(Source: A.M. Neville & J.J. Brooks)

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2.6 Type and quality of aggregate The shape and texture of aggregate particles has an important influence on the

workability of fresh concrete, and hence may affect both the water demand and

the water-cement ratio.

Categorization of aggregate particles by shape (Source: Standard Australia) Flat, flaky or elongated particles not only reduce workability but may also affect

adversely the strength of concrete by their tendency to selective orientation and

bridging (thus forming pockets or honeycombs),

On the other hand, the strength of concrete is affected by the bond between

coarse aggregate particles and the cement paste, and by the interlocking

characteristics of the aggregate.

2.6.1 Aggregates commonly used in Hong Kong 2.6.1.1 Crushed rock aggregates Crushed aggregates can be obtained by crushing rock or stone.

In Hong Kong, the most common aggregate used is crushed granite.

Crushed aggregates tend to be angular and irregular in shape

Crushed coarse aggregates bond well to cement paste and help to produce good

concrete.

Crushed fine demands higher water content then river sand and hence increases

the water-cement ratio.

Round Irregular Angular

Flaky Elongated Flaky and elongated

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2.1.1 Natural aggregate Natural aggregates consist of particles of broken stone deposited by the action of

rivers and streams.

Sands and gravels can be obtained by dredging form river beds.

Aggregates, in particular sands and gravels, should be washed to remove

impurities such as silt and clay.

The surfaces of coarse natural aggregates are too smooth to bond with cement

paste that will affect the strength of concrete. (It is rarely used in Hong Kong.)

Fine natural aggregate (river sand) absorb less water than crushed fine, it is a

better choice for high strength concrete.

3. Tensile strength of Concrete Tensile strength of concrete is important to resist crack due to changes in

moisture or thermal movements.

It varies form 1/8 of the compressive strength at early ages to about 1/12 later.

There is a close relationship between them but not a direct proportionality.

Ratio of the two strengths depends on the general level of strength of concrete.

Generally, ratio of tensile to compressive strength is lower if the compressive

strength is higher.

(Methods to measure the tensile strength of concrete will be discussed later.)

4. Shear Strength Shear strength of concrete is always accompanied by compression and tension

caused by bending.

In testing, it is impossible to eliminate an element of bending.

It is not common for shear strength to be tested.

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5. Deformation of concrete independent of load Besides deformation due to applied load, volume changes due to shrinkage and

temperature variation are also of importance.

5.1 Plastic Shrinkage Most concrete, after it is placed, bleeds, i.e. water rises to the surface as the solid

particles settle.

The bleed water evaporates and there is a loss of total volume - the concrete has

'settled'. This contraction is known as plastic shrinkage.

If there is no restrain, the net result is simply a very slight lowering of the

surface level.

However, if there is something near the surface, such as reinforcing bar, which

restrains part of the concrete from settling while the concrete on either side

continues to drop, there is potential for a crack to form over the restraining

element.

Differential amounts of settlement may also occur where there is a change in the

depth of a section, such as at a beam/slab junction.

Generally, the cracks are not deep but, because they tend to follow and penetrate

down to the reinforcement, they may reduce the durability of a structure.

Different Settlement Cracking (Source: Standard Australia)

Settlement Cracking (Source: Standard Australia)

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5.2 Drying Shrinkage and Moisture Movement Drying shrinkage is caused by withdrawal of water from hardened concrete

stored in unsaturated air.

Part of this shrinkage is irreversible and should be distinguished from the

reversible part, or moisture movement.

That is, when concrete which has been allowed to dry and subsequently placed

in water will swell back partly.

For usual range of concrete, the reversible moisture movement represents 40 –

70 % of the drying shrinkage.

5.2.1 Factors affecting drying shrinkage a. Water/cement ratio

The higher the w/c ratio, the larger the shrinkage.

b. Aggregate

Aggregates restrain the amount of shrinkage.

Use of larger aggregate permits the use of leaner mix at a constant w/c ratio,

thus reduces shrinkage.

c. Relative humidity

The lower the relative humidity, the larger the drying shrinkage.

d. Curing

Adequate curing can also reduce formation of drying shrinkage cracks as it

will increase the tensile strength of concrete.

5.2.2 Drying shrinkage crack Drying shrinkage cracks in large sections may be induced due to internal

restraint caused by differential shrinkage between the surface and interior of

concrete.

Normally shrinkage cracks can be controlled by reinforcement placed near to the

surface, used of low shrinkage concrete and proper curing.

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5.3 Carbonation Carbonation is the process where Carbon Dioxide in the air reacts with Calcium

Hydroxide of the concrete to form Calcium carbonate.

Ca(HO)2 + CO2 CaCO3 + H2O

In association with the process, concrete experiences a contraction known as

Carbonation Shrinkage.

Concrete with a high water/cement ratio and inadequately cured will be more

prone to carbonation.

Carbonation results in increase in strength and decrease in permeability. (This

is possible due to water released by carbonation aids the process of hydration

and the CaCO3 formed deposits in the voids within the cement paste.)

Carbonation proceeds slowly from the surface inwards, and the rate of

carbonation decreases with time because of the decrease of permeability on the

outer surface due to carbonation.

The bad effect of carbonation is that it neutralises the alkaline nature of the

concrete, thus the protection of steel from corrosion is vitiated.

The extent of carbonation can be detected by treating a freshly broken surface

with phenolphthalein. The free Ca(OH)2 is coloured pink while the carbonated

portion is uncoloured.

5.4 Thermal Movement Concrete has a positive coefficient of thermal expansion and about 0.00001 per

C.

Thermal expansion or contraction, if restrained, may induce stresses in

structures and cause cracking.

The remedy is to provide movement joints at suitable locations for large

structures.

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6. Deformation of concrete under load 6.1 Elastic Deformation

Every material deforms when under load, E = strain

stress

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strain = stress / E.

That is, to estimate the deformation of concrete when under load, its Young’s

modulus must be known.

However, the elastic property of concrete is non-linear.

Theoretically, the tangent to the curve at the origin can be measured, and this is

known as the initial tangent modulus.

Practically, the secant modulus corresponding to a stress equal to one-third of

the compressive strength of the concrete is taken as the Modulus of Elasticity of

the concrete.

unloading

Secant modulus

Initial tangent modulus Stress

Strain

Typical Stress-Strain Relationship of Concrete

Determination of Modulus of Elasticity of Concrete

(Source: ELE)

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6.2 Creep Creep can be defined as the increase in strain under a sustained stress.

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A sustained load removed results in the decrease in elastic strain immediately.

This strain is generally smaller than the initial elastic strain because of the

increase in modulus of elasticity with age.

The instantaneous recovery is followed by a gradual decrease in strain, called

creep recovery.

Creep recovery is always smaller than the preceding creep, thus there is a

residual deformation.

6.2.1 Factors affecting creep a. Strength

Increase in strength usually leads to a reduction in creep.

b. Mix proportion

Creep decreases as the w/c ratio and the volume of cement paste decreases.

c. Aggregate

Creep increases as the aggregates become finer;

it is generally greater when porous aggregates are used.

d. Curing

Creep decreases as cement hydration proceeds;

Concrete kept continuously wet will have creeps less than cured in air.

e. age

Rate of creep decreases as the concrete ages.

6.2.2 Effect of creep Creep increases the deflection of R.C. beams and cantilever.

On the other hand, creep reduces internal stresses; thus there is a reduction in

cracking.

Reference A.M. Nevelle & J.J. Brooks (1990), Concrete Technology, Longman Cement and Concrete Association of Australia, Standard Australia (1994), Guide to Concrete Construction.