An Introduction to Concrete Technology

Topics Covered

• Constituents of concrete

• Properties of good concrete

• Strength

• Workability

• Durability

• Grading of Concrete, Acceptance Criteria

• Mix Design

• Stress Strain response of concrete

What is Concrete?

• Concrete is a mixture of cement, water, aggregate (fine and coarse) and admixtures.

• When reinforcing steel is placed in the forms and fresh concrete is placed around it, the solidified mass is called reinforced concrete.

• Since concrete is a composite, the properties of concrete depend on the properties and relative proportions of its constituents.

• The properties of concrete also depend on the temperature and humidity at which it is ‘placed’ and ‘cured’

Constituents• Each constitutent of concrete has an important

contribution to the overall properties of the composite.

• None of the constituents can be wholly replaced by the other.

• For instance, could we use a combination of cement and water alone as a building material and not use any aggregate? The answer is most surely NO.

• The reason why this is not desirable is not just because of economics (cement is much more expensive than aggregate)

Constituents• It is because hydrated cement paste undergoes large

volume changes due to shrinkage and creep

• Thus a building material made of cement paste will contain a large number of cracks and pores due to shrinkage and creep, which would make it practically useless.

• Each constituent of concrete plays an irreplaceable role in the admixture.

• Next we will examine briefly each constituent and its individual properties.

Cement• Cement is a material which acquires cohesive and

adhesive properties in the presence of water.

• In the presence of water, bonds form between individual cement particles, as well as between cement particles and other constituents of concrete such as fine and coarse aggregates.

• Since water is essential for cement to acquire its cohesive and adhesive properties, such cements are called hydraulic cements.

• They consist chiefly of silicates and aluminates of lime obtained from limestone and clay.

Chemical constituents of Cement• The mixture of limestone and clay is grounded, blended and fused together in a kiln at about 1400oC to form a produce called clinker.

• The clinker is then cooled and ground with gypsum to get cement.

• The main chemical constituents of cement are:

Triacalcium Silicate 3CaO.SiO2

Dicalcium Silicate 2CaO.SiO2

Tricalcium Aluminate 3CaO.Al2O3

Hydration of Cement• As mentioned, for cement to acquire its adhesive and

cohesive properties water is essential. Why is it so?

• This is because in the presence of water, the silicates and aluminates in cement form products of hydration or hydrates.

• These hydrates, with time, produce a firm and hard mass – which is the hardened cement paste.

• The hydration reactions for tricalcium silicate and dicalcium silicate are as follows:

2C3S + 6H → C3S2H3 +3CA(OH)2 + heat 2C2S + 6H → C3S2H3 + CA(OH)2 + heat

Hydration of Cement• The amount of tricalcium aluminate in most cements is comparatively small.

• However the hydration reaction of tricalcium aluminate with water is very rapid – it may lead to what is known as “flash set”, or very rapid setting of cement. Gypsum is

added to cement clinker to prevent this.

• The hydration reaction of tricalcium aluminate is:

C3A + 6H → C3AH6 + heat

Heat of Hydration of Cement• Heat released during hydration causes rise in temperature during concrete hardening

• If temperature rise is sufficiently high and concrete undergoes nonuniform heating or cooling thermal stressesmay arise due to internal as well as external (support)restraints to thermal movement.

• Tensile stresses thus generated may result in significantCracking in concrete

• However high heat of hydration is useful if concrete isbeing cast in near freezing temperatures. Prevents iceformation in the pores and enables the hydration reactions to occur and concrete to gain desired strength

Portland Cement• The cement that we have talked about till now is usually

known as Ordinary Portland Cement (OPC), because of its resemblance upon hardening to Portland stone

• This cement is the most commonly used type of cement and it is used most extensively.

• Comprises about 59-64% lime (CaO), 19-24% silica (SiO2), 3-6% alumina (Al2O3), 1-4% iron oxide (Fe2O3)

• However there are various other types of cement which are also used – many for specialized purposes.

• One of the other commonly used cements is Portland Pozzolona Cement

Portland Pozzolona Cement• Pozzolana, a silica based material, is used to replace a portion of cement in a concrete mix. This can result in

significant economy.

• Pozollona by itself possesses no cementitious properties. However in finely divided form, in the presence of water, it can react with calcium hydroxide to possess compounds

with cementitious properties.

• Recall calcium hydroxide is formed during the hydration of dicalcium silicate and tricalcium silicate.

• Portland pozollona cement is prepared by grinding cement clinker with pozollona. IS:1489 requires that the

% age of pozollana vary between 10%-25% by weight of cement.

Portland Pozzolona Cement• The Pozzolona mostly comprises fly ash

• Fly ash is one of the residues generated in combustionand comprises fine particles that rise with flue gasses. It is generally produced by the combustion of coal

• Depending upon the type of coal burnt, the componentsmay vary considerably but usually contains substantial amounts of silicon dioxide (SiO2) and calcium oxide (CaO)

• In the past fly ash was generally released to the atmosphere: pollution control requires that this be capturedprior to release. About 43% is recycled and used forPortland cement production

Rapid Hardening CementOther types of specialized cements include:• Rapid hardening cement: more finely ground than OPC, but same chemical constituents.

• Since more finely ground more surface area available for formation of hydration products.

• Thus it acquires cohesive and adhesive properties faster: 24 hour strength is nearly equal to 3 day strength of OPC

• Allows quicker removal of shuttering – suitable for road work or bridge construction

Portland Slag Cement• Portland slag cement: Blast furnace slag contains oxides

of lime, alumina and silica and is used to replace clay used in the manufacture of OPC.

• Economical, since it allows reuse of blast furnace by product while replacing some of the ingredients of OPC.

• The properties are similar to OPC. However it has lower heat of hydration (the heat generated during the

hydration reactions) and is more durable.

• It is often used in mass concreting structures such as retaining walls – since the lower heat of hydration allows maintenance of proper placing and curing temperatures.

Hydrophobic cement

• Hydrophobic cement formed by adding water repellant chemicals during the grinding process to clinker.

• These chemicals from a coating over cement particles and prevents moisture and air from being absorbed by the cement during storage for long periods in wet climatic conditions.

• However coating is broken during the mixing of concrete and normal hydration processes occur to the

same extent as in OPC

High Alumina Cement

• Clinker for high alumina cement consists mostly of alumina (Al2O3) and lime (CaO) with smaller quantitites of iron oxides, slica (SiO2)

• High durability against chemical attacks, high early strength and high heat of hydration.

• High heat of hydration – useful for concreting in freezing weather.

• High durability against chemical attack – useful in concreting in hostile environments.

Super Sulphated cement• Cement formed by grinding a mixture of 80-85%granulated slag, with 10-15% calcium sulphate and about5% Portland cement clinker

• Used for aggressive conditions such as marine works, mass concrete jobs to resist aggressive waters,concrete construction in sulphate bearing soils, undersideof rail bridges and sewer pipes

• Prevents attack due to formation of sulphuric acid which by dissolves and removes a part of the hydratedcement paste

• This leaves behind a soft and very weak mass and ishence detrimental to strength

Aggregate• Aggregates occupy about 80% of the total volume of concrete. Thus their properties greatly influence the behavior of concrete

• Aggregates are classified as fine aggregates (materials passing through an IS sieve that is less than 4.75 mm gauge) and coarse aggregates (material that do not

pass the 4.75 mm IS sieve)

• For maximum strength and durability of concrete, the aggregates must be packed and cemented as

compactly as possible.

• Thus the gradation of particle sizes is important.

Grading of Aggregates

Aggregate• Aggregates should span a range of sizes – thus

smaller aggregate particles can occupy the interstitial spaces between largely aggregate particles and reduce the volume of void.

• Good gradation may reduce the cement requirement for the concrete – since it reduces the available void space that must be filled with cement.

• Aggregates must also have good strength, durability and resistance to harsh weather.

• Their surface must be free of impurities e.g. organic matter which may weaken the bond with cement paste.

Aggregate classification by Weight• Aggregates are often classified into lightweight

aggregates, normal weight aggregates and heavy weight aggregates.

• Light weight aggregates e.g. slag and sintered fly ash are primarily used for insulating purposes or masonry units.

• Normal weight aggregates include natural material such as sand, gravel and crushed rock such as granite,

basalt and sand stone. Artificial material such as broken brick and air cooled slag are also used.

• Normal aggregates are classified based on particle shape and surface texture e.g. smooth, granular,

honeycombed and porous.

Aggregate classification by Weight• Heavy aggregates are used for making heavy weight

concrete manufactured for specialized purposes such as nuclear reactors where there is a need to screen out harmful radiation.

• They typically consist of heavy iron ores or hard rocks such as barite, crushed to suitable sizes.

Water• Water is an essential requirement in concrete: it is

essential during both the mixing and curing of concrete

• About 25% of the weight of cement is required for the hydration reactions

• Additional water is required to ensure proper workability of concrete

• The water used must be free from injurious chemicals and substances that may be harmful to concrete or reinforcing steel.

• Potable water is considered satisfactory for mixing concrete.

Admixtures• Materials known as admixtures are sometimes added

to the concrete mix, just before or during mixing, to modify the properties of the concrete.

• Some commonly used admixtures include:

• Accelerators such as calcium chloride to increase the rate of strength development in concrete at early ages

(recall Rapid Hardening Concretes)

• Retardants to retard the initial setting time. This is useful when mixes have to be transported or pumped over large distances.

Admixtures• To increase workability without increasing the water

content – pozzolanic materials such as fly ash are used.

• To decrease heat of hydration

• Superplasticizers to reduce the water requirement. These are chemicals which can produce concrete of a given workability at a lower water cement ratio than ordinary concrete.

• Superplasticizers result in concretes with desired workability but higher compressive strength.

Properties of ConcreteA good concrete must have three basic properties:

• Strength

• Workability

• Durability

The purpose of any concrete mix design is to achieve all three goals to the extent desired by the usage of the concrete.

Strength• The concrete must be able to bear stresses arising

from the design loads

• It is uneconomical to design a concrete mix stronger than required – however it must have the minimum strength required.

• Factors influencing the strength of concrete are: cement quality, water cement ratio, grading of aggregate, degree of compaction, efficiency of curing,

temperature during curing etc.

• Water cement ratio is by far the most important factor determining the strength of concrete.

Strength vs water cement ratio

Strength vs water cement ratio• Compressive strength decreases with water cement ratio

• The reason is that the amount of water required for hydration of cement is limited and is small compared

to the water required for workability.

• The water in excess of that required for hydration of cement improves workability.

• However over time the excess water may evaporate leading to increased porosity in the paste.

• Thus higher water cement ratios lead to lower strength.

Strength vs cement content• The strength of concrete will not increase merely by increasing the quantity of cement unless the water- cement ratio is decreased.

• However as will be seen later, the amount of water influences the workability of concrete. Reduction of water cement ratio thus adversely affects the

workability of concrete

• Thus an optimum water cement ratio is required that will give maximum workability as well as strength

Other factors influencing strength: void ratio

• The total volume of voids in concrete affects its strength – gel pores, capillary pores and entrapped air all adversely affect the strength of concrete. Each percentage of air void by volume decreases strength by as much as 5%.

• The total volume of voids depends on the water cement ratio – as evaporation of excess water leads to void creation.

• To reduce the presence of voids, it is vital to achieve the maximum possible density.

Dependence of strength on density

Compaction of concrete• It is well known that increase in density in concrete

leads to an increase in compressive strength.

• Compaction in concrete aims to minimize bubbles of entrapped air in the concrete.

• Immediately upon placing concrete in the form work, the concrete is compacted by hand tools or vibrators.

• Hand compaction by tamping rods requires the concrete to flow readily around the reinforcement bars – thus it usually requires the concrete to be sufficiently wet i.e. possess a higher water cement ratio.

Compaction of concrete: use of vibrators

• Compaction by high frequency power driven vibrators allows compaction to occur without having to increase the water cement ratios – thus they allow the use of stiffer mixes.

• Compaction using power driven vibrators ensures more impenetrable and dense concrete, as well as a higher bond between concrete and reinforcement

• Internal needle vibrators need to be immersed in the concrete, while external vibrators act on the form work.

Other factors influencing strength: age

• The age of concrete also influences the strength. With increase in age, the degree of hydration generally increases so strength also increases.

• However the rate of increase in strength decreases with age.

Workability of Concrete• Workability refers to the amount of energy necessary

for compaction of concrete.

• In compacting concrete, energy is spent in two ways: to overcome the internal friction between the particles of the mix and in overcoming friction between concrete and form surface or the surface of the reinforcements.

• In more general terms, workability of freshly mixed concrete is that property which determines the ease and homogeneity with which it can be mixed, placed, compacted and finished.

• Workability of concrete is different from “consistence” of a concrete mix. Consistence refers to the ease with which a concrete flows.

Workability of Concrete• A concrete that can easily flow, is not necessarily more

workable. Concretes of the same ‘consistence’ can vary in workability.

• Usually however, wet concretes are not only more consistent i.e. can flow easily but also, within limits possess more workability.

• Workability increases with the maximum size of the aggregate, because as the aggregates increase in size, the total surface area of the aggregates reduces.

• Thus less water necessary to wet the surface of the aggregate (to ensure hydration reactions and bond formation at the aggregate surface).

Workability of Concrete• More of the water content is then available to act as a lubricant which reduces the friction between the particles

as well as between the aggregates and the formwork

• This improves workability since it ensures more easy compaction of the concrete.

• Workability also improves if the aggregates have smooth texture – since this reduces frictional forces and ensures easier compaction.

• Workable concretes should not show segregation: ie. separation and local accumulation of coarse aggregates from the cement paste

Test for workability: slump test• A mould, which is a frustum of a cone, is placed with its

base on a smooth surface.

• The frustum is 305 mm and high and has a base diameter of 203 mm.

• It is then filled with concrete in three layers.

• Each layer is hand tamped 25 times with a standard diameter steel rod to ensure proper compaction.

• Immediately after filling the mould is slowly lifted.

• The unsupported concrete now slumps – hence the name of the test.

Slump Test• The decrease in the height of the center of the slumped

concrete is called the slump and is measured to the nearest 5 mm.

• The slump is sensitive to variations in workability, the extent of the slump determining the degree of workability

• Following ranges are commonly adapted:

Degree of Workability Slump (mm)

Very Low 0-25

Low 25-50

Medium 25-100

High 100-175

Slump Test

Durability of Concrete• Concrete must be durable with respect to weather conditions e.g. temperature and humidity variations,

chemical attacks and action of atmospheric gases

• Factors influencing durability of concrete are: quality of cement, water and aggregate as well as the water cement ratio and temperature.

• Poor quality cement and high water cement ratio both lead to porous concrete. High water cement ratio may result in ‘bleeding’ when excess water comes up to the surface of concrete resulting in several small pores in the concrete surface.

• Porous concrete is more susceptible to harmful gases in the atmosphere and salts that can cause its disintegration.

Durability of concrete• Thus durability depends essentially on the porosity of

concrete

• Water cement ratio should be as low as possible to reduce pores and increase durability.

• Durability also depends on the cement content. Thus IS 456 recommends minimum cement content for different surrounding conditions including the possibility of sulphate attack.

• The prescribed minimum cement content ensures sufficient alkalinity (pH value) to provide some resistance to corrosion of reinforcement

• Cement content should also be sufficient to fill the voids in compacted concrete.

Need for curing• Fresh concrete gains strength most rapidly during the

first few days: 70% of 28 days strength is attained at the end of first week after placing

• Final strength attained strongly depends on the conditions of moisture and temperature during this initial period.

• The maintenance of proper moisture and temperature conditions during this period is known as curing

• Loss of water content in concrete due to premature drying out of the concrete can prevent sufficient hydration of the cement – 30% of strength can be lost due to premature drying.

Ways of curing• Moist curing which consists of application of water

directly to the surface of the concrete or by means of continuously covering the concrete with sand, straw or hessian saturated with water

• Surface application of calcium chloride which prevents evaporation of water from concrete and also absorbs moisture from the atmosphere

• Membrane curing – where certain chemicals are sprayed on the surface of the concrete to form an impervious film which prevents evaporation of moisture

• Moist curing is the most widely adopted curing method for ordinary constructions.

Grading concrete• Concrete is graded according to its compressive

strength.

• The grades of concrete are stipulated in IS 456 in terms of their characteristic strength e.g. M20 concrete refers to a concrete mix with characteristic strength of 20 N/mm2

• The characteristic strength is obtained by finding the compressive strength of 15 cm cubes, 28 days after casting

• It is defined as the compressive strength value below which not more than 5% of the test results are expected to fall.

Stress –strain response of Concrete: compressive strength

• Under confinement compressive strength of concrete increases.

• Confining pressure acts to prevent crack propagation and leads to more ductile response.

c

c

Uniaxial Compression

Biaxial Compression

Triaxial Compression

Concrete: main damage mechanisms

• In a concrete system, “weaker planes” occur at the interface of the concrete and the aggregate: as a result of bleeding, shrinkage etc.

• The micro cracks that appear at the interface tend to propagate along the aggregate surfaces. These micro cracks can combine to form macro cracks.

• In addition there can be “mortar cracks” which run through the matrix material, as well as “aggegrate cracks” which tend split apart the aggregates.

Concrete: main damage mechanisms

Casting Direction

Accumulation of weak planes or voids under large aggregate

particles

Concrete: softening behavior• The propagation of internal micro-cracks and micro-voids

is reflected in the macroscopic stress-strain behavior of concrete

• For instance, under uniaxial compression, growth of micro cracks aligned to the direction of loading leads to stress softening

c

c

Direction of external loading

tensile crack

Concrete: tensile behavior• Under uniaxial tension, propagation of micro cracks

along a plane normal to the loading direction leads to strain softening behavior.

• The presence of steel reinforcement reduces crack widths and has a stiffening effect on the post peak behavior.

t

t

Concrete: material modelA constitutive law for concrete must satisfy the following

requirements:• It must account for material nonlinearity: the stress

strain relationship for concrete must not only be nonlinear it must also be dissipative.

.

c

c Elastic strain recovered on

unloading Irrecoverable plastic strain

e p

Summary• Concrete is an extremely complex material

• Designing a good concrete mix still involves some ‘art’

• Analysis of concrete is also extremely complicated:

the stress strain response is nonlinear and inelastic

• However whether for design or analysis, a good understanding of concrete technology is essential.