Conetnt of This Article
Properties of concrete : compressive strength,work-ability,Workability depends upon the following factors, The work ability can be measured by the following tests, Workability Requirements for Different Works,Durability Different Exposures with Normal Weight 20 mm Aggregates,Tensile Strength, Modulus of Elasticity, Poisson’s Ratio, Creep, creep coefficient, Shrinkage, Freezing and Thawing, percentage of en trained air for nominal maximum size of aggregate, Sulfate, Attack Various Types of Environment Exposure Conditions
Properties of Concrete
The properties of concrete depend upon the properties and proportions of its ingredients. The following are the important properties of concrete which are used in the design :
The compressive strength of concrete is determined by the cube test. The characteristic compressive strength of concrete is defined as the compressive strength of 15cm cubes at 28 days in N/mm2, below which not more than 5% of the test results are expected to fall. It is represented as fck. Cube samples are taken at the batching plant and are cured for 28 days. The compressive and tensile strength in bending and modulus of elasticity of concrete can be correlated with the characteristic compressive strength. So the compressive strength of concrete is an important indication of its overall strength.
The strength of concrete is greatly affected by the water cement ratio. Higher the water cement ratio, lower is the strength. As per Abram’s law, the compressive strength of concrete is inversely proportional to the water-cement ratio, provided the mix is of workable consistency.
The water content in a mix also influences the workability of the mix. Therefore, an optimum water cement ratio is required to give a mix of desired workability and maximum strength. The reduced water cement ratio also results in enhancement of density and impermeability of concrete and reduction in shrinkage and creep.
The workability of concrete is defined as the ease by which it can be mixed, placed, compacted and finished. A workable concrete should not bleed or segregate.
Workability depends upon the following factors :
(1) Water cement ratio (3) Shape of aggregate
(2) Size of aggregate (4) Ratio of fine to coarse aggregate
The workability can be measured by the following tests :
(1) Slump test
(2) Vee Bee consistometer test
(3) Compaction factor test
The following Table 1.2 gives the workability requirements for various types of works.
TABLE 1.2. Workability Requirements for Different Works
|1.||Binding concrete, shallow sections, pavements using pavers||Very low||Workability to be determined by compaction factor (0.75-0.80)|
|2.||Lightly reinforced beams, slabs, walls, columns floors, lining of canal, strip footing, mass concreting||Low||25 mm – 75 mm|
|3.||Heavily reinforced slabs, beams, walls, columns pumped concrete||Medium||50 mm – 100 mm|
|4.||Trench fill, in situ piling||High||100 mm – 150 mm|
|5.||Tremite concrete||Very High||Workability to be determined by determinant of flow|
The concrete should be durable to the environment it is exposed to, during its life. The various types of exposure conditions are listed in Table 5 of IS 456:2000 which specifies the maximum water cement ratio and minimum cement content under different conditions of exposures and is given in Table 1.3.
TABLE 1.3. Minimum Cement Content, Maximum Water Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight 20 mm Aggregates
|Sr. No.||Exposure||Reinforced Cement Concrete|
|Minimum Cement Content kg/m3||Maximum free water cement ratio||Minimum grade of concrete|
The main characteristic of durability is its permeability to the ingress of water, oxygen, carbonidioxide, chloride, sulphate etc.
As written earlier, the tensile strength of concrete can be correlated with the characteristic compressive strength of concrete. IS 456 gives the following correlation which can be used in the design.
Modulus of Elasticity
The short term static modulus of elasticity can also be expressed in terms of the characteristic compressive strength and may be written as follows :
Poisson’s ratio is defined as the ratio of lateral strain to the longitudinal strain. The Poisson’s ratio varies from 0.1 to 0.3 for concrete. It can be taken as 0.2 for all design calculation purposes.
Creep is caused under sustained loading. It is seen that when concrete is subjected to sustained loading, strain keeps increasing with time, even without any increase in the load. This strain which is caused due to sustained stress and is time dependent, is called as creep. It depends upon the following factors :
(a) Stress level
(b) Age at loading
(c) Duration of loading
In the absence of actual data, following values of creep coefficient may be taken for design considerations
|Age at Loading||Creep Coefficient|
Concrete shows shrinkage due to loss of moisture by evaporation. Shrinkage also causes strain in concrete. The shrinkage strain depends mainly upon the amount of water present at the time of casting. In the absence of any data. IS code 456 recommends a value of 0.003 mm/m as the ultimate shrinkage strain and it can be used for all design calculations.
Freezing and Thawing
As per IS 456, if freezing and thawing conditions exists, then durability of concrete should be enhanced by adding suitable air entraining admixtures. IS 456:2000 recommends the percentage of entrained air for nominal maximum size of aggregate as follows :
|Nominal maximum size of aggregate||Entrained air (%)|
The air entrainment results in reduction in the strength of the concrete mix and hence it is to be taken while designing mix.
Concrete exposed to sulphate attack such as in coastal environment, should be made from sulphate resisting cement. Table 4 of IS 456 (Table 1.4) gives recommendations regarding the type of cement, maximum free water/cement ratio and minimum cement content etc., required at places subjected to sulphate attack.
TABLE 1.4. Various Types of Environment Exposure Conditions
|1.||Mild||The surface of the concrete is protected against weather or aggressive environment (not coastal areas)|
|2.||Moderate||The surface of concrete is sheltered from severe rain or freezing whilst wet. The concrete surface exposed to rain and condensation, concrete continuously under water, concrete in contact or under soil or ground water. The surface of the concrete sheltered from saturated salt air in coastal areas.|
|3.||Severe||The surface of the concrete exposed to severe rain, alternate wetting and drying, sometimes freezing whilst wet or severe condensation.|
|4.||Very Severe||Concrete exposed to coastal environment, sea water spray, corrosive fumes, severe freezing. Concrete immersed in sea water. Concrete in contact or under aggressive soil or groundwater.|
|5.||Extreme||Concrete surfaces in tidal zone. Concrete in direct contact with aggressive chemicals.|