Physical Properties Of Concrete
Concrete is formed by solidifying the mixture of cement, sand, gravel and water in predefined proportions.
The properties of concrete changes on the basis of the quality and proportions of the components utilized in the mix.
Concrete is strong against compression, but weak in tension. As a result of this, concrete is weak in bending, shear and torsion. Therefore, it is recommended not to use the plain concrete where great compressive strength and weight are mainly required and where tensile stresses do not exist or considerably low.
Properties of Concrete
The followings are some vital properties of concrete for creating the design of concrete mix :-
The unit weights of plain concrete and reinforced concrete formed with sand, gravel of crushed natural stone aggregate are considered as 24 KN/m3 and 25 KN/m3 respectively.
(ii) Compressive Strength
With given properties of aggregate, the compressive strength of concrete is mainly influenced by the age, cement content and the water cement ratio. Characteristic strength depends on the strength at 28 days. The strength at 7 days is about two-thirds of that at 28 days with ordinary port land cement and usually good indicator for retaining the exact strength.
(iii) Increase in strength with age
It is usually gain of strength beyond 28 days. The quantum of increase is dependent on the grade and type of cement curing and environmental conditions etc.
(iv) Tensile strength of concrete
The flexure and split tensile strengths of different types of concrete are provided in IS 516:1959 and IS 5816:1970 respectively when the designer wants to utilize an estimate of the tensile strength from compressive strength, the following formula should be applied.
Flexural strength, fcr=0.7√fck
N/mm2 (v) Elastic Deformation
The modulus of elasticity is mainly dependent on the elastic properties of the aggregate and to lesser extent on the conditions of curing and age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is generally associated with the compressive characteristic strength of concrete.
Here Ec denotes the short-term static modulus of elasticity in N/mm2
fck denotes the characteristic cube strength of concrete in N/mm2
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(vi) Shrinkage of concrete
Shrinkage is the time dependent deformation, usually compressive in nature. The ingredients of concrete, size of the member and environmental conditions impact the shrinkage of concrete. However, the total shrinkage of concrete is primarily based on the total amount of water found in the concrete while mixing it for a given humidity and temperature. The estimated value of the total shrinkage strain for design is considered as 0.0003 in the absence of test data.<
(vii) Creep of concrete
Creep is another time dependent deformation of concrete by which it continues to deform, generally under compressive stress. The creep strains recover partially when the stresses are discharged.
The creep of concrete is dependent on the following factors
- Properties of concrete
- Water/cement ratio
- Humidity and temperature of curing
- Humidity during the period of use
- Age of concrete at first loading
- Magnitude of stress and its duration
- Surface-volume ratio of the member
Often aggregates are examined with combined grading of fine and coarse materials proportioned as intended in the proposed concrete mix. It creates an estimate of how the mix will function in concrete. Every region contains its own deficiencies in aggregates, but as soon as combined aggregate gradation is plotted (percentage retained vs. sieve size), these deficiencies are easily recognized and resolved. Substitute aggregate sources or supplementary aggregate blending should be used to process the elusive "ideal" gradation that maintains the best workability, pumpability, reduced shrinkage, and economy.
(f) Thermal expansion of concrete
The thermal expansion of concrete plays a vital role since it is arranged and remains in service at a wide range of temperature in several countries with very hot or cold climates.. The coefficient of thermal expansion is based on the nature of cement, aggregate, cement content, relative humidity and size of the section.
Workability and Durability of Concrete
Workability and durability of concrete are also very important properties of concrete. The workability of a concrete mix provides a measure of the ease with which fresh concrete are arranged and condensed. The concrete should flow smoothly into the form and spread and cover the reinforcement, the mix should maintain its consistency and the aggregates should not be separated. A mix with high workability is required where sections are thin and/or reinforcement is impenetrable and congested. The workability is mainly affected by the water content of the mix. Admixtures raise the workability but may decrease the strength. Thesize of aggregate, its grading and shape, the ratio of coarse to fine aggregate and the aggregate-to-cement ratio also impact the workability to some extent.