What are the Properties of Concrete?
Many elements influence the properties of concrete, the most important of which is the percentage of cement, sand, aggregates, and water in the mix. The proportions of these elements determine the various concrete qualities listed below:
- Workability
- Strength of concrete
- Durability
- Creep
- Shrinkage
- Unit weight
- Modular ratio
- Poisson’s ratio
Workability
Workability is one of the properties of concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted, and finished. It is the portion of energy to overcome friction. It may also be defined as the amount of useful internal work required to be done by the concrete to achieve full compaction (cannot be determined in absolute terms).
This property of concrete depends on several factors such as water content, mix proportion, size of the aggregate, the shape of the aggregate, texture, grading, and admixtures. The workability of concrete can be increased by increasing the water content and corresponding cement content. This is done in order to make the W/C ratio constant. Bigger size aggregate leads to higher workable concrete than the smaller size aggregate as the lesser surface area is required to be lubricated in this case. Workability can be measured by slump test, compaction factor test, flow table test, and Vee-bee consistometer test.
Strength of Concrete
Strength of concrete is one of the most crucial properties of concrete. The strength of concrete differs significantly for the same concrete mix. As a consequence, a single typical value called characteristic strength is used. The characteristic strength is defined as the strength threshold below which no more than 5% of the test results should be expected to fall.
Strength comes under properties of concrete at its hardened stage Concrete strength is measured in N/mm2 and is defined as the characteristic compressive strength of a 150mm cube at 28 days when tested using universal testing equipment.
Compressive strength of concrete | 0.67 f_{ck} |
Tensile strength of concrete | 0.7f_{ck} |
Concrete has a very low tension. The size and extent of cracks in the structure are affected by another important property of concrete called tensile strength. The tensile strength of good concrete should be 1/10 of the compressive strength. Tensile strength of concrete can be measured by splitting cylinder test or Brazilian test.
Durability
The property of concrete by virtue of which it is able to withstand disintegration and degradation is called durability. Hence, the durability of concrete is its resistance to deteriorating influences that may reside inside the concrete itself, or which are inherent in the environment to which concrete is exposed. The external agents of deterioration are weathering, bacterial growth, attack by natural or industrial liquids or gases, etc. and the internal agents of deterioration are the presence of sulphates and chlorides, volume changes due to non-compatibility of thermal and mechanical properties of aggregate and cement paste, alkali-aggregate reactions, etc.
The permeability of concrete to the rise of water and other potentially harmful materials is one of the most important features impacting its durability. The desired low permeability in concrete is achieved by using sufficient cement, a low water/cement ratio, complete concrete compaction, and proper curing.
Creep
The property of concrete where a structure experiences deformation due to a long-term load is known as creep. Concrete can alter shape if it is subjected to long-term strain or stress. This distortion usually happens in the direction of the applied force. As an example, a concrete column compressing or a beam bending. Concrete does not always fail or break apart as a result of creep. When concrete structures are designed, creep is taken into account.
As a result of the imposed load, water within the hardened cement paste is forced to expand. Creep deformation is mostly caused by this moisture migration. The propagation of microcracks causes some movement as well. Creep movement can be larger than elastic strain upon loading. After the load has been applied, creep can continue for a long time (up to 30 years in some circumstances).
Age of Loading | Creep coefficient |
7 Days | 2.2 |
28 Days | 1.6 |
1 Year | 1.1 |
Shrinkage
Shrinkage is the property of concrete where a decrease in either length or volume of a material takes place resulting from changes in moisture content or chemical changes. There are two types of shrinkage: plastic shrinkage and dry shrinkage.
- Plastic shrinkage is the shrinkage of concrete due to absorption of water by aggregates, evaporation of water, and bleeding.
- Drying shrinkage is the shrinkage taking place after the concrete has been set and hardened.
Shrinkage is primarily determined by the length of exposure. If this strain is not avoided, it causes tensile tension in the concrete, which leads to cracking. Surface cracks are the result of shrinkage. With the increase in w/c ratio and cement content, shrinkage also increases. The rate of shrinkage decreases exponentially with time.
Unit Weight
Density is a measure of mass per volume. The average unit weight of an object equals its total weight divided by its total volume. The weight of concrete is determined by the density of the materials used, such as cement, sand, and aggregate, as well as the water proportion, the availability of void % in the mix, the type of mix proportion, and shrinkage. Concrete can be classified into four types based on the property of concrete called unit weight.
Type of Concrete | Bulk density |
Heavy concrete | over 2500kg/m^{3} |
Dense concrete | 1850-2500 kg/m^{3} |
Lightweight concrete | 500 - 1850 kg/m^{3} |
Extra lightweight concrete | less than 500 kg/m^{3} |
Modular Ratio
For calculating deflections of structural concrete elements, the modulus of elasticity is one of the most important properties of concrete. The modulus of elasticity is given by the ratio of stress and strain. The modular ratio is the modulus of elasticity of steel divided by the modulus of elasticity of concrete m=E_{s}/E_{c}. Modular ratio changes because the modulus of elasticity of concrete varies with the age of loading, time, etc. Hence, this property of concrete takes into account the creep and shrinkage and is divided into two: short-term modular ratio and long-term modular ratio
- Long-term modular ratio:m=280/3f_{ck}
- Short-term modular ratio:m=E_{s}/E_{c}
Poisson’s Ratio
Concrete compression expands in a direction perpendicular to the direction of the application of load. One of the last properties of concrete thus can be defined as the ratio of lateral strain to longitudinal strain. Poisson’s ratio can be determined by the fundamental resonant frequency of longitudinal vibration of the concrete sample using the Ultrasonic pulse velocity method. The ratio increases with the richness of the mix.
For high-strength concrete, the Poisson's ratio is 0.1 while for weak mixes it is 0.2. For strength design, it's usually 0.15, and for serviceability criteria, it's usually 0.2. The value of Poisson’s ratio varies between 0.10 to 0.20 for normal concrete.
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