Concrete is a building material made from a mixture of cement, water, sand and gravel. Performance of concrete is evaluated from mechanical properties which include shrinkage and creep, strength and modulus of elasticity.
To design concrete one should have deep knowledge of these properties in fresh and hardened concrete. Fresh concrete is those which are prepared newly while hardened concrete is those whose final setting time has finished and is ready to bear the load on it.
Perhaps most widely known property of concrete in its hardened state is its strength, which is split into four types:
The compression strength of concrete is a measure of the concrete’s ability to resist loads which tend to compress it. The maximum load at which the specimen breaks is taken as a compressive load that it is measured by crushing cylindrical concrete specimens in compression testing machine. The compressive strength of the concrete cube test provides an idea about all the characteristics of concrete. By this single test one may judge that whether concreting has been done properly or not.
Concrete compressive strength for general construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial structures. Compressive strength has the same degree of importance when it comes to the evaluation of existing structures. Structural engineers used it for modeling different structural properties, such as tensile strength, and to predict the flexural and shear capacity of the elements. Other structural characteristics such as ductility and stiffness can be also defined as a function of compressive strength.
Compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face on which load was applied.
Compressive Strength = Load / Cross-sectional Area
In general, the compressive strength of concrete increases over the time. Therefore, the older concrete has the higher compressive strength:
Concrete strength is affected by many factors, such as quality of water/cement ratio, raw materials, coarse/fine aggregate ratio, age of concrete, compaction of concrete, temperature, relative humidity and curing of concrete. The most important ones are according blow:
Tensile strength of the concrete is defined as the ability of the concrete to resist the tensile force applied on it or the ability of the concrete to withstand the pull force applied on it without causing failure. The Tensile strength of the concrete is measured in the force per cross-sectional area. The unit of Tensile strength is MPa or N/mm2. The Tensile strength of the concrete ranges between 8% to 12% of its compressive strength. The true tensile strength of concrete is difficult to determine. The split-cylinder test (ASTM C496) has been used to determine the tensile strength of concrete and is generally accepted as a good measure of the true tensile strength.
The flexural strength of concrete is one measure of the tensile strength of unreinforced concrete. It refers to the ability of the concrete beam or slab that is being tested to resist bending. The results of flexural test on concrete expressed as a modulus of rupture which denotes as (MR) in MPa or psi. The flexural test on concrete can be conducted using either three points load test (ASTM C78) or center point load test (ASTM C293). Modern concrete technology utilizes a wide range of materials such as glass or steel fibers to improve the flexural strength of the concrete. These modified concretes often require the use of special tests and equipment.
Shear force is a force that tends to create a cut in the cross section of the object. This cut is parallel to the direction of the applied force. In structural and mechanical engineering, shear strength is of great importance for designing the dimensions of materials and structural members (such as beams, plates, and screws). In reinforced concrete beams, the most important purpose of using spacers is to increase shear resistance. Direct determination of shear is very difficult. Hence researchers have assumed the shear strength of concrete about 12% of the compressive strength.
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Zist Yar Varena Company(Private Joint Stock Company) was established in 2014 and has started its activity in the field of production, research and development of modern concrete admixtures in the civil industry and It intends to take steps to improve the quality of concrete and move towards sustainable development by cooperating with the national and international researches and academic centers by using the latest technologies in the world.
