Compressive strength refers to the ability of a certain material or structural element to withstand loads that reduce the size of that material, or structural element, when applied. A force is applied to the top and bottom of a test sample, until the sample fractures or is deformed.
Materials such as concrete and rock are often evaluated using a compressive strength test and in these cases, fracturing occurs.
Materials such as steel can also be tested for compressive strength, and in the case of ductile materials, deformation tends to occur. Initially, a ductile material will accommodate the applied load by adjusting it’s internal structure– a process referred to as plastic flow.
Once the deformation is concentrated in one area, the plastic flow stops and the material breaks. For ductile metals, tensile strength is usually the preferred indicator for measurement and comparison. This is because tensile stress measures the forces needed to pull a material apart, which is better suited to the plastic flow phenomenon.
The compressive strength of concrete is often tested to evaluate if the actual concrete mix meets the requirements of the design specification. The test is usually conducted in batching laboratories.
In order to conduct the compressive strength test, a small sample of concrete mix is first cast in a cube or cylinder form and allowed to age for 28 days. For concrete samples that contain additional material, a longer curing time of 56 days is recommended. If the design engineer wants to test an existing structure, then drilled core samples are taken from that structure.
The sample is then placed between the two platens of a concrete testing machine and a load is applied to opposing sides of the sample until it fractures. The loading rate is important since a loading rate that is too low has the potential to cause creep.
Factors such as mix proportions, the water/cement ratio and curing conditions all affect the compressive strength of the concrete.
The formula used to calculate compressive strength is:
F = P/A
F = The compressive strength (MPa)
P = Maximum load (failure load) applied to the specimen (N)
A = Cross-sectional area of the specimen resisting the load (mm2)
Standard applications usually require the concrete to meet a compressive strength requirement of 10 MPa to 60 MPa, whereas for certain applications higher strength is needed and concrete mixes can be designed that meet a strength requirement of 500 MPa. Concrete that meets this strength requirement is referred to as ultra-high-strength concrete.
The compressive strength of steel and other ductile materials can be determined using a universal testing machine. The ductile material under test is placed between two level plates and compression occurs until a specific load is obtained or the material breaks.
The key measurements that would be evaluated in this case are the maximum force achieved before breakage or the load at displacement. The loads are applied either mechanically or hydraulically.
Within the brittle material group, materials such as rock tend to have higher compressive strengths of 140 MPa. Softer variations such as sandstone tend to have lower compressive strengths of around 60 MPa.
The compressive strength of ductile materials such as mild steel used for most structural purposes is around 250 MPa.
In terms of concrete, ultra-high-strength concrete can be used to construct structures that have to be able to withstand heavy loads and stresses such as highway bridges, whereas for standard, domestic paving use, the concrete can have a lower compressive strength of 30 MPa.
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