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Concrete is a building material made from sand, water, gravel, and Portland cement. As this mixture hardens, or cures, it forms a very strong and durable material. Because of its strength and resilience, concrete is widely used to construct buildings, roads, and a variety of other structures. When building with concrete, it is important to understand whether it can stand up to the compressive forces in the structure where its being used. One way to measure this is by testing the concrete's compressive strength, or how well it holds up to the compressive pressures around it.
While concrete contains very few ingredients, the exact mix ratio must be chosen carefully to ensure the concrete will perform as intended. If too much or too little water is added, for example, the cured concrete is likely to crack or show signs of surface damage. In some cases, the wrong mix ratio can lead to structural failure, which may result in serious safety issues.
Installers and independent testing agencies use a number of testing standards to check the integrity of freshly-poured concrete. One of the most widely used benchmarks is a test of concrete compressive strength. Concrete compressive strength measures the material's ability to withstand compressive forces. For example, a second-floor deck is subject to compressive force from beams positioned above and below the deck. By testing the concrete after it's poured, installers can ensure this floor is strong enough to withstand these forces and keep occupants safe.
The most common testing method for concrete compressive strength requires installers to pour three cylinders from the same mixture used on the structure itself. The three cylinders are placed in tubes or containers and given 28 days to dry before they are sent to testing labs. At the lab, each cylinder is placed in a machine that applies compressive force from above and below the cylinder. Once the cylinders break, or fail, agents measure the amount of strength each cylinder was able to withstand, then compare these results to the specified strength rating for the concrete to see if they match.
People in different parts of the world use different rating systems for measuring concrete compressive strength. In the United States, concrete strength is measured using pounds per square inch, or PSI. A standard concrete mixture used on a sidewalk may measure 3,000 PSI on average, though exact ratings must be determined by a structural engineer. Countries that use the metric system measure concrete compressive strength using megapascals, or Newtons of force per square meter. A concrete mixture that measures 3,000 PSI is equivalent to approximately 20 megapascals.
Concrete compressive strength should not be confused with tensile strength. Tensile strength measures the ability of concrete to resist lateral forces, or to resist being pulled apart from either side. While concrete has a fairly high compressive strength, it generally has a poor tensile strength. Many installers add reinforcement steel or rebar to improve the tensile strength of concrete. To increase compressive strength, it is often necessary to change the mix ratio or add special hardeners.