How do we figure out the properties of concrete?

These are tests that civil engineers use to learn about concrete.

For this test, concrete is poured into a 6 x 12 in. cylinder (6 in. diameter, 12 in. height). The cylinders are usually plastic, like the ones to the right. The cylinders filled with concrete are allowed to cure at 70 degrees farenheit for 28 days. Then each cylinder is loaded axially in a laboratory: the cylinder is stood up in a loading machine and an arm of the machine pushes down on the top of the cylinder until the cylinder breaks. The maximum amount of force the machine used on the cylinder is important. The compressive strength of the concrete, f'c is given by:

f'c=P/A

where P is the load at which the cylinder failed
A is the cross-sectional area of the cylinder,
in this case, pi*r^2
where r is the radius of the cylinder

This test uses the same type of cylinders as are used in the compressive test from above. But instead of standing up in the loading machine, the cylinder lays on its side, as shown to the right. The machine pushes down on the free side of the cylinder. The cylinder will split in two halves. Based on the load at which the cylinder split, you can compute a tensile strength (*), fct, of the concrete. The equation is:

fct=2P/(pi)dL

where P is the load at which the cylinder failed
d is the diameter of the cylinder
and L is the length of the cylinder

photo from Cornell University/NSF Project

This is another type of tension test. For this test, the concrete is in the shape of a beam, usually with the dimensions 6 in. x 6 in. x 30 in. (square cross-section, 30 in. long). The beam sits horizontally in the loading machine. It is loaded at two points as shown below until it cracks. After some calculation, you can get the modulus of rupture, fr.

These methods for calculating the tensile strength of concrete are not very accurate. We do know that the tensile strength of concrete is much smaller than the compressive strength (concrete is much better if it is pushed than if it is pulled). So the experts decided to say that the direct tensile strength, f't, is equal to about 4?f'c, after they did many, many tests.

The experts (whoever they are) use these properties of concrete to help them design buildings, bridges, dams, and other things that are made of concrete. These properties of concrete tell them how much and what kinds of load a concrete structure can hold. But what about if the concrete is cracked? Well, that's a whole 'nother ballgame.

Concrete is poor in tension and it cracks whenever it feels too much tension, or pulling, on it. Concrete cracks quite often. So it is important to know if that crack is going to grow and cause the whole structure to fail or if the crack is harmless. So we need to study the crack.