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.