Compression Tests on Hardened Concrete

Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

There are two distinct stages in a construction project involving concrete. The first stage involves batching, transporting, and casting fresh concrete. At this stage, the material is viscous, and the workability and finishability are the key performance criteria. The second stage occurs when the hydration process begins shortly after the concrete is placed in the form, and the concrete will set and begin to harden. This process is very complex, and not all of its phases are well understood and characterized. Nevertheless, the concrete should achieve its intended design strength and stiffness at about 14 to 28 days after casting. At this point, a series of tests will be conducted on concrete cylinders cast at the time of placement to determine the concrete's compressive and tensile strengths, as well as on occasion, its stiffness.

The objectives of this experiment are threefold: (1) to conduct compressive cylinder tests to determine the 7-, 14-, and 28-day strength of concrete, (2) to determine the modulus of elasticity at 28 days, and (3) to demonstrate the use of a simple non-destructive test to determine in situ concrete strength.

Compression Test

Remove the concrete cylinders from the storage area or curing room, and surface dry the cylinders.
Select six cylinders for this test, and measure the diameter of each of the cylinders.
Ensure that the ends of the cylinders are as level as possible. As the top of the cylinders are probably not very flat, one must (a) grind the concrete cylinder ends with a mason's rubbing stone to remove surface irregularities and cast an asphaltic cap at both ends of the cylinder, or

The cylinders in compression tended to fail along an inclined plane, at roughly 45 degrees. This feature indicates that the failure was not driven by pure compression (crushing of the cylinder) but rather by shear forces, or more precisely by splitting tension stresses.

The compressive test results are computed by dividing the maximum measured load (P_max) by the measured area. The compressive strength value is taken a

Compression tests on concrete cylinders were performed, as well as measurements of Young's modulus, Poisson's ratio, and a demonstration of NDT measurements in concrete. Compression test results from the cylinder tests, like those carried out in this laboratory exercise, are relatively simple to conduct and produce results with acceptable variability. Measurements of Poisson's ratio and Young's modulus are difficult to make, and these properties are often calculated by empirical formulae from the compression strength r

Transcript

-- Views