Compression Tests on Hardened Concrete

Compression Test

1. Remove the concrete cylinders from the storage area or curing room, and surface dry the cylinders.
2. Select six cylinders for this test, and measure the diameter of each of the cylinders.
3. 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 (b) place a neoprene end cap on each end. In this lab, we will use neoprene end caps, as this method is by far the simplest. However, even using this technique, major surface imperfections must be removed beforehand.
4. Apply the compressive load slowly and continuously until the maximum load is reached. The loading rate should be between 20 psi to 50 psi per second (150 lb. to 300 lb. per second). Failure of the cylinder is imminent during the test when the load indicator slows down and finally stops. Allow the compressive load to continue until the cylinder is crushed. Closely examine the type of failure of the cylinder.
5. Record the maximum load and determine the compressive strength for each specimen tested.

Determining Young's Modulus

1. For one of the cylinder compression tests, install a compressometer around the cylinder following steps 2.2 to 2.10.
2. Unscrew the seven contact screws (2 on the upper lock ring, 3 on the lower lock ring and 2 on the middle ring) until the points are flush with the inside surface of the rings.
3. Place the compressometer over the concrete specimen locating the specimen at the center of the ring.
4. Place three equal length blocks under the lower ring. The length of the blocks (cylinders) should be vertical to provide the correct height.
5. Hand-tighten the 3 contact screws in the lower lock ring and the 2 contact screws in the upper ring against the specimen.
6. Hand-tighten the 2 contact screws in the middle ring making sure that the vertical stem of the axial strain dial indicator is midway between the two portions of the middle ring.
7. Remove the two-spacer rods.
8. Remove the three metal blocks from under the lower ring.
9. Zero the axial strain dial indicator with the stem close to the fully extended position.
10. Zero the diametrical strain dial indicator with its stem close to the fully pushed-in position.
11. Apply a series of loads in steps of about 10,000 lbs., up to about 60,000 lbs. At each load step, record the longitudinal and hoop deformations.

Schmidt Hammer Demonstration

1. Mark a 2 ft. x 2 ft. grid on a concrete floor slab, covering an area of 10 ft. x 10 ft. Select a concrete surface that is smooth, dry, and at least 4 inches (or 102 mm) thick.
2. At each grid point, conduct and record a Schmidt rebound hammer test, as given in Steps 3.3 to 3.4.
3. Before the hammer can be used for testing, the piston must be released out of the hammer into the testing position. If the piston is not extended, place the end of the piston against a stiff surface and gently press the Schmidt hammer firmly against the surface. You will hear a click, and the piston will extend into the test position.
4. Gently press the Rebound Hammer against the concrete surface to be tested. When the piston is pressed all the way into the Rebound Hammer, continue to push harder until you hear a rattling sound. Keep the Rebound Hammer firmly pressed against the concrete surface and read the rebound number on the scale.
5. Compute the average and standard deviation for this set of measurements.