5.13: 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.

The strength of concrete used in structures is evaluated using compression tests to meet specific requirements after installation and also to monitor the quality over the lifespan of the project.

When concrete is poured into a form, it will begin to set and harden. The concrete will achieve its design strength and stiffness 14 to 28 days after casting. Concrete test cylinders are cast at the same time that the concrete is put in place. These samples are tested to determine the concrete strength and stiffness.

In this experiment, we will test the 28-day compressive strength of concrete. And use a simple non-destructive test of in-situ concrete strength.

As soon as concrete is put in place, the hydration process starts with the dissolution of cement in water. Which leads to a saturation of ions in the solution. Within a few hours crystals form and the space is occupied by the cement, which will give the material its final structure. The strength of the cured concrete is affected by the mixed design, the curing temperature and humidity, and the uniformity of the product. To measure this strength a hydraulic testing machine is used.

A device called a compressometer is attached to the test specimen to allow for calculations of Young's modulus and Poisson's ratio. Temperature and humidity during storage, the condition of the test specimen during the test, and the way in which the test is performed are all factors that affect test results and must be controlled. While cylinder tests are useful to determine the strength of concrete delivered to the site, in-situ testing is utilized to evaluate the quality in place over the life of the structure.

For this, the Schmidt hammer test shoots a steel weight at the surface of the concrete. The distance that the steel rebounds is measured and related to the strength of the material. Measurements can be made across one surface or many surfaces to evaluate the consistency of the concrete.

In the next section, we will measure the compression strength of test specimens and observe their mode of failure. We will also demonstrate the use of the Schmidt hammer test to indicate material strength.

Compression tests will be carried out using a hydraulic testing machine. For these tests the load capacity must be very high to test high-strength concretes. Remove the concrete cylinder from the mold and dry its surface to prepare the specimen for testing.

Then, inspect the concrete cylinder and remove any major surface imperfections from its ends using a file. After each end is prepared, apply a neoprene cap to ensure that the ends are as flat and level as possible. Center the specimen in the hydraulic testing machine and then apply the compressive load slowly and continuously at a rate between 20 to 50 psi per second. Allow the load to increase until the maximum is reached and the cylinder is crushed.

Failure is imminent when the load indicator slows down and finally stops. When the test is complete, record the maximum load and then closely examine the type of failure in the concrete cylinder. Determine the compressive strength of the specimen and record the fracture mode. Repeat this test for four of the five remaining specimens. For the final specimen install a compressometer so that Young's modulus and Poisson's ratio can be determined for this concrete mix.

First, unscrew all seven contact screws until the points are flush with the inner surface of the rings. Now, place the compressometer over the concrete cylinder and prop it up with three equal height spacers to center it vertically with the specimen. Hand-tighten the three contact screws on the lower ring and the two on the upper ring to secure the specimen concentrically in the compressometer.

When the specimen is secured, hand-tighten the final two contacts screws in the middle ring. Check that the vertical stem of the axial strain dial indicator is midway between the two portions of the middle ring. Confirm that the stem of the axial dial indicator is close to fully extended, and the stem of the diametrical dial indicator is close to fully retracted.

Finally, remove both spacer rods from the sides and the third rod located on the center ring. Lift the assembly by holding the specimen and carefully place it in the hydraulic testing machine and then zero both dial indicators. Apply a series of loads in steps of 10,000 pounds up to a maximum of 60,000 pounds. At each load, record longitudinal and hoop deformations as indicated on the dial indicators.

Find a concrete surface that is smooth, dry and at least four inches thick, and mark a 2 foot by 2 foot grid, covering a total area of 10 feet by 10 feet. If the piston of the Schmidt hammer is not extended place the end against a stiff surface and gently press down until a click is heard. The piston will extend as you pull the hammer away from the surface.

Now, gently press the hammer against the first grid point marked on the concrete surface. Continue to push until a rattling sound is heard. Read the rebound number on the scale and then pull the hammer away from the surface. Repeat this measurement at each grid point marked on the surface and then compute the average and standard deviation for the whole set of measurements.

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 average of the Schmidt hammer readings was 32.4 with a standard deviation of 1.3, which correlates to an in-situ strength of 4,650 psi based on calibration to parallel laboratory cylinder tests.

Now that you appreciate the strength testing methods for concrete structures, let's take a look at how it is applied to assure the quality of structures in our world.

In older bridges increased loading requirements can call for concrete strength testing. In these cases, cores are extracted from existing structures and tested in the laboratory to determine if the structure can carry loads higher than initially designed for.

In between the more destructive but very accurate in-situ core testing and the non-destructive but less accurate Schmidt hammer test, is the Windsor probe. In this test, probes are shot into the concrete surface and the penetration depth is measured to determine concrete strength.

You've just watched JoVE's introduction to compression tests on hardened concrete. You should now understand core testing and Schmidt hammer testing of concrete.

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