Test Cylinders
There are two reasons to make and test cylinders:
- Determine if the concrete meets the design requirements for specified
compressive strength, known as design strength check. - Determine if the concrete that has been placed is of sufficient strength for the forms to be removed or for the concrete to be put into service, known as construction site control.
Making Test Cylinders
Specifications usually require at least two cylinder breaks to be averaged and reported as one “test” to provide compressive strength results, giving a better indication of the strength of the concrete. If you need to test at 7 days and 28 days, you will need to make at least 6 cylinders. You will also need one or more hold cylinders as backups in case the 28-day cylinders are damaged or do not come up to strength.
Most concrete test cylinders made in the United States are 6 inches in diameter and 12 inches high and are known as 6 x 12 cylinders. Sometimes other sizes such as 4 x 8 inches and 3 x 6 inches are used, but these require adjustments of the test results to equate to the 6 x 12 cylinder strengths.
There are several kinds of cylinder molds, including heavy steel molds, sheet metal molds also known as tin can molds, plastic molds, and waxed cardboard molds. Tin can, cardboard, and some plastic molds are single use molds. Heavy steel molds and some sheet metal and plastic molds are reusable, but you must be clean and oil them after each use.
After obtaining a sample of concrete according to ASTM C 172, the procedure to create test cylinders is:
- Make sure the cylinder molds have been cleaned and oiled as needed.
- Place the cylinder molds on a solid base such as a concrete slab or a sheet of plywood, as shown in Figure 8.
- For slumps greater than 3 inches: Fill each mold in three equal layers and use a standard tamping rod to rod each layer 25 times. The rod should penetrate all the way through each layer and into the previous layer about 1 inch. Tap the sides of the mold lightly to close any voids created by rodding.
- Mark each cylinder so it can be matched with the concrete in the corresponding part of the project. Make sure to record the time and date that the concrete was placed.
- For slumps less than 1 inch: Fill each mold in two equal layers. Use an internal vibrator with a diameter of 3/4 inch to 1 1/2 inches, and insert it into the concrete at three locations. Leave the vibrator in the concrete long enough at each location to allow entrapped air to escape. Raise the vibrator slowly and tap the sides of the mold lightly.
- For slumps between 1 and 3 inches: Fill the mold and compact the concrete either by rodding or vibrating, as described in step 3 or 4.
- Strike off and smooth the surface of each cylinder. Cover the top surface of each cylinder with a plate or sheet of impervious plastic as shown in Figure 9.
Curing and Protecting Test Cylinders
Store test cylinders made for design strength check in a moist environment with a temperature of 60° to 80° F for up to 48 hours. ASTM C 31 suggests several ways to maintain the required moisture and temperature.
If the test cylinders will not be transported to the lab within 48 hours, remove the cylinders from their molds within 16 to 32 hours and keep them moist at 70° to 76° F until the time of the test. If the test cylinders are to be sent to the laboratory for standard curing within 48 hours, make sure they remain in the molds and are kept moist until they reach the lab. When the test cylinders reach the lab, the molds are removed and the specimens are placed in standard curing until the time of the test.
Accurate test results rely on care taken in handling, shipping, and storage of test cylinders. ASTM C 31 says that test cylinders must be protected from freezing and moisture loss during transportation and cushioned to prevent them from jarring. Several suggestions are to wrap the specimens in plastic or surround them with wet sand or wet sawdust. Travel time should not exceed 4 hours. Any test cylinders that are prematurely moved, accidentally dropped or kicked, or left in the field too long at the wrong temperature must be discarded.
Keep test cylinders made for construction site control at the jobsite temperature and give them the same curing as the concrete they represent. Specimens made to determine when a structure can be put into service should be removed from their molds at the same time the formwork is removed from the project. Test these specimens in the moisture condition resulting from the jobsite storage. Careful handling and transportation for testing are important for these test cylinders as well.
Testing the Cylinders
Compression tests of concrete cylinders are specified in ASTM C 39. The ends of the test cylinders are ground or capped in accordance with ASTM C 617. The ends of each test cylinder must be smooth, plane surfaces to assure even loading and accurate test results. Various commercial materials are available to cap compressive test specimens. Building codes usually define a strength test as the average of the results of breaking two cylinders made from the same sample and tested at the same designated age.
Job specifications usually require that concrete reach at least 3000 psi, 4000 psi, or some other minimum strength at 28 days. This specified compressive strength is commonly referred to as fc’. Always use a 28-day strength test unless specifications definitely specify otherwise. Cylinders made, cured, and transported according to ASTM C 31 and tested according to ASTM C 39 must average at least as strong as the specified strength for the project.
When the test cylinders reach the age specified for test, place them in a calibrated hydraulic testing machine which applies load at a uniform rate to the flat ends. Increase the load until the cylinder fails under load. Calculate the strength of the concrete by dividing the maximum load by the area of the flat surface. If the cylinder is 6 inches in diameter and 12 inches high, and the maximum load is 90,000 pounds (lb), the strength is calculated as:
Since the loaded area for a 6 inch diameter circle is 28.3 square inches (sq in):
