Concrete formulations vary significantly. Each batch of concrete must be tested to ensure its compressive strength. The traditional way to measure concrete's compressive strength is through test cubes. Small, 100 to 150mm cube samples are either taken from existing structures or made in molds. Specialized concrete compression machines then compress the concrete and precisely measure its compression strength. Concrete compression is often tested in dedicated laboratories. If you have access to a concrete compression machine, you can test concrete cubes yourself.
Things You'll Need
- Permanent marker
- Concrete compression machine
Mark each test concrete cube with a reference number by using a permanent marker. The number of cubes you must test depends on the volume of the concrete from which the sample is taken. Test one cube per 1 to 5 cubic meters of concrete, two cubes per 6 to 15 cubic meters, three cubes per 16 to 30 cubic meters, four cubes per 31 to 50 cubic meters, five cubes for 51 cubic meters or more and an additional cube for every additional 50 cubic meters or part.
Place one test concrete cube on a concrete compression machine's loading shelf. Close the machine's door securely. High strength concrete cubes may shatter into pieces when they reach their peak stress load.
Select your testing options if options are available. Select "cube" to identify the specimen on a compression machine that tests a variety of concrete specimens. Specify the engineering units, such as pounds per square inch or kilograms per centimeter, if desired. Set the rate of application if that option is available. For example, 14 Newtons per square millimeter per minute can be used for standard concrete cube tests.
Operate the machine according to its instructions. Many concrete compression machines operate automatically, compressing at the programmed rate until the concrete cube fractures. Manually operated concrete compression machines, however, require the user to depress the lever steadily. To use a manually operated machine, depress the lever, watch the machine's dial or load indicator and stop depressing the lever when the dial or load indicator shows that the load begins to decrease, which indicates that the concrete cube fractured.
Record the number on the dial or load indicator. If the machine is operated manually, then record the peak load described in Step 4. Automatic concrete compression machines will display the peak stress load on the indicator. Some machines also may display the average load rate applied to the concrete cube over the time of the compression test.
Test each other concrete cube in the same way you tested the first concrete cube.
Compare each individual compression test and the average of all of the compression tests to the industry standard. Both must comply with the standards for the class of concrete.
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