Concrete is very weak in tension, but behaves very soundly under compression. When you put a load on top of a concrete beam, the beam bends into a U-shape. The top of the beam is in compression, and the bottom of the beam is in tension. Tensile strength of concrete depends on mix design, but for many engineering and calculation purposes, it is assumed to be zero. The American Society for Testing and Materials (ASTM) provides standard concrete tensile strength measurement procedures. According to the ASTM, the method is useful for both field and laboratory use.
Obtain or build a concrete test beam. Typically, a 6 inch by 6 inch unreinforced concrete beam, meaning no rebar, with a span length of at least three times the depth is used. Provide supports for the beam on each end.
Ensure the concrete beam is cured and wet immediately before testing. Testing a dry concrete beam can result in a very low and inaccurate tensile strength measurement.
Inspect the test beam for signs of damage. Mishandling a concrete test beam can lead to error in obtaining measurements.
Locate copies of the ASTM C78 standard -- loading at two points, equidistant across the beam length -- and the C293 standard -- one load in center of beam. Copies can be purchased from the ASTM website. This is the only way to be sure you are performing safe and accurate tensile strength tests.
Obtain a Modulus of Rupture value according to the procedures outlined in ASTM C78 and C293. Modulus of Rupture is essentially the maximum stress in a loaded beam at the time of failure. This test measures flexural strength, which is a sufficient way to measure the tensile strength of concrete.
Tips & Warnings
- Seek help from a civil engineer or faculty/staff member at a university with a civil engineering program if you feel unsure of your test scenario.
- Photo Credit concrete beams open to blue sky. image by Steve Johnson from Fotolia.com
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