Test cylinders (ASTM C 31) are cast to verify the specified compressive strength of the mix has been achieved. Typically 6-inch-diameter by 12-inch-tall plastic molds are used. Some projects use 4-inch-diameter by 8-inch-high cylinders.

Fill the 6-inch-diameter molds in three equal layers, rodding each layer 25 times. (Fill 4-inch-diameter molds in two equal lifts.) After rodding each layer, tap the outside of the mold to remove any remaining air voids. Once the mold is filled, strike off the top layer of the concrete with the top of the mold and store the molds at temperatures of 60-80°F, leaving them undisturbed. Good field practice would be to place the set of test cylinders in a cure box (shown here) until it is picked up and brought to a lab for curing until the date of testing. Typically a set of four cylinders are cast, with two tested at 7 days and two tested at 28 days. Specifications can, of course, call for other test dates as needed.

A cure box on a level surface with temperature control is ideal for keeping cylinders within the proper temperature range (60-80°F) prior to pickup, up to 48 hours after casting. (Photo courtesy of PCA.)

Leaving test cylinders in the sun for too long will cause problems later. Cylinders should be placed on a level surface and protected from the elements for up to the first 48 hours, with the tops covered to prevent moisture loss.

Testing tip: Test cylinders that are poorly made, stored, or neglected will cause headaches and may result in the need for costly hardened concrete testing, all to provide the owner information proving that the actual in-place concrete is of sufficient strength and durability. While this procedure is simple, do not take it lightly. There are a number of reasons why cylinder strengths might be compromised by poor practice, as shown in this table "Effects of Selected Testing Errors".

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