A slump test is a method used to determine the consistency of concrete. The consistency, or stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to the requirements for the finished product.
All concrete is a combination of aggregate — gravel and/or sand — cement, and water, but the proportions of the mix can be varied. A larger proportion of water causes a softer, wet concrete mix, which is easier to handle during the pour and requires less care during curing. It also results in a slightly softer and less durable finished product. A smaller amount of water causes a stiffer, wet concrete mix, which is harder to manage but makes a stronger and more durable finished product, one with a higher concrete pounds per square inch (PSI).
Contractors order wet concrete with a slump rating suitable for the job. A slump test is performed when the concrete is delivered to double-check the stiffness of the mix. A separate test is administered to each load of concrete.
The slump test is performed by filling a test mold with the wet concrete. The mold is shaped more or less like a cone that is missing its pointed end. A 5/8-inch (1.5 cm) diameter metal rod is inserted into the filled cone and pumped up and down about 25 times. This removes air bubbles and settles the concrete. The concrete spilling over the top of the mold is leveled off, and then the mold is removed and placed next to the concrete. Without the support from the mold, the concrete slumps down under its own weight.
The metal rod is placed horizontally across the top of the mold and extended over the slumped pile of concrete. A ruler is used to measure the distance between the rod and the concrete, which is the height lost when the concrete slumped down. The amount of height lost, expressed in inches, is the “slump” for that particular batch of concrete.
A slump of 1 to 3 inches (2.5 to 7.6 cm) indicates a very stiff mix, suitable for commercial work. This is sometimes expressed as "1 to 3." A stiff mix like this is difficult to work with, and ordinarily it requires a mechanical vibrator to get it to consolidate properly. It produces the hardest and strongest concrete.
A slump of 4 or 5 is acceptable for residential work, and pouring concrete is easier with this amount of slump. The concrete can be worked by hand, and it still results in strong concrete. Any mix with a slump of more than 7 should not be used at all.
Homeowners who are working with smaller amounts of concrete can perform a slump test without purchasing a mold. They can remove the bottom from a paper cup and invert it on a level surface. The form is filled with concrete, tamped it down gently, and the top leveled. They can then remove the cup and measure the height of the concrete, comparing that height to the height of the paper cup to determine the amount of slump. The slump will be less than it would be if a commercial mold was used, because the overall amounts of concrete are smaller. If the concrete slumps to about 75% of the cup’s height, it is usually a good consistency for pouring. This is not as accurate as a test using a standard mold, but is a good general guide.