Slump is a property of
concrete, which is quite commonly measured on a construction project. It is a measurement of the
consistency of the concrete, and will be indicative of the behavior of the concrete in the situations where it will be placed. The standard slump test is defined by
ASTM C143:
The test is performed with a slump cone (well, actually a frustrum, but who's counting? This is construction after all) and a steel rod. The cone is 12 inches tall, with an 8 inch diameter base, and a 4 inch diameter top. The steel rod is 24 inches long, and 5/8 inch in diameter, with a hemispherical tip. The cone is dampened, then placed on a flat surface. It is then filled in three layers of equal volume. This roughly approximates to filling it to 2 1/2 inches, then 6 inches, and finally to overflowing. Each layer is rodded 25 times, with the roddings ideally penetrating about half an inch into the layer below the one being rodded. After the final layer is rodded, the excess concrete is struck off the top of the mold, and the mold is slowly removed. The mold is then placed next to the concrete, and the rod placed on top of it, parallel with the ground. The distance from the rod to the top of the concrete in inches is the slump of the concrete. As you could probably guess, slump can range from 0 to 12 inches, with 0 being a very stiff concrete, and 12 being a very flowable concrete.
According to
ACI 318, concrete should be tested, including slump tests, at least once per day, and once per 150 cubic yards of concrete or 5000 square feet of slab or wall surface area.
Many construction
specifications require slump tests to ensure that concrete is of theproper strength. This is due to the belief that the slump is a good indicator of
water-cement ratio, a key indicator of concrete strength. However, this is not the case, as many factors influence the slump of a particular mix of concrete, such as coarse and fine
aggregate contents, and the use of
admixtures such as
water reducers or
superplasticizers. Slump is only a valid measurement of the consistency of concrete.