Slump Testing
Slump testing is a means of measuring the consistency of concrete using a slump cone. The cone is made of galvanized metal with an 8 inch diameter base, a 4 inch diameter top, and a 12 inch height. The base and the top are open and parallel to each other and at right angles to the axis of the cone, shown in Figure 6. A tamping rod 5/8 inch in diameter and 24 inches long is also needed. The tamping rod should be smooth and bullet pointed. Do not use a piece of reinforcing bar (rebar).
Take samples of concrete for test specimens at the mixer or, in the case of ready mixed concrete, from the transportation vehicle during discharge.
The sample of concrete from which test specimens are made should be representative of the entire batch. Obtain these samples by repeatedly passing a scoop or pail through the discharging stream of concrete, starting the sampling operation at the beginning of discharge, and repeating the operation until the entire batch is discharged. To counteract segregation when a sample must be transported to a test site, remix the concrete with a shovel until it is uniform in appearance. The job location from which you take the sample should be noted for future reference. In the case of paving concrete, you may take samples from the batch immediately after depositing it on the subgrade. Take at least five samples at different times, and these samples should be thoroughly mixed to form the test specimen.
When making a slump test, dampen the cone and place it on a flat, moist,
nonabsorbent surface. From the sample of concrete obtained, immediately fill the cone in three layers, each approximately one third the volume of the cone. In placing each scoop full of concrete in the cone, move the scoop around the edge of the cone as the concrete slides from the scoop. This ensures symmetrical distribution of concrete within the cone. Then rod in each layer with 25 strokes. Distribute the strokes uniformly over the cross section of the cone and penetrate 1” into the underlying layer. Rod the bottom layer throughout its depth.
If the cone becomes overfilled, use a straightedge to strike off the excess concrete flush with the top. Immediately remove the cone from the concrete by raising it carefully in a vertical direction. Measure the slump immediately after removing the cone.
Determine the slump by measuring the difference between the height of the cone and the height of the specimen as shown in Figure 7. The slump should be recorded to the nearest 1/4” in terms of inches of subsidence of the specimen during the test.
After completing the slump measurement, gently tap the side of the mix with the tamping rod. The behavior of the concrete under this treatment is a valuable indication of the cohesiveness, workability, and placeability of the mix. In a well proportioned mix, tapping only causes it to slump lower. It doesn’t crumble apart or segregate by dropping larger aggregate particles to a lower level in the mix. If the concrete crumbles apart, it is oversanded.
If it segregates, it is undersanded.