Sampling (or obtaining) concrete for the slump test should be accomplished according to ASTM C 172. In this TRAMAN, only the procedure of sampling from a revolving drum truck mixer (TM) or agitator is discussed. If you should ever need to sample from a paving mixer, open-top truck mixer, or other type of equipment, be sure to refer to the most recent ASTM C 172.

Samples taken for the test specimens must be representative of the entire batch. This is accomplished by taking the samples at two or more regularly spaced intervals during discharge of the middle portion of the batch.

Sample by repeatedly passing a scoop or pail through the entire discharge stream. Composite these samples into one sample for testing purposes. Be sure that the first and last portions of the composite sample are taken as quickly as possible, but never exceeding 15 min. If it is necessary to transport the samples away from the mixer to the place where the slump test is to be performed, combine the samples and remix them with a shovel to ensure uniformity.

Perform the slump test according to ASTM C 143. Be sure to start the test within 5 min after obtaining the final portion of the composite sample. In performing the test, first dampen the slump cone and place it on a flat, moist, nonabsorbent, rigid surface. From the composite sample obtained and while standing on the two foot pieces of the cone, fill the cone in three layers, each approximately one third of the volume of the cone. In placing each scoopful of concrete, rotate the scoop around the top edge of the cone as the concrete slides from it to ensure even distribution of concrete within the mold.

Rod each layer with 25 strokes of the tamping rod (using the rounded end), and uniformly distribute the strokes over the en-tire cross section of each layer. Rod the bottom layer throughout its depth. Rod the second layer and the top layer each throughout its depth so that the strokes just penetrate into the under-lying layer. In filling and rodding the top layer, heap the concrete above the mold before the rodding is started. If the rodding results

in subsidence of the concrete below the top edge of the cone, add additional concrete to keep an excess of concrete above the top of the cone at all times. After the top layer has been rodded, strike the surface of the concrete off flush by means of a screeding and rolling motion of the tamping rod. Immediately remove the slump cone from the sample by carefully and steadily lifting it straight up at the rate of 5 2 sec for the height of the cone. Place the cone next to the test specimen. At this point, the entire test from the start of filling the cone to completing the removal of the cone should not exceed 2 1/2 min. In measuring the slump, first place the tamping rod across the top of the cone so that it extends over the test specimen as shown in figure 15-41. Next, measure the vertical distance from the bottom of the rod to the average height of the subsided concrete specimen.

This measurement is known as the SLUMP. If a decided falling away or shearing off of concrete from one side or portion of the specimen mass has occurred, disregard the slump measurement and make a new test on another portion of the sample. If two consecutive tests show a falling away or shearing off, the concrete probably lacks the necessary plasticity and cohesiveness for the slump test to be applicable.

After measuring and recording the slump, you have completed the slump test. As a supplementary procedure, however, tap the sides of the specimen gently with the tamping rod. The reaction of the concrete indicates its cohesiveness and workability. A well proportioned, workable mix gradually slumps to lower elevations and retains its original identity. A poor mix crumbles, segregates, and falls apart.

If the slump testis performed for a trial batch during concrete mix design, then too little, or too much, slump indicates the need for a new trial batch with revised ingredient proportions. When the test is performed as a quality control mea-sure for a construction project, the slump obtained by testing will be compared to the specified slump for the concrete used for that particular project. If too little, or too much, slump has been determined by the test, then the quality control inspector, or other appropriate authority, will determine whether to accept or reject the concrete.