Slump test is a quality control test used to ensure the quality of concrete during manufacture. This test is conducted when the concrete is in the plastic state. It is mainly used to determine the consistency or work ability of concrete. The result of slump test is also taken into consideration while accepting a load of concrete. The first step in slump test is to take a test sample from the batch of concrete within 20 minutes of the arrival concrete in site . The sample is taken in one of the following two ways:
Sampling done after 0.2 m³ of the load has been poured (common method).
Fig.1 Equipment
A Slump cone which is in the form of a frustum with height 30 cm, bottom diameter 20 cm and top diameter 10 cm.
A steel tamping rod with 16 mm diameter and 60cm length which is rounded at one end.
Slump Plate of size 50 cm x 50 cm.
A Ruler or scale
Float and trowel
Procedure
At first the slump cone is placed over the slump plate and both are cleaned and moistened. Then fill one-third of the cone with concrete and this layer is rodded by giving 25 strokes to it. While rodding make sure that the whole area is rodded uniformly. Rodding means pushing the tamping rod in and out of the concrete to compact it. Always rod in a definite pattern by which rodding is done from outside to inside. The slump cone must be held firmly by standing on the foot lugs while adding the concrete during rodding .
Fig. 2
After rodding the first layer, fill the slump cone with a second layer until two-third is full and rod this layer uniformly with 25 strokes just into the top of the first layer. Then fill the slump cone with next layer until it slightly overflows and rod this top layer uniformly with 25 strokes just into the top of the second layer. Then remove the excess concrete from the top using a straight edge so that the cone is exactly filled and the spilled concrete removed from around the bottom of the cone.
Fig. 3
Then lift the slump cone straight up very slowly. The cone is then turned upside down without further disturbing the concrete and the taming rod is placed across the up-turned cone. The distance from the rod is then measured to the top of the slumped concrete. If the top of the slump is irregular, average height is measured. If the slump is too high or too low compared to the specification, another sample must be taken. If this also fails then the remainder of the batch should be rejected.
Result
Slump for the given sample is measured in mm. While carrying out the slump test, following are the shape of the concrete slump that can be observed:
Fig. 4
True Slump: This is the only slump that can be measured in the test. The measurement is taken between the top of the cone and the top of the concrete after the removal of cone as shown in Fig.3.
Zero Slump: This slump is the indication of very low water-cement ratio, which results in dry mixes. Such type of concrete is generally used for road construction.
Collapsed Slump: This slump is an indication that the water-cement ratio is too high. This indicates that the concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.
Shear Slump: This slump indicates that the result is incomplete, and concrete to be retested.
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