Title of Test
Determination of the workability of concrete or slump of concrete mix
Aim of Test
To determine the slump/workability of concrete mix
Theoretical Background
Workability refers to the ease with which concrete can be placed and compacted. The slump test is the most commonly used site method for assessing the workability of a concrete mix. The test entails filling an open-ended conical mould with concrete and measuring the slump or drop in level of the concrete when the mould is lifted up clear. Since the workability of a given concrete increases with the amount of water added, the slump test also provides a means of controlling the water content of successive batches of the same mix provided there is no substantial change in aggregate gradings. Increased water content causes a reduction in strength and durability of concrete, but the workability must be sufficient to allow full compaction. There should be a trade-off between these two opposites.
Apparatus
Slump cone, tamping rod, steel rule, straight edge, scoop, gauging trowels, steel float, mixing tray, measuring cylinder (1000 ml), shovel, cement/aggregate gauging tin and a weighing balance
Materials
Cement, fine and coarse aggregate (graded aggregate), water
Procedure
1. Weigh out about 2.8 kg of cement.
2. Weigh out equivalent masses of fine and coarse aggregate using the mix ration of 1:2:4 (i.e. if the cement is measured using 2 tins, measure out fine aggregate using equivalent 4 tins and coarse aggregate using equivalent 8 tins).
3. Mix thoroughly the cement, sand and aggregate together.
4. Using a water cement ratio (W/C) of 0.7 weigh out the required quantity of water (i.e. W/2.8 = 0.7 >> W = 1.96 kg = 1.96 litres = 1960 millilitres). Add 1960 ml of water the mixture in (3) and mix thoroughly until consistency is achieved.
5. Check the slump cone to ensure that it is clean. Place the wider end of the slump cone as base on a solid, flat, impermeable clean surface.
6. Hold the cone firmly in place and fill it with concrete in three (3) approximately equal layers. Ensure that each layer is tamped or rodded exactly 25 times before filling the subsequent layer. When filling the final layer, overfill the cone slightly.
7. After compacting the final layer, strike off the surplus concrete above the cone and also clean the concrete droppings about the base of the cone. Steadily lift off the cone vertically from the concrete. Once the cone is lifted off, the concrete is expected to slump down to some height before achieving stability again. The level of this slump depends on the amount of water present in the concrete and also to a less level on the constituents of the concrete especially the aggregates.
8. After lifting off the slump cone, invert the slump cone and place it adjacent to the slumped concrete. Place a straight edge over the cone in such a way that the straight edge passes over the slumped concrete. To ensure verticality, drop a plumb from the straight edge over the slumped concrete and measure the height difference between the straight edge (which marks the original height of the cone) and the top level of the slumped concrete in ‘mm’. This height difference is known as the slump of the concrete (see image below).
9. A good slump for concrete should range between 75 mm and 150 mm. A slump below 75 mm means that the water in the concrete is too small to ensure workability. A slump above 150 mm means that there is excess water in the concrete.
