【Haichuan Chemical Construction Technology】What is the slump of concrete? How to measure it? It’s finally clear!

01 What is slump? The slump, which is the key indicator determining whether concrete can flow or not, is an essential measure of its fluidity; it is expressed in millimeters. In simple terms, it quantitatively indicates how easy it is to work with this batch of concrete. The upper diameter of the slump cone is 100 millimeters, the lower diameter is 200 millimeters, and its height is 300 millimeters; it is a conical tube. The concrete mixture is placed into this tube in accordance with specified methods. Once it is filled, the mixture is vibrated layer by layer to ensure compactness. Then, the slump cone is lifted vertically upward, and due to its own weight the concrete settles downward. The height difference between the top of the cone and the highest point of the concrete mixture is measured, and this difference represents the slump value. The greater the slump, the more fluid the concrete is and the better its workability; the smaller the slump, the drier the concrete is and the more difficult it is to pump. A higher slump does not necessarily mean things are easier, nor does a lower slump mean greater strength. Each pumping scenario requires a specific range of slump values, and if these limits are exceeded, measures must be taken to adjust the consistency of the concrete.

02 How is slump measured? One wrong step and all the data is wasted. The procedure for the slump test isn’t complicated, but there are rules for each step. The filling is done in three stages, with one-third of the cylinder filled at each stage. After each layer is filled, a tamping rod is used to tamp evenly from the edges toward the center, 25 times in total; for the bottom layer, tamping does not need to go all the way to the bottom – it should be done at an angle along the backing plate. For the second layer, tamping should reach the depth of that layer, while for the third layer, it is sufficient to tamp it below the surface of the concrete. Tamping should not be done randomly; it needs to be carried out in a spiral pattern in an orderly manner to compact the material, with the aim of expelling air and ensuring that the material fills all the corners. The most crucial step is lifting the cylinder; it must be lifted vertically upward, at a steady pace, and brought to the top within 5 to 10 seconds. There should be no shaking, no tilting, and no sudden stops. If the cylinder tilts or is lifted too quickly, the concrete will also tilt and collapse, and the slump value obtained in this case will not be an accurate figure. Behind the number indicating slump lies a competition between the mix proportions and the conditions on site. If the slump is too low, the mixture is too dry, which results in greater resistance during pumping and an increased risk of pipe blockages. If the slump is too high, the mixture is too thin; in such cases, the stones settle at the bottom and water separates out, leading to stratification once the mixture is poured into the formwork. There are many lessons to be learned from both situations. If the slump on site is insufficient, it’s definitely not advisable to try to increase it by adding water – instead, the mixing plant should be asked to adjust the mixture using admixtures. If the slump is too high, it’s necessary to check the mix proportions, the amount of water used, and the transportation time; it’s not possible to simply keep pouring the mixture without addressing these issues. At the mixing plant, measuring the slump of each truckload is a standard procedure; the lift cylinder is used at regular intervals to simultaneously check the workability. The slump data and the adjustment logs are matched one by one. They regard the slump value as an indicator of the quality of the concrete, rather than just a formality during inspection. The slump test measures not just the consistency of the mixture in wet and dry states, but also the overall condition of the concrete’s aggregates, paste, and admixtures. It tells you whether mixing is possible, how to carry out the mixing process, and what consequences will arise from it.

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