Concrete is traditionally made out of a mixture of cement, water, sand and gravel. Modern concrete mixes also add other ingredients such as fibers, plastics and various chemical admixtures. The ingredients and the mixture proportion determine the properties of concrete, including slump.
Slump is a measurement of the workability or consistency of concrete. In other words, it measures how easy the concrete is to push, mold and smooth out. Accordingly, its slump rating indicates what construction application the concrete is good for. The higher the slump, the more workable the concrete. If the slump of concrete is too low, it won't shape very easily. If it is too high, you run the risk of having the gravel, sand and cement settle out of the mixture, making it unusable.
Slump and Quality
The lower the amount of water in a traditional concrete mix, the lower its slump. Low slump values in traditional mixes generally mean higher quality concrete. The additional ingredients in modern concrete mixes make it impossible to determine concrete quality from slump, however. Concrete with plasticizers and a high slump may actually have less water than a traditional concrete mix with a low slump. Because of these additional ingredients, you can set the slump of a batch of concrete to practically any value while still retaining a high quality mixture.
Slump is tested with a slump cone. This is a 12-inch tall, truncated cone open at both the top and the bottom. The top is 4 inches wide and the bottom is 8 inches wide. To test slump, you fill the slump cone to 1/4 of its height and tamp down with 25 strokes of a 3/4-inch steel rod. Then you fill the cone to the halfway point and tamp it with another 25 strokes. After this, you fill it to the 3/4 point and tamp it again. Filled to the top with one last layer, the mixture gets a final 25 strokes. Measure from the top of the cone to the base. Pull the slump cone up off of the concrete and measure how far the the wet concrete sinks, or slumps, down, by measuring to the top of the pile to the ground. Subtract this measurement from the original height of the slump cone. The distance slumped down is called the slump level.
Do not use slump level to compare the quality of one mixture of concrete to a different mixture. Use it only to compare the quality of different batches of the same mixture. This test does not indicate anything about the strength of the mixture.