A concrete slump range is a measurement of how stiff or fluid the uncured concrete mix is. This test is typically done when concrete comes off a ready-mix truck, before it’s been compacted and before it sets in a form or mould.
The test is typically done by filling a cone with a sample of the concrete, removing the cone and measuring how much the pile has slumped down. The resulting shape of the cone is then observed and measured in order to classify the slump according to European standards (BS 8500).
This is the result you’re looking for and indicates that the concrete has optimum consistency. The mixture will largely retain its shape once the cone is removed, save for some slumping or flattening at the top of the pile.
This shear slump is often a sign that the concrete mix has too much water. The shear slump will cause the concrete to slump dramatically down one side of the pile.
If the concrete completely collapses, it’s too watery and will need to be remixed.
Low Slump Values
A low slump value is a good indication of concrete quality, as it reflects the balance between water content and cement content in the mix. However, it’s important to remember that slump is only a very small part of the overall quality and strength of concrete.
Today, many additional ingredients are added to modern concrete mixes that make it nearly impossible to determine concrete quality based on the slump of the hardened mixture alone. Besides variations in aggregate grading, fines content and other factors, the addition of plasticizers, admixtures and entrained air may significantly impact the concrete’s ability to hold its shape as well as its strength.