Concrete slump refers to the measure of the consistency or fluidity of freshly mixed concrete. It is a standard test that builders perform to test the workability of a concrete mixture. The test for measuring concrete slump involves using a cone, filling the cone with concrete, and observing the amount that the concrete settles or slumps when the cone is removed.
Understanding concrete slump is vital for construction professionals because it helps to assess the suitability of a particular concrete mix for their specific application. It helps to assess the workability and the long-term performance of the concrete mix and allows them to troubleshoot on the construction site in real time.
Types of Concrete Slump
Builders measure concrete slumps in terms of inches or millimeters. This helps to indicate the workability of a particular concrete mix. There are three common concrete slump classification types used to describe varying consistency levels with the final type indicating no slump. These are assessed using a concrete slump test.
A true slump classification occurs when the concrete holds roughly the same symmetrical shape as the cone when someone removes the cone. The top surface remains either slightly convex or flat. While the true slump parameters may differ according to each project and specific type of concrete, typical true slump values fall within the range of 1 to 8 inches (25 to 200 cm).
When builders observe a true slump in their concrete mixture, this indicates that the concrete mixture has good workability and is capable of flowing and filling forms without much segregation or need for excess water. It indicates that the concrete is well-proportioned, well-mixed, and with a good cement-to-water ratio.
A shear slump classification is indicated when the concrete slumps unevenly along one side, with one side shearing off at an angle of 45 degrees or more while the other remains stable. This results in a tilted or lopsided slump shape.
When builders observe a shear slump, this indicates that the concrete mixture has low cohesion and internal stability. It shows that the mix might be prone to segregation, where the heavy particles, like aggregates, separate from the binding paste. This can result in uneven distribution of materials and potential problems with the overall stability of the mix.
The collapse slump type is indicated when the concrete slump collapses or spreads significantly when someone removes the cone. This results in a flat or nearly flat profile with no noticeable cone shape in the concrete.
This profile indicates that the concrete has an excessive amount of water or superplasticizers which results in an extremely high flowability. While excessive flowability is positive in specific contexts, it is almost always undesirable in most construction contexts. Excessive water can negatively impact the strength and durability of the concrete. It causes increased shrinkage and porosity and susceptibility to cracking and other structural problems.
When concrete exhibits zero slump, it means that the mixture retains its shape without any noticeable settling or deformation after the removal of the cone.
This lack of slump indicates a stiff concrete mix with little to no workability. It suggests that this concrete will be difficult to handle, place, and form easily. This might mean that the concrete has a low water-to-cement ratio or it might mean the presence of excessive amounts of coarse aggregates. This type of concrete mix poses a challenge to construction workers because of its stiff workability but also because this mix may not bond or fill forms adequately.
Concrete Slump Ranges for Project Requirements
Countries across the world have different concrete slump requirements for different types of projects. In the United States, the American Concrete Institute (ACI) provides standard ranges for different types of projects.
- General Construction – For general construction projects a standard concrete slump range is between 2-4 in (50-100 mm). This range provides suitable workability and strength for common concrete applications like slabs, beams, and footings.
- Highway and Heavy Construction – In highway or heavy construction applications, there are often more challenging conditions for concrete handling and placement. In this context, the slump range is higher, from 4-6 in (100-150 mm) to facilitate easier placement, compaction, and finishing.
- Mass Construction – For mass projects like dams and foundations, the slump range is lower to help reduce the risk of bleeding and segregation in large pours. For these projects the slump range is 0-2 in (0-50 mm).
- Architectural and Decorative Concrete – With architectural and decorative projects, the applications will vary according to the needs of the artist or architect. This indicates a wider range from 0-4 in (0-100 mm).