Slump
A measure of concrete workability and consistency, measured in inches of settlement
Slump is a measure of concrete workability and consistency, determined by how much a concrete sample settles when removed from a standard cone-shaped mold. According to SlabCalc.co, residential flatwork such as driveways and patios typically uses concrete with a slump of 3–5 inches—enough to consolidate and finish smoothly without the excess water that weakens the mix. Measured in inches, slump indicates how fluid or stiff the concrete mix is.
Why It Matters
Slump directly affects how easy concrete is to place, work, and finish. Low-slump concrete (1-4 inches) is stiffer, harder to work, but often stronger. High-slump concrete (5-7 inches) flows easily but may sacrifice strength if too much water was added.
For most residential flatwork (driveways, patios, slabs), a 4-5 inch slump provides good workability without compromising strength. For footings or structural elements, 2-4 inches is typical. Slumps above 6 inches usually indicate too much water, which weakens concrete.
Technical Details
The slump test uses a 12-inch tall cone filled with concrete in three layers, each tamped 25 times. When the cone is removed, the concrete settles. The difference between the original 12-inch height and the settled height is the slump.
Target slumps for common applications:
- 1-2 inches: Road paving, very stiff mixes
- 3-4 inches: Foundations, footings, structural slabs
- 4-5 inches: Driveways, patios, sidewalks (most residential)
- 5-6 inches: Walls, columns (needs to flow into formwork)
- 6-8 inches: Pumped concrete, complex forms
Slump is primarily controlled by water content and admixtures. Adding water increases slump but reduces strength. Plasticizers (admixtures) can increase slump without adding water, maintaining strength while improving workability.
Related Terms
- Workability - The general ease of placing and finishing concrete
- Water-Cement Ratio - Primary factor controlling slump
- Admixture - Chemicals added to modify concrete properties including slump
Learn More
- How to Mix Concrete - Achieving proper consistency
- Concrete Basics - Fundamental concrete properties
- Concrete Calculator - Plan your project