Understanding Concrete Slump Test for Workability
The concrete slump test is a simple and cost-effective method used to assess the work ability of concrete mixes. It helps ensure consistent quality during construction. This test has been relied upon for work ability assessment since 1922. It is conducted following specific standards like ASTM C143 (US), EN 12350-2 (Europe), and IS 1199:1959 (India).
What is “Slump”?
The term “slump” refers to how fluid or flowy fresh concrete is before it sets. Higher slump values indicate more fluidity. Despite its name, it’s a straightforward test that measures how much a pile of concrete slumps when left to stand.
Factors Affecting Slump Test
- Material properties (e.g., moisture content, fineness, chemistry)
- Aggregate characteristics (e.g., moisture, grading, size)
- Admixture type, dosage, and effectiveness
- Air content in concrete
- Concrete temperature
- Batching, mixing, and transportation methods
- Sampling, testing techniques, and apparatus conditions
- Free water content in the concrete
- Time elapsed since mixing
Apparatus Needed
To perform a slump test, you’ll need:
- Slump test mold: Includes a non-porous base plate, slump cone, measurement scale, and tamping rod.
- Dimensions: 30 cm height, 20 cm bottom width, 10 cm top width.
- Tamping rod: Made of steel, 60 cm long, 16 mm diameter, rounded at one end.
Principles of Slump Test
The test simulates the behavior of an inverted concrete cone under gravity. It evaluates the consistency (wetness) of the concrete, providing insight into its workability.
Procedure for Slump Test
- Clean the mold and apply oil to its inside surface.
- Place the mold on a smooth, flat, pore-free surface.
- Pour the concrete mix into the mold in four equal levels.
- Tamp each layer with 25 uniform strokes using the rounded end of the tamping rod.
- Remove excess concrete with a trowel and level the surface.
- Clear any water or mortar near the mold base.
- Lift the mold vertically from the concrete as soon as possible.
- Calculate the slump by measuring the difference in height.
Interpreting Slump Values
Workability |
Compaction Factor |
Slump (mm) |
Very Low |
0.78 |
0 – 25 |
Low |
0.85 |
25 – 50 |
Medium |
0.92 |
50 – 100 |
High |
0.95 |
100 – 175 |
Analyzing Slump Results
Measure the specimen’s slump in millimeters of subsidence during the test. Record the value for future reference.
Types of Slump Shapes
- True Slump: Measures the subsidence between the cone’s top and the concrete’s surface.
- Zero Slump: Indicates a very low water-to-cement ratio, often used in road construction.
- Collapsed Slump: Signifies excessive wetness or high workability, rendering the test invalid.
- Shear Slump: Suggests unsatisfactory results, requiring retesting.
Precautions in Slump Testing
- Moistening the mold’s inside and base reduces surface friction variations.
- Remove any concrete around the cone’s base before removing the mold.
Recommended Slump Values for Different Applications
Type of Work |
Slump (mm) |
Concrete for Road Work |
25 – 30 |
Ordinary RCC Work for Beam and Slab |
50 – 100 |
Column, Retaining Walls, Thin Sections |
75 – 150 |
Vibrated Concrete |
12 – 25 |
Mass Concrete |
25 – 50 |
Understanding these aspects of the concrete slump test helps ensure the right mix for specific construction needs.
Leave a comment