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.