Slump Test of Concrete – Detailed Explanation

The slump test is one of the most widely used field tests to determine the workability and consistency of fresh concrete.
It provides a quick indication of how easily the concrete can be mixed, transported, placed, and compacted without segregation or bleeding.

The test is simple, economical, and can be performed both on-site and in laboratory conditions, making it the most practical measure of concrete consistency.

Objective

To determine the workability (consistency) of freshly mixed concrete and verify whether it meets the desired design requirements for proper placing and compaction as per ACI and ASTM standards.

Apparatus Required

According to ASTM C143 / ACI 211.3R, the following equipment is used:

1. Slump Cone (Abrams Cone)
   • Top Diameter: 100 mm (4 in)
   • Bottom Diameter: 200 mm (8 in)
   • Height: 300 mm (12 in)
     The cone is open at both ends and provided with foot pieces and handles.
2. Tamping Rod
   • Diameter: 16 mm (5/8 in)
   • Length: 600 mm (24 in)
   • Rounded at one end for uniform compaction.
3. Base Plate
   • Non-absorbent and rigid surface (metal or glass plate).
4. Measuring Scale or Ruler

Procedure (As per ASTM C143)

1. Moisten the cone and base plate before filling.
2. Place the cone on the base plate and hold it firmly with the foot pieces.
3. Fill the cone in three equal layers, each approximately one-third of its height.
4. Tamp each layer 25 times using the tamping rod, distributing strokes evenly over the cross-section.
5. Strike off the excess concrete level with the top surface.
6. Lift the cone vertically in 5–10 seconds, ensuring no lateral or twisting movement.
7. Measure the slump:
   • Place the cone beside the slumped concrete.
   • Measure the vertical difference between the top of the mold and the highest point of the slumped concrete.

Types of Slump

Depending on the way the concrete subsides after lifting the cone, the following types of slump are observed:

Type of Slump	Description	Indication
True Slump	Concrete subsides uniformly, maintaining its shape.	Indicates good workability and cohesion.
Shear Slump	One side of the concrete shears off and slides.	Suggests lack of cohesion or segregation.
Collapse Slump	The concrete completely collapses.	Mix too wet; very high workability.

👉 Only true slump is considered valid for assessing workability.

ACI Requirements and Acceptance Limits

According to ACI 211.3R (“Guide for Selecting Proportions for Concrete”) and ACI 301 (“Specifications for Structural Concrete”), the recommended slump values are:

Type of Construction	Slump Range (mm)	Slump Range (in)
Pavements, slabs	25 – 75 mm	1 – 3 in
Beams and walls	25 – 100 mm	1 – 4 in
Columns	75 – 150 mm	3 – 6 in
Foundations	25 – 75 mm	1 – 3 in
Mass concrete	25 – 50 mm	1 – 2 in
Pumped concrete	75 – 150 mm	3 – 6 in

ACI Tolerance:
A variation of ±25 mm (±1 in) from the specified slump is generally acceptable for field control (as per ACI 301-20, Section 4.2.2.4).

Interpretation of Results

Slump Value	Workability	Typical Use
25–50 mm	Low	Road pavements, footings, mass concrete
50–100 mm	Medium	Beams, slabs, and general construction
100–150 mm	High	Pumped concrete, congested reinforcement areas

Limitations

• Not suitable for very dry (zero slump) or very fluid mixes.
• The test does not directly measure workability, only indicates consistency.
• Test results can vary due to operator technique or testing conditions.
• For more precise workability measurements, tests like the Compaction Factor Test or Vee-Bee Consistometer may be used.

ACI and ASTM Reference Standards

• ACI 301 – Specifications for Structural Concrete
• ACI 211.3R – Guide for Selecting Proportions for Concrete
• ASTM C143/C143M – Standard Test Method for Slump of Hydraulic-Cement Concrete

Conclusion

The slump test is a vital on-site quality control tool that helps ensure concrete possesses the required workability and consistency for effective placement and compaction.
By adhering to ACI and ASTM standards, engineers can maintain concrete uniformity, prevent segregation, and achieve reliable performance in both structural and non-structural applications.