Workability of Concrete: Slump Test and Compaction Factor Test
Workability is the ease with which fresh concrete can be mixed, transported, placed and fully compacted without segregation or bleeding. Concrete that is too stiff leaves honeycombs around reinforcement; concrete that is too wet loses strength and segregates. The slump test is the simplest and most widely used site check for workability in India, and every civil engineering student performs it in the concrete lab. This article explains the factors that affect workability, the full slump test procedure, how to interpret the result, and the compaction factor test for low-workability mixes.
Factors Affecting Workability
| Factor | Effect on workability |
|---|---|
| Water content | More water increases workability but reduces strength |
| Aggregate shape | Rounded aggregate is more workable than angular crushed stone |
| Aggregate size | Larger maximum size needs less water for the same slump |
| Grading of sand | Very fine sand needs more water; well graded sand improves flow |
| Admixtures | Plasticisers and superplasticisers raise slump without extra water |
| Time and temperature | Slump drops with time after mixing, faster in hot weather |
The key point is that workability should be achieved by good grading and admixtures, not by adding water. A water-cement ratio increase from 0.45 to 0.55 can reduce 28-day strength significantly, even though the concrete looks easier to place.
Slump Test Apparatus
The slump test is described in IS 1199, the Indian Standard on methods of sampling and analysis of fresh concrete. The apparatus is simple and portable.
| Item | Specification |
|---|---|
| Slump cone (frustum) | Bottom diameter 200 mm, top diameter 100 mm, height 300 mm |
| Tamping rod | 16 mm diameter steel rod, 600 mm long, rounded at one end |
| Base plate | Smooth, rigid, non-absorbent, levelled |
| Scale | Steel rule or measuring tape marked in mm |
| Scoop and trowel | For filling and striking off |
Slump Test Procedure
- Clean the inside of the cone and the base plate and moisten them lightly. Place the cone on the plate with the narrow end up.
- Stand on the foot pieces to hold the cone firmly in place.
- Fill the cone in four layers, each roughly one quarter of the height.
- Tamp each layer 25 times with the rounded end of the rod, spreading strokes evenly over the area. For the second and later layers, the rod should just penetrate into the layer below.
- After the top layer, strike off the surface level with the top of the cone using the trowel. Clean any spilled concrete from around the base.
- Lift the cone slowly and vertically, without twisting, within about 5 to 10 seconds.
- Place the empty cone upside-down beside the concrete, lay the rod across its top, and measure the drop from the underside of the rod to the highest point of the slumped concrete.
- Complete the whole test within about 2.5 minutes of starting to fill the cone.
rod across inverted cone ______________________________ | | ^ | cone | slumped | slump (mm) | | concrete _____v____ | | / \ |______|__________/_____________\____ base plate Slump = 300 mm - height of slumped concrete
Reading the Result
The shape of the slumped concrete matters as much as the number.
- True slump: the concrete subsides evenly and keeps its shape. Only a true slump is recorded as a valid result.
- Shear slump: one side shears off and slides down. It indicates a lack of cohesion; repeat the test with a fresh sample. If shear persists, the mix is harsh.
- Collapse slump: the concrete flattens completely. The mix is too wet or is a very high workability mix; the slump test is not suitable and a flow test should be used instead.
IS 456 gives guidance on the degree of workability for different placing conditions. Typical slump values are summarised below.
| Placing condition | Degree of workability | Slump (mm) |
|---|---|---|
| Mass concrete, lightly reinforced slabs, beams, walls, strip footings | Low | 25 to 75 |
| Heavily reinforced slabs, beams, walls, columns | Medium | 50 to 100 |
| Pumped concrete, slipform work | Medium | 75 to 100 |
| Trench fill, in-situ piling | High | 100 to 150 |
Worked example
For an RCC beam with congested bars, the specified slump is 75 mm. On site, the slumped concrete height measured from the base is 215 mm, and the shape is a true slump.
Slump = 300 - 215 = 85 mm Specified range for heavily reinforced beam: 50 to 100 mm Result: acceptable
If the slump were 160 mm, the concrete should be rejected or investigated. Adding cement to a truck that arrives too wet is not a site fix; the mix must be corrected at the plant.
Compaction Factor Test
For stiff mixes with low slump, the slump test is not sensitive enough. The compaction factor test uses two hoppers placed one above the other and a cylinder at the bottom. Concrete is dropped from the upper hopper to the lower hopper and then into the cylinder, which partially compacts it under gravity. The cylinder is weighed, then refilled with fully compacted concrete and weighed again.
Compaction factor = W1 / W2 W1 = mass of partially compacted concrete W2 = mass of fully compacted concrete Example: W1 = 11.40 kg, W2 = 12.30 kg CF = 11.40 / 12.30 = 0.93
A compaction factor closer to 1.0 means more workable concrete. Values around 0.8 indicate a very stiff mix suited to vibrated pavements, while values above 0.9 indicate medium to high workability. The Vee-Bee consistometer test is another option for very dry mixes, measured in seconds.
Common Mistakes
- Lifting the cone with a twist or jerk, which causes shear slump.
- Testing concrete from the first or last discharge of a transit mixer; take the sample from the middle of the discharge.
- Measuring to the edge rather than the highest point of the slumped concrete.
- Using slump to compare mixes with different admixtures without also checking cube strength.
Summary
Workability measures how easily concrete can be placed and compacted. The slump test, using a 300 mm cone filled in four layers with 25 strokes each, gives a quick site reading in millimetres, and only a true slump is valid. The compaction factor test suits stiff mixes. Match the slump to the placing condition and achieve it with grading and admixtures, never with extra water.