Measurement of Workability - Slump Test

A concrete is said to be workable if it can be easily mixed, placed, compacted and finished. A workable concrete should not show any segregation or bleeding. Segregation is said to occur when coarse aggregate tries to separate out from the finer material and a concentration of coarse aggregate at one place occurs. This results in large voids, less durability and strength. Bleeding of concrete is said to occur when excess water comes up at the surface of concrete. This causes small pores through the mass of concrete and is undesirable.

There is no universally accepted test method that can directly measure the workability of concrete. The difficulty in measuring the mechanical work defined in terms of workability, the composite nature of the fresh concrete and the dependence of the workability on the type and method of construction makes it impossible to develop a well-accepted test method to measure workability. The most widely used test, which mainly measures the consistency of concrete, is called the slump test. For the same purpose, the second test in order of importance is the Vebe test, which is more meaningful for mixtures with low consistency. The third test is the compacting factor test, which attempts to evaluate the compactability characteristic of a concrete mixture. The fourth test method is the ball penetration test that is related to the mechanical work.

Slump Test

Unsupported fresh concrete flows to the sides and a sinking in height takes place. This vertical settlement is known as slump. The slump is a measure indicating the consistency or workability of cement concrete. It gives an idea of water content needed for concrete to be used for different works. To measure the slump value, the fresh concrete is filled into a mould of specified shape and dimensions and the settlement or slump is measured when supporting mould is removed. The slump increases as water content is increased. For different works different slump values have been recommended.

Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work where nominal maximum size of aggregates does not exceed 40 mm. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability and it always representative of the placability of the concrete. It is conveniently used as a control test and gives an indication of the uniformity of concrete from batch to batch. Repeated batches of the same mix, brought to the same slump, will have the same water content and water cement ratio, provided the weights of aggregate, cement and admixtures are uniform and aggregate grading is within acceptable limits. Additional information on workability and quality of concrete can be obtained by observing the manner in which concrete slumps. Quality of concrete can also be further assessed by giving a few tappings or blows by tamping rod to the base plate. The deformation shows the characteristics of concrete with respect to tendency for segregation.

Tools and Apparatus Used for Slump Test (Equipments)

• Standard slump cone (100 mm top diameter x 200 mm bottom diameter x 300 mm high)
• Small scoop
• Bullet-nosed rod (600 mm long x 16 mm diameter)
• Rule
• Slump plate (500 mm x 500 mm)

(The thickness of the metallic sheet for the mould should not be thinner than 1.6 mm.)

Fig. 1 Slump Testing Equipment

Fig. 2 Slump Cone

Procedure

• Clean the internal surface of the mould thoroughly and place it on a smooth horizontal, rigid and non-absorbent surface, such as of a metal plate.
• Consider a W-C ratio of 0.5 to 0.6 and design mix of proportion about 1:2:4 (It is presumed that a mix is designed already for the test). Weigh the quantity of cement, sand, aggregate and water correctly. Mix thoroughly. Use this freshly prepared concrete for the test.
• Fill the mould to about one fourth of its height with concrete. While filling, hold the mould firmly in position
• Tamp the layer with the round end of the tamping rod with 25 strokes disturbing the strokes uniformly over the cross section.
• Fill the mould further in 3 layers each time by 1/4^th height and tamping evenly each layer as above. After completion of rodding of the topmost layer strike of the concrete with a trowel or tamping bar, level with the top of mould.
• Lift the mould vertically slowly and remove it.
• The concrete will subside. Measure the height of the specimen of concrete after subsidence.
• The slump of concrete is the subsidence, i.e. difference in original height and height up to the topmost point of the subsided concrete in millimeters.

Fig. 3 Slump Test

The pattern of slump indicates the characteristics of concrete in addition to the slump value. If the concrete slumps evenly it is called true slump. If one half of the cone slides down, it is called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence. Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation. It is seen that the slump test gives fairly good consistent results for a plastic mix. This test is not sensitive for a stiff mix. In case of dry mix, no variation can be detected between mixes of different workability. In the case of rich mixes, the value is often satisfactory, their slump being sensitive to variations in workability. IS 456 - 2000 suggests that in the “very low” category of workability where strict control is necessary, for example, Pavement Quality Concrete (PQC) measurement of workability by determination of compacting factor will be more appropriate than slump and a value of 0.75 to 0.80 compacting factor is suggested.

Fig. 4 Types of Slump 

The above IS also suggests that in the “very high” category of workability, measurement of workability by determination of “flow” by flow test will be more appropriate. However, in a lean mix with a tendency of harshness a true slump can easily change to shear slump. In such case, the tests should be repeated. Despite many limitations, the slump test is very useful on site to check day-to-day or hour to- hour variation in the quality of mix. An increase in slump, may mean for instance that the moisture content of the aggregate has suddenly increased or there has been sudden change in the grading of aggregate. The slump test gives warning to correct the causes for change of slump value. Table 1 shows the nominal slump value for different degrees of workability.

Table 1 Nominal Slump Value for Different Degrees of Workability

Sl.No.	Slump Value (in mm)	Degree of Workability
1	0 – 25	Very Low
2	25 - 50	Low
3	50 – 100	Medium
4	100 - 175	High

The Bureau of Indian standards, in the past, generally adopted compacting factor test values for denoting workability. Even in the IS:10262 of 1982 dealing with recommended guide line for Concrete Mix Design, adopted compacting factor for denoting workability. But now in the revision of IS:456 – 2000, the code has reverted back to slump value to denote the workability rather than compacting factor. It shows that slump test has more practical utility than the other tests for workability.

Although, slump test is popular due to the simplicity of apparatus used and simple procedure, the simplicity is also often allowing a wide variability and many time it could not provide true guide to workability. For example, a harsh mix cannot be said to have same workability as one with a large proportion of sand even though they may have the same slump.

Factors Affecting Slump Test

• Material properties like chemical composition, fineness, particle size distribution, moisture content and temperature of cementitious materials
• Size, texture, combined grading, cleanliness and moisture content of the aggregates
• Chemical admixtures dosage, type, combination, interaction, sequence of addition and its effectiveness,
• Air content of concrete
• Concrete batching, mixing and transporting methods and equipment
• Temperature of the concrete
• Sampling of concrete, slump testing technique and the condition of test equipment
• The amount of free water in the concrete
• Time since mixing of concrete at the time of testing