Usefulness of Slump Test

Slump test can be employed either in laboratory or at site of work for measuring consistency of concrete. This test is used conveniently 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.

Figure: Typical Mould for Slump Test

If concrete slumps evenly it is called true slump. If one half of the cone slides down, it is called shear slump. In case of 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. If concrete breaks and spread in all directions, it is called as collapse slump.

Figure: Pattern of Slump

Procedure of slump cone test: 

1. Clean the mould from inside and apply oil to it. 

2. Place the mould on smooth horizontal, rigid and non-                       absorbent surface or the center of metallic tray. 

3. Fill the mould with the concrete to be tested in four layers,              tamping each layer 25 times with the tamping rod, taking care       that the strokes are evenly distributed over the c/s. 

4. Remove the mould by one smooth continuous vertical motion.

5. The concrete subsides and this subsidence is called “slump.”          Measure the slump in mm by using a metric scale from top of         cone. 

6. Based on measured slump, the degree of workability is                    designated as follows

Sr. No.	Degree of Workability	Slump (mm)
01	Very Low	----- (Compacting factor is suitable)
02	Low	25-75
03	Medium	50-100
04	High	100-150
05	Very High	----- (Flow table test is suitable

This test gives fairly good consistency results for a plastic mix but for a stiff mix it is not sensitive. In other words, slump test is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability.  

Despite many limitations, the slump test is very useful on site to check day to day or hour to hour variation in quality of mix. An increase in slump, may mean for instance that the moisture content of the aggregate has sudenly has been increased or there has been sudden change in the grading of aggregate. This test gives warning to correct the causes for change of slump value.

As per IS 1199, for different placing conditions of concrete suggested ranges of workability of concrete (slump) are as follows:

Placing Conditions	Degree of Workability	Slump (mm)
Blinding concrete; Shallow sections; Pavements using pavers	Very Low	----- (Compacting factor is suitable)
Mass concrete; Lightly reinforced sections in slabs, beams, walls, columns; Floors; Hand placed pavements; Canal lining; Strip footings	Low	25-75
Heavily reinforced sections in slabs, beams, walls, columns; Slipform work; Pumped concrete	Medium	50-100
Trench fill; In-situ piling; Tremie concrete	High	100-150
	Very High	----- (Flow table test is suitable