Slump test on concrete : theory and type

 

 

There are two phases of concrete we can name them as fresh concrete and hard concrete. Fresh concrete is the phase when concrete is in the state of having consistency and fluidity through which we can mold, place and compact. Hard concrete is after addition of water with passage of time concrete gains strength and becomes hard, losses its consistency known as hard concrete.

The fresh concrete has an important property known as workability. Workability means ability to work with; it is a measure of amount of effort required to place, compact and finish concrete. The less the effort required the more workable concrete is said to be and the more the effort required the less workable concrete is said to be.

One of the well-known test to measure the workability of concrete is Slump test. Slump test is well known for its simplicity of apparatus and simple procedure.

 

Apparatus:

16 mm diameter and 600 mm long temping rod

Hollow metallic cone having size as given below;

Top diameter: 100 mm

Bottom diameter: 200 mm

Height of cone:  300 mm

Base plate

 

Slump apparatus

Procedure:

The cone is being placed on a level & non water absorbent base plate, sample of concrete to be tested is firmly filled in.

The concrete shall be filled in four layers approximately one quarter in each layer.

Every layer shall be tamped with temping rod of 16 mm diameter for 25 strokes. The strokes shall be evenly distributed over the area of cone.

As the fourth layer is tamped, the top surface is leveled with rolling of temping rod or trowel.

Slump cone is filling in four layers with temping 25 times each

Now raise the cone carefully in vertical manner; the fall of concrete made slump is being measured with help of tape or ruler from the top of slump cone to the top of concrete surface.

The test is done and value is recorded in millimetres known as slump value.

Slump value

 

Types of slump

Zero slump – Very stiff mix of concrete, very low to almost no workability, susceptible to honeycombing.

True slump – Ideal mix, retains its surface, can be measured directly.

Shear slump – One side of slump falls down, lack of proper cohesion, perform test again.

Collapsed slump – Too wet or very high workability, too much water content susceptible to bleeding and segregation.

Type of slumps

 

Key identifications from slump value.

Usually only true slump can be measured directly and shear slump and collapsed slump cannot be measured since there is no flat surface is available to directly measure from. Thus if shear slump happens, the test is repeated one time and if results in shear slump also then it shall be noted as shear slump.

 

Recommended values of slump and workability       

 

Workability – Very low; Slump value – 0 – 25

Shallow sections, Rigid pavements.

 

Workability – Low; Slump value – 25-50

Mass concrete, pavements, light reinforced sections.

 

Workability – Medium; Slump value – 50 – 100

Heavily reinforced sections, slipformwork, pumpable concrete.

 

Workability – High; Slump value – 100 -150

Tremie concrete

 

Workability – Very high; Slump value – 150 <