Determine the Slump of Concrete by Slump Test

Importance:

This test is performed to check the consistency of freshly made concrete. Consistency is a term very closely related to workability. It is a term which describes the state of fresh concrete. It refers to the ease with which the concrete flows. It is used to indicate the degree of wetness. Workability of concrete is mainly affected by consistency i.e. wetter mixes will be more workable than drier mixes, but concrete of the same consistency may vary in workability.

It can also be defined as the relative plasticity of freshly mixed concrete as indicative of its workability. So concrete may have the following types of consistency:

1)   Plastic Consistency:

When it can be shaped into a ball between the palms of hands and adheres to the skin.

2)   Semi-Fluid Consistency:

This cannot be rolled into a ball but spreads out without affecting the cohesion of the constituents so that segregation doesn’t take place.

3)   Fluid Consistency:

Which spreads out rapidly and segregation takes place.

Thus different degree of workability is required at different occasions. If the structure is RCC and the steel bars too much close to each other then high workability is required i.e. fluid consistency. While where the inter bars space is larger then concrete of semi-fluid or plastic consistency is required.

Apparatus and Samples:

The mould for slump test is a frustum of a cone:

a) 12 inch in height.

b) 8 inch diameter at the base.

c) 4 inch diameter at the top.

The cone is made up of metal and is provided with handles on two sides of the cone. For firm holding of the cone during the test it is also provided with foot-rests brazed to the mould near the bottom.

The cone is placed on a plane metallic bottom surface. Before using of cone in the test it is thoroughly cleaned inside and also some oil is applied so that during the lifting of cone no friction is provided.

  • Coarse aggregate. (crush).
  • Fine aggregate. (sand).
  • Cementing material. (cement).
  • Clean water.
  • Slump test apparatus.
  • 5/8” tampering rod.

Procedure:

  • First of all we have to prepare a mixture of concrete having ratio of:  1:2:4. That is one part of cement, two parts sand, and four parts of crush.
  • Determine the volume of the cone in cubic feet. It is determined by taking the mean diameter of the cone and with it finding the area of the cross section of cone. Then multiplying it with the height of the cone which is 1 foot, will give the volume of the cone.
  • This is the volume of concrete which have to be prepared so that the cone is fully compacted.
  • Than determine the total weight of the concrete, as the specific weight of concrete is 150 lb/ft3.
  • Then determine the weight of cement by multiplying the weight of concrete by 1/7, as cement is one part out of total 7 parts.
  • Then determine the weight of sand by multiplying the weight of concrete by 2/7, as sand is two parts out of total 7 parts.
  • Then determine the weight of crush by multiplying the weight of concrete by 4/7, as crush is four parts out of total 7 parts.
  • After calculation of dry ingredients, calculate the amount of water as is given in the ASTM Standard water-cement ratio. For 1:2:4 mixture the water-cement ratio is 0.6, so for weight of water to be added, multiply this ratio with weight of cement.
  • After calculation of weights make a homogeneous mixture of dry ingredients, and then add water carefully to make a paste.
  • Then take the slump-test apparatus and clean it from inside also apply oil to it and to the bottom surface. Then place it on the smooth metallic surface, and fix it firmly.
  • Then put one third of the concrete in the cone and press it with the help of a 5/8 inch, round-ended, tampering rod. It should be tampered 25 times.
  • Then add the second one-third potion of concrete, and also tamper it 25 times with the help of tampering rod.
  • At last add the remaining one-third portion, and also tamper it 25 times.
  • For the upper surface to be smooth work it with float, so that during measurement of slump it is easy to take correct readings.
  • Immediately after filling the cone is slowly lifted, and the unsupported concrete will now slump-hence the name of the test. “The decrease in the height of the centre of the slumped concrete is called slump”.

The slump test gives the following three results:

a)    TRUE-SLUMP: if the concrete subsides evenly then it is called true-slump, and is aimed to be calculated.

b)   SHEAR-SLUMP: If one half of the cone slides down , it is called shear-slump and is difficult to measure. It occurs in harsh mixes (mixes deficient in fine aggregate).

c)    COLLAPSE-SLUMP: If the concrete slides down as soon as the mould is removed, it is known as collapse-slump. It occurs in very wet mix.

 Calculations and Observations:

Ratio of the concrete = 1:2:4

Sum of the ratio of concrete =1+2+4 = 7

Volume of the  Cone:

Mean diameter of the cone = Dm = 4+8/2 = 6 inch

Area of the cone =Ac = π*( Dm)2 / 4

=  π/4 *(6/12)2

=  0.1963 ft2

Volume of the cone = Vc = 0.1963 ft2 * 1 ft

= 0.1963 ft3

Specific weight of concrete = γcon = 150 lb/ ft3

Weight of the concrete = Wcon = γcon * Vc

= 150 * 0.1963

= 29.44  lb

 Weights of Dry Materials:

Weight of cement = Wcem = 1/7 * Wcon

=1/7 * 29.4 = 4.2 lb ≈ 4 lb

Weight of sand =  Wsand  = 2/7 * Wcon

= 2/7 * 29.4 = 8.44 lb ≈ 8 lb

Weight of crush = Wcrush = 4/7 * Wcon

= 4/7 *29.4 = 16.82 lb ≈ 16 lb

 Weight of Water:

Ratio of concrete = 1:2:4

ASTM-Standard water-cement ratio = w/c = 0.6

Weight of water = Wwater = 0.6 * Wcem

= 0.6 * 4

= 2.4 lb

Standard Values of Slump:

 

Description

Slump in inches

Concrete for road construction

¾ to 1.5

Slabs

1 to 2

Normal RCC sections, e.g. slabs, beams, columns, walls etc.

2 to 6

Thin RCC structures

4 to 7

Vibrated concrete

½ to 1

Mass concrete

1 to 3

 

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