WORKABILITY TESTS AT SITE & RECOMMENDED VALUES
What is workability of concrete? Which test is generally performed at site for its determination? And what are its recommended values for different purposes?
Workability of concrete describes the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity.
Workability is important because, if
i. the concrete mixture is too wet, coarse aggregates settle at the bottom of concrete mass and as a result concrete becomes non-uniform composition,
ii. the concrete mixture is too dry, it will be difficult to handle and place it in position
Workability of concrete mixture is measured by:
a) Vee-bee consistometer test
c) Slump test
The first two tests are laboratory tests while the third test is the field test.
This test is carried out with a mould called slump cone whose top diameter is 10cm, bottom diameter is 20 cm and height is 30 cm. the test may be performed in the following steps:
1. Place the slump mould on a smooth flat and non-absorbent surface.
2. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
3. Place the mixed concrete in the mould to about one-fourth of its height.
4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.
5. Place the concrete in the mould about half of its height and compact it again.
6. Place the concrete upto its three fourth height and then upto its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second subsequent layers, the tamping rod should penetrate into underlying layers.
7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top.
8. Remove the mould immediately, ensuring its movement in vertical direction.
9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.
Figure: Measurement of slump
Suitability of Slump Test:
The slump test is suitable only for the concrete of high or medium workability.
Recommended Values of Slump for Various Purposes:
Types of concrete
Concrete for road construction
20 to 40 mm
Concrete for tops of curbs, parapets, piers, slabs and wall
40 to 50 mm
Concrete for canal lining
70 to 80 mm
Normal RCC work
80 to 150 mm
20 to 50 mm
Concrete to be vibrated
10 to 25 mm