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
*The concrete mixture is too wet,
coarse aggregates settle at the bottom of concrete mass and as a result
concrete becomes non-uniform composition,
Workability
of concrete mixture is measured by:
- Vee-bee consistometer test
- Compaction factor test
- Slump test
The
first two tests are laboratory tests while the third test is the field test.
Slump
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:
- Place the slump mould on a smooth flat and non-absorbent surface.
- Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
- Place the mixed concrete in the mould to about one-fourth of its height.
- Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.
- Place the concrete in the mould about half of its height and compact it again.
- 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.
- Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top.
- Remove the mould immediately, ensuring its movement in vertical direction.
- When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.
SL NO
|
TYPES OF CONCRETE
|
SLUMP
|
1
|
Concrete
for road construction
|
20
to 40 mm
|
2
|
Concrete
for tops of curbs, parapets, piers, slabs and wall
|
40
to 50 mm
|
3
|
Concrete
for canal lining
|
70
to 80 mm
|
4
|
Normal
RCC work
|
80
to 150 mm
|
5
|
Mass
concrete
|
20
to 50 mm
|
6
|
Concrete
to be vibrated
|
10
to 25 mm
|