 WORKABILITY
The workability of concrete can be
measured on-site by
the 'Slump Test'. This test involves taking a sample of the mix and
filling a cone with four equal layers of concrete. Each layer is
compacted with 25 strokes of the rod, each stroke penetrating the
layer below. This will ensure that even compaction of the material
is achieved.
When the cone is lifted off the
material should 'slump' to provide a measurement of the difference
between the height of the cone and the slumped concrete. The greater
the slump the more workable the concrete.
If the concrete collapses totally
this shows that the mix is too wet and too fluid to compact, it will
cause segregation of the materials with the larger aggregate sinking
to the bottom during compaction.
If the material collapses on one side
only is shows that the material is too dry and will be unworkable.
The optimum slump is when the
material stays together and just bulges from the centre. This then
shows that the material is workable.
 CUBE
TEST
The cube test is used to measure the
strength of concrete. The test involves taking a sample of the mix
and curing is in laboratory conditions to ensure full testing
strength is achieved.
The concrete should be thoroughly
mixed before placing in the oiled cube. The mix is compacted in two
layers, the first being about two thirds full, with at least 35
strokes of the tamping rod on each layer. The cube is then cured at
20°C in a controlled environment such as a curing tank. When a
prescribed time has elapsed the cubes are then subjected to the
'crushing test'
 Crush
Test
Once the concrete has been allowed to
cure it is placed in a crusher to measure the point at which the
cube fractures. This is measured in Newton's per square millimetres
(N/mm²). The cubes are crushed at 7 days and 28 days and the
figures compared to the strength requirements of the concrete design
mix.
The alternative to cubes are to use
cylinders of at least 150mm diameter in the same way.
 Compacting
Factor Test
This test also measures the
workability of concrete in a more precise way than that of the slump
test. It measures the weight of uncompacted concrete and compared it
with the weight of partially compacted concrete, sometimes known as
the 'drop test'.
The top cone is filled with well
mixed loose concrete and weighed. It is then allowed to drop to the
lower cone and then to the bottom cylinder. The bottom cylinder is
smoothed off level and any surplus concrete on the outside wiped
away. The cylinder is then weighed.
The difference between the weight of
the concrete placed in the top cone and that of the cylinder
provides a measure of workability. The higher the weight of the
cylinder to the cone the more workable the concrete, but the
difference should not be more than 1.
Cone weight = W1
Cylinder Weight = W2
Compacting factor = W1÷W2
The higher the value of the
calculation the more workable the concrete. (A maximum of 1 should
be achieved).
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