determination of the workability of fresh concrete (slump test and making test cubes) practical

1. In the determination of the workability of fresh concrete (slump test and making
test cubes) practical, the first objective of the practical is to determine the amount of
collapse, or slump, as an indication of the workability of different type of fresh concrete
mixture. In the experiment, total six experiments are done with different of concrete
mixtures which is 1:3:6, 1:2:4, 1:1
1
2
:3 using
3
4
inch aggregate with water/cement ratio (w/c)
0.6 and 0.8 for the mixes.
Slump test is suitable for the site used as it determines the workability of fresh
concrete in the specific batch. In the experiments, six different results are obtained. The
slump measured is recorded using a truncated cone with a height of 300mm using the
formula,
The slump = the height of the cone – The height of the concrete cone after the
slump.
Slump Shape is divided into three types which are true, shear and collapse.
When a ratio of concrete mixture of 1:3:6 with w/c ratio of 0.6, the height of the
concrete cone drops from 300mm to 299mm, thus a 1mm slump is formed. A true slump
shape is obtained. Meanwhile, with a w/c ratio f 0.8, a 20mm slump is obtained and a
shear slump shape is formed. In the concrete mixture of 1:2:4 with a w/c ratio of 0.6, a
90mm slump shape is formed with a shear shape and a collapse of 95mm slump is
formed in w/c ratio of 0.8. In the fifth concrete mixture 1:1
1
2
:3, w/c ratio of 0.6 gets a
80mm slump in collapse form and a w/c ratio of 0.8 gets a 80mm slump with a shear
shape is formed.
The slumps of 1:2:4 with 0.6 and 0.8 ratio water is lower compared to the slumps
of 1:1
1
2
:3 with 0.6 and 0.8 ratio water as some errors occurred. One of the possible
reasons for errors to occur is due to the leaking of water from the from the 1:2:4 concrete
mixture slump cone causing the slump of 1:1
1
2
:3 is higher as the water is decrease due to
the leaking. The slump shape of the 1:1
1
2
:3 concrete mixture is also affected due to the

2. water leaking as the mixture become dry and it won’t collapse. Thus, error occurs due to
the water leaking from the cone.
Another possible reason is that of the insufficient compression on the concrete
mixture. This causes the mixing of concrete mixture to be less compact. The concrete
mixture is more workable thus not strong. Hence, it tends to collapse easily, forming
higher slumps. In this experiment, 25 strokes using the tamping rod are needed to
compress the concrete mixture. An error will occur is probably due to the inadequate of
strokes or the insufficient force.
Limitations of slump test are the compacting factor which will affect the
workability of concrete mixtures and the slump formed. One of the limitation is it is not
suitable for concrete with an aggregate size >4cm. Thus, sieve of stone has to be done to
eliminate the bigger aggregate. Next, a wet mix will cause a higher slump to occur as it is
wet and a dry mixture is not suitable as slump does not occur. It is difficult to decide the
exact value for the shapes of slump. Temperature will also limit the slump test, and the
weather is humid, the sand and aggregates will be wet, thus affecting the slump.
A collapse slump will generally mean that the mixture is too wet or that it is a
high workability mix. The ideal value of the slump for a very dry mix, a slump will be 0-
25mm, low workability mixes. The concrete mixture having slump 50-90mm is a
medium value of slump while slump value>100 is a wet concrete mixtures. A collapse
slump is an indication of a wet mixture.
Super-plasticizer effects the slump of concrete as the value of slump can be
increased by the addition of chemical admixtures like mid-range or high-range water
reducing agents (super-plasticizers) without changing the water/ cement ratio. Super-
plasticizer enables the cement to become a more effective lubricant, thus increasing the
slump without adding additional water. Slump can be seen as either a water slump or as a
plasticized slump. If the slump is determined only by the water content, it is said that the
slump is water slump, if the slump is determined by water content and the effects of a
chemical admixture, it is said to be a plasticized slump.

3. Among the six mixtures, the concrete mix of 1:3:6 with 0.6 and 0.8 ratio of
water/cement and 1:2:4 with 0.6 and 0.8 ratio of water/cement will produce the concrete
with the highest strength. The concrete mixture of 1:3:6 with 0.6 ratio of water will has
the highest strength among all. It is because the compressive strength of the hardened
concrete as slump test is affected by the water/cement ratio. When a slump is big then
water/ cement ratio is big and the compressive strength of concrete is slow. A lower
water-cement ratio leads to higher strength. The amount of water is enough to mix with
all the cement causing the cement has a dramatic impact on its strength. Aggregates
which contained within concrete consist of small rocks. The aggregates used in concrete
mix affect its strength based on how they interact with the water and cement mixture.
Larger aggregates increase the cost effectiveness of concrete without decreasing its
strength since they have a relatively smaller surface area, requiring less concrete to coat.
Cement is the glue that binds concrete together. Increasing the proportion of cement in
your concrete mixture will increase its strength but it will also make the mixture stiffer
and more difficult to work with.
The mixtures are then transferred to an oiled cube mould. The mould must be
oiled in order to make it easier when we remove the cube. Precautions should be taken to
ensure standard cube specimens in the laboratory. While finishing off the surface of the
concrete, if the mould is too full, the excess concrete should not be removed by scraping
off the top surface as this takes off the cement paste that has come to the top and leaves
the concrete short of cement. The correct way is to use a corner of the trowel and gig our
a fair sample of the concrete as a whole, then finish the surface by trowelling. Once a
specimen has been compacted, it should not be left standing on the same bench as another
specimen that is being compacted.

4. Graphs
a. Graph of strength against w/c ratio.
b. Graph of workability against water.
c. Graph of Cost effective against cement.
Conclusion
The amount of collapse, or slump indicates the workability of different type of fresh
concrete mixtures. The higher the amount of slump, the higher the workability of
concrete mixtures. The amount of slump is affected by a few factors such as w/c ratio and
compacting factor.