The strength of the concrete doesn’t only depend upon the grade of concrete i.e. actual mix ratio of the components but also the application of fresh concrete, its workability, procedures of preparation, and its compaction during its fresh form.
The compaction moreover depends upon the workability and amount of external work done to it during placement. The theoretical values of the design mix might not be accurately used in the practical approach due to various reasons and thus the compaction of the fresh mix might not accurately be measured.
There is a description of the compaction factor test in British code BS 1881—103:1993 and was first introduced in Britain. The test is the basis for determining the general compaction factor of fresh concrete.
This test helps to know the compaction factor of the fresh concrete which means to which extent the prepared fresh concrete can be compacted in comparison to without doing any external work (like rodding, tamping, and vibration).
The compaction factor is nothing but the ratio of densities of the same fresh concrete, one obtains from the test assuming standard conditioning of placement and the other from the full compaction of concrete through vibration or external work.
The determined compaction factor is then used to estimate to which extent the volume of the fresh concrete can be compacted in formwork.
But the test is more popularly is used as a method of comparing the workability of fresh concrete with the analogy of different another workability test (workability scale) and the compaction factor as shown in fig.
Description of apparatus used in compaction factor test
The apparatus is designed in such a way considering the normal method of pouring concrete in the field. The British have adopted two cones of frustum use as hoppers for placement of concrete in the cylinder maintain one above another at definite height (as shown in Figure).
The logic for doing this might be to use the self-compacting ability of the concrete due to its self-weight. The hoppers inside the surface are well-polished in order to reduce friction and contain a hinged door at the bottom which can be open once they are filled with concrete.
The second hopper is smaller than the top hopper but contains the same amount of concrete indicating there is compaction on fall. This is made in order to decrease the influence of the filling top hopper by individual person in a different way. The standard dimensions of the apparatus are as shown in table 1 as prefer by BS 1881-103.
Besides this other apparatus that might be required are, simple shovel, mechanical mixture, trowels or screed plates, and the tamping rod with 16mm diameter 610mm long with the rounded end at the bottom of the mechanical vibrator arrangements.
Procedure for compaction test
First of all the concrete of required workability with a modified amount of water content (and also the addition of admixture), that are to be used in the field are prepared. The concrete is then poured into the first hopper of the apparatus.
The placing should be very gentle assuming no external work is done to it and no compaction occurs in the process. Then the bottom of the upper hopper is open suddenly and thus is directly collected on the second hopper.
The concrete might be overflow to some extent if the concrete is not kept in a standard way in the first hopper. Then the bottom end of the second hopper is opened and the concrete is allowed to fall into the cylinder.
The excess of concrete in the cylinder is then cut off through the trowel to the top of the cylindrical mold. The density is determined by the help of weighing mass and the known standard volume of a cylinder.
Then the process for calculating the density of fully compacted concrete mass is carried on. This can be done by two approaches, theoretical and practical. The theoretical approach involves calculations of compacted density by the absolute volume of mix ingredient and using the equation and Compaction factor formula below.
The practical approach includes the filling of the same cylinder with concrete in four layers. Each layer is compacted every time with a tamping rod or mechanical vibrator. And thus its density is again determined with the help of its measured weight and standard volume of mould.
Thus the compaction factor is determined by dividing the two densities. The series of tests are performed by altering the workability mass of the fresh concrete and the data can be listed(as in fig 2).
This is required because in actual work the workability is very difficult to maintain as it depends on each individual and different seen or unseen factors. Thus, with help of such data the compaction needed can be controlled.
If W be weight of the cylinder before fill.
Then, the Recorded value of the weight of the cylinder after filling it with the compaction test apparatus. =W1
And recorded value of weight of same cylinder after full compaction = W2
Due to the use of a standard size cylinder, we neglect the volume of the cylinder and thus find the compaction factor with help of weighing.
Compaction factor formula
Therefore, Compaction factor = (W1-W) /(W2-W)
The results of calculations usually lies within range of 0.7-0.9. And with help of this value and reference of other test there workability is estimated as in fig 2.
Limitations of the compaction factor test
- The test shows sensitivities to the workability which means, the compaction factor value decrease as workability increases.
- The dry mixes can adhere to the hoppers and thus poking with a rod is necessary to remove them.
- The compaction factor doesn’t show any relationship between the richness of the mix. But in a practical approach, a leaner mix needs more external work than a richer one.
- Mixes with the same value of compaction factor might need different amounts of external work. So, the work needed to be done doesn’t only depend on the compaction factor.
- The concrete should be carefully loaded in the top hopper in a standard way without external work over it.
- The hoppers surface should be smooth. Sometimes the lubricant also uses so that the concrete doesn’t stick on the surface.
- No external work or compaction is allowed in the cylinder and should be carefully and slowly uplift to measure its weight.
- The concrete should contain a definite amount of water content and workability that are to be used in the actual field.
A compatibility test is developed by BS EN 12350-4 2000, which is a more simple process to determine the compaction factor value also known as the degree of compatibility here.
The method is based on finding the reduction in volume after proper vibration and arrangements of loose particles of concrete. Thus the degree of compatibility is determined by measuring the ratio of the height of the concrete before compaction to height after compaction.
SELF COMPACTING CONCRETE
These are the concrete with an extremely high value of the compaction factor, which have the ability to self compact due to its own weight and gravity when placing. The concrete doesn’t need extra work to be done for compaction.
Moreover such concrete has excellent flowability and high resistant to segregation.
This kind of concrete can be obtained by improving the admixture and components to prepare concrete, like the use of superplasticizers, viscosity controlling agent, good texture and shaped aggregate, more use of extremely finer aggregate and less coarse aggregate, water content maintain at 0.4, etc.
I hope this article on “Compaction factor formula of concrete” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha
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