Compaction Factor Test

The compacting factor test is designed primarily for use in the laboratory but it can also be used in the field. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. Such dry concrete is insensitive to slump test. The compacting factor test has been developed at the Road Research Laboratory U.K. and it is claimed that it is one of the most efficient tests for measuring the workability of concrete. This test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height.

The degree of compaction, called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted. The sample of concrete to be tested is placed in the upper hopper up to the brim. The trap-door is opened so that the concrete falls into the lower hopper. Then the trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. In the case of a dry-mix, it is likely that the concrete may not fall on opening the trap-door. In such a case, a slight poking by a rod may be required to set the concrete in motion. The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades supplied with the apparatus. The outside of the cylinder is wiped clean. The concrete is filled up exactly up to the top level of the cylinder. It is weighed to the nearest 10 grams. This weight is known as ―Weight of partially compacted concrete.

The cylinder is emptied and then refilled with the concrete from the same sample in layers approximately 5 cm deep. The layers are heavily rammed or preferably vibrated so as to obtain full compaction. The top surface of the fully compacted concrete is then carefully struck off level with the top of the cylinder and weighed to the nearest 10 gm. This weight is known as Weight of fully compacted concrete. Usually, the range of compaction factor is from 0.78 to 0.95 and concrete with high fluidity has a higher compaction factor.

Theory

This test is adopted to determine workability of concrete where nominal size of aggregate does not exceed 40 mm and it is as per IS: 1199 – 1959. It is based on the definition, that workability is that property of concrete, which determines the amount of work required to produce full compaction. The test consists essentially of applying a standard amount of work to standard quantity of concrete and measuring the resulting compaction. The compaction factor is defined as the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. It shall be stated to the nearest second decimal place. The relationship between degree of workability and compaction factor are given below.

Table 1 Relationship between Degree of workability and Compaction Factor

Degree of workability	Compaction Factor
Very Low	0.75- 0.80
Low	0.80- 0.85
Medium	0.85- 0.92
High	> 0.92

Compaction factor test is more sensitive and precise than slump test and is particularly useful for concrete mixes of very low workability. Such concrete may show zero to very low slump value. Also, compaction factor (C.F.) test is able to indicate small variations in workability over a wide range. Compaction factor test proves the fact that with increase in the size of coarse aggregate, the workability will decrease. However, compaction factor test has certain limitations. When maximum size of aggregate is large as compare with mean particle size; the drop into bottom container will produce segregation and give unreliable comparison with other mixes of smaller maximum aggregate sizes. Moreover, the method of introducing concrete into mould bears no relationship to any of the more common methods of placing and compacting high concrete.

Compaction Factor Test Apparatus

Compaction factor test apparatus consists of two conical hoppers, A and B, mounted vertically above a cylindrical mould C. The upper hopper A has internal dimensions as: top diameter 250 mm; bottom diameter 125 mm and height 225 mm. The lower hopper B has internal dimensions as: top diameter 225 mm; bottom diameter 125 mm and height 225 mm. The cylinder has internal dimensions as: 150 mm diameter and 300 mm height. The distances between bottom of upper hopper and top of lower hopper, and bottom of lower hopper and top of cylinder are 200 mm in each case. The lower ends of the hoppers are fitted with quick release flap doors. The hoppers and cylinders are rigid in construction and rigidly mounted on a frame. These hoppers and cylinder are rigid and easily detachable from the frame. Other instruments used for this test consists of trowels, hand scoop (15.2 cm long), a rod of steel or other suitable material (1.6 cm diameter, 61 cm long rounded at one end) and a balance.

Fig.1 Compaction Factor Test Apparatus

Procedure

1. Prepare a concrete mix for testing workability. Consider a W/C ratio of 0.5 to 0.6 and design mix of proportion about 1:2:4 (it is presumed that a mix is designed already for the test). Weigh the quantity of cement, sand, aggregate and water correctly. Mix thoroughly. Use this freshly prepared concrete for the test.
2. Place the concrete into the upper hopper up to its brim.
3. Open the trapdoor of the upper hopper. The concrete will fall into the lower hopper.
4. Open the trapdoor of the lower hopper, so that concrete falls into the cylinder below.
5. Remove the excess concrete above the level of the top of the cylinder; clean the outside of the cylinder.
6. Weigh the concrete in the cylinder. This weight of concrete is the "weight of partially compacted concrete", (W1).
7. Empty the cylinder and refill with concrete in layers, compacting each layer well (or the same may be vibrated for full compaction). Top surface may be struck off level.
8. Find cut weight of the concrete in the fully compacted state. This weight is the “Weight of fully compacted concrete" (W2).

The degree of compaction, called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted.