Testing of Fresh Concrete

The following tests are commonly employed to measure workability of fresh concrete:

1. Slump Test
2. Compaction factor Test
3. Flow Test
4. Kelly Ball Test
5. Vee Bee Consistometer Test

Slump Test

It is the most commonly used method of measuring consistency of concrete. This test can be conducted in the field or in a laboratory. However, this test is not suitable for very wet or very dry concrete.

The apparatus for conducting the slump test consists of a metallic mould in the form of a frustum of a cone with a 20 cm diameter at the bottom, 10 cm diameter at the top and 30 cm in height. A steep tamping rod of 16 mm diameter, 0.6 m long with a bullet end is used for tamping.

The internal surface of the mould is thoroughly cleaned and placed on a smooth non-absorbent horizontal surface. The mould is filled in four layers of equal height. Each layer is compacted by giving 25 blows with the tamping rod uniformly. After filling the mould and rodded, the excess concrete is shaken off and levelled.

The mould is lifted upwards from the concrete immediately by raising it slowly. This allows the concrete to subside. This subsidence is referred to as the slump of concrete. The difference in height of the mould and that of the subsided concrete is measured and reported in mm, which is considered to be the slump up of concrete.

The pattern of slump also represents the characteristics of concrete (Fig. 8.2). If the slump of the concrete is even, it is called a true slump. If one-half of the cone slides down, it is called a shear slump. In this case the average value of the slump is considered. The shear slump also indicates that the concrete is not cohesive and reflects segregation. Concrete mixes are classified based on the slump as given in Table 8.12.

Table 8.12 Slump and nature of concrete

Figure 8.2 Types of slumps

Slumps recommended for various works of concrete construction are presented in Table 8.13.

Table 8.13 Recommended slump for concrete works

The slump test can be conducted both in the laboratory and at the work site. The slump test results grant the facility to easily detect the difference in water content of successive batches of the identical mix.

Compacting Factor Test

This is a more refined test than the slump test. The test measures the degree of compaction obtained by using certain energy in overcoming the internal friction of the concrete. This property is a measure of workability.

The test apparatus consists of two conical hoppers with bottom doors and a separate cylinder kept at the bottom. The concrete is filled in the top hopper fully without compaction and released successively through the two hoppers and into the bottom cylinder (Fig. 8.3). After striking off the level in the cylinder the weight of the concrete (W₁) in the cylinder is determined. The same cylinder is filled with the same batch of concrete and compacted to get the maximum weight (W₂). The ratio of the observed weight, W₁, to the theoretical weight, W₂, i.e., W₁/W₂ is the compacting factor. The workability, compacting factor and the corresponding slump are given in Table 8.14.

Figure 8.3 Compacting factor test apparatus (dimensions shown are in mm)

Table 8.14 Workability and compacting factor

The compacting factor test measures the quality of concrete, which relates very close to the workability. This test clearly depicts the workability of concrete.

Flow Test

This test gives an indication of the quality of concrete with respect to consistency, cohesiveness and non-segregation. In this test a mass of concrete is subjected to floating, and the flow or spread of the concrete is measured. The flow is related to workability.

The test apparatus consists of a flow table of 76 mm diameter on which concentric circles are marked (Fig. 8.4). A mould similar to that used in the Slump Test with a base diameter of 25 cm, an upper diameter of 17 cm and a height of 12 cm is used. The mould is kept on a clean table, and concrete is filled in two layers with each layer being rodded 25 times with a tamping rod of 1.6 cm diameter and 61 cm long with a rounded end. The excess concrete at the top of the mould is levelled. The mould is lifted vertically upwards completely. The concrete stands on its own without support.

The table is raised and dropped 12.5 mm with the cam arrangement, 15 times in about 15 seconds. The diameter of the spread concrete is measured in 6 directions and the average value is taken. The flow of the concrete is defined as the percentage increase in the average diameter of the spread concrete to the base diameter of the spread concrete to the base diameter of the mould, i.e.,

The value varies from 0 to 150%.

The spread pattern of the concrete also reflects the tendency of the segregation. The flow test is a laboratory test.

Figure 8.4 Flow table apparatus

Kelly Ball Test

It consists of a metal hemisphere of 15 cm diameter weighting 13.6 kg (Fig. 8.5). The concrete base should be 20 cm depth, and the minimum distance from the centre of the ball to the nearest edge of the concrete is 23 cm. The ball is gradually lowered to the surface of the concrete. The depth of the penetration is read immediately on the stem to the nearest 6 mm. The test can be done in a shorter periods of about 15 seconds. This test gives more consistent results than slump tests.

The test can be performed in the field and it can be performed on the concrete placed on the site.

Figure 8.5 Kelly ball

Vee Bee Consistometer Test

This test consists of a vibrating table, a metal pot, a sheet metal cone and a standard iron rod (Fig. 8.6). A slump cone with concrete is placed inside the sheet metal cylindrical pot of the consistometer. The glass disc is turned and placed on the top of the concrete in the pot. The vibrator is switched on, and the stop watch is started simultaneously. The vibrator is kept on till the concrete in the cone assumes a cylindrical shape. The time is noted. The time required in seconds for the concrete to change from the shape of a cone to the shape of a cylinder is known as the Vee Bee Degree.

This is a good laboratory method and is more suitable for very dry concrete. This test measures the workability indirectly.

Figure 8.6 Vee Bee consistometer-type VBR