Tests on Concrete

The following are some of the important tests conducted on concrete:

1. Slump test.

2. Compaction factor test.

3. Crushing strength test.

1. Slump Test:

This test is conducted to determine the workability of concrete. It needs a slump

cone for test  Slump cone is a vessel in the shape of a frustum of a cone with diameter at bottom 200mm and 50mm at top and 300 mm high. This cone is kept over a impervious platform and is filled with concrete in four layers. Each layer is tamped with a 16 mm pointed rod for 25 times. After filling completely the cone is gently pulled up. The decrease in the height of the concrete is called slump. Higher the slump, more workable is the concrete.

 

2. Compaction Factor Test:

This is another test to identify the workability of concrete. This test is conducted in the laboratory. The test equipment consists of two hoppers and a cylinder fixed to a stand, the dimensions and the distances between the three vessels being standardized.

Vessel A and B are having hinged bottoms whereas cylinder C is having fixed bottom. Top vessel A is filled with the concrete to be tested. As soon as it is filled, the hinged door is opened. Concrete is collected in vessel B. Then the hinged door of B is opened to collect concrete in cylinder C. The concrete in cylinder C is weighted.

Let it be W1.Now cylinder is again filled with the sample of concrete in 50 mm layers, which is compacted by ramming and vibrating. Then the weight of compacted concrete is determined. Let this weight be W2. The ratio W1/W2 is termed as compaction factor.

 

3. Crushing Strength Test: Metallic moulds of size 150 mm × 150 mm × 150 mm are used for

casting concrete cubes. Before filling mould, it is properly oiled on its inner surfaces, so that cubes can be easily separated. Fresh cube is filled with concrete to be tested in 3 layers and kept in the room. After 24 hours, cube is removed from the mould and kept under water for curing.

 After 28 days of curing cubes are tested in the compression testing machine. In this test cubes are placed over the smooth surface which is in contact with side plates of mould. The crushing load is noted and crushing strength is found as load divided by surface area (150 × 150 mm2).

Code specify the desirable strength of concrete for 3 days and 7 days for quick assessment of strength of concrete.Desirable Properties of Concrete Appropriate quality and quantity of cement, fine aggregate, coarse aggregate and water should be used so that the green concrete has the following properties:

(a) Desired workability

(b) No segregation in transporting and placing

(c) No bleeding and

(d) No harshness.

 

Hardened concrete should have

(a) required characteristic strength

(b) minimum dimensional changes

(c) good durability

(d) impermeable

(e) good resistance to wear and tear.

Uses of Concrete

1. As bed concrete below column footings, wall footings, on wall at supports to beams

2. As sill concrete

3. Over the parapet walls as coping concrete

4. For flagging the area around buildings

5. For pavements

6. For making building blocks.

However major use of concrete is as a major ingredient of reinforced and prestressed concrete. Many structural elements like footings, columns, beams, chajjas, lintels, roofs are made with R.C.C. Cement concrete is used for making storage structures like water tanks, bins, silos, bunkers etc. Bridges,dams, retaining walls are R.C.C. structures in which concrete is the major ingredient.