Concrete Technology (General Engineering) (SSC JE)

Aggregate crushing value test–

• This test helps to determine the aggregate crushing value of coarse aggregate as per IS : 2386 (Part 5).

• It gives a relative measure of resistance of an aggregate to crushing under gradually applied compressive load. Generally this test is conducted on single sized aggregate passing 12.5 mm sieve and retained on 10 mm sieve. The aggregate is tested in a surface dry condition.

• The cylinder (internal diameter 152 mm) of the test apparatus is filled with 3 equal layers of aggregate and each layer is tamped 25 strokes by the rounded and of temping rod. The total load should be 400 kN or 40 tonnes.

• IS : 383–1970 requires that the crushing value shall not exceed 45 percent for aggregate used for aggregate used for concrete other than for wearing surfaces and 30 percent for concrete wearing surface such as runway, roads and pavements.

Workability– The term workability may be defined as the ease with which concrete may be mixed handled, transported, placed in position and compacted.
• Several tests which have been developed to measure the workability of concrete are-
→ Slump test
→ Compacting factor test
→ Vee-Bee test
→ Vibro-workability test

Cubical aggregate gives maximum strength in concrete. It has good packing and strength in all direction. It provide very good bend than other and most suitable for high strength concrete and pavements, the requirement of cement paste is relatively more.

The abrasion value of coarse aggregate may be determine by either deval machine or by LosAngles machine.
• Using Los-Angles machine the abrasive charge consists of cast iron sphere or steel spheres approximately 48 mm in diameter and each weighing between 390 and 445 g.
• The test sample consists of clean aggregate dried in an oven at 105⁰ –110⁰C to substantially constant weight.

The compressive strength of concrete is based on test performed upon concrete cube after 28 days of mixing.

Compressive strength test–
• Size of cube → 150 × 150 × 150 mm
Size of cylinder → 150 mm dia, 300 mm height.
• Cube would filled in 3 layers, tempered with tamping rod of 16 mm dia and 600 mm length.
• Rate of loading in compression testing machine = 14 N/mm² /minute.
• Then immersed in water for 7 or 28 days.
Cube strength = 1.25 × cylinder strength

Water cement ratio is the water used to the quantum of cement in the mixture by weight.
• For proper workability the w/c ratio varies from 0.4- 0.6.
• Increase in water cement ratio means porosity increases and strength decreases.