Plain and reinforced concrete - Code of practice — IS 456 (2000) Testing

In IS 456:2000, concrete grades such as M20, M25 and M30 indicate the characteristic compressive strength of 150 mm concrete cubes at 28 days. For example, M20 represents a characteristic compressive strength of 20 N/mm² or MPa.

Characteristic strength is the strength value below which not more than 5 percent of test results are expected to fall. For concrete, IS 456:2000 uses the specified characteristic compressive strength of standard 150 mm cubes at 28 days for grade designation.

The commonly specified concrete compressive-strength test specimen under IS 456:2000 is a 150 mm cube. The measured compressive strength from such specimens forms the basis for determining whether the concrete achieves its specified characteristic strength at the required testing age.

The 28-day compressive strength is an important benchmark for concrete grade classification and quality assessment under IS 456:2000. Testing at this age helps determine whether the concrete has achieved the specified characteristic compressive strength for the selected grade.

Concrete quality control can involve workability testing, compressive strength testing, sampling and inspection, along with checks on constituent materials and durability-related requirements. The appropriate test method depends on the property being evaluated and the applicable referenced Indian Standard.

The slump test is used to assess the workability or consistency of fresh concrete. IS 456:2000 provides suggested workability ranges according to placing conditions, while the actual test is carried out using the applicable test method such as IS 1199.

For heavily reinforced sections such as slabs, beams, walls and columns, IS 456:2000 gives a suggested medium workability range with a slump of approximately 50–100 mm. The required workability should still be selected according to placement and compaction conditions.

IS 456:2000 treats durability as an important concrete performance requirement. It considers factors such as environmental exposure, cover to reinforcement, constituent materials, cement content, water-cement ratio, compaction and curing because these influence permeability and long-term performance.