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Importance and method of compressive strength test

Short answer

Compressive strength fck is the basis of RC design: it defines the design stress block (0.446 fck), elastic modulus (5000 sqrt(fck)), flexural strength (0.7 sqrt(fck)), shear and bond. Test: cast and cure 150 mm cubes, at 28 days load on side faces at 14 N/mm^2/min, fc = P/A, average three cubes. Acceptance: group mean >= fck+0.825s, individual >= fck-3 N/mm^2.

Write the importance of the compressive strength of concrete in the design of reinforced concrete structures. Explain the method of determining compressive strength of concrete.

Importance in RC design

  • Compressive strength is the basis of structural design: the grade (M20, M25 ...) gives the characteristic strength fckf_{ck}, which sets the design stress of 0.446fck0.446 f_{ck} in the limit-state stress block (IS 456, Cl. 38.1) and the permissible bending compressive stress σcbc\sigma_{cbc} of about 0.33fck0.33 f_{ck} in working stress design (IS 456, Table 21).
  • Other properties are derived from it: Ec=5000fckE_c = 5000\sqrt{f_{ck}} (Cl. 6.2.3.1), flexural strength 0.7fck0.7\sqrt{f_{ck}} (Cl. 6.2.2), shear τc\tau_c and bond τbd\tau_{bd} (Cl. 26.2.1.1).
  • It is the principal index of the quality of concrete, since it also reflects density, durability and permeability.
  • It is the easiest to test and is the criterion of mix design (IS 10262), quality control and acceptance (IS 456 Cl. 16).
  • Concrete resists compression in columns, the compression zone of beams and slabs, footings and walls, which is its main role in RC members.

Compressive strength test (IS 516)

  1. Sampling and casting: take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
  2. Curing: keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
  3. Testing: take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on two opposite faces of the cube as cast (not on its top and bottom as cast). Apply the load without shock at 14 N/mm2^2 per minute (about 140 kg/cm2^2/min), until the specimen fails. Record the maximum load PP.
  4. Result: fc=P/Af_{c} = P/A, where A=150×150=22500A = 150\times150 = 22500 mm2^2. The test strength is the average of three specimens; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
  5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; fcyl≈0.8 fcubef_{cyl} \approx 0.8\,f_{cube}.

✓ Answer checked by Claude Sonnet 5.5

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