Popularity of compressive test; tensile methods

Reasons for the popularity of the compressive strength test

1. The test is simple and quick and needs only a cube mould and a compression machine, which every site laboratory has.
2. Concrete is mainly used in compression, so compressive strength is the main design property.
3. Other properties (tensile, flexural, shear, bond, modulus) can be estimated from $f_{ck}$ (IS 456 Cl. 6.2).
4. It is a good index of overall quality: durability, density and impermeability improve as strength rises.
5. Results are reproducible and the standard (IS 516, IS 456 Cl. 15 and 16) is universal, so the test is used for mix design, quality control and acceptance.
6. Compressive strength is much higher than tensile strength, so it can be measured with a smaller relative error.

Methods of obtaining tensile strength

1. Direct tension test. A dog-bone or cylindrical specimen with embedded end grips is pulled axially. It is rarely used, because it is hard to avoid eccentricity and secondary stresses from the grips, and the result scatters. Direct tensile strength is about $0.35\sqrt{f_{ck}}$ to $0.45\sqrt{f_{ck}}$ (MPa).

2. Splitting tension test (Brazilian test, IS 5816). A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Advantages: it is simple, uses the same cylinders and compression machine as the compression test, and gives a more uniform and reproducible result than direct tension, with no grip problem.

3. Flexure test (modulus of rupture, IS 516). A plain concrete beam 150 × 150 × 700 mm is supported on a 600 mm span and loaded at the third points (two-point loading) at 0.7 N/mm$^2$/min extreme-fibre stress rate. If the crack occurs in the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^{2}}$$

If the crack is outside the middle third but within 5% of the span from the load point, $f_{cr} = 3Pa/(bd^2)$ ($a$ = distance of the crack from the nearer support); otherwise the result is rejected. For single (centre) point loading, $f_{cr} = 3Pl/(2bd^2)$. IS 456 (Cl. 6.2.2) gives the flexural strength $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$. The modulus of rupture is greater than direct tension (by about 1.5 to 2 times) because the stress distribution is assumed linear while the actual stress block is plastic.