h/d ratio effect; splitting tension test

Effect of height/diameter ratio

In a compression test the platens restrain the lateral expansion of the specimen ends (platen restraint). A short specimen (low h/d) is mostly under this confinement and gives a higher strength; a taller specimen has a free central zone. Hence the measured strength falls as h/d increases from 1 to 2 and is nearly constant beyond 2.

The measured strength is multiplied by this factor to get the strength of the standard cylinder (h/d = 2). For example, a specimen with h/d = 1 shows about 15 to 25% more strength than the standard. For very large h/d (above 3), buckling and eccentricity reduce the strength.

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.