Compressive strength test of masonry prism

Compressive strength test of a brick masonry prism (IS 1905 Appendix B)

Aim: to determine the compressive strength of masonry $f_m$ built with the actual brick, mortar and workmanship, so that the basic compressive stress can be fixed.

Specimen: a prism of bricks laid in a stack bond or the actual bond, at least 400 mm high, with height-to-thickness ratio ($h/t$) of 2 to 5 (preferably 4 to 5), joints and mortar as in the work. Moisture content of the bricks and mortar consistency are the same as will be used on site. Test after 28 days.

Procedure

1. Build prisms on a flat base; cure under damp cloth and polythene for 28 days.
2. Cap the bed faces with sheets of plywood (4 mm) slightly larger than the prism.
3. Centre the prism in the compression testing machine with a spherically seated platen so that load is applied evenly over the whole top and bottom surfaces.
4. Apply load at a rate of 350 to 700 kN/m per minute (uniform) to failure; record failure load $P$ and the mode of failure (vertical splitting).
5. Test at least 5 prisms and average.

    P
    v
  +-----+ <- ply/cap
  |=====|
  |brick|   h
  |=====|   (h/t = 2 to 5)
  |brick|
  +-----+
  ply cap

Calculation

$$f_m = \frac{P}{A}\times k,\qquad f_b = 0.25\,f_m$$

where $A$ = bed area, $k$ = correction factor for $h/t$ (Table 12): 0.73 for $h/t=2.0$, 0.80 for 2.5, 0.86 for 3.0, 0.91 for 3.5, 0.95 for 4.0 and 1.00 for 5.0. The basic compressive stress is 0.25 times the prism strength.