Compressive strength test of brick wall

Compressive strength test of brick masonry (wall/prism) in the laboratory

Purpose: to find the compressive strength $f_m$ of masonry (made of the same brick, mortar and workmanship), from which the basic compressive stress is derived (IS 1905 Appendix B).

Specimen: a wall panel or prism built in stretcher or English bond, at least 400 mm high, and height-to-thickness ratio ($h/t$) between 2 and 5. Tested after curing for 28 days.

Procedure

1. Construct the specimen with the actual mortar thickness, bond and moisture content of units to be used in the structure.
2. Cap the top and bottom with mortar or plywood sheets, 4 mm thick (to give uniform bearing).
3. Place in the compression testing machine with a spherically seated upper platen, centre carefully.
4. Apply load gradually at about 350 to 700 kN/m of the bed area per minute until failure; record the failure load $P$ and the failure pattern (vertical splitting cracks).
5. Optionally fix gauges to measure strain and find the stress-strain curve and modulus of elasticity.

   | load P
   v
 +-------+  <- steel platen (spherical seat)
 |  cap  |
 |=======|
 | brick |  h = 400 mm min
 |=======|  mortar joints
 | brick |
 +-------+
 | cap   |
 =========  <- machine bed

Calculation

$$f_m = \frac{P}{A}\times k_h$$

where $A$ is the loaded bed area and $k_h$ is the correction factor for the slenderness of the prism (IS 1905 Table 12, brickwork): $h/t = 2.0, 2.5, 3.0, 3.5, 4.0, 5.0$ gives $0.73, 0.80, 0.86, 0.91, 0.95, 1.00$. Basic compressive stress of masonry $f_b = 0.25\,f_m$ (IS 1905 B-2.1).

Bricks (separately): compressive strength of bricks is found by testing a brick (flat, frog up, with mortar bedding) in the compression machine; the strength is the average of five bricks, e.g. 3.5 to 10 N/mm$^2$ for common burnt clay brick classes (IS 3495).