Calculation of Basic Compressive Stress of Masonry by Prism Test
B-1 Determination of Compressive Strength of Masonry by Prism Test
When compressive strength of masonry ( ) is to be established by tests, it shall be done in advance of the construction, using prisms built of similar materials under the same conditions with the same bonding arrangement as for the structure. In building the prisms, moisture content of the units at the time of laying, the
consistency of the mortar, the thickness of mortar joints and workmanship shall be the same as will be used in the structure. Assembled specimen shall be at least 400 mm high and shall have a height to thickness ratio (h/t) of at least 2 but not more than 5. If the h/t ratio of the prisms tested is less than 5 in case of brickwork and more than 2 in case of blockwork, compressive strength values indicated by the tests shall be corrected by multiplying with the factor indicated in Table 31.
PART 6 STRUCTURAL DESIGN — SECTION 4 MASONRY
| Table 31 Correction Factors for Different h/t Ratios(Clause B-1) | ||||||
| Ratio of Height to Thickness (h/t) | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 5.0 |
| Correction Factors for Brickwork | 0.73 | 0.80 | 0.86 | 0.91 | 0.95 | 1.00 |
| Correction Factors for Blockwork | 1.00 | — | 1.20 | — | 1.30 | 1.37 |
Prisms shall be tested after 28 days between sheets of
nominal 4 mm plywood, slightly longer than the bed area of the prism, in a testing machine, the upper platform of which is spherically seated. The load shall be evenly distributed over the whole top and bottom surfaces of the specimen and shall be applied at the rate of 350 kN/m to 700 kN/m. The load at failure should be recorded.
B-2 Calculation of Basic Compressive Stress
Basic compressive stress of masonry shall be taken to be equal to where is the value of compressive strength of masonry as obtained from prism test.