Compressive Strength Tests (IS: 3495 (Part 2)) :
The crushing affords a basis for comparing the quality of bricks but is of little value in determining the strength of a masonry wall, since the latter depends primarily on the strength of mortar. Six bricks are taken for the compressive strength test although it may be found that an individual brick varies by 20% or more from the average, the permissible stresses allowed for load bearing walls take account of this, being based on an average strength of six bricks. It is, therefore, both unnecessary and uneconomical to insist that every bricks is above a certain strength. As a criterion of structural strength for brick, the transverse failure in a wall or pavement is likely to occur on account of improper bedment. For testing bricks for compressive strength from a sample the two bed faces of bricks are ground to provide smooth, even and parallel faces. The bricks are then immersed in water at room temperature for 24 hours. These are then taken out of water and surplus water on the surfaces is wiped off with cotton or a moist cloth. The frog of the brick is flushed level with cement mortar and the brick is stored under damp jute bags for 24 hours followed by its immersion in water at room temperature for three days. The specimen is placed in the compression testing machine with flat faces horizontal and mortar filled face being upwards. Load is applied at a uniform rate of 14 N/m2 per minute till failure. The maximum load at failure divided by the average area of bed face gives the compressive strength.
Compressive strength N/mm2
The average of results shall be reported. The compressive strength of any individual brick tested in the sample should not fall below the minimum average compressive strength specified for the corresponding class of brick by more than 20 percent.
Note :
1. A considerable variation may be found in the strengths of individual bricks from the average strength of the sample.
2. Generally compressive strength of clay bricks decreases with increasing porosity.
3. Compressive strength of clay bricks is influenced by clay composition, firing and flaws.
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