Why does masonry fail by the units splitting?

The mortar expands laterally more than the brick, inducing lateral tension in the unit

The brick expands more than the mortar and crushes it

The mortar fails in shear before the brick

What does a designer usually do, given this behaviour?

Raise unit strength first and use a standard mortar

Use the weakest unit and strongest mortar

Raise mortar and unit strength equally

2076 Chaitra · 4 marks

Effect of unit and mortar strength on masonry

Short answer

Masonry capacity depends on both unit and mortar strength, but unit strength has greater influence. Failure occurs by lateral tension in units; increasing unit strength raises basic compressive stress almost proportionally, while raising mortar grade gives only 10-20% gain. For design, increase unit strength first with standard mortar.

How does the strength of masonry unit and grade of mortar affect the capacity of a masonry?

The compressive capacity of masonry (wall, pier or column) depends on both the strength of the unit and the grade of the mortar, but the unit has the greater influence.

Masonry fails when the units split in tension: the mortar is softer and tends to expand laterally under load more than the brick, which induces lateral tension in the unit. So the prism strength rises with the unit strength.
A stronger mortar expands less laterally and gives a higher masonry strength, while a very weak mortar (e.g. lime) gives a low strength. But the gain from mortar is small compared with the gain from unit strength; e.g. doubling the mortar strength raises the masonry strength by only about 10 to 20 percent.

IS 1905 Table 8 shows this directly. Basic compressive stress $f_b$ (N/mm $^2$ ):

Unit strength (N/mm²)	M2 (1:6)	M1 (1:5)	H2 (1:4)	H1 (1:3)
5.0	0.44	0.50	0.50	0.50
7.5	0.59	0.74	0.74	0.75
10	0.81	0.96	0.96	1.00
15	1.03	1.13	1.19	1.31
20	1.17	1.27	1.41	1.59

For a given mortar, $f_b$ increases almost in proportion to the unit strength up to about 10 N/mm $^2$ ; for a given unit, going from M2 to H1 raises $f_b$ by only about 15 to 35 percent. Hence the designer usually raises the unit strength first, uses a standard mortar, and does not use a mortar stronger than needed. The mortar must also bond well, be workable and be properly cured.

✓ Answer checked by Claude Opus 5.5 (high effort)

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