--- title: "Effect of unit and mortar strength on masonry" subject: "Concrete Technology and Masonry Structure" chapter: 8 key: "semester_5/concrete_technology_and_masonry_structure:unit-mortar-strength-effect" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/unit-mortar-strength-effect site: BCT Engineering --- # Effect of unit and mortar strength on masonry Concrete Technology and Masonry Structure · Chapter 8: Design of masonry walls for gravity loads - Asked in: 2076 Chaitra, 4 marks - Video: [Bulding mortars: types, requirements, guniting and grouting](https://www.youtube.com/watch?v=pl3i0LDx2C0) — Mounika Meduri - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/unit-mortar-strength-effect - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-8 ## Question How does the strength of masonry unit and grade of mortar affect the capacity of a 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. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which has the greater influence on masonry compressive capacity? A. Mortar grade B. Strength of the masonry unit C. Both equally D. Thickness of the joints Answer: B. Unit strength has the greater influence; mortar grade has a smaller effect. (Key point: Unit has greater influence) 2. Why does masonry fail by the units splitting? A. The mortar expands laterally more than the brick, inducing lateral tension in the unit B. The brick expands more than the mortar and crushes it C. The mortar fails in shear before the brick D. Water in the joints freezes Answer: A. Softer mortar expands laterally more than the brick, putting the unit in lateral tension so it splits. (Key point: Failure mechanism) 3. Doubling the mortar strength raises masonry strength by about: A. 100 percent B. 50 to 60 percent C. 10 to 20 percent D. No change at all Answer: C. The gain from mortar is small, about 10 to 20 percent, compared with the gain from unit strength. (Key point: Effect of mortar strength) 4. What does a designer usually do, given this behaviour? A. Use the weakest unit and strongest mortar B. Raise unit strength first and use a standard mortar C. Always use lime mortar D. Raise mortar and unit strength equally Answer: B. Since unit strength matters most, the designer raises it first and avoids mortar stronger than needed. 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