--- title: "Compressive strength test and classification of bricks" subject: "Design of Timber and Masonry Structures" chapter: 8 key: "semester_5/design_of_timber_and_masonry_structures:brick-compressive-strength-test-classification" source: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/brick-compressive-strength-test-classification site: BCT Engineering --- # Compressive strength test and classification of bricks Design of Timber and Masonry Structures · Chapter 8: Testing of Masonry Elements - Asked in: year not given, 6 marks - Video: [Compressive Strength of Bricks](https://www.youtube.com/watch?v=GkrLjzYPCEY) — UP RED Department - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/design_of_timber_and_masonry_structures/brick-compressive-strength-test-classification - Whole chapter: https://www.bctengineer.com/md/semester_5/design_of_timber_and_masonry_structures/chapter-8 ## Question Briefly describe the procedure for the compressive strength test of a brick (IS 3495). Five bricks of size 230 mm × 110 mm × 75 mm were tested flat with the following failure loads: 205, 190, 218, 176 and 199 kN. The bearing area is the full 230 × 110 mm bed. Find the compressive strength of each brick, the average strength, the standard deviation and the strength class (class 7.5 requires an average of at least 7.5 N/mm²; class 10 requires at least 10 N/mm²). A brick weighing 3.05 kg when dry weighed 3.54 kg after 24 hours in water: find the water absorption. ## Short answer Strengths: 8.10, 7.51, 8.62, 6.96, 7.87 N/mm²; average 7.81 N/mm² (s=0.62), class 7.5. Water absorption = (3.54-3.05)/3.05×100 = 16.1%. ## Answer #### Test procedure (IS 3495, Part 1) 1. Select five representative bricks. Remove unevenness and fill frogs/depressions with 1:1 cement–sand mortar. 2. Immerse the bricks in water for 24 hours, then drain surface water. 3. Place the specimen flat (largest face) between plywood sheets or plates in the compression testing machine. 4. Apply load axially at a uniform rate of about 14 N/mm² per minute until failure. 5. Compressive strength $= \dfrac{\text{maximum load}}{\text{loaded area}}$; the average of the five bricks is reported. #### Calculation Area $A = 230 \times 110 = 25\,300$ mm². | Brick | Load (kN) | Strength = $P/A$ (N/mm²) | |---|---|---| | 1 | 205 | 8.10 | | 2 | 190 | 7.51 | | 3 | 218 | 8.62 | | 4 | 176 | 6.96 | | 5 | 199 | 7.87 | $$ \begin{aligned} \bar f &= \frac{8.10 + 7.51 + 8.62 + 6.96 + 7.87}{5} = 7.81\ \text{N/mm}^2 \\ s &= \sqrt{\frac{\sum (f_i - \bar f)^2}{n-1}} = 0.62\ \text{N/mm}^2 \quad (\text{coefficient of variation} = 8\%) \end{aligned} $$ #### Classification Average strength $7.81 \ge 7.5$ N/mm² but $< 10$ N/mm², so the bricks are **class 7.5**. The classification is based on the average strength of the five bricks. #### Water absorption $$ \text{Absorption} = \frac{W_w - W_d}{W_d}\times100 = \frac{3.54 - 3.05}{3.05}\times100 = 16.1\% $$ This is below the 20 percent permitted for common bricks of this class, so it is acceptable. **Answer: Strengths 8.10, 7.51, 8.62, 6.96, 7.87 N/mm²; average 7.81 N/mm² (s = 0.62), class 7.5; water absorption 16.1%.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the brick compression test done (IS 3495)? A. Dry bricks tested on edge at a sudden load B. One brick tested on its end C. Bricks immersed 24 hours, tested flat between plywood with uniform load until failure D. Bricks tested in a hot oven Answer: C. Five bricks, frogs filled, soaked 24 hours, loaded flat at a uniform rate to failure. (Key point: Test procedure) 2. What is the strength of the brick that failed at 205 kN? A. 205 000/25 300 = 8.10 N/mm^2 B. 205 000/(110 x 75) = 24.8 N/mm^2 C. 205/230 = 0.89 N/mm^2 D. 205 000/(230 x 75) = 11.9 N/mm^2 Answer: A. A brick tested flat uses the bed area 230 x 110 = 25 300 mm^2. (Key point: Strength calculation) 3. What class is given and why? A. Class 10, since one brick exceeds 8 B. Class 7.5, since the average 7.81 is >= 7.5 but < 10 C. Fails, since one brick is under 7.5 D. Class 5 since s is high Answer: B. Classification uses the average of the five bricks, not individual values. (Key point: Classification) 4. How is water absorption computed? A. (W_d - W_w)/W_w x 100 B. (W_w - W_d)/W_w x 100 C. W_w / W_d x 100 D. (W_w - W_d)/W_d x 100 = 16.1 percent Answer: D. Absorption is the gain in mass as a percentage of the dry mass: 0.49/3.05 = 16.1 percent. 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