Tests
on bricks:
1. Water absorption: Brick units immersed in
water for 24 hours has,
i) upto
12.5 N/mm2 strength and water absorption should not be greater than
20%
ii) for
higher classes, water absorption should not be greater than 15%
2. Efflorescence: Leaching of water soluble
salts (white coloured) under efflorescence. Test for efflorescence is done as
per IS3495 (Part III) -1976. The brick is kept in a
dish with water height as 25mm and the time for water
absorption and evaporation is noted. This value is compared with the same dish
with 25mm water height kept for evaporation alone. Based on the code,
efflorescence in brick is reported as nil, slight, moderate, heavy and serious.
Hardness: For the brick to be hard, it should
create no impression by finger
3. Hardness: For the brick to be hard, it should create no impression by finger nail.
4. Soundness: When two bricks are struck, it should not break and produce a clear ringing sound.
5. Compressive
test: 3.5 -40 N/mm2
6. Flexure
test: Rarely done
Classification of brick based on structure and usage:
1. Solid
wall
2. Cavity
wall
3. Faced
wall
4. Veneered
wall
Based on loading, walls are classified as,
1. Axially
loaded walls [Load applied at centre t/2]
2. Eccentrically
loaded walls
3. Laterally
loaded walls [Loading applied at sides]
Design
procedure:
Slenderness (Least of le/t & he/t)max
=:27)(?
(? =
60 for RC
columns, nbraced?= RC45walls)&
30 for brace
1. Actual
stress on the brick masonry wall is found based on the load from slab and self
weight of wall.
The
permissible compressive stress for masonry based on the type of mortar and
compressive strength of brick unit is taken from Table 8, IS1905-1987. This
table is valid for slenderness ratio ? ? 6 and eccentricity
3. Corrections
are applied for slenderness ratio, eccentricity (if any), shape and size of
brick unit. Shape modification factor and cross sectional area of masonry (area
reduction factor).
4. Slenderness
ratio is found as the least of le/t or he/t, where, (le
= Effective length and he = effective height).
Effective length is found from Table 5, IS1905 -1987 and
effective height is found from Table 4, IS1905-1987.
Table
4 -Effective height:
Support
condition Effective
height
Fixed
-Fixed 0.75H
Fixed
-Hinged 0.85H
Hinged
-Hinged H
Fixed
-Free 1.5H
The permissible maxvalue) for cement of mortar?(OPCis&
PPC),27 given(? in Table 7, IS1905-1987.
5. Eccentricity
of loading is determined (for axial loading e = 0). Eccentricities for various
other cases are to be checked as per Appendix B of IS1905-1987.
6. For
the permissible stress adopted, shape modification factor is found based on
height to width ratio of each brick unit given in Table 10, IS1905-1987.
7. Area
reduction factor is applied for elements having cross section less than 0.2m2.
The area modification factor, k = 0.7 + 1.5 A
8. After
applying modification factors, the actual stress is verified with a modified
permissibleact< perstress,? ?
The permissible stress (strength of the wall) depends
upon the following factors: i) Compressive strength of masonry unit
ii) Compressive strength of mortar used iii) Slenderness
ratio of the wall
iv) Eccentricity in loading
v) Shape and size of brick
unit
vi)
Cross sectional area of masonry
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