About Cavity Wall

1. Cavity walls
Cavity walls consist of two "skins" separated by a hollow space (cavity).[1]
The
skins are commonly masonry, such as brick or concrete block. Masonry is
an absorbent material, and therefore will slowly draw rainwater or even
humidity into the wall, as well as from the inside of the house as from outside.
The cavity serves as a way to drain water back out through weep holes at the
base of the wall system or above windows. The weep holes allow wind to
create an air stream through the cavity and the stream removes evaporated
water from the cavity to the outside. Usually weep holes are created by
intentionally leaving several vertical joints, also open head joints, open about
two meters apart at the base of in every story. Weep holes are also placed
above windows to prevent dry rot of a wooden window frame. A cavity wall with
masonry as both inner and outer skins is more commonly referred to as a doubl
masonry wall
A cavity wall is composed of two masonry walls
separated by a continuous air space in between
the outer and the inner wall. The outer wall is
the brick wall that faces the outside of the
building structure. The inner wall, separated
from the outer wall by a continuous air space or
the cavity, is the wall on the interior of the
building structure. The inner wall may be
constructed of masonry units such as concrete
block, structural clay, brick or in some other
cases reinforced concrete. These two walls
have to be fastened together with metal ties
or bonding blocks. The ties will allow the cavity
wall to be strong.
The water barrier is a thin membrane that will
keep moisture away from the cavity side of the
interior wall.
Weep holes in masonry wall
In cavity walls the flashing component is very important for the overall water management
of the wall.[4]
The main purpose of the flashing is to direct water out of the cavity. Metal
flashing will usually extend from the interior wall through the outer wall and a weep hole
with a downward curve should be provided in order to get water out. Flashing system in
cavity walls are typically located close to the base of the wall, so that it will collect, on the
bottom, all the water that goes down the wall.
Weep holes are drainage holes left in the exterior wall of the cavity wall, to provide an exit
way for water in the cavity

2. Rat trap bond. Rat trap bond is a brick masonry method of wall construction, in which bricks are placed in
vertical position instead of conventional horizontal position and thus creating a cavity (hollow space) within
the wall.
INTRODUCTION TO RAT TRAP BOND MASONRY (RTB)
A “Rat-Trap Bond” is a type of wall brick masonry bond in which bricks are laid on edge (i.e. the height of each
course in case of a brick size 230x110x75 mm, will be 110 mm plus mortar thickness) such that the shiner and
rowlock are visible on the face of masonry as shown below.
This gives the wall with an internal cavity bridged by the rowlock. This is the major reason where virgin materials
like brick clay and cement can be considerably saved. This adds this technology to the list of Green building
technologies and sustainability for an appropriate option as against conventional solid brick wall masonry.
This cavity adds an added advantage as it adds a Green building feature of help maintain improved thermal
comfort and keep the interiors colder than outside and vice versa.
The Rat trap bond construction is a modular type of masonry construction. Due care must be taken while
designing the wall lengths and heights for a structure. The openings and wall dimensions to be in multiples of the
module. Also the course below sill and lintel to be a solid course by placing bricks on edge. The masonry on the
sides of the openings also to be solid as will help in fixing of the opening frame.
B. MATERIALS SELECTION AS PER NEED AND DESIGN
Bricks
 As far as possible, use of fired clay bricks should be checked with alternative bricks i.e. fly ash bricks,
bricks from construction waste etc. as firing of bricks is in general highly energy consuming and air
polluting.
 These alternate bricks provide better strength and durability than fired clay bricks and finishing cost of
your wall can be saved with a better quality and aesthetically soothing environment in the interiors.
Mortar:
 Lime should be explored as a material for mortar as it is locally available in many parts of the country
and helps reduce the consumption of cement.
 Lime as a binding material along with fly ash can result in to make lime pozzolona mortar which saves on
your cost of cement. It also adds another Green building feature, use of industrial waste (i.e. flyash) as a
raw material for your construction.
C. ADVANTAGES OF USING RAT TRAP BOND TECHNOLOGY

3.  By adopting this method of masonry, you can save on approx. 20-35% less bricks and 30-50% less
mortar; also this reduces the cost of a 9 inch wall by 20-30 % and productivity of work enhances.
 For 1 m3
of Rat trap bond, 470 bricks are required compared to conventional brick wall where a total of
550 bricks are required.
 Rat trap bond wall is a cavity wall construction with added advantage of thermal comfort. The interiors
remain cooler in summer and warmer in winters.
 Rat-trap bond when kept exposed, create aesthetically pleasing wall surface and cost of plastering and
painting also may be avoided.
 Rat trap bond can be used for load bearing as well as thick partition walls.
 All works such as pillars, sill bands, window and tie beams can be concealed.
 The walls have approx. 20% less dead weight and hence the foundations and other supporting structural
members can suitably be designed, this gives an added advantage of cost saving for foundation.
 Service’s installations should be planned during the masonry construction if not exposed.
 Virgin materials such as bricks, cement and steel can be considerably saved upon by adopting this
technology. It will also help reduce the Embodied Energy of virgin materials and save the production of
Green House Gases into the atmosphere.
 In case for more structural safety, reinforcement bars can be inserted through the cavity till the
foundation.

4. Demonstrating various Types of Pointing
Mortar for pointing/Ratio and proportion
Cement is a greyish coloured powder that is mixed with sand and/or other
aggregates to create concrete or mortar. Mortar is a mixture of
fine sand particles, cement, water and sometimes lime. It's typically used for
brick laying and pointing. A good mix for a cement-based pointing mortar would
be one part cement plus one part lime to six parts soft sand. Adding
hydrated lime to water to a creamy consistency and then using sharp, rather
than soft, sand can make a true limemortar, suitable for softer and older
brickwork. Mix the mortar quite stiff.
When masonry structures are first constructed, mortar is applied as a thick paste which sets hard as it cures, creating
a tight seal between bricks and blocks to prevent air and moisture entering into the construction. Depending on the
age of the building, the type of masonry, and the nature of the overall wall construction, mortar used
for pointing will be either lime mortar or cement mortar.
The finished profile of the mortar joint at is external surface is known as ‘pointing’. This profile can be varied
depending on exposure or to create a specific visual effect.
Pointing procedure
Firstly mix mortar in necessary ration of cement sand such as (1:4 or 1:6). Fill the mortar carefully on joints of bricks
horizontally and vertically. Make surface smooth by sapping it.
Types of pointing:
1. Flush pointing: Flush pointing is where the mortar between the
brickwork is flush with the surface of the brick. To achieve this
finish allow the applied mortar to almost dry and then rub
between the joints with a piece of wadded sacking or other coarse
material.
2. Struck joint or pointing: Striking and Pointing brickwork. The difference between 'striking' and 'pointing' is
that: Striking is finishing the mortar joints between newly laid bricks. Pointing is applying a small amount of
mortar into the face joint between bricks (either newly laid or in old brickwork).q
3. Weather struck and cut pointing: Similar to flush except the upper edge is pressed back inside the face to
create a sloping profile. This is time-consuming and requires practice, but the effect can be aesthetically
pleasing.
4. Rounded or tooled pointing: Penny roll: A recessed line is created in the middle of the joint to give a tidier
impression when the joints are wide or degraded.
5. Recessed or raked: Mortar is pressed back from the face of the masonry units by 5 mm or more. This creates
a shadow gap between the masonry units.
6. Tuck pointing: Using two colors of mortar (one similar to the color of the masonry, used at the edges) to give
the impression that the joints are very thin.
7. V-joint pointing: Similar to rubbed but with a sharp inward V-groove rather than rounded.
8. Beaded, concave, half round or bucket handle: A more rounded type of pointing with a concave edge. This
can look aesthetically pleasing, but it may damage more easily than other profiles.
9. Rubbed: Also known as keyed or grooved, this is a rounded groove formed in the middle of
the pointing using a suitable tool.
10. Tuck-pointing: Penny roll: A recessed line is created in the middle of the joint to give a tidier impression
when the joints are wide or degraded.
11. Strap or ribbon: A neat strip of mortar sits proud of the face of the masonry. This can be used to give a
neater impression when joints are degraded.

5. Generally, mortar is structurally weaker than the blocks or bricks it bonds, creating a sacrificial layer that is more
easily repaired than defects would be in the bricks or blocks themselves. As a result, mortar joints can decay over
time, due to weathering, frost damage, and so on. When this happens, repointing is undertaken to renew them