Rat trap bond:

Definition:
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.
OR:
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)
OR:
Rat trap bond is a modular type of masonry bond in which the bricks are
placed in a vertical position which creates a cavity in the wall while
maintaining the same wall thickness as that of the conventional brick
masonry wall. It is also known as a Chinese brick bond.

Explanation:
The strength and elastic properties of masonry are not only affected by
the properties of masonry units and mortars, but also by the way the
units are assembled. The
geometrical pattern of the disposition of units, mainly to break the contin
uity of vertical joints inmasonry, is often referred to as bond. Common
examples of such disposition are the
Stack bond, Stretcher bond. Header bond, English bond, Flemish bond.
Rat-trap bond etc. Rattrap-bond (RTB) is a brick wall construction
technique in which bricks are laid on edge such that the shiner and
rowlock are visible on the face of the masonry (brick cross) creating an
internal cavity bridged by the rowlock. It is a modular brick walling
technology. The origin of name, "Rat Trap" is not known, but is probably
due to the trap formed inside the wall by brick arrangement. RTB
construction has been proved to be an effective and feasible earthquake
resistant construction system. The reinforced rat-trap bond construction
system practically enables to place reinforcement in lateral and vertical
directions along with the in-situ concrete grout, without any necessity for
a special/additional shuttering. . Reinforced rat-trap-bond masonry also
ensures proper confinement, an integral action, and provides better
ductility during earthquakes.
The rat trap bond is a masonry technique, where the bricksare used in a
way which creates a cavity within the wall, while maintaining the same
wall thickness as for a conventional brick masonry wall. While in a
conventional English bond or Flemish bond, bricks are laid flat, in a Rat
trap bond, they are placed on edge forming the inner and outer face of
the wall, with cross bricks bridging the two faces. The main
advantage of Rat-trap bond is reduction in the number of bricks
and mortar required as compared to English/ Flemish bond
because of the cavity formed inthe wall. The cavity also makes
the wall more thermally efficient. This also reduces the embodied
energy of brick masonry by saving number of bricks and the
cement-sand mortar. It is suitable for use, wherever one-brick thick
wall is required. Since its original dissemination in Kerala in the 1970sby
architect Laurie Baker, rat trap bond has been extensively used in every
category of building from large institutional complexes, community
buildings. Government offices/village panchayats, individual homes
both for high income and middle income and also in government
supported EWS housing programs. The following figure shows the
basic layout difference in the traditional English/ Flemish Bond Masonry
methods Vs Rat Trap Bond Masonry. The rat trap bond is a masonry
technique, where the bricks are used in a way which creates a cavity
within the wall, while maintaining the same wall thickness as for a
conventional brick masonry wall. While in a conventional English bond or
Flemish bond, bricks are laid flat, in a Rat trap bond, they are placed on
edge forming the inner and outer face of the wall, with cross bricks
bridging the two faces. The main advantage of Rat-trap bond is
reduction in the number of bricks and mortar required as compared to
English/ Flemish bondbecause of the cavity formed in the wall. The
cavity also makes the wall more thermally efficient. This also
reduces the embodied energy of brick masonry by saving number of
bricks and the cement-sand mortar. It is suitable for use, wherever one-
brick thick wall is required. Since its original dissemination in
Kerala in the 1970sby architect Laurie Baker, rat trap bond has
been extensively used in every category of building from large
institutional complexes, community buildings. Government
offices/villagepanchayats, individual homes both for high income and
middle income and also in government supported EWS housing
programs. The following figure shows the basic layout difference in
the traditional English/ Flemish Bond Masonry methods Vs Rat Trap
Bond Masonry.The Rat-trap bond is a masonry technique in which the
bricks are laid in such a manner that a discontinuous cavity is formed
between two faces of the wall. Headers and stretchers laid on edge.
Typically, a 75-mm cavity is formed in a 230-mm thick wall. This is done
by placing the bricks on edge in a modular fashion. For the purpose of
housing, this system can be used for in-fill walls (in an RCC frame) in
multistore housing or for single storied row housing. It is also possible
to construct single storied housing in reinforced load-
bearing. Rat-trap masonry. It can be used for building houses of fewer
than 3 stories and is still used as an economical bond, as well for
the insulation properties offered by the air cavity. With this
technique, there is reduction in cost of the wall by 25%, as with
conventional English bond (9’’thk wall) 350 bricks are required per cu. m
whereas in Rat-trap bond only 280 bricks are required and also the
reduced number of joints reduces the mortar consumption.
Introducer:
Architect Laurie Baker introduced it in Kerala in the 1970s and used
it extensively for its lower construction cost, reduced material
requirement and better thermal efficiency than conventional
masonry wall, without compromising strength of the wall.

Purpose:
Rat-trap bond substantially observes the same pattern as Flemish bond,
but consists of rowlocks and shiners instead of headers and stretchers.
This gives a wall with an internal cavity bridged by the rowlocks.
The purpose of using this type of masonry bond is to reduce the number
of bricks and mortar required as compared to the English/Flemish bond
because of the cavity formed in the wall.
Also for its lower construction cost, reduced material requirement and
better thermal efficiency than conventional masonry wall, without
compromising the strength of the wall.
Hence these are the objectives:
•Understand the basics of rat-trap bond masonry practice •Limitations of
rat-trap bond
•Locating openings in rat-trap bond
•Understand significance of working in modules
•Understanding modular nature of rat-trap bond
•Estimation on material saving by the use of rat-trap bond, over
conventional bond.
•Calculating embodied energy saved by the use of rat-trap bond, over
conventional bond
•Distinguish old and new construction practices

Comparison of performance of rat trap brick bond

with the conventional brick bond:
Energy efficiency and sustainability are the basic needs of modern era.
In order to achieve this there are several building construction
techniques and environment friendly materials have been introduced.
But there is a need to use these techniques and material in a proper
combination so that they could be adoptable and economical and play a
vital rule in the improvement of energy efficiency and sustainability. One
such building technique to save energy is the use of 'Rat Trap
Bond'(RBT) masonry. Contrary to other technologies, this amazing
building technology is not new. RTB was first introduced in 1970. Since
then, it has been used in many buildings and small houses. In this
modern age, we have overlooked this extremely useful technology
which, while providing the same strength to the walls also saves us time
and labor and also material cost to the extent of about 23% when
compared with a standard brick masonry wall. In this research an
attempt is made to compare the conventional brick bond wall and RTB
brick wall. The cost, energy use and thermal comfort of a house is
compared by using both types of brick bond wall. It is concluded that
RTB brick wall performed very well in saving energy and reducing the
electricity bill cost.
Construction of Rat Trap Bond:
1. The bricks are placed in a vertical position so that 110 mm face is
seen from front elevation, instead of the 75mm face (considering
brick of standard size 230 X 110 X 75 mm).
2. As the width of the wall is kept as 230mm, a cavity is created
inside the wall.
3. However, the first and the last layer of the masonry is constructed
as the convention sold masonry.
4. In the sill, lintel and sides of openings are made of solid masonry
(without cavity) for fixing of frames.
5. To strengthen the masonry, vertical and horizontal reinforcement
bars are provided in the cavities.
6. Electrical conduits and plumbing pipes, with prior planning, can be
put inside the cavity for better aesthetics.
Dos:
 A solid course of brick on edge should be laid at plinth, door and
windowsill/lintel level and roof level.
 The first course should be set right.The brick pattern should be
kept straight and vertical.
 The size of openings should be kept in multiples of the rat trap
module size.
 Services should be planned in advance so as to avoid exposed
wiring and plumbing.

DON’Ts:
 There should be no water penetration in to the cavity. For
that joint should be properly tooled in with mortar.
 Vertical joints should be avoided.
 No untrained masons should work in rat trap bond walling
to avoid wastage of mortar falling into the gap.
CRITERIA FOR DESIGN:
Rat-trap bond masonry can be used to construct a small double
storeyed residential building in load bearing construction, using the
specific construction details which are followed in this technique. The
principal requirement for rat-trap brickwork is the availability of good
quality bricks.

The Guiding Principle:

The following can be taken as guiding principle for strength of

bricks for Rat-trap brickwork: The data presented here is for Short
Span not exceeding 4.2 meters, and Roof/ Floor Loads as per IS 875.
The Criteria for Selection of Bricks:

 Although regional variations in the size of bricks are inevitable, the

acceptable sizes for the bricks (in the Indian context) are —Length
220-250 mm, Width 100-115inm and Height 65-75mm .
 The brick should be rectangular with straight and sharp edges.
 It is important that there should be uniformity in the brick size, so
that the brickwork can be designed and constructed in a modular
pattern and has good strength and finish.
 For mortar, the cement-sand proportion depends on the quality of
the brick and the building design which determines the load on the
masonry. As a guiding principle, a 1:5 (cement: sand) mortar is
recommended for compressive strength of brick not less than
50kg/cm2 and 1:4, if the strength is between 35-50 kg/cm2. A
thickness of 1/2" is recommended for the mortar joints, however, a
slight reduction in the thickness of vertical joints is allowable
because the contact area is much smaller at the side of the brick
than at the bottom and top. Care must be taken to ensure that the
entire vertical joint is filled with mortar; otherwise the brick masonry
wall will be prone to leakages.
 As per design principles of conventional masonry, the corners and
the openings represent the weak areas in masonry and should
be designed for adequate strength, depending on factors like
building design and the seismic strengthening requirements.
Therefore, all masonry corners and ends of openings in rat-trap
brickwork are constructed solid, without any cavity.

Modular Design:
To ensure maximum advantage of the technique, it is
preferable that the masonry is designed in a modular pattern
at the design stage itself, after the prevailing brick size
available for use has been ascertained. For best rat-trap brickwork,
there should be no half bricks/ quarter bricks used in brickwork,
unlike their common use in conventional brickwork. This will
disturb the staggering of joints in rat-trap brickwork and affect
the integrity of brickwork.
 Site plan:

Estimation on Rat-Trap Bond:

Estimation (for 1m^2of masonry):

Conventional Bond:

Rat-trap Bond:

Estimated reduction in bricks =16 Nos:

So, it makes us to estimate that in conventional bond 550 bricks are

used in 1m3, but in rat-trap bond 470 bricks are used in 1m^3.

Cement Mortar:

Conventional Bond:
Rat-trap Bond:
Reduction in quantity of mortar = 0.0252 m^3

Estimation of room:

Volume of brick work in plan = 27.8m^3

Area of brick work in plan = 92.57m^2

Saving:

 14.32 % in bricks
 53.23 % in cement
 55% in sand
Embodied Energy Calculation (for 1m2of masonry):

Construction industry started to incorporate the idea of energy efficiency

in infrastructure and design a few decades ago. This review paper
specifically deals with one of such energy efficient masonry technique
called Rat-trap bond. For this purpose, body of knowledge is explored
high and low and only those articles are selected that deal particularly
with Rat-trap bond. The thorough study of this selected literature shows
that the bond is not only energy proficient but also cost saving. With the
ability to reduce cooling and thermal load, the bond is structurally stable
as well. The compressive strength test of rat-trap bond with C.M. 1:5 is
49% (0.87 MPa) stronger than C.M 1:4 (1.30 MPa) with and without roof
slab respectively. The review is an attempt to suggest future
recommendations by finding the research gap in this field of knowledge.
It has been found that although the bond has a promising ability to be
practically adopted yet it has not enjoyed the eminence it deserves. Only
a limited number of case studies are present for realization of this technique. It is
recommended that this masonry technique, owing to its less construction cost and
excellent energy efficiency, can be primarily utilized by low income groups and in
residential constructions

Saving of Embodied energy is about 27%

Thermal Comfort Analysis Using Ecotect:

From study of different research paper,it has been found that Cavity
produces thermal comfort. Its R-Value is 0.70m2K/W which is twice in
comparison with English & Flemish bond.The air cavity can be filled
with some other insulating material to achieve even more thermal
resistance. For analyzing the house for its cooling and heating load use
the Ecotect software. This is an efficient way of analyzing which is
provided with the ease of selection of weather data for which building is
analyzed. There are number of options of materials that can be applied
on building.
The Rat Trap bond is not only good to save the electricity consumption
but also very good in the thermal load reduction. The below figure shows
the clear difference between the thermal loading of both types of brick
bonds.
Advantages of rat-trap bond:

1. 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.
2. For 1 m3 of Rat trap bond, 470 bricks are required compared to
conventional brick wall where a total of 550 bricks are required.
3. 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.
4. Rat trap bond can be used for load bearing as well as thick
partition walls.
5. Rat-trap bond when kept exposed, create aesthetically pleasing
wall surface and cost of plastering and painting also may be
avoided.
6. All works such as pillars, sill bands, window and tie beams can be
concealed.
7. 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.
8. 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.
9. In case for more structural safety, reinforcement bars can be
inserted through the cavity till tile foundation.
Disadvantages of rat-trap bond:
There are some issues though which have to be dealt with or kept
in mind before commencement of construction work during the
planning stage. These are mentioned below:

1. Service's installations should be planned during the masonry

construction if not exposed.
2. It is most suited where good quality bricks with straight and
sharp edges are available -better avoided when good quality
and uniform size bricks are not available.
3. If the mason is not skilled enough, cement mortar can get
wasted by falling into the wall cavity.
4. Needs pre-planning in case of concealed electrical conducting
because chasing brickwork, like in conventional practice, is not
possible. However, this can be taken care of by identifying
location of wiring and plumbing in the design and planning
stage, so that solid courses of brickwork may be provided in
masonry where the conduits will run.
5. Excavation of soil needed for manufacturing of bricks may lead
to environmental problems.
6. Building can be built up to 3 storey height only.
Performance Validation:
 Rat trap bond can be very easily adapted for
earthquake/seismic strengthening i.e. provision of
horizontal tie bands and vertical reinforcement in the
brickwork cavity.
 The rat trap bond technique has been validated by the
Department of Civil Engineering, Anna University
Chennai. The results conclude that "the rat-trap bond wall
can be safely used for low cost housing having 2 storey
with short span not exceeding 4.2 m and with storey
height not exceeding 3 m, using bricks of minimum
compressive strength 50 kg/cm2 with cement mortar 1:3.
 Rat trap bonded brick masonry has been widely used in
all parts of the country. Its excellent weathering over the
past 30 years is the best performance guarantee.
Importantly, an unplasterbrickwall in rat trap bond
masonry requires very little recurring maintenance cost
since there is no external plaster/painting.
Validation of rat-trap bond for cost effective housing:
In India most of the houses constructed in villages and towns are either
one storey or two storey high. The spans of rooms are 2.5 or 3m.Under
these circumstances, the stress in the brickwork is nominal. However
most of the structures use solid 230mm brickwork using English
Bond. In recent times HUDCO, a techno financing organization
sponsored a project for considering the use of Rat trap Bond wall,
which has a cavity in the wall. This type of construction is very
advantageous in a tropical humid climate. This paper deals with a series
of tests on full-scale wall specimens made with 23 cm solid wall
as well as 23 cm Rat-trap hollow wall specimens. The specimens
were loaded and tested to destruction. A computer analysis was
also made to simulate the behaviour of the wall. The material presented
shows that the Rat-Trap bond wall can be successfully adopted for
normal buildings and will result in 25% saving in bricks and more than
30% saving in mortar. The dead load on the foundation would also be
reduced by at least 20%.In addition this type of wall does not
need thick plastering as both front and rear face of the wall are fair
faces. Examples of Structures built with this type of bond are
illustrated. The wall also lends itself to be reinforced when
structures are built in earthquack or cyclone prone areas.

Experimental program:
To study the behaviour and to find the allowable compressive
stress of the Rat-Trap Bond wall, eight specimens of size
920x920x230 mm were constructed. Six specimens were
constructed with conventional bricks, one using conventional bricks
for stretchers and flyash-lime-gypsum (FAL-G) brick as headers and
another with conventional bricks as stretchers and with wire tied
conventional bricks as headers. To compare the behaviour with
the English bond wall, three specimens of the same size
with conventional bricks in English bond wall were constructed. The
experimental setup for the specimens are shown in the Figure.
Experimental study of structural behaviour of Chinese
bond brick masonry: A new trend of durable &
economical construction in Pakistan:
A variety of structures are constructed in Pakistan using different
construction materials. Cement-lime mortar is used widely worldwide but
not commonly in Pakistan. Cement-lime mortar is a technique that
doesn’t require water for its curing purposes. Structural durability,
economy and energy efficiency are the basic parameters of a modern
construction. In order to attain these parameters, several building
construction techniques and environment friendly materials have been
introduced. One such building technique to save energy, material and to
resist the applied loads is ‘Chinese Bond Brick Masonry (CBBM)’. The
structural properties, construction cost, energy efficiency and thermal
comfort were compared by testing English and Chinese bond masonry
prisms. Chinese bond walls performed excellent in saving materials, less
penetration of external heat waves and resistance against compressive
and lateral loadings. In this study, it has been tried to explain the
importance of CBBM methodology along with construction details. It has
also provided some comparative calculations to highlight the requisite
strength against failure of structure, durability and cost savings that can
be achieved against the conventional brick masonry. Compressive
strength of Chinese bond prisms did not increase over time but shear
and diagonal compressive strength increased. Thus, it was found that
compressive strength was related to strength of bricks while shear
strength, that was representative of masonry unit, was more related to
mortar strength. It is also noted that, addition of lime to the mix
increased the strength of mortar after 28 days and water requirement for
the curing of masonry structures was also reduced efficiently.

Conclusion:
1. The average ultimate stress for the English bond wall is 1.564
N/ mm2and for Rat-Trap bond wall is 1.212 N/mm2. The
allowable compressive stress using a factor of safety of 1.50
works out to 1.042 N/ mm2and 0.808 N/ mm2respectively.
2. The failure of the Rat-Trap bond wall is due separation of the
two leaves of the wall caused by splitting of the header bricks,
which fails primarily in shear.
3. The computer analysis made and the test results indicate that
for normal buildings rate trap bond wall can be used with
advantage provided wall span in less than 3.5 m.