RAT TRAP BOND MASONRY

RAT TRAP BOND MASONRY

INTRODUCTION: 


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 the Rat-trap bond is the reduction in the number of bricks and mortar required as compared to English/ Flemish bond because 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 a 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.

Fig.6: Various Bonds in Brick Masonry

 CONSTRUCTION DETAILS:

The following Flowchart explains the general schematic of the wall construction process

using Rat Trap Bond Masonry:

                        Chart 1:  Rat Trap Bond Masonry Construction Process

 BENEFITS & ISSUES:

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 the conventional brick wall where a total of 550 bricks are required.
3. Rat trap bond wall is a cavity wall construction with an added advantage of thermal comfort. The interiors remain cooler in summer and warmer in winters.
4. Rat-trap bond when kept exposed, create aesthetically pleasing wall surface and cost of plastering and painting also may be avoided.
5. Rat trap bond can be used for load bearing as well as thick partition walls.
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 the 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 the case of more structural safety, reinforcement bars can be inserted through the cavity till tile foundation.

There are some issues though which have to be dealt with or kept in mind before the 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 conduiting because chasing brickwork, like in conventional practice, is not possible. However, this can be taken care of by identifying a 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.

DOs & DON’Ts: 

Before starting the actual construction work, and during the construction progress, there are some checklist items to be observed with care. The critical factors that affect the final quality of the Brickwork Wall made in RAT TRAP BOND are:

1. Curing

2. Quality of Bricks

3. Not used for buildings having more than two floors

4. Quality of sand

5. Availability of skilled mason

Table 4: Dos & Don’ts

Sr. No.	Dos	Don’ts
1	Curing should be done for seven days	Fine sand should not be used for Masonry
2	Cavity should be broken after six layers	Mortar should not be left in cavities
3	Brick should be completely soaked in water before using for masonry
4	Mortar should be used within 30 minutes of adding water to the mix
5	Brick bats should be used at corners and T — Junctions

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 stories with a 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. It's excellent weathering over the past 30 years is the best performance guarantee. Importantly, an unplastered brick wall in rat trap bond masonry requires very little recurring maintenance cost since there is no external plaster/painting.

DESIGN AND CONSTRUCTION

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 the 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.

Table 5: The Strength Requirement Guide

Sr. No.	Type of Building Construction	Recommended Compressive Strength of Bricks
		Best Practice	Minimum Allowable
1	1 Load bearing, double storied	More than 50 kg/cm2	40 kg/ cm2
2	Load bearing, single-storied	More than 40 kg/ cm2	35 kg/ cm2
3	Infill masonry in frame structure, no restriction on number of storey	Minimum 35 kg/ crn2	--

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.

Fig. 7: 1 Module of Rat trap Bond Masonry

MASONRY AT JUNCTIONS: 

To improve earthquake resistance, the Rat-trap masonry walls can be reinforced with steel bars at all corners, around openings (door and windows), T-junctions and filled with a M20 concrete. The diameter of reinforcement depends on the number of storeys, storey height, seismic zone, importance of building and soil type. It should not be less than 12mm.

Fig. 15: Masonry & Reinforcement at 'T' & Junctions

In addition to vertical reinforcement, reinforced concrete bands can be provided at three levels —sill, lintel, and roof level. These bands, typically with 2 bars of 8mm dia, can be provided like an open or a concealed element to tie the structure together with vertical reinforced masonry for earthquake resistance.

CURING:

The walls are mostly exposed to wind and sunshine which will dry out the moisture in the mortar joints which will prevent it from hardening adequately. To prevent this from happening, all brick masonry needs regular curing for at least 7 days. A brick wall needs to be cured for at least 7 days.