Rat trap bond masonry is a unique and efficient method of wall construction that reduces the number of bricks and mortar needed compared to traditional brickwork. In this modular bond, bricks are placed vertically, creating cavities within the wall while preserving the standard wall thickness. This type of bond is often referred to as the “Chinese brick bond.” Introduced in Kerala, India, by British-born architect Laurie Baker in the 1970s, rat trap bond masonry became popular for its cost-effectiveness, material efficiency, and enhanced thermal insulation without compromising wall strength.
Material Criteria and Brick Selection
Brick selection is essential in rat trap bond masonry, as the construction requires fewer bricks, making each brick’s quality critical to the structure’s integrity. The size of the bricks must be standard, ensuring consistency in the masonry. In India, the acceptable brick dimensions range from 220-250 mm in length, 100-115 mm in width, and 65-75 mm in height. Uniformity in brick size is crucial since rat trap bond relies on a modular design for structural stability and a smooth finish. Each brick’s edges and corners should be sharp, straight, and perfectly rectangular to maintain alignment in the masonry.
Material Strength Requirements and Mortar Ratios
The strength of the bricks and the ratio of mortar play significant roles in the structural integrity of rat trap bond masonry. Different types of construction have specific strength requirements:
- Double-storied, load-bearing walls: Recommended brick strength of 40-50 kg/cm², with a mortar ratio of 1:5.
- Single-storied, load-bearing walls: Recommended brick strength of 35-40 kg/cm², with a mortar ratio of 1:4.
- Infill masonry (within frame structures): Minimum brick strength of 35 kg/cm², with a mortar ratio not less than 1:4.
These guidelines ensure that the walls remain stable and durable, with the appropriate compressive strength for each building type.
Construction Process of Rat Trap Bond
The construction process for rat trap bond masonry involves vertical brick placement so that the 110 mm face is visible, rather than the 75 mm face. This design creates a 230 mm wall width, forming cavities within the wall. The first and last layers of masonry are built as solid walls, providing structural stability.
In areas requiring additional stability, such as windowsills, lintels, and door frames, solid masonry (without cavities) is used to allow for secure frame attachment. Reinforcement bars are often added vertically and horizontally within the wall cavities for enhanced strength, especially in load-bearing applications. Furthermore, with careful planning, these cavities can accommodate electrical conduits and plumbing pipes, which not only makes the construction aesthetically pleasing but also prevents the need for surface-mounted utilities.
Advantages of Rat Trap Bond Masonry
Rat trap bond masonry offers numerous benefits, making it an appealing choice for sustainable construction:
- Thermal Insulation: The cavities act as insulators, keeping interiors cooler in the summer and warmer in the winter, enhancing the building’s energy efficiency.
- Cost Efficiency: By using fewer bricks and mortar, rat trap bond construction can reduce masonry costs by as much as 30%.
- Reduced Brick Use: For each square meter of wall, only about 470 bricks are used compared to the 550 bricks typically needed in conventional masonry.
- Versatile Applications: Walls constructed using rat trap bond can serve as load-bearing walls or as thick partition walls.
- Aesthetic Appeal: Rat trap bond masonry can be left exposed, creating a visually appealing wall surface. This eliminates the need for plastering and painting, further reducing costs.
- Lower Dead Load: The presence of cavities reduces the dead load of the structure by approximately 30%, allowing for lighter structural supports such as columns and footings.
- Enhanced Structural Safety: Reinforcement bars can be added within the cavities, providing additional stability and safety.
- Proven Durability: Many buildings constructed decades ago using this technique have demonstrated the durability and low maintenance requirements of rat trap bond masonry.
Disadvantages of Rat Trap Bond Masonry
Despite its advantages, rat trap bond masonry does have some limitations:
- Limited Sound Insulation: The cavities in the masonry can reduce sound insulation, making this technique less effective in noisy environments.
- Need for Skilled Labor: Constructing rat trap bond masonry requires skilled masons familiar with this technique, which may increase labor costs.
- External Surface Maintenance: If the wall is left exposed, frequent cleaning may be needed to maintain its appearance.
- Design and Construction Precision: The modular nature of this bond requires careful planning and attention during both design and construction to ensure stability and aesthetics.
Rat trap bond masonry is a compelling option for sustainable and cost-effective construction. This technique, popularized in India by Laurie Baker, uses fewer resources, maintains structural integrity, and offers better thermal efficiency than traditional brick bonds. While it has some limitations, especially in sound insulation and labor skill requirements, rat trap bond masonry remains a valuable alternative in regions where reducing construction costs and materials is essential. As an eco-friendly building solution, rat trap bond masonry not only provides economic and structural benefits but also supports a more sustainable approach to architecture.