Stress-Strain Behaviour of Fly Ash Brick Masonry with Rat Trap Bond

Abstract

This research examines the compressive strength and stress-strain behaviour of fly ash brick masonry built using the rat trap bond technique. Due to its distinctive arrangement of vertically oriented cavities, the rat trap bond offers notable thermal performance and material efficiency. Three masonry prisms were built using fly ash bricks and a cement-sand mortar mix with a ratio of 1:5. After curing for 28 days, the specimens were subjected to axial compression testing with a Universal Testing Machine. The experimental results indicated that although the compressive strength of rat trap bond masonry was lower than that of conventional bond masonry, it was still within acceptable limits for non-load-bearing and low-load applications. The stress-strain curves showed a linear-elastic behaviour until a certain load level was reached, after which nonlinear deformation and brittle failure occurred. According to the study, rat trap bond masonry using fly ash bricks provides a sustainable and economical option for low-rise buildings, particularly in developing areas. Nonetheless, additional studies are advisable to evaluate its performance when subjected to lateral and cyclic loads, and to advocate for its incorporation into building codes and design guidelines.

Introduction

Brick masonry is a widely used construction method, especially in developing countries, due to its affordability, fire resistance, and material availability. A sustainable variation called the rat-trap bond (RTB)—which arranges bricks on edge to create internal cavities—offers advantages such as reduced material use, improved thermal insulation, and lighter structural weight. Originally from the UK, RTB was reintroduced in India by architect Laurie Baker in the 1970s to promote eco-friendly building.

Traditionally, clay bricks are used in RTB masonry, but their production consumes topsoil and emits carbon. As an alternative, fly ash bricks—made from coal byproducts—are more uniform, less water-absorbent, and energy-efficient. However, their performance in RTB systems remains under-researched.

This study explores the mechanical properties of RTB masonry using fly ash bricks, focusing on stress-strain behavior under axial compression. Experiments involved constructing masonry prisms with standard fly ash bricks and a 1:5 cement-sand mortar mix, then curing and testing them using ASTM standards.

Key Findings:

• Compressive Strength: The average compressive strength was 500.02 psi, which is lower than conventional masonry due to RTB's hollow structure. However, it is adequate for non-load-bearing and low to moderate load-bearing applications.

• Stress-Strain Behavior: The masonry showed brittle failure, with initial linear elasticity followed by micro-cracking, particularly at the brick-mortar interface and unsupported sections. Cracks concentrated in central areas, and failure occurred suddenly after peak load.

• Implications: While not ideal for high-load structures, RTB with fly ash bricks is suitable for sustainable, low-rise buildings, offering material efficiency and environmental benefits. This method supports green construction practices.

Conclusion

1) Due to internal cavities, the compressive strength of rat trap bond masonry was lower than that of conventional bonds; however, it was still sufficient for non-load-bearing and low-load applications. 2) Stress-strain curves revealed: An initial linear elastic phase, a nonlinear softening phase leading to brittle failure. 3) Utilizing fly ash bricks with a rat trap bond resulted in reduced material usage (less mortar and fewer bricks), enhanced thermal performance (due to air gaps), improved sustainability, and greater cost efficiency.

References

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Copyright

Copyright © 2025 Muhib Ur Rahman. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.