Burnt Clay Bricks Beat Greener Blocks on Strength but Fail More Brittle in Quake Tests

When an earthquake strikes a city built largely of reinforced concrete frames filled in with brick walls, the walls themselves often decide whether people walk away or whether floors pancake. A new experimental study from Bangladesh, published in the Bulletin of Earthquake Engineering, has put that intuition to a rigorous test, comparing how two very common masonry infill materials change the seismic behavior of multi-story, multi-bay reinforced concrete frames. The research team, led by Mohammad Raihan Mukhlis and M. A. Rahman Bhuiyan of Chittagong University of Engineering and Technology together with colleagues at the Housing and Building Research Institute in Dhaka, built three small-scale frames, pushed them through cyclic lateral loading until they were badly damaged, and then tried to reproduce what they saw with two widely used numerical macro-models. The results carry a message that is both encouraging and uncomfortable for engineers across the seismic world.

The two materials at the heart of the study could hardly be more different in character. Burnt clay bricks, or BCB, are the conventional workhorse of masonry construction in South Asia and much of the developing world, fired in kilns at high temperature and prized for their compressive strength. Unburnt sandcrete blocks, or USB, made from a sand-cement mix and cured without firing, represent a greener alternative because they avoid the energy intensity and emissions of brick kilns. In many rapidly urbanizing countries, both materials sit side by side in the same neighborhoods, often in the same buildings, yet their influence on how a frame behaves in an earthquake has rarely been compared head to head under controlled laboratory conditions. That gap is precisely what the Bangladeshi team set out to close.

The experimental program involved three small-scale multi-story, multi-bay reinforced concrete frames, each partially infilled with a different masonry unit, subjected to quasi-static cyclic loading that mimics the back-and-forth demands of a strong earthquake. The researchers evaluated seismic performance through hysteretic behavior, the looped force-displacement curves that encode a structure’s strength, stiffness, ductility and capacity to dissipate energy. These four quantities are the currency of earthquake engineering: strength determines the load a frame can carry, stiffness governs how much it sways, ductility measures how far it can deform before collapse, and energy dissipation reflects how effectively the structure absorbs the earthquake’s input rather than passing it on to its occupants.

The headline finding is that the type of infill masonry matters far more than many design codes assume. The frames with burnt clay brick infills delivered the highest lateral load capacity of the three specimens, confirming the material’s reputation for stiffness and strength. But that strength came at a cost. The BCB-infilled frames showed sudden degradation of their initial stiffness and a brittle post-peak response, meaning that once the infill panels began to fail, the loss of strength and stiffness was abrupt rather than gradual. In seismic terms, brittleness is dangerous because it gives occupants and structures little warning and little reserve capacity after the peak demand has passed.

The unburnt sandcrete block infills told a different and arguably more nuanced story. Although the USB-infilled frames reached a slightly lower maximum lateral capacity, they exhibited improved ductility and the highest post-peak cumulative energy dissipation of the studied frames. In other words, the greener material did not simply lose to the conventional one; it traded a modest amount of peak strength for a more forgiving failure mode, deforming further and absorbing more cumulative energy after its peak load had been exceeded. For engineers weighing sustainability against safety, the study suggests that the trade-off is not one-sided, and that softer, weaker infills may actually enhance the survivability of a frame in a severe event, provided the reduced strength is accounted for in design.

Beyond the material comparison, the study examined failure mechanisms in detail and found extensive damage concentrated in the frame members and infill panels near the exterior beam-column joints. This observation matters because exterior joints are precisely the locations where infill frames are most vulnerable in real earthquakes: the panels interact with the surrounding columns and beams, transferring forces that the bare frame was never designed to resist. The concentration of damage near these joints echoes a long-standing concern in the literature, dating back decades to pioneering work on infilled frames, that infills can create unintended load paths, short columns and local failure concentrations that compromise an entire building even when the code-designed frame itself is adequate.

The second half of the study turned from the laboratory to the computer. The team evaluated two available macro-models of masonry infilled reinforced concrete frames, assessing how reliably they could predict the measured seismic performance. Macro-models, typically built from equivalent diagonal struts that stand in for the infill panel, are the workhorses of seismic assessment because modeling every brick individually is computationally prohibitive for whole buildings. The verdict, however, was sobering. Implementing the two macro-models proved difficult, and neither reproduced the experimental behavior with acceptable accuracy. One model depended on substantial empirical calibration, effectively requiring test data to be tuned before it could predict test data, while the other tended to overestimate both stiffness and energy dissipation, painting an optimistic picture that real structures may not match.

These modeling shortcomings have practical consequences. Nonlinear static and dynamic analyses of infilled buildings underpin seismic retrofit decisions, loss estimates and code provisions worldwide, from FEMA guidelines to Eurocode 8 and national codes such as the Bangladesh National Building Code. If a macro-model overestimates energy dissipation, it will underpredict displacements and could lead engineers to conclude a building is safer than it is. If it requires extensive calibration, its predictive value for buildings that have never been tested, which is to say nearly all of them, is limited. The study’s experimental dataset, generated with support from the Housing and Building Research Institute and Chittagong University of Engineering and Technology, therefore serves a dual purpose: it documents how two common infill materials behave, and it provides a benchmark against which future models can be honestly judged.

The broader significance of the work lies in the collision between sustainability goals and seismic safety. As countries seek to phase out energy-hungry brick kilns, unburnt alternatives such as sandcrete blocks are being promoted on environmental grounds, sometimes with little attention to their structural implications. This study suggests that such a transition need not be seismically harmful and could even improve post-peak performance, but it also warns that the two materials produce measurably different frame responses in stiffness degradation, strength, ductility and energy dissipation. Codes and assessment procedures that treat all masonry infills as equivalent, or that ignore infills altogether, are missing behavior that the experiments show to be significant and, in the case of brittle burnt brick panels, potentially hazardous.

For a region that sits near the active boundaries of the Indian and Eurasian plates and carries the memory of devastating earthquakes, research of this kind is not academic. The study’s combination of physical testing and critical numerical evaluation offers engineers a clearer picture of what actually happens inside an infilled frame when the ground shakes, and a candid reminder that the computational tools used to certify safety still lag behind the messy reality of masonry-concrete interaction. As Mukhlis and his colleagues demonstrate, the humble brick in the wall may be the most important structural element nobody designed.

Subject of Research: Seismic performance of reinforced concrete frames infilled with burnt clay brick and unburnt sandcrete masonry

Article Title: Seismic performance of RC frames with burnt and unburnt brickwork masonry infills: experimental and macro-model evaluation

Article References: Seismic performance of RC frames with burnt and unburnt brickwork masonry infills: experimental and macro-model evaluation. (n.d.). https://doi.org/10.1007/s10518-026-02670-y

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02670-y

Keywords: reinforced concrete frames, masonry infill, burnt clay bricks, sandcrete blocks, seismic performance, hysteretic behavior, stiffness degradation, ductility, energy dissipation, macro-modeling, earthquake engineering, Bangladesh