Repairing earthquake-damaged walls could depend on how they cracked

Istanbul, Türkiye – When an earthquake causes a masonry wall to crack, simply covering the cracks isn't enough to restore strength. Walls can slide along joints, rock at their base, or develop staircase-like diagonal cracks in their mortar joints. Additionally, a repair that works well for one type of masonry or damage may not be effective for another.

Researchers from Istanbul Technical University (ITU) and the University of Illinois Chicago have experimentally explored an approach that accounts for these differences. Instead of only studying how to strengthen intact walls, they intentionally damaged masonry walls under repeated sideways loads and then designed repair methods based on the failure modes observed. They combined a special cement mortar with short polyvinyl alcohol (PVA) fibers and a grid of continuous basalt fibers. In some cases, the reinforcement was anchored into the wall's concrete foundation. Tests revealed that this method increased the lateral load capacity of damaged concrete-block walls by 42% and clay-brick walls by 21.83%. Ductility, or the ability to deform before failure, improved by about 30% and 24%, respectively. These results are published in the International Journal of Civil Engineering.

Repairing the damage, not just strengthening the wall

Unreinforced masonry is particularly vulnerable during earthquakes because brick or concrete blocks and their mortar have limited ability to resist tension and sideways forces. Researchers have examined different strengthening materials, such as fiber-reinforced polymers, engineered cementitious composites, and textile-reinforced mortars. These methods improve the strength, deformation capacity, and seismic performance of masonry walls. This study specifically investigated already damaged walls: how they behave when compromised and whether repairs can be customized based on the type of damage.

Previous research focused on masonry repaired with fiber-reinforced cement materials under repeated loads and on damaged masonry strengthened with textile-reinforced systems. The current experiments include chopped fibers and continuous basalt textiles in a cement-based hybrid composite, testing various damage types, levels of vertical compression, and common masonry materials. Instead of assuming the wall was undamaged before reinforcement, the researchers used the existing damage to guide their repair strategies.

Recreating earthquake-like damage in the laboratory

The team built walls using two common materials: hollow concrete blocks and hollow clay bricks. These walls experienced simultaneous vertical compression and repeated horizontal forces, mimicking earthquake-induced loads. By varying the vertical compression, they observed different failure modes: lower compression allowed for rotation or sliding, while higher compression increased the likelihood of diagonal shear cracks. After identifying damage patterns, the researchers devised repair methods using an engineered cementitious composite mortar with short PVA fibers and embedded basalt textile grids. The short fibers prevent crack growth, while the textile maintains wall integrity and bears tensile stress. Some walls were fully covered with the hybrid composite, while others received localized reinforcement. They also tested anchoring the textile into the foundation. The repaired walls underwent further cyclic horizontal loading tests.

Fixing one weak spot can reveal another

Strengthening a cracked wall is like patching the weakest link in a chain: once that part becomes stronger, stress may shift elsewhere. The researchers observed this in their experiments. The new material effectively stopped the diagonal cracks that previously split the masonry. However, instead of vanishing, the effects of the repeated lateral force appeared elsewhere, especially where the wall contacts its foundation. This indicates that reinforcing just the wall surface might be insufficient. When the researchers extended and anchored the textile reinforcement into the foundation, the walls performed better. The anchoring maintained the interaction between the wall and foundation, preventing rocking or sliding at the base. These findings emphasize a key lesson for repairing earthquake-damaged masonry: addressing a single crack isn't enough. Effective repairs should anticipate how seismic forces might transfer elsewhere. Proper anchoring with the hybrid material transformed large, brittle cracks into many smaller, more evenly distributed ones, reducing sudden damage. 

Concrete blocks and clay bricks did not respond equally

The experiments also highlighted a significant limitation. The cement-based hybrid composite bonded better to the damaged concrete-block walls compared to the clay-brick walls. When subjected to high repeated lateral loads, the composite on the brick wall started to detach from its surface. This indicates that there might not be a universal composite repair suitable for all masonry buildings, as factors like surface type, material, existing damage, and reinforcement connections influence effectiveness.

As Didem Donmez from the Department of Civil Engineering at Istanbul Technical University states, “Our experiments show that repairing a damaged masonry wall is not only about adding a stronger material. We need to understand how the wall failed, how the repair material bonds to that particular masonry, and how the strengthened wall transfers forces to its foundation.”

From cracked walls to decisions about what can be saved

After an earthquake, deciding whether to repair or replace a damaged building is a complex issue. For some severely damaged structures, rebuilding might be the safer or more practical option. However, replacement isn't always ideal, for instance, a historic building may have cultural and architectural significance that would be lost if demolished. In such cases, it’s crucial to find ways to reinforce damaged masonry while preserving as much of the original structure as possible.

The latest study does not offer a definitive method for this decision but enhances the engineering knowledge needed for repairs. It emphasizes understanding how a wall failed, its materials, and how it connects to the foundation before selecting a repair approach. Findings show that hybrid composites work well on damaged concrete-block masonry with proper anchoring, though their weaker bond with clay brick indicates an area for further research. More work is necessary to adapt this method to entire buildings, as current experiments are limited to specific wall types under laboratory conditions.

The researchers recommend additional experiments, larger datasets, and numerical modeling. Nonetheless, the study encourages a more nuanced approach to post-earthquake building assessment: cracks are not merely scars but clues to structural failure. For historically valuable buildings, these clues can help engineers determine the best way to restore damaged walls and extend their life.