As owners of older brick homes know all too well, the mortar joints between bricks will begin to loosen and crack in structures that are about 50 years old. Tuck pointing, which involves replacing the damaged mortar, is labor-intensive and very costly.

Now a UB engineering professor has developed a method of significantly strengthening the mortar that is used to "glue" the bricks together, resulting in a much stronger and longer-lasting bond.

The method is applicable to existing brick structures, as well as to new construction.

Deborah D.L. Chung, Niagara Mohawk Chair of Materials Research and professor of mechanical and aerospace engineering, has found that when short carbon fibers are added to cement mortar, the brick-to-mortar bond strength increases by as much as 150 percent.

The research will be reported in a paper co-authored by Mingguang Zhu, who conducted the work as a doctoral candidate in the Department of Mechanical and Aerospace Engineering, in the November issue of Cement and Concrete Research.

"The strength of the bond or joint between brick and mortar determines the overall integrity of the wall," said Chung. "When a brick wall cracks, it does so at the joint between the bricks. That's the weak link."

Several variables can affect mortar strength, she continued, but a critical one is the drying shrinkage of mortar.

"As it dries, the mortar shrinks while the adjoining bricks do not," she said. "This weakens the bond between them."

In her experiments, Chung found that the carbon fiber-reinforced mortar reduces drying shrinkage.

"There is a degree of bonding between the carbon fiber and the cement and this bonding acts to restrain the cement matrix from shrinking as much," she said.

By adding short carbon fibers at five-tenths of one percent of the cement weight to mortar, she was able to improve the brick-to-mortar bond strength by 150 percent under tension or pulling force and by 110 percent under shear, a sliding force.

Chung estimated that the addition of carbon fibers to mortar would increase its cost by approximately 30 percent.

"If one considers lifespan and life-cycle repair costs, then it's very worthwhile," she said.