Repointing mortar joints cracked by the expansion of stone-veneered concrete retaining walls on the Mountaintop Campus of Lehigh University was nothing new. It had been tried before, and always with the same results: new mortar cracked; stone caps on the wall loosened and were retained only by metal pins and straps sunk into the now damaged mortar joint; silicone used as a substitute for mortar failed to adhere to the stone as the joints opened; mortar that did withstand expansion forces failed to match original joints.The solution would be to treat the cracks as "natural" expansion joints, eliminating the mortar and replacing it with materials that would withstand the forces exerted on the joints. The answer was a one-part urethane sealant.Urethane sealant has more generous expansion and contraction characteristics than nonurethane caulks, and it has excellent adhesive properties. Urethane presented only one major problem: aesthetics. Even with a custom mixed color, the texture of the sealant would not match that of the original mortar joints. The solution: mix sand to match the hue and texture of existing mortar, then sprinkle it on the sealed joint before the urethane cures.
Repairing the Joints
To repair the joints, crews removed loose and damaged mortar in a conventional manner with a toothed masonry chisel. They chiseled joints deep enough to accept extruded polyethylene backer rod and a bead of sealant as deep as the joint is wide, up to 1/2 inch.