Rarely does a repair project go exactly as planned, which is why it’s critical to have a contingency plan. And sometimes the contingency plan itself needs a contingency plan.
Such was the case on a structural rehabilitation project that RAND designed and administered for a social services facility in East Harlem. RAND was originally hired to evaluate the overall structural integrity of the six-story building. As part of our evaluation, we installed crack monitors in the facade and found that the cracks were getting wider and deeper, extending through the facade to the back-up concrete masonry unit (CMU) walls.
It was clear from our findings that an extensive repair program would be needed to make the building structurally sound. Our original scope of work specified completely demolishing and rebuilding the bulkhead walls, which were the most damaged. This plan worked for the boiler room bulkhead, but not for the elevator machine bulkhead, where during demolition we discovered severe structural problems.
Faced with the prospect of completely demolishing and rebuilding the CMU walls at an estimated cost of $3 million to $5 million, which would have required relocating the building’s residents and workers, we proposed another option: an innovative construction technique known as core-drilling.
Core-drilling entails drilling holes in masonry blocks and installing re-enforcing bars into the holes. Nylon-mesh “socks” are then lowered into the holes and injected with grout. As the socks expand, the grout seeps out and adheres to the re-enforcing bars and to the masonry blocks, providing structural stability. For this job, 2½-inch-diameter holes were drilled through the masonry blocks the entire height of the building, from the roof 60 feet deep down to the foundation.
The advantages of using core-drilling on this project were significant. Construction costs were approximately one-fifth of the cost of completely demolishing and rebuilding the elevator shaft walls. Core-drilling also decreased the amount of time the elevator was out of service and the potential damage to interior finishes. It also minimized the disruption to residents, who did not have to relocate.
For more on how we used core-drilling to reduce construction costs and restore the building’s structural integrity, see our video on the technique.
Jason Boral is a project engineer at RAND.
2 Responses to Hard Core-Drilling Softens Costs