Q.: We've had much success using polyurethane grouting to stop active leaks through concrete cracks and joints. Sometimes, however, the water flow is so high that the grout is pushed out of the crack or joint before it can react and stop the leak. (We've had this problem even when adding a grout accelerator.) In addition, there are times when we've stopped a leak but the leak returns within a few days. Are there any methods we can use to correct these problems?

A.: To stop high-volume leaks, Joe Solomon of Concrete Restoration Services Ltd. often uses a technique called throttling to divert water away from the crack. This method involves drilling holes (these may be the same as injection holes) to intersect the crack and divert the water through isolated tubing installed in the holes (see photo). The tubing is equipped with valves to control water flow. After the leak in the crack or joint is stopped, the tubing is filled with grout.

There are many potential causes for recurring leaks, but Don Mack, technical director for Avanti International, says that failing to inject enough grout is probably the most common cause. He says that the leaking structure should be thoroughly filled with grout; do not stop injecting just because the leaking stops. The activated grout should emerge from the crack, and injection pressures should rise to ensure the structure is filled. Glenn Smoak, principal of Ecostructural Systems, believes that a poor injection sequence is the most common cause of recurring leaks. He says that some contractors make the mistake of using traditional epoxy-injection procedures when injecting polyurethanes. They start injecting at the lowest hole and move up the crack when leakage stops or grout appears at the next port. Smoak believes that such a procedure often leaves only low-density foam in the crack.

Smoak has achieved better results using a method of split spacing, similar to that used in cementitious foundation grouting, to position injection holes. With this procedure, primary holes are drilled along the leaking crack or joint (spacing depends on the thickness of the structure). The hole is then water-tested (injected with water to make sure resin can penetrate), and then injected with grout. Secondary holes are then drilled halfway between the primary holes, water-tested and injected. This process is repeated with tertiary or quaternary holes until the leak is stopped and the crack refuses to accept more resin. With this procedure, the primary holes create restraining boundaries to resin travel along the crack and allow injection of the secondary and subsequent holes at higher pressures, which results in higher foam densities.