The Patch That Finally Held

Repairing concrete is easy. Making the repair last is not, and the place it is hardest is a dam's stilling basin, the concrete apron below a spillway where the flow dumps its energy. When that flow carries sand, gravel and broken rock, the debris is caught in the turbulence and ground against the floor like a slurry in a mill, and it hollows the concrete out. The instinct is to fight abrasion with a harder aggregate. The counter-intuitive truth, learned the hard way, is that under abrasion the weak link is usually not the stone but the paste that binds it. Strengthen the paste, and the whole surface becomes harder to grind away.

The concrete in the stilling basin of Kinzua Dam, on the Allegheny River in western Pennsylvania, had been ground away by exactly this process, and it had been patched before without lasting success. So the U.S. Army Corps of Engineers took the problem to its Waterways Experiment Station, and the laboratory came back with an answer that was about the binder, not the aggregate: a very high-strength concrete made with silica fume, a particle so fine it packs the spaces between the cement grains and drives the paste strength up.

The concrete in the stilling basin slab of Kinzua Dam in western Pennsylvania was severely damaged by abrasion erosion.

ACI 234R, "Guide for the Use of Silica Fume in Concrete."

In late 1983 the Corps placed over two thousand cubic yards of it, about fifteen hundred cubic metres, specified at 86 megapascals, roughly double an ordinary structural concrete.

The 18-percent silica-fume concrete achieved a compressive strength over 13,000 psi (90 MPa) at 28 days ... and over 16,000 psi (110 MPa) at three years.

ACI 234R, "Guide for the Use of Silica Fume in Concrete."

When divers inspected the basin in the summer of 1990, the wear was there, but it was a small fraction of what the earlier concretes had suffered. The repair had held. But the mix was only ever going to be half the answer, because a repair, however strong, does not outlive the mechanism that destroyed the concrete before it.

The lesson: A durable repair is not the strongest patch; it is the one that answers why the concrete failed. For abrasion that means two things at once. Match the material to the damage, which here meant a high-strength, low water-content, silica-fume matrix rather than a harder rock. And wherever you can, reduce the cause, so the new concrete is not fed back into the same grinder. The same logic holds for every repair. The bond to the sound concrete, the compatibility of shrinkage and stiffness, and the diagnosis of the original cause decide whether a repair lasts, far more than the headline strength of the material you place. About half of concrete repairs fail, and they fail mostly on those points, not on the patch.

Read more: Concrete Repair Materials for Dam Rehabilitation →