This Santa Rosa, Calif., patio was previously a series of concrete slabs and pavers. After the pavers were removed and new concrete was poured between the older slabs, Concrete Staples were used to join new and old. Photos courtesy of Allan Firestone
The Concrete Staple is my invention, an engineered piece of metal that bridges cracks to hold two pieces of concrete together. Overlay can be installed right over the top.
It works while other methods generally fail because it addresses the specific problems of cracked concrete.
The Staple is made from steel with a high-yield strength to overcome the incredible weight and pressures of 4-inch and 5-inch thick slabs of concrete. It’s placed across the crack to not only hold the existing crack together but also to keep the adjacent microcracks from failing and opening up.
It has right angle bends on each end to grip the concrete on either side of the crack. As a result, it does not have to depend on friction only to hold the two pieces of concrete together.
When installed, the Staple bonds the concrete together to counteract against the multidirectional forces that concrete slabs are subject to.
Finally, it’s factory-made from high-yield material. The angled ends have the needed sharp bends to counteract springiness and are precisely lined up so the stresses are balanced.
Critiquing the competition
One way to understand how and why my Staple works is to understand why competing methods eventually fail.
Take the fiberglass tape approach as an example. Concrete is extremely heavy. One yard of 4-inch thick concrete (about 9 by 9 feet) weighs more than 4,000 pounds. Concrete and the ground under it contract and expand with heat and cold, moisture and extreme dryness. Driveways are also subjected to thousands of pounds of vehicles going over them. How can we expect a small piece of fiberglass tape to hold all that weight together? When you think about it, common sense tells you fiberglass tape could not possibly work. And yet, even though it continually fails, some makers of overlay products are still telling contractors to use fiberglass tape.
What about epoxy? Epoxy is super-strong, how could it fail? Once again, with a little knowledge of how concrete breaks and some common sense, you can understand the problem.
When trying to use epoxy to glue concrete together, we have to face some obvious facts. Not only is it nearly impossible to get the crack clean enough for good adhesion, it is very difficult to avoid having hollow areas in the epoxy between the two pieces of concrete.
However, even if you could overcome these problems, epoxy still doesn’t work consistently. Why? Because when concrete does break under pressure, creating a crack, there are a number of microcracks created by that same pressure, running parallel along the visual crack. The epoxy won’t actually fail, but adjacent microcracks will fail, usually on both sides, and it will appear as though the epoxy failed.
How about stitching with pieces of rebar? Like epoxy, stitching is a step in the right direction. It just doesn’t address all the problems you need to overcome to keep a crack in concrete from coming back. Stitching depends totally on friction. The epoxy and the piece of rebar have only friction to hold them from slipping. Over the years it has not proven to be consistently adequate.
The Santa Rosa, Calif. project after removing the pavers.
Concrete staples combine the new and old concrete.
The secret of the bends
Our Concrete Staple has a sharp bend on each end that acts like a double-ended foundation bolt. It doesn’t depend on friction only. In order to match the strength of those ends with friction only you would need a considerable length of rebar on either side of the crack. Trying to match the strength of the Concrete Staple with long pieces of rebar would be too expensive and require too much work.
The Staple locks both the visual crack and the microcracks together. Like stitching, it goes across the crack, and we still advise using epoxy during the staple installation and in the crack itself. The Staple not only keeps the concrete from pulling apart, it also addresses the side-to-side, lateral and heaving pressures concrete slabs are subjected to. In short, it ties the concrete back together.
Why not simply bend rebar into the same shape and use that? We tried that. When I first thought of the idea of a steel rod with a bend at both ends, I went to the store to buy one. I was sure something like that had already been thought of. However, no one had them or had ever heard of anything like it. So I went to a steel fabricator and had them made. The fabricator decided we needed a very strong piece of metal with high-yield strength to do the job. They worked perfectly. However, they were rather expensive.
So when we ran out of the ones they had made, we decided to make our own out of 3/8-inch rebar. When those failed, we went to 1⁄2-inch rebar. They also failed. Working with a testing laboratory, we discovered the reasons for the failures.
It turns out schedule 40 rebar doesn’t have the needed yield strength to do the job and grade 60 was too difficult to work with. Yield strength determines when the steel begins to stretch. When it stretches under pressure naturally, the crack opens up again.
There was also a physical problem with the bends. When you hand-bend rebar, the bend is rounded. This roundness allows for more stretch, which only helps the crack reopen under pressure. We were also advised that if the bends were not lined up with the preciseness of a factory right-angle bend you would lose even more efficiency of yield.
The final answer was to get a factory-made staple using a high-grade, high-yield-strength metal. Our Concrete Staple tests out at a higher yield strength than grade 60 rebar.
Allan Firestone is owner of Castle Care Concrete Inc., a decorative concrete installation company in Santa Rosa, Calif. Castle Care manufactures the CPR Concrete Staple. Allan can be reached at firstname.lastname@example.org.