Keyhole Basics: Coring 101

By E. Marshall Pollock | February 2010 Vol. 65 No. 2
Truck-mounted coring unit with stabilizers

There are several key factors that need to be considered when choosing the proper core reinstatement compound. Most important, it should be easy to use and achieve a very strong, waterproof bond with the remaining pavement. To minimize traffic delays and public inconvenience, it should also have the ability to gain that strength quickly so that the road can be safely reopened to traffic as soon as possible after the repair. That is important for municipal approval, as is the ability of the bonding compound to create a mechanical joint that restores the road to its pre-excavation strength and load sharing ability.

Generally speaking, the core bonding product should create a solid bond that can support a single wheel load of at least 30,000 pounds – which, for safety, is three times the AASHTO H-25 minimum load bearing standard. It should also do so as quickly as possible. There are some commercially available bonding products that gain full strength in just 30-minutes. The bonding compound should also be easy to store and transport and easy to mix with the exact amount of water required. Too much water and the strength of the bond will be dramatically reduced. Too little water and the material will not gain sufficient fluidity to pour and completely surround the core with no voids. Always use a product that eliminates the guesswork on how much water is needed.

Off the shelf, run of the mill, bulk concrete mixes or grouts are really not suitable for core reinstatement. They are unpredictable and there is no way to accurately measure their strength gain. Neither are epoxy type products which are difficult to use in the field. The best strategy is to use only those core bonding compounds that have been specially formulated to deliver all of the performance characteristics needed for this application. Otherwise, you run the risk of reinstatement failure – and failure of the entire keyhole process. Remember, the price differential between good and bad is usually small.

As with any task, having the right tools to do the job is important. As shown, there are many different components involved but, when properly chosen, they can make this innovative construction process worker friendly, easy to perform and, most importantly, cost-effective. And remember, whether it is truck, trailer or skid-steer mounted, if you choose a coring unit that is suited to your coring needs from a reputable and experienced manufacturer, and operate and maintain it in a responsible fashion, it will give you many years of satisfactory coring service.

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