The choice of power to drive the coring drill is another element in the proper selection of a coring unit. Most truckmounted units use a hydraulic pump driven by a power take off (PTO) from the engine of the truck. Using a PTO makes maintenance simpler, there's only one engine to service and maintain, and more cost-effective because most of today's trucks come with a multiyear drive-train warranty. Skidsteer-mounted coring units also require no special fittings and are able to use the equipment's auxiliary hydraulic connections.
Coring units mounted on a trailer or pickup truck require an auxiliary power source, such as a small gasoline or diesel engine, to drive the hydraulic pump. This need for a second engine adds to the overall cost of maintenance and can add substantially to the purchase price.
The coring is performed with a cylindrical, steel core drum about 22 inches long, fitted with carbide- or diamond-encrusted teeth. The usual diameter for keyhole work is 18 inches but some coring units can accommodate larger 24-inch diameter drums. Depending on the pavement type, a typical coring unit can cut at about 1.5 minutes/inch of pavement. High-quality core drums can core between 80 and 100 holes, although the teeth will eventually wear down and need to be replaced; a good quality drum can be retoothed or retipped several times.
Some core drills are fitted with a center pilot bit that, like the center bit in a typical hole saw, cuts a hole at the same time the core is being cut. This center pilot hole has several advantages: it centers and stabilizes the coring drum, ensuring a neat and accurate cut; it also makes it easy to insert a core puller for removing and reinstating the core. Other techniques for pulling the core are less safe or efficient. Also, when filled with bonding compound the pilot hole provides additional bonding surface (about 11% more than the outer surface alone) and adds vertical stability to the reinstated core.
Other features of good keyhole coring and reinstatement units include a water tank, a core hoist, a high-pressure washer for washing down the finished repair, and some onboard storage.
Water is used to cool and lubricate the coring operation. A typical coring operation will consume about 10 gallons (about 1 gallon/inch of coring depth). Most coring units therefore have a 100-gallon tank. The supply of water reaching the coring drum and pilot bit needs to be adjustable to properly lubricate the coring operation and flush the coring residue or slurry out of the kerf, but not to flood the site or run off into storm sewers.
A typical 18-inch diameter core is 8 inches deep, weighs about 170 pounds, and can usually be extracted by a two-person crew with a pry bar inserted through the eye of the core puller. For larger cores some type of hoist will be needed. Some coring units are equipped with a hoist that can handle up to 500 pounds.
The Reinstatement Process
Although the cutting equipment is important, it is only part of the process. Equally important, if not more so, is the process of reinstating the core in the pavement. This is a key element in avoiding repaving costs and is the only true measure of the effectiveness of the process. Experience teaches us that to do a proper reinstatement in the shortest practical time, you need a field-proven, fast-setting bonding compound.
Because poorly repaired utility cuts are a prime source of potholes and premature pavement failure, most jurisdictions have established rigid testing standards for keyhole core replacement. For example, the city of Toronto requires that the bonding compound be capable of generating a waterproof bond that, within 30 minutes of application at 70º F, achieves an equivalent traffic loadable condition that is at least two times greater than the American Association of State Highway and Transportation Officials Standard H-25 (now HM-27) or 30,000 pounds.