Keyhole Basics: Coring 101

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

Too much down pressure leads to premature diamond loss and could even cause the segments to break-off or the drum to jam. The cutting process through typical pavement should take approximately one minute per inch of pavement thickness. If it takes longer, then likely the segments being used in your specific application or your coring procedures are not correct.

Method for extracting the core
Now that you have selected the appropriate equipment to successfully complete the coring portion of the job, the next challenge is how to safely extract that heavy core from the roadway.

An 18-inch diameter core, 10 to 14-inches deep, can weigh almost 175 pounds. Besides the weight, pavement cores sometimes have the tendency to delaminate – or separate --between the asphalt top layer or layers and the concrete base. Sinking a lag bolt or the like into the top of a core only lifts out that top layer and is an unsafe procedure as it can easily pull out of the core and cause injury. This is most prevalent on warm days when the asphalt surface becomes soft.

Core Extracted with Core Puller

The best way to extract the core is to drill an access hole through the center of the core and use a core lifting device. This can be done with a bar hole or hammer drill either before or after coring process. The better approach is to use a coring drum that is fitted with a pilot drill in its center that can cut the center hole at the same time that the core is cut. (Think of a hole saw for the installation of a door knob assembly). This central pilot hole, not only stabilizes the coring operation, but allows for the insertion of a special core puller tool that extends through all the layers, right to the bottom of the core. It has a rubber stopper that expands, friction-tight, inside the pilot hole, allowing the worker to quickly and easily lift the core out by himself. If it is too heavy, a pry bar can be inserted through the ring at the top of the core puller, and the lifting load shared with a co-worker. For really heavy cores (a 24-inch diameter core, 16-inches deep can weigh 450 pounds), some coring units come with an electric hoist or crane.

Bonding compound to reinstate the core
The next step is the process of core reinstatement after the underground work has been completed.