The rule of thumb here is the smaller the drum diameter the faster the rotation, and hence the large the drum diameter the slower the drum rotations. If a drum is set at too high a rotation speed for its diameter, the diamond segments will be a "glazed", or polish, and new diamonds will cease to be exposed, and the cut will take too long. Running at too slow a speed will result in premature erosion of the diamond segments and will reduce the bits life, and result in a higher cost per cut.
Below are some general guidelines for setting the drums R.P.M. to get you the best penetration rate and the longest core drum life.
12" diameter core drum: 200 rpm to 250 rpm
18" diameter core drum: 170 rpm to 210 rpm
24" diameter core drum: 150 rpm to 180 rpm
Coring Drum Segments
All coring drum segments are essentially a matrix which is impregnated with synthetic diamonds. As the bit is driven into the sub straight the segments grind away the material being cut. Obviously in a keyhole environment we are often cutting through two very different materials: asphalt and concrete. The asphalt cutting creates slurry that is very abrasive, and contrary to common sense, even though asphalt is a softer material than concrete it will be more abrasive on the segments than that of concrete slurry.Utilicor, through years of in-field testing, has developed a segment that provides its end users with the best of both worlds. Our ProCor coring drums are equipped to handle both asphalt and concrete and will provide quick and efficient core cutting action with extended drum life. This results in more cuts per drum and reduced cost per core cut.
Feed and Speed
Feed and speed are the two variables the core cutter has to keep in mind when cutting a core. The optimal rates will differ depending on what you are cutting through. With this in mind, all of Utilicor’s equipment comes with a feed gauge and a rotational pressure gauge. Due to our equipments ability to isolate the pressure of the coring drum to the cutting action, we look for 500-700 PSI of down pressure and 800 - 1200 PSI of rotational back pressure on the drum. This will result in a penetration rate of approximately 1 inch per minute, through asphalt, concrete or composite roadways and sidewalks. If you are taking longer than this, one or more parts of your cutting equation need to be adjusted.
Water for drum cooling and slurry removal
Core drums can be formulated to "dry" cut, but in the keyhole process we recommend always using water to cool, lubricate and clear the slurry away which is created by the cutting action, and keep the dust controlled. How much water is proper amount? Utilicor recommends a ratio of approximately 1 gallon per 1 inch of core cutting. Too much water will clear away all the slurry too quickly, and it’s that slurry that helps to expose the new diamonds in the segment during the coring process. Too little water will result in a thick slurry that will cause to cores getting stuck inside the core barrel. Adjust your water flow to keep the kerf area clear, but not so much that you flood the street. As a rule of thumb, for a 10" thick core, look to use about 10 gallons of water.
As you core it is impossible to "see" when you are through the roadway or sidewalk. But there are a number of "tricks" of the trade that will help you best guess when to stop cutting. The down side of coring beyond the depth of the sub straight is the likelihood of the core becoming stuck inside the drum. More debris will be created as you cut through the loose dirt below the pavement, and that may get jammed up inside the barrel, causing the core to become wedged.We have five senses, and you will use a few of them to tell when it’s time to stop coring and check that your core is ready to be extracted.
As the Utilicor core cutting process is designed to incorporate a center pilot bit which simultaneously cuts a center pilot hole through the center of the core, that pilot bit actually extends approximately a ½" longer than the core barrel. As that center bit cuts through the bottom of the core, there will be an auditory change, and you will know the bottom of the coring drum needs only to cut another ½" or so.
The slurry from the cutting of asphalt or concrete is distinct and consistent. As soon as the core drum cuts through the bottom of the roadway or sidewalk, the slurry coming up to the surface will look different. Watch for the change, and then you will know you are through.
All Utilicor coring equipment is outfitted with a proportional orbital feed system which gives the operator real time feedback from the coring operation. Cutting through different materials as you core will "feel" different. Once the drum penetrates through the asphalt or concrete the orbital feed system will become easier to turn, and you will know you are through. To check that the core is ready for extraction, stop coring and stick a pry bar down the center pilot hole in the middle of the core and gently try to rock it back and forth. This will ensure the cut is all the way through, and it will help beak the suction that occurs between the bottom of the core and the base of the roadway.
Extracting the Core
Cores are heavy.And larger diameter deeper cores are very heavy. Always use caution when extracting these from the roadway.Utilicor’s specially designed core puller is the safest way in the industry to extract a core. The rubber stopper will expand inside the pilot hole and lock in place creating a solid hold on the core. Through the eye bolt mounted on the top of the core puller insert a 5 foot pry bar and get a crew member on either side to lift. Make sure you lift with your legs - and never with your back. For cores too heavy for two crew members to lift, use a truck mounted hoist or hook the core puller to small skid steer or backhoe and lift.
Storing and Moving the Core
Cores are round, and as such they roll. This is helpful to keep in mind when you need to move one from the middle of the road to the side of the road. Gently tip on its side, and roll away. If you need to store the core for an extended period of time, and it is a hot day, store it upside down on a flat surface. The uneven nature of the bottom of the core inherent with most cores will result in full asphalt cores sagging and deforming on hot days.
Not every core cut will be perfect. Roadways, being what they are, have often been overlaid (resurfaced) with new layers of asphalt. Sometimes entire new roadways are laid over old roadways. Every time you core it’s a bit of a crap shoot as to what you will find. In the case of a core that has separated from its base, be that many different layers of asphalt, or the asphalt has separated from a concrete sub base, use the core puller stopper to span the separated pieces, and pull the core out in one piece.
Marking Cores for Reinstatement
Because you will be saving the core to be reinstated at a later time, it is important to make its orientation in the roadway before you extract it. We recommend using white marking paint with two intersecting lines that extend over the area of the core. The letter V works best, as there is only one way to put it back to make the lines match up. This is as opposed to an X, which due to being symmetrical can result in misinterpretation of its original orientation.In areas when you will be cutting multiple cores, paint a number on the top of the cored area and mark the roadway with the same number. Reinstatement is simplified when you know which core goes in which location.