The sole purpose of a saw and all its parts is to turn the blade in the cut. The performance of the blade is the most important consideration for cutting concrete, and professional operators know how to maximize blade life while getting the highest level of production.
The life of a blade or bit and the speed at which it cuts depends on a number of conditions. For manufacturers to specify the proper blade or bit for a job, they must be informed of these conditions. It is important for operators to recognize these variables and understand how various combinations of and changes in these conditions can affect blade performance. The following factors have the most influence on concrete cutting.
Type of aggregate. The type of aggregate has a pronounced effect on cutting. Hard aggregates shorten blade life and slow the cutting rate. So cutting concrete with a hard aggregate, such as quartz, will cost much more than cutting concrete with a soft aggregate, such as limestone. Cutting concrete composed of hard aggregates also requires more power. If there is not enough power, bit speed or blade speed should be reduced.
Blades and bits used to cut hard aggregates should have segments with tough diamonds and soft metal bonds. Otherwise, the diamond particles will wear down even with the bond, and the blade or bit will become glazed and unable to cut. Likewise, segments for cutting soft aggregates should have hard metal bonds so that the diamond particles are not lost before their cutting life is used up. Aggregate hardness can be measured using the Mohs Scratch Test, the Los Angeles AbrasionLoss Test or the Shore Hardness Test.
The size of aggregate is also an important consideration. Concrete made with 2inch flint aggregate will be extremely difficult to cut, but concrete with 12inch flint aggregate may be sawed easily. This is because there is more coarse aggregate per unit volume of concrete when a larger maximum size aggregate is used.
Reinforcing steel. It costs more to cut concrete that contains reinforcing steel because blade life or bit life is shorter and cutting rates are slower. If the crosssectional area of the concrete core is 1 percent steel, bit life will be about 75 percent of what it would have been if no steel were present. Concrete with 3 percent steel will reduce bit life by about 90 percent. In tests that assumed time costs of $30 per hour, total drilling cost increased about $2.50 per foot for every 1 percent increase in steel content.
Tip: When steel is encountered, reduce bit speed or blade speed and decrease water flow. Sometimes using interrupted pressure on a drill bit seems to speed cutting as well.
Operating speed. To keep blades from distorting at high speeds, manufacturers create blades in the form of a dish that will straighten when the blade is rotated at optimum speed. The dishing is small, about five ten-thousandths of an inch per inch of diameter. If the designed speed is not achieved, the blade will tend to wander as cutting commences. This dishing is called tensioning. When a blade is said to be out of tension, the amount of dish is not correct and the blade wobbles from side to side while out of the cut and wanders in the cut. An out-of-tension blade cannot be made to cut a straight line.
Manufacturers usually list the recommended operating speed directly on the blade. Recommended operating speeds for diamond blades are based on the blade size and the type of material to be cut. For optimum blade life and cutting speed, the actual operating speed will most likely have to be adjusted for the type of aggregate and the amount of steel encountered. Safe operating speeds can range from 4,536 rpm for an 8-inch-diameter blade down to 756 rpm for a 48-inch-diameter blade.