These days, most joints in pavements and flatwork are sawn when the concrete is green and relatively easy to cut. There are many situations, however, particularly in restoration and repair work, where existing hardened concrete needs to be cut. Often, it's not a question of simply breaking up and removing the material, but rather of making precise cuts. It might be necessary to delineate an area that needs a patch repair, or you might need to create new openings in a wall for windows, doors, or utility lines. Maybe part of an existing structure must be removed to accommodate an addition.
Accomplishing any of these tasks requires the proper tools, techniques, and skills. Manufacturers have produced a variety of specialized saws, blades, and other tools to cut concrete in a range of conditions. Some contractors also have chosen to specialize in this business. The Concrete Sawing and Drilling Association (CSDA), Clearwater, Fla., which represents both groups, offers a variety of services to assist and promote the concrete cutting industry. CSDA has developed standards and offers training to help ensure the work gets done safely and efficiently. Its member directory helps interested parties locate providers of cutting services and equipment. It also publishes guide specifications for various types of concrete cutting projects to help ensure that the work is specified, supervised, and performed correctly. A great deal of useful information can be found at www.csda.org.
Diamond blade basics
One crucial step in the ability to cut hardened concrete safely and efficiently has been the development of diamond blade technology. Diamond crystals are grown synthetically to specific sizes, shapes, and levels of toughness. In the manufacturing process, pressure and heat are employed to embed diamond crystals in a metal matrix and form cutting segments. Then these segments get attached to the perimeter of a saw blade. The size of the diamond crystals, typically called the mesh size, determines the number of diamond cutting points exposed on the surface of a saw blade. In general, a larger mesh-size diamond is used to cut softer materials, while a smaller mesh-size diamond is used to cut harder materials.
The exposed diamonds on the surface of the segment do the cutting, similar to the way sand paper cuts wood. As the blade rotates through the material, the diamonds chip away at the material being cut. The metal matrix is designed to wear away slowly through many blade revolutions, exposing new layers of diamond crystals embedded deeper in the segment. The matrix is designed to wear at a rate that will maximize retention and protrusion of the diamond crystals so they can cut.
Manufacturers design blades for specific purposes and recommend maximum operating speeds based on the blade size, equipment type, and material being cut. The best operating speeds for cutting concrete are about 10,000 surface feet per minute (sfpm)—the surface speed of the diamond cutting segments on the periphery of the blade.
The correct operating conditions will maximize blade performance. In general, higher operating speeds make the blade act harder and tend to lengthen blade life, but slow the cutting. Decreasing blade speed makes the blade act softer, but also decreases blade life. When in doubt about the correct operating speed for a particular material, it's better to choose a lower speed rather than a higher speed. Once the blade is cutting well, you can increase the speed to optimize blade service life.
Choose the appropriate blade
When selecting or recommending a diamond blade, it's greatly beneficial to know as much as possible about the particular concrete to be cut. It's important to know the type of aggregate in the mix, whether rebar will be present, and how deep the cut needs to be. Being relatively soft and abrasive, limestone, slag, and coral aggregates are the least difficult to cut. River gravel and quartzite aggregates present medium difficulty, while flint and granite aggregates are considered the most difficult to cut.
Aggregate size also plays a role in a diamond blade's cutting performance. Concrete made with 2-inch flint aggregate will take a lot of effort to cut, but concrete with ½-inch flint aggregate may be cut more easily. The fine aggregate used to make the concrete also can affect blade selection. Natural quartzitic sands that have been rolled around by water for years are smooth and dull, but manufactured sands usually have sharp, abrasive edges that can wear a blade quickly.
Hard aggregates shorten blade life and slow the cutting rate, so cutting concrete with a hard aggregate will cost much more than cutting concrete with a soft aggregate. Cutting concrete made with hard aggregates also requires more power. To cut hard aggregates, use blades with segments that have tough diamonds and a soft metal bond matrix. Otherwise, the diamond particles will wear even with the bond surface, and the blade will glaze over and be unable to cut. Conversely, segments for cutting soft aggregates should have hard metal bonds, so the diamond crystals are not lost before their cutting life is used up. Aggregate hardness can be measured using the Mohs Hardness Scratch Test.
Tools used for sawing hardened concrete all make use of diamond blade technology, but their configurations and details differ based on the application.