Choosing the right cutting blade is just as important as choosing the right saw. When properly matched to the job and the saw being used, diamond blades cut fast and provide long service life. But using the wrong blade or a saw that turns the blade at improper speeds can shorten blade life and raise cutting costs.
Because diamond blades come in a wide variety of sizes and performance levels, selecting the right blade is not an easy task. Blade costs can vary widely and stretch into the thousands, depending on quality. And although it may be tempting to buy a lower-cost blade, in the end it will likely be wiser to buy a more expensive, high-performance blade that’s designed to last longer.
Several factors affect blade life, including blade quality, characteristics of the concrete being cut, and saw speed. Knowing how a diamond blade works and the characteristics that affect blade life and performance can help you improve productivity and economy.
Diamond Blade Basics
Diamond blades are the go-to choice for cutting concrete, no matter if it’s green or fully cured. They “cut” by grinding away at the surface of the concrete, using diamonds as an abrasive. The metal core, which is a steel disc that attaches to the saw, holds diamond crystals in place at the outer edge. These diamond crystals, which are synthetically grown to specific sizes, shapes, and toughness, are what actually grind through the concrete. The blade’s edge can be continuous, serrated, or segmented. Segmented blades have slots cut into the core that allow water and air to flow, providing faster cooling. The diamonds can be bonded to the blade’s edge in varying concentrations. A blade with a greater diamond concentration will typically cut longer.
As the blade is manufactured, pressure and heat embed the synthetic diamonds into a metal matrix, which holds the crystals in place. The matrix is either laser welded or soldered to the perimeter of the core. Diamonds are set in layers within the matrix from the core outward. With each rotation of the blade, the metal matrix wears away slowly, then new layers of diamonds embedded deeper are exposed. The matrix is manufactured in a range of hardnesses, each meant for different cutting applications. The harder the matrix, the longer diamonds will stay bonded to the blade.
Concrete Affects Blade Wear
The best way to extend the life of your blade and achieve maximum cutting efficiency is to use a blade suited to the material being cut (3). Concrete’s compressive strength, aggregate hardness and size, sand type, and amount of steel, and whether the concrete is green or cured all impact blade wear and performance. The best time to cut a joint into flatwork is when the concrete is green—one to two hours after finishing, or the next day—because it is softer and easier to cut. Green concrete, though softer than the same concrete after it’s cured, is also more abrasive because sand in the mix hasn’t bonded to the mortar. It’s for this reason that blade manufacturers make hard-bonded diamond blades specifically designed for cutting green concrete. Sawing through cured concrete, however, requires some investigation.
Cutting into dense, hard concrete of 6,000 psi or more or into concrete containing hard aggregates causes diamond particles to fracture faster. That’s why the type of aggregate in the concrete mix is an important factor in blade selection.
Hard aggregates like flint and granite are difficult to cut. Dense concrete or concrete with a hard aggregate should be cut using a blade that’s equipped with a softer metal matrix. The soft metal matrix wears more readily, exposing new layers of diamonds, which makes the blade cut more efficiently).
Aggregates like limestone, slag, and coral are relatively soft and abrasive and are the least difficult to cut, as is green concrete. A blade with a hard metal matrix is the best choice here because it will hold the diamond particles longer.
Along with the type of aggregate, it’s also helpful to know its size. The larger the size, the slower the cut and slower the blade will wear. Aggregates smaller than 3/8 inch are easier to cut, but they wear the blade faster. Concrete comprised of large flint aggregate is much more difficult to cut than a mix with small flint aggregate in it, for example. Some finer aggregates can wear a blade quickly. Natural quartzite sands, which have been rolled around by water for years, are smooth and dull, but manufactured sands have sharp, abrasive edges that wear the blade quickly.
Cutting rate is slowed by harder aggregates like flint and granite, which require more power and shorten the life of a blade. Segmented blades built with tough diamonds and a soft metal bond matrix are the best choice for cutting hard aggregates. A blade with a hard metal bond matrix will cause the diamond particles to wear even with the bond surface; the blade will glaze over and will no longer cut. Segments for cutting soft aggregates should have a hard metal bond matrix so that the diamond crystals don’t wear before their cutting life is up.
Wet vs. Dry Blades
Blades are available for either wet- or dry-cutting applications. Dry-cutting blades can usually be used with or without water and are designed to work in higher temperatures. Wet-cutting blades, however, should not be used dry, because they’re designed to be operated with a continuous spray of water. Using a wet blade dry can result in flying debris as segments and teeth become disengaged from the blade.
Wet-cutting is often the best method to use. Water cools the blade, reducing heat build-up and extending blade life. Water also dampens cutting fines and eliminates dust. Some jobs require dry cutting, like when you’re using an electric saw. Cold weather can make wet cutting impractical as well because of potential freeze problems. Wet-cutting is necessary when using a concrete-cutting chain saw.
When cutting dry, don’t cut more than 1 to 2 inches deep in one pass, and allow the blade to cool periodically. Doing so will keep the blade from overheating; overheating can shorten the blade’s life.
Match the Blade to the Saw
While considering the material being cut, you also need to match the blade to the saw in which it will be used. The chart below is a good guide, but in general you can cut faster and extend blade life using lower-performance blades on low-horsepower saws, and higher-performance blades on high-horsepower saws.
Blade-shaft speed is another consideration along with horsepower. A saw that rotates the blade at the wrong speed can cause the blade to wobble, warp, and wear excessively. For most saws, blade-shaft speed (measured in RPM at no load) will be higher than the recommended operating speeds shown in the chart. Under normal sawing conditions, the actual shaft speed will slow under load and fall within the optimum speed range. Before purchasing a blade, tell the supplier the model and rpm of the saw you plan to use.
You can maximize blade performance by using the right blade in the right operating conditions, such as the speed of rotation and the material being cut. Manufacturers design these blades for specific purposes and offer recommended maximum operating speeds based on blade size, saw type, and the material being cut.
Increasing operating speeds can make the blade perform as if it is harder than it actually is and will therefore lengthen its life—but higher operating speeds also slow down the cutting. The converse is true, too: Decreasing the blade speed decreases the blade life. When in doubt, start with a lower speed and work up to an increased speed once the blade is cutting well.
All blades these days come with a lettered Blade Application Code developed by the Concrete Sawing and Drilling Association
(csda.org). The code indicates whether the blade is rated for wet or dry use, the type of surface being cut (cured vs. green concrete, for example), as well as what type of saw the blade is meant for. Blade manufacturers offer blade-buying guidance within their blades’ specs, via an online buying guide with drop-down menus, or both.
Chris Ermides is a senior editor for Tools of the Trade.