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Sawing Concrete - Sidebar

Why Not Do It Yourself?

There are many good reasons not to rent a cutoff saw and tackle the job yourself. A major difference between dry and wet sawing is that a dry blade cuts by grinding. Forcing the blade to cut too deep or too fast will quickly wear it out. In fact, the blade should never become hotter than you can touch with your bare hand, demanding a slow, measured technique and repeated shallow passes. New blades cost $200 and up. Burn one out and you buy it. Unless the rental outfit thoroughly instructs you in the proper cutting techniques, you could end up damaging the tool, the saw blade, or worse, yourself. A hand-held saw weighs 25 to 30 pounds and becomes harder to control as you get tired. And if you should encounter a piece of rebar that just happens to align with the cut, forget trying to get through it. Without the surrounding concrete particles present in a cross-cut to mix with and clear the metal filings from the abrasive segment, you'll soon destroy the blade. But how eager are you going to be to relocate the cut and start over? By contrast, wet sawing is just that — a steady flow of water flushes the slurry from the cut, allowing the diamond abrasives to aggressively and rapidly saw the concrete. It's a faster, cheaper, and higher-quality method. Another issue is that the type of aggregate used in concrete varies in hardness by region. In Florida, for example, soft limestone is the local material, and saw blades go through it like butter compared to concrete mixed in New Hampshire, where hard, igneous rock is plentiful. The saw blade that comes with your rental saw may or may not be suited to the local aggregate. A concrete sawing specialist will have the right blade for local conditions. A concrete saw blade's segments are produced in three hardness groups. Segments are made with a mix of relatively soft metal and industrial diamonds in various degrees of friability. When the segment is properly matched to the material being cut, fresh, sharp diamond surfaces are continuously exposed in the process of sawing. Interestingly, the harder the material, the softer the segment should be. Green concrete calls for a hard segment, because the sand and aggregate are not yet fully settled and locked into the mix and so produce an abrasive grit that would rapidly degrade a too-soft segment. Fully hardened concrete presents a homogenous mass that yields more readily to the cutting action. A soft segment wears better in hard material, exposing fresh cutting edges. It's also important to have a sufficient, directed flow of water in the cut. Merely wetting the surface to control dust can actually create an abrasive slurry in the groove that will kill a blade in short order. A wet saw is designed to throw water into the cut by centrifugal force. The slurry is constantly flushed out, effectively cleaning and lubricating the segments as they cut. The dust produced by dry cutting is a health hazard. Silicosis is a chronic disease of the lungs, caused by inhalation of silica dust, which is often present in the aggregates used in concrete. A good dust mask is a must when you're grinding concrete. Protective eyewear is just as important.

— C.S. "Jack" Parsons, Jr.


Jack Parsonsis a concrete sawing and drilling consultant for P.M.A.C. and a U.S. agent for Dimas Lamage, an industrial-grade diamond tooling manufacturer. He can be reached via e-mail at jackpmac@capecod.net.