Whether newly laid or poured or old and in need of replacement, concrete and masonry may require cutting for size, shape, expansion, fit, or repair. Learn how to properly cut concrete and masonry with diamond blades.
Reasons To Cut Concrete and Masonry
Newly poured concrete requires both control joints and expansion joints. The terminology can be confusing because cuts that are made to control cracking as drying concrete shrinks are often erroneously called "expansion” joints.
A better term for the shallow cuts made in new concrete is “control joints.” Pros make these cuts in newly poured concrete between 3 and 18 hours after the pour, but never longer than 24 hours after. If the concrete is cut too soon, it will cause “raveling,” jagged edges along the cut.
Control joints create weaker spots in the concrete to isolate the inevitable cracks that occur as concrete dries. The cracks tend to form along the cut rather than in the middle of the newly poured concrete.
Expansion joints allow for motion in the concrete when it heats in hot weather or absorbs moisture from precipitation or humidity. These joints go all the way through the slab and are filled with a compressible or composite material, such as silicone, asphalt, or rubber. The joints allow different sections of the concrete to move independently when they expand due to heat or shift due to movement in the earth beneath.
Choosing the Proper Tools
The hardness of the material you’ll be cutting dictates most of the selection of the tool to use. Cutting with diamond concrete blades is the industry standard. Cheaper blades made of corundum, a mineral composed of a type of aluminum oxide, can manage small jobs with few cuts but wear out quickly, need frequent adjustment, and tend to overheat.
Diamond concrete cutting blades last longer, make more precise cuts, and usually have serrated edges, often with gullets that help withdraw waste from the cut. Over time, they’re more cost-effective than cheaper blades.
Diamond blades have synthetic diamonds fused to the core of the blade. The bond is critical to the quality of the cut. Hard materials require a softer bond because the bond wears away to reveal new, freshly sharp diamond crystals as you use the saw, keeping the cut sharp and precise.
Selecting Wet or Dry Cutting
Concrete diamond saws are made for dry or wet cutting. Dry cutting creates a lot of dust, and the blades can overheat and become clogged with debris unless you periodically lift them from the cut to cool off and shed the material that builds up in the gullets of the blade. Dry cutting is better for small jobs that generate a limited amount of dust.
Concrete dust is toxic to breathe, containing particulates that can lodge deep in the lungs, causing respiratory problems and even cancer. Wet cutting uses water to reduce dust and cool the blade.
To cut concrete pavement, gasoline-powered walk-behind saws are standard. These saws have a reservoir that’s filled with water and a pump that sprays the water continuously on the blade and the cutting surface for cooling and dust control.
Understanding Safety and Personal Protective Gear
Before you do any cutting, determine if the concrete you’ll be working on contains rebar or if any gas, water, or electrical lines run through or under it. Call the local number in your area to ask someone to come out and mark the locations of such lines.
Personal protective equipment (PPE) is essential. Dress in heavy coveralls, steel or composite hard-toed boots, and eye and ear protection. Above all, use a respirator that is rated to protect you from inhaling small particulates.
Inspect the work area for any potential hazards. Clear away obstructions and tripping hazards such as indoor or outdoor furniture and rugs, planters, or non-essential cords. Ensure the work surface is free of debris like gravel, wood chips, or sand.
If you are using an electric-powered saw, ensure you have a ground fault circuit interrupter (GFCI) outlet close enough for the job. If you must use an extension cord, make sure it is one with integrated GFCI outlets.
It is critically important that no children or visitors can enter your work area. You might not be able to see or hear someone walking toward you, and a serious or fatal injury could result if you lift your saw at the wrong moment. Mark off your area with bright work zone tape and barriers.
If you’re working inside, drape the area with thick plastic, turn off the HVAC, and seal any vents that lead to other parts of the building to avoid spreading dust beyond the work area. Outside, cover trees or shrubs that could be coated in dust during your project.
Measuring and Marking
Use thick chalk lines (especially if wet cutting, as the chalk will rinse away as you go) to mark your lines of cutting. Use a guide board to further indicate the edge of the line you want to cut.
Cutting the Concrete
Unless you’re using a large, 14-inch or larger diameter walk-behind saw capable of deep cuts, make several passes of shallower cuts that cumulatively add up to the depth you need.
Set the depth on your saw to initially make a shallow cut. Make sure the saw is not touching the surface of the concrete or masonry when you turn it on. Set your saw to its lowest RPM setting, turn it on, and gently lower the saw to the cutting surface.
Let the saw do the work without pushing forward or down. Just move the saw gently along the guide board for 45 seconds or until you’ve completed one continuous shallow cut, whichever comes first.
Remove the saw from the cut and allow it to spin to shed debris and cool down. Then, proceed in 30- to 45-second intervals of cutting and lifting until you finish your guide cut.
Once your first cut is done, you can remove the guide board and use your straight cut as the guide for additional cuts. Do this until you achieve the depth you need, adjusting the depth setting of the saw as you go.
If you’re cutting concrete to remove damaged material and add new concrete alongside it, you may need to switch to a sledgehammer and a chisel to get all the way through the slab near the cut. Turn your saw off and store it carefully, ensuring cords are properly coiled and stowed before you bring out the manual tools. New concrete will adhere better to a jagged edge.
When your project is complete, and your saw safely stored, go over your work area again with a shop vac. Remove chunks of concrete you cut up and dispose of them properly according to local solid waste disposal regulations.
Properly cutting concrete and masonry with a diamond saw blade requires attention to safety and detail. If you don’t have the necessary time or lack the inclination or confidence to do the job yourself, hire a pro to ensure the project is done safely and correctly.