1. Cutting Concrete with the Right Diamond Blade
For maximum cutting speed and blade life, you should match the blade as closely as possible to the material you're cutting. Characteristics of the concrete you need to know include the compressive strength, the size and hardness of the aggregate, and the type of sand.
Blade manufacturers consider concrete with a compressive strength of 3000 psi or lower a soft material and concrete with a strength above 6000 psi a hard material. Therefore, you should use a blade with a hard bond to cut the lower-strength concrete and a blade with a soft bond to cut high-psi concrete. Similarly, hard aggregate (such as trap rock, basalt, and quartz) dulls diamond particles quickly, so use a blade with a softer bond to allow new diamonds to be exposed as needed.
The size of the aggregate in concrete primarily affects blade performance. When cutting through larger aggregate (3/4 inch and up), the blade cuts and wears more slowly. Pea gravel (smaller than 3/8 inch) is easier to cut, but the blade will wear faster.
The type of sand determines the abrasiveness of concrete, with sharp sand being the most abrasive and round sand the least. To determine the sharpness of the sand, you need to know where it's from. Crushed or river bank sand is usually sharp while river sand is round and nonabrasive. The more abrasive the sand, the harder the bond requirement.
What if you will be cutting more than one type of concrete? As a general rule, manufacturers recommend choosing the blade based on the material you'll be working with most often or the material for which top blade performance is most important. Most diamond blades can cut a range of materials.
2. Cutting Concrete at the Right Time
If you're placing new concrete, you have the option of cutting control joints while the concrete is still green (about 1 to 2 hours after finishing) or the next day after the concrete has hardened. The timing of the cut will dictate the type of blade you select.
Some decorative concrete contractors prefer to cut concrete while it's still green because it minimizes the occurrence of ugly random cracking (especially in warm weather, when concrete hydrates faster) and permits shallower joint depths of an inch or less. However, green concrete will be softer and more abrasive than the same concrete in a cured state. That's because the sand in the mixture hasn't yet bonded to the mortar and it acts as an abrasive. Blade manufacturers offer hard-bonded diamond blades specifically for cutting green concrete.
3. Wet Cutting Concrete vs. Dry Cutting Concrete - How to Decide
Often the decision of cutting wet or dry depends on your preference and job requirements. Dry cutting eliminates messy wet slurry and the need to equip saws with water tanks and hoses. Using a blade wet, on the other hand, reduces dust but makes it necessary to contain or clean up the slurry. For indoor jobs where you need to keep the work area dry, a dry-cutting blade and compatible saw may be your only option.
The main difference between wet and dry blades is the weld (as described in Step 1). Dry-cutting blades have segment welds that resist heat and don't require water for cooling. They are usually intended for intermittent cutting and for use on handheld, low-horsepower saws. If you're sawing decorative pattern lines in concrete, dry-cutting blades are often the best choice for making crisp, clean cuts (see Sawing and Patterning). These decorative cuts are typically only 1/16 to 1/4 inch deep and do not function as control joints.
Wet-cutting blades are typically used with walk-behind saws for cutting joints in cured concrete flatwork because water cooling permits deeper cuts. Although it's possible to use most dry-cutting blades with water, never use a wet-cutting blade dry. Always continuously cool the blade with water to avoid segment loss and blade warpage.