Getting the best from your Concrete Saw blades
Concrete Saw Abrasive Blades and Diamond Blades – what's the difference?
Unlike wood sawing, which uses toothed blades to chip away and actually cut the wood, concrete sawing is accomplished by grinding away at the concrete. This means that both the blade and the concrete will wear down as they grind away at each other.
Abrasive blades can be used and are the most cost efficient for short jobs. The blade will start out at it's full diameter, which is normally 12-16", and wear down in size as it cuts. Eventually it will be too small to cut at the desired depth and it is discarded, then a new blade is put on the saw.
Diamond blades, on the other hand, do not wear down in size appreciably as they are used. The diamonds are laser welded on in segments to a steel hub. As the diamonds wear out they drop off into the cut and a new row appears. Eventually the segment wears down to the point where the blade must be replaced. Diamond blades are very expensive compared to abrasive blades but they do last quite a bit longer and they hold their depth of cut so on larger jobs they are the way to go. Diamond blades are also thinner than abrasive blades. This means that you'll be cutting faster because you have less concrete to remove when cutting due to the narrower swath. Because there is less concrete being removed there is also less dust being generated – although when cutting with water it's doubtful you'd notice the difference.
Concrete, like anything else, expands and contracts when it gets hot or cold. Then it will crack because concrete isn't all that flexible. For this reason control joints are cut into the slab every 10' or so that there is a weak spot for the crack to find. A rule of thumb is to cut 1/5th of the slab thickness so normally a 4" slab is cut at a depth of 1". When the slab wants to crack it will find the weakest spot and crack right beneath the control joint. Then the cracks aren't as visible.
When cutting concrete it is important to know how deep you need to make the cut. Generally it is not necessary to cut all the way through the material. If you are removing existing cured concrete the saw cuts are normally only 2" deep on a 4" slab. This sufficiently weakens the cement and leaves a smooth joint at the surface where it is visible. Once you use an electric breaker or air hammer the rest of it will break off right beneath the saw cut. Furthermore, if you plan on replacing the removed concrete with new concrete later on the rougher edge at the base of the cut will help bind the new slab to the old slab and prevent frost heaves which leave a lip where the two slabs meet.
Asphalt saws quite differently from concrete. It is softer and more abrasive. In addition, it gets hot and melts in, pinching the sides of the blade. Water will help quite a bit in cutting asphalt but it can be cut dry if need be. Diamond blades are available for asphalt and are of a different matrix than the concrete cutting blades. For short-term users abrasive blades are a good choice. Asphalt over concrete can use a general purpose blade but asphalt over gravel takes extra care to cut. If you cut through the blacktop and the blade contacts the gravel underneath the blacktop the gravel will tumble and act as a grinding wheel dresser and wear out the blade very fast. If you set the blade depth to not quite go all the way through the blacktop it will easily break off when you remove it and your blades will last much longer.