Types of Saws
There's basically two kinds of concrete saws - hand held and walk-behind. The handheld saws are lightweight and more portable than a walk-behind saw. They can be used to cut wall openings as well as smaller surface jobs like curbing or access holes for placing posts in an existing slab. Their ability to make straight cuts on longer runs is not as good as the walk-behind floor saws and they also mean you have to bend over and get closer to the dust when working on floors, which adds to operator fatigue.

Walk-behind saws allow you to stand up while sawing. They also have faster cutting rates because they have more horsepower and you can apply more force when cutting. The heavier weight of the floor saws helps to hold the saw down into the cut rather than floating up out of the cut like a lighter hand-held saw would. Because they have wheels and a guide pointer it is much easier to follow a chalk or crayon line and maintain a straight cut. Walk-behind saws do require a bit of room behind the blade to place the saw on. In order to cut this last 4' piece you'll have to turn the saw around later on to finish up the last 4'. If you are cutting short runs, such as control joints on a sidewalk, you will be better off with a hand-held saw. Also, walk-behind saws have the blade mounted to one side so that they can cut reasonably close to a wall. Note that if you do need to do this you won't be able to turn the saw around later to cut that last 4' or else the blade will be 2' away from the wall. If you have an open slab or are not cutting that close to a wall that is not a problem though.

Concrete saws are normally fitted with gasoline powered engines. This means they produce carbon monoxide fumes and can't be used in a basement or other confined space. Carbon Monoxide fumes settle and by the time they reach the height of a basement window you'll be dead. Fans don't help when the window is high. If you have a garage floor, factory area, or any other area where floor level ventilation is found you can use fans to control the exhaust fumes.

If you are not sawing that much, a hand-held electric cutoff saw is available. This eliminates any exhaust fume problem. However, because it is electric, you can't use any water to hold down dust so you will be cutting dry. The dry dust does get all over the place so it's not a good choice in basements. You can use fans to ventilate but you'll find that the saw creates more dust than the fan can remove so you'll spend most of your time stopping for the dust to clear so that you can see where you're cutting.

A better bet for sawing in enclosed spaces is to use a pneumatic saw. Pneumatic saws use compressed air to operate and are available in both hand-held and walk-behind versions. They can be used wet or dry, but typically are used wet for better dust control. The disadvantage is that you need a large towable air compressor to power the saw, which adds to the rental cost. This unit sits outside and large diameter air hose is run to the basement or other sawing location. This is the best way to saw concrete in an enclosed area.

If you need to cut completely through a poured concrete wall you may want to use a chain saw. No, not your average chain saw :)  We have a pneumatic powered chain saw with a diamond chain that is designed strictly for cutting concrete. The chain saw can cut perfectly square corners and plunge cut through a 15" wall in one pass.

Saw Blades
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 14", and wear down in size as it cuts. Eventually it will be too small to cut at the desired depth and it is discarded and 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.

Helpful Cutting Hints
When cutting concrete it is important to know just how deep you need to 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 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.

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.

Asphalt Cutting
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.