Saw Cuts in Concrete
Saw cuts are a way to generate joints in concrete. Saw cuts should be done at predetermined spacing to control cracking due to shrinkage. It is really important to cut concrete after it has obtained enough strength to keep it from raveling but before cracks could be initiated internally. Timing of saw cuts is essential to avoid cracks in concrete before cutting. Determining when to saw cut concrete depends on the type of admixtures, temperature, mix design and aggregate type.
When to Saw Cut
Saw cut timing depends on several factors such as hardness, blade type, weather condition, concrete mix design and aggregate size. If you start saw cutting concrete to early, you might be causing raveling, while sawing too late will result in uncontrolled cracking. Under hot weather, saw cutting should start after an hour depending on how the concrete is reacting. The saw operator could try to perform trial cuts to determine if the concrete is ready to be saw cut. Saw cutting should start as soon as the raveling stops during these trial cuts. If the slab of concrete is too large, you might want to have several saws to work in several areas at the same time. Some contractors delay sawing to protect their equipment and saw blade to reduce blade abrasion. There are several types of saw blades that can be used depending on the concrete used and how fast these cuts could be started. Avoidjoint raveling by:
- Pushing the blade too hard
- Saw cutting at high speed
- Using a saw with a ben spindle
- Using the inappropriate saw blade
Determine Where to Saw Cut
Before you start saw cutting concrete, is important to select where these cuts will be done. It is recommended to start saw cuts on or at the center of column lines. Joints shall be spaced at 24 to 36 times the slab thickness but this need to be confirmed by a structural engineer. Joint spacing normally ranges between 10 to 18 feet depending on the amount of reinforcement the slab has. If you are using high shrinkage concrete, you might want to decrease the cut spacing. Here are other factors to will help you determine where to saw cut:
- Try to form square patterns
- Make saw cuts continuous
- It is important to select an area that doesn’t have continuous steel reinforcement from one slab or square to another
- It is better to have smaller cracks than to increase cost by having and maintaining several joints.
- Under hot weather conditions you might want to try forming larger square and then cutting interior joints. By doing this you will be controlling fast setting concrete cracks in larger areas.
How to Saw Cut
Once you know the joints to be saw cut, mark them using a chalk line. If using water cutting equipment, make sure that the water is running all the way down to the blade. Allow the blade to get its required depth and start walking or moving the equipment following the chalk like mark. Do not twist the saw blade. Do not let the blade spin in the cut, as this will increase wear on the bond. When cutting concrete with heavy rebar use blades with soft metal segment bonds. Use required PPE.
Saw Cut Depth
A good rule of thumb is to cut the joint one-quarter to one-third the slab thickness. Check that the saw cut depth is sufficient and according to the structural engineer recommendation. If the joint is too deep, aggregate interlocking will not be enough to transfer loads. However, when the saw cut is too shallow, random cracking might occur. Factors such as curing techniques, slab thickness, slab length and base type shall be analyzed before selecting where these joints should be cut.