Crack management

Joints cut in concrete manage the inevitable cracking process. Here’s how to help concrete crack where you want it.

“A concrete joint is really a controlled crack,” explains Rick Norland, trainer for the Concrete Sawing and Drilling Association. “The trouble is, cutters are the last to go on the slab and often get blamed for everything that goes wrong with the slab, even though it was not their fault,” he says.

Norland speaks from experience. He’s a mechanical engineer by training and has cut miles of concrete as a trade. With all of those hours behind a concrete saw and through his follow-up work with contractors and owners, he can identify the problems projects face when the inevitable cracking process ends up somewhere other than at the straight joint.

Joints “make” the weakest point
“The joint is the Achilles heel of a slab. It provides a plane of weakness to ensure that when the slab shrinks during curing, the crack develops in a straight, predictable line. It is a sign to the concrete to ‘crack here,’” he says.

However, making a joint in concrete can also make the slab vulnerable. Improperly cut joints can spall, creating a lip or larger gap that makes for a rough ride or a safety hazard. A misplaced joint will not prevent the concrete from developing a crack elsewhere in the slab, resulting in a crooked, weaving fracture that affects the concrete’s beauty and longevity.

Factors affecting joint cutting
For best results, concrete must be cut at the right time in the curing process. However, many factors affect concrete curing. That can make joint cutting a slow, less precise process. “If you ask questions upfront, you will have a better idea how fast the concrete will cure before it cracks. This affects the joint-sawing window,” he says.

The most common factors affecting cure time include sudden temperature changes during placement or curing; or additives such as fly ash, ground granular blast furnace slag (GGBFS), set retarders or accelerators.

A little bit of knowledge can go a long way in understanding the joint-sawing window. “Ask if fly ash or other additives that can affect the cure rate have been added. Ask if the concrete is coming from the same plant or different plants. If fly ash is used in the mix, there will be inconsistencies between the two plants and it will affect cure time,” he says.

Hitting the sawing window takes some experience. Norland suggests taking a pocket knife, key or nail and do a scratch test on the curing concrete. “If the surface aggregate can’t be scarified, it is time to cut,” he says, “If the joints begin to ravel during cutting, you need to wait.”

The slab cures from the bottom to the top. If the air temperature is very cool, Norland recommends cutting every third or fourth cut to relieve stress, then come back and make the other cuts.

“Fly ash slows the curing process; so does GGBFS, but when it cures, it cures fast. Other set retarders push out and lengthen the curing window while set accelerants speed up the process, so you may need more saws and more manpower on large jobs,” he says.

Accelerants are often used in late-season pours. If the concrete has been covered to help the curing process, you can pull the blanket back, make the cut, then put the blanket back over the concrete.

Joint depth drives cracks
Depending on the application, joint depth will vary. “Typically, joints are designed to be D/3, D/4 or D/10 cuts where D equals the thickness of the concrete. D/10 cuts are usually made with dry-cutting equipment. The deeper cuts are made with walk-behind wet-blade units that can cut two or more inches into the concrete,” he says.

Norland says research reveals that a D/6 and D/7 joint is the least amount of cutting depth needed to prevent uncontrolled parallel cracking over time.

The challenge of making an effective joint is cutting it to the best depth and width without it spalling. To do that with a dry-cut saw, use the biggest saw possible.

If you are doing a shallow cut, such as D/6 or D/10, an up-cut saw should be used. If you are sealing the joint, the cut must be 1/4" wide; an up-cut saw also removes more dust from the crack for better sealant bonding.

The cutting process
Diamond saws wear as part of the cutting process. As the blade wears, the operator must adjust the blade to assure it is making the cut to the proper depth. “This is a big problem because some operators forget to check cutting depth. That can lead to cracks in the wrong areas,” he says.

Blades that act hard will last longer, but they cut slower and cause more spalling. Blades that act soft cut faster but wear out faster, so you must balance speed with blade cost and spalling concerns.

Depending on the cutting method and joint depth, step cutting may be needed to reach the proper depth.

“If you are green-cutting concrete, don’t step cut because you can’t cut straight on the second cut.

“In cured concrete, the saw should be operated to get 10' of cutting per minute. That is the most efficient cutting speed. If you go slower than that, the operator’s attention span suffers; if you go faster than that, the saw blade wears too fast,” he says.

Manage spalling
Spalling may be one of the few things the saw operator can control in the whole paving operation. “Spalling is more affected by saw condition and cutting window than by depth of cut,” says Norland.

With a down-cut saw, concrete will spall when there is not enough concrete to hold the aggregate in place. “It is exacerbated when the saw is poorly maintained. If the saw pounds while down-cutting, the unit is misaligned and likely needs maintenance. Worn bearings, worn wheels and spindles that are out of alignment or are worn can also cause cutting problems,” he says.

On up-cut saws, concrete will spall if the exit forces go into uncut concrete. Up-cut saws tend to cause less spalling than down-cut saws because the cut fully supports the aggregate. Smaller-hp units are typically up-cutting saws.

Hydraulic undercutting can also cause spalls. “This happens when too much water is applied to cool the blade. The blade spins at 16,000 sfm, which creates a pumping action of the water into the crack at pressures as high as 3,000 psi. This much pressure can blast a cavity into the cut and weaken the concrete next to the cut so that it eventually spalls. To reduce spalling, use the least amount of water needed to cool the blade,” he says.

Pushing the saw too hard can cause spalling. “You know Bubba is pushing too hard on the saw if only one side of the cut is spalled. Operators should push on the left side of the machine to accommodate the pull of the saw on the right side of the machine,” he says.

 

Concrete sawing tips In the many years that Rick Norland has been working with concrete, he has identified several tips that can make concrete work faster and more accurate. Follow these tips and you’ll likely have fewer or no cracks where you don’t want them. • If the slab will be installed over a lean concrete base (large aggregate, little cement) that has not had joints installed, reflective cracks will appear in the slab regardless of joint location and spacing. A joint should be installed in the lean concrete base, and the joint in the slab should be directly over the lean concrete base joint. • If you are laying new concrete over old, working cracks in the old concrete will reflect through the new concrete. You must tie down the base or make cuts that account for them in the new material. • If the concrete is only going to be 2" or 3" thick over asphalt, you will need to saw extensively to accommodate cracks in the asphalt. • If there are any changes in foundation thickness, a joint will be needed where the thickness changes. • Decorative radius cuts must be treated as such: they are for decoration only. A decorative cut isn’t a crack-control joint, so it’s important to put the crack control joints in first. To make the decorative radius cut, come back with a narrow diamond blade on a saw with its rear wheels positioned as close as possible to the blade. • Interior posts or other building materials that create a sharp notch in interior concrete will create a crack. To manage the cracking on floors inside buildings, put an isolation joint around the column as shown at right.

Published in the November/December 2003 issue of Contractor Tools and Supplies magazine.

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