Placing and finishing
The remaining steps in fixed form pavement construction include placement, finishing, and curing. Although most highway pavements today are placed with a slipform paving machine, the most common type of concrete pavement construction remains fixed-form work, which can be as simple as setting up 2x4s as side forms. Finishing the concrete surface in this type of setup can be equally simple, screeding with a 2×4 or a vibrating screed. The main way to spread out concrete is directly from the truck chute, keeping a uniform level in front of the straightedge or vibrating screed.
After the initial strike off, and a waiting period for the bleed water to rise, the pavement surface should be textured. Texturing is typically accomplished in one of three ways: 1) using moistened or wet burlap dragged across the surface, 2) using artificial turf turned upside down and drug across the surface, or 3) using a broom to texture the surface. A high-traction texture on concrete pavement surface is important to avoid a slick surface. “The point here is to create some friction but keep it smooth enough so that it’s not actually rough, noisy, or uneven—smooth, but not glassy,” says Waalkes.
Importance of curing
After texturing, comes curing. Waalkes recommends, and most specifications require, that a layer of wax-based, membrane-forming curing compound be sprayed on the surface. This is liquid, typically pigmented white for easy visibility. Once it dries, the wax-based membrane seals in the water so that the concrete near the surface will not dry out but have plenty of water to complete its hydration. If the surface of the slab is not adequately cured, the water will evaporate before it has a chance to fully react with the cement, resulting in a lower-strength concrete near the surface where most of the wear takes place.
“Curing is critical to getting durable concrete,” says Waalkes. “Typically it simply involves spraying that compound on and sealing in all the water, and then you are good to go. This is a key point in the work.” But on some projects, such as a driveway pour, the contractor might choose to cure using a sprayer, fogger, or sprinkler once the concrete has started to gain a little bit of strength. This is a good way to cure, but the advantage of the curing compound is that it may be sprayed on as soon as the texturing of the concrete has been completed. “I personally think the curing compound is easier to use, quicker, and more reliable. Unfortunately it is not used as much for smaller jobs such as sidewalks. I wish it was. It would help us avoid a lot of problems,” says Waalkes.
Joints key to controlled cracking
Properly forming or cutting joints into the concrete slabs is one of the final steps on a project. The key is that the spacing between the joints is typically around 24 times the thickness of the slab, up to a maximum of 15 feet. So, on a driveway with a 5-inch slab, the maximum spacing between joints would be 5×24—120 inches or 10 feet.
The purpose of the joints is to control the natural cracking that occurs in the concrete due to shrinkage. “If joints were not used,” says Waalkes, “the cracks would just occur randomly, and we’ve found that typically they happen at about 24 times the concrete’s thickness. For this reason it’s good to put a joint at that location. The ultimate purpose of a joint is so that the concrete that you’ve poured will have no uncontrolled cracks.”
As soon as possible after placement, joints should be sawed into the slab to a depth of one-fourth to one-third the slab thickness. Joints should be just deep enough so that when the concrete starts to shrink it will want to crack there where the concrete is thinner.
Ready for traffic
Waalkes recommends that pavement generally be opened up to traffic in three to seven days. “Some people think that you have to let the pavement set up for a whole month before you can walk or drive on it,” says Waalkes. “The truth is, concrete can be opened up as soon as it has strength to withstand whatever is going to be driving on it. The traffic speed on the concrete after the pour depends on many factors, including temperature (the reaction occurs quicker in hot temperatures), mix design, and cement type.”
Whatever the requirements for the project, a concrete mix can be designed and proportioned to these specifications so the mix will gain strength at the right time. “If you want to drive on your concrete in four hours, there’s a mix that can do that,” says Waalkes. “There is an infinite number of possibilities. Just tell the concrete producer in your area what your needs are and you should be able to get that mix.”
—Pete Hildebrandt is a freelance construction and business writer based in South Carolina.
