Today's concrete pavements are being built smoother than ever before, providing an excellent riding surface while retaining good surface texture and skid resistance. And the advent of stringless paving and stringless trimming are making construction easier, thanks to laser or optical guidance systems. Well-graded concrete mixes have shown excellent properties, both in the fresh state, which helps constructability, and in the hardened state, which enhances durability and performance.
But, even with all the technological advances, understanding the basics remains critical. Steve Waalkes, managing director of technical services for the American Concrete Pavement Association (ACPA) spoke with us and provided a primer for basic fixed-form concrete pavement construction—as in driveways, sidewalks, and city streets.
Quality concrete pavement starts with adequate preparation of the existing soil surface. The soil should be properly compacted and graded to the correct elevation for the project. Depending on various factors, you might need to place a layer of base material on top of the subgrade.
“The subgrade soil type does affect your pavement design,” says Waalkes. “It also determines whether or not you are going to put a base material down to help cushion the slab. Certain clays are very susceptible to moisture changes, expanding when wet, and shrinking when they dry out. If you have a layer of material in between your pavement and your clay subgrade it will help minimize that effect.”
Any type of graded stone can be added as base material, like rounded river gravel, quarried stone, crushed stone, or any material that can achieve the proper density with as few voids as possible when compacted. Often this material consists of whatever is available locally. It doesn't have to be especially dense; it just needs to be stable. “Most specs require approximately 95% of the material's maximum density at a given water content,” says Waalkes. “When all is said and done, this material must have enough stability to support the construction traffic that will be driven on it before the pavement is completed. If that's the case, it will also be fine when it's underneath the concrete.”
Some contractors rework soft spots in the subgrade material, mixing quicklime (hydrated lime) or fly ash into the material to help dry it out or bind it together. These two substances not only dry up soft and muddy spots but also work as a binder similar to cement or asphalt, stabilizing the soil.
Insuring the use of quality concrete
Concrete for pavements should be designed to reach 4000 psi compressive strength at 28 days. But an even more important characteristic of quality concrete is durability. It must stand up over time, through all kinds of traffic, wet and dry weather, and freeze/thaw cycles, if these are a factor. Things that contribute to durability include quality aggregates, a well-graded mix (for low permeability) adequate strength, and proper air-entrainment (well-dispersed and typically around 6%).
Air entrainment protects the concrete when freezing occurs. These tiny bubbles in the concrete mix serve as air valves when freezing takes place. Air entrainment in nonfreeze areas, such as the Florida Keys, also makes the concrete more workable. “Entrained air acts like little ball bearings when the concrete is being worked fresh, before it's hardened,” says Waalkes. “Concrete will naturally contain air bubbles, but if it is controlled, it can make the work easier, as well as counteract freeze/thaw problems. It's important to remember that concrete contains water throughout its life. The entrained air bubbles themselves do not fill with water. The water is elsewhere in the concrete's microstructure. During freezing, the air bubbles are an important buffer because ice has 9% more volume than water. As the water expands, air bubbles serve as miniature ‘pressure valves' in the concrete. If they are not present, the concrete will easily break apart.”
Slump measures the workability and fluidity of the concrete. A good workable pavement mix for fixed-form construction will have a slump of 4 to 5 inches. If the slump of a batch of concrete is too low, adding water is not a solution. “The issue that some engineers, inspectors, or owners have is that they think the real high-slump mixtures were achieved with extra water that was added, but that's not necessarily the case. You don't have to add water. Chemicals can be added to make the mixture more workable without affecting the strength of the concrete or causing all the negative aspects that come about as a result of adding water.”
If the slump is too low, and the mixture is too stiff, Waalkes advises adding a water-reducing admixture, which will increase the slump without using water. Adding water would increase the water-cement ratio, which should be 0.45 or lower. “That figure is a good target to aim for. You don't want to be much higher than that,” says Waalkes. Higher water-cement ratios reduce the concrete's durability.