optical guidance systems. Well-graded concrete mixes have shown excellent properties, both in the fresh state,

Transcription

1

2 :rzr, 7nrf,aI e~,c??rt explains what works best Construction Basics oday'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. Subgrade preparation Quality concrete pavement starts with adequate preparation of the existing soil surface. The soil should be ;h the con crete surface. itt? Concrete Construction t December 2005 t 47

3 Top, left: Proper grade preparation and compaction is essential to achieve stability. Top, right: Air content is typically measured using the pressure (air pot) method. Above: Texture can be applied by hand or by using a machine, as shown. Below: A good application of curing compound is essential for keeping water from evaporating during the critical curing period. 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 freezdthaw 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

4 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 freezelthaw 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 A joint cut into the concrete surface has resulted in a "controlled" crack just below the surface--not at an undesirable place on the surface of your concrete.

5 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. Placlng 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 2x4 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 waxbased, 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.

6 "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 5x 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. B time and within budget ringing your project in on 1 4: 1 ; 1 I* I Ideal for high strength anchorappl~catlons, fiten HD ~nstalls in standard d~ameter holes with less torque than other concrete anchors are the things that make you look like a superhero. Simpson Anchor Systems offers products that meet specifica tions and install auicklv to save High powered a" I'ghfWelght, you time and money, Our experi- i i S~mpsons PA tools are deslgned for enced sales reps provide the speed, reliability and comfort on the I kind of service and training that lobslte A full selection of pws, loads and accessones available help you speed up your work on the jobsite. Find all these products and more at your local Simpson Anchor Systems dealer rod for overhead applications Saves and you can look like a super- tme over, process of mta111ng rod hero on your next project! after concrete cure a WORLD OF CONCRETE w EXH'B'TOR I, I ] : j ~ ~ j ~ : y!, ~ p j j : ~ This rod hanging system WIII change 7 the Way You lookat hanging threaded ANCHOR SYSTEMS SIMPSON STRONG-TIE COMPANY, INC. 0 m h t ms rnp~bmgne~o IM circle 283 on reader service card 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." W -Pete Hildebrandt is a freelance construction and business writer based in South Carolina.