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By Peter Hildebrandt |
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David Hand, president of O.L. Thompson Construction based in Charleston, SC, has poured concrete in all shapes and sizes, from driveways to the newly completed Cooper River Bridge, the longest cable-stay bridge in North America. Hand says the basics of concrete are fairly simple. For someone who is used to doing vertical concrete pours or for those who have always been subcontractors and might now be interested in developing their concrete flat work, there are some basic points to remember before getting started in pouring concrete.
Like most building projects, the starting place is a sketch of the project, with approximate dimensions and the amount of lumber and concrete needed. Hand says the first priority in planning is finding a concrete supplier who knows what he’s doing. You should be able to tell this supplier what you’re planning, whether the concrete is to be pumped, dumped, or wheelbarrowed, and how fast it is needed, and he will know exactly what to send you. “If you tell potential suppliers the basics of what you are doing and they start asking you good questions and show interest, they are a good supplier. A lot of it is a gut feeling. This important start will make or break you in a lot of ways.” Wooden stakes should be driven into the ground and string lines set up to mark out the size and shape of the pour. Sod or earth to a depth of at least 4 inches should be removed inside the area to be poured. For greater load-bearing pours such as driveways, the depth of removal should be at least 6 inches with the use of wire mesh or reinforcing rods. The soil in the pour area can be leveled and made uniform with a pullcrete, and a tamping tool can compact loose soil. A light spray of water over the area prior to placing the concrete helps with the curing. Wooden frames level with the string line can then be constructed and nailed together with support stakes outside the forms. They can be made from wood two-by-fours or two-by-sixes, or more sophisticated metal or plastic. Leveling will ensure that forms are in line, level, and sloped properly to allow for adequate drainage.
Be sure that the truck you are using can reach the spot where you are pouring concrete, or if it cannot reach there, be sure transfer vehicles are available to transport the concrete. “This is a ‘forest for the trees’ issue,” says Hand. “Say you have a long driveway and there are trees down both sides right up against the forms. How is the piece 100 feet away going to be poured without driving through the first 80 feet? The farthest out that a concrete truck can reach is about 15 feet. Logistics is a big issue—and bites people in the butt more than anything else. Sometimes all it takes is being sure you have a pump on hand to get the concrete to the spot where you are starting the pour.” The level of quality involved in making something look like a finished product instead of just a piece of concrete is directly linked to the quality of the finishers that you have. “You need to have at least one person very skilled and knowledgeable in finishing the concrete,” says Hand. “The lead guy needs to have experience or have done enough research to know what he’s doing. I recommend the experienced person, but admit they are hard to find. The rest just comes down to people with strong backs and the willingness to work in the sun.” Once the concrete is in place, the floating or rough grading or flattening out of the concrete must start. When the concrete reaches a point at which and consistency in which you can still leave footprints on its surface but don’t sink in to your ankles, the final finishing needs to be done. Hand cautions that if the final finishing is done too fast, as the concrete is bleeding and breathing, the sealing-off on the top with a curing compound will lead to a layer of air and water on the surface. Within six months the surface will delaminate if this occurs. “You can read about this in books all you want,” says Hand. “But until you actually do this it is a very touchy-feely thing. Here is where experience comes in over ‘book smarts.’” When it is poured, concrete should form a fairly loose pile, standing up more like cookie dough than wet oatmeal. “Most workers, however, like to make things easier on themselves, and they will want more pourable concrete,” says Hand. “This is an issue that is always on everyone’s mind: fine-tuning that concrete that is brought in. The person responsible for the pour needs to be there to make sure that the laborers aren’t just making things easier for themselves—but at the same time destroying the project in the long run with poor-consistency concrete. This, and whether the truck can reach where it needs to go, are the two places where there are always fights in concrete construction!” Whoever is ordering the concrete needs to be very aware of how much work is involved when the concrete arrives. If too much concrete arrives for the workers, your workers will not be able to complete the job. Start small and build up to bigger jobs. Break up the job into several days of work. “It’s a very big domino effect in knowing how to keep up,” says Hand. “Starting out slow will really help your learning curve. In about two days you will know how things are working out. Remember that 10 yards of concrete equals 50,000 pounds of concrete to move. Think of what your workers will have to pick up and move. A pump truck will help you do more work.” How’s Your Subgrade? The amount of preparation of the subgrade involved depends on the application. Depending on the soil, either gravel or sand can be brought in and used for a sub-base. “If fresh, newly exposed soil is used, and a simple excavator, grader, or skid-steer is being used to remove this material, many operators will drag out below grade and then come back in to set their forms,” says Luke. “The forms will be above it and the bottom will be filled with sand or rock to get a good subsurface in there. Many contractors will use a two-by-six form and then use an inch or two of the sub-base to get it up to the proper level to place the fill material. Rather than simply soil, they’d like to have something in there that’s going to give a good surface to the structure." The American Concrete Pavement Association’s (ACPA’s) Web site (www.pavement.com) contains a number of helpful articles from its recent newsletters that may be read online. These articles mention that, unlike asphalt, concrete will distribute its load’s weight through slab action or spreading the load over a large area, so the importance of a well-prepared sub-base cannot be overemphasized. Tools of the Trade Once the wet concrete is poured to the top of the form, use the pullcrete to spread it, being sure to push the concrete completely into the corners, as well as to pour some extra in the middle to compensate for low spots.The screeding or leveling process on a pour uses three people, two for the screeding and one to keep leveling the excess concrete to fill low spots ahead of the screeding work going on. A screed board, as a rule of thumb, should extend 10 to 12 inches beyond the form during work. Tops of form boards should be kept free and clear of obstructions. The best way to proceed is to start at one end and, using a sawing motion, pull the screed back and forth over the wet concrete, always being sure that the screed board stays flat on the top of the forms. Excess concrete is pushed ahead of the screed while the pullcrete is used to pull concrete away from spots where it has become too thick. Low spots can be filled with more concrete and rescreeded. With a bull float or channel float the entire concrete area should be finished to an even finer texture. For larger areas, use bull or channel floats with extension handles for quicker and easier floating, while a hand float may be used for smaller, hard-to-reach spots. Floating submerges bigger rocks and at the same time causes a water paste to rise to the top, resulting in a more finished, smoother concrete surface. Running a trowel between the poured concrete and the form allows for a separation so that a rounding edger may be used to round corners for better resistance to breaking or chipping of the hardened concrete. More fine-tuning may be done with a hand trowel in larger, harder-to-reach spots. This makes for an even finer surface. Work should begin at one side of the slab, using large sweeping arches, at the same time keeping the front edge of the trowel raised slightly. Grooving should be done before the concrete sets up for installing expansion joints, no farther than 8 to 10 feet apart. The generally accepted depth of the groove is 25% of the thickness of the poured slab—or a 1-inch groove for a 4-inch poured slab. A groover bit on a bull float, a groover bit for a Fresno trowel, or a long-handled groover are all tools that will work. To make the final surface slip-free, brush the concrete surface lightly with a soft bristle broom. Some manufacturers offer adapters so that the broom can be attached to a standard bull float handle as well. Form Varieties and Stamping Steve Morici, owner of Morici Bros. Concrete of New Berlin, WI, works on smaller projects such as patios, driveways, sidewalks, flat work, and stamped and color concrete jobs. Morici switched to metal and poly forms five years ago and found they cut his setup time by half. “Another benefit of the forms is the ability to adjust easily during the course of the job,” says Morici. “If you have to change grade during the work, this can be done by adjusting a coupling on the poly forms in a matter of minutes, versus pulling out nails and renailing on the wooden forms.” Morici finds that cleanup is much easier as well. With the form oil in place, once the job is finished the only thing that needs to be done is to wash off the forms. Being certain that the grade of the pour is pitched away from the house is important, according to Morici. “You never want standing water when you are finished with your job,” he says. “You can have a great-looking pour. But if it’s pitched wrong, you are going to be tearing your work out. No one wants water up against their building. But actually every step in a concrete pouring job is important.” To deal with pouring concrete on an unusually hot day, decrease the amount of concrete in your pour. “If your three-person crew is used to pouring 9 yards of concrete but it is very hot outside, it might help you to pour only 7 yards,” says Morici. “The reason for this is that when it is really hot outside, the concrete starts setting up quicker, people get behind, steps get missed, and water ends up being placed on the top surface to compensate, and that only ruins the pour. “Once concrete is mixed, it is going to dry no matter what is done to it, as a chemical reaction is taking place, not a physical drying process. After the concrete is poured and is curing, for small jobs it helps to water the concrete for approximately one week to slow the curing down—or the escape of water—and avoid cracking and scaling.” For Morici, having the proper tools is another important given in concrete work. “For example, I have excellent joint-cutting tools where I don’t even have to walk on top of the concrete surface to do this job,” says Morici. “Sometimes a homeowner may lay down a two-by-four to be able go out on the concrete surface to cut a joint. But I have walking tools where I can cut the joint from off the concrete, and this saves a lot of time. Instead of knee boards to go out on the concrete for finishing, we have excellent walking trowels that let us work from the edge of the concrete to get things looking very nice. Tools make a big difference.” Morici’s company, like others today, has moved into the stamped concrete market. Stamping is done by going out on the concrete when it is still wet and stamping patterns on the surface to make it look like brick or stone. “Many people are building $400,000 to $600,000 houses,” says Morici. “They don’t want their house to look like everyone else’s. This is a way to make their house stand out—with some decorative concrete.” When asked what is new on the horizon for smaller concrete pours, Colin Lobo, vice president of engineering for the National Ready Mixed Concrete Association, says there are many new decorative options for sidewalk construction. The big question with that type of concrete work is whether or not the municipality or development will permit that type of work to be done. “If there are no problems with one of the new decorative options, using one of these patterns, stamps, or colors for a sidewalk project can be done relatively easily and will add value to the project,” says Lobo. Lobo’s first concern with smaller concrete projects is the climate where the pour is to be done. “If the work is to be done in an area prone to extensive freezing and thawing, in general you want to be sure that you are using at least a 3,500- to 4,000-psi concrete, which should be air-entrained,” says Lobo. “You should also make sure that you are pouring at a 3- to 5-inch slump and be sure not to get your concrete mix too wet. Definitely do not try to water down the concrete too much or you will eventually end up with problems with scaling.” For sidewalks, it is also best not to work them too much after the initial pour to get a trowel-finish. As little finishing as necessary should be adequate. “Overworking will generally cause some type of delamination or scaling of the surface,” says Lobo. “It’s the same story when working on exterior pads. Garage pads might be slightly different, and if you plan to do hand-trowel finishing, make sure you use non–air-entrained concrete as long as it is in fact going to be used in an interior area—out of any freeze/thaw zones.” When it comes to curbs, if the concrete is going through a slope form paver or a slip form machine, it’s important for the concrete to be at a low slump, or fluidity. “Because the curb-forming machine is moving as the concrete is being formed, you want a relatively stiff concrete,” says Lobo. “If the stiffness is not less than 2 inches, the concrete may fall off as the machine moves on.” Lobo says curing is vital on smaller jobs. He recommends using curing compounds after the concrete is put in place. “Curing maintains sufficient moisture and temperature within the concrete itself. A curing compound is essentially a spray-on film on the concrete that prevents water from evaporating.” The American Concrete Institute (www.concrete.org) is a good source for guidance and reference on anything involved with finishing concrete. It has a book on slabs on grade construction, as well as a flat work finishers’ certification program. The National Ready Mixed Concrete Association’s Web site is www.nrmca.org. A click on the Research and Engineering/Concrete in Practice headings will bring up 38 one-page summary sheets providing general guidance on many of the various aspects of concrete work, such as why you might get scaling. The Concrete in Practice Series can be read online in English or Spanish (the summaries are not printable, though) and may be purchased in hard-copy form in sets of 50 in either language as well. The topics offered range from “Dusting Concrete Surfaces” to “Pervious Concrete.” “Between those two organizations, and two others, the Portland Cement Association [www.cement.org] and the American Society of Concrete Contractors [www.ascconline.org], there is enough information that interested individuals can educate themselves with and have a successful project,” says Lobo. “Though all projects are included, there is plenty for contractors preparing for the smaller concrete pours and jobs.” Peter Hildebrandt writes extensively on engineering and scientific subjects.GEC - January/February 2006 |
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