Article No: 18
Paving the way to quality surfaces
By: KYLE DALTON
The histories of concrete homes and concrete pavers — or interlocking segments of concrete pavement — could not be any more different. Concrete homes, although ever-increasing in their popularity, are relatively new to the scene compared to pavers, which have existed for many years and have been used in some form, going back to the days of cobblestone streets in many of the older cities throughout the world.
Despite these decidedly different pasts, the two have one important thing in common — their composition. Pavers are composed of concrete as well as fine and coarse aggregates. Each individual segment is made in factory-controlled conditions that apply pressure and vibration to create a high-strength product as durable as the standard concrete used in driveways, sidewalks or patios.
Also, like concrete homes and their use of a variety of insulating concrete forms in which the concrete is poured, pavers come in numerous styles that vary in color, texture, size and shape. Variety hasn't always been the norm. Fortunately, interlocking paving technology has made great strides since the Netherlands and Germany began developing it more than 50 years ago in the 1940s and '50s.
Color is one of, if not the first, design element a homeowner considers when deciding to install pavers. Having the paving match the exterior color of the home is often a high priority. This should be of little concern to the contractor because paver manufacturers across the country offer numerous colors and blends that will match or complement the exterior of any house. Contractors should understand that pavers of certain colors or blends might not be available in certain regions simply because they are not the "top sellers" for that region. However, with enough research and a little shopping around, the ideal color or blend can be found. It's also worth noting that the trend in colors today consists of soft tones that create more of a natural stone appearance.
While colors are important in achieving the natural stone look, the texture of the stone can go a long way in achieving the same effect. One texturing process that helps accomplish this appearance and is commonly used today is tumbling. This consists of putting the pavers into a large cylindrical drum and rotating it. While the cylinder rotates, the pavers will collide against each other and the side of the cylinder walls. The end result is a worn and somewhat weathered look on the edges and face of each unit. Some manufacturers might introduce water and aggregate into the tumbling process to enhance the effect. Although the process sounds severe, manufacturers have tailored it so that the individual pavers are not damaged or distorted beyond the point of use.
One Texas paver manufacturer, Jewel Concrete Products, uses its own proprietary "weathering" process and is leading the way in the natural, weathered look with its Antiques Collection, which is part of the Belgard line of products. According to Belgard sales manager Charles Keys, the Antiques Collection is their answer to the latest trend in weathered finishes. "People want the distressed-pavers look right now because they want them to mimic natural stone. In 10 years, the trend for pavers may go back to the brighter colors and the non-distressed look."
Another manufacturer, Pavestone, also offers pavers with natural surface textures through its Heritage SeriesT. This line includes the Holland Stone, which offers a rustic medieval appearance.
Sizes and shapes are also important factors when it comes to paver solutions. The standard residential grade thickness of pavers is 6 centimeters (2-3/8 inches). For vehicular grade, which is required on roadways, the thickness standard is 8 cm (3-1/8 inches). Keys said 6 cm pavers can be and are normally used on driveways.
The width and height of pavers is as varied as the colors and textures. Most commonly used, square or rectangular shapes can create an array of pattern designs. These designs can be enhanced by the homeowner's choice of paver placement. The running bond pattern is like the standard brick house design where the pavers are placed horizontally across the layout and the paver in each successive row spans the joint of the pavers located in the rows above and below.
Another popular pattern is the herringbone where the pavers are arranged horizontally and vertically through the layout. While the running bond and herringbone patterns are simple and creative, another type of paver allows even more flexibility in pattern design. Modular pavers are four-piece systems that are comprised of one square piece and one rectangular piece, as well as another square and rectangular piece of different sizes. These four different pieces are engineered to allow random placement throughout the layout. "You can randomly place them, and the end result is a random-looking paved area that is very easy to lay out with a minimal amount of cutting," Keys said. Two modular systems include the Mega-Bergerac and the Dublin Cobble Modular, both part of the Belgard line.
After the future homeowner has decided on the style of paver, it's time for installation. Installation will take more time than the standard monolithic concrete pour because of the labor involved. This labor includes steps quite similar to a concrete pour. Depending on the use of the pavers — i.e., patios, driveways or walkways — the steps will vary.
Consider paver installation for a driveway, for example. Base preparation is first and key. Just as for a monolithic pour, there must be a solid, level foundation. The flex base is commonly used for paver installation. Some flex bases consist of stabilized sand, which includes Portland cement and sandy soil, while others include concrete or a cement-treated base (CTB), which includes a combination of Portland cement with the base soil.
Another base type is the flowable fill that is comprised mainly of Portland cement and is similar to the monolithic pour. After a screen is dragged across the flowable fill, the base sets in a matter of minutes. Depending on the area and the code, some bases require a concrete slab on driveways. Whatever base is used, a slope of two degrees from the center crown is required for optimal drainage. After the base is complete, next up is the perimeter.
The entire paver field must be contained with some type of edge restraint to prevent the sand from spreading out. For a driveway there are several options including steel edging, a poured concrete curb or a wetset perimeter, where pavers are placed on the edge and are held in place by a toe that is mortared with concrete. All these perimeters are suitable for a surface that must stand up to a car potentially driving off the edge. For patios or walkways, plastic landscape edging works well.
After the base and perimeter are securely in place, a one-inch layer of bedding sand must be put down. The purpose of the bedding sand is to achieve lockup, which is the quality of the sand between each segmental paving unit that locks the pavers in place — not rigidly, but firmly enough so they can transfer the load to their neighbors and work in unison to disperse the load. Sand that is sharp-washed and suitable for the manufacturing of concrete is preferred because this will enhance the lockup of the pavers once they are in place. After the sand is down, a screening bar or plank is drawn across the bedding sand to obtain a level setting medium for the pavers.
Finally, the pavers are placed in whatever pattern allowed by the paver design. Some require the running bond method, while others are more liberal and allow haphazard placement throughout. After the pavers are firmly in place, more sand is poured on top of the pavers. The purpose of this layer of sand is to fill in any of the joints that might be lacking in sand and to provide additional lockup. Using a vibrator over the pavers, the sand fills the joints. When the vibration process is complete, the excess sand can be swept off and the surface is ready for use.
\While installation is a labor-intensive process, maintenance is not. If maintenance is ever required on the paver surface, the most likely cause will be base foundation failure. "Pavers are very strong and they won't fail," Keys said. "If a failure occurs, 99 percent of the time, it will be as a result of a base failure."
Keys' statement sounds bold, but when you take a closer look at the specifications required of each individual paving unit, it's not bold at all. All pavers must meet ASTM 936 specification, which requires each unit to go through a set of stringent tests. In one of those tests, an individual unit must achieve a minimum compressive strength of 7,200 pounds per square inch (psi), while multiple units must achieve 8,000 psi. In addition, each unit must pass freeze-thaw durability tests where the paver is submerged in water, frozen and then thawed. This is done over a period of several cycles. Depending on the percentage of the paver lost during the cycles, the paver will either pass or fail. Keys said only a very small percentage of the paver is allowed to disintegrate during the process before the unit is classified as failing.
There is also a test for the absorption factor. If the paver completes the test with an absorption factor of less than 5 percent, it is considered freeze-thaw resistant. In severe climates such as Canada and some states in the Northeast, where pavers are more commonly used, the tests are even more stringent and involve a freeze-thaw test requiring submersion of the pavers in saline solution.
If at some point a paver surface does fail, repair is minimal, according to Keys. The section that is in disrepair — possibly as a result of washout from improper drainage or roots growing underneath the surface — can be corrected in a short time. First, the affected pavers must be removed and the underlying foundation cleared. After the problem is corrected, Keys noted, the initial installation process is repeated, including installing the base, the bedding sand, the pavers and the final layer of sand.