Concrete Overlay Installations
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A question commonly asked is: Can pavers/DevonStone be installed on top of a poured existing concrete pad? The answer is YES. However certain criteria must be met. Installing segmental system concrete pavers or slabs on top of concrete causes a flexible system to no longer be flexible. Normal installation requires a modified 2A base material which allows pavers to spread their load over a broader area. Concrete does not allow for this flexibility and results in the inability to carry the same loads as segmental pavers. A key consideration is to determine if the concrete pour is in good condition, as well as the depth of the existing concrete, and if the area will allow for the additional depth of pavers and/or slabs. These are all issues that should be addressed when considering an overlay of paving stones and slabs onto an existing concrete pad. Existing Concrete: Assess the existing concrete pad. Does it has any cracks, either larger or small; is it heaving, is the top surface breaking down or in bad condition? All cracks or breakage have to be addressed. In whatever condition the crack is, it should be filled in with a polyurethane caulk to prevent any migration of water. If water settles into the cracks, and it freezes, it will expand the crack, widen, and the concrete slab has the potential of heaving and deteriorating the base surface. If the concrete slab has the potential, the paving stones or slabs overlaid also will follow. Cracks must be filled with a polyurethane caulk even if stabilizing fabric is being used. If the top surface of the concrete is breaking down or uneven, once the pavers are seated into the bedding sand they will set unevenly. Therefore, the results could be improper pitch, puddling, and damage to the product itself during tamping. On the other hand, if the pavers or slabs are adhered to the top surface concrete and the surface is chipping, flaking or finished unevenly it will more than likely make the concrete pad sprawl. This sprawl will cause the overlay product to shift or teeter on high spots of the concrete pad and create an uneven surface. If the concrete that is proposed to be over laid is in poor condition, it should be removed and either re-poured in place or a typical 2Amodified base should be used for the application in troubled areas. Thickness of Concrete: A pour (concrete pad) should be no less than four inches in depth to perform properly in a residential application, and for non-vehicular use. For a commercial project, it should be more than four inches in depth and the project's engineer should make final decision on the depth. Concrete Pad Function: The final use of the project area will determine the installation process. Will it have a hot tub on top of it in a fixed location or will there be vehicular traffic? An area that is supporting a hot tub or a driveway for limited vehicular use should be no less than four inches in thickness. This is the standard thickness for this type of application without pavers or slabs set over the top. The addition of paving stones should not change the concrete that is supporting the weight. Slabs should not be used in vehicular applications, even when over laying on concrete. They will break under the weight due to their aspect ratio which is greater than 4:1. Depth of finished area: Will the area allow for the thickness of the paver/slab and setting bed? If a patio is proposed to be over laid, there must be an allowance for doors and entryways. Additionally, it may create an elevation problem near window wells or crawl space vents. What if a driveway is proposed to be over laid? Will there be a height difference at the garage floor elevation, or by the apron at the street? If installing pavers/slabs near an existing swimming pool the finished product changes the height of the first step riser into the pool based on the thickness of the pavers/slabs. Installation Process: After inspection of the project area and all issues have been addressed, installation is as follows: Geo-textile fabric should be used to prevent future concrete sand migration. The fabric should be cut to extend to the outer perimeter by six inches and overlap by twelve inches where the working area exceeds the length of roll. To ensure the concrete sand setting bed does not migrate, make certain that all areas are covered by fabric and that the fabric is taunt. Once the fabric is set out, screed one (1") of concrete sand with one (1) screed pipe and a 2" x 4" of aluminum rail. The sand is to meet ASTM Standard C33. Masonry sand or bar sand is not suitable for this application. The sand should not exceed one inch (1") in thickness. If the sand area does exceed one inch (1") in thickness the area has the potential to tamp unevenly and can create problems with water run off. In addition, the area has the potential to collect water in the bedding sand. That specific location could heave and buckle once it goes through freeze – thaw cycles. If it is a small area of three-eighths of an inch (3/8") or less, thin set can be applied to raise the area to its proper elevation. Pro-pack also can be applied as a setting bed in areas that are greater than one inch (1") but keep in mind during the tamping process Portland Cement dust may force its way up through the joints and can stick to the top surface of the pavers. Using a piece of Mirafi 500Xt stabilizing fabric will help reduce the chances of this happening. Only screed an area that you can lay pavers on the work day, check the weather forecast to make sure any unfinished areas will not be vulnerable to unfavorable weather conditions and never let screed sand be rained upon. Once the area is screeded, lay pavers or slabs in the same manner of any other standard paving stone/slab installation. Determine the 90 degrees starting point and install the pavers with the click and drop method. Take care not to run heavy equipment over the area. Paver carts and wheelbarrows are ideal for these applications. Finish all areas that are installed for the day, including cuts, edge restraints and final tamping. Edge restraints are installed in the following manner: Plastic edge restraints can be used as a restraint for concrete over lays. By using a 3/8" inch masonry drill bit and drilling holes every two feet (2') on center and on straight runs to enable a galvanized spike to be driven into the hole and every one foot (1') on center on radius areas. If available a 3 1/8" paving stone can be adhered directly to the concrete on the project; making sure the adhesive is applied in pencil thick beads perpendicular to the edge to allow for drainage. Using a 3 1/8" or 80mm paving stone on the perimeter will have a + ¾ inch height difference that will allow for screeded concrete sand under the field of the project. With either application the 500XT Mirafi fabric must be inverted up against the inside edge of the 3 1/8 / 80mm paver boarder to stop migration of the bedding sand from working its way out. Final tamping and joint sand installation is the same as a typical installation over 2A modified base. Remove any debris or dust and clean the area for the first tamping. Joint sand can be either masonry sand or concrete sand to fill the joints as long as they are within graduation standards ASTM C 33. Polymeric sands should no be used on concrete over lays. The bedding sand retains more moisture than over a typical 2A modified base material and the polymers will remain constantly moist inhibiting proper curing. Therefore the product may stain the top surface of the pavers or slabs creating halos or hazing on or around individual pieces. Keep this mind. Joint sand is applied after the initial tamping to insure a proper seating of paving stones in the sand bed. Starting from the outside perimeter tamp to the center of the area. If slabs (DevonStone) are installed, set the pieces by hand with a 3' foot long 2"x 4" and rubber mallet or dead blow hammer. Slabs (DevonStone) cannot be tamped and will break under the pressure of a tamper. Once the initial tamping is completed, joint sand can be applied to fill the joints until they are full. Make sure to tamp in between each application of joint sand in different directions to help migrate the sand into the joints. Joint stabilization sealers can be used to help control joint sand migration. When the installation is complete, all exposed fabric should be cut to the height of the edging so it will not be seen. After fabric has been cut back, use a propane torch on the outer edge to create uniform custom finish. Back fill all areas around the project and the job is complete.
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