DEAR TIM: Is it possible to add an additional layer of thin concrete on top of an existing concrete surface? My concrete patio and front porch are scaling and there are patches where the top surface has deteriorated. The rest of the concrete looks fine. Can the average homeowner successfully complete such a job? Marcia D., Helena, MT
DEAR MARCIA: Resurfacing concrete with an overlay over existing concrete surfaces is a fantastic way to extend the life of a driveway, sidewalk, patio or porch. The new surface will be durable and certainly more long lasting than the original finish if you diligently follow a few simple steps. The best part is that the material cost is minimal and you can do the work on different days to suit your schedule. I believe you can tackle this project if you take your time.
The most important step in the process is to make sure that all loose concrete material is removed. Weak concrete can be removed by hand with a chisel and hammer or you can rent a lightweight demolition chipping hammer that blasts away material. Industrial strength power washers can also be used. The concentrated stream of water removes weak concrete. The surface that will receive the overlay must be clean, oil and dust free.
Before you purchase the materials for the job you need to decide how thick the overlay will be. If the thickness will be 3/4 inch or less you will not need any small stone aggregate. The size of stone in concrete mixes is very critical. The maximum diameter of any stone in a concrete mix should never exceed 1/3 the thickness of the pour. Small pea gravel often has stones larger than 1/4 inch in diameter, thus it can only be used in overlays that are greater than 3/4 inch thick.
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Overlays can be as thin as 1/16th inch thick. In these instances you would use fine sand mixed with Portland cement. Sand purchased from a traditional gravel pit or building supply yard often is available in three grades: fine, medium and coarse. Coarse sand has very small stones in it that can be as large as 1/8 inch in diameter. This sand is perfect for one half to 3/4 inch thick overlays.
Before you mix any material you need to set up some small form boards. Use furring strips that are braced against the sides of the existing concrete work. The top of these pieces of lumber should be set at the final height of the overlay. It is not necessary to attach them to the existing slab. Simply drive stakes next to the furring strips to pinch them against the existing concrete work.
If your overlay will just contain sand and cement, make the mixture rich. Simply mix the two parts sand and one part cement together before you add any water. If you decide to do a thick overlay, mix three parts rounded gravel, two parts sand, and one and one half parts cement. Feel free to add another half measure of cement for an extra strong overlay. You should add enough water to the mix to make the mixture plastic. It should flow out of a wheelbarrow like pancake batter pours from a bowl.
To permanently bond the overlay to the old concrete, apply a cement paint to the old concrete. This paint is made by simply mixing Portland cement and water until it is the consistency of paint. Do not let the cement paint dry on the existing concrete. It must be covered with the overlay mix within minutes of being applied.
The best weather to perform this work is when the temperatures are in the 50 - 60F range. Overcast days are ideal. You want the resurfacing to set up slowly. You apply your final finish to the overlay when the material is somewhat firm. Do not sprinkle water on the surface as you finish it. This will weaken the top skin of concrete. Tightly cover the finish slab with plastic to cure it or spray a clear curing compound on it so that the overlay cures very slowly. This will allow the concrete to attain maximum strength.