What is an overlay? An overlay in decorative concrete lingo is a cementitious topping applied in various degrees of thickness from less than 1/16" up to 3/8". The overlay may be put down with a trowel to create a thin textured finish over concrete or with a hopper gun to create a Knock Down or splatter texture. Another technique is to apply it a little bit thicker and then imprint it with texture mats to create very realistic reproductions of stone, slate, tile and even more sophisticated textures such as seashells, coral and wood. There is no limit regarding color and designs, which allow the contractor to offer and deliver, a finished product not available before the advent of this very versatile product. That’s the reason why Decorative Overlays are enjoying a tremendous surge in popularity and increase on the number of square feet installed to levels never seen before. What’s an Overlay made of: Most decorative overlays consist of Portland cement, sand, and a latex cement modifier. Additional admixtures such as water reducers, fibers, accelerants, retarders, air entrainers and defoamers may or may not be present in the formula depending on the manufacturer. In order to understand better how ingredients work, we’ll get a little more in depth into our analysis starting with the sand. Sand: Sand needs to be clean and properly graded. A granulometry analysis will determine which particle size distribution will work better for a specific product. This analysis takes into account sand origin, size, distribution, water demand and cement content in the mix. Sand size and distribution are key factors in an overlay’s performance and more important than most people think, too many fines will increase the water demand, weaken the mixture and promote shrinkage cracking, while too much coarse aggregate will provide a sandy, hard to smooth undesirable finish. An overlay with a properly balanced sand blend makes a difference; as the smaller grains fit between the larger grains it creates a more cohesive paste, a denser mix with higher wear resistance, higher compressive and tensile strengths and a more uniform finish. In manufacturing DecoTop we use a cross of three pre-blended sand grades, which range from a #30 to #200. The way the finished product looks after installation is paramount to the decorative concrete contractor success, DecoTop mixes allow installers to easily achieve strong, eye catching finishes either with our stamp grade or thin finish toppings. Cement: Cement is the glue that binds all the ingredients together. There are many classes and subclasses of cements but the most commonly used is Portland Type I. Choosing which type and brand of cement to use is a very delicate matter, just because you buy cement from the same company doesn't mean the cement you get is the same every time. There is a very high cement demand in the world today, with most companies struggling to maintain their stocks full enough to satisfy demand. Because the total cement production in the US is not nearly enough to satisfy our own domestic consumption, most companies bring cements from overseas in huge barges and ships and bag them locally. Even very large companies such as Lafarge and Lehigh sell cements that come from many different parts of the world; the problem for the manufacturer of overlays and the contractor is that one day you get a cement from China, then next time it’s from India and the next time it’s from South America, therefore the manufacturer or contractor may not have any knowledge or control over it. Cements, especially White Portland will have different characteristics and behaviors based on their origin and composition, which may be critical to the performance of the end product. Some cements will work better for some applications and not so well for others, which leads to contractors getting different results from the same bag mix from the same company without being aware that the problem may lie with the cement used in the mix, and the company that sold them the product may not know either. Here at Decosup we manufacture our own products, which gives us a great advantage over companies that buy their overlays from Toll Blenders. We can choose and pick which cement and sand we use on our products, we don’t have to worry about the Toll Blender changing cements on us without our knowledge and most important: we are free to switch and replace any raw material in a heart beat if needed. We have tested many cements and had a hard time finding a source that would assure us of the quality and continuity we demand. After much research and product evaluation we settled with the only company in the US that would guarantee us that the white Portland cement we use now would be from the same manufacturing plant and origin every time. This option is a little bit costlier than others but we like to have full control on our products down to the very last detail so we can assure our customers that they can count on quality and consistency from us. Polymer Cement Modifier: there are many different cement-modifying polymers used on overlays today, they range from Acrylics to Vinyl Acetates to Styrenes, Versatate blends, EVAs, VEAs, VeoVas, SBs and many more. Each polymer has its own benefits and use; there are also many differences among the same type polymers regarding their hardness, solvency, elasticity and special properties. Polymers are added to overlays to increase bond strength, lower permeability and add workability. Cement modifiers also increase the compressive strength by reducing the water demands of the overlay. Among the many different polymers available, a careful examination and testing needs to be done to determine which modifier will work better for a specific system as there are many variables and factors that need to be explored. Two of these variables have a great impact on determining the suitability for a specific application. The first one is called Tg C° (Glass Transition Temperature), Tg C° is the point on which the polymer transforms from a rubber like material to a glass or crystal like state, the higher the number, the harder and more brittle the film will be, a lower Tg C° indicates more elasticity. The other variable to look at is the MFFT (minimum film forming temperature), which is the minimum temperature at which the film will coalesce. If a polymer has a low MFFT it will easily form a continuous film but may have undesirable tendencies such as increased tackiness and less durability. A higher MFFT will have better performance but may not be used in lower temperature applications without the use of a coalescing agent. A high Tg doesn’t necessarily translate into a better and harder product, as with the increase in the modulus of elasticity, the impact and wear resistance may actually be reduced. The MFFT is important when the application temperature will be low. Polymers work by forming a continuous film within the voids left by the mix water evaporation, if the wrong polymer is used or no coalescent is added, at a low temperature the overlay will still get hard but will lose many of the benefits of using a polymer and no continuous film will be formed within the matrix, though this will not be visible to the naked eye and there will be an apparent hardness to the product, it will not perform as desired and the end result will be a weaker mix. In view of the many problems and pitfalls caused by miss formulated products no stone can be left unturned when manufacturing an overlay. At Decosup we spend a great many hours doing research and testing because we know that there is a lot more to formulating a product than just putting some raw materials together. We have found a good balance between Tg C° and MFFT, coalescing additives, sand blends and a great quality cement that results in an overlay with a higher performance, durability and workability. Additives: Part of engineering a good overlay mix involves the use of additives or admixtures, though there are many additives that can be used in an overlay mix, some are more important than others. Each company conducts their own evaluation to determine what may or may not work for their products. Here are some of the additives that may be used on overlays. Water Reducers: As their name suggests Water Reducers lower the water demand by making a workable mix with the desired consistency and workability with less water. This may very well work for concrete but with overlays it’s a little more complicated. Overlays have a much higher cement content than concrete and lack coarse aggregate. The use of the wrong plasticizer may have some undesirable effects such as excessive set retardation, making the mix too sticky, hard to finish and may cause stamps to stick to the surface. A common side effect of most water reducers is that they cause excessive set times, which may be problematic especially when the temperature is below 70º. Accelerants: An accelerant may be used when the temperature is low, but using accelerants in overlays is a bit more complicated than in concrete as there are more restrictions. Calcium chloride is the most affective accelerant but it must be added on site, if color and steel corrosion are an issue then a non-chloride accelerant may be used but here is where the restrictions come into play. First, it may take too much accelerant to get results, possibly as much as one lb. per 60 lb. bag of overlay. If not carefully tested and monitored, excessive accelerants may cause many problems such as increase in drying shrinkage, cracking, creep and even cause marked retardation of the mix. Non-chloride accelerants are harder to predict and their effectiveness is remarkably linked to the amount of Gypsum in the cement used. If the Gypsum to Ca3 ratio is not at the optimum level the accelerant may be totally ineffective leading to a weaker mix, no accelerating at all and even retarding the mix. There are much more effective ways to accelerate an overlay by using a preset package of High Aluminate cements with or without Li-Carbonate, but this package must come from the manufacturer and should be approved and tested to be used with their overlay. Air Entrainers make concrete easier to finish and give it more freeze and thaw resistance, this may not be the same with overlays as overlays are usually mixed with a drill and a paddle or in paddle blenders which already introduce a lot of air in the mix, excessive air content will translate into a weak mix and reduce wear resistance. If an air entrainer is used, a defoamer should also be added to control excessive air. This sounds like a contradiction but it does work. Fibers: Fibers are used as active shrinkage cracking control by keeping the matrix together as the overlay loses water. Fibers for concrete are made from a number of raw materials such as: Nylon, Polypropylene, Polyester, Fiberglass and even metal. They come in all sizes from 1/4" to 2" or more. To use fiber in an overlay the fiber must meet certain criteria; the most important is that it cannot show on the surface of the product, nobody likes a hairy finish. They must also be effective as crack control. Some fibers absorb water from the mix and some don’t. Depending on the overlay formulation one may be better than the other, but the formulator must be aware of the consequences of using the wrong type. After testing several fibers for DecoTop we chose to use a premium grade Nylon fiber, these fibers have proven to be very effective for shrinkage crack control especially during the hot summer months. They never show on the finished surface even at only 1/8" thickness and they also increase tensile strength and freeze thaw resistance. There are many other additives that can go into an overlay but I just wanted to cover the main ones. Conclusion: Engineering and formulating an overlay takes a lot of trial and error, quality raw materials, production control and lots and lots of testing. It is not recommend to add anything to commercially available overlays unless the manufacturer approves as the results may be disastrous. Stick with manufactured products, do your homework, ask a lot of questions, go to seminars, follow directions and your overlaying experience should produce good results. Art Pinto Decosup Inc. Copyright © 2005 Decosup Inc. All rights reserved. No portion of this article may be reproduced without the express consent or permission from Decosup Inc. | Home | Products Page | Decorative Concrete Gallery | Training | Technical Data | About us