QUESTION: We have a contract to install a decorative concrete overlay. There’s a fair amount of flexibility on the timing of the installation, so I want to make sure to install the overlay when it will have the highest chance of success. What should I consider?
ANSWER: With a variety of overlay systems from which to choose, you will discover that many products have their own set of performance criteria—as well as specific recommendations as to the best conditions and procedures for application. Often these criteria will help you determine the best system for a particular project. Whatever system you choose, I can’t stress enough the importance of reading the manufacturer’s technical data sheets and specifications. This is where you’ll find vital information about mix ratios and mixing times, the best temperatures for installation, surface preparation procedures, and much more.
Most important, the manufacturer’s specifications will describe when or when not to install their system based on such factors as exposure conditions, subfloor types, the age and moisture content of the slab, and the condition and profile of the existing substrate. Do the necessary groundwork beforehand to find out what site conditions could affect the system you’re installing. Often these issues are impossible to correct once the overlay goes down.
Whether you are working indoors or out, one of the most important considerations when installing overlays is temperature. Quite simply, the warmer the air temperature and the substrate temperature, the faster the material will set.
Installing an overlay in direct sunlight during the hottest part of the day will greatly reduce your working time and jeopardize the quality of the results. In the heat of summer, keep your raw materials cool and install the overlay mix in the morning or during the coolest part of the day. In the cooler winter months, you may find that your topping sets too slowly. Also, control air movement during installation to avoid premature drying.
Inevitably, questions arise about the performance of overlays or underlayments over wood subfloors. Again, read the technical data sheets! Some manufacturers strongly warn against applying their products over wood subfloors while others say it’s OK if certain precautions are taken. The main consideration when going over wood is the amount of deflection the floor exhibits, which is usually determined by the joist spacing and the thickness of the plywood. On wood floors requiring more rigidity, we have had good success screwing down ¾-inch plywood on 12- to 16-inch centers.
Condition of the existing substrate
As a general rule, most overlay manufacturers recommend curing new concrete a minimum of 28 days before applying their products. This waiting time permits some of the moisture to escape from the slab. If toppings are installed prematurely, moisture vapor passing through capillaries of the concrete can lift it. For some overlay systems, manufacturers will stipulate a maximum moisture vapor emission rate.
Bob Harris, founder of the Decorative Concrete Institute, Temple, Ga., and senior decorative concrete consultant for Structural Services Inc., has over 25 years of experience in the construction industry. He conducts training seminars in architectural and decorative concrete worldwide, is involved with numerous associations, and is a popular speaker at World of Concrete and other events. The information in this article is based on his book, Bob Harris’ Guide to Concrete Overlays & Toppings, which now comes with a DVD that provides step-by-step instructions for rejuvenating floors and exterior flatwork. Visit www.decorativeconcreteinstitute.com.