Disadvantages of Modified Concrete Overlay

Requires Adequate Surface Preparation

• While modified concrete overlays can be used to resurface concrete and other materials, such as wood, they cannot be placed over all surfaces. First, the application surface, whether it is a public roadway or a patio in your backyard, must be cleaned thoroughly and then primed. Primer binds to the substrate, fills small cracks and provides an ideal surface for the overlay to adhere to. Skipping the primer may result in areas that break away or because of poor adhesion. If your surface is severely cracked, spalling or resting on unstable soil which causes it to shift, you may not achieve satisfactory results from the application of a modified concrete overlay. Large, deep cracks in a concrete surface indicate that settlement has occurred or that freeze and thaw cycles have caused heaving or sinking in the concrete. Patching or hiding these with a modified concrete overlay will provide only a temporary fix.

Moisture-Sensitive

• According to Pavement Interactive, most polymer-modified concrete overlays are made with thermoplastics because of the rigid nature of these binders. However, despite their durability after curing, modified concrete overlays cannot be applied in all weather conditions. Rain, fog, frost and dew can affect the moisture content in the substrate, which affects the quality of your final finish. This is because polymer-modified concrete overlays are sensitive to moisture and any water present in the surface you’re covering will become trapped in the polymer-concrete mix where it interferes with polymerization. This leads to a poor quality finish that won’t be as durable as concrete overlay applied in dry weather. To ensure ideal conditions in exterior applications, such as driveways or roads, modified concrete overlays should only be applied after a period of at least 24 hours of warm, dry weather.

Temperature and Gelling Time

• Cure and gel times of polymer-modified concrete overlays are greatly affected by temperature. Gel time refers to the amount of time it takes for the polymer-modified concrete to thicken. Warmer conditions speed up gel time, but cold conditions will slow it down. Modified concrete overlays that take longer than normal to gel because of cold temperatures and this lengthens the amount of time it takes the material to gain strength as well. In regions where the temperature fluctuates, this limits the ideal conditions for application.

Emissions

• Some types of polymer-modified concrete overlays can give off a strong chemical odor. In indoor applications, such as a garage floor, this smell is concentrated, making it seem much more pungent. According to Pavement Interactive, this odor is caused by styrene, which is a monomer emitted during the polymerization process. These are not usually present at levels that are harmful, but styrene is flammable, and it is a carcinogen which is toxic. This is a potential concern when using polymer-modified concrete overlays in residential applications.