Polymer-modified concrete is commonly used in many applications. You should know what polymers you are using and their strengths and weaknesses. Making the right choice could determine the success or failure of an installation—and the resultant liabilities.

Today latex polymer concrete mixes are being used increasingly as concrete patching materials, impermeable toppings over bridge decks, grout for installing tile, additives for plaster, stucco and EIFS, and a wide variety of decorative thin topping finishes.

Only five polymers are commonly used with concrete: acrylics, styrene-acrylics, vinyl acetate ethylene (VAE), polyvinyl acetate (PVA), and styrene-butadiene resin (SBR). In liquid form they come in two parts, liquid and powder, to be mixed together. In dry form they are prepackaged materials blended for specific applications. Whether in liquid or dry form, each polymer has strengths and weaknesses and recommended uses.

There are many reasons to add polymers to concrete overlay mixes. Polymers improve adhesion to the old surface, flexural strength, tensile strength, and freeze/thaw durability. They also reduce permeability, the intrusion of chlorides, salts, and carbon dioxide. They increase abrasion resistance and can be applied in very thin cross sections.

No one polymer performs best to achieve these characteristics; each has strengths and weaknesses. In addition, some polymers resist ultraviolet light (UV) better, transmit water vapor more efficiently, and do not re-emulsify when they are re-wetted.

Many characteristics of portland cement remain the same when polymers are added.

There are many different formulations of monomers and each manufacturer combines them to create polymers with specific characteristics.

Acrylic polymers have the best water vapor transmission rates (breathability) of all the polymers.

Styrene acrylics are very similar to acrylics in their performance with improved water resistance and less UV stability.

Vinyl acetate ethylene (VAE) can be packaged either wet or dry and is the most-used polymer for prepackaged products.

Styrene-butadiene copolymer resin (SBR) has the best adhesion of all the polymers mentioned, the best resistance to water, and the best abrasion resistance. It is also most cost-effective for thick overlays, but SBR polymers have poor UV stability.

Polyvinyl acetate (PVA) is the least expensive and the most re-wettable of the group. It's subject to hydrolysis in wet alkaline environments causing breakdown of the polymer.

The best adhesion to the base slab results when a bonding agent is first applied.

Most installation failures are bond related—poor preparation of the base slab. The International Concrete Repair Institute has published a book titled Guideline for Surface Preparation, which has excellent preparation guidelines for each type of patch and overlay installation.

There are a number of other potential problems.

Even though the experts disagree about the properties of polymers and how they act in concrete, you should know their strengths and weaknesses and make careful selections for your project to avoid expensive failures.