Flooring and concrete with a high Fly Ash content don't mix, or do they?

Is there a problem with flooring glued to concrete with a high Fly Ash content?

If you talk to enough flooring professionals on the subject of site preparation and related issues, eventually the question of concrete, high Fly Ash content and adhesive bond failure will crop up.

I've heard the question from all corners of the commercial flooring industry, and there are many concerns but few definitive answers. As a result, many commercial flooring contractors are not warrantying their installations over such concrete. Instead, they add a disclaimer in their 'terms and conditions’ stating that no installation warranty is offered when a certain percentage level of Fly Ash in the concrete mix is exceeded. Some say 15%, others 20 to 25%, some say more. Such disclaimers won't protect the flooring contractor if there is a failure and things turn nasty.

Concrete with a high Fly Ash content results in a denser, less porous product. This in turn can interfere with the flooring adhesive’s (or hydraulic cement underlayment’s) ability to mechanically bond. Hard troweling of the concrete surface to a super smooth finish adds to the problem, and introduces the need for Shot Blasting.  Shot Blasting requires time and money, both of which are in short supply at the end of the project when the flooring is scheduled.

As concrete mixes are proprietary to the concrete supplier, it can be difficult to confirm exactly how much Fly Ash is present in any one mix. If this is the case or where the concrete is super smooth, unusual in color, or if you are just not sure, then perform a droplet test and/or a bond test prior to installation.

Place a dime size droplet of water on the cleaned concrete surface, if it is not absorbed after 60 seconds (or in accordance with ASTM F-3191), you could be facing an adhesive bond issue. If this is the case then you need to Shot Blast to a Concrete Surface Profile (CSP) of 1 or 2, or per adhesive manufacturers’ requirements depending on the floor covering to be installed. (A CSP 2 for example, is similar to 60 grit sandpaper)

Change to an adhesive that is recommended for non-porous substrates. Consult the manufacturer and again, perform a bond test. Remember that the floor covering manufacturers’ installation requirements will likely be met but what about the adhesive manufacturers’? This is the important one, because any flooring system is only as good as its bond to the sub-floor.

Also, old emulsifiers and cutting oils hiding in the concrete can cause all kinds of problems if they aren't identified prior to installation. Bond tests won’t always show this up as they can take time to eat away at the new adhesive.

Calcium Chloride tests are part of the problem

The industry is talking about the Anhydrous Calcium Chloride moisture test method not being reliable over such concrete because of its density.

The recommended test method is the RH, In-Situ Probe (ASTM 2170-16). This test gives a true moisture reading at a 40% depth in the slab.

In many cases both test methods are called for as they are designed to measure different moisture related characteristics of concrete and are important when establishing proof that acceptable conditions existed prior to flooring installation.

Better still specify a concrete treatment product that can be introduced at time of pour. Some products claim to eliminate the need for moisture testing altogether (while also reducing slab curl by over 90%). If this is the case, problem solved!

Because Fly Ash is a by-product of burning coal, using this waste product in concrete makes financial and environmental sense and so it's easy to see why its use will likely increase over time.

Meanwhile the advice remains the same, proceed with caution, ask questions and conduct proper testing before you install.