Caution: Flooring & High Fly Ash In Concrete

Contributed by Chris Maskell

​Is there a problem with flooring glued to concrete with a high fly ash content?  Fly ash is the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gasses. It is used as a replacement for Portland cement in concrete and in some cases can add to the final strength, increase its chemical resistance and durability and can significantly improve the workability of concrete.

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 water absorbency test in accordance with ASTM F-3191 and/or a bond test prior to installation.

Place dime sized droplets of water on the cleaned concrete surface, if they are 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 be affected, as the emulsifiers and oils 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 (ASTM F1869) method not being reliable over such concrete because of its density.

The recommended test method is the RH, In-Situ Probe (ASTM F2170-16). This test gives a true moisture reading at a 40% depth in the slab. (Editor's Note: you can read about the latest on F2170 in this post)

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.

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

Meanwhile, the advice remains the same:

• Proceed with caution
• Ask questions
• Conduct proper testing before you install

(Editor's Note: Chris Maskell is the CEO of The National Floor Covering Association (NFCA) in Canada, which promotes industry standards for resilient, carpet, hardwood, laminate, cork and bamboo floor covering installations. Their mission is to engage professionals in the construction industry through education and compliance to national floor covering installation standards which provide a quality assurance platform to ensure successful installations on commercial projects.