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Caution: Flooring and High Fly Ash In Concrete



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 percent, others 20-25 percent, some say more. Such disclaimers won't protect the flooring contractor if there is a failure and things turn nasty.

Friedrich / Getty Images

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.

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 manufacturer’s installation requirements will likely be met, but what about the adhesive manufacturer’s? 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 percent 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.

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; and
  • Conduct proper testing before you install.


About the Author

Chris Maskell is the President of The National Floor Covering Association 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.


Let's Fix Construction is written by a collective group of construction professionals involved in, an online impartial platform to provide forward-thinking solutions to many longstanding issues that have plagued construction. Organizers and contributors seek to better the industry by sharing knowledge, while creating open and positive communication and collaboration. Many of the posts have appeared first on and are republished on Durability + Design with permission. Author information is available at the bottom of each blog entry.



Tagged categories: Architects; Business management; Construction; Consultants; Contractors; Design; Designers; Developers; Engineers; Good Technical Practice; Specifiers; Asia Pacific; concrete; Concrete; Concrete defects; EMEA (Europe, Middle East and Africa); Latin America; North America

Comment from Robert Bullard, (12/21/2018, 10:08 AM)

This is not a problem for the concrete industry to fix, rather for the floor adhesive industry to fix, and I think that the secret lies in one or more 'primers'. I suggest you head to the World of Concrete ( next month and talk to the concrete chemistry, coatings and admixture mavens exhibiting about this problem. One that I have worked with in the past with good results is Aquron.

Comment from Avi Dwivedi, (3/19/2019, 1:12 PM)

Tobslak offers a full range of products to decorate, protect, waterproof, repair, and restore concrete, masonry, brick, stone and wooden substrates. In addition to providing the highest quality products, we are devoted to supporting our clients with technical expertise and practical know how gained over many years of on-the-job experience. We follow global standards in building decoration, protection and in waterproofing, floor, facade paint and wood protection,also we provide complete product systems at one source, from cellar to roof. The company has gained market leadership globally based on local focus because of the strength of the Tobslak system, which comprises our companies and our nearly 21 partners worldwide. For more than three decades, Tobslak has specialized in all types of Facade Paints, Wood coating, Epoxy flooring and Car park coatings, Waterproofing, high quality, long lasting external facade and internal paints and coatings. Our coating systems are not just robust and reliable, they are also gentle on the environment and are physiologically safe.

Comment from Benjamin Sellers, (12/22/2020, 10:56 PM)

We've noticed a lot of the individuals who are laying a stamped concrete driveway these days.

Comment from Marcos Chino, (2/9/2021, 6:50 AM)

There's a lot of fly ash when it comes to using material like concrete. This article is very helpful on handling the fly ash, and this will relatively reduce the hazards.

Comment from Gregory Johnson, (2/6/2022, 10:28 PM)

I'm going to tell my concrete company about this article. Thanks!

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