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Forging a Bond: Surface Prep’s Tie to Strong Adhesion

A D+D Online Feature published November 11, 2010



More items for Good Technical Practice

by Jayson L. Helsel, P.E.

Performance shortcomings of coatings applied to concrete-floor surfaces are certainly not uncommon, and can be manifested in the form of coating delamination (loss of adhesion), blistering, and other modes of failure or weakness....
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Tagged categories: Concrete coatings and treatments; Floor coatings; Surface preparation

Comment from Jay Barstow, (11/12/2010, 10:43 AM)

Shot blast? ... I don't THINK so! You'll end up with a topographical map of Tanzania. Adhesion is never consistent, some areas adjacent to others, for some quirk of fate, will exhibit greater adhesion. These will initially resist the blasting effect, whilst the poorest adhering will be whisked away. What happens is that the exposed concrete will be costantly abraded while the adhered product will protect the coating underneath. By the time ALL the coating is removed, the depth of blast will be all over the place. Use a planetary with PCD blades or a Pearl head with carbide chips. Besides, manuvering a blast machine in confined areas is sure to engender foul language.


Comment from Jerry LeCompte, (11/12/2010, 12:01 PM)

I must agree with Jay, shot blasting can be way too aggressive for coating removal with the possible exception of resurfacing. Shot blasting does little for the removal of contaminants like grease, oil and soot from fire. Also the blast media is recycled and can become contaminated, serving as another source for exacerbating potential coating failure. Soda Blasting may be considered especially when a verity of complex surface materials comes into play.


Comment from Tony S. Knoedl, (11/12/2010, 1:44 PM)

I do agree shotblasting would not be the method I would employ for removals. In defense of Mr. Helsel, I think he would expect a quality applicator to remove exising coatings utilizing either planetary grinders or mechanical scrapers(wether pneumatic hand held or battery/propane ride on) followed with proper preparatory methods-shotblast being the preferred for open areas and vacuum attached 7" handheld diamond grinders for areas inaccesible to large equipment. We have all seen this phenomonon, only not on our job. I feel the article itself stresses the importance of proper surface preparation as well as sound application procedures. I wonder who was the applicator? Too many fly by nights.


Comment from Kevin Malone, (11/12/2010, 2:12 PM)

YES on shotblasting only if in prep for high build/thick mil epoxy systems and slab is deemed suitable for mechanical prep and such a coating system in the first place. Concerns of inconsistant depth would then be invalid and mil thickness/leveling cancels out any issues. If only a coating system is being removed then yes carbon blades can be used to some success if slab is smooth underneith. It will then take a myraid of non-permenant coatings like wax/acrylic but not at a strengh level compared to actually profiling a slab. Best of all worlds in terms of acheiving an even depth yet proper profile for adhesion of both thick build or thin mil urethanes or acrylics/stains is by way of using diamond grinding methods. Regardless, no mechanical method is sound on it's own if dust/powder is not removed via a wash or autoscrub. An 100-120 grit profile will prevent a good deal of the possible adhesion strength by way of trapped powder in it's scratch pattern. The peaks and valleys of a scratch pattern being totally clean is of utmost importance. Sweeping or dy vacuuming doesn't cut it...been there done that. Even though a dry prep grind or blast method will outperform a smooth scrape method everytime they could offer even better performance if cleaned proper prior to coating. This of course prevents the fast in/out waterless business models being marketed or demanded upon contractors. Many of us have prescribed to this plan until we learn from experience that it is not best. With proper neutralization/rinse there is also nothing really wrong with old fashioned acid etching unless the slab is reinforced with rebar at shallow depth or if an acid stain is to be used in short order. Acid will effect the protective coating surrounding the rebar that is formed by chemical reaction when originally installed so it is not advised if rebar depth is unknown. Any of the 3 profiling methods of blasting, grinding, or acid etch can work fine but all are subject to weak slabs void of proper composition, moisture/vapor transmission from underground sources. Hydrostatic pressure will kill any coating system. . Such is the case of many a older slab wanting a new coatings. It's like putting lipstick on a pig. This field is all about clarifying expectations and preventing talk of high hopes and permenance. Only thing permenance applies to in respect to a coating system is the difficulty in which it may be to remove it. All systems require proper ongoing maintenance to look good or stay intact. Put a scrape to even the best slab and coating system and watch the ingress of water and contaminates work it's way into delaminating the surrounding area...just my two cents :) The coating system in this article showing residue can be either a result of subpar dry prep methods or a weak slab to begin with or likely a combination thereof.


Comment from Kevin Malone, (11/12/2010, 2:17 PM)

++1 Tony..I agree that Helsel did not equate shot blast prep to a coating removal procedure. The importance message that should be taken away is that sacraficing a proper profiling or determination of slab suitability is an outline for disaster.


Comment from Jay Helsel, (11/13/2010, 7:50 AM)

Many good points have been raised – shot blasting should be used cautiously. Although blasting may be aggressive, a variety of outcomes can be achieved based on the size of the shot and operation of the machine. Preparation of test sections showing the intended surface preparation (and coating application) are always a good idea. The coating manufacturer of the intended coating should also be in agreement on the surface preparation method. Shot blasting is generally more appropriate for preparation when applying thicker film coatings.


Comment from Roy Guzzio, (11/14/2010, 1:10 AM)

I agree that surface prep is crucial to adhesion, but I have not seen any test results from calcium cloride tests or any PH tests done on this failed area. Has anyone addressed the MVT on this slab?? As coatings pro's and contractors, we must protect ourselves against vapor drive as being a cause for failure and determine the appropriate flooring system that should be installed based on the slab as well as the customers expectations of the system being installed (thin mil coating, decorative coating, cementitious urethane coating)?. Only after these questions are answered can you determine the appropriate process of concrete preparation.


Comment from ROBERT BOONE, (11/14/2010, 10:52 AM)

Just a thought...is anyone who is commenting actually well versed at applying these type of products? The main issue is to get a supplier to "verify" the substrate is sound/suitable enough for the coatings application. The multi-national suppliers have experts paid to assist the contractor in this arena, and then move forward from that point on. If you are in the trade, the relationship built with your supplier is an integral part of the job, dialing in the right system whether its blastrac TM and or grinding the substrate to (lightly profile) the substate are two popular ways to ensure proper bonding characteristics. Due diligence in testing the "existing conditions" for any unforseen problems, and a proper "test patch" layed down with a 30 day (waiting period) as well for the client/contractor realtionship will avoid most nighmares. Also some products are more forgiving than others ...just my 2 cents > Boone Coatings Incoporated. By the way the last project/failure i was brought into diagnose was due to MVT as per the previous comment from Roy >thanks Robert


Comment from Jay Helsel, (11/16/2010, 6:30 AM)

Moisture was not an issue in this case - although it is the most common reason for problems with concrete floor coatings in our experience.


Comment from Ian Rolfe-Vyson, (11/16/2010, 3:45 PM)

I agree,using blastrac to remove old coatings or for that matter preparing new concrete floors is to harsh leaving the surface to rough for most coating systems. The object of surface preparation is to create proper adhesion of a coating over a concrete substrate. Adhesion is the key to coating effectiveness and it determines whether the coating is merely a thin sheet of material lying on the surface or whether it becomes an actual part of the substrate. Diamond grinding is the preferred method and the surface should resemble P180 grit sandpaper in texture. The floor should be thoroughly clean, dry and dust free before painting. If these conditions are met, the overall coating should encounter no application problems, and should be tightly bonded, providing protection for a long time.


Comment from David Grove, (11/30/2010, 12:06 PM)

Jay, you made no comment about the poor surface preparation being the responsibility of the contractor since the specification stated "should be performed if necessary". All manufacturer's product application data would identify the required surface preparation criteria. Therefore, wouldn't the rework to dictated by the warranty terms of the contract? Removal of the coatings would be to his decision provided it made minimum interference to the owner (by owner's approval) and met the criteria of the manufacturer's product appliction data and recommendations. We can all make comments about how to do the rework, but I have always understood that the first step should always be as defined in the contract.


Comment from David Grove, (11/30/2010, 12:06 PM)

Jay, you made no comment about the poor surface preparation being the responsibility of the contractor since the specification stated "should be performed if necessary". All manufacturer's product application data would identify the required surface preparation criteria. Therefore, wouldn't the rework to dictated by the warranty terms of the contract? Removal of the coatings would be to his decision provided it made minimum interference to the owner (by owner's approval) and met the criteria of the manufacturer's product appliction data and recommendations. We can all make comments about how to do the rework, but I have always understood that the first step should always be as defined in the contract.


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