Concrete is hard, but not always hard enough. Some concrete floors wear out before their time. To make them last longer, the marketplace offers a wide range of products, including dry-shake hardeners, sealers, and concrete admixtures.
But, a critical question: Do they work?
The theory
Liquid hardeners are supposed to work like this:
1. Concrete normally contains some free lime—calcium carbonate—which contributes nothing to strength and durability. It’s there because the chemical reactions that make concrete hard don’t reach every last bit of cement.
2. Every concrete surface contains pores, which weaken it.
3. Certain chemicals, when dissolved in water, react with free lime to form C-S-H gel. These chemicals consist of silicate ions combined with one of the metals from the left side of the periodic table: lithium, sodium, potassium, or magnesium. The silicate ions are the key to the reaction. The metallic ions are relatively unimportant, though they may play a role in how the solution penetrates the concrete.
4. The C-S-H gel fills pores in the floor surface, making the concrete stronger and more resistant to wear.
That’s the theory, and it makes sense. Part of it—the chemical reaction between silicate ions and free lime—is beyond dispute. But theories require proof before they can be accepted as fact and this is where our story grows murky. Though liquid hardeners have been around for many years (Albert Moyer proposed their use in 1910 in The American Contractor magazine), no one has yet proven that they improve the wear resistance of actual concrete floors.
Manufacturers’ claims
Evidence in support of liquid hardeners is easy to find, but almost all of it comes from companies that sell hardeners. The source of the evidence doesn’t invalidate it, of course, but test results that appear only in manufacturers’ literature, and often only in abbreviated form, deserve special scrutiny. And when you scrutinize the manufacturers’ claims, three problems appear.
The first is that the reported results vary greatly. Graph 1 shows all the manufacturer’s claims I was able to find in August 2013. I show each result as the percentage decrease in wear depth compared to untreated concrete (see sidebar below). Where the source document presented the result as an increase in wear resistance, I converted it to a decrease in wear depth. The range is wide even if you count only the claims manufacturers make for their own products,ranging from 20% to 91%.
The range grows wider still when you add the reports in which one manufacturer looks at another manufacturer’s product (see Graph 1). The two sets of test results scarcely overlap. Manufacturers consistently report good results for their own products, and bad results for the competition. The average of all the reported values for the manufacturers’ own products is 48%. In contrast, the average for competing products is just 1%. Some manufacturers even report results below zero for competing products. If true, that means a product sold as a hardener actually made concrete softer. If we look at all of the manufacturers’ reports,both sides of Graph 1,the results range from a 35% increase in wear depth to a 91% decrease.
There may be good explanations for the wide range in test results, but such explanations are out of reach because of the second problem with the manufacturers’ claims: the lack of information about the test conditions. Some claims leave out the most basic facts, such as the name of the test method. Some reports provide more information, but never as much as you would expect.
I can hardly overstate the importance of the test conditions. Take the example I discussed above, in which Companies A and B reported vastly disparate results for Company A’s hardener. Do you suppose the two companies might have conducted their tests under different conditions? I’d bet on it.
The third problem with the manufacturers’ claims involves the distinction between laboratory and field testing. In every case where the test location was made clear, the work occurred in a lab. While lab and field tests are both useful, one cannot replace the other. If you want to learn exactly how a hardener works, the laboratory is the better place. But if you want to determine whether hardeners work in the real world, you have to test them there.
U.K. research
What we need is testing performed by people who don’t sell hardeners. As far as I can tell, there hasn’t been much of it. But at least two independent researchers, Massud Sadegzadeh and Roger Kettle at Aston University in the U.K.,investigated liquid hardeners in the 1980s and published the results both in Europe and America.
Sadegzadeh and Kettle used a Chaplin abrasion tester, which relies on rolling steel wheels to create wear. They tried two liquid hardeners, a sodium silicate and a magnesium fluorosilicate, and they applied both to three concrete mixes having different water-cement ratios. The testing was done in the lab, but on slabs big enough that they had been finished with power trowels, using methods similar to those a contractor would employ on site.
Graph 2 summarizes what they found. When you compare these results to the manufacturers’ claims, two things stand out. Sadegzadeh and Kettle’s reported values fall in a much narrower range, and they are near the low end of what the manufacturers report when testing their own products. The U.K. tests show, on average, a 28% reduction in wear depth, with practically no difference between sodium silicate and magnesium fluorosilicate, and an overall range of 15% to 38%. In contrast, the manufacturers claim, on average, a 48% reduction for their own hardeners, with a range of 20% to 91%.
Interestingly, Sadegzadeh and Kettle investigated other surface treatments, some of which performed much better than the two liquid hardeners. Rock-based dry shakes did poorly. But a metallic dry shake reduced wear depth by 63%, on average, with a range of 50% to 71%. Three polymer-resin sealers were even more effective, reducing wear depth by an average of 81% within a tight band of 72% to 88%. Sadegzadeh and Kettle also looked at curing methods, reporting that a seven-day cure under polyethylene did more to reduce wear than either of the liquid hardeners they tried.
The UK research provides independent confirmation that liquid hardeners can indeed harden concrete, at least in a laboratory. It does not, however, tell us how liquid hardeners perform on real construction sites.
My experience
I’ve been measuring the wear resistance of concrete floors since 1988, when I bought one of the earliest Chaplin abrasion testers offered to the public. At the start I had no plan to investigate liquid hardeners, but opportunities arose over the years. Eventually, I was able to measure the effect of liquid hardeners on six concrete floors, and a clear pattern emerged: The hardeners didn’t do much.
The six hardeners I tested made no significant improvement in wear resistance. Sometimes the treated surface tested a little better than plain concrete, and sometimes it tested worse. But the differences were always small compared to the normal variation from test to test.
My experience spans a range of floors—good and bad, new and old, troweled and polished. All were built by concrete contractors under normal site conditions. All but the last were real working floors, not laboratory specimens. Here is the whole list:
1. A warehouse floor showed excessive wear, mainly in the form of scratches from dragged loads, after one year of use. The problem was blamed on lack of curing. The builder tried two remedies: an acrylic-resin sealer and a sodium-silicate hardener. The acrylic worked, cutting wear depth by 70%. The sodium-silicate did not work, reducing wear depth by a negligible 2%.
2. The floor in a food-processing plant was dusty and easily scratched. The surface was so weak I did not finish the standard 15-minute Chaplin test, stopping after five minutes to avoid damaging the machine. The problem was blamed on carbonation, caused by the use of unvented kerosene heaters during the concrete pours. A sales representative proposed a sodium-silicate hardener. The plant owner agreed to a small trial, insisting on wear tests before and after. The representative applied the hardener himself, soaking the porous surface at over three times the normal application rate. I asked the representative how long I needed to wait for the hardener to do its job, and was told the effect would be instantaneous. Nevertheless, I gave it a week. But once again I had to abort the test after five minutes. With or without hardener, the wear depth exceeded 3 mm, the worst I’ve seen on any floor. Other remedies proved equally useless, and in the end the contractor overslabbed the whole floor with 4 inches of new concrete.
3. A factory floor got a polished finish, followed by a lithium-silicate hardener. To answer questions about the effect of polishing, several different finishes were tried in a non-critical area. I tested the floor a month later, and Graph 3 shows what I found. The troweled finish and the 800-grit polish tested about the same. The 1500-grit polish did much better, but the lithium silicate made no difference.
4. A warehouse floor was treated with a liquid hardener right after construction, with one small area left bare. When the floor was about three months old, the hardener manufacturer hired me to test the floor. The hope, of course, was that my test would show less wear on the treated surface, but the opposite happened. The plain concrete actually tested better, though the difference was tiny. You will not be surprised to learn that my report was never distributed.
5. A warehouse floor showed premature wear, with severe dusting and ruts in the forklift paths. The cause remains a mystery. After a year, the designer-builder hired me to evaluate three proposed fixes: grinding, an acrylic sealer, and a sodium-silicate hardener. Grinding didn’t help. The acrylic did, though the owner eventually decided against it on the grounds that it might not last. The sodium silicate had no measurable effect.
6. A concrete contractor placed a test slab of about 800 square feet, made of everyday 4000-psi concrete. The concrete was power-troweled to a burnished finish and cured under polyethylene for seven days. When the concrete was 28 days old, I treated part of it with a sodium-silicate hardener, applying two coats at a rate of about 300 square feet per gallon, per coat. Two hours later I measured the floor’s wear resistance, making six tests on plain concrete and three on hardened concrete. Graph 4 shows the results for all nine tests. The individual readings varied widely, but the averages for plain and treated concrete were practically the same: 0.109 mm for plain concrete, and 0.114 mm—5% worse—for concrete treated with sodium silicate.
Conclusions
It’s time to circle back to the main question: Do liquid hardeners help concrete floors resist wear?
Going just on my own testing, I would have to answer no. I’ve tested hardeners on six floors, and not once did the hardener significantly reduce wear depth.
Liquid hardeners are not new, however, and I am not the only one to have tested them. I cannot ignore the test results that contradict my own. Sadegzadeh and Kettle found that liquid hardeners modestly improved wear resistance, under certain conditions. The manufacturers report even more improvement, at least when talking about their own products.
Can we reconcile those contradictions? I suspect we can, but it will take a combination of laboratory and field tests over a wide range of conditions. The results could point the way toward a more efficient use of liquid hardeners, or they might lead to the general abandonment of the products. Until that research is done, anyone deciding whether to use a liquid hardener will have to choose which evidence to rely on.
George Garber is a concrete flooring consultant with Face Consultants, Lexington, Ky.
Measuring Wear Resistance
We care about wear resistance, but we measure wear depth. One is the inverse of the other. I have tested wear depth with the Chaplin abrasion tester, a machine with steel wheels that runs for a fixed length of time or a fixed number of revolutions, wearing a groove in the floor surface. The test result is the depth of the groove, in millimeters or inches. When reporting wear depth, high numbers are bad, and low numbers are good. Some manufacturers prefer to talk about wear resistance rather than wear depth. When you look at wear this way, high numbers are good, and low numbers are bad.
—George Garber
