We can actually assess the effect of relative humidity that the adhesive seize by doing testing.
This picture shows a whole series of standard test blocks of concrete set over standard relative humidity solution. And what we do, is take these standard blocks of concrete; place them on top of panels of saturated salt solutions. The water then provides a standard environment of 75% humidity or 85% or 95%. And then we let those equilibrate.
And then we go along and show the next picture, and we can actually pull the adhesive on the flooring off of the concrete by doing what’s called “Adhesive Bond Test”, “Tensile Bond Test” or “Shear Bond Test”. And those give us the strength of the bond after the concrete has been exposed to moisture for some length of time. By doing this, we can actually measure how the moisture’s affecting the adhesive bond as shown in the graphs on this next slide.
There are two different adhesives that we tested here as examples that we’re showing. The vertical left-hand axis shows the Tensile Bond Strength. 20, 40, 60 up to 140 pounds per square inch of bond strength. Across the bottom we see different exposures to humidity, 75%, 85%, 97%.
Let’s look at the left-hand graph first, the Adhesive #1. When an adhesive was exposed to 75% humidity as shown in the last photograph of the experimental set-up, we found we could get a 100 to 120psi bond strength. Some of that same adhesive was exposed then to 85% relative humidity in the concrete and the strength went down to between 80 and 100psi, lost about 20% of its strength. The third experiment was measuring down at 97% relative humidity and you see the bond’s strength has been cut in half. So, here’s an example of an adhesive that’s strongly affected and we can measure that effect very precisely and very accurately. And we’ve seen that it went from a 100 or 120psi down to around 60psi. As the humidity in the concrete went from 75%, to 85%, to 97%.
So, as the relative humidity increase, bond strength decreases. And now, the manufacturer can take this data and set a limit. And say, okay, 75% is good, 85% we’ve loss 20% of our bond strength, and we’re going to set a limit of 75% for this particular adhesive. And that’s a precise scientific technical way of doing it.
So, we’re passed the days of simply saying 3 pounds for sheet final and 5 pounds for VCT. We can actually establish very accurately and with the scientific validity, the relative humidity number to use for a particular adhesive in floor covering combination.
The second graph results are very different, over the right-hand side for Adhesive #2. We see that the strength of the adhesive bond, first of all, is quite a bit lower, around 25 to 30psi. And the reason is that the VCT adhesive where we don’t need a lot of tensile bond strength. We need better in plain shear strength, but the tensile strength is not so important. With this particular adhesive, we see there’s virtually no change in its bond strength, going from 75% to 85% to 97% humidity. This is an adhesive that is very moisture resistant.
By looking at this data, the manufacturer can say, we know we’re to work up to 95, 97% humidity, we’ll put a safety factor in there and we’ll sell this adhesive up to 90% relative humidity.
And this is how we can determine a safe number for the performance of an adhesive in a scientifically valid fashion. And that’s what adhesive manufacturers are having us do in our facility.
So, in summary, this next slide we see a few bullet points that point out the relative humidity limit for an adhesive in a four-covering system will vary with the product and the particular adhesive. So, we can have vital composition tile, which will perform differently, depending upon the adhesive that’s used. And the opposite is true; we may have various adhesives that are suitable at different moisture levels for a particular flooring product.
It can be determine scientifically in a valid way through testing. We can evaluate the bond strength versus adhesive at a safety factor. And now, we can come up with maybe a tiered system of adhesive that is suitable for different floorings. And the beauty of the system, is going to be that we can go out to a job site and measure the actual moisture condition in the floor. Depending on that moisture condition, we may then select the most appropriate adhesive to get good performance and to have a warranty. And this seems to be the direction that the industry is moving.