Repair front porch paint

snipped-for-privacy@snyder.on.ca;3076053 Wrote: >
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How do you know this?
The PVA resins used to make general purpose primers and inexpensive paints are generally LESS alkali resistant then acrylic resins used to make specialized primers and higher priced paints. That is, a general purpose PVA primer will crack and peel when painted onto fresh concrete before an acrylic paint or primer would.
But, if high alkalinity supposedly makes PVA more moisture resistant, supposedly by promoting denser crosslinking, then the resulting PVA film would be stronger because of that denser cross linking as well, and so it should hold together better than an acrylic primer or paint film. But, in fact the opposite is true. Acrylic primers and paints are MORE alkali resistant than PVA primers and paints.
So, what you're saying about alkalinity enhancing the moisture resistance of PVA just doesn't seem to ring true at first glance.
You should also know that the only reason fresh concrete is highly alkaline is because it's made with lime, or calcium hydroxide. Calcium hydroxide is Ca(OH)2, and it's those hydroxyl (OH) groups in the concrete that make it alkaline. Gradually, over the course of one or two years, most of the calcium hydroxide near the surface of the concrete reacts with carbon dioxide in the air and is converted to calcium carbonate, CaCO3, also called "limestone" or "chaulk". As that chemical reaction happens, those OH groups disappear and the alkalinity of the concrete subsides. That conversion of hydroxide to carbonate is part of something called the "Lime Cycle", and it happens in every cementatious material made using lime, including tile setting thin set mortars, brick mortar, stuccos and real lime based plasters. Lime, Ca(OH)2 is highly alkaline, but chaulk is only a weak alkali; so weak in fact that we swallow it in the form of TUMS tablets to neutralize excessive stomach acidity.
'What is Lime? | Products | Graymont' (http://www.graymont.com/what_is_lime.shtml )
So, whomever told you that the pH of concrete affects PVA's moisture resistance didn't know that only FRESH concrete is highly alkaline. Old concrete is not highly alkaline, and so the PVA resins that get painted onto the old concrete would NOT be affected by the alkalinity the way any PVA resins actually mixed in to the fresh concrete might.
snipped-for-privacy@snyder.on.ca;3076053 Wrote: >
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Believe it or not, no one actually knows what actually happens when you mix water with a mixture of portland cement, lime and stones to transform it into the hard waterproof compound we call "concrete". We've known what we have to mix together to make concrete from Roman times, but we don't actually know why that particular mixture of materials, when mixed with water, cures to a compound that's harder than any of the constituent materials used to make it, or why the resulting compound doesn't dissolve in water. Obviously, chemical bonds form that are stronger than the hydrogen bonds that would cause it to dissolve in water, but what actually happens at a detailed technical level is unknown. It's one of the most mysterious substances that we're completely comfortable using because we've learned how to use it very well over the past 2000 years.
So, what happens inside concrete when you add PVA resins is also unknown, but if I had to guess, I'd certainly say that the PVA plastic remains a separate material within the concrete. That is, there would be no chemical reactions between the concrete and the plastic.
Since you use a concrete bonding agent by painting it onto the surface of the old concrete you want the new concrete to stick to, AND, that the concrete bonding agent can dry before you apply the fresh concrete and the moisture from the fresh concrete will reactivate the adhesive quality of the bonding agent, then I'd say that the concrete bonding agent sticks to the old concrete and diffuses into the fresh concrete. So, what you have inside the concrete at the repair would be a matrix that goes from pure concrete to pure PVA over a distance of less than 1/4 of an inch, and the PVA plastic on the pure plastic side is sticking to the old concrete. Based on the way concrete bonding agent is used, that's what makes the most sense to me.
snipped-for-privacy@snyder.on.ca;3076053 Wrote: >
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> portland cement.
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If you can point me to any website that confirms that the high initial pH of concrete has any effect at all on the moisture resistance of the PVA resins used in white wood glue, I'll spend some quality time on that web site.
snipped-for-privacy@snyder.on.ca;3076053 Wrote: >
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> bonding agent for decades with good results on porches and garage floors > - and also in foundation parging.
I have nothing but respect for your neighbor, but like so many other people of his generation, he learned his trade from others who knew exactly what to do, but never understood the science behind what they were doing and why they were doing it. I fully expect that he put Bondfast wood glue into lots of the concrete he used for repairs with the full knowledge and approval of his boss.
But, I also fully expect that neither of them knew that there were different KINDS of PVA resins, and that if he had used a different PVA resin which would have crosslinked after it dried to produce a completely waterproof glue, then those repairs would have been done even better than they were.
Finding old guys with 600 years experience that can't explain why they're using one chemical in one situation and another chemical in a different situation is NOT uncommon because they learned their trade from master craftsmen that didn't know those things themselves, and therefore couldn't explain it to their underlings. Nowadays, with many of the people doing repair work on concrete (whether amateurs or professionals) having university science backgrounds, those people are asking the technical questions that the previous generation never did, and so the old explanation "But, we always did it that way!" is more and more being questioned by people who understand the science and chemistry of it all and can tell a good answer from a lousy one.
And, of course, the internet is causing an explosion in public knowledge just like the printing press, the radio and the television all did in their respective times. People are learning from each other over the internet because regardless of their educational background, they can recognize a correct explanation when they hear one because it MAKE SENSE to them.
These are both things that have happened just over the past 20 to 40 years since your neighbor was learning his trade, so it's not surprising that there's a totally different mind set between people doing the same kind of work only one generation apart. Your neighbor probably never knew what was in the Bondfast he was adding to his concrete; only that it was sticky and made new concrete stick to old concrete. Nowadays, just one generation later, you and I are discussing why you'd use one kind of PVA resin for assembling real wood furniture but a different kind for fixing an outdoor concrete step. Learning and communicating that knowledge is something we humans do extremely well; it was absolutely pivotal in our evolution from apes.