Wednesday, September 1, 2010

Painting Old Concrete

Paint doesn't work very well on bare concrete.... it always seems to be peeling off and the concrete underneath is always a bit decomposed.  When I was in college, Rick, one of my TA's in chemistry was working on polymers that could be used to preserve rock and concrete surfaces.  Since that time there have been many products of that sort placed on the market.  My house is built on a concrete pad and concrete on the sides of the house, where exposed to the weather, has surface problems.  It occurred to me that I could stabilize that concrete with one of the concrete sealers that can be found on today's home improvement market.  If they are anything like the compounds that Rick was working on, they might do a pretty good job.

Concrete is a mixture that contains Portland cement which reacts with water through a process called hydration.  This process continues after the cement is hard, so the older concrete is, the harder it becomes.  Portland cement (or OPC) consists mostly of calcium compounds; pretty basic in the acid-base scheme of things.  Air contains carbon dioxide which, when mixed with water, produces a weak acid solution.  Over time, in my case about 50 years, this very slow chemical reaction can significantly impact any exposed concrete on a structure.

So here's the problem.  A scratch in the paint exposes concrete to air where carbon dioxide in the air reacts with the concrete.  This expands the imperfection in the paint, exposing more concrete to the air; a viscous circle but a slow one.  My thought is that the concrete sealer will prevent or at least slow down the reaction process and then paint can be applied over the treated concrete for cosmetic appearance.

First step in this process was a good scraping of the old paint.  I worked the scraper into the borders of paint to make sure all the loose paint was removed.  I followed this with a pressure wash of the concrete to be painted.  Care should be employed here, as the pressure washer can dig out the concrete if pressure is too high.  The concrete was then allowed to dry completely before the sealer is applied.  I used a spray bottle and drenched the concrete until product started to run off.  When we finally got an August day with little chance of afternoon showers, Gena and I finished painting the pad.  I should know in a couple of years how well this worked but I'm sure it will be better than a simple coat of paint.... time will tell but hey!  Thanks Rick!

EDIT:  It is now January, 2016.  The paint on the concrete treated with sealer is still solid.  The paint is now peeling in places where I painted over untreated concrete, and on other surfaces the wood is beginning to show on the fascia boards on the edge of the roof.  I would say that treating the concrete was very successful.