"My driveway and sidewalks are crumbling"
Column #836 03/05/2011
On The Level
Q. Our house is three years old in a planned development. During the first winter the developer hired a company to shovel snow and they applied salt deicer to the concrete walks, driveway and front porch. In the spring we noticed the surface of the concrete was crumbling in some areas of the driveway and on the corners of the porch. The developer replaced part of our driveway in the summer and in the fall had someone put a light gray concrete type coating on the porch. Over this winter the porch coating cracked where we walked and it is now coming off. The surface of the concrete that was underneath this area is crumbling.
We don’t know what type of salt was used by the developer the first winter. Our community hired a contractor this past winter who claims he used only calcium chloride. My question is was the salt used either the first or second winters to blame for the damaged concrete, or was the concrete itself faulty? The developer wants to send someone to recoat the porch when the weather warms up. What chance is there that this will adhere and stop the damage? Do you have any other solutions?
A. Unfortunately the prognosis isn't good. The condition your concrete is in is known as spalling or scaling and is the result of a variety of causes. De-icing chemicals and freeze-thaw cycles accelerate the problem but the root cause is the concrete itself. I’m sure you’ve seen concrete sidewalks and driveways or roads that get salted every year, year in and year out, that don’t scale up. Imagine the beating concrete on interstate highways Like I-97 or Route 50 gets. Calcium chloride is less aggressive than sodium chloride--rock salt-- but if the concrete is susceptible either one will help the process along.
The problem can begin when concrete is batched and poured. We all view concrete as tough stuff but the truth of the matter is that each step of preparation, handling, placing, finishing and curing concrete bears directly on its future performance. It's unlikely the batch was bad from the plant in the sense that it was mixed improperly or mixed with adulterated materials. The concrete plants have too much invested to send out a product that wont perform from a basic mix point of view. It’s possible but so rare I’ve never seen it.
If the concrete was finished when the weather was hot and the top wasn't protected from the sun that could have weakened the top. Or conversely, if it was too cold outside and the top froze slightly while curing that will ruin the finish. It could have been too wet, or it may have begun to set up too quickly and the finishers sprinkled water on the surface to keep it workable weakening the surface in so doing. The list goes on. It tends to come down to workmanship and experience on the job.
Any concrete intended for exterior use should have a tiny amount of microscopic air bubbles mixed into it at the plant which helps the concrete resist the stress of freeze-thaw cycles. If this was omitted, the concrete wont last long in Maryland winters. If the job is as new as yours then somewhere there will be a record of the load leaving the plant and on the ticket will be all the information you’d need to know about the mix including air-entrainment.
Initially, spalling and scaling concrete still does its job, that is to provide a surface over which to walk or drive but it’s ugly and is a signal that it’s a bad concrete pour. The surface will eventually scale to the point that it will become hazardous to walk on and you’ll be tracking concrete dust into the house.
There are concrete patching products on the market to do spot repairs and surface topping such as Top’n Bond by Sakrete® but the bad news is that I have never seen them work successfully over the long run. You might get a summer or even a year out of a patch job but that's about it, as you’ve learned. Epoxy top coats-- the type you see placed around swimming pool decks-- if well applied may last longer.
I recently read a case study where 20 year-old concrete scaled after calcium chloride was used and the owner tried to sue the calcium chloride supplier. A petrographer-- a materials engineer specializing in concrete analysis--was called in and was able, through chemical and microscopic examination following ASTM protocols, to tell that the original concrete was improperly placed and all the chemical deicer did was push it over the edge.
The only long term solution is to break out the bad concrete and replace it. When you do that hire a contractor who knows what he is doing. Discuss such things as weather, air entrainment, mix, placement, finishing and curing steps. If the job is done right the pour will outlive us all. I've seen concrete that the Romans poured over two thousand years ago in Bath, England that's hardly worse for the wear. They did the job right.
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