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Noticed the surface of the concrete was crumbling
30 April 2005


Q. I live in a home completed in September 2003. During the first winter the developer hired a company to shovel snow and they applied salt 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 grey concrete type coating on the porch. Over the next 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 now crumbling.

We do not 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. 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. Iím sure youíve seen concrete sidewalks and driveways or roads that get salted every year, year in and year out, that donít scale up. Imagine the beating the concrete on interstate highways 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 often begins when the 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 of the material 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 around here are pretty good.

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 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.

Any concrete intended for exterior use should have a tiny amount of microscopic air bubbles injected into at the plant which helps the concrete resist the stress of freeze-thaw cycles. If this was omitted, the concrete wont last long in the 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 youíd 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 itís ugly and is a signal that itís a bad concrete pour. The surface will eventually scale to the point that it will become hazardous to walk on.

There are concrete patching products on the market to do spot repairs and surface topping 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 youíve learned.

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 de-icer 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 that's hardly worse for the wear. They did the job right.

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