How do I fix my cement driveway?
September 2, 2010 6:33 PM   Subscribe

How do I fix my cement driveway?

Before I went overseas for three months, I had a cement driveway put in. It was all in a bit of a rush, but the job seemed good when I left town three days later.

Coming back, the surface of the cement is slightly soft and chalky - not hard as you'd expect from cement. When I walk on in, the bottoms of my shoes pick up the finest powder. My young son has managed to gouge it with a metal tool. When it gets wet, I can hear air bubbling out from underneath it.

Without biasing any explanation for what's gone wrong, I should add this is in Brisbane, Australia - ie a warm climate.

I'm keen to explore fixing it myself, or at least have some knowledge of what can be done. Can I roll or spread some other product onto it? A sealant or hardener or something? If its totally unfixable (eg, can only be replaced or concreted over again), then I'll start chasing the original contractor or talking to other professionals.

The driveway has a flat area for the car, and a slope downhill to the roadway. I've also noticed a low point that collects water in the flat area - any solution to the above that also could help me address that problem too would be most welcome.
posted by jjderooy to Home & Garden (6 answers total)
Sounds to me like the concrete dried out while it was curing. (Not a good thing)

1. Did they put cement blankets down after the concrete was poured and troweled?

2. Did they spray the concrete with water and no blankets?

3. Did they put down concrete onto bone dry dirt?

The heat from the curing process will evaporate the water in the mix, this is normal. But not blanketing the concrete or allowing the concrete to set to quickly without controlling the process can result in a bad pour i.e. soft or chalky concrete.

Have a few other concrete contractors come out and give you their opinion. You might get lucky as sometimes this can be still salvaged depending the amount of damage.

Don't pour anything on the concrete (sealer or hardner) until you can talk with a professional.
posted by Gravitus at 6:46 PM on September 2, 2010 [1 favorite]

We have had a lot of concrete put in, slabs and finished/stamped work both. My understanding, albeit limited to what a layperson would know from a lot of reading and talking to his contractors over the years, would suggest you might have a bad pour as far as your ratio of cement to sand or gravel. If the concrete was poured on a really hot day and cured too quick you would see surface cracking and maybe worse, but "soft" concrete means, to me, that they used a bad mix.

I would get ahold of the contractor. If necessary get an opinion from a testing service. Worst case scenario, the contractor has to pay a cutting service to haul the old stuff away and then repour it, preferably with both rebar underlayment and mesh or fiber in the pour to minimize cracking.

And as Gravitus said, if you can, wait to pour until fall or spring, when a nice sunny day comes but with cooler weather, or else demand they keep it covered and sprayed to limit the curing.
posted by docpops at 7:36 PM on September 2, 2010

There are products made to lay over existing concrete, essentially polymers that are hard as hell, but I would imagine you need a solid substrate. The fact that you have a low point is also a sign that someone didn't do their job. Either the subsoil and gravel was compacted or graded poorly, or the contractor didn't pitch the pour properly for runoff to occur.
posted by docpops at 7:38 PM on September 2, 2010

I think getting advice from a testing company is a good idea, as well as contacting the original contractor.

A standard time period for testing is 28 days, so perhaps they could come and cut a core and measure the crushing strength of it. The slab is probably not very deep so I imagine it would be a non-standard test but it could tell you how bad the concrete is.
posted by a womble is an active kind of sloth at 10:53 PM on September 2, 2010

You may want to go ahead and have an independent lab test the concrete. If it comes down to making your contractor replace the driveway, an independent analysis could be very valuable.

It sounds to me like the water to cement ratio in the mix was wrong. Too much water can lead to symptoms like those you describe. It's also the cause of what's called "spalling," in which small pieces of the top of the slab flake off.

It could be that there was too much water applied to the very top of the slab to make finishing easier, and that the compressive strength of the slab as a whole is still good. Nonetheless, it shouldn't be chalky or easily gouged. At the very least you're going to have cosmetic problems, and your contractor should rectify the situation.

I don't know of any type of sealer that will help this situation. The problem sounds to me to be structural- as in within the concrete itself.
posted by Shohn at 5:29 AM on September 3, 2010

the only way to fix bad concrete is to take it out and replace. Any of the sealers or epoxy finishes require a good surface to adhere to. You almost certainly have a case of fast curing seeing as you live in a warm, dry climate. Like Gravitus said above about the curing. I have inspected a lot of concrete over the years and poured a little of my own (it really is a fine art)and what you describe has to be replaced and should be at the contractors expense. Here in the US you would almost certainly have to go to small claims court to get this done (contractors are notourisly shifty here-no idea about there). An option that might work and be cheaper than remove and replace is to 'blacktop' it, meaning put a couple of inches of asphalt on top of it if that is an option. If you you can dig down to a harder layer under the spalling you might be able to get a good pour on top of it also although i doubt it, and the work requried to remove the soft top layer is probably the same as just removing the whole slab.
posted by bartonlong at 8:14 AM on September 3, 2010

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