So why do fresh concrete pavements / sidewalks eventually go dark?

March 15, 2006 3:19 PM

So why do fresh concrete pavements / sidewalks eventually go dark?

Concrete starts out light grey when freshly poured and then goes gradually darker. And don't say it's dirt because it happens to concrete indoors, under things as well as outside!

Another seemingly google-proof question.

posted by snailer to grab bag (16 comments total)

I'd guess it's probably some form of oxidation. Oxygen is a very nasty element. It reacts with practically everything, which is why we use it to power our metabolisms.
posted by Malor at 3:27 PM on March 15, 2006

This page on concrete weathering and restoration is pretty good at talking about the causes of these issues.

Googling concrete discoloration turns up zillions of relevant pages, I think.

Google fu is weird, isn't it?
posted by ikkyu2 at 3:28 PM on March 15, 2006

durr... sorry should have searched for that. in other news, if you add washing up liquid to wet concrete it will dry harder and quicker, apparently.

cheers dudes. will do better at google-fu, ikkyu2-san!
posted by snailer at 3:31 PM on March 15, 2006

I'm assuming that you are referring to portland cement concrete?

There might be some oxidation for a time after the concrete is poured, but that would cease once the cement is cured (no more than 3 weeks. Some concretes cure faster because a catalyst is added to the cement). Unless the indoor concrete you are referring to is in a hermetically sealed environment, I'd say dirt is a big factor.

I work for a highway agency, and one of our project was to grind off 1/4" of an existing cement concrete highway (noise reduction, in case you are curious). The freshly exposed roadway (old, but previously unexposed to the elements) was so bright that you couldn't see the white paint stripes, so maintenance had to restripe it by painting the lane line area black then painting the white stripe on top of that.
posted by luneray at 3:37 PM on March 15, 2006

no offense intended, snailer. AskMe is full of google fu gurus, before whose titanic skillz I must bow in tremulous admiration.
posted by ikkyu2 at 3:41 PM on March 15, 2006

Concrete cures very slowly. The initial set takes less than a day, but it doesn't reach full strength until after about 28 days. During the curing process, it continues to absorb water as it gains strength.
posted by electroboy at 5:09 PM on March 15, 2006

Continues to absorb water?!

Does this mean I ought to be putting a tarp over my poured-last-week, interior concrete project? Spraying it down with water?

Humidity in the house hit 50% during the first few days I had it drying. And I was using a couple of box fans to circulate the air. Did I do wrong?
posted by five fresh fish at 5:41 PM on March 15, 2006

Does this mean I ought to be putting a tarp over my poured-last-week, interior concrete project? Spraying it down with water?

Actually, a concrete slab should be sprayed with water periodically for it to cure correctly, for at least seven days after the pour. I'm not sure how much or how often during those seven days. You don't need to hothouse it by covering it with a tarp, though.
posted by LionIndex at 5:48 PM on March 15, 2006

Use linseed oil to prevent water being absorbed into concrete. Every street paving project uses a form of linseed oil to help it cure properly. Ideally, you should use linseed oil after pouring any exterior concrete.

Concrete discolors mostly either from sun bleaching or from wear. Concrete is made from a combination of lime cement, sand, and gravel or stone. Generally when finishing the concrete with a float, the surface is mostly a cement cream. The cement is very white in color. When this starts to wear down, it will show the sand and stone which is usually more of a yellowish color depending on the local materials used. Sometimes a colorant is used in the concrete which can vary depending on whether it is mixed with the concrete or sprinkled on the surface after pouring. Surface colorants reflect wear more easily and are much less desirable. It is more desirable to use a colorant mixed with concrete but this can be more expensive. Redi-mix companies charge more for this because they need to completely wash the trucks afterwards to prevent later batches from being contaminated from the color used in a particluar batch.
posted by JJ86 at 6:01 PM on March 15, 2006

ikkyu2's links mostly relate to blotches due to poor concrete mixes and not general discoloration due to age. Also as electroboy alluded to, concrete will also appear darker when it has absorbed more water or is damp. In a basement this could be a sign of some problems that may need to be addressed with a dehumidifier.
posted by JJ86 at 6:20 PM on March 15, 2006

Well poopstains. I hope I haven't fubared my cement work.
posted by five fresh fish at 12:16 AM on March 16, 2006

Don't forget to write your name in it.
posted by camworld at 5:37 AM on March 16, 2006

five fresh fish: Only time will tell! Assuming you poured some new flooring in the basement, I wouldn't be too worried. average basements have no problem keeping a higher humidity so it isn't like the concrete would have been prematurely and completely dried out. The additional moisture you could retain by covering the work with plastic will increase the concrete's strength, but unless you will be driving heavy forklifts on it or keeping a herd of elephants in your basement, that won't be a problem for the average home. Obviously for a driveway, which would get heavier loads, this would be an issue.

Also just to be clear, I wouldn't put linseed oil on concrete poured inside, the fumes aren't noxious but they aren't altogether pleasant either.
posted by JJ86 at 5:41 AM on March 16, 2006

Use linseed oil to prevent water being absorbed into concrete. Every street paving project uses a form of linseed oil to help it cure properly.

Well, that just goes against everything I know, where you actually want your concrete to absorb water so that it will cure properly. The concrete specifications section at my firm stipulate a "water-cure" method for our floor slabs (which are generally interior). Why the linseed oil for exterior slabs?
posted by LionIndex at 7:51 AM on March 16, 2006

There is more than enough water in just about any concrete ready-mix. There are also a huge amount of admixtures or air-entrainment that depend on different amounts of water being present in the mix. In most cases, adding extra water during finishing will cause scaling and spalling of concrete. Linseed oil mixtures help protect the surface from absorbing extra liquids, salts, and helps to make the concrete more durable.

One thing to remember is that not all admixtures, cements, and types of concrete work best for all circumstances and purposes. Different areas of the country require mixes to match their seasonal variations. Talk to your ready-mix supplier for the best mix to use for your needs whether it be exterior or interior use. Most contractors have a good idea of how to handle concrete to prevent problems but many do not!
posted by JJ86 at 8:23 AM on March 16, 2006

Well, I suspect I put a little too much water in my mix, and the sharp sand base was reasonably moist, so it'll all work out. Also, this particular bit of work in underneath a staircase, where it will seldom be walked upon; the only real weight will be a gross of wine bottles. Which are heavy, to be sure, but are also going to be spread out such that there's not going to be much weight at any given point on the floor.

While doing all this work, it occured to me that concrete is a lot like cake or bread recipes: there is really very little room for slack. Fail to measure it correctly, and you end up with a disaster.
posted by five fresh fish at 10:28 AM on March 16, 2006