Why is my concrete two different shades?
November 20, 2021 10:38 AM   Subscribe

I just had some concrete done and one section is darker. I'm curious as to why this is.

I had a section of sidewalk replaced (in the top of this photo), and then they came back a few days later to pour a little more concrete to smooth the transition to the driveway (at the bottom of the picture). They used the same Quickcrete for everything, but the transitional section is darker and slightly shinier than the rest. It's totally dry at this point, so I'm just wondering what would account for the difference in color/shine. Would it just come down to the proportion of water being different when they did the second part?
posted by jonathanhughes to Home & Garden (3 answers total)
Best answer: I think it usually happens when extra water is added to the concrete mixture to delay the setting time. The concrete has started to 'go off' (i.e. set), and rather than throwing it away and mixing a new batch, they may have just added a bit more water. This isn't a terrible thing, but it will affect how the concrete cures, and the colour when dry. Another reason might be that the two areas were poured in different conditions (humidity, temperature), which caused them to cure at different rates.
posted by pipeski at 2:06 PM on November 20, 2021 [3 favorites]

Best answer: How much the concrete is worked can change the appearance and colour. If one section was troweled more, especially once the water started to rise, it can result in a different colour.

Batch differences in the quickcrete can also be a factor. If the bags of quickcrete aren't from the same batch there could be a difference in age, aggregate or cement content all of which can change the colour.

Concrete is also sensitive to proper hydration. EG: adding excess water to make concrete more workable or placing concrete on a dry wicking surface (like dry earth) without wetting it down are common errors that can cause all sorts of problems including discolouration.

If it's been less than a month the darker concrete could just be greener that than the lighter concrete. Concrete continues to cure for years after placement but typically reaches 90+% of full strength after a month. 1 week old concrete has as little as 50% ultimate compressive strength. Concrete is darker at the beginning of the curing process and lighter later.

You may find the colour evens out after a few months.
posted by Mitheral at 2:09 PM on November 20, 2021 [3 favorites]

Response by poster: Awesome answers. Thanks!
posted by jonathanhughes at 2:21 PM on November 20, 2021

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