Hi, I've concreted for the first time, mixing my own in a proper mixer, to lay a new slab alongside a slab that already existed. I used 1 part cement to 5 parts balast. The new slab has now been curing for 6 days (with a couple days of intermittent heavy rain and my having to cover the cement with plastic sheeting) but the new slab is a darker shade, may be a browny shade, where as the existing slab is lighter and is a shade of grey.
My questions are - Does all concrete wind up roughly the same shade of grey? Will my new slab eventually look the same (ish) as the existing one after curing is complete? How long does it take to fully cure?
Concrete always looks darker when it's wet and during the curing process.
The final colour of concrete depends on the colour of the aggregate it's made from. If the new batch is made from the same ingredients in the same proportion as the old batch, then the two batches will end up the same colour.
Concrete takes an indefinite time to fully cure and does so by a curve called "exponential decay"[1]. It is considered to be "set" after 24 hours, safe to walk on after 48 hours, safe to build on after 7 days and after 28 days will have reached well in excess of 90% of its final strength.
Structural engineering calcs take the 28-day strength as being equivalent to fully-cured, so for all intents and purposes concrete is fully cured after 28 days.
[1] As an approximation, have a look at this graph:
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the dark blue line. The x-axis (horizontal) represents time after pouring in weeks. The y-axis represents (vertical) "how much of the concrete still has not cured", where 1.0 means all the concrete is un-cured
0 means that none of the concrete is un-cured.
You can see that after a couple of days, the concrete is 25% cured, after a week, the concrete is over 60% cured and after 4 weeks, it's pretty close to being fully cured. The nature of the exponential decay curve is that the dark blue line never actually gets to zero, it just "tends to a limit of zero over an infinite time-scale".
Yes I've experienced the same thing myself with aggregate obtained from different suppliers - the colours were still quite noticeably different when fully cured. Didn't matter to me as mine was going to be hidden uder floor covering, but assuming yours isn't, maybe you could use floor paint (ie grey, concrete-coloured?)
My mix will definitely be of a different aggregate so I'm sure the colours will noticeably remain different. Oh well it's a utility area anyway for sheds so no huge deal. I'm just a perfectionist and like to have consistency. Paint might be the answer.
It would be nice if it stopped raining enough to allow it to cure properly too.
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