Dundonald wrote:
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?

Thanks

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:
https://secure.wikimedia.org/wikiped...tial-decay.svg

Consider 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".