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

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".

On 09/09/2010 19:36, Dave Osborne wrote:
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?

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.

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?)

David

On Sep 10, 8:03*am, Lobster wrote:
On 09/09/2010 19:36, Dave Osborne wrote:





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?
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.

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?)

David

Thanks both posts.

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.

On 10 Sep, 23:08, Dundonald wrote:
On Sep 10, 8:03*am, Lobster wrote:



On 09/09/2010 19:36, Dave Osborne wrote:

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?
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.

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?)

David

Thanks both posts.

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.

The rain shouldn't affect it. Curing is not drying, it's a chemical
process. Concrete sets under water as the Romans discoverd.

Jonathan

Jonathan wrote:
On 10 Sep, 23:08, Dundonald wrote:
On Sep 10, 8:03 am, Lobster wrote:



On 09/09/2010 19:36, Dave Osborne wrote:
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?
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.
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?)
David
Thanks both posts.

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.

The rain shouldn't affect it. Curing is not drying, it's a chemical
process. Concrete sets under water as the Romans discoverd.

Jonathan

Rain *will* affect concrete curing in the first 24 hours or so as it
will weaken the mix at the surface and will increase the amount of laitence.