WAS: How Thick Cement Slab for Bottom Stair

Posted by Manjo on September 1, 2006, 8:19 am

I poured a new concrete bottom stair, replacing several layers of
asphalt that was used to reach the original concrete block at the
bottom of the wooden stairs. It looks pretty good.

I have a few questions on how to finish off the original block and the
new concrete bottom stair:

1. There were "curing" instructions on the concrete bag. It said to
either add a curing additive to the mix, or keep the concrete wet for 5
days. Since there was no rush to use the stair, I kept it wet. My
question is, why does keeping the concrete wet cure it, and why doesn't
just letting the concrete dry normally work?

2. I would like the new stair to match-up with the old concrete stair
just above it. I bought a bag of top cement to spread a thin layer
over both stairs to make them look alike. I was hoping to make a 1/4"
layer, but there are some good sized pebbles in the mix that would make
it hard to smooth out the cover layer. My question is, can I
screen-out these pebbles and use the remainder to spread over the two
stairs? Will this sand and cement layer be strong enough to not crack
or break off?

TIA for any tips, suggestions, comment.

Manjo

Posted by carolyn on September 1, 2006, 10:59 am

Manjo wrote:

> I poured a new concrete bottom stair, replacing several layers of
> asphalt that was used to reach the original concrete block at the
> bottom of the wooden stairs. It looks pretty good.
>
> I have a few questions on how to finish off the original block and the
> new concrete bottom stair:
>
> 1. There were "curing" instructions on the concrete bag. It said to
> either add a curing additive to the mix, or keep the concrete wet for 5
> days. Since there was no rush to use the stair, I kept it wet. My
> question is, why does keeping the concrete wet cure it, and why doesn't
> just letting the concrete dry normally work?

Curing cement/concrete is not a drying process but a chemical process. The
cement mixture needs the water as part of the chemical process. If it
dries, it loses the water for the chemical process. The result is a poorer
quality final product. Same goes if it is too hot or cold while the
concrete is curing. Technically it is only really important for the first
few days, maybe even the first week. Concrete takes upwards of 100 years
to finish curing, but it is useable within days of starting the process.

> 2. I would like the new stair to match-up with the old concrete stair
> just above it. I bought a bag of top cement to spread a thin layer
> over both stairs to make them look alike. I was hoping to make a 1/4"
> layer, but there are some good sized pebbles in the mix that would make
> it hard to smooth out the cover layer. My question is, can I
> screen-out these pebbles and use the remainder to spread over the two
> stairs? Will this sand and cement layer be strong enough to not crack
> or break off?

I don't want to answer this one, as I know I don't know enough to give a
reasonable answer.

> TIA for any tips, suggestions, comment.

Carolyn
--
Carolyn Marenger

Posted by Manjo on September 1, 2006, 11:29 am

Carolyn,

Thanks for the info on curing. I recently saw a docuemenatary on the
Hoover Dam. The engineers ran cooling pipes through the 16 x 16 poured
"blocks" that made up the dam to cool it. If they hadn't cooled it,
the heat developed during the chemical drying process would have
delayed full curing for 136 years.

Regards,

Manjo

carolyn wrote:

> Manjo wrote:

> > I poured a new concrete bottom stair, replacing several layers of
> > asphalt that was used to reach the original concrete block at the
> > bottom of the wooden stairs. It looks pretty good.
> > I have a few questions on how to finish off the original block and the
> > new concrete bottom stair:
> > 1. There were "curing" instructions on the concrete bag. It said to
> > either add a curing additive to the mix, or keep the concrete wet for 5
> > days. Since there was no rush to use the stair, I kept it wet. My
> > question is, why does keeping the concrete wet cure it, and why doesn't
> > just letting the concrete dry normally work?

> Curing cement/concrete is not a drying process but a chemical process. The
> cement mixture needs the water as part of the chemical process. If it
> dries, it loses the water for the chemical process. The result is a poorer
> quality final product. Same goes if it is too hot or cold while the
> concrete is curing. Technically it is only really important for the first
> few days, maybe even the first week. Concrete takes upwards of 100 years
> to finish curing, but it is useable within days of starting the process.

> > 2. I would like the new stair to match-up with the old concrete stair
> > just above it. I bought a bag of top cement to spread a thin layer
> > over both stairs to make them look alike. I was hoping to make a 1/4"
> > layer, but there are some good sized pebbles in the mix that would make
> > it hard to smooth out the cover layer. My question is, can I
> > screen-out these pebbles and use the remainder to spread over the two
> > stairs? Will this sand and cement layer be strong enough to not crack
> > or break off?

> I don't want to answer this one, as I know I don't know enough to give a
> reasonable answer.

> > TIA for any tips, suggestions, comment.

>
> Carolyn
> --
> Carolyn Marenger

Posted by mrsgator88 on September 2, 2006, 2:38 am

> Carolyn,
> Thanks for the info on curing. I recently saw a docuemenatary on the
> Hoover Dam. The engineers ran cooling pipes through the 16 x 16 poured
> "blocks" that made up the dam to cool it. If they hadn't cooled it,
> the heat developed during the chemical drying process would have
> delayed full curing for 136 years.

Its not a drying process. It is a chemical reaction. Drying means that the
water evaporates. So when you mix water with flour to make pasta, you let
it sit so the water can evaporate.

When you mix water with concrete mix, the water never evaporates. It
becomes part of the concrete, allowing it to form crystals, giving it
hardness and strength. Concrete can set and harden even underwater.

S

Posted by Manjo on September 2, 2006, 9:00 am

mrsgator88 wrote:

> > Carolyn,
> > Thanks for the info on curing. I recently saw a docuemenatary on the
> > Hoover Dam. The engineers ran cooling pipes through the 16 x 16 poured
> > "blocks" that made up the dam to cool it. If they hadn't cooled it,
> > the heat developed during the chemical drying process would have
> > delayed full curing for 136 years.

> Its not a drying process. It is a chemical reaction. Drying means that the
> water evaporates. So when you mix water with flour to make pasta, you let
> it sit so the water can evaporate.
> When you mix water with concrete mix, the water never evaporates. It
> becomes part of the concrete, allowing it to form crystals, giving it
> hardness and strength. Concrete can set and harden even underwater.

S,

Thanks for the info. I never realized the water bcame part of the
concrete. Most critical to process or there would be no process.
After 9 days, I can still see dark portions towards the back of the
stair versus the white towards the front part. Still curing, I guess.

Best regards,

Manjo

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