RichToyBox wrote:
Concrete, whether truck mixed or gunnite/shotcrete is made of Portland
Cement, sand, water, coarse aggregate (gravel or crushed stone) and
admixtures. The sand, water and coarse aggregates should not pose any
problems. Most of the admixtures are used in such small quantities and are
bound in the concrete, so I don't see a problem with them, either. I will
cover some admixes that you might want to use later.

When Portland Cement hydrates (reacts with water) it produces the calcium
silicate hydrate gel which is the stuff that makes the concrete stong and
water tight. It also produces as by-products heat which can lead to
expansion and then on cooling contraction that will cause cracking, and it
produces calcium hydroxide. The calcium hydroxide is in the form of pore
water, which comes to the surface, the pH of which is about 13. In air, the
calcium hydroxide solution reacts with carbon dioxide to form calcium
carbonate which is essentially just limestone, neutral pH. The calcium
carbonate blocks the pores making the concrete both stronger at the surface
and more watertight. It takes time exposed to air to get the carbonation
layer formed. With the high water table in NO, I don,t think it would be
possible to keep an empty concrete pond to allow the drying, since they make
ships of concrete. The longer concrete cures the strionger and more
watertight the concrete becomes. Curing requires a reasonable temperature,
which you have, and a constant supply of moisture. So as soon as the pond
has been poured, keep the concrete moist for a few days and then fill it
with water.

Water is the best and worst thing for concrete. During mixing and placement
the less water the concrete has, the stronger and more water tight the
concrete. Some of the mix water will leave the concrete through what is
referred to as bleeding, where the solids settle and the water flows to the
surface, leaving pores that are an avenue for water and chemicals to get
back into the concrete and cause problems. The concrete will set up before
all this mix water can leave, so some of it is just spreading the solid
ingredients out and not providing any strength in the end. After placement,
the concrete should not be allowed to dry, since the hydration reaction
requires water and this reaction, suipposedly goes on forever. So the
longer the concrete can be protected from drying the better.
The concrete ponds that I have seen that do not have some form of
waterproofing membrane painted on are usually allowed to sit for a year
before any fish can go in, doing acid additions, water changes, and pH
monitoring to see when the pH stabilizes at a safe level. The leaching of
the calcium hydroxide will continue for a long time, but will decrease in
rate, so the pond water will finally stabilize.

As far as admixtures, there are 3 or 4 that you might want or need. Air
entraining admixture is a must in northern climates to provide freeze thaw
protection to concrete, but it also provides some improvement in the
watertightness of concrete. Set retarder slows the initial harding of the
concrete to provide a little extra time place and finish the concrete, and
the warmer the concrete the faster it sets, so this one I think would be a
must. Set retarders will usually allow for less water in the mixture.
Water reducers, especially super water reducers, will allow the flow of the
concrete to be much higher with the use of less water or for the same flow,
reduce the water significantly. Now you can also get plastic fibers mixed
with the concrete that will help to reduce any cracking by providing some
small scale reinforcement of the surfaces, don't use it to replace the real
steel reinforcement.

Sorry for the length of the dissertation, but I felt a complete answer
required this. Hope it helps.

You sound quite knowledgeable about concrete. Perhaps you could
enlighten me about my pool to pond conversion.

I have a plastered Shotcrete pool built in 1987. Looks very good with a
few spalling pits in the plaster. I plan to overpour concrete in the
8'1/2" deep end to raise the entire bottom to about 4' deep. I plan to
put packed dirt over the plaster and pour 4" of concrete. Do I need to
knock off the plaster around the sides so the edges of the new concrete
floor mate with the Shotcrete? For fish, do I need to coat the new
concrete and old plaster with Epoxy or will normal pool paint be alright?

Chip