I am a designer trying to build a concrete sink for a project I'm
working on. I have workers that are familiar with concrete forming,
but we aren't sure how to mix the concrete so as to make it
waterproof. Is there an additive you put in it, or is there a certain
of ratio of cement to water that needs to be achieved. Any help would
be greatly appreciated.
Thanks,
Donovan

Hi Donovan,

You are correct in thinking that cement to water ratio is a factor and
there is an additive that you can use. You need to learn about proper
curing in order to prevent or minimize capillaries and using an
additive or "admixture".

Curing

Curing is the process whereby this mixture of cement, water, sand and
aggregates become concrete. This does not happen all at once, but is a
chemical process that takes time. The time it takes depends primarily
on the temperature and humidity (water vapor content) of the air.
Because we have little control over these environmental conditions, we
need to control the process by other means. The reason we want to
control the process is that if the curing takes place too quickly, the
concrete created will be fill with numerous capillaries. The same is
true if you use a high water to cement ratio (more water). Either way,
capillaries are extremely undesirable because the render the concrete
weak and porous."

Capillaries

"Capillaries are formed during the curing process. They are unwanted
small tunnels left behind after excess water (water not needed
chemically but needed for flowability) has evaporated from the
concrete. No matter how high a quality of concrete you make, there
will always be capillaries formed if no other measures are taken. The
goal is to keep them to a minimum, and of course eliminate them all
together if possible. Luckily this last option is now available to us,
as we will see later."

Why do capillaries make concrete porous?

"Because they are filled with air until water or water vapor comes in
contact with them from the ground or the air. They will actually wick
moisture from the surrounding environment and deliver it to the top of
the concrete slab just underneath flooring materials. There the water,
or water vapor when it condenses, attacks flooring adhesives.
Capillaries form the transit system for water and water vapor
(moisture) migration."

(Source for the above information: Moxie International's Primer on
Concrete http://www.moxie-intl.com/tutorial.htm, Moxie International,
Copyright © 1975-2002)

Based on the info, concrete becomes porous due to the capillaries or
tiny tunnels created due to excess water and/or incorrect curing.
Capillaries also make concrete weak. At this point you may think that
by keeping the water to cement ratio to a minimum (mixing less water
to cement) and by using longer curing time, you can prevent the
formation of capillaries. That is not the case and this is where you
need to use an additive or "admixture". The same source states that
"even if you were to make the best concrete you possible could, (low
water to cement ratio and a long controlled curing process) with out
an additive or "admixture", the concrete would still have some degree
of capillary formation."

Here is where the admixture comes in and here are two waterproofing admix products.

Moxie International
Moxie 1800 Super-admix
http://www.moxie-intl.com/1800-BENEFITS-AND-ADVANTAGES.htm

Grace Construction Products
Water Repellents - Darapel
http://www.na.graceconstruction.com/product.cfm?mode=a&id=12&did=1


More info regarding admix can be found here:

Concrete Admixtures - FAQs
http://www.na.graceconstruction.com/template.cfm?page=/concrete/concrete_faqs.html#segregation


Finally, here's another product that will help you achieve your desired result:

Aquron Corporation
Aquron® 6312? - elastomeric clear waterproofing/oil repellant system
http://www.aquron.com/products/6312.htm

You can find other related information from the search links below.
With regards to the products, including the product description, etc.,
I'm in no way representing any product or company in this answer. The
material serve only to inform you and help you to make your decision.


Search strategy:

"waterproof concrete" mixture
://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&safe=off&q=%22waterproof+concrete%22+mixture&btnG=Search

darapel
://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&safe=off&q=darapel&btnG=Search

"waterproof concrete" "clear coating"
://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&safe=off&q=%22waterproof+concrete%22+%22clear+coating%22&btnG=Search


I hope this helps you. If you have a question, please feel free to
post your clarification and I'll attend to you as soon as possible.
Thanks for asking.

Best regards,
Feilong