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Everyone I talk to tells me that there is no product on the market that can be applied to concrete as a non-toxic waterproofing agent.

Can anyone shed some light on this? 

How do these folks like "Morningstar" who build concrete tanks in 3rd. world countries solve this problem? 

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Stuart,

Have you looked at thoroseal FX100? Also ...what reinforcement did you use in your tanks? Mesh? Fiber? Rebar?   I am looking at using a pre-made septic tank as my sump. Any thoughts?

We used to use Thoroseal. The problem was it tends to brake down within acouple of years. Aquatic environments are very hard on this product. If you use any rubber product, be sure to acid wash your tank first with muratic acid. BE CAREFULL!! Ware protective clothing ang a respirator! With deli gent preperation Ruberize It should last much longer. The other problem with Thoroseal is it is not inert and will raise ph as it brakes down.

Thanks Bill...Great info....Sounds like the rubber is the ticket....all things considered.

Bill Swearingen said:

We used to use Thoroseal. The problem was it tends to brake down within acouple of years. Aquatic environments are very hard on this product. If you use any rubber product, be sure to acid wash your tank first with muratic acid. BE CAREFULL!! Ware protective clothing ang a respirator! With deli gent preperation Ruberize It should last much longer. The other problem with Thoroseal is it is not inert and will raise ph as it brakes down.

There are paint on 2 part liquid rubber liners that i have used on old farm ponds but i do not know if it is food grade.

I visited a commercial tilapia breeder who makes all his tanks from concrete, and does not seal them with anything. When the pH falls, it leaches the lime out of the concrete, buffering the pH. He has to resurface his tank every few years because of this.

If the pH is neutral it shouldn't leach from your tanks, so he adds lime to try to keep his pH up. Despite this he still resurfaces every so often, although I don't remember how often.

I've used paraffin wax before with no problems in an aquaponic system.

I have used rubberizit on my grow bed.  Last year when I took down my system, I fiberglassed the inside of my wood grow bed then used a paint roller to apply the rubber.  After letting it cure for 7 days, I replaced the gravel and flooded the bed.  No leaks so far.  The system has been down for the last year with the gravel sitting in the GB.  No problems so far.  I'll be fiberglassing and rubbering my FT and ST this spring to start the system back up.  I'll let you know how the rubber has held up over the winter.

The reason why concrete in itself is NOT water tight, is because once it's casted and the curing process starts, the chemical reaction between cement and water liberates heat, which in turn pushes air out of the mix; the tiny air bubbles traveling all the way to the surface create conduits, through which water can flow. This makes concrete behave like a porous rock.

On top of this, water tanks specifically have another weak point: corners or joints. Regardless of the steel rebar that you use, the joint between the floor slab and the tank wall will ALWAYS fracture and water will escape (sometimes massively!) through that gap. This will inevitably happen, even if you cast the floor and the walls at once. It's not a matter of casting technique, it's simply a matter of material behavior.

Fortunately, civil engineers (me being one of the sort) have found several cures to this "natural"behavior of concrete.

- Additives: these substances are added when concrete is mixed. There are many kinds of additives, working in various ways, but their final purpose is to restrain air from flowing out of the mix while curing. This will prevent the creation of the conduits through which water can flow. In other words, concrete will not be porous anymore, securing water tightness. The most popular additive brands are SIKA and HILTI, widely available at your nearest home depot.

- Joint sealing tapes: even with water tightening additives, concrete tank joints will fracture anyhow. The solution is to install a sealing tape specially design to prevent water flow through this gap. This implies that the tank cannot be casted in one single step, but rather it HAS to be casted in two steps: first the floor slab, making sure that one halve of the sealing tape is embedded all along the contact surface of the walls; and then the walls, making sure that the other halve of the sealing tape is properly located in the halfwidth of the wall. These tapes can also be found at your local home depot, and again SIKA and HILTI are popular brands.

Once you have your concrete tank casted, you may also use internal liners, all of which have been mentioned here, with the exception of fiberglass and watertight textiles. You can also use mortars with water sealing additives as internal liners.

In case some cracks are detected, you may also use, on the external face of the wall, mortar mixed with water sealing additives. This can obviously only be done on walls, since it's virtually impossible to detect cracks on floor slabs, let alone fixing them. This is also why in the case of tanks below floor surface, all water containment tests have to be done before the backfill is put in place.

Finally, in particular for concrete tanks partially or completely under floor surface, its always good to install perimeter drain system that allow monitoring of water loss out of the tank.

I hope this helps!

CARLOS A

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