The surcharge weight of the water inside the tank plus the lateral soil pressure (usually at-rest as opposed to active due to the stiffness of the ringwall) creates tension in the ringwall. Whether or not the concrete is in tension depends on the modulus of rupture of the concrete, the section...
Personally, I think the use of Xypex for a water storage tank is overkill. We specify a concrete mix with 25% fly ash replacement and a maximum 0.41 w/(c+p) ratio. This mix is, for all practical purposes, impermeable. I feel Xypex is best used in areas where you are concerned with chloride...
The concrete does not carry any hoop tension. The reinforcing steel in the ringwall carries all the tension. Use your steel area, not the concrete area.
There are three companies in the US that I am aware of that manufacture and install post-tensioning systems.
1. VStructural (www.structural.net)
2. Dwyidag (www.dywidag-systems.com)
3. Suncoast PT (www.suncoast-pt.com)
Any concrete slab requires a relatively stable foundation. If you have...
Regarding the concrete aspect of your repair: 1. Pressure washing is a good idea. I would recommend a pressure of approximately 4000 psi as you want to open up the existing concrete and create a rough profile on the surface. 2. If the contractor does a good job on the pressure wash I would...