Linear horizontal cracks following internal reinforcing.
Internal surface of a concrete tank showing rust spots following internal reinforcing along with a visible high water line where the plaster lining above the water is still in good condition.
Why is my concrete tank leaking?
Most of the leaking concrete rainwater tanks we install Tankliners in have failed for the same reason.
Rainwater collecting in a tank arrives at a pH somewhere between 6.5 and 7.5.
As it sits there, it stays in constant contact with the internal surface — usually a plaster render or bare concrete.
That surface is bound with Portland cement. During manufacture, hydrating the cement forms portlandite (calcium hydroxide crystals), which water low in calcium — rainwater — leaches out of the cement-mortar lining.
Over time more lime compounds leach from the walls, removing layer after layer of material. The sand aggregate falls away, eventually exposing the reinforcing steel set within.
Once that steel sits wet, it oxidises in place — the obvious rust stains we see around the internal surface of the tanks we work on.
As oxides form, rust jacking (oxide jacking) takes hold: the oxides can expand to up to 10× the original volume of the steel.
This rust jacking is what causes the cracks and holes in a concrete tank's sidewall — often quite linear, following the horizontal and vertical reinforcing (see image inset).
Anyone who's dealt with rust on a vehicle will tell you: unless you cut out every trace before repairing and sealing, the rust comes back.
In a tank, cutting out the rust isn't possible. There's no feasible way to reach that oxidised steel without cutting the tank to pieces.
A plaster layer over the rust might hold in the short term, but it eventually suffers the same fate. Oxide jacking gradually pries the surface beneath it apart, and the plaster has no ability to stretch across the widening crack.
This is where geomembrane Tankliners come in — they really are the only way to remedy the issue when you measure in years or decades.
A Tankliner is fixed loosely at the top of the sidewall only, and not adhered to the internal surface at all. The water it contains holds it flush against the sidewall and floor.
That lets the liner bridge over cracks and voids without relying on those cracks staying exactly where they are.
The Permaflex fPE500 polyethylene geomembrane Membrane Dynamics uses for Tankliners is rated to 1000% elongation at break — a 10× stretch before tearing. In other words, a 1 mm crack would need to become a 10 mm crack to be of concern.
Secondary to bridging cracks and gaps, an installed Tankliner removes fresh water from contact with the concrete surface — halting the tank's degradation and extending the lifespan of the whole system.
Diagram showing mechanism for leaching of Calcium Hydroxide from cement based tank sidewall.
Source: CEED Seminar Proceedings (2016) Investigation of the Effect of Water Quality and Lime Dosing on Mild Steel Cement Lined Pipes, J. Le, J. Leggoe, G. Ash & T. Christen. https://ceed.wa.edu.au/wp-content/uploads/2017/02/9.-Le-Water-Corporation-Water-Quality-Lime-MSCL-Pipes.pdf