There are many ways to waterproof concrete foundation walls. The traditional method involves installing elastomeric vapor-retarding membranes against the outside face of the concrete. These membranes must be able to expand and contract with the concrete without failure. A primary protection support for membrane systems is accomplished by installing drains alongside footings to collect and then direct water away from the foundation, which reduces hydrostatic pressure against the membrane.
There are a number of repair techniques employed when foundations develop problems and leak water into a structure. When cracks develop in the concrete and water seeps in, contractors can inject epoxy or moisture-reactive urethanes to fill them. Another fix frequently employed is to install drains and a sump pump along the inside of the footings to reduce hydrostatic pressure on walls and move water away from the building—a costly repair that involves removing the perimeter portion of the floor slabs to install the drains. But a method called electro-osmotic pulse (EOP) is starting to be used for both new construction and repair work. Once the system is installed, it can be operated for the same cost as running a 50-W lightbulb and you can “waterproof” your basement walls. That's less than the cost to operate a sump pump.
What is EOP?
Similar to cathodic protection for corrosion prevention of steel reinforcement in structures, an EOP protection system has an anode and a cathode plus a flow of electricity between them to create an electric field within the concrete. The anode is of mixed metal oxide titanium and attached to the concrete. A copper-coated steel rod cathode is driven into the soil backfill region outside, but near the wall. A small DC-pulsing current passes between the anode and the cathode and moves moisture to the outside face of the wall. When concrete is moist or wet, it is very conductive and more electricity is used. When concrete is dry, very little electricity is consumed.
How the system works
In moist concrete, there are molecules of calcium cations, molecules of hydroxyl anions, and molecules of calcium hydroxide (Ca(OH)2) forming and splitting apart all the time. When electricity flows through moist concrete, it causes calcium cations to slowly move in the direction of the cathode toward the outside of the wall. Hydroxyl anions slowly move in the direction of the anode on the inside face of the wall. This activity occurs in the pores and canal structures of the concrete, which is also where moisture can be found.
The cations move to the outside face of the wall in solution with water where they concentrate in the micro pores. Because concrete is a semipermeable membrane, by osmosis water is attracted to the calcium cations, thus creating osmotic pressure. The resulting effect is that the relative humidity (RH) toward the inside of the concrete wall is lowered while the RH at the outside concrete regions is raised. When an EOP protection system is engaged for a period of time, the outside wall RH approaches 100% while the RH on the inside of the wall can become as low as 45%.
The goal is for the hydrostatic pore pressure in the outside region of the concrete to be higher than the hydrostatic pressure of wet soil against the wall. In a properly balanced EOP system, this happens: water flows through the concrete in one direction with osmotic pressures as high as 20 to 30 psi, which is much higher than soil hydrostatic pressure associated with residential construction (no higher than 4 psi). The anions have no real affect on the process, except to offer some protection to reinforcing steel due to the higher alkalinity.
