About Concrete Tank Deterioration

To the editor:

According to the Portland Cement Association, “ettringite (calcium sulfoaluminate), is found in all Portland cement concretes.

“Any form of attack or disintegration of concrete by freeze-thaw action, alkali-silica reactivity (ASR), or other means, accelerates the rate at which ettringite leaves its original location in the paste to go into solution and recrystallizes in larger spaces such as voids or cracks.

“Both water and space must be present for the crystals to form. The space is often provided by cracks that form due to damage caused by frost action, ASR, drying shrinkage, or other mechanisms. Ettringite crystals in air voids and cracks are typically two to four micrometers in cross section and 20 to 30 micrometers long. Under conditions of extreme deterioration, the white ettringite crystals appear to completely fill voids or cracks.”

It appears that the attack of hydrogen sulfide, which would condense to form a weak sulfuric acid, may lead to the migration or translocation of the ettringite found in concrete. This may be a byproduct and marker of deterioration, or part of the process.

In New Hampshire and Maine, it is common to see deteriorated concrete outlet baffle structures and deteriorated concrete distribution boxes. It is logical to assume that gases originating in the soil disposal area would be the cause, or a significant contributor, to this phenomenon, as in most cases the rest of the tank is much less affected.

Gases from the field area would likely accumulate in the distribution box and in the concrete outlet chimney, condensing there to form weak sulfuric acid, which would attack the concrete. This attack could lead to the translocation of ettringite. This ettringite would thus be a marker of the attack.

Sometimes water treatment is associated with this. We have high iron in many of our wells. The common iron removal system uses salt brine, and the brine backwash ends up in the septic system. There is overwhelming anecdotal evidence of poorly performing septic tanks associated with this.

Black, murky, poorly stratified, emulsified tank contents with effluent strength in the range of 400 to 500 ppm of BOD and TSS is common. Some speculate that the salt may also play a part in the weakening. It is perhaps more likely that the higher-strength effluent will lead to greater gas production during the natural decomposition in the disposal area that produces these gases.

Tom Canfield

Thomas Canfield Earth Construction/Earthcon

Earth Construction

Rochester, N.H.