William Hime
William Hime
Bernard Erlin
Bernard Erlin

We have to be careful on this one, because we are experts in litigation involving it. So we'll tread somewhat softly, carefully, and cautiously, but carry as big a stick as possible. We begin with a bold statement: "In this phenomenon, there are no chemical reactions involving any sulfate salt with any component of the concrete. It stands alone and is its own worst enemy. It needs no help from anyone or anything, except ambient temperature and humidity."

The outward manifestations are seen usually on the exterior surfaces of slabs, often several inches above ground level on vertical concrete surfaces, several inches in from leading edges of garage slabs and usually in line with garage doors. The phenomenon is typically a line of white, fluffy salts that have an acrid taste—if you are so adventuresome to taste it—and sometimes accompanied by some loss of the concrete surface, resulting in a coarse sandpaper texture. The common salts are usuallysodium sulfates. It is observed in many states, but mostly in western areas. An accepted scientific name for it is the "rising damp" because it can climb upward several feet, depending upon environmental circumstances. On the Alamo, one of our nation's official shrines, the rising damp wicks upward 7 to 8 feet, and the source of the salt is thought to be gunpowder.

One interesting source of physical sodium sulfate attack was the fly ash used as a replacement for portland cement. It was rich in sodium sulfate that migrated to the outside exposed slab and wall surfaces, and wicked up surfaces and left them with sandpaper textures. Experiments we have performed demonstrate that sodium sulfate, or at least its solubilized form, has a remarkable ability to "climb" on many different material surfaces. It can climb up concrete, steel, aluminum, tin, and glass, many of which essentially have no porosity. The phenomenon occurs because sodium sulfate is very soluble in water that moves in a capillary action. It exists in two different mineralogical states: (a) thenardite (Na2SO4), the anhydrous form; and (2) mirabilite (Na2SO4·10H2O), the decahydrate hydrous form. Remarkably, a transformation from one mineral to the other occurs at temperatures and relative humidities reached almost daily in some areas of the country. When transforming in situ from solid anhydrous to the decahydrate form, it understandably occupies a greater volume, and stresses and crumbles whatever encases it.

Whether it is naturally occurring or manmade, it knows no difference and superficially "eats" its way into the concrete surface. The sodium sulfate transformation from the anhydrous to hydrous states is also common to sodium carbonate, which exists in several forms: Na2CO3 (thermonatrite); Na2CO3·21/2H2O (no mineral name); and Na2CO3·10H2O (natron). It is reported that sodium chloride (halite, table salt, NaCl) has a two-watered form (NaCl·2H2O) around the freezing point of water. Maybe that helps explain sandpaper surfaces associated with some sodium chloride exposures. Leonhardite, a zeolite, at ambient temperatures and humidity also undergoes solid-to-solid transformation to a higher and more voluminous watered state called laumontite, which has caused surface and deeper-seated distress to concrete.

Getting back to sodium sulfate physical distress, coating at least the base of the potential "climb areas" with a slick, tightly adhering coating may mitigate its movement. The American Concrete Institute in its 201 Committee Report states a maximum water-cement ratio of 0.45 improves durability. The Argentineans report that a low water-cement ratio doesn't improve durability. Whoa, who is right? And the Australians like using an epoxy coating or a sacrificial surface layer. Directing lawn sprinklers away from concrete surfaces also helps, as does trenching around the concrete. The distress always occurs above-grade. Ignoring the problem generally also works if a little surface damage is acceptable.

The solution to the Alamo problem was correcting roof drains so water didn't splash on building walls, directing sprinklers away from walls, and trenching 12 feetaround the building so any water that tries to get close to the walls is drained away.

Unlike chemical sulfate attack, where standards have been developed to counter it, there are no official standards for mitigating physical sulfate salt attack. But there are ways for handling it, such as isolating the rising damp using slick bands that interfere with its climbing prowess and by keeping water away. If youwant some excitement (at your expense), take your pick about using low water-cement ratios. A little attention and you may have it beat—but there is also the other possibility.

-William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 55 years ago.

-Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for more than 48 years.