Most everyone has read about, or heard of, problems besetting concrete. These include alkali silica reaction, alkali carbonate aggregate reaction, sulfate attack, D-cracking, cyclic freezing, premature and other improper finishing, steel corrosion, excessive air, low strength, delamination, cracks of all sorts, popouts of all kinds, mortar flaking over aggregate particles, delayed ettringite formation, excessive bleeding, blisters, rusting, rutting, excessive wear, reinforcement corrosion, and their variations. Maybe some of these are not as major so you have neither heard of nor encountered them. Their variations are sometimes lesser known.
For example, sulfate attack can result from concrete exposure to sulfate soils, sulfate-enriched water, and some fertilizers. It also can result from internal sources such as aggregates that contain gypsum; inadvertent addition of gypsum plaster; or exposure to sulfuric acid from industrial wastes, derivatives of sluggish sewage effluent, and over-sulfated cements.
How about some of the lesser-known evils? These can be cataloged into several groups: (1) volume unstable compounds that cyclically increase and decrease in volume due to changes in ambient temperature and relative humidity; (2) anhydrous compounds that imbibe water and then hydrate and, in situ, enlarge in size; (3) a variety of compounds that degrade and form substances that adversely react with neighboring compounds; (4) water solutions that dissolve paste compounds; and (5) miscellaneous compounds.
Group 1 includes: (a) the zeolites laumontite and leonhardite where sodium or potassium can substitute for calcium; (b) the sodium sulfate salts thenardite and mirabilite; (c) magnesium sulfate salts kieserite and epsomite; and (d) a variety of sodium carbonate salts from trona to thermonatrite and natron. In each case, the lower-watered compound absorbs water and “metamorphoses” into a higher-watered, larger compound that exerts stress to flanking paste followed by an in situ conversion to the lower-watered compound. The conversion from one form to another is cyclic as ambient temperature and humidity change so there is repetitive stress to neighboring materials until rupture eventually occurs.
Group 2 includes hydration of anhydrous compounds such as free lime, periclase, and magnesium wustite. These compounds irreversibly react with water and form in situ hydrates that have larger solid volumes. Calcium oxide forms strained calcium hydroxide (epizet) that is almost double the size of calcium oxide. Periclase forms magnesium hydroxide (brucite) that is slightly more than double the size of periclase. The magnesium part of wustite forms magnesium hydroxide (brucite) and an attendant volume increase. The destructive effects of these conversions result because the hydrates form on the surface of the anhydrous particles creating stress to neighboring encasing materials. The in situ phase changes of free lime and periclase to their respective hydroxides (epizet and brucite) have been reported capable of creating stresses up to 25,000 to 30,000 psi.
Group 3 includes iron sulfates pyrite, marcasite, and pyrrhotite. They oxidize at ambient temperature and humidity to form sulfuric acid and iron sulfate compounds. The sulfuric acid destructively reacts with calcium compounds in the paste and forms gypsum that precipitates in voids while some of the iron compounds further oxidize to form unsightly rust. If the gypsum is solublized, its sulfate component can react with calcium aluminates in uncarbonated paste and result in internal sulfate attack. The iron sulfates enter concrete as components of aggregates.
Group 4 includes aggressive solutions deficient in calcium that leach it from paste; rainwater, water containing bicarbonates and carbonic acid (e.g., swamp water); and solutions containing ammonium salts such as from some fertilizers. Peculiarly, because carbonated paste is more soluble in cold versus warm water, paste dissolution in lime-deficient waters may occur only during winter.
Group 5 includes miscellaneous mischievous things that pop up from time to time. Like mysterious pink-colored concrete, corrosion of aluminum-embedded materials, failure to set due to lead contamination or trace amounts of some unusual chemicals, air-entraining chemicals rendered ineffective due to organic materials in aggregates, ammonium-like odors emanating from concrete, and high air contents resulting from the reaction of aluminum (scraped from hoses used for pumping concrete or from aluminum stake-body trucks used to haul concrete) and cement alkalies.
Now it is time to call it quits on this fascinating subject about concrete ills and ailments, and hope that you suffer less than the concrete we have been discussing.
Bernard Erlin is president of Erlin Co. (TEC), Latrobe, Pa. and has been involved with all aspects of concrete for more than 48 years.
William Him is principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 54 years ago.