This month's analytical development ties together all of the facts of the streak and etching problems. With that information in hand, cores from the precast facade were examined using ASTM C 856, “Standard Practice for Petrographic Examination of Hardened Concrete.” The examinations revealed an expected thin, lime-leached layer of paste up to 1/64 inch thick at the concrete surface; an unanticipated dense, low water-cement ratio paste (less than 0.30) up to 1/16 inch thick below the leached layer; a superabundance of residual portland cement particles showing very restricted hydration in the dense, low w/c ratio zone; and microsized rips and tears in the leached layer, in the low w/c ratio zone, and in paste flanking aggregate particles.
Those observations explained the continuing stains but not why the other symptoms were present. Investigations of the concrete manufacturing process revealed that in order to deeply expose aggregate particles, a strong retarder of cement hydration was applied to the surface of the facedown form. Concrete placed into the forms came in contact with the retarder. The concrete was stripped from the forms and the aggregate exposed using pressurized water hoses and stiff-bristled brushes to remove soft paste between aggregate particles. During this process, aggregate particles and paste were jarred slightly causing rips and tears. Petrographic examinations revealed a chemical effect of the form retarder was a thin transition zone of low w/c ratio paste at the exposed aggregate surface.
Efflorescence of concrete products usually slows and terminates. Termination results because the cycling transgressing solutions leach soluble materials to sufficient depths so nothing is left to solubilize, or whatever is solubilized precipitates below the surface. The plaintiff's petrographic expert's examinations revealed an abundance of residual portland cement particles in the low w/c zone at the concrete surface. Those particles provide a reserve of calcium for the acids to feed on and are a continuous supply of calcium to the aggressive sulfuric acid solutions that easily penetrate the concrete surface region via the rips and tears. The now neutralized calcium- and sulfate-enriched solutions migrate down window surfaces where they precipitate gypsum and continue to leave white streaking on the windows. Subsequent cyclic wetting and drying of the gypsum deposits solubilize the window glass leaving rivulets etched in the glass surface.
During litigation, the precaster relied upon five experts to counter the plaintiff's conclusions, including a concrete petrographer, chemical engineer, cement/concrete chemist, consulting materials engineer, and glass expert knowledgeable about etching. Cores selected by the defendant's experts were sent to the defendant's consulting petrographer who cut the cores into longitudinal halves and sent one half to the plaintiff's consulting petrographer. All of these experts had an explanation for the etching that coincidentally absolved their client.
The plaintiff's expert showed photomicrographs of the residual cement particles and the rips and tears; commented about the excellent mathematical model derived by the chemical engineer, yet pointed out that five of the seven assumptions were incorrect or not wholly applicable so that the evaluation was inappropriate; and agreed with the cement/concrete chemist that water alone could cause the rivulet problem, but said that did not fully explain the gypsum streaks directly associated with the rivulets.
In the end, the defendant was found responsible for the window distress. The plaintiff's petrographer uncovered all of the facts of what happened. For whatever reason, the defendant did not have at his grasp enough positive analytical data for identifying the “truth” so he misconstrued the events that actually occurred to cause the stained and etched window panes—his maneuvering of some of the data was unsuccessful in light of all of the facts.
William Hime was a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 58 years ago.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for 52 years.