Last month's analytical data set the stage for this month's conclusions: The obvious and overwhelming evidence rendered a summary judgment against the aggregate supplier. But it wasn't over: A trial still ensued over a damage award. The defendant's engineers again asserted that drying shrinkage was responsible for the cracks. To support this, the defendant's experts used a shotcrete core to demonstrate that only a few gypsum particles were present and insufficient gypsum remained to cause expansion. Using a companion core inches away, the plaintiff's expert presented pictorial evidence of more than 40 separate, positively identified, sand-sized gypsum-containing particles in a 26-cm2 (2-in.2) area of the shotcrete.

The evidence was clear. Internal sulfate attack caused the cracking, and the shotcrete component responsible for the cracks was gypsum introduced by the aggregate. The following chemical, mineralogical, and physical evidence led to that conclusion.

  • The shotcrete had an unaccounted-for high sulfate content.
  • Gypsum and gypsum-containing particles in the aggregate were abundant.
  • There was partial-to-complete dissolution of gypsum in aggregate particles.
  • Ettringite was intergrown with calcium silicate hydrates in the paste.
  • There was progressive widening of cracks and development of new cracks in the shotcrete.
  • Similar cracking did not occur before and after the six-month window of pool construction.
  • Gypsum is known to be present in the pit from which the aggregate was mined.
  • A production switch occurred at the beginning of that time window to an area in the aggregate pit where gypsum content is known to be high.

The defense presented only those items that might exonerate its client. A key part of the evidence was misrepresented as very little gypsum. The shrinkage theory was not founded on facts, thus it was not convincing. The jury awarded the contractor 100% of the damage claim, plus an additional amount for future damages.

White stains in the panes

In another case, the windows on a high-rise building in a large city were plagued by white vertical streaks that were difficult to remove. The building was constructed of a concrete frame and lightweight concrete floor slabs with precast concrete column covers and lintels with exposed quartz aggregate surface finishes and inset windows.

Window washers could not remove the white streaks, and etched rivulets below the streaks remained on window surfaces even after a variety of cleaners were applied. The final resort was a window cleaner that contained buffered hydrofluoric acid, which created an even worse etching problem, and eroded the aluminum swing stages. New stages were bought to support the window washers and so their arduous task to clean the windows continued using less corrosive solutions.

The rivulets associated with the white streaks were etched into the glass. They created optical distortions, so looking through the stained and etched glass panes was difficult. The building owner sought restitution from whoever was responsible, but everyone connected with the building construction claimed innocence. The concrete precaster—assuming the white stains were typical alkali-salt efflorescence (usually sodium and potassium sulfates)—advised the owner that the stains would go away after several years, however the stains remained.

Eventually, samples were sent to several laboratories. Only one properly identified it as mainly gypsum with minor amounts of atmospheric particulates, such as fly ash, carbon residues, and grit (including calcite, feldspar, and quartz). The cause of the stains was a reaction between the exposed aggregate concrete surface and atmospheric sulfurous and sulfuric acids (or acid rain). These acid solutions react with portland cement paste, dissolving calcium, which after drying leaves a residue of white gypsum. Minor amounts of ammonium sulfate from reactions of ammonium (from lightning strikes or cleaning solutions) with acid rain are intergrown with the gypsum deposits. Next month's column will continue this story.

Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for 52 years.

William Hime was a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 58 years ago.