Shotcrete cross section. Within the yellow circle is caulk at the top of a “repaired” crack. Within the white circle is a No. 3 reinforcing bar. (Scale in inches)
Bernie Erlin Shotcrete cross section. Within the yellow circle is caulk at the top of a “repaired” crack. Within the white circle is a No. 3 reinforcing bar. (Scale in inches)

More than 100 inground outdoor swimming pools and integral spas—constructed using reinforced shotcrete shells faced with nominal 12.5-mm-thick (½-in.-thick) intimately bonded white mortar veneer—had cracked after a few years of use. The major cracks had polygonal patterns and became noticeable after the pools were flooded. Subsequently the cracks widened, even while the shotcrete was underwater. The cracked pools and spas were constructed during a continuous six-month period. Pools constructed before and after that period did not crack; any problems were minor and resolved during normal maintenance. Continuing last month's discussion, the litigation involving the pools constructed during the troublesome six-month period ensued.

The shotcrete was dry mixed, with water added and controlled by a nozzle person who gauged the water to minimize the amount of overspray and rebound. The shotcrete was normal for the locality (see photo), with common and typical variegated cross sections. The variegations were mainly the result of variable water-cement ratios due to mixing water adjustments by the nozzle person. The shotcrete contained typical small areas of micro honeycombing and numerous small, isolated honeycombed areas below (or behind) reinforcing bars.

The natural siliceous-calcareous pea-gravel-sized sand shotcrete aggregate contained rocks and minerals, including granite, granodiorite, diorite, gabbro, diabase, volcanic rock, quartz, feldspar, mica, magnetite, mafic minerals, limestone, gypsiferous limestone, and selenitic and multicrystal gypsum.

Bernard Erlin
Bernard Erlin

The mortar veneer used white portland cement and very fine fragmental marble aggregate applied onto the shotcrete substrate and the veneer adhered. The pools were flooded and maintained underwater.

After the pools were in use for several months, cracks with polygonal patterns on nominal 0.3-m (1-ft.) centers began to develop. They became more prominent as they progressively widened. The contractor provided a lifetime warranty, so cracks were repaired at his time and expense. Some pools required extra repairs because old cracks reopened after first-, second-, third-, and sometimes even fourth-time repairs—direct evidence of continuing expansion of the always-moist shotcrete.

Petrographic examinations indicated that inconsequential alkali silica-aggregate reactions occurred. In areas of the paste, acicular ettringite (3CaO·Al2O3·3CaSO4·32H2O) lines voided and were intergrown with calcium silicate hydrates. There also were remnants of gypsum (CaSO4·2H20) aggregate particles that evidenced dissolution by solutions. Additionally, some sites where limestone aggregate particles were vestigial remnants of clay and calcite, occasionally with traces of residual gypsum, also evidenced dissolution.

William Hime
William Hime

Experts agreed that gypsum particles were present in the aggregate. However, they disagreed on several factors, including the number of gypsum particles, the high overall gypsum particle content, and the effects of the ettringite on concrete expansion. The situation was complicated by several engineers for the aggregate-supplier defendant who maintained the cracks were from drying shrinkage—after the pool shells were determined to be structurally adequate. It is difficult to envision drying shrinkage since the cracks grew wider when exposed underwater and were always damp.

Detailed petrographic examinations by at least four petrographers identified the gypsum-containing aggregate particles and ettringite intergrown in the paste. Chemical data for sulfate (SO4) was used to demonstrate that it was present in amounts far greater than contributed by the portland cement. Stay tuned next month for more.