Freezing and Thawing and Alkali-Aggregate Reaction Weakens Lightweight, Structural Concrete in North Dakota

Abstract

The Bureau of Reclamation may replace a transfer deck that is deteriorating in North Dakota. The transfer deck serves as the parking deck and roof for a pumping plant. In late June 2001, portions of the lightweight concrete failed and fell to the pumping plant floor during a storm. The concrete failed along a continuous fracture about 1 cm above the ceiling surface. The fracture penetrated all aggregate particles encountered. Petrographic examination of 15 cores indicated about 18% of the concrete below a waterproofing membrane and 3% of the lower surface of the concrete was damaged by freezing and thawing deterioration and minor alkali-aggregate reaction (AAR). The water saturating the lightweight concrete apparently penetrated the field-manufactured seams in the membrane. Saturated aggregates froze during the winter as the depth of frost penetrated the full thickness of the deck and structural concrete. Evidence indicates that ceiling insulation held water in contact with the lower concrete surface, while freezing and thawing deterioration and AAR caused the concrete to fail. Structural analysis indicates the transfer deck loads be restricted to 40.7 kPa (850 lbsf) unless further deterioration is anticipated or additional damage is observed. The lightweight concrete cannot be repaired and will have to be replaced eventually.