Durability of Prestressed Concrete Beams

Abstract

The durability of pretensioned and posttensioned concrete beams is being studied using laboratory-prepared beam specimens. A group of beams containing pretensioned strands, some made with airentrained concrete and others with non-air-entrained concrete, was exposed to natural weathering. Sixteen beams were subjected to freezing in air and thawing in seawater at Treat Island, Maine, and three beams were exposed to sulfate attack in warm seawater on the Florida coast. In Maine the nonair-entrained concrete beams failed during the first winter; the air-entrained beams remain in good condition after five winters. No significant results have been observed at the Florida exposure. A group of posttensioned beams, all made with air-entrained concrete, was also exposed in Maine. Four posttensioning systems with either external or flush end anchorages, and 12 types of end-anchorage protection were used. The auxiliary steel reinforcement had a nominal 3/a in. concrete cover. After two winters of exposure, two of 40 end-anchorage protections have become detached, two have become loose, and several have developed a narrow crack at the bond with the beam. In all of the beams that show evidence of failure, portland cement concrete was used to enclose external anchorages. It is concluded that (1) Concrete for prestressed structural elements that are to be exposed to freezing and thawing in a moist condition should contain entrained air. (2) The concrete cover over reinforcing should be greater than '3/a in. when exposure to seawater is involved. (3) Epoxy concrete is superior to portland cement concrete for end protection. Flush anchorages appear to be more effectively protected than external anchorages.