Nature of Bond in Pre-Tensioned Prestressed Concrete


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Title: Nature of Bond in Pre-Tensioned Prestressed Concrete

Author(s): Jack R. Janney

Publication: Journal Proceedings

Volume: 50

Issue: 5

Appears on pages(s): 717-736

Keywords: no keywords

Date: 5/1/1954

Methods and findings of an investigation of bond in pre-tensioned prestressed concrete members are discussed. Two types of tests were involved. Bond near the ends of a prestressed member after release of the wire pre-tension was studied using prismatic specimens. Beam specimens were used to study flexural bond and the interrelation between this flexural bond and the bond resulting from the transfer of prestress. Principal variables considered were diameter, surface condition, and the degree of pre-tension of the wire rein-forcement. A variation in anchorage length and general shape of the stress transfer distribution was noted for wires of different diameters and for different surface conditions ranging from rusted to lubricated. Similar observations were observed when the pre-tensioned steel was released to concretes of different strengths. An elastic analysis of the deformations occurring when pre-tensioned steel is released confirms the test results in suggesting that the prestress transfer bond is largely a result of friction between concrete and steel. Beam tests indicate that reliable values for flexural bond stress after cracking are not obtained by the expression generally accepted for calculation of bond stress. High bond stresses develop only after cracking has occurred. Conse-quently, if loss of bond is the cause of beam failure, a prestressed beam will carry a greater ultimate load than an unprestressed beam reinforced with the same steel.