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51316-7121-Corrosion Induced Bridge Tendon Failures Resulting from Deficient Grout: Part I – Model Development

There are advantages to Post-tensioned (PT) concrete construction compared to conventional reinforcement.  However, corrosion caused tendon failures have recently been reported d/t chemically and/or physically deficient grout. A predictive model has been developed that projects onset and rate of wire and strand fractures.

Product Number: 51316-7121-SG
ISBN: 7121 2016 CP
Author: William Hartt
Publication Date: 2016
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Grouted post-tensioning has evolved to become the construction method of choice for many structures, including bridges. However, instances of corrosion induced fracture of wires and strands and failure of tendons have occurred as a consequence of either chemically or physically deficient grout (or a combination of these), where the former involves elevated levels of chlorides or free sulfates (or both) and the latter some combination of soft, chalky, separated, segregated grout with free water. In the extreme, there is the possibility of structure collapse. In response to this, a methodology has been developed whereby, first, the initial occurrence of wire and strand fracture and tendon failure and, second, the subsequent rate of fractures/failures can be projected. This was accomplished by relating occurrence of fractures and failures to the extent of wire cross section loss and its progression with time, as affected by severity of a grout deficiency or deficiencies. Primary inputs to the model include the mean and standard deviation of, first, wire corrosion rate and, second, wire fracture strength. Results indicate an initiation period as corrosion progresses for onset of fractures and failures followed by a progressively increasing fracture/failure rate up to an inflection point beyond which these rates moderate. Results from a simulated tendon are presented which serve to qualify the modeling approach.
Key words: downloadable, Post-tensioning, corrosion, bridges, grout, deficiencies, fracture, failure.

Grouted post-tensioning has evolved to become the construction method of choice for many structures, including bridges. However, instances of corrosion induced fracture of wires and strands and failure of tendons have occurred as a consequence of either chemically or physically deficient grout (or a combination of these), where the former involves elevated levels of chlorides or free sulfates (or both) and the latter some combination of soft, chalky, separated, segregated grout with free water. In the extreme, there is the possibility of structure collapse. In response to this, a methodology has been developed whereby, first, the initial occurrence of wire and strand fracture and tendon failure and, second, the subsequent rate of fractures/failures can be projected. This was accomplished by relating occurrence of fractures and failures to the extent of wire cross section loss and its progression with time, as affected by severity of a grout deficiency or deficiencies. Primary inputs to the model include the mean and standard deviation of, first, wire corrosion rate and, second, wire fracture strength. Results indicate an initiation period as corrosion progresses for onset of fractures and failures followed by a progressively increasing fracture/failure rate up to an inflection point beyond which these rates moderate. Results from a simulated tendon are presented which serve to qualify the modeling approach.
Key words: downloadable, Post-tensioning, corrosion, bridges, grout, deficiencies, fracture, failure.

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