Yi Sun and Rigoberto
Burgueño
Department of Civil and Environmental Engineering , Michigan State University, 3546 Engineering Building, East Lansing, MI 48824-1226
Strand debonding is a common procedure used in prestressed concrete members for limitation of compressive and tensile concrete stresses near the element ends, i.e., in the anchorage zone. However, a recent problem faced by the Michigan Department of Transportation (MDOT) on the production of pre-tensioned box girders with debonded strands has raised questions on the adequacy of this manufacturing the design practice. While many issues are associated with the effects of debonded strands, little research has been conducted on this topic, and the available design guidelines from the AASHTO-LRFD specifications (limit to the number of unbonded strands and their arrangement) are quite simple and their wide applicability is questionable. However, no guidance exists on manufacturing practice, debonded length limits, reinforcement within the unbonded region, layout, or cutting sequence. Therefore, a better understanding of the effects of debonded strands, particularly those that can lead to distress, is needed for improved design and manufacturing practice.
The idea of debonding is to delay the stress transfer between prestressing strand and concrete due to bond. Since the stress transfer between strand and concrete starts further into the concrete element, the stresses at the end regions are reduced. Strand debonding is normally achieved by placing plastic sheathing around the strand and two types of debondng material are currently used (both made of plastic-type materials, namely, flexible split-sheathing and a more rigid preformed tube.
Experimental and numerical approaches were conducted in this study in order to achieve a further understanding of strand debonding. Twenty-four small-scale prestressed concrete beam units were tested and used for the calibration of nonlinear finite element models simulating concrete-strand bond behavior, while three models of AASHTO box girders were established to investigate an incident of end cracking encountered in the manufacturing of a bridge girder. The numerical simulations were in good agreement with the experiment data and damage evidence on prestressed girders production indicating that the lack of bonding will maximize the dilation of strand after release in the debonded region and that such dilation may cause concrete damage in the debonded region if there is tight contact between concrete and strand. It was also found that such problem will be eliminated if enough room is provided for the strand dilation. Thus, the use of “rigid” or oversized debonding material is recommended for strand debonding practice.
Department of Civil and Environmental Engineering , Michigan State University, 3546 Engineering Building, East Lansing, MI 48824-1226
Strand debonding is a common procedure used in prestressed concrete members for limitation of compressive and tensile concrete stresses near the element ends, i.e., in the anchorage zone. However, a recent problem faced by the Michigan Department of Transportation (MDOT) on the production of pre-tensioned box girders with debonded strands has raised questions on the adequacy of this manufacturing the design practice. While many issues are associated with the effects of debonded strands, little research has been conducted on this topic, and the available design guidelines from the AASHTO-LRFD specifications (limit to the number of unbonded strands and their arrangement) are quite simple and their wide applicability is questionable. However, no guidance exists on manufacturing practice, debonded length limits, reinforcement within the unbonded region, layout, or cutting sequence. Therefore, a better understanding of the effects of debonded strands, particularly those that can lead to distress, is needed for improved design and manufacturing practice.
The idea of debonding is to delay the stress transfer between prestressing strand and concrete due to bond. Since the stress transfer between strand and concrete starts further into the concrete element, the stresses at the end regions are reduced. Strand debonding is normally achieved by placing plastic sheathing around the strand and two types of debondng material are currently used (both made of plastic-type materials, namely, flexible split-sheathing and a more rigid preformed tube.
Experimental and numerical approaches were conducted in this study in order to achieve a further understanding of strand debonding. Twenty-four small-scale prestressed concrete beam units were tested and used for the calibration of nonlinear finite element models simulating concrete-strand bond behavior, while three models of AASHTO box girders were established to investigate an incident of end cracking encountered in the manufacturing of a bridge girder. The numerical simulations were in good agreement with the experiment data and damage evidence on prestressed girders production indicating that the lack of bonding will maximize the dilation of strand after release in the debonded region and that such dilation may cause concrete damage in the debonded region if there is tight contact between concrete and strand. It was also found that such problem will be eliminated if enough room is provided for the strand dilation. Thus, the use of “rigid” or oversized debonding material is recommended for strand debonding practice.
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