Control of Concrete Cracking in Bridges (2017)

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3 Background Despite many advances in bridge design, concrete technology, and corrosion-resistant reinforce- ment, cracking of concrete members continues to be a concern for bridge owners, particularly for bridges exposed to severe environments. The presence of cracks provides a direct path for water and chlorides to penetrate concrete and reach the reinforcement, a process that can lead to degradation of the concrete or corrosion of steel reinforcement. The AASHTO LRFD (load and resistance factor design) Bridge Design Specifications (AASHTO 2017) provide requirements for minimum amounts of reinforcement and maximum spacing of reinforcement to control crack widths. Some requirements are based on in-depth research; others are based on experience. Nevertheless, bridge owners find the need to supplement the AASHTO specifi- cations with their own requirements. The control of cracking for aesthetic, durability, and structural reasons becomes increasingly important as service-life goals are extended and higher strength concrete, higher strength reinforcement, and different types of reinforcement are used in bridge construction. A need existed to compile the latest information about the state of the practice for control of concrete cracking in bridges and to identify the significance of cracking on long-term durability. oBjectives and scope The objectives of this synthesis are to • Provide a compilation and discussion of methods used to control concrete cracking in bridge superstructures and substructures, and • Present information on the influence of cracking on long-term durability. The scope includes information about the effect of the following on control of cracking: • Concrete mix design and performance requirements, • Construction practices, • Structural design requirements, • Steel reinforcement with yield strengths from 60 to 100 ksi, • Corrosion-resistant steel reinforcement, and • Fiber-reinforced polymer reinforcement. The discussion includes background study and experience in practice, as well as proven inno- vative methods. The relationship between environmental exposure conditions and crack-related parameters, such as width, depth, spacing on concrete degradation, corrosion, and service life, is addressed. Both lightweight and normal-weight concrete and concrete decks on steel and concrete beams are discussed. The scope of this synthesis does not include segmental box girder bridges and cracking caused by corrosion of reinforcement; detrimental concrete chemical reactions, such as alkali-aggregate reactivity; or adverse concrete performance from environmental exposure conditions, such as freeze- thaw damage. chapter one introduction

4 study approach Information for this synthesis was obtained from a literature review, surveys of state departments of transportation (DOTs), surveys of provincial and territorial agencies in Canada, and input from selected individuals who have in-depth information. The literature search provided many references related to concrete cracking and concrete shrinkage; such references were too numerous to all be included in this synthesis. Consequently, the synthesis concentrates on publications from 1990 onward, those related to bridges, and those that provide practical recommendations. Information on international practice, except for that of Canada, is not included. In addition, many laboratory research projects on shrinkage and concrete cracking have been performed but are not referenced in this synthesis. Information gathered in this synthesis provides a basis for understanding the causes of concrete crack- ing in bridges and helps to establish the most practical and efficient methods for reducing the occurrence of cracking and controlling cracking when it occurs. report organization The text of the synthesis is organized as follows: • Chapter two provides an overview of the different types and causes of concrete cracking that can occur in concrete and steel bridges. The crack types are those associated with flexure, tor- sion, shear, splitting, plastic shrinkage, plastic settlement, autogenous shrinkage, drying shrink- age, and temperature changes. • Chapter three provides information about approaches to reduce the potential for cracking through the selection of concrete constituent materials and use of internal curing. • Chapter four provides information about the effects of construction practices on cracking, includ- ing curing, weather conditions, and construction sequence. • Chapter five includes information about the effects of different types of reinforcement on crack- ing. It discusses the effects of using steel reinforcement with specified yield strengths from 60 to 100 ksi, corrosion-resistant steel reinforcement, and fiber-reinforced polymer (FRP) rein- forcement. Specifications for reinforcement to control cracking and practices to supplement the specifications are described. • Chapter six discusses the influence of cracking on long-term durability, including permissible crack widths, bar spacing, and service life. • Chapter seven provides four case examples showing how the states of California, Kansas, Pennsylvania, and Washington have reduced the severity of cracking in their concrete bridge decks. • Chapter eight provides a summary of the key findings of the synthesis project, including the state of the practice for control of cracking in bridges and “effective practices.” Key findings and con- clusions are summarized. Suggestions for future research are offered. • Appendices provide the survey questionnaire (Appendix A), a compilation of the responses to the questionnaire (Appendix B), a cross reference table between the article numbers used in the sev- enth and eighth editions of the AASHTO LRFD Bridge Design Specifications for those articles referenced in this synthesis (Appendix C), and a research problem statement (Appendix D). The eighth edition of the AASHTO LRFD Bridge Design Specifications contains a reorganized version of the seventh edition’s Section 5: Concrete Structures. Because of the reorganization, many articles have been renumbered. Article numbers used in this synthesis are based on the eighth edition, with the corresponding number for the seventh edition included in parentheses.