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41.1 Background Use of high-strength prestressed concrete girders and high-performance bridge decks has become accepted prac- tice by many state highway agencies because of their tech- nical and economic benefits. These girders and decks are generally constructed with concrete made with natural normal weight aggregates. Use of manufactured lightweight coarse aggregates (e.g., expanded shale, slate, and clay) to produce lightweight concrete offers the benefit of reducing the weight of the superstructure, leading to reductions in the size of girders, substructure, and foundations. These size and weight reductions facilitate shipping, handling, and construction or replacement of bridge elements and result in economic benefits. Recent advances in high-performance/high-strength lightweight concrete have had limited application in bridge construction because of the lack of design and construction guidelines and concerns about material properties and their impact on performance. Research was needed to address the factors that significantly influence the design, constructa- bility, and performance of high-strength prestressed con- crete bridge girders and high-performance bridge decks and to recommend changes to the AASHTO LRFD bridge specifications. These modified specifications will provide highway agencies with better guidance for considering lightweight concrete mixtures that are expected to yield economic benefits. 1.2 Project Objectives and Scope This research focused on developing recommended changes to the AASHTO LRFD Bridge Design Specifications (2010) and the AASHTO LRFD Bridge Construction Speci- fications (2010) relevant to high-strength lightweight con- crete girders and high-performance lightweight concrete decks. This research dealt with mixtures made with normal weight fine aggregates and manufactured lightweight shale, clay or slate coarse aggregates to produce concrete with equilibrium density, as determined according to ASTM C567, of not more than 125 lb/ft3. To accomplish these objectives, the research included work to ⢠Develop mix designs and material properties for light- weight concrete used in bridge decks and precast, pre- stressed bridge elements; ⢠Identify and evaluate key design parameters for lightweight concrete; ⢠Propose relevant changes to the AASHTO LRFD Bridge Design Specifications and AASHTO LRFD Bridge Construc- tion Specifications; and ⢠Perform design examples to investigate the effect of the proposed changes on design practice. 1.3 Research Plan and Methodology The following tasks were performed to achieve the project objectives: 1. Review of relevant design standards and recent research results on the use of lightweight concrete in bridges. 2. Synthesis of survey results from state departments of trans- portation concerning use of lightweight concrete in bridge decks and girders. 3. Development and execution of an experimental investi- gation which included the proportioning of lightweight concrete mixtures and the manufacture and testing of lab-cast beams, full-scale beams, and interface shear test specimens. 4. Development of proposed changes to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications. C h a p t e r 1 Introduction
5 5. Preparation of design examples showing the effect of proposed design specification changes on typical super- structure designs. 6. Preparation of a report that documents the entire research. 1.4 Organization of the Report This chapter presents the background, objectives, meth- odology, and scope of the project. Chapter 2 summarizes the findings of the research, including the material testing and structural testing portions of the project. Chapter 3 provides background for the research program and the details of the testing program. Chapter 4 presents a review of the inter- pretation and appraisal of the project results. Chapter 5 pro- vides the project conclusions and suggested future research. Attachments A and B give the suggested changes to the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications. Attachments C, D, and E contain a detailed literature review, survey results, and a literature summary and the approved work plan, respectively. Attachments F through M provide details of the experi- mental program that were not able to be included in the body of this report. Attachments N through Q present design examples of bridges containing lightweight con- crete and details of the parametric study. Attachment R is a detailed reference list. Attachments C through R are not included herein but are available by searching for NCHRP Report 733 on the TRB website.
