High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks (2013)

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1 S U M M A R Y High-Performance/High-Strength Lightweight Concrete for Bridge Girders and Decks Background Use of high-strength prestressed concrete girders and high-performance bridge decks has become accepted practice by many state highway agencies because of their technical 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, sub- structure, 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 is needed to address the factors that significantly influence the design, constructability, and perfor- mance of high-strength prestressed concrete bridge girders and high-performance bridge decks and to recommend changes to the AASHTO LRFD bridge specifications. These modi- fied specifications will provide highway agencies with better guidance for considering light- weight concrete mixtures that are expected to yield economic benefits. Project Scope This research focused on developing recommended changes to the AASHTO LRFD Bridge Design Specifications (2010) and the AASHTO LRFD Bridge Construction Specifications (2010) relevant to high-strength lightweight concrete girders and high-performance lightweight concrete decks. The research (and resulting specifications and recommendations) 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 lightweight concrete used in bridge decks and precast, prestressed bridge elements; • Evaluate key design parameters for lightweight concrete; • Propose relevant changes to AASHTO LRFD Specifications; and • Perform design examples illustrating the effect of the proposed changes on design practice.

2Overview of the Project The research performed in this project began with an extensive literature review and prac- titioner survey conducted as Phase I of the project. Results of the literature review and prac- titioner survey are provided in Attachments C and D, respectively (available by searching for NCHRP Report 733 on the TRB website). The literature review and practitioner survey gave the researchers insight into the areas of investigation needed to achieve the project objectives and provided the basis and justification for the resulting Phase II work plan. The results of the project are recommended changes to the AASHTO LRFD Bridge Design Specifications and Construction Specifications, provided in Attachments A and B. The Phase II work plan (provided in Attachment E available by searching for NCHRP Report 733 on the TRB website) included both material property characterization and structural performance testing. During the material property characterization portion of this project a selection of lightweight aggregate sources was made, representing regional variations and a range of aggregate qualities, to identify a finite set of aggregates to be used for further investigation under this study. These aggregate sources were used to develop mixtures exhibiting a range of compressive and tensile properties. A typical normal weight mixture was selected for comparison purposes. Compressive, splitting tensile, and modulus of elasticity tests were conducted on all lightweight concrete mixtures used in this project. Based on the results of these tests, several mix designs were selected for use during the structural testing portion of this project. In addition, while structural testing was occurring, time-dependent material properties (e.g., creep and shrinkage) as well as other material properties (e.g., coefficient of thermal expansion and permeability) of representative light- weight concrete mix designs were investigated. The structural testing portion of the project included the following: • Push-off specimens (non-prestressed) to investigate interface shear strength in light- weight concrete bridge decks and girders; • Laboratory-cast prestressed beams to investigate prestress loss, transfer length, develop- ment length, deflections, and camber; and • Full-size prestressed girders to investigate shear strength, prestress losses, camber, and deflections. Specimens containing normal weight concrete were fabricated and served as control specimens. Comparative deck and girder designs were performed to investigate the changes in struc- tural design that resulted from the use of lightweight prestressed concrete girders with lightweight concrete decks instead of normal weight prestressed concrete girders with a normal weight concrete deck. Research Findings The major findings of this research are summarized below. These findings are applicable only to the materials used in this project. • Lightweight concrete with a compressive strength of 7000 psi and a unit weight less than 125 lb/ft3 can be produced with a 0.30 w/cm and 800 lb of cementitious material with expanded shale and slate aggregates. Concrete mixtures containing slate aggregate con- sistently produced higher compressive strength concretes than mixtures made with other lightweight aggregates. • The AASHTO LRFD equation for modulus of elasticity (with K1 = 1.0) is appropriate for lightweight aggregates. Predictions of modulus can be improved by calibrating the K1 value for each aggregate type.

3 • On average, the modulus of rupture of the lightweight concrete ranged from 0 26. ′f c to 0 36. ′f c , with an average of 0 31. ′f c . • ′f c is more appropriate for use in shear strength calculations for sand lightweight con- crete than 0 85. ′f c which is required in AASHTO LRFD Section 5.8.2.2. • Based on a reliability analysis of interface and beam shear, a phi factor of 0.85 is appropri- ate for shear design of sand lightweight concrete. • The AASHTO equations and the Ramirez and Russell equation for transfer length and development length provide a reasonable upper bound to the measured transfer lengths in the lightweight and normal weight girders. • The current AASHTO refined method for calculating prestress losses is appropriate for lightweight girders with lightweight decks. • The AASHTO model for creep and shrinkage results in good estimates of prestress loss and is appropriate for use with lightweight prestressed concrete girders. • The PCI Bridge Design Manual improved multiplier method, using the AASHTO creep and shrinkage models, provides a good estimate of camber at the time of erection.