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

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65 This chapter presents the major conclusions of the research effort and provides suggestions for future research. However, it should be recognized that these findings are based on the results of limited tests conducted using limited material sources. Other research may be necessary to validate these findings. 5.1 Material Properties Ninety-five laboratory test batches and 15 production batches of lightweight concrete were tested for a wide range of material properties. Based on these tests, the following conclusions are made: • Two lightweight aggregates used in concrete mixtures for large-scale test specimens yielded test results consistent with what is needed for structural concretes. • 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 shales and slates. The slate (SL1) consistently produced the highest strength concretes. • The AASHTO LRFD equation for modulus of elasticity with K1 = 1.0 is appropriate for lightweight aggregates. Pre- dictions of modulus can be improved by calibrating the K1 value for each aggregate type. • The average splitting tensile strength of the lightweight concrete mixtures was 0.25 ′f c which exceeded ′f c /4.7. • On average, the modulus of rupture of the lightweight concrete was 0.31 ′f c , with a lower bound of 0.26 ′f c . • Low-permeability lightweight concrete was produced through the use of supplementary cementitious materials. Lowest permeability values were achieved with silica fume as the cement replacement product. • The AASHTO model for shrinkage generally predicted the shrinkage of lightweight concrete better than ACI 209 or CEB MC90. • The AASHTO model for creep generally predicted the creep coefficients of the lightweight girder mixtures better than ACI 209 or CEB MC90. The creep coefficients of the deck concrete mixtures were considerably higher than pre- dicted by the AASHTO model and were better predicted by the ACI 209 model. 5.2 Interface Shear Strength Nine sets of three specimens each were tested to evaluate interface shear strength. Concrete strengths for the specimens ranged from 5.73 ksi to 6.25 ksi for the deck concretes and from 7.78 ksi to 11.1 ksi for the girder concretes. Push-off tests were used, with three different reinforcement ratios for the interface shear reinforcement, and three different com- binations of concrete type (normal weight deck on normal weight girder, lightweight deck on lightweight girder, and lightweight deck on normal weight girder). Analysis of the test results indicated the following: • The bias of the measured shear strengths to the nominal shear strength computed with the AASHTO equation for a concrete deck placed on the top flange of a girder that was intentionally roughened was 1.16 for N-N, 1.29 for L-N, and 1.26 for L-L. • The AASHTO equation is less conservative with increasing reinforcement ratios, which indicates the friction coeffi- cient may be too high. • Based on a reliability analysis, normal weight and light- weight concrete should have the same strength reduction factor for interface shear. 5.3 Shear Tests Two shear tests were performed on each of six full-scale prestressed beams. Five beams were fabricated with light- weight concrete and one with normal weight. Compressive strengths of the concrete used in the full-size girders ranged C h a p t e r 5 Conclusions and Suggested Research

66 from 8.5 ksi to 10.3 ksi. Based on the results of these 12 tests, the following observations and recommendations are made: • The factor, lv, has an insignificant effect on the calcu- lated shear strength of prestressed girders, when using the AASHTO sectional or simplified shear design approach. • The bias of measured shear strength to calculated shear strength for normal weight and lightweight prestressed girders is approximately the same. • Modification of the ′f c term in shear calculations for lightweight concrete is not necessary. • The f factor for shear design of sand lightweight concrete of 0.85 is appropriate. 5.4 Transfer and Development Length Testing Twelve lab-cast beams were constructed and tested to eval- uate transfer and development length in lightweight concrete prestressed girders. Concrete compressive strengths of these beams ranged from 7.39 ksi to 8.54 ksi at 28 days of con- crete age. Analysis of test results showed that the AASHTO equations and the Ramirez and Russell (2008) equations for transfer length both provide a reasonable upper bound to the measured transfer and development lengths in the lightweight and normal weight girders studied in this project and others found in the literature. 5.5 Time-Dependent Behavior Prestress Losses The twelve lab-cast beams and six full-scale beams were instrumented with vibrating wire gages to track changes in strain at the level of the strands. The concrete compressive strengths found in these beams and girders ranged from 7.39 ksi to 10.3 ksi. Based on comparisons of measured and calculated prestress losses, the following conclusions are made: • The current AASHTO refined method for calculating pre- stress losses is appropriate for lightweight girders with lightweight decks. • The majority of the difference between calculated and mea- sured prestress loss occurs during the time between release and deck placement. The AASHTO method consistently pre- dicts higher losses than were measured during this period. • Of the three creep and shrinkage models allowed by AASHTO (AASHTO, ACI 209, and CEB MC90), the AASHTO model results in estimates of prestress loss closest to those measured and is appropriate for use with lightweight prestressed concrete girders. Camber The twelve lab-cast beams and six full-scale beams were instrumented with a taut-wire measuring system to track changes in camber. The concrete compressive strengths found in these beams and girders range from 7.39 ksi to 10.3 ksi. Based on comparisons of measured and calculated cambers, the following conclusions are made: • Of the methods evaluated in the project, the PCI Bridge Design Manual improved multiplier method, using the AASHTO creep and shrinkage models, provides an appro- priate estimate of camber at the time of erection. • For estimates of camber after casting of the deck, the AAEM method provides good prediction of end-of-service cambers in composite girders. 5.6 Design Examples and Parametric Studies Based on comparative designs of bridge superstructures with normal weight and lightweight concrete in the prestressed girders and deck, the following observations and recommen- dations are made: • For identical configurations, the lightweight girder and deck example required 10% fewer strands than the normal weight example. • The current strength reduction factor for a shear of 0.70 for lightweight girder results in almost twice the amount of shear reinforcement required for the normal weight example. • A change in the strength reduction factor to 0.85 will result in required amounts of shear reinforcement similar to that required for normal weight beams. 5.7 Recommendations for Future Research The following recommendations are made for future research related to the use of lightweight concrete in bridge girders and decks: • Six aggregate sources from around the United States were used in this study. However, there are numerous other lightweight aggregate sources in the United States. Therefore, state and local transportation authorities need to perform material property testing for locally available lightweight aggregate to verify that other available light- weight aggregates yield concrete mixtures with structural material properties similar to those found in this research. The most important structural properties to be verified are modulus of elasticity and tensile strength. Addition-

67 ally, testing of Resistance to Abrasion (ASTM C944) and Resistance to Scaling (ASTM C672) of concrete mixtures containing lightweight aggregates should be performed in order to ensure proper resistance to these effects. • The reliability study for interface shear strength was per- formed on a very limited data set. Further tests of interface shear strength of lightweight concrete will help to better quantify the reliability index. • The evaluation of shear strength and strength reduction factors of lightweight girders was based on a total of less than 25 tests. Further tests of full-size prestressed girders will help to verify or modify the recommended f value.