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

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6The research began with an extensive literature review and practitioner survey, both of which are provided in Attach- ment C (available by searching for NCHRP Report 733 on the TRB website). Based on the literature review, practitio- ner survey, and review of the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications, those aspects of the specifications that could be improved to better reflect the behavior of lightweight concrete were identified. It was found that the areas of high- est importance were shear strength of composite lightweight prestressed concrete girders, estimation of prestress loss and camber in bridge girders, bond of prestressing strand to light- weight concrete, and numerous material properties (strength, modulus of elasticity, creep, shrinkage, and durability). The work plan included both material property character- ization and structural performance tests designed to inves- tigate these areas of highest importance. Specific findings derived from both the material characterization and structural performance testing portions of this project are provided in this section. The resulting proposed changes to the AASHTO specifications are presented in Attachments A and B. 2.1 Materials Testing The mix design and material property portion of the project consisted of two phases. The first phase focused on lightweight aggregate selection and developing mix designs for concrete used in bridge decks and girders. Ninety-five concrete batches were mixed in the laboratory to evaluate the relative performance of six aggregate sources across a range of mixture designs. Tar- geted variables in the screening mixture design matrix included aggregate source, w/cm (0.30 or 0.40), supplementary cementi- tious materials, and total cementitious content. The second phase involved the determination of the material properties of the developed mix designs when used on a production basis. Based on findings from the laboratory screening tests, mixtures were identified for further large-scale testing in laboratory and lab-cast beams, full-size girders, and deck segments. Two lightweight aggregates were selected for use in the large-scale test specimens. These aggregates, when used in laboratory mixtures, yielded test results consistent with what is needed for structural concretes. Three categories of lightweight mixtures were selected, representing moder- ate and high-strength lightweight girder mixtures and a light- weight deck mixture. The lightweight concrete mixtures were required to have a unit weight less than 125 lb/ft3. To account for variability in concrete properties, all mixtures were designed for a tar- get fresh unit weight of 122 lb/ft3. The measured unit weight ranged from 119 to 125 lb/ft3. The compressive strength of concrete mixtures containing certain lightweight aggregates was consistently higher than those made with other lightweight aggregates. The binder pro- portions that provided the best combination of workability and strength incorporated either slag cement or silica fume in addition to portland cement. Mixtures containing slag cement were typically very workable and easily provided the desired air content. Lightweight concrete with a compressive strength of 7000 psi and a unit weight of less than 125 lb/ft3 was produced with a 0.30 w/cm and 800 lb of cementitious material. The AASHTO LRFD Bridge Design Specifications Sec - tion 5.4.2.4 (2010) provides the following equation to predict the modulus of elasticity based on compressive strength, unit weight of concrete, and aggregate source used as follows: E K w fc c c= ′33 000 1 1 5, . where K1 = correction factor for source of aggregate wc = unit weight of concrete (kcf) f ′c = specified compressive strength of concrete (ksi) A statistical analysis of the modulus of elasticity test values obtained from the laboratory and production mixtures was C h a p t e r 2 Findings

7 performed in order to determine the appropriate K1 factor to be used. The analysis showed that using a value of K1 = 1 in the AASHTO equation for modulus of elasticity gave good correlation with the test results for lightweight concrete. The relationship between splitting tensile strength fct and ′f c was used to evaluate the effects of lightweight aggregates on splitting tensile strength of concrete. This was done by determining the factor “a”, where f a fct c= ′, for the light- weight mixes tested in this study. The value for factor a ranged from about 0.23 to 0.27, with an overall average of 0.25 for these lightweight mixtures, well above the value of 0.21 inferred from the AASHTO specifications. This indi- cated the need to consider modifying Section 5.8.2.2 of the AASHTO specifications. The measured values of flexural strength were compared to the predictions from AASHTO specifications. For sand light- weight concrete, AASHTO specifications (Section 5.4.2.6) require using f fr c= ′0 20. , where fr and f ′c are expressed in ksi, for calculating modulus of rupture. For normal weight concrete, factors before the radical include 0.20 for crack- ing moment when calculating Vci, and 0.24 for calculating deflections. Comparison of the predicted results for flexural strength with the measured results at 28 days showed the fac- tor for sand lightweight concrete significantly underestimates flexural strength. The concrete creep test results were compared with the AASHTO, ACI 209, and CEB-FIP Model Code 1990 (CEB MC90) models. For the concrete mixtures tested, creep coeffi- cients for normal weight and lightweight girders were best pre- dicted by the AASHTO model. Therefore, use of the AASHTO creep coefficient calculation method is recommended for esti- mating prestress losses and deflections of lightweight concrete girders. The lightweight high performance concrete (LWHPC) deck mixtures exhibited considerable variation in creep coef- ficients, which were best predicted by the ACI 209 model. How- ever, because the creep behavior of the deck is secondary in importance to the girder creep characteristics, continued use of the AASHTO creep coefficient for calculation of the deck properties is appropriate. 2.2 Interface Shear Strength Precast girders with cast-in-place decks is a common type of bridge construction. Developing composite action between the two components is a key requirement of this type of design, and providing adequate horizontal shear strength at the inter- face of these components is necessary for developing this com- posite action. Tests were performed to examine the interface or horizontal shear strength of lightweight concrete. The cur- rent AASHTO LRFD Bridge Design Specifications (5th Edition) design equation for interface shear strength of cast-in-place concrete decks on precast concrete girders suggests more variability of lightweight concrete’s behavior in shear than of normal weight concrete. To reflect this difference, a resistance factor of 0.7 is used for lightweight concrete and a resistance factor of 0.9 is used for normal weight concrete. Push-off tests were conducted to compare the interface shear strength of lightweight concrete to that of normal weight concrete. These tests investigated (1) the ratio of the area of interface shear reinforcement to the area of con- crete at the interface of the bridge deck and bridge girder and (2) combinations of deck and girder concretes. Four levels of this shear reinforcement were investigated: no horizontal shear reinforcement, the minimum allowed, the maximum allowed, and an intermediate amount. The combinations of deck and girder concrete were a lightweight deck cast on a lightweight girder, a lightweight deck cast on a normal weight girder, and a normal weight deck cast on a normal weight girder. These test variables resulted in 12 configurations of push-off tests, all of which were repeated three times to provide information on the variability of test behavior. Details of the testing program are provided later in this report. The following observations are made based on the inter- face shear tests: • The current AASHTO LRFD design equation (with a strength reduction factor of 0.9) underestimated interface horizon- tal shear strength of all of the lightweight deck-lightweight girder specimens, 8 of 9 of the normal weight-lightweight specimens, and 8 of 9 normal weight-normal weight speci- mens. A strength reduction factor of 0.8 is more appropriate for predicting interface shear strength of these specimens. • On average, design calculations were more conservative in predicting the strength of the lightweight concrete than the strength of specimens containing normal weight concrete. • The normal weight girder with lightweight deck exhib- ited behavior similar to that of the lightweight girder with lightweight deck. The interface shear strength of girder/ deck combinations with lightweight concrete was greater than that of normal weight concrete deck on a normal weight concrete girder. • The average post-crack interface strength for normal weight/ normal weight specimens was greater than for lightweight specimens when interface shear reinforcement was provided. Specimens without interface shear reinforcement had slightly higher post-crack interface strengths when lightweight con- crete was used. • As the amount of shear reinforcement at the interface increased, the ratio of measured horizontal shear strength to LRFD calculated strength decreased. • Resistance factors calculated from the reliability analysis performed on test data indicated that the same resistance factor used for normal weight concrete could be used for lightweight concrete. The reliability index for lightweight

8Section 5.8.2.2). The second is the use of a resistance fac- tor (f) for shear design of lightweight concrete girder of 0.7 (compared to 0.9 for normal weight concrete girder). Based on the test results from the six girders and full-size girder test results found in the literature, the following conclusions were made: • The AASHTO modification factor (lv) applied to ′f c term in shear strength calculations is not needed for sand lightweight concrete prestressed concrete girders. This con- clusion is primarily based on the results of full-size girder tests; it is also supported by the finding from the material portion of this study that the tensile strength of lightweight concrete was similar in magnitude to that of normal weight concrete. • In Section 5.5.4.2, the AASHTO LRFD Bridge Design Speci- fications require that a f of 0.7 be used for calculation of the shear strength of lightweight prestressed concrete girders as compared with 0.9 for normal weight prestressed concrete girders. Based on analysis of all available full-size girder test results, the f required for use with lightweight prestressed concrete girders is 0.85. 2.5 Time-Dependent Behavior of Lab-Cast and Full-Scale Beams Two aspects of time-dependent behavior were studied: prestress losses and deflection. Each plant-cast girder and lab-cast girder had a vibrating wire gage installed at midspan at the level of the centroid of the prestressing force. This gage monitored changes in strain in the concrete at the centroid of the strand from just prior to release of prestress to the time of testing of the girders. In addition, each beam was equipped with a taut-wire deflection measurement system to measure beam deflection at the quarter points and midspan. The ini- tial displacement reading was made just prior to release of prestress, and readings were taken regularly from release to testing of each girder. Measurements of strain changes, as well as the initial cam- ber and changes in camber with time, were compared to the calculated prestress loss and camber changes obtained from several methods. The improved multiplier method (PCI Bridge Design Manual, (1997)) and a general method using the age adjusted effective modulus (AAEM) were used for camber calculations. The AASHTO refined method (Section 5.9.5.4 in AASHTO LRFD) and the AAEM method were used for prestress loss calculations. Three creep and shrinkage models, AASHTO (Section 5.4.2.3 in AASHTO LRFD), ACI 209 (1997), and CEB MC90 (1990), were used with the camber and pre- stress loss calculations. concrete interface shear strength tests was higher than that for normal weight concrete tests, when analyzed for the same resistance factor. 2.3 Laboratory Beam Tests Laboratory test specimens were T-shaped beams and con- tained either three 0.5-in.- or three 0.6-in.-diameter prestress- ing strands. A total of 12 beams were made (10 with lightweight concrete and 2 with normal weight concrete) which resulted in 24 transfer length and development length tests. In addition, the prestress loss and camber of each beam were monitored over time. Direct pullout tests on strand specimens were performed on each strand size to determine strand bond quality. All tested samples for both sizes met the National Associa- tion of Strand Producers (NASP) requirements for strand bond with measured slip values far less than expected at the required load values. Large Block Pullout Tests were also performed on strand samples following the procedures rec- ommended by Logan (1997). All strand samples exceeded the minimum pullout load requirements proposed by Logan. The measured transfer lengths for all lightweight and nor- mal weight beams were less than the current AASHTO LRFD specifications (Section 5.11.4) and the proposed specification changes by Ramirez and Russell (2008). Based on this data and the data from Meyer (2002), both the current AASHTO LRFD specification of 60-strand diameters and the proposed changes to the AASHTO specification by Ramirez and Russell provide conservative predictions of transfer lengths of high- strength lightweight concrete (HSLWC) beams. The measured development lengths for the lightweight concrete beams were slightly longer than measured for the normal weight beams. However, all measured development lengths were substantially less than those calculated using the AASHTO Specifications (Eq. 5.11.4.2-1) as well as those calculated using the proposed Ramirez and Russell equation (Ramirez and Russell (2008)). This data confirms the findings of earlier studies that the AASHTO LRFD equation for development length provides a conservative estimate for HSLWC. 2.4 Shear Tests Six full-scale prestressed girders with cast-in-place com- posite decks were tested to investigate shear strength of lightweight prestressed concrete girders. The AASHTO LRFD Bridge Design Specifications include two reductions to calculated shear strength of lightweight concrete pre- stressed girders. The first is a 0.85 modifier for the ′f c term used in calculating concrete shear strength (given in

9 prestress loss more than the AASHTO creep and shrinkage models. • Initial cambers and elastic shortening can be estimated using the AASHTO equation for modulus of elasticity with a measured compressive strength. • The PCI improved multiplier method, used with the AASHTO creep and shrinkage model, provides reason- able estimates of camber at the time of erection, but not of camber growth after the composite deck is placed. Based on comparisons of calculated values and measured values, the following conclusions can be drawn regarding pre- stress losses and camber growth in lightweight concrete girders: • The AASHTO refined method for prestress loss calculation using the AASHTO creep and shrinkage model overestimates the measured losses. • The ACI 209 and CEB MC90 models for creep and shrink- age used with the AASHTO refined method overestimate