Browsing by Author "French, Catherine"

Now showing 1 - 13 of 13
  • Thumbnail Image
    Item
    Anchorage of Shear Reinforcement in Prestressed Concrete Bridge Girders
    (Center for Transportation Studies, University of Minnesota, 2014-10) Mathys, Brian; French, Catherine; Shield, Carol
    The Minnesota Department of Transportation has typically used epoxy-coated, straight-legged stirrups anchored in the tension zone as transverse reinforcement in prestressed concrete bridge girders. This configuration is readily placed after stressing the prestressing strands. American Concrete Institute (ACI) and American Association of State Highway and Transportation Officials (AASHTO) specifications require stirrups with bent legs that encompass the longitudinal reinforcement to properly anchor the stirrups. Such a configuration is specified to provide mechanical anchorage to the stirrup, ensuring that it will be able to develop its yield strength with a short anchorage length to resist shear within the web of the girder. AASHTO specifications for anchoring transverse reinforcement are the same for reinforced and prestressed concrete; however, in the case of prestressed concrete bridge girders, there are a number of differences that serve to enhance the anchorage of the transverse reinforcement, thereby enabling the straight bar detail. These include the precompression in the bottom flange of the girder in regions of web-shear cracking. In addition, the stirrup legs are usually embedded within a bottom flange that contains longitudinal strands outside the stirrups. The increased concrete cover over the stirrups provided by the bottom flange and the resistance to vertical splitting cracks along the legs of the stirrups provided by the longitudinal prestressing reinforcement outside the stirrups help to enhance the straight-legged anchorage in both regions of web-shear cracking and flexure-shear cracking. A two-phase experimental program was conducted to investigate the anchorage of straight-legged, epoxy-coated stirrups, which included bar pullout tests performed on 13 subassemblage specimens that represented the bottom flanges of prestressed concrete girders, to determine the effectiveness of straight-legged stirrup anchorage in developing yield strains. Additionally, four girder ends were cast with straight-legged stirrup anchorage details and tested in flexure-shear and web- shear. The straight leg stirrup anchorage detail was determined to be acceptable for Minnesota Department of Transportation (MnDOT) M and MN shaped girders as nominal shear capacities were exceeded and yield strains were measured in the stirrups prior to failure during each of the tests.
  • Thumbnail Image
    Item
    A Camber Study of MnDOT Prestressed Concrete I-Girders
    (Minnesota Department of Transportation, 1998-01) Woolf, Douglass; French, Catherine
    This project investigated the relationship between predicted and measured girder cambers. For more than three years, researchers collected camber data on girders of various depths and lengths from the time of strand release until shipment to the job site. Researchers used three camber prediction methods to compare with the measured values: PCI method, Branson's timestep approach, and "CRACK" analysis program by Ghali et. al. The Branson time-step approach resulted in the closest predictions to the measured cambers. The PCI method, although simple, gave reasonable long-term camber results compared with the more detailed methods.
  • Thumbnail Image
    Item
    Corrosion of Coated and Uncoated Reinforcing Steel in Concrete
    (1992-05) Lorentz, Thomas E.; French, Catherine; Leon, Roberto T.
    An experimental program designed to investigate the effects of various material properties on the corrosion of reinforcing steel in concrete was conducted at the University of Minnesota. The test specimens were constructed to promote macrocell corrosion. A total of 96 prism and cracked slab specimens were subjected to an accelerated corrosion process for periods ranging from 35 to 48 weeks. The impact of the following variables on the corrosion of reinforcing steel in concrete was monitored in this program: 1) water/cementitious ratio; 2) addition of condensed silica fume; 3) percentage of entrained air in the concrete; 4) type of reinforcing steel and coating; 5) cracked concrete. The corrosion current, specimen resistance, driving potential, and CuCuS04 half-cell potential were monitored regularly to follow the corrosion process. The most significant variables determined in the University of Minnesota experimental program were the concentration levels (7.5% vs. 10%) of condensed silica fume (CSF), the significance of cracked concrete on the corrosion of reinforcing steel, and the lack of any notable corrosion resulting in concrete specimens containing bars with significantly damaged epoxy-coatings, despite high levels of chloride contamination.
  • Thumbnail Image
    Item
    Crack and Concrete Deck Sealant Performance
    (Minnesota Department of Transportation, 2009-03) Johnson, Karl; Schultz, Arturo E.; French, Catherine; Reneson, Jacob
    The objective of this project is to define the current state-of-the-art regarding the use of bridge deck sealants and crack sealers to extend the life of reinforced concrete bridge decks. The report includes the information generated from a literature review and survey which focused on current and significant studies in the field of deck and crack sealing. The intent of the survey was to determine common practices for the use and application of these sealers in different States throughout the United States. Based on the information collected from the literature review and the survey, the best sealant materials and application practices are recommended for use in Minnesota and throughout the Midwest. The report consists of four sections including: (1) a synthesis of the literature review on the background, application, and performance of concrete deck sealants and crack sealers; (2) a summary of the survey conducted by Mn/DOT to determine the current selection criteria, materials, application practices, and findings from different states in United States; (3) an assessment of selection criteria, materials, application practices, and performance; and (4) conclusions, recommendations and areas in need of further research.
  • Thumbnail Image
    Item
    Debonded Strands in Prestressed Concrete Bridge Girders
    (Minnesota Department of Transportation., 2019-07) Osman, Mahad; French, Catherine
    There are three potential options to reduce end stresses in prestressed concrete bridge girders: drape strands, debond strands, or a combination of the two. In the draping option, a portion of the strands are raised from harp points within the girder to reduce the strand eccentricity at the girder ends. Large vertical reactions are required at the hold down points within the girder to resist the uplift of the draped strands. In addition, end cracking that follows the draped strand pattern is often observed, particularly in deeper sections. In the debonding option, a portion of the strands are debonded toward the girder ends to reduce the resultant prestress force. Concerns with debonding are its potential to reduce shear strength and to cause corrosion issues if moisture and deicing chemicals make their way into the girder ends along the debonded path. Due to potential corrosion concerns, MnDOT has prohibited strand debonding. However, as a means to eliminate some of the end cracking observed during fabrication with draped strands, this study was conducted to explore the use of debonded strands and to develop design recommendations. To this end, an extensive literature review was conducted regarding debonded strand research, and state Departments of Transportation with similar climates and fabricators were queried to learn from their experiences. Design recommendations and potential material specifications to protect debonded strands from corrosion are presented in this report.
  • Thumbnail Image
    Item
    Discrepancies in Shear Strength of Prestressed Beams with Different Specifications
    (Minnesota Department of Transportation Research Services Section, 2010-01) Dereli, Ozer; Shield, Carol; French, Catherine
    Although Mn/DOT inspection reports indicate that prestressed concrete bridge girders in service do not show signs of shear distress, girders rated with the Virtis-BRASS rating tool and Load Factor Rating (LFR) have indicated that a number of the girders have capacities lower than design level capacities. One of the reasons for the discrepancy was suspected to be conservatism of the rating methods (i.e., LFR). Other suspected reasons included potential flaws in the rating tools used by Mn/DOT (i.e., Virtis-BRASS software) including neglecting possible additional shear capacity parameters (e.g., end blocks). As a consequence, the rating methods have made it difficult to discern the cases for which shear capacity may be a real concern. In order to identify the reasons for the discrepancies and inconsistency in rating results relative to observed performance of the prestressed bridge girders, an analytical research program was conducted. The report provides a brief description of the models that provide the basis for the AASHTO shear design provisions and descriptions of the provisions through the 2002 AASHTO Standard specifications. This is followed by a description of the Virtis-BRASS rating tool, which was verified with example bridges provided by Mn/DOT. To investigate prestressed bridge girders within the inventory that might be most at risk for being undercapacity for shear, 54 girders were selected from the inventory for further evaluation. Some of the 54 girders were found to have larger stirrup spacings than required at the time of design. These girders were subsequently rated and evaluated per the 2002 AASHTO Standard Specifications to determine the adequacy of the designs based on the LFR inventory and operating rating methods. Potential sources for increased shear capacity were identified and reviewed.
  • Thumbnail Image
    Item
    Full-Depth Precast Concrete Bridge Deck System: Phase II
    (Minnesota Department of Transportation, 2012-10) Halverson, Max; French, Catherine; Shield, Carol
    The Minnesota Department of Transportation (MnDOT) has developed a design for a precast composite slab-span system (PCSSS) to be used in accelerated bridge construction. The system consists of shallow inverted-tee precast beams placed between supports with cast-in-place (CIP) concrete placed on top, forming a composite slab-span system. Suitable for spans between 20 and 60 ft., the MnDOT PCSSS is useful for replacing a large number of aging conventional slab-span bridges throughout the United States highway system. The PCSSS has particular durability, constructability, and economical concerns that affect its value as a viable bridge design. To address these concerns, the performance of existing PCSSS bridges was evaluated and a review of a number of PCSSS design details was conducted. The field inspections demonstrated that design changes made to the PCSSS over its development have improved performance. A parametric design study was also conducted to investigate the effects of continuity design on the economy of the PCSSS. It was recommended that the PCSSS be designed as simply supported rather than as a continuous system.
  • Thumbnail Image
    Item
    Monitoring and Analysis of Mn/DOT Precast Composite Slab Span System (PCSSS)
    (Minnesota Department of Transportation, 2008-09) Smith, Matthew; Eriksson, Whitney; Shield, Carol; French, Catherine
    The Mn/DOT Precast Composite Slab Span System (PCSSS) was initially designed by Mn/DOT with input from the University of Minnesota researchers and local fabricators. The bridge system consisted of a series of precast prestressed concrete inverted tee bridge elements that also served as stay-in-place formwork for the cast-in-place (CIP) portion of the deck placed in the field. One of the Mn/DOT implementations, located in Center City, MN, was instrumented, and subsequently, monitored for 24 months to investigate reflective cracking and continuity over the piers since the deck was cast. The data obtained from the field study indicated that cracking had initiated in the bridge at the locations of some of the transverse gages in the CIP just above the longitudinal flange joint at midspan and some of the longitudinal gages near the support. The cracking was determined to be the result restrained shrinkage and environmental effects rather than due to vehicular loads. Transverse load distribution was evaluated with a static truck test. In addition, a two-span laboratory specimen was constructed and load tested to investigate effects of variations in flange thickness, bursting reinforcement, horizontal shear reinforcement, and flange surface treatment. The positive restraint moment that developed at the pier was also monitored for the first 250 days after continuity was made. The data from the laboratory and field tests were analyzed with respect to reflective cracking, transverse load distribution, pier continuity, bursting, and restraint moment, and design recommendations were made.
  • Thumbnail Image
    Item
    Shear Capacity of Prestressed Concrete Beams
    (Minnesota Department of Transportation, 2007-11) Runzell, Brian; Shield, Carol; French, Catherine
    The shear provisions of the American Association of State Highway and Transportation Officials bridge design code have changed significantly in recent years. The 2004 Load and Resistance Factor Design (LRFD) and 2002 Standard shear provisions for the design of prestressed concrete bridge girders typically require more shear reinforcement than the 1979 Interim shear provisions. The purpose of this research was to determine whether or not bridge girders designed according to the 1979 interim shear provisions were underdesigned for shear and develop a method to identify potentially underdesigned girders. Two shear capacity tests were performed on opposite ends of a bridge girder removed from Mn/DOT Bridge No. 73023. The stirrup spacing in the girder suggested it was designed according to the 1979 Interim shear provisions. The results from the shear tests indicated the girder was capable of holding the required shear demand because the applied shear at failure for both tests was larger than the factored shear strength required by the 2004 LRFD HL-93 and 2002 Standard HS20-44 loading. The results of a parametric study showed that girders designed using the 1979 Interim were most likely to be underdesigned for shear near the support and that the girders most likely to be underdesigned in this region had smaller length to girder spacing ratios.
  • Thumbnail Image
    Item
    Strength and Stability of Prestressed Concrete Through-Girder Pedestrian Bridges Subjected to Vehicular Impact
    (Minnesota Department of Transportation, Research Services Section, 2007-01) Baran, Eray; Schultz, Arturo; French, Catherine
    Two issues regarding the prestressed concrete through-girder pedestrian bridge system are investigated. The first issue concerns the ductility of prestressed concrete girders in these bridges because the section that is typically used may be considered to be over-reinforced according to AASHTO LRFD Bridge Specifications. Response of the section, including neutral axis location, strand stress at ultimate capacity, and moment capacity, predicted by AASHTO Standard and AASHTO LRFD Specifications are compared with the sectional response determined from nonlinear strain compatibility analyses. Modifications are proposed to the AASHTO LRFD procedure to rectify the errors in predicting sectional response. The second issue that was investigated concerns the strength and stability of prestressed concrete through-girder pedestrian bridges when subjected to impact by over-height vehicles. Three-dimensional finite element models of entire bridges and subassemblages were used to evaluate the strength, stiffness, and ductility characteristics of the bridge system and connection details. Accurate representation of the bridge details in the finite element models were assured by utilizing experimentally determined load-deformation characteristics for the connections. Results showed that significant improvements in the lateral load-deflection behavior of the bridge system could be obtained by implementing alternate connection schemes, and that concrete side-walls should be provided at girder ends.
  • Thumbnail Image
    Item
    Thermal and Mechanical Fatigue Effects on GFRP Rebar-concrete Bond
    (1997-12) Retika, Annie Christine; Shield, Carol; French, Catherine
    This report summarizes an experimental program conducted to investigate the thermal and mechanical fatigue effects on the bond between Glass Fiber-Reinforced Plastic (GFRP) rebars and concrete. Variables included in ihe study were rebar diameter (No. 6 and No. 4 GFRP, No. 6 steel), material (steel and GFRP) and manufacturer (two different manufactures for the GFRP rebar). For comparison, specimens were also fabricated using steel rebar as a control. The embedment lengths were chosen so that a splitting failure was assured in all specimens. A total of 30 inverted half-beam specimens were cast in 15 beams. Six specimens were mechanically tensiontension cycled, twelve specimens were thermally cycled and twelve were used as control specimens (no thermal or mechanical fatigue). During thermal fatigue, the rebars were also subjected to a constant tensile load to simulate dead load. During the bond tests, specimens were loaded continuously until failure while monitoring load, free-end slip, and loaded end slip. The results were evaluated to compare the bond performance of GFRP rebar to steel rebar, and to determine the effects of thermal and mechanical fatigue on bond. It was found that mechanical fatigue has more detrimental effect on steel that on GFRP specimens while thermal fatigue has more effect on GFRP than on steel specimens. The effect of thermal fatigue was more in GFRP M2 than in Ml specimens. The difference in bond performance between No. 6 and No. 4 GFRP Ml cannot be determined form this study due to inferior product quality of the No. 4 M bars.
  • Thumbnail Image
    Item
    Understanding and Mitigating the Dynamic Behavior of RICWS and DMS Under Wind Loading
    (Minnesota Department of Transportation, 2020-06) Linderman, Lauren; Guala, Michele; French, Catherine; Schillinger, Dominik; Finley, Nicole; Heisel, Michael; Nguyen, Lam; Stoter, Stein; Vievering, Josh; Zhu, Qiming
    Dynamic Messaging Signs (DMS) and Rural Intersection Conflict Warning Signs (RICWS) are roadside signs that feature much larger and heavier signs than are typically placed on their respective support systems. The excess weight and size of these signs, in conjunction with their breakaway support systems, introduces vibration problems not seen in the past. The AASHTO 2015 LRFD Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (SLTS) does not yet address vibration design for these nontraditional roadside signs. DMS and RICWS were instrumented in the field and numerically modeled to explore their wind-induced behavior. A dynamic numerical model was validated with experimental field data and used to evaluate the fatigue life of the DMS support system instrumented in the field. The resulting fatigue life differed significantly from the equivalent static pressure analysis prescribed in the AASHTO specification. The fatigue life of the DMS instrumented in the field was conservatively estimated to be 23.8 years. Based on data collected from a RICWS instrumented in the field and experiments done on a scaled model of the RICWS at the St. Anthony Falls Laboratory, vortex shedding was identified as the predominant wind phenomena acting on the RICWS structure. Three modifications were proposed to reduce the impacts of vortex shedding. The investigation of these newer sign types highlights the importance of considering the impact of dynamic behavior and vortex shedding on the structural design.
  • Thumbnail Image
    Item
    Validation of Prestressed Concrete I-Beam Deflection and Camber Estimates
    (Minnesota Department of Transportation, 2012-06) O'Neill, Cullen; French, Catherine
    The camber at the time of bridge erection of prestressed concrete bridge girders predicted by the Minnesota Department of Transportation (MnDOT) was observed to often overestimate the measured cambers of girders erected at bridge sites in Minnesota, which, in some cases, was causing significant problems related to the formation of the bridge deck profile, the composite behavior of the girders and bridge deck, delays in construction and increased costs. Extensive historical data was collected from two precasting plants and MN counties and it was found that, on average, the measured cambers at release and erection were only 74% and 83.5%, respectively, of the design values. Through data collection, analysis, and material testing, it was found that the primary causes of the low camber at release were concrete release strengths that exceeded the design values, the use of an equation for concrete elastic modulus that greatly under-predicted the measured values, and thermal prestress losses not accounted for in design. Fourteen girders were instrumented and their camber measured and the program PBEAM was used to evaluate the influence of various time-dependent effects (i.e., solar radiation, relative humidity, concrete creep and shrinkage, length of cure and bunking/storage conditions) on long-term camber. Once investigated, these effects were included in long-term camber predictions that were used to create sets of both time-dependent and singlevalue camber multipliers. The use of these multipliers, along with modifications made to the elastic release camber calculations, greatly reduced the observed discrepancy between measured and design release and erection cambers.