Pretensioned Bent Caps to Improve Bridge Substructure Design and Performance
Presented By: Anna Birely
Affiliation: Texas A&M University
Description: Precast bent caps can play a critical role in accelerated bridge construction by minimizing construction activities over roads or sensitive environmental habitats, providing increased worker safety, and accelerating commencement of superstructure construction. With appropriate connections and details, prestressed, pretensioned caps can offer improved performance over reinforced concrete caps, at the same time providing increased quality control during fabrication. Here, recommendations for the analysis, design, and construction of precast, pretensioned bent caps are drawn from findings of an experimental test program that included full-scale precast, prestressed bent caps. Flexural design recommendations focus on the introduction of dead load stress limits to ensure that cracks close upon full removal of live load. A case study illustrates how pretensioned bent caps can be utilized to optimize the design and performance of substructures, including a reduction in the number of columns to reduce the overall foundation cost of a project.
On the Application of Basalt-Fiber Reinforced Polymer (BFRP) Bars to the Pre-stressed Concrete Industry
Presented By: Liberato Ferrara
Affiliation: Politecnico di Milano
Description: Basalt-Fiber Reinforced Polymer (BFRP) bars have been recently proposed to be used to pre-stress precast concrete elements. Their mechanical properties, combined with the potential low production cost and the low carbon footprint, make this application promising, given the problems related to anchorage and relaxation persist. Applications at present are limited to very few laboratory tests. The paper discussed how the Serviceability Limit State (SLS) and Ultimate Limit State (ULS) verifications of pre-stressed elements employing this technology vary with respect to elements pre-stressed with steel tendons. Furthermore, an attempt is made to investigate the potential application into the precast concrete industry, by analyzing several elements (roof slabs, floor slabs, and beams) typical of the precast concrete industry and having different shape. This investigation highlights which type of element could be more advantageously switched to the use of pre-stressed BFRP bars, and at which cost in terms of structural performance.
Mildglass: GFRP Strand Prototype Development
Presented By: Marco Rossini
Affiliation: University of Miami
Description: Corrosion of steel reinforcement is the primary cause of durability problems in aged prestressed concrete (PC) structures. In the case of the transportation infrastructure in coastal areas, corrosion problems are mainly experienced by bridge substructures, sheet pile bulkheads and seawalls. In the State of Florida alone, about 3,600 coastal miles are armored with aging sheet piles with an estimated $21B Maintenance, Repair and Replacement (MRR) liability. The construction industry has only partially answered the rising demand for corrosion-resistant prestressing technologies, offering expensive, complex and sometimes ineffective solutions. This study focuses on developing a Glass Fiber Reinforced Polymer (GFRP) strand prototype to be applied in Mild-Prestressed Concrete (MPC) elements. Glass FRP bars are effective corrosion-resistant reinforcing solutions, but a glass FRP prestressing strand is not available in the marketplace. Glass fibers are an efficient alternative to carbon fibers in applications that do not require high levels of concrete prestressing such as sheet piles for seawalls, bearing piles, and retaining walls. Limiting the initial level of prestress in the strands addresses the main constructability issues observed with carbon FRP. It guarantees compatibility with standard steel chucks and conventional tensioning techniques. At the same time, the reduced cost of glass fiber makes the GFRP strand a competitive and durable alternative to standard low-relaxation high-strength carbon steel. The technology implements E-CR glass fibers and thermoplastic resin to ease manufacturing of a twisted 7-wire geometry. In this presentation, the challenges associated to the development of the GFRP strand prototype will be discussed; experimental validation of the final product will be presented; and, technology readiness and economic implication will be analyzed. Figure 1 shows a single GFRP wire and the GFRP strand in its final configuration.
AASHTO-LRFD Design Specifications for Beams Prestressed with CFRP Systems
Presented By: Abdeldjelil Belarbi
Affiliation: University of Houston
Description: Carbon Fiber Reinforced Polymers (CFRP) are becoming a recognized alternative to traditional construction materials with a wide range of applications in current civil engineering practice. An example of such applications is the use of CFRP cables or bars as prestressing reinforcement for prestressed concrete bridge girders, especially in aggressive environments where steel prestressing strands are susceptible to corrosion. Despite its promise, prestressing CFRP has not been used frequently for bridge construction worldwide. Apart from a limited number of guides, manuals, and commentaries, there is currently no standard or comprehensive design guideline available to bridge engineers in the U.S. for the design of concrete structures prestressed with CFRP systems. A comprehensive research study was conducted as part of NCHRP 12-97 project to develop design guidelines for the use of CFRP materials in prestressed concrete bridge girders. The proposed guidelines and research findings are expected to further advance and facilitate the use of CFRP systems in bridge applications. In order to implement the research findings, “Guide Specifications for the Design of Concrete Bridge Beams Prestressed with CFRP Systems” was formulated along with the companion material specifications. The specifications are supported by a commentary explaining the background, rationale, and limitations of the provisions mentioned therein. This presentation will provide the research details and methodology used to develop the AASHTO-LRFD design specifications using CFRP.
Nonconventional Materials (CNTs) Application in Prestressed concrete to Improve Concrete Tensile Strength
Presented By: Robabeh Jazaei
Affiliation: University of Wisconsinplatteville
Description: Concrete is cement-based material which is recognized by its high compression strength. However, concrete must be reinforced by steel rebars or various class of synthetic and natural fibers to compensate its low tensile strength and ductility. In prestressed concrete, rebar contribution is to carry tensile loads with neglecting concrete tensile strength. However, concrete shrinkage and crack propagation along rebar is an important issue in prestressed concretes. Using unconventional materials reinforcements such as carbon nanotubes (CNTs) is a novel alternative to produce homogeneous cement paste. Cement-based nanocomposites incorporating carbon nanotubes with advanced fabricating technology offers higher concrete tensile strength. The main objective of this experimental research was to synthesize surface treated multi-walled carbon nanotubes (MWCNTs) in water solution and cement matrix to assess tensile strength. In this research, batches with 0.2-0.6wt% of MWCNTs, and control batch without MWCNTs were made and examined. Since strong Van der Waals forces cause MWCNTs agglomeration, ultrasonic sonicator is used to disperse MWCNTs in water. Water-cement ratio of 0.5 were used due to water reduction through sonication process. Field Emission Electron Microscope (FESEM) was used to evaluate the morphology of MWCNTs bridge with cement matrix. The results of tensile splitting test indicated that the tensile strength of cement-based nanocomposite reinforced by 0.4wt% of MWCNTs were notably enhanced. Additionally, MWCNTs reinforcement affected the failure mechanism of cementitious nanocomposite under tensile loading with providing higher ductility.
Use of Flexible Filler in U.S. Post-tensioned Bridges: An Overview of Testing To-date and Lessons Learned
Presented By: Natassia Brenkus
Affiliation: The Ohio State University
Description: The use of flexible fillers – greases and waxes – as post-tensioning (PT) tendon filler materials is a new practice to U.S. bridge construction. The use of these materials, in lieu of the more-common cementitious grouts, has implications to construction practices, structural behavior and durability. The Florida Department of Transportation began moving towards the adoption of these materials in select applications in 2014, completing a large research investigation in 2017; follow-up research work is currently underway. The implementation of flexible fillers has progressed into industry, with injections of an approach slab and a bent cap; a training seminar has been developed to educate practitioners and injection personnel. This paper is a summary of efforts to-date. It briefly summarizes the first research effort encompassing proof-of-concept and structural testing conducted to-date, including PT injection mock-ups, flexural static and cyclic testing of members with both internal and external tendons, and fatigue testing of PT tendons in diabolo-type deviators. Recent examples of injections in full-size members will be provided. Lessons learned and changes made to standard grouting practice are highlighted and future work required for the successful implementation of flexible fillers is discussed.