Degree Type

Thesis

Date of Award

2020

Degree Name

Doctor of Philosophy

Department

Civil, Construction, and Environmental Engineering

Major

Civil Engineering (Structural Engineering)

First Advisor

Sivalingam (Sri) Sritharan

Abstract

Ultra High-Performance Concrete (UHPC), owing to its superior mechanical and durability properties, presents a unique opportunity for innovative use in unbonded post-tensioned floor systems. Two unique focus areas, anchorage in unbonded post-tensioned (PT) slabs and composite UHPC-steel truss system for long-span floors, are identified and researched as part of this dissertation. In unbonded PT slabs and beams, the use of cast-in-place steel confined ‘UHPC Bond Anchors' (UBA) has been suggested to anchor steel prestressing strands for better durability, increased strand ductility, cost-effectiveness, and ease of installation. Also, a ‘UHPC-Steel Composite Truss Assembly' (UCTA) has been created to reduce the building weight and material consumption by over 25 percent compared to traditional long-span PT beam-slab systems. This floor system optimizes the UHPC use such that the weight reduction results in smaller column and foundation sizes and reduction in the lateral force-resisting system, thus maintaining or reducing the direct short-term building construction costs while improving the long-term durability performance.

Conventional mechanical steel bearing anchors require expensive encapsulation and installation methods to inhibit corrosion-related strand failure. Cost-cutting in new regions adopting unbonded PT technology has resulted in the use of low-quality encapsulation and installation methods. The use of mechanical anchors also results in premature strand fracture at low-strains between 1% and 2%. A phased experimental testing program resulted in a steel confined UBA that adequately addresses all the above issues. A 150 mm long and 3 mm thick steel conical confining device was developed as one of the main components of the UBA. The device resists hoop tension and eliminates splitting cracks in the UHPC during prestress transfer. It also helps to reduce the anchorage length. A non-linear iterative analytical method using the classical thick cylinder theory was developed to assess the stresses in the confining device and the UHPC due to prestress transfer. It also assesses the crack-widths and their extents in the UHPC along the anchorage zone. The prestress transfer into the UBA was simulated using the above-developed method as well as using the finite-element (FE) method. High average bond stress helps reduce the UBA length, and consequently, the material consumption. The bond stress at the strand-UHPC interface was increased by intentionally roughening or indenting the strand. A procedure for strand indentation was developed as part of the process. The tests also revealed that the steel fiber contribution to the bond was negligible, thereby helping to reduce the fiber quantity in the UBA.

With superior mechanical and durability properties, UHPC is an ideal choice for shallow depth, long span beams in building floors. However, such an application has not been realized due to significantly high material cost making UHPC volume reduction critical for a cost-effective and optimal design solution for long-span beams. The program initially explored the possibility of developing precast, prestressed UHPC Double Tee beams—a concept that has been successfully used with concrete precast, prestressed concrete. Design runs revealed that UHPC Double Tees were at best only 20 percent lighter than their concrete counter-part, making them economically unviable. The double-tees were then further explored by making them open-webbed. Additionally, monolithic open-web and truss type UHPC beams with wider top flange representing the floor were also explored. The findings suggested that the open-web UHPC beams reduced material consumption by over 40 percent, but their webs required significant passive reinforcement to prevent them from experiencing premature tension failure. Addressing this concern cost-effectively, the UCTA was developed. The UCTA is a UHPC-steel composite open-webbed truss floor system made of pre and post-tensioned UHPC elements connected with structural steel web members. UCTA's can be either topped with a precast pretensioned normal concrete deck or a cast-in-place post-tensioned concrete slab compositely to create a cost-effective floor system. A complete step-by-step design methodology with an application to a real-life structure is presented.

DOI

https://doi.org/10.31274/etd-20200624-229

Copyright Owner

Satish Hansmukhlal Jain

Language

en

File Format

application/pdf

File Size

275 pages

Available for download on Thursday, June 16, 2022

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