Adaptive Prestressing System for Concrete Structures using Shape Memory Alloys
Advisor: Professor Bassem Andrawes
Abstract
This research aimed to study a novel method for prestressing concrete structures that can avoid the shortcomings of conventional prestressing methods. The prestressing of concrete has been widely used since the 1950s to counteract the tensile stresses induced in concrete due to external loads. The prestressing force is applied through the stressing of high-strength steel (HSS) reinforcement embedded in the concrete and tensioned at either one end or both ends using a mechanical hydraulic jacking system. The currently used prestressing system makes it inevitable that the prestressing force is applied throughout the whole domain, even where prestressing is unnecessary, such as at the end regions and compressive zones. This study addresses this issue by adopting a new system that can apply prestressing force at small and targeted regions. Another issue the new system will help to address is the susceptibility of existing prestressing systems to excessive prestress losses and premature failures resulting from the early prestress force release. This issue is particularly a major concern in pretensioned members that are typically mass-produced in precast plants equipped with a casting bay. To increase the production of precast pretensioned members, the manufacturing process is shortened by steam-curing the concrete to accelerate early-age concrete maturity. The early application of prestressing force can cause significant prestress losses due to creep and shrinkage of the concrete and end-splitting cracks due to the immaturity of the concrete.
An adaptive concrete prestressing system using shape memory alloys (SMAs) was studied to address the issues mentioned above of the conventional prestressing system. SMAs are a new class of material with a unique characteristic that can apply a prestressing force only at a target region that was not achievable with conventional steel. The objectives of this dissertation are: (1) Prove the concept of applying localized prestressing using SMAs experimentally and numerically on small-scale concrete rail crossties. (2) Develop an adaptive prestressing system (APS) as a hybrid system that merges SMAs and HSS reinforcement. The APS is sought as a cost-effective solution where only a small fraction of SMA material is used as a prestressing fuse. (3) Full-scale prototyping of the concrete rail crossties using APS. (4) Characterize the thermomechanical behavior of cost-effective Fe-based SMAs and hot-rolled unannealed NiTiNb bars to study their potential large-scale application in the APS. (5) Explore the feasibility of producing more efficient designs of concrete members prestressed with the APS using topology optimization. A topology optimization framework based on a hybrid mesh approach was developed. Using the developed framework, a topology of the continuous precast prestressed concrete bridge girder was optimized using different levels of material reductions. (6) The optimized design was analyzed to evaluate its feasibility in a real-world application. The behavior of the optimized girder was analyzed to confirm whether the design conformed to the current design guidelines. Two other design cases using conventional prestressing methods were designed and their behavior and material usage were compared with the optimized design's.
