Evaluating Prestressing Strands and Post-Tensioning Cables in Concrete Structures Using Nondestructive Evaluation (NDE) Including Joint Shear Wave Analysis

Prestressed beams are commonly used in highway bridge construction and segmental posttensioned bridges are becoming more widely used for medium and long span bridge construction. These types of structures rely heavily on steel prestressing strands and posttensioning cables for strength and durability. Additionally, it is very important during the construction of these bridges to implement an effective quality control plan to ensure proper placement of the strands and to ensure that the ducts are fully grouted in accordance with the design. Methods to determine the overall conditions of these strands are critical to verify the overall integrity and safety of these structures. Because the strands are embedded in concrete, and often the area is complex and congested, nondestructive evaluation (NDE) methods are needed to assess the condition as well as the placement of the strands. In particular, exposed structures like highway bridges are more vulnerable to environmental deterioration. The deterioration of prestressed and post-tensioned concrete members is highly related to the concrete durability and corrosion of the steel reinforcement. In the case of prestressed structures the corrosion products may cause localized concrete damage leading to delamination and spalling. In the case of post-tensioned structures the corrosion is typically associated with regions of voids in the otherwise grouted ducts . Yet, common for both types of structures is that the onset of damage occurs without visual clues such as cracks and stains on the concrete surface. However, a significant indicator of the potential for 'hidden' deterioration is the formation voids or presence of voids (gaps) ranging in size from the micro to the meso level. These voids may form either as a result of the corrosion product, delamination and cracks in the prestressed concrete beam, or voids due to lack of proper grouting of the post-tensioning ducts. A second indicator is the change in magnetic resistance of the strands due to the loss of cross section area. The research team proposes a comprehensive two year study to investigate and recommend NDE methods for assessing the conditions of prestressed and post-tensioned concrete members. The proposed study will include: a literature review, a national survey, an analytical and experimental work on laboratory concrete members, and a detailed field exploratory work. The outcome of this investigation will provide the Michigan Department of Transportation (MDOT) with the most suitable NDE methods to implement as a mandatory inspection technique in the bridge inspection protocol for prestressed and post-tensioned concrete bridge systems. Detailed field exploration will be conducted on selected prestressed and post-tensioned bridges either using members from decommissioned bridges or onsite evaluations. MDOT engineers will assist in defining the selection criteria and the selection of the candidate bridges. The project deliverables are a comprehensive report on the research activities and recommendations for rapid and flexible NDE methods for deployment. The research team will also host a demonstration session on the use of the selected NDE methods.