ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/768999
SAFETY & SERVICEABILITY Enhancing Durability with Precast Concrete Structures by Greg Banks, BergerABAM Inc. “The structure is our friend, and within limits it is tolerant of our shortcomings. Loads, such as dead, live, wind, earthquake, temperature, and creep, are our enemies. It behooves us to learn the enemy’s plan of attack and develop defensive tactics, not always by frontal resistance but sometimes by flanking movement.” Halvard W. Birkeland Wisdom of the Structure Our aging highway infrastructure has allowed us to witness concrete damage due to corrosion of reinforcement steel, which has led to significant costs in maintenance and repair. As Halvard Birkeland had stated, it is now our duty as engineers to develop strategies and mitigate costs associated with preservation of existing infrastructure. Rising to the challenge, significant efforts have been deployed to develop more durable design strategies. One such effort, developed through the Strategic Highway Research Program 2 (SHRP2), is a service life design approach. The service life design approach provides a systematic way to assess the service performance of a given structure with the goal to find new and better ways to design structures that will last longer and require less maintenance. The approach considers a wide range of factors such as project location, environmental conditions, materials, and anticipated load demands with the primary objective being to avoid development of degradation mechanisms. For concrete structures, degradation first occurs when substances from the surrounding environment penetrate into the concrete via cracks and either accumulate over time within the outer concrete layers or penetrate inward toward the reinforcement. Carbonation, chloride penetration, and s u l fate accumulation accelerated by cyclic wetting and drying are examples of such means of penetration. After time, the protective layers break down and/or critical levels of detrimental substances are reached and corrosion commences. The focus of this article is not to go through the service design approach (readers are directed to the references for details), however, to introduce an innovative partially prestressed concrete concept that could be used to enhance the durability of our transportation infrastructure. The concept being introduced stems from research conducted by the U.S. Navy, which invested in a multi-year project with the goal to develop a fully prestressed concrete pier system that minimizes concrete cracking to produce more durable pier structures. U.S. Navy Partially Prestressed Concrete Pier Concept Historically, pier structures in the Pacific Northwest have consisted of reinforced concrete flat slab decks supported by precast, prestressed concrete piles. Due to the significant investment being devoted to maintenance and repair of these structures, the U.S. Navy invested in a multi-year research program with the goal of developing a prototype for fully prestressed concrete pier and sea wall designs with similar features to their existing reinforced concrete pier designs, that would minimize cracking to produce a more durable coastal structure. Due to constructability issues related to the use of post-tensioning in some areas, the age of precast concrete element integration, curing conditions, or other concerns, a fully prestressed (pretensioned with posttensioned) system using gravity-induced prestressed concrete was developed (Figure 1). This system was developed with the following objectives: • Reducing the routine dependency on post-tensioning • Using precast precompressed concrete elements for all structural members exposed to saltwater, thus reducing cracking on the exposed faces • Improving performance of precast, prestressed concrete elements while simplifying fabrication and installation • Increasing the modularity of elements to provide the designer with flexibility to configure the pier to meet structural and functional requirements • Reducing cost to make the new pier concept comparable to current Navy pier designs