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/306900
ASPIRE , Summer 2007 | 15 P E R S P E C T I V E Concrete is a durable material for bridges that can last a long time with minimal maintenance if designed and used correctly for a particular application. Concerns for safety, limited resources, and the desire to reduce lane closures or delays have prompted attention to the durability of concrete bridges worldwide. W ith new concrete technologies, structures can be built to last a long time. Whereas, bridges were designed previously for a service life of 50 years, this has now been extended to 75 years and more. In other countries, 100 years of service life are planned and structures are designed and built accordingly. The durability of concrete largely depends on its ability to resist the infiltration of aggressive solutions. Concretes that are protected from the environment and that stay dry can last centuries. There are many good examples of longevity in Roman structures, such as the Pantheon, which was built around 126 A.D. and still remains intact. Today, structural applications use a combination of concrete and reinforcement for efficient load-carrying capacity, and this introduces new challenges in terms of durability. In this composite system of modern times, corrosion of the reinforcement in concrete exposed to water and aggressive solutions has become the most widely experienced distress in concrete structures. It is not uncommon for poorly executed concrete exposed outdoors to experience reinforcement corrosion, cracking, and spalling within a couple of decades. The same concrete situated indoors and away from the aggressive solutions could have lasted for centuries. Chlorides that reach the steel reinforcement accelerate the corrosion process necessitating costly repairs. Low permeability concretes can resist the penetration of chlorides and thus minimize the damage due to corrosion. In addition, low permeability concrete minimizes other environmental distresses due to cycles of freezing and thawing, alkali-silica reactivity (ASR), and sulfate attack because, in each case, water or aggressive solution entering the concrete is part of the problem. For damage due to cycles of freezing and thawing, another important factor is the proper entrainment of air voids in the concrete. However, low permeability and a proper air-void system do not always ensure durability if the concrete contains excessive cracks that facilitate the intrusion of aggressive solutions. This cracking can be due to many factors related to both environmental effects and structural loads (TR Circular E-C107, 2006). Building durable bridge structures requires innovation and the use of available resources in an efficient manner. An ideal durable structure needs to have a low permeability concrete with a proper air-void system, no cracks, and not be subject to deleterious chemical reactions. These characteristics are discussed below in relation to design practices, material selection, construction practices, and specifications. Design Practices Certain design parameters can assist in achieving durable structures. Good drainage details can minimize ponding and prolonged exposure of bridge components to solutions. Leaking deck joints enable chloride solutions to penetrate onto the substructure elements such as the beam ends, pier caps, and columns causing corrosion. Jointless bridges can minimize such occurrences. Bridge decks supported by more rigid concrete beams exhibit less cracking compared to decks supported by flexible steel beams. Thicker concrete cover provides more resistance to the penetration of solutions to the level of reinforcement. Avoidance of skews on structures can aid in durability as this design feature introduces torsional stresses that lead to diagonal cracking at the corners near the abutments. The selection of reinforcement is important as well; bars made of tough and intrinsically corrosion-resistant materials can minimize the corrosion potential (Clemena and Virmani, 2004). Bars clad with stainless steel and solid by Dr. H. Celik Ozyildirim Durable Concrete for Bridges Proper consolidation by internal and external vibrators is essential for strength and permeability. ASPIRE_Summer_2007.indb 15 5/15/07 11:40:21 AM