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/575053
PERSPECTIVE Are We Really Designingfor a 100-Year Service Life? by Mike Bartholomew, CH2M Recently, requests for proposals for several large bridge projects have had requirements for the structure to have a service life of 100 years. With no U.S. standards or guidelines in place to establish performance criteria for service life design (SLD), it is often unclear what is being requested and what is being achieved. SLD for concrete structures has developed over the past 30 years in Europe, and has been documented in several international standards and specifications: * Model Code for Service Life Design by the International Federation of Structural Concrete (fib) (Bulletin 34) * Model Code for Concrete Structures 2010 by fib * Durability-Service Life Design for Concrete Structures (ISO 16204) by the International Standards Organziation (ISO) In the United States, SHRP2 research project R19A, "Service Life Design for 100 Years and Beyond," was completed in 2013. Currently, the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA) are sponsoring the SHRP2 R19A Implementation Action Program to promote the use of SLD. Four state departments of transportation (Iowa, Oregon, Pennsylvania, and Virginia), along with FHWA Central Federal Lands Division, are developing bridge projects using SLD principles. As these projects are completed, training materials will be developed and published to show the need for using SLD. SLD is based on the way materials deteriorate in their environmental exposure conditions. Concrete structure deterioration is generally due to corrosion of the reinforcing steel. Chlorides from seawater or deicing chemicals represent the most severe environmental exposure zones. Other deterioration mechanisms include reinforcement corrosion after concrete carbonation, concrete damage due to alkali-silica reaction, and abrasion on bridge decks. Chloride ingress deterioration has been well defined. Chlorides applied at the surface of the concrete diffuse with time through the outer concrete cover layer. When chloride ions reach a critical concentration threshold at the level of the reinforcement, corrosion initiates. Corrosion by-products are expansive, and coupled with concrete's low tensile capacity, this expansion creates surface cracking. As the corrosion continues, it leads to spalling and loss of steel cross section. The deterioration mechanism consists of two phases * An initiation phase, where there is no visible physical change to the structure * A propagation phase, where the structure experiences damage from corrosion A mathematical model based on Fick's 2nd Law for diffusion, outlined in fib Bulletin 34, defines the initiation phase. Currently, there are no generally accepted models for the corrosion propagation phase. Fick's 2nd Law can be evaluated as a load and resistance factor equation similar to traditional structural design.