Question:
What is service life and how is it predicted?
Answer:
The AASHTO LRFD Bridge Design Specifications defines service life as the period of time that the bridge is expected to be in operation. The design life is defined as the period of time on which the statistical derivation of transient loads is based. Though the subject specifications prescribe transient loads based on a design life of 75 years, they are silent on the extent of the expected service life.
A bridge’s ability to fulfill its intended function can be compromised due to degradation. Major causes of degradation are high transient loads and severe environmental conditions. Proper structural design addresses the effects of transient loads through adequate member proportioning and design details.
Environmental conditions that cause degradation include carbonation, sulfate attack, alkali-silica reaction, freeze-thaw cycles, and ingress of chlorides and other harmful chemicals. Adverse environmental conditions, if not properly addressed, typically cause chemicals to invade the concrete’s pore structure and initiate physical and/or chemical reactions causing expansive by-products. The most damaging consequence of these reactions is depassivation and eventual corrosion of reinforcing steel causing cracking and spalling of concrete. The end of the service life of the structure occurs when the accumulated damage in the bridge materials exceeds the tolerance limit. However, the service life is typically extended by performing periodic repairs to restore the serviceability of the structure.
Chlorides from deicing salts and salt water penetrate concrete by several transport mechanisms: ionic diffusion, capillary sorption, permeation, dispersion, and wick action. During the last several years, computer models have been developed to predict the service life of concrete bridges exposed to chlorides. Several service life prediction models assume diffusion to be the most dominant mode of transport for chloride ions. The time taken by chlorides to reach reinforcing steel and accumulate to a level exceeding the corrosion threshold is known as Time to Initiation of Corrosion (TIC). Typically, TIC is computed by modeling chloride ingress according to Fick’s Second Law of Diffusion. TIC depends on many factors; major among them are diffusivity of concrete, concrete cover, temperature, and the degree of exposure. The Propagation Time—from initiation of corrosion to intolerable accumulation of damage—also depends on many factors including environmental conditions and corrosion protection strategies.
The following is a list of some of the service life prediction models now available:
Life-365: Computer software developed by M. D. A. Thomas and E. C. Bentz, University of Toronto for W. R. Grace, Master Builders Technologies, and Silica Fume Association. Addresses time-dependent diffusion of chlorides. Predicts service life and life-cycle costs for various protection strategies.
CIKS: Computer-Integrated Knowledge System developed by D. Bentz, NIST. Predicts chloride ion diffusivity coefficients and TIC.
Duramodel: Developed by W. R. Grace. With the help of effective diffusion coefficients, the model accounts for mechanisms other than pure diffusion.
ConFlux – A Multimechanistic Chloride Transport Model: Developed by A. Boddy, E. C. Bentz, M. D. A. Thomas, and R. D. Hooton, University of Toronto. PCbased program accounts for diffusion, permeability, chloride binding, and wicking.
ClinConc: Developed by L. Tang, Chalmers University of Technology, Goteborg, Sweden. Chloride penetration model is based on mass balance and genuine flux equations. Promising for predicting chloride profiles in submerged parts of structures.
HETEK Model: AEC Laboratory, Denmark. Applicable to marine structures and salt water splash zones. Ten-step spreadsheet calculation for service life.
Further reading: Frohnsdorf, G., “Modeling Service Life and Life-Cycle Cost of Steel-Reinforced Concrete,” NIST/ACI/ASTM Workshop, Gaithersburg, MD, November 9-10, 1998.
Answer contributed by Shri Bhidé of the Portland Cement Association. He may be contacted at [email protected] or 847-972-9100 for further information.