Guide Specification for Service Life Design of Highway Bridges (2020) / Chapter Skim
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 136 C H A P T E R 3 Development of the Guide Specification for Service Life Design of Highway Bridges This chapter describes the development of each section of the Guide Specification. For reference, the table of contents from the Guide Specification is included in Appendix C
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From page 144... ...
NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 137 Environmental Exposure From a service life standpoint, the environment surrounding a bridge and its components represents the load on the structure. Standards and specifications typically define the environmental exposure on an element in terms of exposure zones and exposure classes.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 138 in Figure 52. The splash and spray zone also exists above the tidal zone and is subject to splash and spray from waves.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 139 The second figure (Figure 52 below) in the article on exposure zones defines the roadway splash/spray zone limits, x and y.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 140 Table 63. Concrete Exposure Classes [Guide Specification Table 2.2.2.1-1]
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 141 Table 63. Concrete Exposure Classes (continued)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 142 Table 64. Steel Exposure Classes [Guide Specification Table 2.2.2.2-1]
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 143 and inspection, and construction oversight. There are a number of factors that affect service life that must be addressed by the designer at the planning stage.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 144 An article on documentation is included at the end of Section 3, and provides guidance on the development of a service life design manual and a preservation plan. The purpose of the service life design manual is to document all assumptions made by the designer related to service life including: • design assumptions – exposure conditions, deterioration mechanisms, material properties, etc.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 145 Table 65. Concrete Material Requirements (CSA 2009, ACI 2014, BSI 2014)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 146 with corresponding U.S. units when relevant.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 147 Based on guidance provided by fib Bulletin 34, a confidence level of 90% that corrosion will not be initiated within the targeted corrosion initiation time (which, in this case, is equal to the target service life) was used for modeling.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 148 0( )
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 149 Table 66. Input Parameters for Exposure Conditions.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 150 which extends a distance Δx below the concrete surface. CS,Δx and Δx account for the deviation of chloride diffusion from Fick's second law within the convection zone due to wetting and drying cycles (Figure 54)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 151 Source: fib (2006) Figure 55.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 152 Source: fib (2006) Figure 56.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 153 • GGBS: Ordinary Portland Cement with 36%-65% ground granulated blastfurnace slag grade 100 or higher by mass of total cementitious • GGBS+SF: Ordinary Portland Cement with 36%-65% ground granulated blastfurnace slag grade 100 or higher by mass of total cementitious and with 5%-8% silica fume by mass of total cementitious. Table 67.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 154 The aging exponent is a critical material property used in the chloride diffusion calculations and represents the decrease in the apparent diffusion coefficient over time (fib 2015)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 155 Table 69. Influences on the Critical Chloride Threshold for Mild Steel Reinforcement (after Glass and Buenfeld 1995, Böhni 2005)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 156 Solving Equation 31 for the standard deviation and combining it with Equation 30 results in: min 0.05 1.64 nom devc c c u σ − ∆ = = (32) Equation 32 was used to define the standard deviation used in the calibration.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 157 While values for the target service life integers are not presented in Table 70 or in the identical table in the Guide Specification, it was necessary to select values in order to perform the calculations. The following assumptions were made for each service life category: • Normal = 75 years.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 158 Table 70. Calibrated Design Provisions for the Limit State of Chloride-Induced Corrosion [Guide Specification Table 4.2.4.2.2-1]
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 159 Table 70. Calibrated design provisions for the limit state of chloride-induced corrosion (continued)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 160 Table 70. Calibrated design provisions for the limit state of chloride-induced corrosion (continued)
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 161 Steel Structures The section on steel structures, Section 5, leads with a discussion of the two most common deterioration mechanisms for steel bridge components: corrosion and fatigue. This is followed by an article on the protective measures for each mechanism.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 162 Specification for additional guidance where appropriate, such as the provisions of Section 4 when designing a concrete foundation element. The following elements are included: • Spread footings • Driven piles • Micropiles • Drilled shafts • Nongravity cantilever walls • Anchored walls • Soil nails • MSE walls A brief discussion on durability considerations for approach embankments is also included at the end of Section 6.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 163 well as guidance on how to apply the method in design. This section focuses on the implementation of the probabilistic method for corrosion on a project specific level, such that a designer could use different assumptions than those presented in Section 4 (i.e., concrete mix type, cover dimensions, and type of reinforcement)
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