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Guide Specification for Service Life Design of Highway Bridges (2020)

Chapter: Chapter 4 Conclusions and Suggested Research

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Suggested Citation:"Chapter 4 Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2020. Guide Specification for Service Life Design of Highway Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25672.
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Suggested Citation:"Chapter 4 Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2020. Guide Specification for Service Life Design of Highway Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25672.
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Suggested Citation:"Chapter 4 Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2020. Guide Specification for Service Life Design of Highway Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25672.
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Suggested Citation:"Chapter 4 Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2020. Guide Specification for Service Life Design of Highway Bridges. Washington, DC: The National Academies Press. doi: 10.17226/25672.
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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 164 C H A P T E R 4 Conclusions and Suggested Research Conclusions The preceding chapters detailed the development of the proposed AASHTO Guide Specification for Service Life Design of Highway Bridges. A comprehensive literature review and survey of industry professionals were performed to gauge the current state of practice of service life design. The majority of service life design provisions contained in existing specifications are deemed-to-satisfy or avoidance of deterioration approaches. The results of the survey highlighted that corrosion is by far the most prevalent deterioration problem for both structural steel and reinforced concrete elements that leads to reduced service lives. The Guide Specification was developed around a three-tiered methodology of “good-better-best” design practices. Exposure zones are defined in order to classify bridge elements based on the severity of the surrounding environment. Design provisions in the form of deemed-to-satisfy and avoidance approaches are then provided for common materials and bridge elements to resist said environmental conditions. For the limit state of chloride-induced corrosion of reinforced concrete, the deemed-to-satisfy provisions were calibrated using the full probabilistic method defined in fib Bulletin 34. Widely accepted details that avoid deterioration and extend service life are also presented for each material and element type. A framework for future refinement and/or expansion of the design provisions to be founded on additional probabilistic methods is provided in the form of an appendix to the Guide Specification. This appendix currently outlines the fib Bulletin 34 probabilistic method for chloride-induced corrosion of reinforced concrete. The appendix is formatted to facilitate the addition of limit states when sufficient supporting data and research is available, such as freeze-thaw damage of reinforced concrete and corrosion of structural steel. Two case studies were developed that illustrate the implementation of the Guide Specification. The bridges are designed using the developed provisions and guidance. The bridges are then hypothetically relocated and redesigned to demonstrate the effect of changes in exposure classes. Comparisons are made between the resulting designs. Suggested Research During the creation of the Guide Specification, it became apparent that there was a lack of supporting research for certain topics that hindered the development of specific sections and articles. The following sections discuss topics related to service life design of highway bridges where additional research would be beneficial in advancing the Guide Specification.

NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 165 Concrete Structures Chloride-induced Corrosion While the creation of the probabilistically calibrated deemed-to-satisfy provisions in Section 4 of the Guide Specification are an advancement in service life design specifications, there are still a number of unresolved limitations and research needs, as given in the following sections. Reinforcement Critical Chloride Threshold The challenges associated with selecting a value for the critical chloride threshold were introduced in this report, with the large number of influences on the threshold value given in Table 69. One difficulty in establishing a value for the critical chloride threshold for use in service life design is the lack of a universally accepted standard test method (or set of test methods). Reported values often vary depending on the testing procedure that was used. Four general approaches have been used to estimate the critical chloride threshold, none being able to replicate the entirety of field conditions seen by an actual structure (Nilsson et al. 1994, Frederiksen 2000, Böhni 2005): • Field testing of laboratory cast concrete specimens of various composition (binder type, W/CM ratio) with small covers. Misses the effects of practical cover dimensions and variable workmanship. • Laboratory testing of reinforcement embedded in concrete of various composition with small covers and under submerged conditions. Sometimes the environmental impact is simulated using potentiostatic controlled steel potentials. Misses the effects of practical cover dimensions, variable workmanship, and (usually) varying microclimate. • Field testing of concrete from existing old structures, typically with practical covers but higher W/CM ratios (≥ 0.5) and only OPC as a binder. Misses the effects of modern W/CM ratios and binder types. • Laboratory or field testing of concrete with cast-in chlorides (e.g., in the mix water), allowing for the use of low W/CM ratios, various binder types, and practical cover dimensions. Misses the effects of steel passivation in chloride-free concrete and different chloride binding and hydration mechanisms compared to penetrated chlorides. This is considered the most erroneous method. Common laboratory test methods used to determine the critical chloride threshold include immersion of reinforcement in simulated pore solution (Scully and Hurley 2007, Hartt et al. 2007) and various ponding tests (Clemeña 2003, Draper et al. 2009, O’Reilly et al. 2011, Darwin et al. 2013). Other tests have been developed specifically for the measurement of the critical chloride threshold (Trejo and Halmen 2009), but none have been standardized to date to the knowledge of the research team. The acceptance of a set of standardized tests that account for the probable in field conditions would serve to refine service life modeling. One of the groups of influences from Table 69 are external factors, which are measures of the surrounding environment. The environment represents the load on the reinforcement and is determined by factors such as moisture content, oxygen concentration, chloride type and concentration, and temperature. The critical chloride content is a measure of the material’s resistance to the environment. However, the relationship between the critical chloride threshold and environmental conditions is not well understood. Research in this area would be beneficial to service life modeling for bridges and their components, as the exposure zones between components can vary significantly at the same site (e.g., buried versus atmospheric). This would allow for the critical chloride threshold to be varied by exposure class, either deterministically or probabilistically depending on the extent of data. The standardized testing needs outlined above may also help in this area.

NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 166 The threshold influences from Table 69 only pertain to typical mild steel reinforcement. Other types of steel (e.g., stainless), alternate material types (e.g., composites), and coatings used for reinforcement (e.g., epoxy coating) could have very different influences. Critical chloride threshold testing has been performed on these reinforcement types (Clemeña 2003, Trejo and Pillai 2004, Scully and Hurley 2007, Hartt et al. 2007, Hartt et al. 2008, Draper et al. 2009, O’Reilly et al. 2011, Darwin et al. 2013), albeit limited and subject to the same test method issues outlined previously. These reinforcement types would benefit from additional critical chloride threshold testing. This would serve to assign reinforcement types into the reinforcement classes in Section 4 of the Guide Specification, as well as to refine the associated critical chloride threshold values assigned to Class B through D. Chloride Diffusion Parameters Estimates for the chloride diffusion parameters used in the fib Bulletin 34 model, particularly the chloride migration coefficient and aging exponent, were originally based on research and testing of European concrete mixes. While there are comparable mixes used in the U.S., testing on U.S. mixes would be beneficial in refining the values used for these parameters. Several States have performed chloride migration testing per NT Build 492 (Presuel-Moreno et al. 2014, Naito et al. 2016, Bales et al. 2018, Riding et al. 2018), and testing by additional States would serve to increase the size of the available database. A fuller understanding of the statistical distribution of these parameters is obtained with a large number of samples. Exposure Condition Parameters As previously discussed in this report, assumptions for the surface chloride concentration in buried and nonaggressive environments had to be made in order to create the calibrated cover provisions in Section 4 of the Guide Specification. More accurate estimates of the surface chloride concentration within these environments would help to refine the associated calibrated cover provisions, whether it be through testing of existing structures or laboratory testing to develop the necessary chloride adsorption isotherms. Cracking The effect of cracking is not explicitly considered in the fib Bulletin 34 model. While it is difficult to anticipate cracking in design, it’s potential occurrence should be recognized. One possibility would be to provide an allowance for cracking via a factor in the model or through a resistance factor if a partial factor design approach is ever implemented. Issues to address include the effect of crack properties (e.g., width, depth, frequency, orientation, and nature) on the diffusion and corrosion process, the differences in cracking susceptibility between concrete mixes, and cracking that arises as a result of construction practices versus in design. Overlays and Surface Treatments There is a large number of overlays and surface treatments available for reinforced concrete. However, there is a lack of long-term service life performance data for these protection strategies. Service life modeling would benefit from means to account for various surface protection strategies that are based on measured in-service performance. Concrete Cover While defining the distribution for concrete cover to be used in the fib Bulletin 34 model, there was an absence of literature on as-built cover dimensions for bridges in the U.S. This information would be beneficial in confirming the most applicable distribution for concrete cover as well as the associated characteristic values (i.e., mean and standard deviation).

NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 167 Freeze-Thaw and Sulfate Attack The design requirements for the two other limit states in Section 4 of the Guide Specification, freeze- thaw and sulfate attack, are based on a deemed-to-satisfy provisions from other specifications (e.g., ACI). The majority of these provisions were originally developed for the building industry. A set of exposure class conditions and associated design provisions that are specific to bridges would be beneficial. fib Bulletin 34 includes probabilistic models for freeze-thaw damage, one for internal frost damage and the second for salt-frost induced surface scaling. However, the data that is needed to define the input parameters for these models is scattered and insufficient. Testing to quantify the input parameters for different concrete mixes under various environmental conditions is needed in order to implement design provisions similar to the probabilistically calibrated provisions for chloride-induced corrosion. Steel Structures Current service life design provisions for structural steel take the form of deemed-to-satisfy provisions. The majority of these provisions use sacrificial thicknesses based on estimates of corrosion rates from observed field performance. More efficient and economical designs could be achieved by developing a deterioration model similar to the fib Bulletin 34 model for corrosion of reinforcing steel in concrete. Ideally, the model would be based on fundamental principles of steel corrosion and validated against in- service performance. This would allow the steel section to be sized based on the service life demands (e.g., environmental exposure conditions) in addition to the structural loads. The model would have to account for the type and grade of steel. Provisions within the model for other protection strategies such as coatings would also be beneficial. Foundations and Retaining Walls Many of the design provisions in the Guide Specification for concrete foundation and retaining wall elements will be improved when some of the areas of research need presented above are addressed, including improved estimates for the critical chloride threshold of reinforcement and measurements of the surface chloride concentration for buried components. In addition, the current singular exposure class for buried elements (C-B) could be split into multiple classes using a set of micro exposure zones that are defined by the condition of the surrounding environment. Environmental conditions that could be considered include chloride concentration, sulfate concentration, and pH level, among others. Research is needed to (1) define the set of exposure zones and (2) set the appropriate limits on the relevant environmental conditions for each zone.

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The majority of instances of explicit consideration of service life design has been limited to signature bridges and other projects where extended service lives (in other words, greater than 100 years) are specified by the owner. Many state departments of transportation and other transportation agencies have recognized the importance of implementing service life design for typical highway bridges; however, no specification or standard has been developed to date in the U.S.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges provides a new guide specification on the service life design of highway bridges for adoption by AASHTO, including a set of case studies that demonstrate its application.

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