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NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks CONCLUSIONS AND SUGGESTED RESEARCH Effectiveness of Approach Optimizing concrete mixtures for durability is a challenge that must be dealt with on a local basis. The raw materials used in such concretes, particularly the SCMs, which are included because of the great potential for improved performance, are likely to vary significantly depending on their source and may not be universally available. The long-term deterioration mechanisms and the design requirements are different for different service environments, which are locally determined. SCMs add a significant level of complexity to such mixtures, especially if used as part of ternary or quaternary cementitious mixtures, and the exact mechanisms by which they influence the properties of the concrete are not well-enough understood to allow reliable mechanistic modeling. Because of these issues, there is no single set of guidelines for selecting mixture proportions. Instead, the optimum mixture proportions can only be determined for each situation separately, based on an experimental investigation. The Methodology developed in this research project provides a step-by-step process for conducting just such an investigation. Evaluating the performance of concrete relative to the potential range of deterioration mechanisms, such as freezing and thawing, scaling, chloride induced corrosion, ASR, and drying shrinkage and thermal cracking requires a large program involving many separate tests. The concept of desirability and the desirability function has been introduced to provide a framework for evaluating the combined significance of all of these performance measures. The overall desirability permits the comparison of mixtures and the modeling that identifies the optimized mixture proportions. Because of the large scale of durability-related investigations, statistically based experimental design procedures have been adopted to efficiently investigate as many combinations of materials as possible with the minimum number of tests. The data generated by this test program are used to develop models that predict the performance of mixtures for any combination of the tested levels. This Methodology is flexible and the user selects the responses to be included in the evaluation of the mixtures, designs the desirability functions for each response to reflect the importance and reliability of the test result, and chooses the factors to be evaluated. These factors can be Amount, Type, or Source factors. This flexibility makes it useful in a range of mixture proportioning applications. The Hypothetical Case Study, based on a realistic set of mixture objectives and conducted with a set of locally available materials, showed that the approach laid out in NCHRP Report 566 can be used to identify an optimum concrete mixture proportion. Predictions based on empirical modeling of each response were used to predict a BPC that was produced and tested. Excellent agreement was observed between the individual responses and the overall desirabilities of this concrete as predicted and actually tested. While this was only the first experiment conducted with this Methodology, the accuracy of the modeling demonstrated the far-ranging potential of this approach. 37
NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks Suggested Research and Implementation During this project, the experimental design approach outlined in the Methodology was successfully implemented for one situation, the Hypothetical Case Study. NCHRP Report 566 was developed to be a stand-alone guide to allow this approach to be applied in a wide range of situations. However, implementation of the Methodology is a significant undertaking for even experienced users and additional work is warranted to facilitate this process. The following tasks are suggested to further develop the Methodology and to support its implementation: ⢠Methodology Validation and Development ⢠Software Development and Support ⢠Implementation Support Each of these tasks is discussed individually below. Methodology Validation and Development To fully evaluate the effectiveness of the approach, additional validation testing in a range of applications is recommended. The Methodology has been designed to reflect the fact that the experimental constraints for each individual application will vary widely throughout the country. The effectiveness of this strategy should be proven through its implementation in other mixture optimization projects. The modeling accuracy should also be evaluated based on data collected from design matrices constructed with different numbers and categories of factors, and for different sets of raw materials. Finally, as is the case with the field implementation of any new technology, long-term monitoring of actual bridge decks constructed with mixtures developed using this Methodology is recommended. This performance should be compared with bridges produced with concrete mixtures selected by conventional means to evaluate whether the extra effort involved in the optimization process is, in fact, justified. The process outlined in NCHRP Report 566 requires significant input from the user in selecting and evaluating test methods and materials to include in the experimental program. The best future use of this Methodology will include consideration of new test methods and additional materials. Therefore, the continued development of this Methodology must be based on the knowledge and understanding of the engineers or scientists in a position to use it. Software Development and Support In this project, good progress was made towards providing a computational tool package that will support the Methodology, including significant efforts in terms of decision making, documentation, data analyses, and optimization modeling. This package, SEDOC, has been established within the framework of Microsoft® Excel. This approach was selected since Excel is widely available and provides calculation and plotting tools that are familiar to most users. However, the SEDOC: Analysis tool is based on the specific workbook developed to perform the analysis conducted for the Hypothetical Case Study while that analysis process was being developed. As a result, it does not contain all the functionality that could be included in a package designed from the ground up, and additional work to develop the SEDOC tool further would be worthwhile. In addition, to fully realize the potential of the Methodology, a distribution and support framework for the tool is needed. 38
NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks The current user-interface is spreadsheet-based and requires the user to fill in appropriate cells, which can be confusing and difficult to locate for users unfamiliar with the Methodology or the software. The ease of use of the tool could be improved by incorporating dialogs that prompt the user to perform tasks in the correct sequence and query the user for information where needed. The SEDOC package, in its present form, contains specific workbooks for a limited number of individual design matrices. While a user of this Methodology could develop spreadsheets for a specific unsupported matrix based on the information provided in NCHRP Report 566, the additional time required for that task would make the implementation of this Methodology a formidable task. The specific matrices that are currently supported were chosen to span the range of possibilities and include 9- and 18-mixture designs. Because the shape of the experiment in the current tool is limited to be one of those supported, additional development is needed to make SEDOC a viable tool for all practical experiment designs. While the optimization routine implemented in the SEDOC package is a simple brute-force evaluation of all possible combinations, other optimization routines could be applied that would identify multiple mixtures worthy of consideration for selection as the BC. This would be of interest if multiple, local optima in the overall desirability response surface occur with significantly different combinations of materials but similar overall desirabilities. The selection of multiple, locally optimized mixtures would be based on thresholds for the overall desirability that would need to be defined by the user. Identifying a range of mixtures (if they exist) that provide similar performance, instead of a single optimized mixture, would provide additional flexibility to deal with specification and other construction-related issues. A single program able to perform all the steps in the experimental setup and analysis process, able to deal with each of the potential orthogonal designs and incorporating the above modifications would be an improvement on the existing program. This could be achieved within Microsoft® Excel using Visual Basic routines or as an independent PC-based application created specifically to perform this task. The PC-based application will provide the most flexibility for the developers, allowing dynamic definition of the arrays depending on the design matrix selected for use, which would simplify the process of supporting multiple designs from a single interface. Dynamic array definition is also possible within Excel but was not incorporated in the SEDOC tool because of the substantial computer programming effort required, which was not a focus of this study. Developing an independent PC-based application has the disadvantage of requiring building many features from the ground up, including some that Excel already provides, like plotting and regression analysis. An effort to develop a simpler and more robust software application would require at least the three following types of expertise: software development, statistical experimental design, and concrete mixture expertise. A team with a wide breadth of experience will be needed to complete this development. The first step in such a project will be establishing the scope of the application by balancing what is desirable for the experimental design and mixture development processes with what is feasible for the software development. Implementation Support The Methodology developed is a sequence of steps that requires users to evaluate their specific performance demands and materials and to design and conduct a unique experiment. This can be a difficult process to apply since each situation will be different, and the specific example provided in NCHRP Report 566 shows only one of the many ways in which the process 39
NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks can be applied. Therefore, a support network to help potential users learn the Methodology and operate the computational tool should be established. To help interested engineers and scientists implement this process, a short course (two to three days long) would be useful to teach potential users how to work through the Methodology. Each step of the process could be reviewed in detail, and specific questions relative to individual situations could be covered. On-going implementation support should include an internet-based bulletin board for specific questions on using the Methodology and a help contact for the SEDOC package, where questions related to performing the statistical-analysis could be answered. The ultimate success of this research project will be based on the number of people and organizations that are able to make effective use of this process that was developed to select optimized mixture proportions for a local set of conditions and materials. That number will be maximized by ensuring that potentially interested parties are aware of and know how to use this Methodology. 40
