Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks (2007)

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NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks INTRODUCTION AND RESEARCH APPROACH Problem Statement Premature deterioration of our nation’s concrete bridges has been a persistent and frustrating problem to those responsible for maintaining those bridges as well as to the traveling public. The deterioration typically consists of concrete delamination and spalling due to various mechanisms, including corrosion of embedded steel reinforcement, repeated freezing and thawing, deicing salt-induced scaling, or reactive aggregates. The rate of these mechanisms is primarily dependent on the resistance of the concrete to the ingress of moisture and aggressive substances and on cracking of the concrete. Since nearly all concrete deterioration processes are driven in some manner by the ingress of water and water-borne agents, such as chloride and sulfate ions, one way to minimize these problems is to make the concrete less permeable. Concrete is primarily made less permeable by densifying the cementitious paste. This is achieved by lowering the water-cementitious materials ratio (w/cm) and by adding supplementary cementitious materials (SCMs), such as silica fume, fly ash, ground granulated blast furnace slag, or metakaolin. However, regardless of how impermeable the concrete cover is, if the concrete cracks, aggressive agents may reach the interior of the concrete and the reinforcing steel and promote deterioration. Excessive cracking can result from freezing and thawing action, alkali/silica reactions (ASR), corrosion of reinforcement, plastic shrinkage, restrained drying shrinkage, or thermal stress. Early-age cracking became relatively common within the past 30 years as practitioners strived to use less permeable concrete made with extremely low w/cm and high dosages of some SCMs, such as silica fume. These mixtures often produced very high-strength concrete that was prone to thermal, drying, and plastic shrinkage cracking. As these problems have surfaced, researchers and practitioners have developed materials, mixtures, and construction practices to combat these problems and prolong the life of concrete in bridges. It is now better understood that to make durable concrete, high strengths are not necessarily required. High strengths may in fact be detrimental due to the concomitant high modulus of elasticity and low creep, which can result in the development of restraint-induced stresses sufficient to produce cracks. Instead, the mixture performance can be balanced to minimize permeability and shrinkage/thermal cracking while enabling ease of placement, consolidation, and finishing. The use of SCMs, such as fly ash, silica fume, slag, and natural pozzolans, in concrete bridge decks has become a widely accepted practice by many state highway agencies seeking to maximize durability. This practice is justified by a great deal of research that has been performed on properties of concrete containing one or more supplementary cementitious materials. However, this prior research, necessarily conducted on individual SCM sources, has not provided clear nor universally applicable conclusions concerning the optimum use of these materials. A “one size fits all” approach to concrete mixtures does not achieve the goal of maximizing long-term durability. This is because the quality of local materials used to produce the concrete strongly influences mixture properties and performance. There are large variabilities within, and interactions between, concrete raw materials, and these influence the short-term properties and long-term durability of the concrete. In addition, service environments and the associated deterioration mechanisms vary with geographical location. As a result, concrete mixtures cannot be truly optimized without direct testing of local materials and evaluating the 4

NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks concrete produced with those materials relative to local performance demands. Therefore, research was performed under the National Cooperative Highway Research Program (NCHRP) Project 18-08A to develop a statistically based experimental methodology to efficiently determine the optimum mixture proportions of concretes based on locally available materials and performance requirements. Scope of Work The objective of the research was to develop a methodology for designing hydraulic cement concrete mixtures incorporating supplementary cementitious materials that will result in enhanced durability of concrete bridge decks and this objective has been met. The process outlined is based on an experimental program aimed at evaluating the performance relative to an anticipated service environment, using the best materials available and structured around a test matrix that lends itself to statistical analysis. The individual results of the test program are combined based on the desirability-function concept, which provides a consistent framework within which to evaluate various types of performance, and are modeled to predict the optimum combination of materials. Finally, to confirm the model predictions, confirmatory testing is required so that the best concrete for the particular situation and materials can be chosen with confidence. This Methodology is presented in NCHRP Report 566, which outlines the following six step process: Step 1: Define Concrete Performance Requirements (and appropriate tests for measuring this performance) Step 2: Select Durable Raw Materials Step 3: Generate the Experimental Design Matrix Step 4: Perform Tests Step 5: Analyze Test Results and Predict the Optimum Mixture Proportions Step 6: Perform Confirmation Testing and Select Best Concrete While the testing program (Step 4) is the largest and most time-consuming part of the process, before the testing can be initiated, several other important steps must be completed. The criteria against which the concrete performance will be evaluated must be determined (Step 1), and the range of locally available candidate materials most likely to achieve the performance objectives must be identified (Step 2). A decision-making system for defining appropriate test methods required for the service environment, selecting durable raw materials, and selecting proper ranges and combinations of SCMs was developed to support these processes. This system is based on flowcharts and background guidance summarizing the available literature. This guidance provides the user with a frame of reference to make intelligent decisions, and it provides sufficient information to allow the Methodology to be adapted to the user’s specific application. In the background covering the definition of the service environment and performance requirements, the following topics are covered: cyclic freezing and thawing resistance, salt scaling resistance, chloride penetration resistance, resistance to abrasion, cracking resistance, workability, finishability, and the effects of SCMs on each of these properties. The concrete property requirements most likely to produce durable long-term performance, test methods for evaluating those properties, and target values for those test results are also discussed. Background information on selecting likely candidate raw materials includes topics 5

NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks such as: aggregates (including ASR testing), cement, Class C fly ash, Class F fly ash, ground granulated blast furnace slag (GGBFS), silica fume, metakaolin, and chemical admixtures. Recommendations were also developed for appropriate additional raw materials testing, where needed, as well as target values for these tests. A range of statistically based experimental approaches was evaluated. The fractional orthogonal design method was chosen for implementation in this Methodology, since this approach provides a means for obtaining useful information over a large test space while testing the minimum number of possible combinations of variables. Guidance has been provided for selecting a feasible number of mixtures to be tested that is consistent with this orthogonal approach. The number of variables (or factors) and levels that can be investigated is governed by the number of mixtures that can be tested within available resources, and a discussion of the significant aspects of this selection is provided. According to the Methodology, at the completion of Step 4, statistical analysis of the data is performed to identify concrete mixture that performed best relative to the performance requirements and to predict the optimum concrete mixture that will produce the best overall performance relative to those same requirements. (These mixtures are known as the Best Tested Concrete (BTC) and Best Predicted Concrete (BPC), respectively). The method for performing this analysis is presented in detail. The final Step in the Methodology involves confirmatory testing of the BTC and testing the BPC to verify improved mixture performance, as predicted. This is necessary due to the high variability inherent with concrete materials testing, and it provides an assessment of the repeatability of the procedures. From the analysis of the full testing program, the optimum mixture is recommended. To provide a basis for evaluating the effectiveness of this Methodology and to serve as a tool during its development, a case study was investigated. A hypothetical northern, Midwest environment subject to freezing and thawing and deicing salt exposure, was chosen. Performance requirements were developed, and locally available materials were obtained and used in an experimental program in which eleven concrete mixtures were produced and tested. The results of this test program were analyzed using the process outlined in the Methodology, and the effectiveness of the modeling was evaluated. Finally, to aid potential users in the implementation of this Methodology, a computation tool called Statistical Experimental Design for Optimization of Concrete (SEDOC) based in Microsoft® Excel was developed. This tool leads the user through each of the Steps in the Methodology and performs the statistical analysis and modeling. Outline of Project Deliverables NCHRP Project 18-08A, “Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks,” generated the following products: • NCHRP Report 566: Guidelines for Concrete Mixtures Containing Supplementary Cementitious Materials to Enhance Durability of Bridge Decks • NCHRP Web-Only Document 110, the project report and hypothetical case study appendix • Microsoft® Excel-based computational tool titled Statistical Experimental Design for Optimization of Concrete (SEDOC) and user’s guide 6

NCHRP Web-Only Document 110: Supplementary Cementitious Materials to Enhance Durability of Concrete Bridge Decks This document, NCHRP Web-Only Document 110, provides a condensed description of the Methodology and the process by which it was developed. It discusses the scope and capabilities of the Methodology and how and where it may be best applied. To distinguish table and figure numbers in NCHRP Report 566 from those in this document, NCHRP Report 566 table and figure numbers are prefaced by an “I” for Introduction or by an “S” followed by the Step number. For example, Table S1.1 indicates Step 1, Table 1 (the first table in Step 1), of NCHRP Report 566. Figures and tables discussed in this document are numbered sequentially. 7