U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-05-052
Date: September 2005
Materials and mixtures considerations include issues related to mix design parameters, materials specifications and selection, mixture proportioning, type and number of admixtures, and more.
As described in chapter 1, the challenges resulting from today’s increasingly complex concrete mixtures are a major reason for developing the CP Road Map. The public demands fast construction and long-asting pavements, which require specific mix additives and more durable materials. In addition, environmental, social, and economic pressures on the industry have introduced a host of new, sometimes marginal aggregates, cements, and mineral and chemical admixtures. Singly or in combination, these materials can improve pavement performance in specific situations. They also can introduce new, unforeseen problems.
It is expected that in the next decade even more materials options will be available. The industry needs integrated solutions for optimizing complex concrete mixtures to ensure just the right mix for each specific job.
If developed within the right framework, integrated mix optimization solutions will benefit not only materials engineers and suppliers, who face day-to-day materials selection and mix proportioning decisions; they also will benefit everyone involved in the complex sequence of events in a pavement project, from design to construction to inservice maintenance and performance management. Integrating materials and mix proportioning with development of design specifications, for example, might provide a rational basis for a more objective specification, including items such as minimum cement content and pozzolanic replacement. And a better understanding of the interaction among specific mixtures and construction practices and the environment should result in improved quality control during construction. Inservice pavement forensics and pavement inventories can be integrated in this overall process, providing valuable feedback to improve future materials selection and mix proportioning decisions.
The need for new, integrated materials solutions is intensified by changing roles and responsibilities in the highway community. Traditionally, many State DOTs have relied on skilled, experienced staff not only to identify mix design parameters, but also to control mix proportions, type and number of admixtures, and much of the operations based on detailed specifications. The trend is to move these responsibilities to the contractor and specify only performance requirements. State agencies and contractors alike need well-developed national mix proportioning and materials selection guidelines to ensure successful implementation of performance-based specifications.
The CP Road Map clearly identifies the research and implementation steps needed to ensure that the proposed mix design system is technically sound, fully evaluated, and clearly understood by public and private sector engineers, technicians, and contractors through initial and continuing training and outreach programs. An important capability of the mix design system will be its ability to interact fully and provide key inputs to the structural design procedure.
The overall mix and materials goal of the CP Road Map is that innovative concrete mix material selection and mix design procedures will result in economical, compatible, and optimized concrete mixes integrated with both structural design and construction control.
This goal can be reached by achieving the following specific objectives, which are addressed in various tracks of the CP Road Map:
End users’ needs are ultimately the driving force for mix design and materials selection and proportioning, because users dictate the functional requirements, or specifications, of the finished pavement. All projects, however, begin with the individual material components of the concrete mix. The proportions and interaction of the materials will influence mix properties like strength and workability. Mix properties dictate production requirements, while production itself affects the as-produced pavement.
Numerous efforts have been made to address individual components of materials selection and proportioning. The CP Road Map is unique in that it includes tasks that link, or integrate, all of the elements of a paving project in a logical fashion. The next generation of materials selection and mix proportioning tools and procedures will therefore integrate a series of laboratory tests, analytical tools, and mechanistic models that will:
Following are brief discussions of some critical issues related to materials and mixtures for concrete pavement included in the CP Road Map.
Standards drive how concrete pavements are built, including the selection of materials. Material specifications have evolved over time, and typically have become more restrictive. As problems have appeared on various jobs, new specifications commonly were established to attempt to eliminate their recurrence. Unfortunately, the effect of the new specifications on other properties was not considered. This has led specifiers to intentionally seek out materials that meet one condition without knowing if the materials can meet additional conditions. For example, a finely ground cement may meet early-age strength requirements, but lack the required long-term strength gain requirement.
Fortunately, as a result of these issues, this trend has been reversed somewhat. By recognizing that specifiers should focus on the end result rather than on process details, researchers have an opportunity to explore and develop a more performance-based approach to materials selection and concrete mixture proportioning.
Optimizing a mixture in terms of cost, performance, and durability requires mathematical and computer models to relate all variables to each other. For example, the material components and relationships should relate to mix properties so that a mixture with certain properties can be developed by computer before testing for verification in the laboratory. These variables should ultimately be related through models to the functional performance for the highway user. For example, the types of aggregates used in the mixture will in turn affect the friction, noise, and smoothness characteristics of the concrete pavement over the design period.
For decades, it has been recognized that concrete mix design is really a misnomer. Current methods base proportioning only on highly empirical relationships of mix properties, which are not directly tied to performance or function. The industry needs to explore the selection and proportioning of mixes that ultimately are tied to user demands. Achieving this goal, however, requires a fuller understanding of the connectivity of the following issues:
The complexity of PCC mixes makes the trial-and-error process of mix design in the laboratory even more time consuming and labor intensive than it already is. Means are needed to model the behaviors of concrete mixes without actually having to mix all of the possible combinations and cast specimens in the laboratory. Work has begun on developing computer simulations of concrete to optimize proportions and properties. This work needs to be continued so that most mix design details can be worked out through such simulations, with only small-scale laboratory followup testing needed to verify predictions. These models should be capable of designing concrete mixes incorporating recycled materials, as well as special mixes for maintenance or rehabilitation activities.
The ever-increasing complexity of concrete mixtures has made recipe specifications and empirical mix design rules less reliable for obtaining appropriate concretes for high-performance concrete pavements. The range of chemical and mineral admixtures used and the potential for compatibility problems have added to this complexity. Improved tests are needed that better characterize the materials involved in terms of their effect on the performance of the concrete produced. This is particularly true in the case of aggregates, whose potential influence on concrete performance has not been investigated or categorized sufficiently. A suite of tests should be developed to evaluate any waste, byproduct, or recycled material with the potential for use in paving concrete.
Material components of concrete pavement are tested and approved before pavement construction begins. Often, however, the materials used will change during the course of the project. Substituting materials changes the concrete mix characteristics and can affect its durability. An important future consideration is developing accelerated methods for testing long-term concrete durability. Correlation should be made to current durability tests, as well as to concrete pavement field performance. These tests should be simple enough to use in the field, preferably on a construction site, and produce results in a matter of hours to help workers evaluate the effects of proposed materials changes. Current test methods, such as American Society for Testing and Materials (ASTM) C666, measure freeze-thaw resistance of a concrete sample, but are time consuming and should be performed in a laboratory.
The ability to predict the performance of concrete pavements is critical to meeting service life requirements. A comprehensive durability (service life) design model for concrete pavements that fully addresses multiple chemical and physical environments could result in extended structural life, lower life cycle cost, and increased energy efficiency. New families of embedded sensors and monitoring devices should be developed to provide the base data necessary to build and validate these predictive design models.
Future research should develop an integrated system that not only specifies a mix based on empirical relationships, but also takes into consideration all available materials, construction requirements for specific projects, and performance requirements for the finished product. Using a knowledge base, computerized guidelines, and innovative laboratory tests, designers will be able to determine the optimum mix design for each specific project quickly and efficiently.