Today's cement and concrete specifications are based on the 1948 work,” Long-Time Study of Cement Performance in Concrete”. While the 1948 study established early specifications, modern cements differ significantly from their 20th century counterpart’s due to market-driven needs for rapid strength gain, reduction in CO2 emission, and increased use of non-cementitious materials. It is increasingly difficult to extrapolate these early specification criteria to modern cements. This project will provide the industry with valid data to replace the 1948 study and foster new material specifications. Predictive statistical models will be developed using cements from the Cement and Concrete Reference Laboratory (CCRL) proficiency test data and completing this information with internally-generated materials characterization and performance data. The resulting predictive models will be validated and promoted within ASTM and then used to further develop and validate the NIST cement hydration models.
Objective - To develop and promote new standard test methods and specifications for cement and concrete materials based upon a comprehensive assessment of mineralogical, chemical and textural properties.
What is the new technical idea? Current cement specifications are based on materials from the mid-20th century. It is increasingly difficult to apply existing criteria on modern cement, because phase-related specification limits are based upon an inherently biased estimates of compositional and particle fineness and outdated (70 years old) methodologies. The collective biases of these measurement technologies, simple linear models, and characteristics changes in modern cements make the historic relationships between material properties and performance tenuous. Aggregate constitutes up to 70% of a concrete and aggregate selection is important because it affects concrete performance. One example is the selection of aggregates to avoid alkali-silica reaction (ASR), which is currently based on empirical expansion measurements made on mixtures having no relationship to the concrete mixture used in the field. As a result, there are still cases of failure due to ASR. This project would provide methodologies to base the selection on detailed characterization of the aggregates using its morphology or composition and its influence on ASR. Thus, an alternative approach emphasizing the mineralogical and microstructural characteristics associated with differing durability issues need a more comprehensive understanding if guides and specifications are to improve. This information would be paramount to establish methodologies and database of materials properties that could be used for new engineered material selections.
This project will address two constituents of concrete: the cement and the aggregates. To examine the cement, a large volume of cement performance data available from the Cement and Concrete Reference Laboratory (CCRL) proficiency program will be utilized. This program includes data from ASTM standard tests for 19 chemical tests and 19 physical test results on over 100 cements from over 200 laboratories. These same cements were used in the concrete testing program, comprised of eight tests from over 300 laboratories. Using direct measurements of cement phase and texture characteristics by X-ray diffraction, electron microscopy, and laser diffraction, these cements will be analyzed to identify all (up to 12) crystalline phases and polymorphs, and the results correlated to performance from the CCRL data. These results will provide a basis for robust predictive statistical models as an alternative to the currently used Bogue method in cement specifications, which estimates only four primary phases in cements. Priority is placed in developing statistical models for heat of hydration, setting time and sulfate resistance.
Aggregates are typically characterized by sieve size distribution and general rock classification (limestone vs. granite). This limited classification scheme has restricted the development of relationships between material characteristics and performance in key areas, including alkali-aggregate reaction (AAR). Guides and standardized test methods for a detailed characterization of aggregate mineralogy will provide a more comprehensive assessment of aggregate characteristics for durability. Ultimately, a priori characterization of the aggregates will permit a more accurate prediction of their potential performance in various concrete mixtures.
What is the research plan? The research plan will concentrate on two aspects: 1) data collection and data management for cements and aggregates and 2) development of statistical models to correlate the material characterization with performance. Initially, the focus on this project will be on cements. In future years, aggregates characterization will also be explored.