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specification limits are based upon an inherently biased estimates of compositional and particle fineness and outdated (70 years old) methodologies.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.
Overall Project Plan: The breadth of existing cement and aggregate test methods is extensive. A preliminary study is needed to identify the test method revisions having the greatest impact, identifying the commonalities among those test methods, and identifying and articulating a new fundamental approach to developing standardized test methods. Before beginning a revolutionary approach, the Project Plan will need to be developed with stakeholder engagement, and presented to a wide audience. Industry buy-in will be critical to the success of this project. Introduction of the protocols at the Cementitious Materials Characterization Workshop and through presentations at ASTM and ACI will gain stakeholder awareness of the potentials of this new approach. Furthermore, the complexity of this effort needs to be flushed out thoroughly and articulated because it will, most likely, involve using analysis methods that are not familiar to the vast majority of the industry.
Data Collection and Data Management: With over 60 CCRL cements and test data available, a materials characterization and performance database will be established, enhancing the CCRL data. This new NIST database will contain mineralogical test results, particle size distribution data, and SEM imaging data (phase concentration, surface area) and the CCRL test results for both the chemical and physical tests. Once completed, this database could be used to develop statistical models to predict performance properties of cement from its composition. As a proof of concept, predictive models for heat of hydration, setting time, and sulfate resistance of cements will be developed and evaluated against existing test methods and specification limits. ASTM C1012 and the NIST mini-bar test for sulfate resistance (submitted to ASTM) will be performed using a subset of the CCRL cements to establish a data set to explore the relationships between cement properties and sulfate resistance. As new CCRL cements are issued (4 cements per year), they will be characterized and all data added to the NIST database. The methodologies to characterize the cements developed in this project will be proposed to ASTM.
Statistical Modeling: The comprehensive materials characterization together with improved, non-linear combination statistical data exploration techniques will be used to explore predictive models for cement performance. Sophistication in model selection techniques for both multi-linear candidate models and selected non-linear extensions for multi-linear models has improved tremendously. Establishing a protocol combining these techniques to explore the relationships between mineralogy, particle size distribution, texture and performance will result in an improved understanding of the combinations of material properties that affect performance. One such approach is to employ all possible subsets regression with alternating conditional expectation to determine which variables exhibit the highest potential predictive power for nonlinear models for cement performance. These new statistical models will be used to facilitate a new generation of cement qualification and specification criteria for a new engineered cementitious material.