Direct Assessment of Concrete-Making Materials for Standards and Specifications

Summary

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. Moreover, the performance of the aggregate can depend, in part, on the properties of the cement. 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 cement specifications. Predictive statistical models will be developed using cements from the Cement and Concrete Reference Laboratory (CCRL) proficiency test data and complementing this information with internally generated materials characterization and performance data. The resulting predictive models and cement specification limits will be validated and promoted within ASTM.

Description

Objective - To develop and promote new ASTM standard test methods and ASTM specification limits 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 to modern cements because of phase-related specification limits based upon inherently biased estimates of composition and particle fineness, using outdated (70 years old) methodologies. The collective biases of these measurement technologies, simple linear models, and changes in modern cements mineralogical and physical characteristics make the historic relationships between material properties and performance tenuous. Meanwhile, aggregate constitutes up to 70 % of the concrete’s volume, and aggregate selection is important because it affects concrete performance. Despite aggregate selection based on empirical expansion measurements related to alkali-silica reactivity (ASR) and a petrographic analysis, there are still cases of failure due to ASR. This project will provide methodologies to base the selection on detailed characterization of the aggregates, including mineralogy and microstructure and their influences on ASR potential. This information is necessary to establish methodologies and database of materials properties that could be used for new engineered material selections, forming the basis for new screening practices and standard specification limits.

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 results from ASTM standard tests for 38 chemical and 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 crystalline phases and polymorphs, and the results correlated to performance from the CCRL data. These results will provide more comprehensive and accurate estimates of compositional and textural characteristics of cements compared to the currently applied Bogue method in cement specifications. These data form the basis for robust predictive statistical models, with priority 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?  Project efforts are focused on the following three areas: 1) testing for cement performance in sulfate environments, 3) data management for cements and aggregates and 4) development of statistical models relating physical and mineralogical characteristics to performance.

1. Overall Project Plan: The breadth of existing cement and aggregate test methods are 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 testing program will need to be developed with stakeholder engagement and presented to a wide audience as industry buy-in will be critical to the success of this project. For example, 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. The statistical model may then be tested against the long-term testing performance and in examination of the suitability of Bogue C3A limits in the cement specification. If a more rigorous predictive model is established using the cement materials properties database, it will be promoted within the proper Subcommittees of ASTM and the existing cement specification limits will be addressed.
   
   Aggregate durability issues stem from deleterious interaction of specific mineral and textural types with the concrete pore solution. Identifying specific mineral and textural combinations of reactive aggregates will form the basis improved standard guides for evaluation of aggregate for durability. 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 efforts need to be fleshed out thoroughly and articulated because it will involve using analysis methods that are not familiar to most of the industry.
2. Data Collection and Data Management: With over 60 CCRL cements and test data available, a materials characterization and performance data set will be established to enhance the CCRL data and will be amended with new CCRL cements as they are issued. This new cementitious materials data repository contains mineralogical test results, traditional Bogue-calculated values, and particle size distribution data and the CCRL test results for both the chemical and physical tests. Once completed, this data repository 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. The methodologies to characterize the cements and aggregates developed in this project will be proposed to ASTM in the form of guides and standard test methods.
3. 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 new engineered cementitious materials.

Major Accomplishments

 

• Collect data for a proficiency study for XRD analysis of commercial Portland cements
  • Complete
• Complete sulfate expansion testing for statistical modeling of cement performance
  • Moved forward to FY21 due to lack of lab assistance for mortar casting and expansion testing
• ASTM C01 Reporting on the Proficiency study for XRD analysis of commercial portland cements using ASTM C1356.
  • Complete – Presented progress report to C01.23 at December meeting
• Journal paper through WERB on proficiency study for XRD analysis of commercial Portland cements
  • In progress. Analysis is complete and draft paper in review
• Draft of NIST Quality Manual (QMIII) for developing Standard Reference Material clinker
  • Complete
• Journal paper on ruggedness testing of XRD measurements of portland cement
  • Moved to FY21 due to delays with lab access; in analysis of data phase with the data I currently have
• Complete statistical data exploration for heat of hydration, initial and final set times, false set and strength development
  • In progress with Alan Heckert of SED