Early-Age Performance of Concrete Project

Successfully delivering the desired early-age performance of concrete, upon which long-term prediction of sustainable performance depends, requires careful selection of mutually compatible constituent materials, understanding and using the rheological properties of concrete to assure flow and pumping performance, and proper curing of the concrete, all guided by accurate prediction of in-use concrete performance. This project will remove major technical barriers to successful delivery of the desired early-age performance of concrete by:

1. Improving the reliability of high volume use of fly ash in concrete mixtures by developing the measurement science basis for new standards and models for more accurate and useful characterization of fly ash; and
2. Assuring the accurate utilization of concrete flow properties by developing standard reference materials and using accurate models that support the standardized use of concrete rheometers.

Objective: By FY2014, to enable industry to deliver the desired early-age performance of concrete by developing the measurement science basis for new standards for characterizing and classifying fly ash; developing a suite of standard reference materials, standards, and models for calibrating concrete rheometers, and developing and validating models of in-service early-age performance of concrete incorporating fly ash.

What is the new technical idea? Unexpected behavior of current concrete mixtures often can be attributed to unanticipated interactions during construction or during the strength-gaining phase of the chemical reactions. The desired early-age performance of concrete can be reliably achieved only by:

1. Carefully selecting mutually compatible constituent materials, including cement, industrial by-product materials like fly ash, sand, gravel, and chemical admixtures;
2. Properly quantifying the rheological properties of concrete to assure flow and pumping performance; and
3. Properly curing in-place concrete (this item was completed in an earlier project).

To base these three aspects of assured delivery on sound engineering principles, they must be guided by accurate predictions of in-service concrete performance. The first two of these areas will be addressed in this project, using the new technical idea of combining experiments and models that both measure and use fundamental material parameters. This project will link together microscopy, modeling, and experiment to support the development of new standards and test methods for assuring the early-age performance of concrete. Constituent materials will be characterized both physically and chemically to assure performance during construction and strength gain over time. The modeling will inform the microscopy of what properties to characterize. The experiments will validate the models, and provide the data required to promulgate new standards.

What is the research plan? The research plan is divided in two main thrusts: 1) the characterization and modeling of fly ash and its interaction with cement, and 2) develop mortar and concrete rheology standard reference materials and standards using a combination of experiment and rheological models.

X-ray diffraction, X-ray fluorescence, field emission scanning electron microscopy, and quantitative analysis of leachate solutions will be brought to bear both for characterizing the starting powders and for tracking the chemical and structural development in cement + fly ash paste microstructures. The goal is to identify a critical set of fly ash material parameters that will enable forecasting of undesirable interactions in proposed mixes that lead to delayed setting or poor strength development. The sufficiency of these parameters will be validated by (1) testing to see if they can distinguish fly ash materials that do and do not cause extensive retardation in a given mixture; and (2) using existing models, which will be adapted to high-volume fly ash systems, of chemical, structural, and property development at the microstructural scale, to simulate their early hydration and determine if the simulated behavior is sensitive to variations in these parameters. Identifying a sufficient parameter set will serve as the technical basis for new standard tests to characterize fly ash that can be formulated and proposed to augment/replace ASTM C 618 and ASTM C 311. Improved fly ash characterization that is relevant to its compatibility with cement binders is a measurement capability gap in materials characterizations identified at a 2010 FHWA workshop^[1].

In addition, the measurement methods and simulation tools that will be developed will apply directly to concrete binders containing crushed waste glass, which is a potential industrial by-product material that is available in quantities comparable to the annual usage of fly ash in concrete.

Rheometer response is sensitive to the flow geometry and operating principles of the instrument, and the response often is not related directly to any one rheological property of the mixture. Therefore, rheometer behavior must interpreted using a combination of accurate simulations and standard reference materials (NIST term is SRMs)^[2]. Rheometers based on vane geometry are the most commonly used concrete rheometer used in industry, so the unique NIST capability of accurately simulating flow in a realistic vane rheometer will be essential for interpreting the rheological parameters obtained from these instruments as they are used in the industry. Building upon the recent development of SRM 2492 for cement paste rheology, an SRM for mortar will be produced and validated using a combination of modeling and experimental measurements. The “sand” for the mortar SRM will be comprised of small glass beads, with larger beads used as coarse aggregates for the anticipated concrete SRM. Inter-laboratory studies will be organized to ensure industry impact in collaboration with the American Concrete Institute (ACI) Technical Committee 238^[3].

Each higher-scale SRM is based on the previous lower-scale one to better control the rheological properties of the matrix fluid and ensure robustness of measurements. Collaboration with industry through the American Concrete Institute (ACI) 238 Workability of Concrete committee and the CRÈME consortium is essential to ensure that this approach is in line with the roadmap developed in the workshop^[4]  conducted at NIST, and is converging toward an integrated vision of future construction.

[1] 2010 FHWA Workshop on “Fly Ash Research and Specifications for Use in Highway Concrete Pavements and Transportation Structures”; http://www.fhwa.dot.gov/publications/rtnow/10novemberrtnow.cfm

[2] C. Ferraris, L. Brower editors, “Comparison of concrete rheometers: International tests at MB (Cleveland OH, USA) in May 2003”, NISTIR 7154, September 2004 (http://ciks.cbt.nist.gov/~ferraris/PDF/DraftRheo2003V11.4.pdf)

[3] ACI 238 http://www.concrete.org/COMMITTEES/committeehome.asp?committee_code=0000238-00

[4] “Measurement Science Roadmap for Workability of Cementitious Materials” held at NIST on March 18, 2011 – NIST Technical note 1704.

Outputs:

• Created website to disseminate all information related to CRÈME (Concrete Rheology: Enabling MEtrology) http://concrete.nist.gov/CREME.html
• Ferraris,  Li, Zhang, Stutzman. “Development of a Reference Material for the Calibration of Cement Paste Rheometers” accepted for publication to ASTM-Advances in Civil Engineering Materials (2012), which is an extended study of the technology behind the cement paste SRM 2492
• N. Martys - invited speaker at the international workshop "Micro-structure, setting and aging of cement: from soft matter physics to sustainable materials" (Switzerland, 8/12).
• J.W. Bullard, D.P. Bentz, P.E. Stutzman et al., l, “Simulating Solution Chemistry and Phase Evolution in Early-Age Concrete Materials.”, invited talk at ACI Spring 2012 Symposium on Early-Age Properties of Concrete
• Organized and facilitated special meeting at ACI Spring 2012 Convention in Dallas, TX on the use of virtual testing in cement plants using the NIST Virtual Cement and Concrete Testing Laboratory software (will also appear as an article in Concrete International).
• Talk and paper at the Transportation Research Board meeting, 1/2012: Gurney, Bentz, Sato, Weiss, “Using Limestone to Reduce Set Retardation in High Volume Fly Ash Mixtures: Improving Constructability for Sustainability”. This paper showed how the addition of fine limestone can be used to successfully mitigate set retardation in high volume fly ash concrete  mortars.
• Gustashaw, Chancey, Stutzman, and Juenger, “Quantitative Characterization of Fly Ash Reactivity for use in Geopolymer Cements”, Proc. 13th International Congress on the Chemistry of Cement (ICCC), Madrid, July 3-8, 2011; awarded the Best Poster Award.

Outcomes:

• Department of Energy awarded the NIST Engineering and Information Technology Laboratories  22 computer million processor hours at Argonne National Laboratory to carry out large scale simulations of suspension flow in complex geometries like the vane rheometer.
• Lafarge, the world’s largest building materials company, has entered into a Cooperative Research and Development Agreement (CRADA) with NIST to study the role of aggregate shape on flow in cement-based materials.
• The NIST-led industrial consortium CRÈME, with its main topic being grout pumping, began in FY12, with three starting members: FHWA, Sika, and BASF.

Impacts:

• ASTM C1749-2012: “ASTM C1749 - 12 Standard Guide for Measurement of the Rheological Properties of Hydraulic Cementious Paste Using a Rotational Rheometer” was approved by ASTM in FY12.

Standards and Codes:

Chiara Ferraris is a member of ACI 238 Workability of Concrete and the chair of ASTM C01.22 Workability. As member of ACI 238, she is able to recruit collaboration for necessary inter-laboratory studies for the development of the SRMs for rheometers. As the chair of ASTM C01.22, she is able to champion standard tests needed for the usage of the SRMs.

Paul Stutzman is coordinating the efforts to develop and ballot a standard method for X-ray fluorescence for hydraulic cements, as member of C01.23. The needed data were provided to develop the test precision and bias statements and Stutzman will champion the test through the process.  Jeff Bullard has requested ACI to form a new task group on hydration within the ACI 236 Materials Science of Concrete technical committee, which will use the recent FHWA vision document for hydration^[5] to assess the state of the art in measurement and prediction of hydration and will propose strategies for filling gaps in critical measurement technologies. Dale Bentz is a member of ASTM C09.48/C01.48 Performance of Cementitious Materials and Admixture Combinations. He is helping the committee finalize the “Standard Practice for Evaluating Hydration of Hydraulic Cementitious Mixtures Using Thermal Measurements”, and participated in an ASTM task group to update the C1608 Chemical Shrinkage standard test method.

[5] In WERB review at NIST, August, 2012