Early-Age Performance of Concrete Project

Successfully delivering the desired early-age performance of concrete, upon which long-term 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 new standards and models for better characterization of fly ash; 2) assuring the accurate utilization of concrete flow properties by developing and producing standard reference materials (SRMs) and using accurate models for calibrating concrete rheometers, and 3) creating a grout pumping station test-bed to evaluate mixture designs and to develop new test methods to assure pumpability. This technology will be incorporated into performance-based standards for grout and concrete.

Objective:  To enable industry to deliver the desired early-age performance of concrete by developing new standards for characterizing fly ash (FY2014), develop an SRM for calibrating concrete rheometers (FY2013), make available to the industry a test bed and standards for quantifying grout pumping performance (FY2014), and develop standardized models for predicting in-service performance for concrete with fly ash (FY2014).

What is the new technical idea?  Delivering the desired early-age performance of concrete requires: careful selection of mutually compatible constituent materials, including cement, supplementary cementitious materials (SCM) like fly ash, sand, gravel, and chemical admixtures; 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-service concrete performance  Each of these requirements and their associated technical barriers will be addressed in this project using the new technical idea of a combination of experiments and models that measure and use fundamental material parameters.

The assured use of fly ash, even at high volumes, will be enabled by developing new standard tests. This standard will facilitate prediction of fly ash and cementing incompatibilities through a series of material characterization tests such as calorimetry, X-ray diffraction, and inductive coupled plasma spectrophotometry. Combining these tests with a thermodynamic model (THAMES) adapted to fly ash mineralogy will allow accurate prediction of in-service performance of a cement and fly ash concrete from their mineralogical composition.

The placement of concrete by pumping will be addressed by investigating the key parameters that determine pumpability[1] of grouts. This will be achieved by building an instrumented grout station that can be used initially for post-tensioning grouts (collaboration with Florida DoT). Grouts themselves are of great industrial interest, and also are ideal model systems for eventual concrete pumping development. These key parameters will form the basis for designing new standards that will eventually be scaled up for concrete in a combination of tests and numerical models. One key parameter that affects pumpability and other forms of placement is the flow or rheology of the material, measured by rheometers. Results from rheometers need to be interpreted using a combination of accurate models and standard reference materials (SRMs)[2]. Rheometers based on vane geometry are the most common for concrete, therefore using the unique NIST capability of accurately modeling a realistic vane rheometer will be essential for the calculation of rheological parameters as used in the industry. Multi-scale SRMs will be developed to calibrate concrete rheometers: one for paste (FY2011), one for mortar (FY2012), and one for concrete (FY2013). Each higher-scale SRM is based on the previous lower-scale one and carefully selected model aggregates such as glass beads are added at ever increasing size. Collaboration with the industry through the American Concrete Institute (ACI) 238 Workability of Concrete committee is essential to ensure that this approach is in line with the roadmap outlined in the workshop[3] conducted at NIST, and is converging toward an integrated vision of future construction. 

What is the research plan?  The research plan is divided in three main thrusts that will progress in parallel while end-user involvement will be used to ensure that SRMs and new standards will be adopted. The ultimate goal is to enable the assured delivery of the desired early-age performance of concrete by developing standards and SRMs that will define and measure concrete constituents, concrete rheology, grout and concrete pumpability, and standard prediction of in-use concrete properties. The three thrusts are: 1) characterize and classify fly ash and its interaction with cement, adapt existing models, and incorporate into new standards, 2) develop mortar and concrete SRMs, based on the previous FY2011 cement paste SRM, and 3) develop a test bed for grout pumping to facilitate industry innovation and new test methods.

A procedure was developed (FY2011) that combines various techniques such as calorimetry, X-ray diffraction, and inductive coupled plasma spectrophotometry to characterize the mineralogy and chemistry of fly ash that partially replaces cement in concrete. In preparation for developing a new standard, a number of fly ashes (at least seven) having a wide range of field performances will be identified in collaboration with industry and characterized. This characterization will be used to improve the THAMES model, which will be used to accurately predict in-service performance for concretes containing fly ash.

Building upon the recent development of SRM 2492 for cement paste rheology (FY2011), 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 (FY13). Round-robin inter-laboratory studies will be organized to ensure industry impact in collaboration with ACI 238. The members of ACI 238 are ideal partners with their goal[4] of fostering the development of an SRM for concrete rheometers and being primary end-users.

A combination of tests and modeling will be used to identify key factors that affect the pumpability of a grout, which at least include tribology and rheological properties. The majority of parts necessary for an instrumented grout pumping station test bed is available at NIST and the parts will be assembled and tested. The results of this project will be incorporated into standards to determine concrete and grout performance for pumpability. Partnership with Florida Department of Transportation (DoT) will enable the development of standards for post-tensioning grout conduits that are placed by pumping. Grouts are of technical interest by themselves, since they are used in numerous industrial applications from repair to mining and are placed by pumping. Most of the issues with pumpability in concrete can be studied by the use of grout as a model pumping material. Later in the project the grout pumping results can be expanded to concrete, an operation that cannot be performed without partners due to the need for concrete trucks and large scale pumps.

Once the concrete is placed, proper curing is necessary to ensure early and late-age properties. Extensive research on curing has been finished in previous years, and a draft standard has been written (see Standards and Codes section). The standardization process is on-going in the American Society of Testing Materials (ASTM).

[1]Knut Kasten, Putzmeister, "Developing efficient and effective methods for determining pumpability" and "Defining main characteristics parameters for simulating concrete flow in pipelines" Slides presented to "Measurement Science Roadmap for Workability of Cementitious Materials" held at NIST on March 18, 2011 – NIST Technical note under WERB review

[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] "Measurement Science Roadmap for Workability of Cementitious Materials" held at NIST on March 18, 2011 –  NIST Technical note under WERB review

[4] ACI 238 website posted goals

Outputs:

• State-of-the-art review on internal curing published as NIST Interagency Report #7765, featured in NIST News on the NIST homepage, included in the REACT web site, and picked up by numerous news services, including the April 2011 issue of Engineering New Record (ENR).
• Conceived and organized a proficiency test program for quantitative X-ray diffraction analysis of hydraulic cements for ASTM, with results to be presented at ASTM June 2011 meeting. This program will serve to ensure laboratories' performance using the ASTM C1365 quantitative XRD analysis standard test method for cement characterization.
• Three articles were published that strengthen the technical foundations for experimental and computational concrete rheometry and fly ash characterization.
• Successfully conducted a workshop "Measurement Science Roadmap for Workability of Cementitious Materials" held at NIST on March 18, 2011. Over thirty people attended from industry and academia from USA, Europe and Asia. The results of the workshop are used as a guide for the structure of this project.

Outcomes:

• Produced and delivered SRM 2492 "Bingham Paste Mixture for Rheological Measurements" for calibration of cement paste rheometers and to be used as the first step for the development of a concrete SRM. (July 2011)
• Program co-PI was asked to serve on two technical advisory panels for Oregon DoT projects on early-age performance of bridge decks. His role will result in Oregon DoT modifying concrete specifications to include internal curing.
• ASTM standard C 1738-2011was approved by ASTM C01 Cement in March 2011: Standard Practice for High-Shear Mixing of Hydraulic Cement Pastes". This standard will be an integral part of the instructions for preparing the paste SRM 2492.

Standards and Codes: Chiara Ferraris is a member of ACI 238 Workability of Concrete and the chair of ASTM C01.22 Workability, where she is moving the new "Test Method for Rheological Properties of Hydraulic Cementious Paste Using a Rotational Rheometer" through the balloting process. The item is under ballot and received only two negatives. It is expected to be approved by December 2011. Membership in ACI 238 is used to recruit partners for the round-robin for SRM 2492 as well as the new FY2012 mortar and FY2013 concrete SRMs. As chair of ASTM C01.23 Compositional Analysis, Paul Stutzman is coordinating the efforts to develop and ballot a standard method for X-ray fluorescence (XRF) for hydraulic cements in FY2011, which will support project work with fly ash. Dale Bentz is a member of ASTM C09.48/C01.48 Performance of Cementitious Materials and Admixture Combinations, where the committee is balloting "Standard Practice for Evaluating Hydration of Hydraulic Cementitious Mixtures Using Thermal Measurements" (four negatives to be resolved), which is based on previous project research. He has provided technical knowledge based on program research and written the draft for "Standard Specification for Lightweight Aggregates for Internal Curing Applications" for ASTM C09.21 Lightweight Aggregates and Concrete, whichis proceeding with final drafting and balloting.