Development of reference materials for cement paste and mortar : calibration of rheological measurements

Olivas, Alex
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This thesis presents the results of five interrelated projects conducted to advance the progress of studies in concrete workability. Specifically, work conducted towards the development and certification of Standard Reference Materials (SRMs) for cement paste and mortar rheometers are presented in this work. Other SRMs developed at NIST have served a multitude of services, but these SRMs target the concrete industry by providing a way for commercially available rheometers to be calibrated economically and with good accuracy. However, problems regarding reproducibility of the paste SRM, microbial contamination of the paste SRM, and accuracy of measurement have limited the development of the concrete SRM. Thus, this thesis includes studies that were conducted to address these issues. In Project 1, the SRM preparation method was modified to improve reproducibility. The major outcome of this project was the development and re-certification of a standard reference material for cement paste. This recertification includes new rheological characteristics and statistical analyses. The goal of Project 2 was to extend the shelf life of the SRM since it was discovered that after 10 days the rheological properties of the SRM was not stable It was found that use of biocides, such as sodium propionate, extends the stability of the SRM. In Project 3, the development and certification of a SRM for mortar is provided with rheological properties and statistical analyses. Description of a model that was developed to predict the behavior of the SRM is also provided. In Project 4, a literature review regarding why industrial rheometers experience slippage issues was conducted. Key findings were that slippage issues depends on the rheometry choice and boundary conditions (free surfaces). Project 5 presents the results of a critical analysis conducted to evaluate the effect of rheometry systems for calibrating mortar-type rheometers. Rheological behaviors of two SRMs were measured experimentally and the results were compared to a computer simulation models. Discussion on the best rheometry system to avoid slippage is provided in that project. Overall, the outcomes of the work conducted in fulfillment of this thesis serve as the initial steps towards developing a reference material for concrete rheometers.