Electrical Resistivity and Temperature of Fresh Cement Paste as Indicators for Setting Time

Abstract

The process, rate, and quality of cement hydration (a complex physical-chemical process) affect the properties development of concrete, such as its compressive strength, penetrability, and setting. Among all, setting time is one of the important properties for the concrete industry, as it is the factor in determining the consolidation, finishing, and curing time of a concrete pour. Inadequate determination of concrete setting time will result in improper finishing, formation of cold joints between layers of concrete, and curing time delay. Therefore, short- and long-term performance of concrete, concrete durability, and its life span will be affected. Concrete setting time proceeds from an initial stage (end of concrete pouring and consolidation) to a final stage (end of finishing process and beginning of curing) with the hydration of cement particles in the matrix of concrete. The accurate determination of the setting time in concrete projects has always been a question. Setting time is measured in laboratory setups by the traditional method of the Vicat needle (as standardized in ASTM C191). However, the field condition of a concrete pour is different from a lab set up, so the importance of replacing a lab-based setting time test with a field applicable method is out of question. This research project studies the feasibility of using a practical and non-destructive technique to detect the actual concrete setting time and its development in a reasonable amount of time. In this research, the electrical properties and internal temperature of cementing-based mixtures are monitored over the first 24 hours of hydration. The electrical properties of concrete and cement-based materials and corresponding internal temperature are influenced by many factors affected by cement hydration, similar to the setting time. The main goal of this research project is to establish a predicting method for the fresh cement paste setting time by monitoring either the electrical properties or the internal temperature of the mixture. To isolate the effect of the specimen geometry, electrical resistivity has been chosen as the electrical properties’ representative in this study. It is measured by a novel resistivity-meter setup. However, to develop a time-saving and accurate method, microstructure factors affecting the electrical resistivity development of fresh cement paste need to be studied. These factors are (1) the ionic concentration and ionic variation in the pore solution (in the liquid phase of the cement paste microstructure), and (2) the hydration products formation and crystallization (or the physical characteristics of the cement paste microstructure). Since formation of hydration products and crystallization are exothermic, the changes in the internal temperature of the materials are also monitored. Finally, reliable predicting indicators, electrical resistivity and internal temperature values, which are influenced by not only the fresh stage ionic concentration, but also by the development and changes in the physical characteristics of microstructure are proposed to the industry.