Jeffery S Volz, Oklahoma DOT
Missouri University of Science & Technology
Additives, Admixtures, Cracking, Fly ash, Freeze thaw durability, Surface treating, X-ray computed tomography
Concrete mixtures that contain a high volume of fly ash (greater than 30%) have become more desirable for the increase in sustainability and economy of these mixtures. Extensive work has been completed by the principal investigator (PI) Volz for the Missouri Department of Transportation under several previous projects to show that there are many applications where high volume fly ash can be used to provide satisfactory performance. However, several additional additives had to be included in the concrete mixture to improve the reactivity and subsequent strength gain of the mixture. While these additives did improve the performance of the mixtures, there was still a measurable difference between the high volume fly ash mixtures and concretes that use traditional volumes of fly ash. Furthermore, during the durability testing of these mixtures, there was poor performance of these mixtures in the ASTM C 672 salt scaling test. These differences in performance in these materials provide significant barriers between low and high dosages of fly ash that will not allow their usage in bridge decks, pavements, sidewalks, or any element with a time sensitive strength gain. Recent work has been completed by Silva, Cheung, and Roberts of W.R. Grace to introduce a new and promising method of pre-treating fly ash by soaking it in water or other solutions before using it in concrete. This treatment allows the fly ash mixtures investigated to show improved early and later age reaction rates and subsequent strength gain. This methodology needs to be investigated with a larger number of fly ash sources and also the mechanisms need to be better understood. If the benefits of this method could be achieved in a practical manner, then this would greatly improve the ability to use high volume fly ash concrete and would address several of these critical issues with delayed strength and setting. Even if these issues are addressed then there will still be problems with the subsequent scaling of these mixtures. While there are a number of different ways to address this problem, it would be better if there was a greater understanding of the mechanisms of salt scaling with high volume fly ash and why it is more severe than low volumes. In order to investigate this, the research team will use X-ray computed tomography techniques to image samples during freezing and thawing cycles. This technique can make 3D maps of materials with a scale of 1 micron and has been a significant focus of research of PI Ley at OSU. This technique will allow direct observations to be made of the crack initiation sites and their subsequent growth under freezing cycles. Once the scaling mechanism is better understood, then this will give much greater insight to guide future solutions.