Early age strength development of fly ash mixes as affected by temperature

• Main Author: Elsageer, Mohammed A. Abdalla
• Published: University of Liverpool 2011
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.631690

This thesis investigates the effect of temperature on the strength development of fly ash (FA) concretes, particularly its effect in enhancing early age strength development. The research also provides the basis for the development of models to predict in situ concrete temperature and strength development. Fly ash concretes, have not gained popularity in construction applications where high early age strength is the main requirement, due to their slower strength development at early ages and at standard cube curing temperatures. The fly ash reaction rate could increase at early age by an elevated ambient temperature and by elevated temperatures inside structural elements. Both factors appear to provide the activation energy required for the reaction to kick-in fast increasing the early age strength. This would allow the early removal of forms or the need for post-tensioning application. Hence it reduces the overall cost of concreting. The research used the normal building research establishment (BRE)[I] and the modified maximum density theory (MMDT)[2] concrete mix design methods to obtain Portland cement concrete and concrete containing fly ash with cement replacement levels of 15, 30 and 45%. This strength design concrete, which is of 40, 60 and 90 grades were cured under a range of temperature to enhance their strength development experimentally. The result was compared against maturity and ultrasonic pulse velocity (UPV) methods predicted values. Furthermore, the result was validated against the values generated by finite element analysis (FEA). The study which was carried out under controlled laboratory condition indicates that high curing temperature enhances the early age strength development of FA concretes significantly. It also shows that the higher the level of FAin a concrete, the earlier the development. A new UPV application was examined and validated that used UPV data for the determination of apparent activation energy. The obtained activation energy compared well with values obtained from strength data for some of the concretes investigated. The research presents a comprehensive study of the effect of temperature on the strength development of FA concrete. It could be used as guide in the application of the concrete in fast track constructions.