Microwave curing of concrete repair

Large number of concrete structures have approached or are approaching a state where repairs are necessary. During repair of concrete, curing is an important criteria and widely recognised to assure adequate performance of cement based repair materials. Curing is the process for promoting hydration of cement that leads to the strength development. Strength development of cement based repair materials is slow and often takes several days to develop fully. In addition, fresh concrete exposed to cold weather experience significant loss of strength, permeability and durability in long term due to frost damage. Thermal curing is an effective way to develop higher early age strength of cement based repair materials and it can be used to accelerate construction works, reopen roads soon after repair and maintain construction work in cold weather. However, most of the thermal methods are inefficient and rarely provide a controlled temperature increase to concrete and often are not suitable for site. This project investigates the microwave curing of concrete repair as an effective method of thermal curing in order to accelerate curing. Initially, laboratory investigations are carried out to determine the important parameters of microwave and cement based repair materials such as power of microwave, duration of heating, temperature, type and volume of repair materials, and then, their relationships are determined empirically. These relationships were derived from laboratory investigations and led to the development of a Fixed Time Power Regulating (FTPR) algorithm which can be used for the automatic control of any microwave system to enable it to deal with endless variation of repair material formulations encountered on construction sites. This research programme also deals with the effect of microwave curing on the properties of different commercial repair materials. This includes the effect of microwave curing on the hydration characteristics, porosity and pore structure, the bond between substrate and repair patch, strength, shrinkage and finally bond of steel rebar in patch repairs. Results show that microwave curing accelerates the heat of hydration and brings forward the peak time. It also increases the porosity of the bulk matrix and interfacial Transition Zone (ITZ) relative to normal curing. In addition, microwave curing prevents bond loss of concrete substrate with repair mortar applied at a sub-zero temperature (-5 ⁰C) while microwave curing reduces the bond strength of plain (undeformed) steel bar embedded in repair materials by 10 to 40 %. Furthermore, results also show that microwave curing reduces the shrinkage by 7 to 32% relative to normal curing. Finally, this thesis also describes a pre-industrial prototype microwave system which has been developed by Fraunhofer IGB (Germany) based on the recommendations of the research. It also includes the results from field trials to validate the prototype and design equations by testing slab elements of four commercial repair materials and CEM II concrete. Slabs of dimensions 1 m x 1 m and depth up to 80 mm were cast and microwave cured to a temperature ranged between 41.9 to 76.5 ⁰C for a predetermined time. The results of the research show that microwave curing is an effective method of thermal curing for concrete repair. Relationships between the key parameters relating to the repair material properties and microwave energy parameters have been derived to enable design of microwave curing system.