Abstract
This thesis records the results of an investigation designed to obtain a better understanding of the processes involved in the fracture of cement and concrete. To this end two approaches have been taken. Firstly the characteristics of the fracture process have been studied using mechanical testing methods. Secondly, attempts have been made to develop techniques for studying in a realistic way the microstructure of hardened cement paste and its relation to the fracture process. The investigation was initially restricted to hardened cement paste to reduce somewhat the complexity, and the effect of aggregate addition was studied briefly in the later stages. Specimens containing cut in slits were tested to failure in flexure and direct tension, and the results analysed in terms of various models of fracture. A fracture mechanics approach showed the measured fracture toughness to vary with specimen size and this variation together with the relative notch insensitivity of the material suggested the existence of a region in the vicinity of the slit tip, in which the material did not behave in an ideal linear elastic manner and it was hoped that the microstructural investigation would shed further light on the fracture processes. The standard methods employed in an attempt to detect changes taking place in the microstructure under stress, ranged from simple visual observation to high resolution electron microscopy. However, normal optical microscopy shows up no changes in the microstructure prior to complete failure, and the specimen preparation techniques necessary for electron microscopy introduce large drying out stresses which make interpretation of the results difficult. A new diffuse illumination optical microscopy technique was developed which enables a specimen to be studied in a controlled environment and this technique has shown for the first time that stable micro-cracks occur in stressed samples of hardened cement paste prior to complete failure. Finally an attempt is made to explain the mechanical testing and microstructural observations by means of a tied crack model.