Concrete Repair, Rehabilitation and Retrofitting II – Alexander et al (eds) © 2009 Taylor & Francis Group, London, ISBN 978-0-415-46850-3 131 Investigations on the PCC-microstructure after mechanical load A. Flohr, A. Dimmig-Osburg & K.A. Bode F.A. Finger-Institute of Building Material Science, Bauhaus-University Weimar, Weimar, Germany ABSTRACT: For PCC as a constructional material two parameters are of particular importance: the deformation behaviour and the stiffness evolution at mechanical loading. The microstructure and the composition are the main differences between normal concrete and polymer modified concrete. To understand the deformation behaviour of PCC it is necessary to survey the behaviour of the polymer matrix under mechanical load. There are two ways how concrete is influenced by polymers under mechanical load. Firstly, strong polymer films demand a higher load to open the micro-crack edges so that the tensile strength increases. Secondly, a small par- ticle size raises the specific adhesion between the matrix and the sand grains, which also leads to an increasing tensile strength. In fact, it is a co-action of both ways, depending on the kind of polymers, their particle sizes, their film tensile strengths, and their film formation temperatures. 1 INTRODUCTION The requirements on long-life cycle and mechanical properties of concrete constructions are continually increasing. In practice often durability problems arise with reinforced concrete constructions due to rising environmental impacts. Polymer modified cement concrete and mortar are mainly used in the form of thin layers within the field of restoration of already exist- ing structures. As construction material it is hardly used and little investigated up to now. Therefore one research aim is to investigate how changed hardened concrete properties due to the polymer modification are applicable for construction purposes. Normally polymers in the structure of hardened concrete reduce the elastic modulus and the compressive strength and increase at the same time the flexural strengths, tensile strengths and the durability (Ohama 1995, Beeldens et al. 2005, Dimmig 2002, Justnes et al. 1998). Großkurth and Konietzko (1989) concerned them- selves with this problem already early. They detected that, due to their visco-elastic behaviour, polymers can only be used to increase deformability and reduce the elastic modulus of the concrete. However, a suf- ficient knowledge of the long-term strength behav- iour and time dependent deformation behaviour of those concretes would be essential. By investigating the tensile and compressive load bearing behaviour of PCC (Großkurth & Konietzko, 1989) they found out, that temperatures above the glass transition tempera- ture of the used polymer led to an increase of the con- crete ductility and at the same time to a decrease of the concrete strength. Within another research project Großkurth and Steinmetz were engaged in the “Struc- tural behaviour of polymer modified cement concrete (PCC)” (Steinmetz 2001, Steinmetz et al. 2004). They revealed that there was a formation of crack-bridging structures due to the film formation of the polymers. Thus, the fracture behaviour was influenced. The load induced crack formation was affected favourably. As a result of that improvement of ductility, the crack distribution was favourably influenced, i.e. the cracks became finer. That in turn is of positive influence on the durability of the concrete. Explicitly, further investigations had been recommended, in which the direct bond between hardened cement paste, polymer matrix, grains and steel should be microscopically assessed. Lohaus and Anders (2004) described effects of a polymer modification on mechanical properties and fracture behaviour of high performance concrete (HPC), which should be used for structural members under dynamic load. Polymers on basis of styrene/ acrylate and epoxy resin had been used. In addition to standard fresh and hardened concrete properties, load deformation curves had been determined and frac- ture energy as well as ductility had been calculated. Polymers on basis of styrene/acrylate did not increase the ductility and the fracture energy. That was only achieved by using the epoxy resin. Those results cor- related with those of the hardened concrete investiga- tions. Some effects were contradictory and could not be identified. Further investigations especially of the structure formation as well as interactions between