Behaviour of cement matrix composites in various service conditions

Ordinary concrete elements cast in situ or in a pre-cast plant have to support their own weight at a relatively early age, together with the dead weight of other parts of the structure and of non-structural elements. In contrast, the service loads and different other actions due to environmental conditions are usually applied much later and then they are combined with the dead weight. During exploitation, the structures are loaded slowly and only a part of the peak stresses in structural elements is caused by rapid actions, for example, traffic on bridges or runways, which induce dynamic effects. 

In normal conditions, it is expected that structures support loads and various actions over a long period of time, expressed in tens of years for ordinary structures a period of 50 years is usually considered and for special structures longer periods are required. This implies long-term resistance of materials and durability of structures and their parts. On the other extreme end there are temporary structures and situations in which loads are imposed with very high rates, for example, due to accidents of various kinds. 

A system of microcracks appears during the initial period of hardening of the cement paste. It is later subjected to the action of external loads and imposed deformations, which vary over time. Local stress concentrations contribute considerably to crack propagation also under relatively low to average stresses. In certain conditions, the microcracks may exhibit healing (cf. Section 9.7). In most cases, however, they remain and may develop into major cracks leading to disintegration of elements. 

Several processes are caused by the action of detrimental agents from the environment. Corrosion of reinforcement and cement matrix, leaching, excessive abrasion and cavitation, overloading and impacts these are examples of degradation mechanisms acting simnltaneously with normal wear from external loads, and are accelerated by existing crack systems. 

High rates of stress or strain encountered in the cement-based materials appear in various accidental situations, for example, earthquake events, impacts of vehicles and projectiles, explosions, particular kinds of exploitation in different industrial facilities or the action of waves on off-shore platforms and bridge piers. The influence of dynamic effects in concrete may be considered either in terms of rate of loading or strain rate. In the case of rate of loading, the rate is indicated in MPa/s and in the case of strain rate, in s h The relation between these two groups of values is always approximate as the notion of the Young s modulus is not well defined. From the engineering 

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Behaviour of cement matrix composites in various service conditions... 

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