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- KurzbeschreibungAutogenous shrinkage is the major shrinkage component of concretes that contain much less<br>water than would be required for complete hydration. This mainly applies to ultra-high<br>strength concrete and, to a lesser extent, to high strength concrete. Both have particularly low<br>water-cement ratios. The relative surplus of cement leads to an internal drying, irrespective of<br>whether the concrete dries out to the ambient air or not. This process of so called selfdesiccation<br>is associated with autogenous shrinkage which, if restrained, can lead to cracks, potentially impairing the in many respects outstanding durability of these kinds of concrete. Hence, to fully benefit from the advantages of high and ultra-high strength concrete, it is essential to minimize the risk of autogenous shrinkage cracking. Attempts to do so, however, require a reliable method for assessing this risk. Presently, there is no such method.<br>Cracks are the result of relatively complex processes, in particular at early age as concrete<br>properties change rapidly. A dependable assessment of the cracking risk requires comprehensive<br>testing and a thorough understanding of the interacting parameters. Early age cracking in<br>cementitious systems is not a new problem; cracking due to restrained drying shrinkage and<br>thermal contraction has been examined at length. However, the investigation and prediction of<br>stresses and cracks due to autogenous shrinkage brings about new challenges. The essential<br>issue is the onset of stresses at very early age. This greatly increases the influence of creep<br>and relaxation. Especially at stress levels close to failure this influence is highly non-linear<br>and difficult to quantify, experimentally as well as mathematically.<br>Another challenge is the fact that temperature strongly influences the autogenous shrinkage<br>and, presumably, the cracking risk as well. From isothermal tests at different temperatures it<br>appears that this influence cannot be accounted for by formulas conventionally used to describe<br>the temperature dependency of cement hydration. The lack of clarity in this regard in part is a consequence of a series of methodological issues, most importantly the large number of different test methods and the difficulties in defining the onset of the autogenous shrinkage. The measurement of autogenous shrinkage, yet error-prone at constant temperatures, becomes<br>particularly demanding at realistic temperature histories. The thermal deformations that inevitably<br>superimpose the shrinkage strains are difficult to compensate for. At present there is no general agreement on how to measure the autogenous shrinkage under non-isothermal conditions
- AutorSören Eppers
- VerlagVerlag Bau+Technik
- Seiten181 Seiten
- Gewicht356 g
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