ALBERT

Beschreibung / Inhaltsverzeichnis: Summary The shrinkage of concrete is generally determined using the standards. When shrinkage is measured over a long period of time, deformation is found to be larger than predicted by the standards. Therefore it seems to be interesting to measure shrinkage on different samples for a relatively long period of time. In this paper, we do not suggest a new model, but present the experimental measurements that can be used to check or develop any model. Our samples are divided into five series according to the examined parameters: (I) cylinders with different diameters; (II) prism with different dimensions; (III) different quality of cement paste; (IV) concrete with same workability but different cement content; (V) variability. The following empirical Equation 1 was used to link shrinkage and time of drying: ɛ=a (t/b+t) n . The coefficientsa, b andn were fitted using the least-square method. This allows us to see the influence of some factors on shrinkage. All samples were cast using the same aggregates and the same cement type (CPN). In some cases we also used a superplasticizer. Both axial and superficial shrinkage have been measured. As the data seem to fit correctly Equation 1 the following conclusions can be drawn: the final shrinkage,a, is decreasing linearly with increasing diameter. The slope depends on the concrete mixture. It is obvious that for a large diameter final shrinkage will reach an asymptotic line; the timet 1/2 needed to obtain half of the final shrinkage is increasing with the diameter ϕ and, as a first approximation, we gett 1/2=Aϕ+Bϕ2; therefore the theory of diffusion does not apply for small diameters; the coefficientn increases also with the diameter, it reaches asymptotically the value of 1. It varies between 0.45 and 0.8 as seen by our measurements, and the functionn=1−C e−dϕ fits correctly. If we assume that Equation 1 describes realistically the behaviour of concrete during drying, the coefficientsa andn should not vary with the sample dimension. We should not forget that shrinkage is a complex phenomenon and therefore cannot be described by such a simple equation. From our measurement we can draw the following conclusions: (i) there is no significant difference between the shrinkage of samples cast or sawed; (ii) when the volume of cement paste is constant, shrinkage is almost uninfluenced by the W/C ratio; (iii) the superplasticizer increases shrinkage slightly, but this increase is compensated by the decrease in W/C ratio; (iv) the cement content does not influence the shrinkage, if the workability is constant; (v) variability can be drastically reduced when precautions are taken to measure deformation during the first minutes of drying.