engleski

Grouts for Pre-stressed Structures and Ground Anchors: A Critical Review of European Standards and Guidelines

A COST 534 group project “ Improvement of properties of grouts for prestressing tendons and/or ground anchors” is concerned with the rheology of grouts and with the improvement of grout performance for prestressing tendons and ground anchors. Important properties are rheology, stability of suspension with respect to bleeding and settlement, expansion, etc. The respective standards such as EN 445 or fib-guideline contain requirements but no guidance on grout formulation. Therefore there is a need to study the influence of the additives of selected producers, both added as a single additive and in combination with other additives. Test methods and evaluation criteria of standards are critically discussed and it is shown that some of the prescribed test methods and requirements are not representative of the behaviour and performance of grouts in use. Furthermore, the information given by the producers in the product data sheets is partly insufficient. In order to get the necessary insight in the correlations tests were carried out with the use of scientific instruments. For example, rheology was measured in a rotational instrument using a program which starts, after a pre-shear phase, at a shear rate (1/s) of practically zero, increases up to 200 and goes back to zero again. Six such measurement cycles of 23 min each were carried out one after another. Therefore one measurement was completed after 138 min and the shear rate/shear-stress curves so obtained contain information on thixotropy, structural changes of the grout with time, etc. The information content of such measurements is higher than that of a single point measurement as prescribed in the standards. Another example showing that basic studies are necessary is expansion. Grouts should expand sufficiently to fill the duct completely and form a protective encasement to the tendons. However, a volume contraction is permitted by the EN 447 (1996) and the fib guidelines (2002) on desirable properties of grouts and the EN 447 requires the expansion in the grout to be measured at an age of 24 hours only. It will be shown that several products caused a fast expansion which was more or less completed after less than one hour, i. e. after a time at which the grout is not necessarily fully filled into the duct. The final goal of this study which will be completed in 2007 is to define rules for grout formulation and how the requirements can be met.

Grout; Rheological parameters; Bleeding Time; Volume Change

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