Bei der Herstellung von massigen Bodenplatten können trotz des Einsatzes von regelgerechten Betonen Schwierigkeiten bei der Betonage und ebenfalls nach der Fertigstellung auftreten. Aufgrund der anspruchsvollen Baustellenlogistik und der besonderen Anforderungen bei der Ausführung der Betonage ist neben der Betonzusammensetzung eine sehr sorgfältige Arbeitsweise bei der Fertigstellung notwendig. Im Rahmen dieses Beitrags werden Untersuchungsergebnisse hinsichtlich der Herabsetzung der Schubkraftübertragung durch ungewollte Betonierfugen vorgestellt. Zudem wird die Beeinflussung des Verbundverhaltens infolge unzureichender Nachverdichtung sowohl bei weichen als auch bei fließfähigen Betonen aufgezeigt. Die Funktionsweise einer Weißen Wanne kann durch diese Erscheinungen stark beeinträchtigt werden.
Regular concrete for floor slabs – is that enough?
In spite of the use of regular concretes, difficulties can arise during the concreting of solid floor slabs and also after completion. Demanding building site logistics and the special requirements when carrying out concreting work mean that, in addition to the correct concrete composition, a very careful procedure is needed during the finishing. This article describes the results of investigations into the reduction in the transfer of shearing forces due to unintentional concreting joints. The effect on the bond behaviour as a result of inadequate re-compaction with both soft and free-flowing concretes is also indicated. These phenomena can have a very detrimental effect on the functioning of watertight concrete. The article demonstrates that the use of non-conforming concretes can lead to difficulties during the production of floor slabs that can have not only an adverse effect on the load-bearing ability but also a great influence on the impermeability to water. Consideration has been given to the following two problems that, among others, can occur during the concreting of solid components. Firstly, the influence of a concreting joint on the transfer of shear forces at the joint was examined and, secondly, trials were also carried out to classify the impaired bond behaviour between reinforcement and concrete due to inadequate re-compaction. During the investigations into the transfer of shear forces at a joint it emerged that the time when an upper layer of concrete is placed and the associated drying out of the surface is of decisive importance. It is not critical whether there is a skin on the fresh concrete or if, for example, a retarder has been used. As soon as the surface has dried out too far there is a significant drop in the shear force that can be transferred at the joint. Particular care should therefore be taken in future concreting designs for solid components that the surface is protected from drying out. With free-flowing concrete there is often inadequate re-compaction, with the result that the concrete can sag below the reinforcement and the bond is adversely affected. Sagging was also observed with concretes that were not free-flowing. As a rule, this can be attributed to inadequate re-compaction. Apart from the adhesive bond at very small pull-out distances this leads to a significant drop in the bond stresses than can be transferred. In both cases, adequate re-compaction prevents failure of the bond. It was also shown that subsequent grouting represents a very good option for restoring the bond. The results were even better than the joint stresses in the reference series.