Creep in FRC – From Material Properties to Composite Behavior

Although being the subject of numerous studies during the last 5 decades, fiber reinforced concrete (FRC) only very recently started to penetrate the construction market to a larger extent. This is mainly due to the lack of appropriate standards in the past years and the recent availability of structural codes such as fib Model Code 2010, the German DAfStb guideline “Stahlfaserbeton”, the Italian code, or codes under development, such as the Eurocode 2. However, FRC still struggles with some issues that potentially hinder, at least at a first glance, a further penetration into structural applications. Creep under tensile stress is one of these major obstacles. The creep response of a structural member in tension results from the interaction of the creep of the fiber material itself and the creep of the bond between fiber and surrounding matrix. The latter at last leads to the fact that creep is also a subject of further investigation for steel fiber reinforced concrete which shows that this is not an exclusive problem for synthetic fibers, as publicized in common literature. Unfortunately, commonly agreed test methods to investigate creep are missing. In this regard more research is required in order to specify which load levels and crack openings should be chosen to provide a realistic scenario for creep tests on FRC in general.

Testing of creep in FRC is a very complex and time-consuming matter. Therefore, the subject of this paper is an investigation of a possible correlation of the creep properties of a single filament and the bond behavior of a single fiber in mortar. The most important subject of this paper is whether it is possible to derive the overall creep response of the composite material FRC from the determined component-specific characteristics. Based on a thorough test plan, two distinctly different performing polypropylene fibers (moderate to high-performing) are tested and compared. The determination of short- and long-term behavior of the filaments in this test plan is carried out in varying temperatures in a range from −10 ℃ to 60 ℃. Results indicate that the creep performances of a filament and the bond between a filament and a mortar correlate with the overall creep of the corresponding FRC. Moreover, a broad range of creep performance is revealed which strongly suggests that the performance of synthetic fibers and, in particular the creep performance, cannot be generalized.