Experimental study on Steel Fiber Reinforced Concrete and Reinforced Concrete elements under concentrated loads
Graphical abstract
Introduction
The transmission of a heavy load acting on concrete elements, distributed over a small portion of its surface is a typical problem of structural engineering, occurring for instance at the ends of post-tensioned prestressed concrete beams or under the bearing of bridge piers as well as in footing of foundations. High concentrated forces are also exerted by hydraulic jacks of Tunnel Boring Machines (TBMs) over the thrust shoes having a smaller area than the precast tunnel segment [1], [2]; similarly, in the final loading stage for segmental lining, high compressive normal ring forces are transmitted through the small contact area of longitudinal joints between segments belonging to the same ring [1], [2].
In all aforementioned cases the diffusion of high loads through concrete elements determines a biaxial or triaxial state of stress characterized by very high compressive stresses under the loading area (contact pressures) and transverse stresses in the nearby zone: compressive and tensile [3], [4], [5]. The latter are generally defined as splitting or bursting stresses and occur in the disturbed region (D-region), which extends for a distance equal to a specific length of the concrete element [3], [4], [5]. High contact pressures could cause local crushing failure, due to the achievement of confined compressive strength underneath the loading area. Tensile transverse stresses could determine splitting cracking phenomena that has to be withstood by specific reinforcement in order to avoid an early splitting failure [6]. In this regard, steel reinforcing bars consisting in local stirrups or ties [7], [8] are typically adopted by designers, even though there is growing interest in the scientific community and among practitioners on Fiber Reinforced Concrete (FRC) for use as splitting reinforcement, especially in case of precast tunnel segments [1], [2].
One of the first studies on the local splitting behavior of FRC elements was carried out by Schnütgen [9]; more recently, it was demonstrated that post-cracking residual strengths exhibited by FRC enables a progressive stable development of splitting cracks, which guarantees the increase of the applied load after cracking [6], [10]. The effectiveness of FRC was stated in other research works [11], [12], [13], [14], even though further studies are needed to have more data which are necessary when dealing with the local complex stress distribution occurring in the D-region, especially in the post-cracking stage. To this aim, it is a matter of discussion in the scientific community the spatial development of splitting crack underneath the load as a function of post-cracking strength exhibited by FRCs. In fact, for concrete reinforced with traditional rebars, it is reasonable to localize the stirrups in the D-region, calculated according to typical elastic solutions [3], [4], while fiber reinforcement is spread everywhere and the corresponding resistant mechanism is expected to be different. To the contrary, Schnütgen [9] and current DAfStb [15] arbitrarily assumed for Steel Fiber Reinforced Concrete (SFRC) elements, a region over distributing post-cracking tensile strengths corresponding to the elastic solution, without significant experimental evidences. Moreover, the splitting behavior of samples up to the crushing failure was not experimentally studied neither in [6], [10] nor in [9], [12], leading to a lack of knowledge with regard to the mechanism moving from a possible splitting failure to a crushing one.
A further aspect to consider when using FRC structural elements is the fiber orientation with respect to the expected crack surface, which considerably influences the structural response [16], [17], [18]. The distribution and orientation of fibers depend on several factors such as concrete pouring, the geometry of the formwork, the type of vibration and the production method. With regard to splitting cracking phenomena, the effect of different casting procedures was studied only in few research works both for concrete reinforced by steel fibers [11], [19] and by macro synthetic fibers [10].
Within this framework, a broad experimental program was developed to shed new lights on the behavior of FRC elements under high concentrated loads. The main goals are the evaluation of crack development and the increment of bearing capacity promoted by fibers as well as the assessment of failure mechanisms. To this aim, concrete prisms reinforced with two different types of steel fibers and three different dosages were considered. Particular attention was devoted to the effect of fiber orientation on the splitting phenomenon, by considering two different casting procedures. In addition, the behavior of concrete specimens reinforced by two different amounts of traditional rebars was also studied.
Section snippets
Experimental program
This section describes an experimental campaign on specimens made of Steel Fiber Reinforced Concrete and of traditional Reinforced Concrete (RC). The study is focused on the analysis of the splitting behavior of prismatic elements subjected to high compressive loads applied along a small contact area over the whole specimen depth (concrete partially loaded areas). The adopted loading condition is defined as Line Load (LL) configuration and leads to a two-dimensional stress distribution.
The
Experimental results and discussion
The experimental study allowed to experience both types of failure mechanism for elements subjected to partially loaded areas: splitting and crushing (as reported in Table 5).
According to the experimental evidence, and in agreement with the results described in Conforti et al. [6], the splitting collapse can be schematized in three phases (Fig. 6a). In the first one there is no crack and the element behaves elastically. The second stage begins when the splitting crack forms and fibers start
Concluding remarks
In the present paper the behavior of high loads distributed over a small area of SFRC and RC elements was investigated by means of a broad experimental program. Prismatic samples with a line load configuration (a/d ratio equal to 0.4) were tested and the influence of the reinforcement type (either rebars or fibers), SFRC mechanical properties, fiber types and casting direction were investigated.
Based on the experimental results, the following conclusions can be drawn:
- (1)
SFRC is a very efficient
CRediT authorship contribution statement
Ivan Trabucchi: Methodology, Investigation, Data curation, Writing - original draft. Giuseppe Tiberti: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Antonio Conforti: Methodology, Writing - review & editing. Filippo Medeghini: Investigation, Data curation. Giovanni A. Plizzari: Supervision, Project administration, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The Authors are grateful to the technicians of Laboratory for testing materials Augusto Botturi, Domenico Caravaggi, Andrea Delbarba and Luca Martinelli, for the assistance in performing the experimental program.
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