Although concrete performance has increased significantly over the last 20 years, the durability of concrete structures still remains a major concern. In fact, while concrete permeability has been remarkably reduced, aggressive agents can still penetrate concrete structures through cracks, which are generally present due to shrinkage or tensile stresses. A possible solution for crack control in structural members reinforced with conventional steel rebars is represented by fibre-reinforced concrete. In fact, fibre reinforcement improves the tension stiffening in the concrete portions between cracks and allows tensile stresses to be transmitted between the crack faces; both aspects reduce the crack distance and, as a consequence, the crack width. Within this framework, the aim of this paper is to shed new light on the crack control exhibited by steel-fibre-reinforced concrete (SFRC), having high post-cracking residual strength, in combination with steel rebars. In this regard, 32 tension ties under direct tension were tested. Special attention was devoted to the influence of the SFRC post-cracking performance on crack formation and development. The results confirmed a reduction in mean crack spacing compared with reinforced concrete members. However, beyond a nominal residual strength of around 5 MPa, no considerable reduction of the mean crack spacing was found.
The construction of underground structures is playing a relevant role in modern society. As a result of noticeable tunnel boring machine (TBM) evolution, mechanized excavation methods are gaining increased market shares. TBM driven tunnels are generally used in combination with precast concrete segments as the main support system for reducing or minimizing surface disturbances during construction and to fulfill the project requirements with regards to quality, construction times, budget and safety. Within this framework optimization approaches strongly focused on these process parameters, rather than the general structural performance of the segments. However, improving the performance by using conventional reinforcement, High Performance Fiber Reinforced Concretes (HPFRCs) or a combination of such enables a possible reduction of the lining thickness or the amount of reinforcement. To this aim, this paper presents a new hybrid solution for precast tunnel segments with a reduced thickness, which leads to new perspectives in reducing the total amount of concrete, in decreasing the volume of disposal materials from the boring process as well as to minimize the TBM size. A parametric numerical study is developed for evaluating the effectiveness of proposed hybrid solution. The latter is based on high strength reinforced concrete in combination with HPFRC at the longitudinal joints, whose geometric configuration is properly modified to maximize the segment's bearing capacity.
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