An analytical proposal to calculate splitting bearing capacity of FRC elements under partially loaded area

Concrete elements are often subjected to high compressive forces applied over small contact areas; the consequent load spreading can result in concrete crushing or splitting failures. The latter is due to cracking phenomena caused by tensile stresses perpendicular to the load. In the last decades Fibre Reinforced Concrete (FRC) has been widely studied for many applications (precast tunnel segments, retrofitting, slab, etc.) and different standards now include rules for FRC members. However, only few analytical models allow the calculation of splitting bearing capacity for FRC elements under concentrated loads. To this aim, a new analytical formulation is proposed which enables a good agreement with experimental results collected from the literature regarding FRC prisms exhibiting a splitting failure

Local splitting bearing capacity under high concentrated load of Fibre Reinforced Concrete elements: an analytical model

Fibre Reinforced Concrete (FRC) is an attractive material for segmental lining reinforcement for totally or partially replace the traditional steel bar cage. Over the last decades, numerous research studies have been devoted on the application of FRC to precast tunnel segments, highlighting the effectiveness of fibres as reinforcement, by considering different load cases occurring during segment manufacturing, installation and service condition. One of the critical loading scenarios is the application of high loads on small contact surfaces; this could occur during the excavation process, due to the loads applied by the boring machine on the lining, or in the final stage, when high compressive stresses are transmitted between longitudinal joints. Accordingly, several Authors investigated the behaviour of FRC prisms subjected to high concentrated loads by means of experimental tests. However, only few analytical models exist in the literature that allow the calculation of splitting bearing capacity of FRC elements under concentrated loads. The aim of this paper is to contribute to fill this lack by introducing a new formulation to calculate bearing capacity and crack depth when the failure mechanism is governed by splitting collapse

Actual achievements and future challenges of HPFRC for structural rehabilitation of bridges

The state of road infrastructures in many advanced countries is rapidly changing under the impulse of massive funding from governments, eager to have more efficient and safer transportation systems. The use of well-known materials such as fibre-reinforced concrete (FRC) is finding a growing space for structural rehabilitation of bridges; the adopted material is often defined as High performance fibre reinforced concrete (HPFRC) due to its enhanced performance. The paper presents the principal findings of an EU-funded project that involved the repair of two road bridges in Italy using HPFRC. The project has successfully carried out the jacketing of bridge piers and cap-beams, heavily damaged by corrosion, with a new HPFRC layer of reduced thickness (40-60 mm) and limited use of steel reinforcements. Experimental tests carried out in the laboratory of the University of Brescia on 1:2 scaled specimens have shown the possibility to increase the load bearing capacity of the cap beams (with respect to vertical loading) up to 73%, with moderate effects on the change in stiffness and ductility of the existing structure. Based on field and laboratory experience, the article eventually presents some new challenges for the use of HPFRC in the reduction of environmental impact of construction industry

Local splitting and crushing behavior under TBM hydraulic jacks

During the lining construction process, high concentrated thrust jack forces are introduced in the segments by the Tunnel Boring Machine (TBM) leading to both high compressive stresses under the thrust shoes that can provoke a concrete crushing and tensile transversal stresses; the latter can cause cracks and, eventually, a splitting failure. In order to investigate the main mechanisms involved during either splitting or crushing failure, specific experimental tests on prismatic concrete specimens loaded by concentrated axial forces were designed. The specimens were cast using Steel Fiber Reinforced Concretes (SFRCs). The experimental results were compared with the ones obtained from specimens reinforced only by means of traditional steel rebars

Optimized reinforcement for precast tunnel segments with macro-synthetic fibers

The use of fiber reinforced concrete (FRC) in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially in segmental linings. Nowadays there is a growing interest in the scientific community on macro-synthetic fibers for use in underground structures. Within this framework, the present study investigates the possibility of using macro-synthetic fiber reinforcement in precast tunnel segments in a possible combination with conventional rebars. An experimental program based on 3 Point Bending Tests was carried out on FRCs characterized by different fiber contents in order to assess their post-cracking nominal residual stresses. The corresponding stress vs. crack opening laws, representative of the FRCs investigated, were calculated through inverse analysis procedure. Then, a typical tunnel lining having small diameter was adopted as reference to optimize the reinforcement solution (macro-synthetic fibers and conventional rebars, i.e. hybrid solution). Particular attention was devoted to the TBM thrust phase, in which high-concentrated forces are introduced in the linings

An experimental study on the post-cracking behavior of Polypropylene Fiber Reinforced Concrete core samples drilled from precast tunnel segments

In the scientific community and among designers there is a growing interest on macro-synthetic fibers for use in underground structures, especially for precast tunnel segmental linings. Moreover, in the last decade, important research efforts have been devoted to the development of new types of structural macro-synthetic fibers, which are now able to impart significant toughness and ductility to concrete. The enhanced post-cracking strengths of concrete, which are provided by the presence of polypropylene fibers, can be included in analytical and numerical approaches for designing precast tunnel segments in Polypropylene Fiber Reinforced Concrete (PFRC). However, it is still a matter of discussion the actual post-cracking performances exhibited by PFRC in precast tunnel segments. Within this framework, an experimental study on core samples drilled from precast tunnel segments in PFRC (considering different positions and directions) was carried out in order to shed some new lights on fiber distribution and orientation. The actual PFRC post-cracking performances evaluated by uniaxial tensile tests on drilled core samples from segments, were compared against the results of typical flexural standard tests on notched beams

Concrete with fast setting and hardening process for use in SFRC precast tunnel segments

Segmental lining gained popularity over the years due to the possibility of operating in difficult excavation conditions, high production rates during digging process and high standards of quality. In segmental lining, the tunnel is made by a sequence of rings placed side-by-side, each one composed by precast tunnel segments (from 4 to 10, depending on the tunnel geometry). This tunnel type can be used in different underground applications (Subways, Highways, Railways, Water Supply etc.). As far as tunnel segments production is concerned, it generally requires long curing time (>24h for demoulding a single segment or 8h by adding a specific treatment) and a wide area to allow primary and secondary storage of segments. However, the availability of large storage areas near the building site is often a constraint, which causes high transport costs for the segments and, in the worst cases, hinders the tunnel construction. Speed-up the production of segments is very often essential and to do this a steam curing process have to be adopted. However, the latter is extremely expensive and requires an additional dedicated production line. These criticalities can be overcome by the development of a concrete with a fast setting and hardening process; in fact, it could allow a quick segment demoulding (about 3h) without the use of additional expensive steam curing process and reducing segments storage area (due to the enhanced production rate). The aim of this study is twofold: the first is to develop a concrete with a rapid setting and hardening process adequate for being used in precast tunnel segment industry. The second is to include steel fibres as reinforcement to obtain a Fibre Reinforced Concrete (FRC) having an adequate post-cracking performance to totally substitute the traditional reinforcement (steel bars). These goals were successfully reached by the optimization of a compound based on Portland cement and Calcium Sulfoaluminate Cement (CSA) which presents favourable early strengths development and whose compressive and fracture properties were compared to that of concrete with Ordinary Portland Cement (OPC). The results obtained are very promising making feasible to reduce segments production times as well as corresponding storage areas. Moreover, it is expected to obtain segments less prone to impact damage, lower CO2 emissions and manufacturing costs

Precast tunnel segments reinforced by macro-synthetic fibers during TBM operations: an experimental and numerical study

In the scientific community and among designers there is a growing interest on macro-synthetic fibers for use in underground structures, especially for precast tunnel segmental linings. Segment reinforcement is generally designed according to the design actions on tunnel segments, resulting from segment transportation, placing process and ground pressure in the final state. In particular, during construction, the high-concentrated forces exerted by TBM jacks are the most critical factors, which could lead to undesirable cracks. Within this framework, the present study investigates the possibility of using polypropylene (PP) fiber reinforcement in hydraulic precast tunnel segments. In a first phase, point-load experimental tests on full-scale tunnel segments of a hydraulic tunnel were carried out. In a second phase, based on data initially retrieved, a reliable numerical model was developed. The latter was used for numerically simulating the segment behavior during TBM operations by including Fiber Reinforced Concrete (FRC) post-cracking properties through a non-linear concrete crack model. Different reinforcement configurations (fiber reinforcement and traditional rebars) were considered, as well as possible irregularities that can occur during this temporary phase

Structural behavior of precast tunnel segments with macro-synthetic fibers during TBM operations: a numerical study

The use of fiber reinforced concrete in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially in segmental linings. In the meanwhile, in the scientific community there was a growing interest on macro-synthetic fibers for use in underground structures. Within this framework, the present study investigates the possibility of using macro-synthetic fiber reinforcement in precast tunnel segments by means of a numerical study. Firstly, an experimental program based on three point bending tests was carried out on polypropylene fiber reinforced concretes (PFRCs) characterized by different fiber contents in order to assess their post-cracking residual strength. Secondly, the corresponding stress vs. crack opening laws, representative of the PFRCs investigated, were calculated through inverse analysis procedure. Then, a segment of a typical tunnel lining having small diameter was adopted as reference to optimize the reinforcement solution (macro-synthetic fibers and conventional rebars, i.e. hybrid solution) and to study its structural behavior by numerical analyses. Particular attention was devoted to the Tunnel Boring Machine (TBM) thrust jack phase, in which the TBM moves forward by pushing the thrust jacks on the bearing pads of the latest assembled ring, introducing high-concentrated forces in the lining

An experimental study on the behavior of precast tunnel segments reinforced by macro-synthetic fibers during temporary loading stages

The use of fiber reinforced concrete in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially in segmental linings. The design process of segmental concrete linings in ground conditions generally refers to temporary loading conditions (demolding, storage, transportation, grouting process and TBM thrust jack phase) as well as to the final permanent embedded in ground condition. Within this framework, the present study investigates the possibility of using macro-synthetic fiber reinforcement in precast tunnel segments, which are part of a tunnel lining with an internal diameter of 3.5 m. Full-scale tunnel segments were tested under bending and high point loads representative of those exerted by the TBM during the excavation process. Different reinforcement solutions were considered: macro-synthetic fibers only (PFRC segments); combination of rebars and macro-synthetic fibers properly designed to obtain an optimized reinforcement solution (hybrid solution, RCO+PFRC segments) and the reference RC solution (traditional steel rebars only)