Precast tunnel segments for metro tunnel lining: A hybrid reinforcement solution using macro-synthetic fibers

Engineering Structures 199 (2019) 109628

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Precast tunnel segments for metro tunnel lining: A hybrid reinforcement solution using macro-synthetic fibers

T

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Antonio Confortia, , Ivan Trabucchia, Giuseppe Tibertia, Giovanni A. Plizzaria, Angelo Caratellib, Alberto Medab a b

Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, Italy Department of Civil Engineering, University of Rome “Tor Vergata”, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Fiber reinforced concrete Hybrid reinforcement Macro-synthetic fibers Metro tunnel lining Precast tunnel segments

The addition of fibers has been proven successful to simplify reinforcement in precast tunnel segments, allowing as a function of both segment typology and fiber reinforced concrete (FRC) toughness a total or partial replacement of the conventional reinforcement. Results from an experimental research aimed at comparing the structural behavior of segments made with conventional (only rebars, RC) or hybrid (rebars + fibers) reinforcement are presented. The experimental program consisted of flexural and point load tests (which reproduces the jack actions during TBM operations) carried out on four large-scale precast tunnel segments representative of a metro tunnel lining characterized by an internal diameter of 5.80 m and a thickness of 0.30 m. The main goal of the experimental program was to evaluate the possibility of using macro-synthetic polypropylene fibers (Polypropylene Fiber Reinforced Concrete, PFRC) in combination with a lower amount of conventional rebars (optimized reinforcement, RCO) for guaranteeing the required segment performance. Experimental results indicate that macro-synthetic fibers may be very effective in combination with conventional rebars to withstand the main stresses that arise in a segment both at initial and final phases, proving that the adoption of hybrid reinforcement solution using macro-synthetic fiber is possible for metro tunnel lining.

1. Introduction In the two last decades Fiber Reinforced Concrete (FRC) has been increasingly adopted in precast tunnel linings [1–4] since FRC, with or without conventional reinforcement, represents an efficient solution for fulfilling the need of improving the design requirements of segmental linings in terms of bearing capacity, crack control and, therefore, watertightness. The enhancement of the general structural behavior together with the boost of the industrialized production of precast tunnel segments are probably the two main key-factors among designers, practitioners and contractors for the growing interest in fiber reinforcement as possible alternative solution to conventional steel reinforcing bars. Concerning the fiber type, steel fibers are so far the most studied and used in precast tunnel segments [1–8]. Besides national or international standards regarding the general design of FRC structural elements [9–12], specific documents are currently available for the design of precast tunnel segments in FRC. In fact, the International Tunnelling Association (ITA), as well as the American Concrete Institute (ACI) published guidelines for FRC

segmental linings [13,14]. More recently, fib TG 1.4.1 has prepared design guidelines for FRC precast segments [15]. It is worth noticing that ITA report n.16 [13] and fib bulletin No. 83 [15] refers to the performance-based-design approach suggested by fib Model Code 2010 [12] (hereafter MC2010). In fact, the post-cracking residual strengths provided by FRC are fundamental parameters to be included in the design approach of segmental lining. In addition, ITA report n.16 [13] provides additional design principles and suggestions to complete existing standards and recommendations for the specific case of tunnel linings, while the fib bulletin No. 83 [15] presents a detailed design procedure for FRC precast linings based on-going research or recently developed research. While FRC was initially developed using steel fibers, during the last fifteen years important research efforts have been devoted to the development of new types of macro-synthetic fibers and, in particular, polypropylene (PP) macro fibers. The latter are now able to significantly enhance concrete post-cracking residual strengths in order to make Polypropylene Fiber Reinforced Concrete (PFRC) adequate for structural purposes [16,17]. Therefore, macro-synthetic fibers for use in

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Corresponding author. E-mail addresses: [email protected] (A. Conforti), [email protected] (I. Trabucchi), [email protected] (G. Tiberti), [email protected] (G.A. Plizzari), [email protected] (A. Caratelli), [email protected] (A. Meda). https://doi.org/10.1016/j.engstruct.2019.109628 Received 10 January 2019; Received in revised form 18 July 2019; Accepted 2 September 2019 0141-0296/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

l Mcr MEd Mmax P Pcr Pmax Pspalling Psplitting,#1 Psplitting,#2 t VEd Vf VRd δcr δmax δu ρs ρw

List of symbols As,min b CMOD d fc fc,cube fct fL fLk fR,j fR,jk fu fy

minimum reinforcement area segment width crack mouth opening displacement segment effective depth mean value of the cylindrical compressive concrete strength mean value of the cubic compressive concrete strength mean value of tensile concrete strength mean value of the limit of proportionality characteristic value of the limit of proportionality mean value of the residual flexural tensile strength corresponding to CMOD = CMODj characteristic value of the residual flexural tensile strength corresponding to CMOD = CMODj ultimate strength of steel bar reinforcement yield strength of steel bar reinforcement

segment average length bending moment at Pcr applied moment bending moment at Pmax applied load flexural cracking load flexural maximum load spalling crack load splitting crack load under loading shoe #1 splitting crack load under loading shoe #2 segment thickness applied shear force fiber volume fraction shear strength mid-span deflection at Pcr mid-span deflection at Pmax mid-span deflection at steel rebar failure longitudinal reinforcement ratio transverse reinforcement ratio

presenting a post-cracking class “2e” (according to MC2010 [12]) in controlling local splitting crack phenomena under high concentrated forces applied by the Tunnel Boring Machine (TBM). Full-scale tests on precast tunnel segment carried out by di Prisco et al. [22] and by Conforti et al. [23] have evidenced the opportunities offered by macrosynthetic fibers in combination with a minimum amount of conventional rebars (hybrid solution) for tunnel linings having small diameters (around 3 m, typical of hydraulic tunnels). Nevertheless, in the literature there is a lack of experimental results on PFRC tunnel segments for linings presenting an internal diameter larger than 4 m (e.g. metro tunnel linings). To this aim, the case study of a metro tunnel was considered herein and an experimental campaign on full-scale segments was carried out in order to investigate the use of PFRC in combination

underground structures are raising interest in the scientific community. PFRC could be used when a high resistance to the environmental attack [18] is required together with a long specific lining service life [19] since macro-synthetic fibers do not suffer from corrosion problems that may occur in hydraulic tunnels or tunnels in presence of aggressive soil environments. Concerning the behavior under long term loading (that is the case of embedded ground condition) of PFRC precast tunnel segments, since cracks (either flexural or shear) are not expected to arise in service conditions (the segment is mainly compressed), the effect of fiber creep on crack opening is generally negligible [20], while the PFRC creep in compression is expected to be similar to ordinary concrete. Tiberti et al. [21] have demonstrated the effectiveness of PFRC

Fig. 1. Metro tunnel lining ring and precast tunnel segment adopted. 2

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with an optimized amount of conventional steel reinforcement (RCO + PFRC segment). The typical conventional reinforcement solution generally adopted in practice (RC segment) was also studied and considered as reference. Both reinforcement solutions were analyzed under two different loading conditions:

Before being applied in the lining rings, segments are mainly subjected to provisional phases in which flexural and shear resistance are required. Based on both static schemes and partial load factors reported in Di Carlo et al. [25], the internal actions in the provisional phases are the following:

- three-point loading test to evaluate the flexural bearing capacity and the crack pattern development for reproducing provisional phases (e.g. handling and storage) and providing useful information for the final stage (lining embedded in the ground); - point load test for reproducing segments subjected to thrust jack actions on two loading shoes, in order to study the crack (spalling and splitting) development for increasing load levels.

- demolding and positioning on floor: MEd = 4.0 kN m and a VEd = 10.8 kN (simply-supported scheme with cantilever span of 0.75 m); - storage: MEd = 14.9 kN m and a VEd = 119.6 kN (six segments are piled up in one stack considering an eccentricity of 0.10 m); - transportation: MEd = 16.8 kN m and a VEd = 108.6 kN (three segments are piled up in one stack considering an eccentricity of 0.10 m, as well as a dynamic shock coefficient equal to 2 was considered); - handling: MEd = 8.0 kN m and a VEd = 21.6 kN (slings placed at a defined spacing and with an inclination of 45°, as well as a dynamic shock coefficient equal to 2 was considered).

2. Experimental program 2.1. Specimen geometry The trapezoidal shaped precast tunnel segment considered herein is characterized by a thickness (t) of 30 cm, an average length (l) of 302 cm and a width (b) of 142 cm [24]. This segment is part of a Metro tunnel characterized by an internal diameter of 5.80 m (internal diameter-to-thickness ratio equal to 19.3) excavated by a Tunnel Boring Machine (TBM), with a lining ring composed by six segments having trapezoidal or parallelogram shape and one key-segment (Fig. 1). Two lifting sockets (external diameter of 8 cm) are included in the segment intrados, while other sockets are present on segment sides for the connection with the adjacent rings. The TBM uses the previous placed lining ring as a reaction frame for moving forward, applying 13 loading shoes to the whole ring. The segment under investigation is subjected to the actions of two loading shoes. The design thrust load and the maximum thrust load for each pad is 1160 kN and 2160 kN (leading to total ring load of 15.1 MN and 28.1 MN), respectively.

This leads to have for the adopted segment a design bending moment of 16.8 kN m (transportation phase) and a design shear action of 119.6 kN (storage phase). In addition, as already underlined in Section 1, flexural and shear resistances are also required to the segment at the final stage due to the ground pressure (geotechnical design provides the design actions as a function of soil properties), as well as for earthquake actions and fire events. With regard to the final stage, the following actions were considered: MEd = 90.3 kN m and NEd = 833.5 kN (corresponding to a tunnel overburden of 20 m), while a standard fire scenario having a duration of 120 min according to Eurocode 2 [26] was assumed. The latter could be considered reliable for a metro lining as discussed by Di Carlo et al. [27]. By varying rebar amount and material toughness, two types of reinforcement solutions were studied:

Fig. 2. Reinforcement details of RC segments. 3

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- Conventional reinforcement solution (RC): the typical amount of conventional reinforcement adopted in practice was used (Section 2.1.1). RC segments are also adopted as reference samples; - Hybrid solution (RCO + PFRC): the combination of reinforcing bars and macro-synthetic fibers was properly designed in order to obtain an optimized reinforcement solution based on the main design criteria reported in details in Section 2.1.2.

Table 1 Base concrete mix designs.

2.1.1. RC segments Two full-scale segments were produced adopting this reinforcement solution. More in details, the following reinforcing bars were adopted: - 10Ø10 curved rebars both as compression and tension reinforcement (P1 and P2) to have a double-reinforced section with a typical longitudinal reinforcement ratio (ρs) of 0.22%; U-shaped rebars were adopted at bar ends for having a proper anchorage in concrete. These rebars are placed to mainly withstand flexural actions, circumferential splitting forces, as well as to control spalling cracks; - Stirrups Ø8@15 cm with 6 legs as minimum shear reinforcement according to MC2010 (shear reinforcement ratio

ρw = 0.14% >

(0.08· fc ) fy

Base concrete

PC

PFRC

Cement Type Cement Content [kg/m3] Aggregate 0–20 [kg/m3] Water [L/m3] Water-Cement ratio Superplasticizer [L/m3] PP fibers [kg/m3] PP fibers Vf (%) Slump [cm]

CEM II/A-LL 42.5R 440 1711 185 0.42 4.40 0 0 24

CEM II/A-LL 42.5R 440 1711 185 0.42 6.16 10 1.1 22

Table 2 Segment designations, adopted concretes and material mechanical properties (CV in brackets).

= 0.10%, where fc is the cylindrical concrete

Segment designation

RC (Conventional reinforcement)

RCO + PFRC (optimized reinforcement + PP fibers)

Base concrete fc,cube [MPa]

PC 51.3 (0.04) 42.5 Ø8 = 532 MPa (0.01) Ø10 = 545 MPa (0.01) Ø8 = 620 MPa (0.02) Ø10 = 634 MPa (0.01)

PFRC 52.4 (0.05) 43.5

fc1 [MPa] fy

compressive strength and fy the yield strength of reinforcement). In fact, the value of the design shear forces are smaller than the segment shear resistance (VRd = 140 kN considering the II Level of approximation of MC2010 and a safety factor for concrete equal to 1.5); - Stirrups Ø10@15 cm with 2 legs along all segment sides to withstand radial splitting forces.

fu

1

Estimated from fc,cube.

2.1.2. RCO + PFRC segments Two segments were build using a combination of rebars and macrosynthetic fibers. Clear concrete cover and effective depth were considered once again equal to 4 cm and 25.5 cm, respectively. This reinforcement solution was composed by:

The overall steel content was equal to 100 kg/m3. The clear concrete cover on curved rebars was 4 cm, leading to a segment effective depth (d) of 25.5 cm. Fig. 2 summarizes the reinforcement details of RC segments and provide a view of the segment before casting.

Fig. 3. Reinforcement details of RCO + PFRC segments. 4

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Table 3 Main experimental results of flexural tests on RC and RCO + PFRC segments. Segment designation

RC

RCO + PFRC

Pcr [kN]

108 (Mcr = 54.0 kNm) 0.94 233 (Mmax = 116.5 kNm) 2.16 28.9 53.5

125 (Mcr = 62.5 kNm) 0.87 216 (Mmax = 108 kNm) 1.73 14.5 24.5

δcr [mm] Pmax [kN] Pmax/Pcr [-] δmax [mm] δu [mm]

to use values of ρs smaller than code prescriptions for conventional RC elements. In fact, the longitudinal steel rebar ratio adopted for RCO + PFRC segment was estimated through preliminary simplified analytical approaches [28] for evaluating the segment cross-sectional bending behavior after cracking. The latter was compared to that exhibited by RC solution in order to guarantee the same effectiveness in terms of cracking control. Finally, it is worth noting that chord rebars and fibers are also adopted in order to mutually resist the tensile stresses in the spalling area and for better controlling the corresponding cracking phenomena; - Macro-synthetic fibers for substituting the minimum amount of shear reinforcement (according to Equation 7.7–14 of MC2010 [12]) and for withstanding the local tensile stresses in the splitting zones (in case of FRC presenting a class “2e” or higher).

Fig. 4. Nominal stress vs. CMOD mean curve and result dispersion for PFRC (EN 14651 [29]).

- Top and bottom chord (connected each other at inclined segments sides) made by 3Ø10 curved rebars both as compression and tension reinforcement with stirrups Ø8@20 cm with 2 legs, leading to a double-reinforced section with ρs = 0.13%. The latter was chosen smaller than the minimum amount of flexural reinforcement ref A = 0.26· f ct = 0.17%, where quired by MC2010 for RC members ( bs,·min d

It is worthwhile noticing that RCO + PFRC solution guarantees a satisfactory behavior at serviceability and ultimate limit state with respect to the final embedded in ground condition. The feasibility of the hybrid solution was also verified by considering a typical fire scenario adequate for metro tunnel lining [27] and the simplified 500 °C isotherm method [26]. As first approximation, the post-cracking strengths

yk

the mean value of the tensile concrete strength (fct) can be estimated by Eq. 5.1-3a of MC2010 [12]). In fact, the previous research carried out by Conforti et al. [23] showed that the combination of rebars and macro-synthetic fibers can be very effective in flexure, allowing

Fig. 5. Test set-up and instrumentation details of flexural tests. 5

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Fig. 6. Flexural test: experimental response of RC and RCO + PFRC segments.

Fig. 7. Flexural test: final crack pattern (thrust side, ring side and intrados) of RC and RCO + PFRC segments.

polypropylene (PP) fibers and superplasticizer. The latter was increased in PFRC in order to guarantee a slump of 22 ± 2 cm. PP fibers were added in the amount of 10 kg/m3 (Vf = 1.10%), according to previous (and promising) results obtained with this material in hydraulic precast tunnel segments [23]. Embossed polymer macrofibers were adopted in the PFRC base concrete. Fibers are characterized by a length of 54 mm, an equivalent diameter of 0.81 mm and a consequent fiber aspect ratio of 67. Tensile strengths and elastic modulus are 552 MPa and 6000 MPa, respectively, while the fiber density is 910 kg/m3. The mix-design was provided by a local ready-mix concrete supplier that produced the two concrete batches; both base concretes showed a good workability (Table 1). Segments were casted in layers (up to 30 cm thick) from their centre and consolidated by vibration mould system. After two days, segments were removed from steel moulds and stored at laboratory environmental conditions until testing (after 30 days). In

due to PP fibers contribution were neglected; the use of macro-synthetic fiber is possible in metro lining in combination with traditional steel rebars, since these latter could be exploited in case of fire events. Fig. 3 shows in detail the reinforcing bars adopted in this solution, providing also a picture of a RCO + PFRC segment before casting. The overall steel content is significantly smaller (−60%) as compared to RC segments, as it can be clearly appreciated in Fig. 3. This significantly simplifies the steel cage construction and facilitates its positioning. 2.2. Materials Table 1 shows the mixture proportions of the two base concrete adopted. Conventional concrete (PC) was adopted for RC segments, while a polypropylene fiber reinforced concrete (PFRC) was used in case of RCO + PFRC segments. PFRC differs from PC for both 6

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drop of about 25%, the behavior is characterized by a progressive residual strength increment up to a CMOD of about 3 mm that leads to a residual strength (fR,2, fR,3 and fR,4) higher than fL. The characteristic values of the residual strength parameters are required by MC2010 for design purposes. Considering fR,jk = fR,j ⋅ (1–1.64 CV), the following characteristic values are obtained: fL,k = 3.89 MPa, fR,1k = 2.78 MPa and fR,3k = 3.99 MPa; based on these parameters PFRC can be also classified as “2.5e” according to MC2010. Thus, the PFRC adopted herein widely fulfills the requirements of MC2010 for structural materials. 3. Flexural tests The flexural behavior of segments was evaluated by a three-point loading test characterized by a net span of 200 cm. Fig. 5 summarizes the test set-up, providing also a lateral view of a segment before testing. The two supports were continuous on the entire segment width, while the load was applied at segment extrados by means of neoprene layers (1 cm thick) under four steel plates 22 × 14 cm. In order to ensure a good distribution of the load along segment width, a statically determined loading scheme was adopted for distributing the load applied by the hydraulic jack (loading capacity of 1000 kN). Fig. 5 also shows the instrumentation details and a bottom view of a segments. Three wire transducers were placed on segment intrados to measure the mid-span deflection at segment center (D2) and sides (D1 and D3), while the flexural crack opening at the maximum bending moment was measured by two linear variable differential transducers (LVDT, W1 and W2). All tests were carried out by using a quasi-static loading rate of 0.3 mm/min. During the tests, particular attention was also devoted to the evaluation of the crack pattern development along both segment length and width at different loading steps. 3.1. Experimental results and discussion of flexural tests Both reinforcement solutions led the segments to a collapse characterized by rebar failure before concrete crushing. Table 3 summarizes the main experimental results in terms of: -

Fig. 8. Test set-up and instrumentation details of point load tests.

Flexural cracking load (Pcr); Mid-span deflection at Pcr (δcr); Flexural maximum load (Pmax); Pmax over Pcr ratio; Mid-span deflection at Pmax (δmax); Mid-span deflection at rebar failure (δu).

The bending moments related to these loads are also shown in Table 3 (i.e. Mcr and Mmax). In addition, Fig. 6 shows the experimental curves of load vs. mid-span deflection response and load vs. crack width of RC and RCO + PFRC segments. The mid-span deflection of each segment was calculated as mean value of the deflection measured by the three transducers D1, D2 and D3 (it should be noticed that no significant differences were observed during both tests between D1, D2, D3). Two flexural cracks were measured by W1 instrument after a load of 120 kN in the RC segment, while in the other cases only one flexural crack was captured. It can be observed that the combination of fibers and a low amount of conventional reinforcement allowed the RCO + PFRC segment to reach a maximum bearing capacity (Pmax = 216 kN) comparable to the one of RC samples (Pmax = 233 kN), underlining the efficiency of the hybrid reinforcement. No fiber influence was instead observed on the cracking load (as expected). In Section 2.1, a design moment of 16.8 kN m (corresponding to a load P = 33.5 kN according to the test set-up adopted) was assumed as a reference value for guaranteeing the worker safety in case of exceptional situations. It can be noticed that both reinforcement solutions exhibit a noticeable higher bearing

both concrete batches 12 cubes (15 cm side) were cast to evaluate the concrete compressive strength; ten small beams (15 × 15 × 60 cm) were also produced in case of PFRC to characterize its post-cracking mechanical properties according to EN14651 [29]. Table 2 summarizes the cubic (fc,cube) and cylindrical (fc) compressive strength of both concrete batches, where fc was estimated from fc,cube. It can be observed that both concretes are characterized by a very similar compressive strength, which is around 52 MPa. Steel B500C [26] was used for the reinforcing bars; the yielding and ultimate tensile strength (obtained by testing three samples according to EN 15630-1 [30]) are summarized in Table 2. Concerning the post-cracking behavior of PFRC, Fig. 4 shows the mean experimental curve as determined from beam tests (according to EN14651 [29]), in terms of nominal stress vs. Crack Mouth Opening Displacement (CMOD). Mean values and coefficient of variations (CV) are also reported for the limit of proportionality (fL) and the residual strengths at CMOD values of 0.5, 1.5, 2.5 and 3.5 mm (fR,1, fR,2, fR,3 and fR,4). It can be observed that macro-synthetic fibers provide a significant post-cracking resistance to concrete; in particular, after a nominal stress 7

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Fig. 9. Experimental response of segments (single shoe load vs. mean vertical displacement) subjected to point load test.

4. Point load tests

capacity than the design load. In both segments the behavior after cracking was similar (see the ratios Pmax/Pcr in Table 3) as a result of a similar post-cracking stiffness (Fig. 6a); comparable maximum crack openings (Fig. 6b) were also observed up to the initiation of crack localization (δmax = 14.5 cm) in the RCO + PFRC segment. The crack localization is a phenomenon that may occur in elements reinforced by rebars and fibers after rebar yielding (depending on longitudinal reinforcement ratio and FRC toughness), leading to a crack to widen quicker than the other ones [31]. Both reinforcement solutions provide also significant ductility after segment cracking. The latter can be expressed in terms of displacement ratio between δu and δcr; since δcr was similar in both segments, a direct comparison between the values of δu, shown in Table 3, can be carried out. It can be observed that the RC segment is characterized by a significantly higher ductility as compared to the RCO + PFRC one, even if the ductility provided by hybrid reinforcement solution is adequate for tunnel segment. This ductility reduction is due to the crack localization occurred in the RCO + PFRC segments, which led to an early failure of curved rebars. However, it can be observed that, in RCO + PFRC segment, the load decreased in a stable way in the range between δmax and δu, remaining always higher than Pcr. Fig. 7 shows the final crack patterns of RC and RCO + PFRC segments at the thrust face, intrados and ring side. The crack development in both segments was characterized by multiple flexural cracking, even if these cracks tend to propagate in a different way through the segment width probably due to the different layout of longitudinal rebars. In the RC segment, a progressive bifurcation and deviation of some cracks was observed, leading to a slightly smaller value of the mean crack spacing at the segment central part (13.1 cm), as compared to the sides (14.8 cm); a similar behaviour was observed on wide-shallow beams by Conforti et al. [32]. The RCO + PFRC segment showed a higher number of cracks along the edges (where reinforcement is present) as compared to the central part, due to the mutual collaboration of fiber and rebars in the edges [28]. In particular, a mean crack spacing of 13.0 cm and 12.4 cm were detected on thrust and ring sides, whereas in the central part of the segment it was around 16.5 cm. However, a similar mean crack spacing was observed in both reinforcement configurations (i.e. 14.2 cm and 14.0 cm for RC and RCO + PFRC segment, respectively).

Segments were subjected on the thrust side to the actions of two loading shoes (#1 and #2), as shown in Fig. 8. The ring side of the segments was instead placed on a continuous support made by a steel plate 2.5 cm thick over a reinforced concrete beam. Hydraulic jacks with a loading capacity of 2000 kN were coupled on each shoe, leading to a test in load control (as it happens during TBM operation) with a maximum capacity of the system of 8000 kN (4000 kN per shoe). The load was applied through a self-supporting steel frame and it was measured continuously by means of full bridge pressure transducers. More details about the adopted reaction frame can be found in Di Carlo et al. [25]. Both shoe dimensions (77 cm long and 30 cm wide) and their relative distance (145 cm from center to center) were the same to the ones adopted in the construction of the real Metro tunnel. Due to the presence of joints on segment sides for the gaskets, the area of loading shoes in contact with concrete results 20 cm wide in the radial direction. It should be also noticed that, hereafter, the loads regarding point load tests will be reported as single shoe load. Fig. 8 also summarizes the instrumentation adopted on both intrados and extrados sides. In particular: - four potentiometric transducers (V#1-int, V#2-int, V#1-ext and V#2-ext) were placed under the two loading shoes to measure the vertical shortening; - two LVDTs (Spalling-int and Spalling-ext) were arranged between the two loading shoes to capture the spalling crack opening on both intrados and extrados sides; - two potentiometric transducers (Splitting#1 and Splitting#2) were horizontally settled at a distance of 55 cm thrust side (maximum circumferential tensile stresses are theoretically expected in this zone according to the Iyengar’s elastic solution [33]) on segment intrados to measure the splitting crack opening under both loading shoes. A 10 cm grid was drawn on segment intrados to facilitate the identification of the final crack pattern and its development. In particular, the crack pattern was evaluated at the following load levels: 1000 kN, spalling crack load (Pspalling), 1160 kN, splitting crack load under loading shoe #1 and #2 (Psplitting,#1 and Psplitting,#2), 1500 kN, 2160 kN, 2500 kN, 2750 kN, 3000 kN and 3500 kN. Concerning the loading modality, after two preliminary elastic cycles all tests were 8

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Fig. 10. Point load test: evolution of the crack pattern for RC and RCO + PFRC segment (single shoe load).

that, when reaching the design (1160 kN) and maximum thrust loads (2160 kN), the segments were unloaded (Fig. 9). Fig. 10 shows the crack pattern of both segments at different load levels (see load levels in Section 4), underlining both the crack evolution by consecutive numbers and the load at which the main cracks appeared; it should be noticed that both reinforcement solutions showed a similar crack pattern evolution. The spalling crack (between loading shoes at thrust side) took place at a load of 1210 kN and 1095 kN for RC and RCO + PFRC segments, respectively and progressively propagated until reaching the lifting socket at a load of about 2000–2500 kN. At this range of loads, in both segments, a splitting crack occurred under the loading shoe #2. A splitting crack arose also under the loading shoe #1 at a load slightly higher than 2500 kN. In the meantime, and for higher loads, multiple cracking developed in both segments in the spalling zone, leading to 2 and 3 cracks (in this area) for RC and RCO + PFRC segments, respectively. This confirms that the combination of conventional rebars and fiber reinforcement in the two chords (RCO + PFRC) results in an adequate control of local spalling

carried out using a quasi-static loading rate. Segments were also unloaded after reaching both design (1160 kN) and maximum loads (2160 kN) in order to evaluate the residual crack widths through the instrument measurements and the visual inspection with a crack width comparator.

4.1. Experimental results and discussion of point load tests Fig. 9 shows the load vs. mean vertical displacement (between intrados and extrados measurements) under both loading shoes (#1 and #2) for RC and RCO + PFRC segments. It can be observed that both reinforcement solutions allowed segments to reach the maximum capacity of the loading system, i.e. 4000 kN. The latter is significantly higher than the design and maximum values of thrust loads per shoe (underlined in black dotted line in the Fig. 9) and it was reached by both segments showing a very similar stiffness. In fact, the vertical displacement was about 0.36 mm and 0.38 mm at the maximum applied load for RC and RCO + PFRC segment, respectively. It is worth noticing 9

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Fig. 11. Comparison between RC and RCO + PFRC segments in terms of load vs. mean spalling crack width (a) and load vs. splitting crack width (b).

crack under the loading shoe #2 in Fig. 10). By comparing the splitting crack width in RCO + PFRC against the one of RC samples (Fig. 11b), it can be observed that the splitting crack under the loading shoe #2 was higher in RCO + PFRC segment, since in this segment the crack occurred earlier (2005 kN vs. 2485 kN, RCO + PFRC vs. RC, see Fig. 10). To the contrary, splitting cracks under the loading shoe #1 occurred at similar load levels in both RC and RCO + PFRC segments; in the latter configuration the crack opening resulted always smaller than in the RC one. Therefore, both reinforcement solutions guaranteed the force equilibrium and a good control of splitting cracks up to maximum capacity of the loading system (which is about twice the maximum thrust load).

cracks between the TBM rams. Finally, only in case of RC segment, a further splitting crack was observed under the loading shoe #1 for a higher load level (i.e. 3490 kN). Fig. 11 shows the development of the mean spalling crack (between intrados and extrados measurements, Fig. 11a) and splitting crack (Fig. 11b) widths as a function of the applied load. It can be observed that in both segments the maximum crack opening is always represented by the spalling crack; hence, it is recommended to carry out a control of spalling crack width during thrust jack phase for three main reasons: appearance, leakage and corrosion. In this regard, in both reinforcement solutions, after the unloading procedure at 1160 kN (design load) and 2160 kN (maximum thrust load), the residual cracks were always significantly smaller than 0.05 mm and hardly detectable by naked eyes. The maximum value of spalling crack was comparable in RC and RCO + PFRC segments (Fig. 11a), even if it resulted slightly higher in the hybrid reinforcement solution (+15%). The spalling crack developed in a different way in the two considered segments, probably due to the different loads at which the main cracks appeared (Pspalling, Psplitting,#1 and Psplitting,#2). In fact, both splitting and spalling cracking occurred for lower load levels in the segment with hybrid reinforcement. At the design load, the spalling crack was not present in RC segment while it was very small in RCO + PFRC one and then developed slowly in RC segment as compared to RCO + PFRC (Fig. 11a). After a load of 2500 kN, when both spalling and splitting cracks appeared in both segments and the global crack pattern was almost complete, the spalling crack widths in the two segments were comparable and their subsequent growing were characterized by a similar stiffness. Concerning the spalling crack propagation along segment width, shown in Fig. 10, it can be underlined that a slightly higher crack depth was observed in RCO + PFRC segment (49.8 cm), as compared to RC one (40 cm), even if a more diffused multiple cracking was observed in the RCO + PFRC sample. This confirm that both reinforcement solutions were able to control the cracking phenomenon arising in the spalling area, even if a slightly better performances were provided by traditional reinforcement solution (RC). Splitting cracks are not present at design thrust load but they could appear before the maximum thrust load is reached (Fig. 11b). However, the development of these cracks was well controlled by macro-synthetic fibers (RCO + PFRC segment) by a transverse stress redistribution that continuously provide equilibrium with the external applied load (see the progressively development along specimen width of the splitting

5. Concluding remarks In this paper, the structural applicability of macro-synthetic fibers in precast tunnel segments for metro tunnel lining (internal diameter of 5.8 m) was studied by means of an experimental program on full-scale specimens. Two reinforcement solutions were studied by varying the segment reinforcement: a conventional solution generally adopted in practice with rebars (RC segments) and a hybrid solution made of a combination of rebars and macro-synthetic fibers (RCO + PFRC segments). Based on these experimental results, the following conclusions can be drawn: (1) A combination of FRC class “2.5e” (according to fib Model Code 2010) and a low amount of conventional bars (steel content of about 40 kg/m3) was found to be a possible reinforcement solution of precast tunnel segments for metro tunnel lining. This underlines that a proper combination of the two types of reinforcement is able to guarantee the required segment performance. (2) Under flexure, both reinforcement solutions were able to provide a similar bearing capacity and a significant ductility after cracking. However, the final crack pattern of RCO + PFRC segment resulted different to the one of RC sample, since a higher number of smaller cracks was observed at chord zone (due to bar location) as compared to the central part. (3) During the thrust jack phase, segments reinforced by either conventional or hybrid reinforcement showed a very similar structural response both in terms of bearing capacity, stiffness and crack width control. For the hybrid reinforcement solution, tensile forces 10

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A. Conforti, et al.

in splitting zones are well-resisted by macro-synthetic fibers only, while spalling crack control is guaranteed by an optimized combination of fibers and low amount of curved rebars along the segment edges (even smaller than the minimum amount generally required by codes for RC elements). (4) Point load tests showed that, with both reinforcement solutions, the maximum crack width during TBM thrust jack phase always occurred in the spalling crack, even for high load level. This experimental evidence underlines the importance of controlling cracks in the segment area between loading shoes during TBM operations.

[11]

[12] [13] [14] [15] [16]

Further studies should be carried out introducing in the point load tests possible jack eccentricities as well as possible non-uniform force distributions in order to evaluate the effectiveness of the proposed hybrid reinforcement solution also in presence of unfavorable conditions.

[17]

[18] [19]

Acknowledgements [20] [21]

The Authors are grateful to BASF Construction Chemicals Italy for the financial support; a special acknowledgement goes to Dr. Martin Hunger (BASF Construction Solutions GmbH, Germany) and to Dr. Sandro Moro (BASF Construction Chemicals Italy).

[22]

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