Tunnels are an essential infrastructure which contributes to transportation and shipping in society. The fundamental functionality of tunnel systems depends on the structural integrity and durability of lining systems which serve as protection against geological conditions and significant loads. The case study by Abbas, Soliman, and Nehdi seeks to investigate the structural behavior of precast concrete tunnel lining (PCTL) consisting of reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC). Structural integrity assessments consisted of flexural monotonic, cyclic load, and thrust load tests (Abbas et al. 501).
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PCTL has been increasingly used in tunneling construction due to efficiency and economic benefits that it brings in comparison with traditional in-place lining techniques. PCTL increases the speed of construction and ensures higher quality due to more efficient control and mechanisms during fabrication in precast plants and ultimately has less produced waste. One of the primary concerns in PCTL segments is the durability of steel in RC due to chloride ion exposure. Chloride ion penetrates and degrades reinforcement structures through groundwater and cracks in concrete, thus resulting in corrosion due to moisture and oxygen. Meanwhile, SFRC is a relatively new technology which allows preserving the PCTL structure. The processes of assembly and handling prevent the formation of cracks. Furthermore, the steel fibers are anti-corrosive and considered the best for structural integrity. Therefore, the significance of the case study research was to evaluate the mechanical response of traditional RC techniques in comparison to SFRC lining segments.
The experimental procedure consisted of selecting tunnel segments consisting of RC and SFRC lining. Core segments were taken from the segments to determine the compressive and tensile strengths of cast concrete. Flexural testing was conducted by construction a reaction frame with a span of 3000mm out of long stiffened I-beams welded together. A uniformly distributed load was applied to the frame with incremental loads of 2 kN/min. Cyclic compressive loads were measured in the context of percentage of maximum displacement, increasing until the segment broke apart. The thrust load test was conducted by placing the segment on a surface and applying incremental loads of 10 kN/min to the elongated edge. For both tests, linear variable displacement transducers (LVDTs) were used to measure load and displacement readings.
Results were evaluated based on average readings of five core samples from both RC and SFRC segments. Compressive strength was at 61.4 MPa for SFRC in comparison to 60.0 MPa for RC. The splitting tensile strength was recorded at 9.0 MPa for SFRC over 7.5 for RC. The addition of steel fibers increases tensile strength by 20%. The flexural test showed the free crack load for SFRC at 71 kN with 1.6mm displacement and 0.10mm crack width. Meanwhile, RC showed lower crack load at 45 kN with 1.8mm displacement 0.20mm crack width. Flexural cyclic behavior showed that the SFRC segments are more durable and more efficiently able to distribute energy and load. The RC segment has 2.95 times higher displacement ductility. However, both showed the similar result on thrust load tests.
PCTL segments are designed with the criteria to bear the force created by construction loads, which can depend on soil characteristics. Thrust loads created by TBM are especially critical for structural integrity. Segments may be subject to loads from handling, storage, and lifting processes of construction. During earthquakes, tunnels are better equipped to handle the stress because of its location. While buildings and bridges move independently of the ground motion, a tunnel is restrained. Therefore, design thrust and bending movement remain a critical evaluation for seismic activity. However, both RC and SFRC lining meets appropriate architectural criteria.
In conclusion, SFRC tunnel lining is most effective for serviceability and limiting cracks due to corrosion. This type responds better to handling and installation due to higher crack resistance. Despite having a lower peak load, SFRC segments showed steady decline rather than failing abruptly. Meanwhile, RC segments have higher energy dissipation and displacement ductility with an overall better load-carrying capacity. SFRC remains a promising technology which can be optimized to meet desired characteristics and overall traditional RC segments in comprehensive structural behavior.
Abbas, Safeer, et al. “Mechanical Performance of RC and SFRC Precast Tunnel Lining Segments: A Case Study.” ACI Materials Journal, vol. 111, no. 5, 2014, pp. 501-510.
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