Failure experiment on CFRP-strengthened prestressed concrete cylinder pipe with broken wires

Highlights

• Externally bonded CFRP for repair of PCCP was investigated.

• A full-scale failure test was conducted.

• The performance of repaired PCCP was observed at different wire breakage ratios.

• Repaired PCCPs with 18 % broken wires can withstand a pressure of 1.27 MPa.

Abstract

The prestressed concrete cylinder pipe (PCCP) has been used widely in hydraulic engineering. However, due to hydrogen embrittlement and corrosion, the prestressing wires in PCCP can break and cause pipe failure. In this study, a retrofitting technique of using externally bonded carbon fiber reinforced polymer (CFRP) to repair PCCP was proposed. The technique is particularly useful when shutting down water is not possible. The proposed strategy was evaluated by using a full-scale PCCP with a 2.8 m inside diameter loaded with internal water pressure. First, the PCCP strains before and after repair were compared under different internal pressures. In the presence of CFRP, the strains near the broken wires significantly decreased under the same internal pressure, and the carrying capacity of the PCCP increased. PCCPs with 13 %, 15 %, and 18 % wire breakage ratios were then evaluated and compared. The strain in each material increased with increasing wire breakage ratio, and the maximum strain in CFRP was observed at the location of wire breakage. Considering the PCCP with 18 % wire breakage ratio, the section 2300 mm from the pipe end was in the linear state under an internal pressure of 0–0.9 MPa, and the PCCP reached its serviceability and elastic limit states when the internal pressure increased to about 1.0 MPa and 1.2 MPa, respectively. Finally, when the internal water pressure was increased to 1.27 MPa, which was higher than the internal design pressure of 1.12 MPa, the PCCP failed due to the rupture of CFRP.

Introduction

The prestressed concrete cylinder pipe (PCCP) is a large-diameter water-supply pipe capable of withstanding high internal pressure and being buried deep in soil. The pipe is composed of mortar coating, prestressing steel wires, a steel cylinder, and a concrete core. The performance of PCCP is determined by the compression resistance of the concrete, the tensile properties of the prestressing steel wires, and the impermeability of the steel cylinder. Prestressing wires are generally used to provide prestressing stress to the concrete core of PCCP to improve its bearing capacity. PCCPs can be divided into two types according to the position of the steel cylinder in the pipe: lined cylinder pipe (LCP) and embedded cylinder pipe (ECP) (American Water Works Association, 2014, Ge and Sinha, 2014, Hu et al., 2019c). Since its invention 70 years ago, PCCP has become the main pipe of choice in many water delivery and water diversion systems around the world due to its many outstanding advantages.

Different loads such as internal and external pressures can affect PCCP; so, numerous experiments have been conducted to analyze the mechanical characteristics of PCCP under different loads (Zarghamee et al., 1993a, Zarghamee et al., 1993b, Zhai et al., 2021b, Li et al., 2022, Zhai and Moore, 2023). In several studies conducted by Hu, 2009, Hu and Liu, 2012, Hu et al., 2010, the failure modes of PCCP under overload conditions were analyzed, the influence of concrete cracks on its bearing capacity was discussed, and the specific process and characteristics of failure were expounded. Dou et al., 2017, Dou et al., 2018, Cheng et al., 2020 used the Brillouin optical time-domain analysis (BOTDA) and fiber Bragg grating (FBG) fiber sensing techniques to continuously measure the structural response of each layer in a pressurized PCCP pipeline. Using a full-scale test, Zhang et al. (2018) described the failure phenomena and analyzed the failure mode of jacking prestressed concrete cylinder pipe (JPCCP) under combined load conditions.

After long-term service, the prestressing wires in PCCP are prone to breakage, which in turn induces pipe failure and causes water wastage and economic loss. Therefore, many studies on wire breakage in PCCP have been carried out. Through the finite element method, Zarghamee et al., 2002, Zarghamee, 2003 analyzed the influence of wire breakage on PCCP under loadings of internal pressure, self-weight, and soil. The results show that the final failure mode of the pipe was the progression of wire breaks due to the increase in the stresses in the wires, rather than concrete crushing or major leakage. Ge and Sinha (2015) proposed a theoretical method for calculating the length of prestressing wire loss, which was thereafter validated by the finite element method. Hajali et al., 2015, Hajali et al., 2016a, Hajali et al., 2016b) carried out a series of studies to evaluate the reliability of PCCP with broken wires by considering both the location and number of wire breakages and the interactions between adjacent PCCPs. Hu et al. (2019a) applied both tests and simulations to analyze the effect of wire breakage on the stress re-distribution of PCCP, and they also studied the load-carrying capacity of PCCPs with different wire breakage ratios.

The replacement of PCCPs with broken wires is costly and time-consuming; so, it is considered unsuitable for many projects. Therefore, PCCP reinforcement technology is preferred by many engineers (Zhao et al., 2019). Carbon fiber reinforced polymer (CFRP) is widely used in the field of civil engineering as an external reinforcement since it is lightweight, strong, and simple to construct. It is also used as a lining in the repair of PCCP (AWWA C305, 2018). Lee and Karbhari (2005) studied the mechanical properties of PCCP repaired by lining with FRP under external and internal pressures using PCCP section tests. Gipsov (2013) introduced quality assurance methods and processes for lining with CFRP in various engineering examples. Based on their tests, Wasco et al. (2016) expatiated on the necessary inspections and steps for effectively repairing PCCP by lining with CFRP. Hu et al., 2018, Hu et al., 2019b) employed a full-scale test and the 3D finite element (FE) model to investigate repaired PCCP by simulating the complex CFRP-concrete bonded interface. They also explained how their results can be helpful for strengthening PCCP.

Although the above-mentioned studies have made great contributions to CFRP-lined PCCP, adding the CFRP lining requires the water supply to be interrupted and is infeasible for PCCPs with small inside diameters that limit internal access. Therefore, the method of using externally bonded CFRP to repair PCCP was proposed and applied in some projects (Geisbush, 2013, Geisbush, 2015, Zhai et al., 2019). The mechanical properties of PCCP strengthened by CFRP were studied by Zhai et al., 2020, Zhai et al., 2021c, Zhai et al., 2021a. However, relevant studies on this repair method are rather limited, and the performance changes of PCCP before and after repair are not clear. Also, the failure mechanism of PCCP repaired using externally bonded CFRP is unknown. Therefore, a full-scale study on a PCCP with an inside diameter of 2.8 m was carried out in this study. First, 10 % of the wires in the PCCP were cut, and the PCCP was pressurized to the working internal pressure to simulate its actual working state. After the pressure was relieved, two layers of CFRP were externally bonded to repair the PCCP; thereafter, the pressure was increased to the working state. The strains in the PCCP before and after the repair were compared. The wires were then broken continuously, and the effects of different wire breakage ratios on the CFRP and the PCCP under the working internal pressure were tested. The mechanical characteristics of the materials under different wire breakage ratios were determined. The limit state of the pipe was studied based on concrete microcracking and steel wire stress, and the damage process and failure mode of the repaired PCCP were analyzed.