Numerical Study on Mechanical Properties of Corroded Concrete Pipes before and after Cured-in-Place-Pipe Rehabilitation

Abstract

Cured-In-Place-Pipe (CIPP) rehabilitation technology is widely utilized in pipeline rehabilitation projects and has exhibited favorable results. Nevertheless, the mechanical characteristics of pipelines after CIPP rehabilitation and the effectiveness of CIPP rehabilitation in repairing these mechanical characteristics remain unknown. To address these issues, a three-dimensional numerical model of a corroded concrete pipe before and after CIPP rehabilitation was established in the present study. To authenticate the accuracy of the numerical model, the numerical simulation data were compared with the full-scale test data from prior research, and the comparison outcomes show that the numerical model formulated in this study is reasonable and reliable. To appraise the repair effectiveness of CIPP rehabilitation, the mechanical properties of a corroded pipe, a CIPP-repaired pipe, and a normal pipe under traffic load were computed and compared, and the comparison outcomes demonstrate that the stress in the pipe bell, stress in the pipe spigot, vertical displacement of the pipe crown, and vertical displacement of the pipe invert were reduced by 39.8%, 16.7%, 24.7%, and 24.4%, respectively, after CIPP rehabilitation. Moreover, a series of three-dimensional numerical models were constructed to scrutinize the impacts of factors such as corrosion degree, corrosion angle, and traffic load on the mechanical properties of corroded pipelines before and after CIPP rehabilitation. The findings indicate that the stress on the pipe escalates with increasing corrosion degrees and diminishes with increasing corrosion angles; there are no noteworthy differences between the vertical displacement of the pipe and the von Mises stress of the CIPP liner for diverse corrosion degrees and corrosion angles; the amplification of the traffic load will augment the stress and displacement of the pipe and increase the rotation of the pipe, resulting in a significant upsurge in the stress of the CIPP liner at pipe joints. When the traffic load magnitude rises from 0.7 MPa to 1 MPa, the stress and displacement of the pipe and the von Mises stress of the CIPP liner were increased by 18.9%, 42.3%, and 42.1%, respectively.

1. Introduction

Urban underground pipelines are commonly referred to as “underground lifelines” due to their critical role in facilitating urban sewage and rainwater discharge [1]. As an indispensable part of the infrastructures of cities, the importance of these pipelines cannot be overstated [2]. With the accelerated urbanization process in China, the development of urban underground drainage pipelines has also been rapid [3]. As of 2021, the total length of drainage pipelines in China has reached 872,000 km, with pipelines built prior to 2012 accounting for 50.3% of the total [4]. However, due to long periods of operation and inadequate repair management, these pipelines are susceptible to various defects [5,6]. If not repaired in a timely manner, these defects can lead to pipeline leakage, environmental pollution, and even road collapse, resulting in significant societal and economic losses [7].

Corrosion is considered to be the primary structural defect in drainage pipelines [8]. Corrosion is a prevalent issue that reduces the wall thickness and structural performance of pipes, ultimately impacting the safety of the pipeline [9]. Erickson [10] observed that sewage in pipelines generate hydrogen sulfide under the influence of anaerobic bacteria. The hydrogen sulfide eventually escapes to the crown of the pipe and forms sulfuric acid molecules that dissolve into droplets due to temperature differences. The crown of the pipe then gradually changes from alkaline to neutral, forming a bean curd-like watery gypsum layer under the long-term effect of sulfuric acid. The corrosion process of the drainage pipe is depicted in Figure 1. To investigate the mechanical behavior and failure pressure of corroded pipes, Han et al. [11] conducted numerical simulations of pipes with single and multiple internal corrosion damage locations. Xu et al. [12] utilized the finite element method to study the failure behavior of pipelines suffering from corrosion defects and used artificial intelligence to predict burst pressure. Zhou et al. [13] evaluated the failure probability of pressure pipelines with corrosion defects and analyzed the sensitivity of the failure probability to factors such as time and corrosion through a combination of experiments and simulations. These studies provide evidence that corrosion diminishes the bearing capacity of pipelines, increases the likelihood of pipeline failure, and poses a significant risk to pipeline safety.

Eliminating the impact of corrosion on pipeline safety is an urgent task [14], and CIPP rehabilitation has been recognized as an effective repair method to improve the load carrying capacity and extend the service life of pipelines [15]. CIPP rehabilitation involves pulling a hose impregnated with thermosetting material through a defective pipe, inflating it with inflation equipment, and curing it with UV light to form a new pipe inside of the existing one [16]. This technology has become widely used in trenchless pipeline rehabilitation because of its advantages, such as the lack of need for grouting, short construction time, and minimal flow area loss [17]. As CIPP technology has been increasingly used, many scholars have investigated its application and evaluation. Thomas et al. [18] developed an analytical model to evaluate the long-term structural behavior of CIPP liners subjected to external hydrostatic pressure in a gravity drainage pipe after CIPP rehabilitation. Reda et al. [19] developed mathematical equations between annular flow and annular space in lined pipes based on field tests on different forms of lined rehabilitated pipelines. Li et al. [20] developed an analytical model based on the theory of minimum potential energy to simulate an elastically flexural CIPP liner subjected to external hydrostatic pressure and analyzed the effect of CIPP liner thickness variation due to corrosive liquids or gases on the performance of CIPP liners. Hani Alzraiee et al. [21] investigated the life expectancy of drainage pipes after CIPP rehabilitation by analyzing the results of mechanical damage tests on drainage pipes after CIPP rehabilitation. Argyrou et al. [22] experimentally investigated the effect of large-scale faults caused by earthquakes on the operation of CIPP to assess the seismic performance of CIPP liners. Shou et al. [23] used numerical methods to study the stresses and displacements of buried corroded pipes before and after CIPP rehabilitation under internal pressure and analyzed the effect of CIPP liner wall thickness on the mechanical properties of pipes.

After conducting extensive research of the literature, it became evident that the long-term operation of pipelines without timely repair measures has led to the widespread occurrence of corrosion and other damages, resulting in significant societal losses. To evaluate the impact of corrosion on pipeline operation, numerous scholars have employed numerical methods, experimental approaches, and artificial intelligence techniques to investigate corrosion patterns, failure probabilities, and the expected lifespans of corroded pipelines. In order to mitigate the detrimental effects of corrosion, CIPP rehabilitation technology has been widely adopted in engineering practice. Researchers have conducted various studies on construction improvement, performance evaluation, material properties, life prediction, and thickness optimization of CIPP rehabilitation technology. However, there is a notable lack of theoretical studies addressing the assessment of safety conditions and the prediction of expected lifespans of pipelines after CIPP rehabilitation. In response to these engineering challenges, this paper presents four research endeavors aimed at addressing these issues. First, a three-dimensional numerical model of the corroded pipe before and after CIPP rehabilitation was established using Abaqus finite element software. Second, the accuracy of the numerical model was verified based on full-scale test results from previous studies. Then, the mechanical properties of the corroded pipe, the CIPP-repaired pipe, and the normal pipe were calculated and compared to evaluate the repair effect of CIPP rehabilitation. Finally, a series of three-dimensional numerical models were established to investigate the effects of factors such as corrosion degree, corrosion angle, and traffic load magnitude on the mechanical properties of the corroded pipe and the CIPP-repaired pipe. The results of this research provide a theoretical basis for the evaluation and management of pipelines after CIPP rehabilitation.

2. Three-Dimensional (3D) Finite Element Model

2.1. Model Description

In this study, a 3D finite element model was developed consisting of four components: soil, pipe, gasket, and CIPP liner, as shown in Figure 2. The model dimensions are 17.2 m × 8 m × 7.5 m (length × width × height), with the soil simulated using the Mohr–Coulomb model. The material of the pipeline is C30 concrete, and the Concrete Damaged Plasticity (CDP) model proposed by Lee and Fenves [24] was used for simulation. The material parameters of CDP are shown in

Table 1. Material parameters of CDP.

Dilation Angle	Eccentricity	fb0/fc0	κ	Viscosity Parameter
30	0.1	1.16	0.6667	0.005

, and the uniaxial compression stress–strain and tensile stress–strain curve of the CDP model are shown in Figure 3. The gasket is made of compressible hyperelastic material, and the Mooney–Rivlin strain energy function was used for simulation. The CIPP liner material is a composite made of glass fiber and thermosetting resin, and the linear elastic constitutive model was used for simulation. All components involved in the model are deformable solid parts, and it is assumed that all components have homogeneous properties when assigning material properties to components.

Table 2. Material parameters.

Material	ρ (kg/m3)	E (MPa)	ν	c (KPa)	ψ (°)
Soil	1732	28.9	0.3	17.3	25.2
CIPP liner	1600	8800	0.3	--	--
Pipeline	2300	30,000	0.2	--	--

shows the material parameters used in the model. In the table, ρ is the density of the material, E is the modulus of elasticity, c is cohesion, and ψ is the internal friction angle.

2.2. Mesh Sensitivity and Convergence Analysis

In this study, Hypermesh was utilized for meshing to improve the mesh quality of the model, and the model was then imported into Abaqus as independent meshes for computation. The mesh geometry is depicted in Figure 4. Previous research has demonstrated that the C3D8R mesh type is less prone to shear self-locking under bending loads, yielding more accurate displacement results. Furthermore, when the calculated mesh exhibits distortional deformation, employing the C3D8R mesh type improves calculation accuracy [25,26]. Considering these advantages, the C3D8R mesh type was selected for this study, with mesh hourglass control. To determine an appropriate element size, element sensitivity analysis was conducted in this section, and five element sizes were chosen for numerical calculations, as shown in

Table 3. Mesh dimensions.

Element Number	Element 1	Element 2	Element 3	Element 4	Element 5
Maximum size of soil/m	0.1	0.2	0.3	0.4	0.5
Maximum size of pipe/m	0.01	0.02	0.03	0.04	0.05
Maximum size of CIPP/m	0.001	0.002	0.003	0.004	0.005

. To assess the influence of the element size on the calculation results, the computed results using different element sizes were extracted and compared with the experimental data of Fang et al. [27], as illustrated in Figure 5. In the figure, “C” represents the error at the crown of the pipe, “I” represents the error at the invert of the pipe, and “S” represents the error at the springline of the pipe.

The figure illustrates that the error in the calculation results rose as the element size increased. Notably, Element 4 and Element 5 exhibited considerably higher computational errors compared to the other elements. Conversely, the computational errors for Element 1, Element 2, and Element 3 show smaller differences, indicating that element size stabilized around Element 2. Taking into account both computational efficiency and accuracy, Element 2 was chosen as the element size for the numerical model in this study, following the calculation of element sensitivity.

2.3. Pipe Model

The pipeline model consists of eight sections of pipes with a cover depth of 1m, where P3, P4, P5, and P6 are corroded pipes, and the remaining sections are normal pipes, as depicted in Figure 2 (where P_n denotes the nth section of pipes and J_n denotes the n_th pipe joint). The geometry parameters of the pipe were selected based on the Chinese code for concrete and reinforced concrete sewer pipes (GB/T 11836—2009), as shown in Figure 6; the unit in Figure 6 is mm. Liang et al. [28] conducted an investigation on the mechanical properties of corroded pipes using sulfuric acid corrosion, and they concluded that the concentration of the corrosive medium in the pipe wall decreases with the increase of corrosion depth. The ratio of the concentration of the corrosive medium in the pipe wall to that inside the pipe is defined as the relative concentration of the corrosive medium, and the modulus of elasticity of concrete is negatively correlated with this concentration. When the relative concentration of the corrosive medium in the pipe wall reaches 65%, the concrete loses its mechanical properties entirely; when the relative concentration of the corrosive medium is 0, the elastic modulus of concrete is the initial elastic modulus. When the relative concentration of the corrosive medium is greater than 0 and less than 65%, the elastic modulus of concrete can be calculated using the linear difference method. The modulus of elasticity of the corroded concrete can be calculated according to Equations (1)–(3). Based on the experimental data of concrete corrosion by Liang, this study applied the parameter reduction method to simulate the corrosion of the pipe. The method of simulating corrosion in this study mainly contained the following three steps: first, set the location and depth of corrosion to the crown of the pipe and 25 mm, respectively; second, mesh the element in the corrosion area uniformly into 10 layers, each layer being 2.5 mm; third, set the material parameters of each layer of the element to simulate the different degrees of corrosion along the pipe wall direction with reference to the experimental data of Liang et al., and the modulus of elasticity of the material will decrease from the inner wall to the outer wall of the pipe.

$$X=C/C_0$$

(1)

$$m=\left\{\begin{array}{cc}0& x\ge 0.65\\ 1-x/0.65& 0<x<0.65\\ 1& x=0\end{array}\right.$$

(2)

$$E=m{E}_0$$

(3)

In Equations (1)–(3), C is the concentration of corrosive medium in the pipe wall; C₀ is the concentration of corrosive medium inside the pipe; x is the relative concentration of the corrosive medium; m is reduction coefficient of the modulus of elasticity of corroded concrete; E is the modulus of elasticity of concrete after corrosion; E₀ is the initial modulus of elasticity of concrete.

2.4. Gasket Model

The size of the gasket was selected based on the rubber seal-joint rings for water supply, drainage, and sewerage pipelines. The gasket’s inner diameter is 0.462, outer diameter is 0.472, and height is 0.01. The gasket is in contact with the pipe spigot on its inner wall and with the pipe bell on its outer wall. Therefore, the gasket–bell interface was set on the outer surface of the gasket, and the gasket–spigot interface was also set on the outer surface of the gasket. The gasket model is illustrated in Figure 7.

2.5. CIPP Liner Model

In this study, the pipeline was assumed to be partially deteriorated but still able to provide structural support for the CIPP liner by bearing most of the earth pressure and traffic load. The thickness of the CIPP liner was determined based on the partially deteriorated gravity pipe condition outlined in ASTM F1216 [29]. After calculation, the thickness of the CIPP liner was found to be 8 mm, with a length of 15,900 mm and an outer diameter of 800 mm. The model of the CIPP liner is illustrated in Figure 8.

$$\mathrm{P}=\frac{2K{E}_L}{\left(1-{\upsilon }^2\right)}.\frac{1}{{\left(DR-1\right)}^3}.\frac{C}{N}$$

(4)

where P is the groundwater load; $DR=\frac{D_0}{t}$ is the dimension ratio of CIPP liner; E_L is the elastic modulus of the CIPP; $\upsilon$ is Poisson’s ratio, K is the enhancement factor; C is the ovality reduction factor; N is the safety factor.

2.6. Model Load

The soil load, traffic load, and fluid load inside the pipe are the basic external loads of the pipeline. However, a series of studies have shown that the fluid load inside the pipeline has a limited impact on the structural performance of the pipeline due to the small amount of fluid load. Therefore, in this study, we mainly analyzed the mechanical properties of corroded concrete pipelines before and after CIPP rehabilitation under soil and traffic loads. Soil load is the most important constant load over the pipe and has a huge impact on the mechanical properties of the pipe. In this study, the soil load was applied by gravity with a gravitational acceleration of 9.8 N/m². As a frequent variable load, traffic load plays a significant role in the structural design of the pipe. In this study, the traffic load was simplified as a static load based on the structural design code for pipelines of water supply and wastewater engineering (GB50332-2017). The wheel size was chosen to be 0.213 m × 0.167 m with a distance of 2 m between two wheels. The position of the traffic load is illustrated in Figure 2. The application of the model load consisted of the following three main steps: (1) the geostatic analysis step was established with an analysis step time of 1s, and the gravity force of each component was applied in this analysis step; (2) the geostatic stress of the model was calculated, and the calculation results were imported into the predefined field for ground stress equilibrium; (3) the static general analysis step was established with an analysis step time of 10s, and the traffic load of the pipeline was applied in this analysis step.

2.7. Model Interaction

Normal and tangential contacts were set between the gasket and the pipe. The contact parameters were set with reference to the study of Xu et al. [30]. The normal stiffness K_n was taken to be 10,000 GPa/m, and the tangential shear modulus K_t was taken to be 8000 GPa/m.

A “hard” contact was set in the normal direction, and a “Penalty” contact was set in the tangential direction of the interface of the pipe and soil. The tangential friction coefficient was selected according to Equation (5):

$$\mu =\frac{A}{H/D-B}+C$$

(5)

where H is the cover depth; D is the pipe diameter; A, B, and C are the fitting parameters.

After CIPP rehabilitation, the CIPP liner will be closely bonded with the original pipe and coordinate the deformation; thus, the “tie” method was adopted to simulate the interaction between the CIPP liner and the pipe.

2.8. Boundary Conditions

The model boundary conditions consisted of two main aspects:

(1) Applying displacement constraints in the normal direction to the surrounding faces of the soil model and applying displacement constraints and rotational degrees to the bottom surface of the soil model.

(2) Constraining the axial displacement and rotational degrees of freedom of both the pipe and the CIPP liner.

2.9. Model Validation

In this paper, the accuracy of the established 3D numerical model was verified through comparison with previous studies. Fang et al. [27] conducted full-scale tests to investigate the effects of corrosion defects on pipelines. A corresponding three-dimensional numerical model was established based on Fang’s tests to calculate the mechanical properties of corroded pipes under traffic loading, and the numerical results were compared with Fang’s full-scale test results, as presented in Figure 9. The setting conditions and parameters of the 3D numerical model are consistent with the experiments of Fang et al. In the figure, OUT Sim is the data of the outer wall of the pipe in the numerical model; OUT Test is the data of the outer wall of the pipe in the experimental data of Fang et al.; IN Sim is the data of the inner wall of the pipe in the numerical model; IN Test is the data of the inner wall of the pipe in the experimental data of Fang et al. To evaluate the difference between the data, MAPE was calculated using Equation (6):

$$\mathrm{MAPE}=\frac{100\%}{n}{\displaystyle \sum }_{t=1}^n\left|\frac{A-F}{A}\right|$$

(6)

where A is the value of the full-scale test data, F is the value of the simulated data, and n is the sample size.

The results in Figure 9 demonstrate good agreement between the experimental data from Fang et al. and the numerical simulation data in terms of both trends and values. The MAPE values for the inside and outside of the pipe bell were 8.4% and 9.1%, respectively, whereas the MAPE values for the inside and outside of the pipe spigot were 7.7% and 6.8%, respectively. The calculation results of the MAPE show that there is a small difference between the data of the numerical model established in this study and the experimental results of Fang et al., with an error range of about 10%. These calculation results also indicate that the 3D numerical model presented in this paper is both reasonable and reliable.

3. Test Results and Analysis

3.1. Comparison Analysis of the Mechanical Properties of the Pipe before and after CIPP Rehabilitation

CIPP rehabilitation is a widely adopted technique for repairing pipelines. To evaluate the effectiveness of CIPP rehabilitation, this study utilized a 3D finite element model to calculate the mechanical conditions of corroded pipes, CIPP-repaired pipes, and normal pipes. The contours of the calculation results of P4 and P5 before and after CIPP rehabilitation are shown in Figure 10. Buco et al. [31] conducted a survey of 1800 km of sewer pipes and reported that 26.7% of concrete pipe defects were related to defects at the pipe joints, which are considered to be the most vulnerable locations of the pipes. Xu et al. [30] observed that the asymmetry of the geometry of the pipe bell and spigot at the pipe joint can cause shear displacement at the pipe joints, which can significantly affect the safe operation of the pipe. Therefore, the maximum principal stresses and the vertical displacement of the pipe at the pipe joints under different rehabilitation conditions were extracted and analyzed, as shown in Figure 11 and Figure 12. In the figures, “CP” denotes the corroded pipe, “RP” denotes the CIPP-repaired pipe, and “NP” denotes the normal pipe.

As can be seen in Figure 10, the Mises stress and maximum principal stress of the pipe were greatly reduced after CIPP rehabilitation, which indicates that CIPP rehabilitation can improve the stress states of the pipelines and make them more reasonable. The vertical displacement of P4 (spigot) and P5 (bell) was significantly reduced after CIPP rehabilitation, and the rotation of the pipe was also greatly reduced accordingly, which indicates that CIPP rehablitation has a significant impact on the mechanical properties of pipelines.

Figure 11 demonstrates that the stresses in the bell of the pipe were significantly greater than those in the spigot. This is due to the fact that the bell was exposed to the external load, whereas the spigot was under less stress due to being located inside the bell. The stress curve of the corroded pipe exhibits a substantial abrupt change at the corroded section, and there were no significant differences in stress between the corroded and normal pipes at other locations. This indicates that corrosion has a greater impact on the stress in the corroded area than in other regions. The stresses in the bell and spigot of the corroded pipe increased by 18.9% and 43.1%, respectively, compared to the normal pipe, which indicates that corrosion had a considerable effect on the pipe. After CIPP rehabilitation, the stresses in the pipe’s bell and spigot were reduced by 39.8% and 16.7%, respectively, and the stress curve of the pipe became smoother. This indicates that CIPP rehabilitation can reduce the stress on the pipe, improve the stress state of the pipe, and make the force distribution of the pipe more reasonable.

Figure 12 illustrates that there are no significant differences between the vertical displacement curves of the corroded pipe and the normal pipe. This is because the vertical displacement of the pipe is mainly determined by two factors: the pipeline settlement caused by the external load and the deformation of the pipeline. The external load on both the corroded and normal pipes was the same, and the settlement is comparable. As the pipes were rigid, their own deformation was minimal, and corrosion did not alter their rigidity. Thus, the difference in displacement between the corroded and normal pipes is insignificant. After CIPP rehabilitation, the vertical displacements of the crown and invert of the pipe were reduced by 24.7% and 24.4%, respectively. This is due to the fact that the CIPP liner is a continuous pipe that snugly fits inside the pipe, preventing the pipe from rotating and altering its stress state, ultimately reducing the vertical displacement of the pipe.

In conclusion, CIPP rehabilitation can effectively mitigate the stress and displacement of the pipeline, thereby enhancing the stress state of the pipeline and improving its safety.

In order to further study the repair effect of CIPP rehabilitation and analyze the mechanical state of the pipe–liner structure after CIPP rehabilitation under different defect and load conditions, a series of three-dimensional numerical models were developed using numerical simulation methods to analyze the effects of factors such as corrosion degree, corrosion angle, and traffic load on the pipe–liner structure.

3.2. The Effect of Corrosion Degree

In this section, the corrosion width of the pipe was 60°, the traffic load was 0.7 MPa, and the location of the traffic load was as shown in Figure 2. The modulus of the elasticity reduction method was applied to simulate the effect of corrosion on the pipe. The elastic modulus parameters of different corrosion degrees are shown in

Table 4. The elastic modulus parameters of different corrosion degrees.

Corrosion Case	Number of Layers	1	2	3	4	5	6	7	8	9	10
CP1	m	0	0	0.11	0.33	0.54	0.70	0.81	0.89	0.93	0.95
	E (MPa)	0	0	3.37	9.97	16.06	20.91	24.23	26.54	27.79	28.57
CP2	m	0	0	0.08	0.23	0.37	0.49	0.57	0.89	0.93	0.95
	E (MPa)	0	0	2.36	6.98	11.24	14.64	16.96	26.54	27.79	28.57
CP3	m	0	0	0.04	0.13	0.21	0.28	0.32	0.89	0.93	0.95
	E (MPa)	0	0	1.35	3.99	6.43	8.36	9.97	26.54	27.79	28.57

. In order to quantitatively analyze the effects of factors such as corrosion degree, corrosion width, and traffic load on the mechanical properties of pipelines, the buried depths of all numerical models were fixed. The numerical calculation results are shown in Figure 13, Figure 14 and Figure 15. For ease of analysis, the corroded pipes are categorized as CP 1, CP 2, and CP 3 according to the corrosion degree, whereas the CIPP-repaired pipes are labeled as RP 1, RP 2, and RP 3.

Figure 13 illustrates that the maximum principal stress curve of both the corroded and CIPP-repaired pipes exhibited a pattern of being high in the middle and low on both sides, with significant discontinuities at the pipe joints, indicating that the spigot and bell structure had a significant influence on the pipe stress. Figure 13a indicates that as the corrosion degree increased the maximum principal stress at the crown of the corroded pipe gradually increased at 4–12 m, whereas no significant difference was observed between 0–4 m and 12–16 m. The maximum principal stress distribution at the crown of the CIPP-repaired pipe also conformed to this pattern. Figure 13b shows that the maximum principal stress curves at the invert of the corroded pipe and CIPP-repaired pipe with varying degrees of corrosion are almost consistent. These experimental observations suggest that the corrosion degree has a significant impact on the crown of the pipeline, which is the location of the corrosion area, and has a minimal impact on other locations. As the corrosion degree increased, the stress in the corroded area also increased. However, the stresses in the pipeline with varying degrees of corrosion were significantly reduced after CIPP rehabilitation, indicating that CIPP rehabilitation can effectively alleviate stress in the pipeline.

Figure 14 demonstrates that the vertical displacement of pipelines at the crown and invert with different corrosion degrees did not show any significant differences. This is due to the rigid properties of the pipeline, where an increase in corrosion degree would not alter the rigidity of the pipeline. At 6–10 m, the vertical displacement of the crown and invert of the CIPP-repaired pipe were considerably smaller than that of the corroded pipe; at 4–6 m and 10–12 m, the vertical displacement of the crown and invert of the CIPP-repaired pipe were slightly larger than that of the corroded pipe; at 0–4 m and 12–16 m, the vertical displacement of the CIPP-repaired pipe did not show any significant difference from that of the corroded pipe. These experimental observations indicate that the degree of corrosion has a negligible impact on the vertical displacement of the pipe, and that CIPP can significantly decrease the vertical displacement of the pipe. However, the reduction is negatively related to the distance of the traffic load location.

In Figure 15, it can be observed that the von Mises stress of the CIPP liner remained almost unchanged for different corrosion degrees. This is due to the fact that the stresses in the CIPP liner comprised two main components: the circumferential force transmitted to the CIPP liner by the external load through the pipe and the longitudinal tension on the CIPP liner caused by the rotation and shear displacements formed by the pipe at the pipe joints due to the unevenness of the external loads and pipe structure. Because the corrosion degree does not affect the rigid nature of the pipe, the circumferential forces transmitted by the external load to the CIPP liner are minimally influenced by the corrosion degree. Furthermore, because the external loads of pipes with different corrosion degrees are the same, their rotation and shear displacements are similar, and because the longitudinal tensile forces of the CIPP liner are comparable as well, the Mises stresses of the CIPP liners with different corrosion degrees remain almost the same.

3.3. The Effect of Corrosion Angle

In this section, the corrosion degree of the pipe is labeled CP1, the traffic load was 0.7 MPa, and the location of the traffic load was as shown in Figure 2. In order to study the effect of corrosion angle on the pipe–liner structure, numerical simulation methods were used to calculate the corrosion angles of 30°, 60°, and 90°. The numerical calculation results are shown in Figure 16, Figure 17 and Figure 18. In this study, the corroded pipes were classified as CP 30°, CP 60°, and CP 90° according to the corrosion angle, and the CIPP-repaired pipes were classified as RP 30°, RP 60°, and RP 90°.

Figure 16a reveals that the maximum principal stress at the crown of the corroded pipe decreased as the corrosion angle increased from 4 to 12 m. There were no significant differences in the maximum principal stress at the crown of the corroded pipe for different corrosion angles at 0–4 m and 12–16 m. Figure 16b shows that the maximum principal stresses of corroded pipes with different corrosion angles were similar. The stress of the CIPP-repaired pipe changed in a similar pattern to that of the corroded pipe with increasing corrosion angles. These results indicate that the corrosion angle has a more significant impact on the stress in the corroded area of the corroded pipe and the CIPP-repaired pipe, and that it has a minor effect on other locations. Moreover, the stress at the corroded area decreased with increasing corrosion angles. This is attributed to the phenomenon of stress concentration at the corrosion area. When the corrosion angle was small, the stress concentration phenomenon was evident, and with an increase in corrosion angle, the stress concentration phenomenon was reduced.

Figure 17 illustrates that the vertical displacement of both the corroded and CIPP-repaired pipes was essentially identical under different corrosion angles, which is consistent with the explanation in Section 3.2, as the corrosion angle did not affect the pipe’s rigidity. After CIPP rehabilitation, the vertical displacement of P4 and P5 considerably reduced, whereas that of P3 and P6 slightly increased. Furthermore, the vertical displacement of P1, P2, P7, and P8 remained essentially unchanged. This finding suggests that CIPP rehabilitation can alleviate the displacement difference between pipes, reduce relative pipe rotation, and enhance pipeline safety during operation.

Figure 18 demonstrates that the von Mises stress of the CIPP liner remains essentially constant as the corrosion angle increases. Combined with the test phenomena described in Section 3.2, it can be inferred that internal defects of the pipe, such as corrosion degree and corrosion angle, have a limited impact on the stress of the CIPP-lined pipe. This is due to the rigid nature of the concrete pipe, which can sustain the external load independently, and therefore, the internal defects of the pipe, including corrosion degree and corrosion angle, have little effect on the vertical displacement and relative rotation of the pipe. Thus, the stress of the CIPP liner is generally unaffected by different internal defects.

3.4. The Effect of Traffic Load

In this section, the corrosion degree of the pipe is labeled CP1, the corrosion angle was 60°, and the location of the traffic load was as shown in Figure 2. In order to study the effect of traffic load on the pipe–liner structure, numerical simulation methods were used to calculate the traffic loads of 0.7 MPa, 0.85 MPa, and 1 MPa. The numerical calculation results are shown in Figure 19, Figure 20 and Figure 21. In this study, the corroded pipes were classified as CP 0.7 MPa, CP 0.85 MPa, and CP 1MPa according to the traffic load amplitude, and the CIPP-repaired pipes were classified as RP 0.7 MPa, RP 0.85 MPa, and RP 1 MPa.

Figure 19 illustrates that the maximum principal stresses at the crown and invert of the corroded pipe were more pronounced with an increase in traffic load magnitude at 6–10 m. However, at 1–6 m and 10–16 m, there were no significant variations in the maximum principal stress of the pipe under different traffic load magnitudes. These results suggest that traffic load magnitude has a more substantial impact on the adjacent pipelines situated on both sides of the traffic load location and less on the remaining pipelines.

Figure 20 illustrates that the vertical displacement of the corroded pipe and CIPP-repaired pipe increased as the traffic load magnitude increased. Specifically, when the traffic load magnitude rose from 0.7 MPa to 0.85 MPa, the maximum vertical displacement of the corroded pipe at the pipe crown and pipe invert increased by 1.210 times and 1.212 times, respectively, and the CIPP-repaired pipe experienced a similar increase of 1.210 times and 1.213 times. Moreover, when the traffic load magnitude further increased from 0.85 MPa to 1.0 MPa, the maximum vertical displacement of the corroded pipe at the pipe crown and pipe invert increased by 1.173 times and 1.174 times, respectively, whereas the CIPP-repaired pipe experienced an increase of 1.173 times and 1.175 times. The test results suggest that the traffic load magnitude has a significant impact on the vertical displacement of the pipe.

Figure 21 shows that the stresses in the CIPP liner significantly increased at 8 m with increasing traffic load magnitudes, whereas the difference in stresses at other locations with different traffic load magnitudes was small. This can be attributed to the fact that the CIPP liner fits closely to the concrete pipe and the external loads transfer less circumferential force due to the rigid nature of the pipe. As a result, the CIPP liner had less stress at the pipe barrel and more stress at the pipe joints. The increase in traffic load significantly increased the vertical displacement of the pipe, and the shear displacement and relative rotation of the pipe also increased, which caused an increase in the longitudinal tension of the CIPP liner. Therefore, the stress of the CIPP liner increases significantly with the increase in traffic load magnitudes at the pipe joints.

4. Conclusions

In this study, a three-dimensional numerical model of a corroded pipeline before and after CIPP rehabilitation was established and validated using numerical simulation methods. A series of comparative analyses were conducted to investigate the repair effect of CIPP rehabilitation and the influence of factors such as corrosion degree, corrosion angle, and traffic load magnitude on the corroded pipeline before and after CIPP rehabilitation. The main findings are as follows:

(1) CIPP rehabilitation reduces the stress and displacement of the pipeline, thereby improving its stress state and increasing its safety. After CIPP rehabilitation, the stress in the pipe bell, stress in the pipe spigot, vertical displacement of pipe crown, and vertical displacement of pipe invert were reduced by 39.8%, 16.7%, 24.7%, and 24.4%, respectively.

(2) Internal defects of the pipe such as corrosion degree and corrosion angle significantly affect the stress at the corroded area of the pipe and have a smaller effect on other locations. The stress of the corroded pipe and CIPP-repaired pipe at the corrosion area increases with increasing corrosion degree and decreases with increasing corrosion angle.

(3) Internal defects of the pipe have a minimal effect on the vertical displacement and von Mises stress of the CIPP liner. There is no significant difference between the vertical displacement of the pipe and the von Mises stress of the CIPP liner for different corrosion degrees and corrosion angles.

(4) Traffic load magnitude has a significant effect on the mechanical properties of the pipe-liner structure. The application of traffic loads can lead to an increase in both the stress and displacement of the pipeline. Consequently, this can also result in an increase in the rotation of the pipeline, ultimately causing a significant increase in the stress of the CIPP liner at the pipe joints at the location of the traffic load.