Failure Risk of Bar-Wrapped Pipe with Broken Bars and Corroded Cylinder

Transcription

1 457 Failure Risk of Bar-Wrapped Pipe with Broken Bars and Corroded Cylinder Ali Alavinasab 1, Muthu Chandrasekaran 2, Edward Padewski III 3 1 Pure Technologies, 3322 State Route 22 West, Suite 902, Building 9, Branchburg, NJ 08876; PH (908) ; FAX (908) ; ali.alavinasab@puretechltd.com 2 Pure Technologies, 8920 State Route 108, Suite D Columbia, MD 21045; PH (443) ; FAX (443) ; Muthu.Chandrasekaran@puretechltd.com 3 Pure Technologies, 3322 State Route 22 West, Suite 902, Building 9, Branchburg, NJ 08876; PH (908) ; FAX (908) ; ed.padewski@puretechltd.com ABSTRACT Bar-wrapped pipe ( BWP ) commonly used in pressure pipelines due to its reliability, cost effectiveness and durability. Failure of BWP can occur as a result of long term leakage and subsequent corrosion or as a result of leakage and deterioration of the reinforcing bars over time. The failure can also be the direct result of a transient pressure or other sudden catastrophic events. The consequence of failure may result in a significant disruption of operation and service for a water utility without any warning. This is a concern because assessing the condition of a damaged BWP is very challenging. In this paper, a nonlinear finite element analysis was used to evaluate the performance of a damaged BWP. For the structural evaluation, stresses and strains developed in the damaged BWP were evaluated. Cracking and spalling of the mortar lining will eventually lead to the corrosion of the steel components. In an effort to account for the steel deterioration, the model was adjusted by reducing the thickness of the steel cylinder. This study investigates the behavior of a deteriorating BWP under various levels of distress and various internal pressures. The results based on a 24-inch pipe transmission main, are used to define criteria to evaluate the performance of a damaged BWP. Based upon the finite element results obtained in this study, suggestions for future work are presented and discussed. INTRODUCTION Since 1942, concrete cylinder pipe, commonly known as bar-wrapped pipe or pretensioned pipe, has been used in pipelines due to its reliability, cost effectiveness and durability. Typically, BWP is manufactured in standard diameters of 10 to 72 inches, with design pressures up to 400 psi and varying external earth loads. Although BWP resembles prestressed concrete cylinder pipe ( PCCP ), lined cylinder type, BWP and PCCP utilize different structural components to carry the loads. The steel cylinder in

2 458 PCCP acts primarily as a water barrier, not a structural member. This is not the case with BWP where the steel cylinder is one of the key structural components. BWP is essentially a steel pipe that is stiffened with steel reinforcing bars; therefore, the failure mechanism of BWP tends to be the same as that of a steel pipe. Unlike PCCP, where failure typically involves a bursting of the pipe due to broken wire wraps, the failure of BWP usually involves leaking due to a hole in the cylinder. This may be a flowing leak or a weeping leak, depending on the condition of the concrete lining. An electromagnetic ( EM ) inspection can be a successful means of performing a condition assessment of a pipeline composed of BWP with the understanding that the EM inspection will only provide data about the current condition of the pipeline. To evaluate the design and the risk of failure associated with specific pipe sections, a structural evaluation, based on a finite element analysis, can provide insight into the future serviceability of a pipeline following an EM inspection. Increasing levels of distress can be evaluated to determine the reliability of the pipeline as the condition deteriorates. This type of investigation was used to inform the City of Calgary Water Department whether or not the pipes were adequately designed and to provide data and performance curves to assist the city in making management decisions for the 24-inch Rundle Feedermain BWP. A structural evaluation was performed to determine if any pipe sections were approaching one of the failure limit states, which can help prevent ruptures similar to the one that occurred on July 14, The performance of BWP is directly related to the condition of the rebars and the steel cylinder in the pipes. A Finite Element Analysis (FEA) was performed to predict the failure of PCCP due to an increasing number of broken wire wraps [1-8]. Based on the limited amount of literature available, the effects of broken rebars and corrosion in the steel cylinder have not been thoroughly investigated for BWP as compared to that of PCCP. The objective of this study is to investigate the effect that broken rebar and corrosion in the steel cylinder has on the structural integrity of a distressed BWP using three-dimensional ( 3D ) FEA. A 24-inch (0.61 m) BWP was considered with two (2), five (5), ten (10), fifteen (15) and 25 broken rebars for this investigation. Based on the FEA results, a risk analysis was performed for the 24-inch Rundle Feedermain BWP located in Calgary. MODEL FORMULATION BWP is comprised of a welded steel cylinder that creates a watertight membrane and helically wrapped, pre-tensioned, steel reinforcing bars wrapped around the cylinder to provide additional strength. An internal concrete lining and external mortar coating provide corrosion protection to the steel components. Properties of the 24-inch BWP were obtained during site visits. Figure 1 shows the typical cross section of a BWP.

3 459 Figure 1. Cross section of a typical AWWA C303 Bar-Wrapped Pipe Table 1 represents the field measurements of the inner and outer mortar of the Calgary 24-inch BWP. Table 1 Field measurement of mortar coating, Calgary 24 inch BWP Section Sample 1 Sample 2 Sample 3 Sample 4 Inner mortar (inch) Outer mortar (inch) Based on the data shown in Table 1, the average thickness of the inner and outer mortar used for the structural evaluation was 0.65 inches and 0.85 inches, respectively. FINITE ELEMENT ANALYSIS The finite element method ( FEM ) is a general method of structural analysis in which the solution of a problem in continuum mechanics is approximated by analyzing an assemblage of finite elements which are interconnected at a finite number of nodal points across the solution domain of the problem. 20 feet 24 inch Figure 2. Geometry of BWP.

4 460 As shown in Figure 2, the BWP used in the analysis has a length of 20 feet (6.1 m) and an inner diameter of 24 inches (0.61 m). A commercial finite element software ( ABAQUS )) was used for the numerical analysis. It was assumed that the interface between the rebar and the mortar coating is perfect. A four-node quadratic shell element (S4R) was used forr modeling the undamaged portion of the pipe. This type of element is characterized by three displacement and three rotational degrees of freedom. The FEM model of the pipe with shell elements is shown in Figure 3: Figure 3. 3-D finite element model of BWP Breakage in the rebar is often due to significant corrosion overr time. Corrosion begins when a significant amount of energy is placed into the steel during its extraction from the ores, placing it in a high-energy state. Therefore, steel is thermodynamically unstable and will tend to react with its environment (e.g., oxygen or water) in order to reach a lower energy state (Fe 2 O 3 ). The environment surrounding the rebar and the steel cylinder is the controlling factor in a corrosion reaction. The FEA was performed for a different number of broken reinforcement bars and corresponding loss of thickness in the steel cylinder.. Table 2 presents the values used in the FEA for the number of continuous bar breaks and the percentage of thickness lost in the steel cylinder. The values are based on thee assumption the mortar coating is deteriorating exposing not only the reinforcement bars to a corrosive environment, but the steel cylinder as well. Depending on the environment and, if possible, external or forensic evaluation of damaged pipes in a particular system, the amount of steel cylinder damage can be modified to better represent the actual conditions.

5 461 Table 2 FEA Damage Propagation in the Calgary 24 inch BWP No. of Continuous Percentage of steel cylinder Bar Breaks thickness lost in damaged zone 0 0 % 5 10 % % % % % Material properties for the BWP were obtained using American Water Works Association ( AWWA ) C304 Standard [13]. The Young s modulus, density and Poisson s ratio used in the model are presented in Table 3. Table 3: Material properties of PCCP based on AWWA C304 Standard Young s modulus Density (GPa) / (psi) ( kg 3 m ) / (lb/ft 3 ) Poisson s ratio Mortar Coating 25.1 / 3.64E / Steel Cylinder / 3E / RESULTS AND DISCUSSION The nonlinear response of a BWP to internal pressure and external loading is obtained using a FEA. Cracks occur at the mortar coating where the concrete tensile load exceeds the concrete tensile strength. Since cracking is considered a stage of deterioration which causes corrosion in the steel cylinder, realistic cracking models need to be developed in order to accurately predict the load-deformation behavior. Figure 4 demonstrates the stresses in the rebars, the steel cylinder, and the inner and outer mortar coatings with five broken rebars and a 10% steel cylinder loss in the damaged zone. In these figures, the color gradients indicate the calculated stress for each element in the FEA model. Undamaged Area (No Broken Rebar) Damaged Area (Broken Rebar/Corroded Cylinder) a) Stress in the rebar

6 462 Outer Coating Inner Coating b) Stress in the coating c) Stress in the inner liner Figure 4. Stresses in the 24 inch BWP with five (5) broken rebars and a 10% loss of steel Figure 5 represents the damage in the mortar coating, caused by excessive tension, for the Calgary 24 inch BWP.

7 463 Figure 5. Tension damage in 24 inch BWP with five (5) broken rebars and a 10% loss of steel in the damaged zone. A risk analysis evaluates the impact of a growing number of broken rebars, and the subsequent corrosion of the steel cylinder, on the performance of the pipe and determines the corresponding risk of failure resulting from this damage. Failure risk is expressed in terms of limit states that relate to serviceability (micro cracking and visible cracking), damage, and ultimate deflection of the pipe. The micro cracking limit state is based on the onset of cracking in the inner or outer mortar coating. It is further defined as a 1/16-inch crack at 12 inches in length. The visible cracking limit state is defined as a crack greater than inches wide and 12 inches in length. The damaged limit state is based on structural cracking of the coating, which exposes the cylinder to corrosion, and an increase in deflection of the pipe based on Spangler s equation. The ultimate deflection limit state is based on a critical deflection of the pipe which represents severe damage and changes in the geometry of the pipe section. Based on this analysis, performance curves were generated which denote the aformentioned limit states in terms of the number of contiguous broken rebars and a corroded steel cylinder versus the maximum deflection for different internal pressures and a given external load.

8 464 EXPECTED DETERIORATION OF BWP When steel reinforcing bars break or the steel cylinder corrodes, a pipe's strength is significantly reduced at the area of damage. The ends of the reinforcing bars retract, and then react to the constrictive forces of friction being applied by the mortar coating. These friction forces enable the reinforcing bars to redevelop their tension over a relatively short distance from the point of breakage. The length of the rebar that needs to be firmly encased in the mortar in order to redevelop the tensile strength can vary up to several feet depending on the size of the reinforcing bar and the level of corrosion, as well as the condition and quality of the mortar coating. The redevelopment of the tensile strength is referred to as bond. When the coating becomes soft, cracks, or is delaminated from the steel cylinder, the rebar s bond to the mortar is reduced and the redevelopment length is increased significantly. Since it is not practical to understand the individual rebar s bond for each pipe section, a conservative assumption must be made for modeling purposes. The most conservative assumption is to remove the broken rebars and reduce the thickness of the steel cylinder in the damaged zone. This is overly conservative and unrealistic. In this scenario, according to the structural model, the pipe would fail at the ultimate deflection limit. When actual information of the localized damage is incorporated, the performance risk curves predict that the ultimate strength of the pipe would occur at the ultimate deflection curve. It is important to understand that if conditions exist in a pipe section that lead to a thinner steel cylinder in the damaged zone than what was used for the analysis, the models indicate that the pipe section would fail prior to reaching the curve. If the mortar coating becomes delaminated, the rebars may not be bonded for the entire damaged area. Also, if the mortar is under attack by aggressive soil and groundwater conditions surrounding the pipeline, the bond will be reduced. Based on this analysis, performance risk curves were generated which show the limit states in terms of the number of contiguous broken rebars, the loss of the thickness of steel cylinder steel cylinder and the different internal pressures for a given external load. The performance risk curve for the Calgary 24-inch BWP is shown below in Figure 6.

9 465 Figure 6. Expected deflection for BWP at various levels of deterioration The maximum number of broken rebars and the corrosion of the steel cylinder required to reach the limit states under operational and surge conditions for each pipe was calculated using the FEA performance risk curves. Table 4 summarizes the FEA condition rating based on the quantity of wire breaks. Table 4 - FEA Condition Rating based on the Design Working Pressures Pressure (psi) Limit State Broken Rebars / % of cylinder thickness lost When Microcracking Occurs When Damage Occurs 40 0/0% 22/70% 60 0/0% 17/50% 80 0/0% 10/20% 100 0/0% 0/0% CONCLUSIONS In this study, the numerical FEM was used to determine the performance of BWP at various levels of deterioration. The effects of broken rebars and corrosion in steel cylinder of a 24-in BWP (61 cm) were investigated under combined internal pressures and external earth loads. The results were shown for the pipe with (i) 2 broken rebars and 5% corrosion in steel cylinder, (ii) 5 broken rebars and 10% corrosion in steel cylinder, (iii) 10 broken rebars and 20% corrosion in steel cylinder, (iv) 15 broken rebars and 40% corrosion in steel cylinder, (v) 20 broken rebars and 60% corrosion in steel cylinder and (vi) 25 broken rebars and 80% corrosion in steel cylinder. The results indicates the

10 466 deflection based performance of the pipe change significantly when the level of damage increases. Utility owners can use the proposed method to assess risk of failure of BWP based on condition data. Broken rebars and corrosion in steel cylinder can be obtained by physical testing, electromagnetic inspections or other testing. Based on the collected data and the proposed performance curve, an estimation of remaining useful life in a BWP main can be obtained. REFERENCES [1] Zarghamee, M. S., Eggers, D. W., Ojdrovic, R. P., and Rose, B., (2003). Risk analysis of Prestressed concrete cylinder pipe with broken wires. Int. Conf. on Pipeline Eng. and Construction, ASCE, Baltimore, MD, [2] Zarghamee, M. S., Eggers, D., and Ojdrovic, R. P., (2002). Finite element modeling of failure of PCCP with broken wires subjected to combined loads. Beneath Our Feet: Challengers and Solutions (CD-ROM), ASCE Press, Reston, VA. [3] Erbay, O. O., Zarghamee, M. S., and Ojdrovic, R. P., (2007). Failure risk analysis of lined cylinder pipes with broken wires and corroded cylinder. Int. Conf. on Pipeline Eng. and Construction, ASCE, Boston, MA, [4] Xionga, H., Li, P., and Lia, Q., (2010) FE model for simulating wire-wrapping during prestressing of an embedded prestressed concrete cylinder pipe. Simul. Model. Prac. Theo. 18(5), [5] Lewis, R. A., Wheatley, M., (2003). Prestressed Concrete Cylinder Pipeline Evaluation, A Toolbox Approach. Int. Conf. on Pipeline Eng. and Construction, ASCE, Baltimore, MD, [6] Alavinasab,A., Padewski E., Holley, M., Jha R., Ahmadi, G., (2010). Damage Identification Based on Vibration Response of Prestressed Concrete Pipes. ASCE Pipelines Conf.: Climbing New Peaks to Infrastructure Reliability Renew, Rehab, and Reinvest, Keystone, CO, [7]Alavinasab, A., Padewski, E., Holley, M., Jha, R., Ahmadi, G., Crack Propagation in Prestressed Concrete Noncylinder Pipe using Finite Element Method, ASCE Pipelines Conf.: A Sound Conduit for Sharing Solutions, Seattle, WA, [8]Alavinasab, A., Jha, R., Ahmadi, G., Damage Identification based on Modal Analysis of Prestressed Concrete Pipes, ASCE Pipelines Conf.: A Sound Conduit for Sharing Solutions, Seattle, WA, [9] American Water Works Association (2007), AWWA C304 Standard for Design of Prestressed Concrete Cylinder Pipe. Denver, CO. [10] American Water Works Association (2009), AWWA C303 Standard for Design of Concrete Pressure Pipe, Bar-Wrapped, Steel Cylinder Type. Denver, CO.