The Reinforcement of Insitu Concrete Pressure Pipe with Carbon Fiber Reinforced Polymer (CFRP)Steed Lopez2020-02-11T17:53:05+00:00
Large diameter concrete pipes commonly used in the transfer of water and waste water that have been in use for extended periods of time are starting to exhibit structural distress due to corrosion of both prestress and mild reinforcing steel. The majority of the failure mechanisms have occurred on the outside surface of the pipe. This failure mechanism has forced owners to excavate the pipe and perform insitu repairs or replace the pipe. Both conditions are time consuming and expensive. Structurally reinforcing the pipe internally with FRP has proven to be a cost effective and time saving method. This paper will provide a brief overview of the methods of testing the pipe for local failures and the design and repair using carbon fiber, CFRP
Myles A. Murray, P.E., FACI Chairman and Founder Restruction Corp. P.O. Box 343 Sedalia, Colorado 80135 United States
1. Introduction Large diameter precast prestressed concrete pipe (PPCP) has been used extensively to transfer water and wastewater in the cities and countries of the world. These pipes are subject to deterioration, as they age, primarily from corrosion. These pipes have been in use for more than 30 years. With the desire to have a useful life of 50 years or more an active maintenance program must be incorporated. To address the problem the areas within the pipes that are exhibiting deterioration must be located, identified and repairs initiated.
Some of the earliest failures occurred in the large water transfer pipes used across the Arabian desert in Saudi Arabia and the pipes used to transfer the water from Lake Powell to the City of Phoenix, Arizona in the United States. Those repairs were done with steel liners and from that was born the idea of installing an insitu flexible pipe system with the installation of CFRP inside these large pipes.
a. Inspection To identify and locate the problems the pipes must be inspected on a regular basis. The primary means of inspection requires the pipes to be shut down and inspected visually by qualified engineers. The visual inspection is looking for signs of deterioration due to corrosion and sounding the concrete surface to locate delaminations The visual signs historically are cracks and spalls of the concrete that are the product of corrosion. The corrosion is generally well advanced when failure mechanisims start to appear within the pipe. Spalls resulting from corrosion of the prestressed steel strands occur on the exterior of the pipe. The spalls can not be visually identified without excavating the pipe. A costly and unacceptable inspection procedure.
The prestressed strands that break, due to corrosion, will not reveal the distress visually on the interior of the pipe. To find the breaks in the prestressed strands there is a non-destructive method, electromagnetic remote field eddy current system, that can be performed by sending a black box through the pipe remotely. This system will identify breaks in the prestressed strands as well as their locations. The repairs executed in the following case study were based upon the findings of the electromagnetic remote field eddy current inspection system.
b. Structural Evaluation The loss of structural capacity due to the failure of the presressed starnds must be addressed. The options available to the owner are four fold. 1, do nothing and hope for the best; 2, remove and replace the deteriorated pipe segment; 3, excavate, remove the concrete cover and prestress strand. Install external prestress wires and cover with shotcrete; 4, install a CFRP system on the internal surface of the pipe. The design solution achieved with the CFRP creates the installation of an internal flexible pipe system. This changes the dynamics of the load carrying system but maintains the pipes load carrying capability.
c. Cost Analysis Once the problem has been identified the design engineer can work with the owner to prioritize the repairs. Then in conjunction with the contractor the different costs to execute the design repairs can be evaluated. A cost to benefit analysis will assure the owner that the most cost effective repair solution will be executed. Once these steps have been completed the owner is able to budget repairs in accordance with availability of funds and condition of the pipe. These steps allow the owner to maximize the cost to benefit ratio.
d. Repair Decision The owner, in conjunction with the engineer and based on the results of the non-destructive pipe inspection, elected to use two different repair solutions on this project. Although this paper just addresses the three pipe segments that utilized a CFRP solution one of the pipe segments was repaired by excavating the pipe and installing an external prestress system.
2. Pipe Repair The PPCP is 108” (274mm) diameter pipe and each precast segment is 18’ – 0” (5.5m) long. There were no visual indications of failure on the interior of any of the precast segments. Four segments were repaired with 3 segments being repaired internally with CFRP and one segment being repaired externally with post tensioned reinforcing strands. This paper addresses those precast segments that were repaired internally with CFRP.
The repair of these pipe segments can be achieved internally by accessing the pipe through manholes. This procedure does not require external excavation to achieve access to the pipe segment requiring repair, thereby minimizing the impact on the activities or the people in the immediate area. Although there are demands for equipment and personnel operating on the surface that will be disruptive to the community the physical disturbance is less and the time required to complete the operation is significantly shorter.
a. Surface Preparation In the absence of any required repairs to the interior of the concrete pipe, surface preparation is all that is required before applying the CFRP. The surface preparation used in this project was a handheld hydroblasting device (See Fig. 1). The handheld hydro-blasting tool operated at 35,000 psi (245N/m2) with the objective of removing the surface laitance and exposing the fine aggregate. The hydro-blasting tool reclaimed the water used in the blasting operation so that there was no free contaminated water escaping to the environment. The reclaimed water was collected and vacuumed into a tanker truck that transported the contaminated waste water to an approved dump site.
The objective was to provide a roughened surface that would improve the bonding of the polymer to the concrete and subsequently the carbon fiber fabric to the polymer. The hydro-blasting unit was held against the surface of the concrete by the craftsman. The men would take turns operating the tool. Since the tool was heavy each man would be relieved by a co-worker. When operating the tool overhead each man was relieved between 30 – 60 seconds. The time required to prepare the surface of each segment of pipe was approximately 5 hours with a seven man crew.
Figure 1. Hydro-blasting of the concrete surface
b. CFRP Application Once the surface was prepared and approved by the engineer the application of the CFRP was ready to proceed. A polymer leveling coat was applied to the surface initially. Immediately following the application of the leveling coat the first layer of carbon fiber was saturated with polymer and applied to the surface in the longitudinal direction. (Fig. 2) Once the first layer was applied the second layer was applied in the transverse (lateral) direction. There was a total of three layers applied in the transverse direction for a total of four layers. Each layer was saturated with a polymer and applied in a continuous operation.
The carbon strips were 2’- 0” (610mm) wide. Each strip overlapped the adjacent strip by 1” (25mm). The CFRP hoop strips were overlapped at each end by 1’- 0” (300mm). The application of all four layers in a single pipe segment was completed in a single work day of eight hours. (Fig. 3)
The termination of the fabric at the ends of the pipe required an anchoring system to assure full closure of the system. The concrete cover over the steel liner was cut at a 45o angle to expose 4” (100mm) of the internal steel liner. The exposed steel liner was then covered with three layers of fabric. One layer of glass fiber, one layer of carbon fiber and a final layer of glass fiber. The top layer of carbon fiber was then extended beyond the end of the concrete and laid in on top of the previously installed fabrics.
Figure 2. Application of the longitudinal layer of carbon fiber
The proper application of the carbon fiber is critical to the long term success of the CFRP. A quality control program was set up by the engineer to validate the performance capability of the installed product. A carbon fiber test area on an adjacent pipe was installed for quality control purposes. The carbon fiber installation was subjected to a series of tensile tests. There was one tensile test also taken on the repaired pipe section to compare with the test area.
Figure 3. The completed installation of the four layers of CFRP
Figure 4. The hydraulic compressive prestressing of the stainless steel bar
A stainless steel bar was then installed in the cavity and prestressed in compression on a bed of a rubber seal to assure a positive anchorage of the carbon fiber. Once layed in place the stainless steel bar was prestressed in compression by hydraulic rams and bolted into place. (Fig. 4) It was then bolted through the steel liner and into the outer concrete. The remaining cavity was filled with an epoxy mortar. The primary purpose was to eliminate the possibility of water getting underneath the fiber and causing delamination of the CFRP.
The final step was the application of a polymer tie coat. The purpose of this coating was to provide a protective layer over the CFRP layers and tie the whole system together. (Fig. 5) The application of the tie coat on the invert was the last function the craftsman executed on their way out of the pipe. Once completed and the appropriate cure time elapsed the pipe could be put back in service.
Figure 5. The finished pipe just prior to the final coat on the invert and the test area
3. Conclusion The precast prestressed concrete pipes have been in use now for more than 35 years. The maintenance demands are just beginning to come to light and must be addressed if the useful life of these pipes is to be maximized.
There are several factors that must be evaluated before a repair decision can be instigated. The costs of the different options; the impact of each repair solution on the general public and the subsequent costs indirectly imposed on them; the potential life time costs of the designated repair; and the cost of time imposed on the owner when taking the infrastructure out of service. These factors all create either a direct or indirect cost to the owner.
The CFRP repair option provides the owner with a long term solution while minimizing the downtime to execute the repairs. This repair scheme is the least intrusive and can be completed in the shortest time frame. The CFRP repair option has now been in use for a little over ten years. The number of applications is growing exponentially.