Rehabilitation of Industrial Chimneys and Silos by using Advanced Composites Christoforos Kolyvas , İhsan Engin BAL and Michael Karantzikis Fyfe Europe SA, Athens, Greece Summary
Chimneys lose their structural integrity quite fast, starting from the moment they are built, even before getting into the service. Several existing chimneys may require repair and strengthening not after many years of operation. Apart from the aging effects, seismic performance is another issue. Design codes and seismic hazard studies being updated continuously may require strengthening of a chimney that was designed and built properly per the previous codes and standards.
This paper describes some selected projects in which Tyfo Fibrwrap Advanced Composite Systems were used for repair and strengthening. Advanced composite solutions have the advantage of being faster and more precise as compared to the traditional methods of strengthening chimneys and silos.
1. Introduction
Standards and codes for the design of chimneys, especially for the FRP (Fiber Reinforced Polymers) design, are not many. The design of chimneys in the United States, for example, is typically achieved in accordance with ACI 307 [ACI, 1998], which offers a simple design procedure for RC angular sections under the assumption that the steel reinforcement is uniformly distributed around the section, see Figure 1. When FRP laminates are added to the cross-section, the bending behavior and failure is mostly governed by the properties and the linear tensile behavior of the FRP. Capacity of the FRP upgraded section of the chimney should therefore be determined using integration that accounts for the strain variation in the FRP placed around the chimney section. Since bending due to lateral forces can occur in any direction, the design of sections with openings should account for the worse condition.
The reason FRP solution may be needed for a chimney could be detrimental effects of aging, which manifest itself as deterioration of concrete and/or rebars, upgrade in seismic codes or hazard maps, updates in wind load regulations, mistakes in design and construction phases, or change in use.
Tyfo Fibrwrap Advanced Composite Systems would typically be used more at the bottom levels of the chimney or silo if the fibers were aimed to increase the flexural load capacity. If the purpose of Tyfo application is confinement, or repair of deteriorated concrete, then hoop layers would be requires and they can be applied at any height depending on the requirements.
Figure 1. Distribution of strains along the section height and determination of the FRP strains
This paper focuses on some selected projects were several different cases have been examined to help the reader to better understand the capabilities and advantages of Tyfo Fibrwrap solutions when used for chimney and silo rehabilitation projects.
2. Chimney Strengthening at University of Missouri
A power plant at the campus of University of Missouri has two chimneys, 30 years old, with the height of 105m (Figure 2). A comprehensive assessment study showed that both south and north chimneys would require structural strengthening against flexural actions due to wind loads. Tyfo Fibrwrap Systems provided a fast yet feasible solution and the project was completed successfully in 2011.
25cm wide strips of Tyfo SCH41, unidirectional carbon, were used parallel to the axis of the chimney with the purpose of increasing the flexural capacity (Figure 3).
The application of the vertical SCH41 strips was with a clear distance of 1m in the lower 1/3rd of the chimneys while the distance was doubled in the second level above the bottom by terminating some of the strips.
A layer of SEH51, glass FRP material in the family of Tyfo Fibrwrap Systems, was used for confinement of the vertical strips. The confinement was applied all the way up. The upper parts of the chimneys, where moment capacity is not exceeded by the moment demand, were strengthened only with the hoop layers.
Figure 3. Tyfo Fibrwrap application on the chimneys: Tyfo systems are used for flexural and confinement actions
The application was done only outside of the chimney, and there was a need for proper anchorage of the vertical strips when an obstacle was found.
The design of the Tyfo Fibrwrap System was done according to ACI 307 [ACI, 1998], but FRP strips were treated as steel rebars in the conventional calculations. There were doors at the bottom of the chimney, something that alters the symmetry. The design needs to take into account the worst-case scenario according to the code. The worst-case scenario is when the moments are such that the side of opening is in tension.
The final coating of the system was done by using Tyfo UV polyurethane-based painting.
3. Industrial Stack Strengthening
The industrial stack, shown in Figure 4, was strengthened by using Tyfo Fibrwrap Systems. The reason for the strengthening was seismic detailing. Seismic assessment studies showed that the rebar development length was not enough in some parts of the chimney (see Galati et al, 2011).
Lack of rebar development length is a common problem not only in chimneys and silos but also in bridge piers and RC columns and walls in ordinary buildings. Based on the experiments and the past experience in application, Tyfo Fibrwrap application can be particularly useful in such cases.
Figure 4. Outside view of the stack during the application The application of the Tyfo Fibrwrap Systems was done both outside and inside, but not in a continuous form. The application followed a mapping provided by the designer, which was showing the zones where bar development length was missing and additional strengthening is needed.
One of the main challenges in this project was that since the FRP strips were applied not continuously but as strips, there was a need for proper anchorage of the strip where the strip terminates, as shown in Figure 5.
Figure 5. Inside application of Tyfo Fibrwrap Systems
4. NRG Industrial Stack Strengthening
The repair and strengthening of the NRG stack, which was located in San Francisco Bay Area, was not a structural strengthening but it was rather aimed to prevent concrete spalling on top of the stack.
As happens in aged stacks, the concrete was in its worst shape for the last 10‐ 15m of the stack. Following several concrete repair works, the last 15m of the stack was covered by using Tyfo SEH51A material, something that provided not confinement and integrity to the system. Figure 6. General view of the NRG stack after the application of Tyfo Fibrwrap Systems 5. Amaren UE Stack Strengthening
The project presented here is an example of the use of Tyfo Fibrwrap Systems for strengthening against wind loads. Structural assessment studies have shown that the industrial stack in the Amaren power plant did not meet the currently updated wind load and design requirements.
Tyfo Fibrwrap Systems were used as vertical strips and strips in hoop direction. The vertical strips were installed aiming to increase the flexural capacity leading thus the structure to pass the new design limits under wind loads. Some anchorage was also introduced at the beginning of the bottom layers so that the tensile forces on the Tyfo strips could be transferred to the foundation. The hoop direction layers, consisted of Tyfo SEH51A, aimed to confine the vertical layers and increase the structural integrity in case the section reaches high strain levels.
Figure 7. General view of the Amaren Stack and the surface after Tyfo Fibrwrap application
6. Hamm and Women Hospital Chimney Strengthening Projects
Hamm chimney is located in Vancouver, Canada. There was a requirement for seismic strengthening of this chimney. The chimney consisted of internal and external chimneys, connected to each other along the height. The chimney belonged to a small power plant. The height of the chimney was around 34m. Application of vertical and hoop Tyfo strips, in varying number of layers, was the best and the fastest solution for this strengthening project. Figure 8. Applied Tyfo Fibrwrap SCH41 carbon FRP material on the outside surface of the inside chimney
Figure 9. Tyfo Fibrwrap Strengthening on outside and inside chimneys
The women hospital in Vancouver, Canada, has a small internal plant with a chimney of 48m high. The chimney did not satisfy the new seismic design code. The style of the chimney was similar to that of Hamm Chimney, with internal external chimneys. The seismic strengthening scheme was also similar to that of the Hamm Chimney. Figure 10. General view of the Women’s Hospital chimney in Vancouver and application of vertical and horizontal Tyfo strips
7. Grain Silo Strengthening
The grain silo in Hawaii experiences a grain explosion, a common cause of failure in grain silos (see Figure 11 and Figure 12). The explosion created damage at the bottom levels of the silo, treating thus the structural safety of the overall system.
Figure 11. General view of the grain silo
Figure 12. The damaged section of the silo after the grain explosion
The damaged part of the silo was repaired by using traditional concrete work, but the structural strength was not as it was before. Tyfo Fibrwrap solution was used internally so that the strength of the structure in hoop direction could be increased back to the original stage.
The Tyfo Fibrwrap solutions are generally applied outside of the silos since the battery structure of silos does not allow a full wrap around the system. The internal application was enough to recover the lost strength (Figure 13). The main advantage of Tyfo Fibrwrap Systems is that the internal wrap can be done without decreasing the internal volume capacity of the silo.
Figure 13. Application of Tyfo Fibrwrap Systems inside the silos
8. Conclusions
Existing chimneys, stacks and silos may require repair and strengthening for several reasons. These reasons may be aging effects, increased seismic requirements, wind load updates, deterioration of concrete, corrosion, etc. These structures usually belong to industry where the shut down time need to be minimum, zero if possible, and the project foot print needs to be minimized. Precision in application and timing are other parameters demanded by the industry. Tyfo Fibrwrap Systems can answer all of these demands and provide quick yet efficient and feasible solutions to the industry.
Acknowledgements
Technical contribution of the Engineer Mr. Tomás T. Jiménez, Vice President of Fyfe US, and the head of the Waterfront and Industrial Divisions, is gratefully appreciated. Valuable information about the chimneys at the University of Missury was provided by the Engineer of Record Mr. Jarek Czernikiewicz.
References
ACI (1998), “ACI 307: Code Requirements for Reinforced Concrete Chimneys (ACI 307-08) and Commentary”, American Concrete Institute, USA.
Galati N., and Alkhrdaji, (2011), “Strengthening of RC Chimneys with FRP Composites”, First Middle East Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures, Dubai, UAE.
