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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.

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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).

 

Figure 2. South and North chimneys of the power plant 

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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.

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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.  

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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.  

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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 

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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. 

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 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. 

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 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.