STRESS RIBBON BRIDGE

ABSTRACT

A stress ribbon bridge is a tension structure, similar in many ways to a simple suspension bridge. The stress ribbon design is rare. Few people including bridge engineers are familiar with this form and fewer than 50 have been built worldwide. The suspension cables are embedded in the deck which follows a catenary arc between supports. Unlike the simple span the ribbon is stressed in compression which adds to the stiffness of the structure. Such bridges are typically made from concrete reinforced by steel tensioning cables. They are used mainly for pedestrian and cycling traffic. Stress ribbon bridges are very economical, aesthetic and almost maintenance free structure. They require minimal quantity of materials. At present studies, on combining stress ribbon bridges with cables or arches, to build most economical stress ribbon bridges. It makes the study of features of these particular bridges as an important one.

CONTENTS 

ž   INTRODUCTION

ž   FINSTERWALDER’S STRESS RIBBON BRIDGE THEORY

ž   FORM OF A STRESS RIBBON BRIDGE

ž   COMPARISON WITH A SIMPLE SUSPENSION BRIDGE

ž   CONSTRUCTION TECHNIQUES

ž   STRUCTURAL SYSTEM

ž   MODEL TESTS

ž   ADVANTAGES AND APPLICATIONS

ž   MODIFIED STRESS RIBBON BRIDGES

ž   A CASE STUDY

ž   STRESS RIBBON BRIDGES AROUND THE GLOBE

ž   CONCLUSION

ž   REFERENCES

 

FIGURE LIST

1. Fig 2.1 Ulrich Finsterwalder
2. Fig 4.1 Bodie Creek Suspension Bridge , Falkland Islands
3. Fig 4.2 Maldonado Stress Ribbon Bridge, Uruguay
4. Fig 5.1 Construction Technique
5. Fig 6.1 Structural System
6. Fig 6.2 Structural System
7. Fig 7.1.1 Static model – cross section
8. Fig 7.1.2  Static model
9. Fig 7.1.3 Static model – ultimate load
10. Fig 7.1.4 Wind tunnel test
11. Fig 7.2.1 Prague-Troja Bridge - load test

TABLE LIST

1. Table  11.1 Stress Ribbon Bridges

INTRODUCTION

A stressed ribbon bridge (also stress-ribbon Bridge) is a tension structure (similar in many ways to a simple suspension bridge). The suspension cables are embedded in the deck which follows a catenary arc between supports. Unlike the simple span the ribbon is stressed in compression, which adds to the stiffness of the structure (simple suspension spans tend to sway and bounce). The supports in turn support upward thrusting arcs that allow the grade to be changed between spans (where multiple spans are used). Such bridges are typically made from concrete reinforced by steel tensioning cables. Where such bridges carry vehicle traffic a certain degree of stiffness is required to prevent excessive flexure of the structure, obtained by stressing the concrete in compression.

Stress Ribbon Bridges Philosophers, thinkers, intellectuals all appeal, please build bridges and not walls between different communities, nationalities, countries, languages etc, to achieve universal brotherhood. This can be achieved by constructing stress ribbon bridges.

 Stress ribbon bridges are very economical, aesthetic and almost maintenance free structure. They require minimal quantity of materials. They are erected independently from the existing terrain and therefore they have minimum impact upon the environment during construction. 

 Stress ribbon bridge is the term used to describe structures formed by a very slender concrete deck in the shape of a catenary. They can be designed with one or more spans and are characterized by successive and complementary smooth curves. These curves blend in to natural environment and their forms, the most simple and basic of structural solutions. The stress ribbon bridge can be erected without undue pressure on the environment.

 Stress ribbon bridges looks at how slender concrete deck are used in the design of suspension and cable stayed structures. It looks at their characteristic feature; their rigidity, which is mainly given by the tension stiffness of prestressed concrete decking so much so that movement caused by pedestrians or wind does not register as discomfort by users. As opposed to suspension bridges, where the cables carry the load, in stress ribbon, by tensioning the cables and the deck between the abutments, the deck shares the axial tension forces. Anchorage forces are unusually large since the structure is tightly tensioned.

2. FINSTERWALDER’S STRESS RIBBON BRIDGE THEORY

Stress Ribbon Bridge uses the theory of a catenary transmitting loads via tension in the deck to abutments which are anchored to the ground. This concept was first introduced by a German engineer Ulrich Finsterwalder. The first stress ribbon bridge was constructed in Switzerland in the 1960s. The new bridge at Lake Hodges is the sixth ribbon bridge in North America, with three equal spans of 330 feet is the longest of this type.

The stress ribbon bridge combines a suspended concave span and a supported convex span. The concave span utilizes a radius of about 8200 ft while the convex span, depending on the design speed of the bridge, utilizes an approximate radius of 9800 ft (1965).

The stress ribbon itself is a reinforced concrete slab with a thickness of about 10 inches (25.4cm). This reinforcement consists of three to four layers of 1 inch (2.5cm) to 1 ¼ inch (1.2cm) diameter, high strength steel. The layers are spaced so that the prestressing pipe sleeve couplings can be used as spacers both vertically and horizontally. To resist bending moments from traffic, the slab is heavily reinforced at the top and bottom in the transverse direction.

The high strength steel tendons are stressed piece by piece during erection to produce the

 desired upward deflection radius of 8200 feet (2500m) under dead load of the superstructure plus the pavement. A temporary catwalk is provided to stress the first tendons. The formwork for the bridge is hung from the tendons and then removed once the concrete is cured. Concrete is placed from the middle of the freely hanging 63 suspended concave part and continues without interruption to the supports (Finsterwalder 1965).

Fig 2.1 Ulrich Finsterwalder

3. FORM OF A STRESS RIBBON BRIDGE

3.1 Superstructure

A typical stress ribbon bridge deck consists of precast concrete planks with bearing tendons to support them during construction and separate prestressing tendons which are tensioned to create the final designed geometric form. The joints between the planks are most often sealed with in-situ concrete before stressing the deck. The prestressing tendons transfer horizontal forces in to the abutments and then to the ground most often using ground anchors. The tendons are encased in ducts which are generally grouted after tensioning in order to lock in the stress and protect them from corrosion. Since the bending in the deck is low, the depth can be minimized and results in reduction in dead load and horizontal forces in abutments.

3.2 Substructure

The abutments are designed to transfer the horizontal forces from the deck cables into the ground via ground anchors. Pedestrians, wind and temperature loads can cause large changes in the bending moments in the deck close to the abutments and accordingly crack widths and fatigue in reinforcement must be considered. The ground anchors are normally tensioned in 2 stages, the first step is tensioned before the deck is erected and the rest, after the deck is complete. If stressed in one stage only, there will be a large out of balance force to be resisted by the abutments in the temporary case. The soil pressure, overturning and sliding has to be checked for construction as well as permanent condition.

3.3 Ground Conditions

The ideal ground condition for resisting large horizontal forces from the ribbon is a rock base. This occurs rarely but suitable foundations can be devised even if competent soils are only found at some depth below the abutments. In some cases where soil conditions do not permit the use of anchors, piles can also be used. Horizontal deformations can be significant and are considered in the design. It is also possible to use a combination of anchors and drilled shafts. Battered micropiling is another alternative which can resist the load from the ribbon because of its compression and tension capacity.

4. COMPARISON WITH A SIMPLE SUSPENSION BRIDGE

A stress ribbon bridge is a tension structure similar in many ways to a simple suspension bridge. The suspension cables are embedded in the deck which follows a catenary arc between the supports. As opposed to suspension bridges, where the cables carry the load, in stress ribbon, by tensioning the cables and the deck between abutments, the deck shares axial tension forces. Unlike the simple span the ribbon is stressed in compression, which adds to the stiffness of the structure. A simple suspension span tends to sway and bounce. The supports in turn support upward thrusting arcs that allow the grade to be changed between spans, where multiple spans are used.

Such bridges are typically made from concrete reinforced by steel tensioning cables. Where such bridges carry vehicle traffic a certain degree of stiffness is required to prevent excessive flexure of the structure, obtained by stressing the concrete in compression. Anchorage forces are unusually large since the structure is tightly tensioned.

Fig 4.1 Bodie Creek Suspension Bridge, Falkland Islands

Fig 4.2 Maldonado Stress Ribbon Bridge, Uruguay

5. CONSTRUCTION TECHNIQUES

The construction of the bridge is relatively straight forward. The abutments and piers are built first. Next the bearing cables were stretched from abutment to abutment and draped over steel saddles that rested atop the piers. The bearing tendons generally support the structure during construction, and only rarely is additional false work used. Once the bearing cables were tensioned to the specified design force, precast panels were suspended via support rods located at the four corners of each panel. At this point the bridge sagged into its catenary shape.

The next step was to place post tensioning ducts in the bridge. The ducts were placed directly above the bearing cables and support rods, which are all located in two longitudinal troughs that run the length of the bridge. After the ducts were in place, the cast-in place concrete was placed in the longitudinal troughs in small transverse closure joints. Concrete is poured in the joints between the planks and allowed to harden before the final tensioning is carried out. Retarding admixtures may be used in the concrete mix to allow all the concrete to be placed before hardening occurs. Once the final tension has been jacked into the tendons and the deflected shape is verified, the ducts containing the tendons are grouted.

After allowing the cast in place concrete to cure and achieve its full strength, the bridge was post tensioned. The post tensioning lifts each span, closes the gap between the panels, puts the entire bridge in to compression and transforms the bridge in to continuous ribbon of prestressed concrete.

Fig 5.1 Construction Technique

6. STRUCTURAL SYSTEM

The development of the self-anchored stress-ribbon structure supported by an arch is evident from Fig. 1. It is clear that the intermediate support of a multi-span stress-ribbon can also have the shape of an arch (Figure 1a). The arch serves as a saddle from which the stress-ribbon spans can rise during post-tensioning and during temperature drop, and where the center "band" can rest during a temperature rise.

In the initial stage, the stress-ribbon behaves as a two-span cable supported by the saddle that is fixed to the end abutments (Figure 1b). The arch is loaded by its self-weight, the weight of the saddle segments and the radial forces caused by the bearing tendons (Figure 1c). After post-tensioning the stress-ribbon with the prestressing tendons, the stress-ribbon and arch behave as one structure.

The shape and initial stresses in the stress-ribbon and in the arch can be chosen such that the horizontal forces in the stress-ribbon HSR and in the arch HA are the same. It is then possible to connect the stress-ribbon and arch footings with inclined compression struts that balance the horizontal forces. The moment created by horizontal forces HSR.h is then resisted by the ΔV.LP. In this way a self-anchored system with only vertical reactions is created (Figure 1d).

Fig 6.1 Structural System

It is also obvious that the stress-ribbon can be suspended from the arch. It is then possible to develop several self-anchored systems. Figure 2 presents some concepts using such systems. Figure 2a shows an arch fixed at the anchor blocks of the slender prestressed concrete deck. The arch is loaded not only by its self-weight and that of the stress-ribbon, but also with the radial forces of the prestressing tendons. Figure 2b shows a structure that has a similar static behavior as the structure presented in Figure 1d. To reduce the tension force at the stress-ribbon anchor blocks, it is possible to connect the stress-ribbon and arch footings by inclined compression struts that fully or partially balance the stress-ribbon horizontal forces. Figure 2c shows a similar structure in which the slender prestressed concrete band has increased bending stiffness in the portion of the structure not suspended from the arch.

Fig 6.2 Structural system

7. MODEL TESTS

7.1 MODEL TESTS

 The authors believe that a structural system made up of a stress-ribbon supported by an arch increases the potential application of stress-ribbon structures. Several analyses were under taken to verify this. The structures were checked not only with detailed static and dynamic analysis, but also on static and full aero elastic models. The tests verified the design assumptions and behavior of the structure under wind loading that determined the ultimate capacity of the full system.

Fig 7.1.2 Static model

  

The model tests were done for a proposed pedestrian bridge across the Radbuza River in Plzen, Czech Republic. This structure was designed to combine a steel pipe arch having a span length of 77 m and the deck assembled of precast segments. The static physical model was done in a 1:10 scale. The shape is shown in Figures 3 and 4. Dimensions of the model and cross-section, loads, and prestressing forces were determined according to rules of similarity. The stress-ribbon was assembled with precast segments of 18 mm depth and the cast-in-place haunches were anchored in anchor blocks made with steel channel sections. The arch consisted of two steel pipes, and the end struts consisted of two steel boxes fabricated from channel sections. The saddle was made by two steel angles supported on longitudinal plates strengthened with vertical stiffeners. The footings common to the arch and inclined struts were assembled from steel boxes fabricated with two channel sections. They were supported by steel columns consisting of two I sections. The end ties consisted of four rectangular tubes. The steel columns and the ties were supported by a longitudinal steel beam that was anchored to the test floor.

Fig 7.1.3 Static model – ultimate load

Fig 7.1.4 Wind tunnel test

The stress-ribbon before casting of the joints. During erection of the segments, casting of the joints and post-tensioning of the structure, the deformations of the arch and the deck where the precast segments were made from micro-concrete of 50 MPa characteristic strength. The stress-ribbon was supported and post-tensioned by 2 monostrands situated outside the section. Their position was determined by two angles embedded in the segments. The loads, determined according to the rules of similarity, consisted of steel circular bars suspended on the transverse diaphragms and on the arch. The number of bars was modified according to desired load. The erection of the model corresponded to the erection of the actual structure. After the assembly of the arch and end struts, the monostrands were stranded. Then the segments were placed on the monostrands and the loads were applied. Next, the joints between the segments and the haunches were cast. When the concrete reached the minimum prescribed strength, the monostrands were tensioned to the design force. Before erection of the segments, strain gauges were attached to the steel members and the initial stresses in the structure were measured. The strain gauges were attached at critical points of carefully monitored and the forces in the monostrands were measured by dynamometers placed at their anchors (Figure 4). The model was tested for the 5 positions of live load.  At the end of the tests the ultimate capacity of the overall structure was determined. It was clear that the capacity of the structure was not given by the capacity of the stress-ribbon since, after the opening of the joints, the whole load would be resisted by the tension capacity of the monostrands. Since the capacity of the structure would be given by the buckling strength of the arch, the model was tested for a load situated on one side  of the structure (Figure 5). The structure was tested for an increased dead load (1.3 G) applied using the additional suspended steel rods, and then for a gradually increasing live load P applied with force control using a hydraulic jack reacting against a loading frame. The structure failed by buckling of the arch at a load 1.87 times higher than the required ultimate load Qu = 1.3 G + 2.2 P. The stress-ribbon itself was damaged only locally by cracks that closed after the overloads were removed. The structure also proved to be very stiff in the transverse direction. The buckling capacity of the structure was also calculated with a nonlinear analysis in which the structure was analyzed for a gradually increasing load. The failure of the structure was taken at the point when the analytic solution did not converge. Analysis was performed for the arch with and without fabrication imperfections. The imperfections were introduced as a sinus-shaped curve with nodes at arch springs and at the crown. Maximum agreement between  the analytical solution and the model was achieved for the structure with a maximum value of imperfection of 10 mm. This value is very close to the fabrication tolerance. The test has proven that the analytical model can accurately describe the static function of the structure both at service and at ultimate load. The dynamic behavior of the proposed structure was also verified by dynamic

STATIC AND DYNAMIC LOADING TESTS

The design assumptions and quality of workmanship of the author's first stress ribbon structure built in the Czech Republic and of the first stress ribbon bridge built in United States were checked by measuring the deformations of the superstructure at the time of prestressing and during loading tests. Dynamic tests were also  performed on these structures. Only a few key results of a typical structure are given here. Since the shape of a stress ribbon structure is extremely sensitive to temperature change, the temperature of the bridge was carefully recorded at all times.

Fig 7.2.1 Prague-Troja Bridge - load test

The pedestrian bridge in Prague-Troja was tested by 38 vehicles weighing between 2.8 and 8.4 tons – see Fig.4.7.1. First, the vehicles were placed along the entire length of the structure, and then they were placed on each span. During the test only the deformations in the middle of the spans and the horizontal displacements of all supports were measured 

8. ADVANTAGES AND APPLICATIONS

8.1 Advantages

•          Stress ribbon pedestrian bridges are very economical, aesthetical and almost maintenance free structures.
•          They require minimal quantity of materials.
•          They are erected independently from existing terrain and therefore they have a minimum impact upon the environment during construction.
•          They are quick and convenient to construct if given appropriate conditions, without false work.
•          A stress ribbon bridge allows for long spans with a minimum number of piers and the piers can be shorter than those required for cable stayed or suspension bridges.

8.2 Applications of stress ribbon principle

·         Eco duct: A tunnel which was built as part of a large network of motorways outside Brno. The theory is the same as a self-anchored arch but the geometry is much more complex. It is 50m wide and spans 70m a finite element programme was used in its design.

·         Stuttgart trade fair hall roof: The suspended asymmetric roof comprises a regular repetition of stressed trusses with individual I-beam ribbons of steel between them. The trusses function as strut and tie A-frames based on concrete strip foundations and are tied back to the ground with anchors. The stresses in the ribbons and weight of its ‘green roof’ were used to resist wind uplift.

9. MODIFIED STRESS RIBBON BRIDGES

One disadvantage of the traditional stress ribbon type bridges is the need to resist very large horizontal forces at the abutments. Another characteristic feature of the stress ribbon type structures, in addition to their very slender concrete decks, is that the stiffness and stability are given by the whole structural system using predominantly the geometric stiffness of the deck. At present research on the development of new structures combining classical stress-ribbon deck with arches or cables is being carried out.

9.1 Stress stiffened by arches ribbon bridges

The stress ribbon deck is fixed in the side strut. Both the arches and struts are founded on the same footings. Due to the dead load the horizontal force both in the arch and in the stress ribbon have the same magnitude, but they act in opposite directions. Therefore the foundation is loaded only by vertical reactions. This self-anchoring system allows a reduction in the cost of the substructure.

The arches serves as a saddle from which the stress ribbon can rise during post tensioning and during temperature drop, and where the bond can rest during a temperature rise. In the initial stage the stress ribbon behaves as a two span cable supported by the saddle that is fixed to end abutments. After post tensioning the stress ribbon with the prestressing tendons, the stress ribbon and arch behaves as one structure.

9.2 Stress ribbon bridges stiffened by cables

The second type of studied structure is a suspension structure formed by a straight or arched stress ribbon fixed at the abutments. External bearing cables stiffen the structure both in the

vertical and horizontal directions. Horizontal movements caused by live load are eliminated by stoppers, which only allow horizontal movement due to temperature change and shrinkage of concrete.

Support of the deck in a horizontal direction provided by a stopper was designed and analyzed during the study and development of this structural type. This device allows horizontal movement due to the creep and shrinkage of concrete. At the same time the devices stops horizontal movement due to short term loads like a live load, wind load or earthquake. Deck deflections and bending moments are reduced to zero or very small horizontal movement. Natural frequencies and mode shapes were also determined during dynamic analysis. The influence of the aforementioned structural arrangements on frequencies and mode shapes were studied. The structure allows one to place an observation platform at midspan. But dynamic behavior is influenced by platform positioning, weight and area. For this reason the aerodynamic stability of the structure was checked in a wind tunnel.

10. A CASE STUDY

Location: Over Lake Hodges, San Diego, USA

Length: 3 spans of 330 feet

This is the world’s longest stress ribbon bridge. Earlier, there was only a 9 mile road connecting the north and south sides of the lake. Bicyclists and pedestrians had to use the shoulder for travelling to and fro from work. Now this elegant structure keeps pedestrians and bicyclists of the freeway without exacting a toll on the environment or visual landscape.

The firm behind this evaluated a broad range of bridge types that might be viable for this location. They included the pre-fabricated steel truss design, various concrete girder alternatives, a laminated timber bridge in which glue is used for lamination (“glulam”) and such long-span alternatives as cable-stayed and suspension bridges. The steel ribbon concept was also considered. Steel truss, concrete and glulam have bulky super structure and long span concepts were avoided due to the very high towers. It was quite clear that the chosen bridge type had to have the following features:-

· Minimal environmental effects.

· A long span with a minimum number of piers in the lake.

· An ability to be constructed above water without false-work.

· A visual effect so minimal that the structure would blend into the landscape.

· The design should work well in both dry and wet conditions.

After considering the above options, aesthetically and functionally stress ribbon design was the perfect choice.

11. STRESS RIBBON BRIDGES AROUND THE GLOBE

 

12. CONCLUSION

Stress ribbon bridges are a versatile form of bridge, the adaptable form of structure is applicable to a variety of requirements. The slender decks are visually pleasing and have a visual impact on surroundings giving a light aesthetic impression. Post tensioned concrete minimizes cracking and assures durability. Bearings and expansion joints are rarely required minimizing maintenance and inspections. There are also advantages in construction method, since erection using pre-cast segments does not depend on particular site condition and permits labour saving erection and a short time to delivery. Using bearing tendons can eliminate the need for site form work and large plant, contributing to fast construction programmes and preservation of the environments. There is a wide range of different topographies and soil conditions found and a number of areas which require aesthetic yet cost effective pedestrian bridges to be built: Stress ribbon bridges could provide elegant solutions to these challenges.

REFERENCES

v  Strasky, J (2005) Stress Ribbon and Cable-Supported Pedestrian Bridges. London:

Thomas Telford.

v  Strasky, J (1987) Precast stress ribbon pedestrian bridges in Czechoslovakia. PCI JOURNAL, May-June 1987.



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CONTROL OF CORROSION ON UNDERWATER PILES

 ABSTRACT   

Piles are structures used to transfer loads from superstructure to the sub surface strata. When the subsurface stratum is water based or if we deal with a hydraulic structure, the piles are to be driven into water and under water strata. Piles used in underwater structures are often subjected to corrosion. There is no absolute way to eliminate all corrosion; but corrosion protection measures are employed to control the effect of corrosion. Corrosion protection can be in different ways according to the environment and other factors. Forms of corrosion protection include the use of inhibitors, surface treatments, coatings and sealants, cathodic protection and anodic protection. The control measures explained in this are Protective coatings, cathodic treatment and application of Fibre Re-inforced Polymer (FRP) Composites.

1.      INTRODUCTION

 Corrosion is the destruction of metals and alloys by the chemical reaction with the environment. During corrosion the metals are converted to metallic compounds at the surface and these compounds wears away as corrosion product. Hence corrosion may be regarded as the reverse process of extraction of metals from ore.

         Corrosion and in particular corrosion of metal structures, is a problem that must regularly be addressed in a wide  variety of areas, for example, in the automotive industry, metal parts are often plated or coated to protect them from road salt and moisture in hopes of increasing their longevity. Indeed, many traditional metal parts are currently being used with polymeric components, which are not only lighter but also more cost effective to produce. But these are generally impervious to electrochemical corrosion often experienced by metals. Even with the proper selection of base metals and well-designed systems or structures, there is no absolute way to eliminate all corrosion. Therefore, corrosion protection methods are used to additionally mitigate and control the effects of corrosion. Corrosion protection can be in a number of different forms/strategies with perhaps multiple methods applied in severe environments. Forms of corrosion protection include the use of inhibitors, surface treatments, coatings and sealants, cathodic protection and anodic protection.

1.1 Corrosion Mechanism Of Steel In Sea Water

 On steel piling in seawater, the more chemically active surface areas (anodes) are metallically coupled through the piling itself to the less chemically active surface areas (cathodes) resulting in a flow of electricity and corrosion of the anodic areas. General surface roughening occurs when these local anodic and cathodic areas continually shift about randomly during the corrosion process. Sometimes these active local areas do not shift position end, therefore, the metal suffers localized attack and pitting occurs. In general, the depth of pitting is related to the ratio of the anodic sites to the area of cathodic site in contact with the electrolyte (seawater). The smaller the anode area relative to the cathode area, the deeper the pitting.

      1.2 Zones Of Corrosion Of Steel Piles

                         Examination of corroded marine piles reveals several distinct areas of attack. It is convenient to divide these areas into five zones, each having a characteristic corrosion rate as shown in Fig 2.1

Fig 1.1 zone of corrosion of steel plates

1.3   Corrosion Management

 Before deciding on the methods for control of corrosion to be applied, conceptual and feasibility studies have been carried out. Typically, corrosion management can be divided into three major phases.

        Phase 1 of the program is the programmatic assessment of the project. This phase is the planning stage for a corrosion management program  to take place. It initiates the program to be implemented on structures that are found to be under the threat of corrosion. For the planning stage, three main requirements are sought, namely the strategy, budget and schedule needed to overcome the problem raised from corrosion of reinforcement. This is seen as an important part for an effective management program as feasibility studies are normally conducted to determine the serviceability of the structure after treatment.

        Phase 2 of the program involves physical assessment and actual remediation. Inspections for severity of corrosion are conducted in this phase to determine what strategy or methods are most suitable to be applied. Development of corrosion control strategy would present more option to the management program. Remedial work would be carried out once the proper strategy has been  recognized.

        Phase 3 of the program mainly deals with future monitoring of the repaired structure. Currently and historically, most of the corrosion control programs are driven by response to incident or urgent need, rather than systematically identifying and managing the existing resources. This can be overcome by implementing internal or external monitoring system using current technology practiced in oil and gas industries is as shown in Fig. 2.2.

Fig.1.2 The overall flowchart for an effective corrosion management program

2.      CORROSION PROTECTION METHODS

             2.1  Protective Coating

 In order to protect metals from corrosion, the contact between the metal and the corrosive environment is to be cut off. This is done by coating the surface of metals with a continuous non-porous material inert to the corrosive atmosphere.

Surface coatings are broadly classified into three

a). Metallic coatings

b). Inorganic Coatings

c). Organic Coatings

        Individual coatings are formulated to perform specific functions and must be selected to become components of a total system designed for optimum results considering the environment and service expectations.

The different types of coatings used for under water piles are:

2.1.1 Inorganic Zinc Silicates Primers

Steel structures that are permanently immersed in sea water, such as jackets in the area below the Splash Zone, are typically not coated for various reasons and protected solely by cathodic protection systems consisting of sacrificial anodes or impressed current arrays, which can be maintained as required by underwater contractors. Various anticorrosive pigmented primers are available, some that passivate the steel but the most effective are inorganic zinc silicate primers which essentially become anodic to the steel in a corrosion cycle. The primary advantage of this type of coating is that it will arrest rust creep, or undercutting of the coatings surrounding the damaged area, and confine corrosion to the point of the damage. These coatings also provide a high degree of resistance to heat and chemical spills.

2.1.2 High Build Epoxy Coatings

Epoxies are generally more abrasion and chemical resistant than primers and topcoats and in this case protect not only the substrate itself, but the zinc primer as well from all of these detrimental factors. However, one drawback with epoxy coatings is very poor resistance to ultra violet from sunlight and most will chalk and fade rapidly. This leads to an erosion of the coatings’ film thickness, reducing the barrier protection of the system.

2.1.3   Aliphatic Polyurethane Topcoats

Polyurethane finish coats are generally acknowledged as providing optimum resistance to UV and high degrees of flexibility and chemical resistance. They also help to maintain a very high level of cosmetic gloss and colour retention and can be cleaned very easily, generally with low pH detergents and fresh water pressure washing. Although polyurethane finishes offer no real anticorrosive or barrier protection to the substrate they do provide a high level of protection to the integrity of the coatings system.

2.1.4   Zinc Rich Epoxy Primers

 Zinc modified epoxy anticorrosives will provide a high level of service and are more tolerant to compromised surface preparation and ambient weather conditions provided the zinc loading of the formula is sufficient. Zinc rich epoxy is also most effective in maintaining damaged areas and breakdown of the coatings systems applied at new construction as it is compatible with alternate methods of surface preparation such as power tool cleaning and UHP Hydro Blasting.

2.1.5 Non-Skid Deck Coatings

Coatings specifically designed with anti-slip properties normally incorporate very course aggregates for an exaggerated profile. They are applied in very high film builds and normally without a zinc rich primer. When primers are required they are usually epoxy types.

2.2 Cathodic Protection

The preferred technique for mitigating marine corrosion, based on historical performance   and measurable results, is cathodic protection (CP) - the practice of using electrochemical reactions to prevent the corrosion of steel structures. The reason for increased acceptance: cathodic protection prevents corrosion on underwater structures.

        In theory and practice, the implementation of a CP system is quite simple. Assuming you already have corroding steel in seawater, all you need is an anode, a power supply, and engineering talent. A protective circuit is accomplished between the anode, steel (cathode), power supply and electrolyte (seawater).

2.3 A Typical Anode Delivery System

	Fig. 2.1 Pile mounted anode  Pile mounted anodes are designed for efficient current distribution in and around pilings where the complex geometry of the facility precludes remote placement of the anodes. These delivery systems are suitable for direct attachment to pilings. The Flat Back Pile Mounted Anode was designed specifically for H-Piles, and can also be configured for installation on sheet piling. 2.3.1   Pile Mounted Anodes   Pile mounted anodes are designed for efficient current distribution in and around pilings where the complex geometry of the facility precludes remote placement of the anodes. These delivery systems are suitable for direct attachment to pilings. The Flat Back Pile Mounted Anode was designed specifically for H-Piles, and can also be configured for installation on sheet piling. 2.3.2   Disk Anode The disk anode was designed in conjunction with the U.S. Army Construction Engineering Research Lab for use on navigational locks and dam gates. This anode system is also suitable for use on seawater intake structures, vessel internals, and sheet piling when shore side access is possible. 2.3.3  Retractable Mount Fig. 2.2 Retractable mount For installations where it is deemed necessary to access the anode for periodic maintenance, or when current is only required on a periodic basis, the retractable anode allows the user to easily retrieve the anode. The above illustration is rotated by 90 degrees. 2.3.4   Sled Anode
Anodes mounted on the sea bed typically afford the best spread of protection on a marine structure. Sled anodes can be designed for operation in either seawater or buried in the mud. The Post Tension Sled was developed to insure anode operation out of the mud when resting in silty and soft sea beds. By adjusting the height of the concrete sled, the mesh anode sled can also be designed for operation out of the mud. The advantage of this type of sled is its low profile, thereby limiting the potential for, damage by anchors fishing nets, etc.

Fig. 2.3 Sled anode

2.3.5  Suspension Anodes Suspension Anode Delivery Systems allow for strategic placement of anodes in and around a marine facility, providing optimum distribution of current. Many suspended anode systems are also suitable for mounting on pilings, or other structural steel. Fig 2.4 Suspension anode 2.3.6  Rod Anode Although incorporated into a variety of anode delivery systems, the rod anode is most commonly used for the cathodic protection of seawater intake structures and vessel internals.

 2.4   Application Of FRP Composites

        Fibre reinforced polymers (FRP) have long been used for the repair and retrofit of         concrete structural elements. Their lightweight, high strength and resistance to chemicals offer obvious benefits. In fabric form, they provide unparallel flexibility. Moreover, as fibres can be oriented in any direction, their use can be optimized. This makes FRP particularly suited for emergency repairs where damage can be multi-directional and speed of strength restoration critically important.

Fig 2.5   Repair and retrofit of concrete structural elements using FRP composites

        The emergence of new adhesives that allow FRP to be bonded to wet concrete surfaces makes it possible to economically conduct emergency repairs on sub-structure elements. Fig.3.6 shows impact damage that led to both cross-section loss and breakage of the spiral ties. Conventional repairs will require the cross-section to be enlarged to accommodate new ties. If instead, FRP were used it would only be necessary to re-form the cross-section and apply bi-directional layers that could restore lost tensile capacity while providing equivalent lateral support to the longitudinal steel. Moreover, the application of a protective UV (ultra-violet) coating on the wrap of the right colour will render the repaired pile indistinguishable from other undamaged piles. The aesthetics of FRP repair is one of its unheralded benefits.

Fig.2.6  Impact damage that led to both cross-section loss and breakage of the spiral ties

 2.5    Case Study For Application Of FRP Composites

 2.5.1 Allen Creek Bridge

 Allen Creek Bridge is located on the busy US 19 highway connecting Clearwater

and St. Petersburg, FL. The original bridge built in 1950 was supported on reinforced

concrete piles driven into Allen Creek. In 1982, the bridge was widened and this new

section was supported on 35 cm (14 in.) square prestressed piles.

        The waters from Allen Creek flow east into Old Tampa Bay that in turn joins then Gulf of Mexico to the south. The environment is very aggressive; all the reinforced concrete piles from the original construction had been rehabilitated several times. At low tide, the water level in the deepest portion of the creek is about 0.76 m (2.6 ft). Maximum high tide is about 1.89m (6.2 ft). This shallow depth meant that the underwater wrap could be carried out on a ladder.

2.5.1.1 Preparatory work

Pile surfaces were covered with marine growth  that had to be scraped off. Additionally, two of the four corners that were not rounded but chamfered had to be ground using an air-powered grinder. This was a difficult operation particularly for sections that were below the water line. A quick-setting hydraulic cement was used to fill any depression, discontinuities and provide a smooth surface. Just prior to wrapping the entire surface was pressure washed using freshwater to remove all dust and marine algae.

2.5.1.2 Instrumentation

Instrumentation was installed to allow linear polarization and corrosion potential measurements to be made. An innovative instrumentation scheme was developed that eliminated the need for wiring and junction boxes. This was an important consideration since the piles were located in relatively shallow waters that were accessible on foot. Several piles supporting the structure had been defaced and the probability of vandalism was very real. FRP wrapping -- Two different schemes using two different materials were evaluated. In each scheme four piles were wrapped with two other instrumented piles serving as controls. In the first scheme, cofferdam construction was used and the piles wrapped using a bi-directional FRP in a wet lay up under dry conditions. As this was wrapped under ‘perfect’ conditions, its performance provided means for evaluating piles that were directly wrapped in water using a new water activated resin The latter scheme was a pre-preg system developed by Air Logistics. The pre-preg was easy to install since all the material came in labelled hermetically sealed packets. After applying an initial epoxy layer, the packets were opened according to the layout scheme and the FRP material applied. A shrinkage wrap was applied at the end to allow the FRP to cure. On an average, it took between 30 minutes to 45 minutes to wrap a pile over a 1.5 m depth depending on the number of layers of material that had to be applied.

2.5.2   Friendship Trails Bridge

 This is the oldest of the Gandy Boulevard bridges crossing Tampa Bay. It was originally constructed in 1956 and was slated for demolition in 1997. Thanks to community activists, the bridge was saved, refurbished and rehabilitated. In 1999, the bridge was re-opened as a pedestrian bridge and re-christened as the “Friendship Trails Bridge”. The 4.2 km (2.6 mile) structure is now the longest over-water recreational trail in the world. The bridge has 275 spans supported by 254 reinforced concrete pile bents and 22 column type piers located at the main channel crossing. Seventy seven percent of the 254 piers supporting this bridge have needed to be repaired indicating that the environment is very aggressive.

2.5.2.1 Preparatory work

All piles wrapped were 50.8 cm x 50.6 m (20 in. x 20 in.) reinforced concrete piles and wrapped over a depth of 1.5 m that extended all the way to the underside of the pile cap. The waters are approximately 4.88 m (16 ft) deep. This meant that ladders could no longer be used to apply the FRP in this situation. An innovative scaffolding system was designed and fabricated. It was lightweight, modular  yet sufficiently rigid when assembled to support 4-6 people. The scaffolding was suspended from the pile cap and extended 2.74 m (9 ft) below. Its mesh flooring provided a secure platform around the pile that allowed the wrap to be carried out unimpeded in knee deep waters Fig. 2.7.

Fig 2.7   Mesh flooring around piles

2.5.2.2 Instrumentation

Unlike the Allen Creek Bridge where vandalism was a real concern, the piles of the Friendship Trails Bridge are located in deeper and more turbulent waters. Moreover, as the majority of the piles supporting this bridge had been repaired and some were instrumented, the element of novelty was absent making vandalism less likely. In view of this, an instrumentation system developed by the Florida Department of Transportation was selected. This required both wiring and junction boxes. The scheme uses rebar probes Fig. 3.8 that are installed at different elevations close to the reinforcing steel. Changes in the direction of the corrosion current between these locations can indicate if the FRP is working as expected. Reductions in the measured current compared to unwrapped controls were also expected to provide an index of the efficacy of the FRP wrap. The drawback with this system is that it takes time for the equilibrium state around the probe to be attained. Until this time, data may not be meaningful.

Fig. 2.8 Use of rebar probes in instrumentation

2.5.2.3 FRP wrapping

Two different FRP systems were used. One was the same pre-preg system with a water-activated resin used in the Allen Creek Bridge. The other was Fyfe’s system that used resins that cure in water. The pre-preg system was used to wrap four piles – two using carbon and two using glass. The wet-layup system from Fyfe required on-site saturation of the fibres. Two piles were wrapped with fibreglass using this system. Of the two, one was an experimental FRP system that combined wrapping with a sacrificial cathodic protection system. Two other unwrapped piles in a similar initial state of disrepair were used as controls to evaluate the performance of the wrapped piles. Application was facilitated through the use of a scaffolding system mentioned earlier Fig. 2.7.   

Fig. 2.9  Wrapping of FRP material around piles

 

        The pre-preg system was applied as in the Allen Creek Bridge and posed no problems. The Fyfe system was more challenging since the FRP material had to be saturated on-site. Access to foundations of an adjacent bridge provided a convenient staging post for the on-site impregnation Fig. 2.9. On an average the operation took 90 minutes to complete.

Fig. 2.10 On-site saturation, Friendship Trails Bridge, Tampa

3.    STANDARDS AND CODES

There are no Indian standards codes as such for the control of corrosion. The latest editions of the following organizations’ standards, codes, and guidelines shall be used for the design of corrosion control systems:

• NACE International (formerly The National Association of Corrosion

Engineers)

• RP0169 – Control of External Corrosion on Underground or Submerged

Metallic Piping Systems

• American Society for Testing and Materials (ASTM)

• ASTM D512 – Standard Test Methods for Chloride Ion in Water

• ASTM D516 – Standard Test Method for Sulfate Ion in Water

• ASTM G51 – Standard Test Method for measuring pH of Soil for Use in

Corrosion Testing

• American Railway Engineering and Maintenance-of-Way Association

(AREMA)

• Federal Highway Administration (FHWA)

• Publication FHWA-NHI-00-044 – Corrosion/Degradation of Soil

Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Soil

Slopes

4.  CONCLUSION

Though there is no absolute way to eliminate all corrosion on under water piles, there are some effective measures to control them. The cathodic protection is found to be quite simple to employ and mostly used in  marine conditions. The protective coatings are used in vast and expensive structures. The FRP composites have many advantages over conventional methods such that they are light weight, possess high strength and chemical resistance and moreover have incomparable flexibility.

         Of the various ways of wrapping of FRP composites, transverse wrapping is found to be the easiest as otherwise, the longitudinal pieces are awkward to handle and difficult to position. Bi-directional material is the best option. Scaffolding measures during the application of materials ensures safety and simplifies installation. Out of the two system of FRP application, the pre-preg system is easier to use. On-site FRP saturation can be problematic. High winds and high tides should be avoided during the process.

5.  REFERENCES

• R. Sen, G. Mullins, K. Suh and D. Winters: ‘FRP application in underwater repair of corroded pipes’, FRPRCS-7 (the 7th International Symposium on Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures), November 7-10, 2005 New Orleans Marriott  
• http://www.quakewrap.com/frp%20papers/FRPApplicationinUnderwaterRepairofCorrodedPiles.pdf

• Underwater steel structures: inspection, repair and maintenance

http://www.corrdefense.org/Technical%20Papers/UNDERWATER%20STEEL%20STRUCTURES%20INSPECTION%20REPAIR%20AND%20MAINTENANCE.pdf

• Marine cathodic protection

http://www.farwestcorrosion.com/fwst/anodimpr/elg04.htm

·        L. Van Damme, W. Vreulst: ‘Low Water Corrosion Of Steel Pilings’, Pianc Bulletin No.101,1999.

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