A Seminar Report On Portable Structure
A Seminar Report On Portable Structure
A Seminar Report On Portable Structure
On
CONTROL OF CORROSION ON
UNDERWATER PILES
Submitted in partial fulfillment of
BACHELOR OF TECHNOLOGY
In
CIVIL ENGINEERING
By
RAM SAYAN Kr. YADAV
I take this opportunity to remember the Almighty and our parents, who
bestowed strength, courage to perseverance to undertake the present
course of study and complete it successfully.
Last but the least, I would like to thankful my family & relatives, our
institute and my friends for helping me in my Endeavour.
Content
INTRODUCTION
Corrosion Mechanism Of Steel In Sea Water
Zones Of Corrosion Of Steel Piles
Corrosion Management
CORROSION PROTECTION METHODS
Protective Coating
Types of coatings used for under water piles
Inorganic Zinc Silicates Primers
High Build Epoxy Coatings
Aliphatic Polyurethane Topcoats
Zinc Rich Epoxy Primers
Non-Skid Deck Coatings
Cathodic Protection
Suspension Anodes
Rod Anode
Application Of FRP Composites
Case Study For Application Of FRP Composites
Allen Creek Bridge
Preparatory work
Instrumentation
Friendship Trails Bridge
Preparatory work
Instrumentation
FRP wrapping
CONCLUSION
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.
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.
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.
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
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: -
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 are:-
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).
Fig.3. sled anode
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.
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
Fiber 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.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 color will render the repaired pile indistinguishable from other undamaged
piles. The aesthetics of FRP repair is one of its unheralded benefits.
Fig.6. Impact damage that led to both cross-section loss and breakage of the spiral ties
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.
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. 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.
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.
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.
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.7.
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.
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.7.
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.9. On an average the operation took 90 minutes to complete.
Fig.10. On-site saturation, Friendship Trails Bridge, Tampa
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.