Underwater repair for bridge

Transcription

1 IABSE-JSCE Joint Conference on Advances in Bridge Engineering-II, August 8-10, 2010, Dhaka, Bangladesh. ISBN: Amin, Okui, Bhuiyan (eds.) Underwater repair for bridge D.S. Ramachandra BASF Construction Chemicals India Pvt. Ltd., India ABSTRACT: With the rapid increase in vehicular traffic, there is a demand for increase in infrastructural facilities. Due to various constraints the process is still at snails pace and hence the demand for maintenance of existing bridges is becoming important. With most of the bridges being constructed across the waterlines there is a need for underwater evaluation and repair of bridge components. With the occurrence of a series of unfortunate incidences, the importance of underwater evaluation and repair of submerged bridge components is taking centre stage. In USA guidelines for carrying out the evaluation and repair of submerged components was prepared (FHWA DP 98 1) and being followed since long time. The following paper discusses the various methods of evaluation of bridges and repair methodologies for different defects found. 1 INTRODUCTION Most of the bridges across the water line have their foundations in the submerged area and hence are critical for evaluation. These may be constructed of Concrete, Steel, timber or combination of above. Most of these elements being submerged under water are prone to fail without any indication (visible by naked eye). The changing conditions of waterline from the time of construction viz., pollution, and sudden floods cause severe defects in the hidden part of the bridges. All these conditions lead for formulation of strategic methodology for evaluation and repair of submerged bridge elements. The unavailability of technical diving personnel calls for an understanding of the subject by bridge engineers. This would help in guiding the available diving personnel to carry out the necessary operation. The equipment required for inspection apart from diving equipment is expensive and require training to operate. The current paper discusses the following points pertaining to this topic. 1 Evaluation methods. 2 Diving systems. 3 Inspection tools 4 Defects in structures. 5 Repair methodologies & materials. 6 New technologies 2 EVALUATION METHODS As per FHWA standards the inspection methods are categorized under three levels. 2.1 Level I inspection visual & tactile inspection A level I inspection involves a close visual or tactile examination, using large sweeping motion of hands where visibility is limited. Although the Level I inspection is often referred to as Swim-By inspection it must be detailed enough to detect obvious major damage or distress due to over stress or severe deterioration 576

2 or corrosion. It should confirm the continuity of the full length of the all members, detect undermining and exposure of normally buried elements. A level I inspection is normally conducted on the total exterior surface of the underwater element be it pier, abutment, pile bent or bulk head. A level I inspection may also include limited probing of the substructure and adjacent stream bed. The Level I inspection provides general overview of the substructure conditions and verification of the as built drawings. The Level I inspection can also indicate the need for Level II or Level III inspections and aid in determining the extent and location of more detailed inspections. 2.2 Level II inspection detailed inspection with partial cleaning A Level II inspection is a detailed inspection which requires that portions of the structures be cleaned to make them free from marine growth. Cleaning is a time consuming method and hence it is restricted to the critical areas. For pile type structures a 10 band shall be cleaned at three locations generally at the low water line, above the mud line and at mid point between the low water line and mud line. For rectangular piles at least three sides shall be included and for octagonal piles at least six sides and for circular piles at least three fourths the perimeter of the pile. On an H-pile, at least outside faces of the flange and one side of the web. On large solid faced elements such as piers or abutments a 1 x 1 patches at three different locations shall be selected. The selection of areas for cleaning shall be determined so as to minimize the potential for damage of the structure. Damaged areas shall be measured and extent and severity of damage shall be documented. The Level II inspection is aimed at determining the damaged and deteriorated areas hidden by surface biofouling. The thoroughness of the cleaning shall be determined by what is necessary to discern the condition of the underlying element. 2.3 Level III inspection highly detailed inspection with NDT A Level III inspection is a highly detailed inspection of a critical structure where extensive repair or possible replacement is being contemplated. The purpose of this inspection is to determine the hidden or interior damage or loss of cross sectional area and to evaluate the material homogeneity. This level of inspection includes extensive cleaning, detailed measurements and Non-destructive or Partially destructive test such as Ultrasonics, Core Sampling and Boring, Physical material sampling and insitu hardness testing. The use of testing techniques are generally limited to key structural areas, areas which are suspect or areas which are representative of the underwater structure. 3 DIVING SYSTEMS Within air diving, there are two primary systems viz., Scuba in which the diver carries his air supply with him in a cylinder and Surface applied in which air source is kept on a boat or shore. The appropriateness of system depends on various factors depth, bottom time, inspection tasks, waterway, environment and the experience and capability of the diver. Both the systems have their own advantages and disadvantages. SCUBA: Scuba is an acronym for Self Contained Underwater Breathing Apparatus. Scuba is generally recognized today as an open circuit. The diver inhales air directly from a container and exhales into the surrounding water. Scuba generally utilizes high pressure aluminum or stainless steel cylinders through two stage regulators that supply air to divers. The cylinders are filled by means of High pressure compressor. The First stage regulator is connected directly to the valve of the high pressure bottle. The first stage reduces the high pressure air in the cylinder to the intermediate pressure and Second stage regulator reduces the intermediate pressure to ambient level to make if comfortable for diver. The diver activates a valve when inhales which supplies airflow to him. The advantage of scuba is it ease of maneuverability and its disadvantage is the limited bottom times due to cylinder capacities. Surface to diver communications is possible in scuba using Hard wire or Wireless systems. SURFACE APPLIED SYSTEMS: The surface applied systems are two types. The hard hat and light weight. The hard hat system consists of metallic helmet connected to a breast plate with heavy weight dress and shoes weighing around 200 pounds. The hat is connected to the surface supplied air source by means of air hoses. The light weight equipment consists of Full face mask, safety harness, weight belt and fins. Demand regulators similar to that of Scuba were fit inside the helmets for comfortable breathing. In both the systems, the helmet or hat is directly connected to the Low pressure compressor or series of reservoirs. The air flow source is connected through a filtration system to the reservoirs which are connected to the diver s Umbilical. Um- 577

3 bilical is a combination of air hose, safety line, communication line and pneumo-fathometer. Pneumofathometer gives the depth at which diver is working that will help the tender at surface to adjust the pressure and determine the bottom times accordingly. The major disadvantage of this system is limited maneuverability of divers due to umbilical and advantage is unlimited bottom times of course with in the decompression limits. SECOND STAGE REGULATORS FOR SCUBA KMB HELMET 4 INSPECTION TOOLS DIVER COMMUNICATION RADIO For Level I inspection: Since the Level I inspection consists of scanning the entire structure using swim by method, as such not much additional tools are required for inspection apart from diving equipment. As already explained earlier, divers not being technical a on line CCTV system connected to an underwater camera would be highly handy in such type of inspections. This system enables the bridge inspector to see the surface of the submerged components on the monitor provided at shore or boat. The monitor is connected to the hand held camera by the diver. Since the medium through which images reflect on camera is water, the inspection is highly dependent on water visibility or turbidity or clarity. This is a major set back since most of the bridges are across the river bodies which seldom have clear water. Thus picture would be highly hazy and inconclusive in these cases. Hence one has to adapt a tactile inspection of the structure where the diver will try to feel the surface with his hands and would report the same to the shore through hard wire communication. Alternatively, the same can be negated by the use of CLEARWATER BOX. The clear water box is a rectangular box having a transparent glass or acrylic sheet at the front with clean water filled in it. The camera is installed in side the clear water box and connected to the monitor. The diver is asked to run the clear water box along the peripheral or surface line of the component. The picture is much better than normal, but the size of the area covered per frame is reduced to the size of the transparent plate at the front. For practical reasons, the size of the box can not be very high. 578

4 Level II Inspection: In this inspection, small areas on the surface of the elements need to cleaned to make them free from biofouling before doing the inspection as per Level I. Thus some tools for effective cleaning of the same are required for this inspection. Hand Tools: Almost all hand tools can be used in underwater but they require better care and maintenance. Typical tools that are used include hammers, screw drivers, scrapers, ice picks, hammers, axes, wire brushes, pry bars and hand saws. Divers routinely drop tools during operation, hence securing the same to the diver with a lanyard to save time searching the dropped tools in soft silt. The working with hand tools is cumbersome, strenuous and time consuming and hence can not be applied for large areas. Pneumatic / Hydraulic tools: Most of the mechanical tools used in underwater are pneumatic or rarely hydraulic. The pneumatic tools may become very ineffective beyond depths of 30m. Typical pneumatic tools like hammers, chippers, scrapers or rotary brushes are used for this. Although pneumatic tools are designed for underwater use the same can be adapted to perform required tasks. Pneumatic tools are produce a stream air bubbles which might obstruct the diver s vision. Underwater Hydraulic tools are modified versions of tools used on land. Providing an hydraulic source can be costly and tools themselves can be highly fatiguing since they produce torque or vibrations during operation. Also the stiff connecting hoses add to diver s difficulty. The biggest advantage with hydraulic tools are that they don t produce bubbles like the pneumatic tools. The most convenient and advantageous preparation tool is water blaster or popularly known as high pressure water jet. This delivers water through designed nozzles at a pressure of higher than 150 to 200 kgs/sq.cm that would make the working easier. HIGH PRESSURE WATER JET UW NDT ON CONCRETE Level III Inspection: This is a more detailed & expensive inspection of the elements which is incorporated when it was found that Level I and Level II inspections are found to be inconclusive. Various Non-destructive and Partially destructive test methods are used in this depending on the type of structure. Typical testing methods are discussed below: Steel: The most important aspect to be determined for steel structures is the remaining cross-section of steel members available after corrosion. The same can be measured using graduate scale (highly approximate), Calipers (better precision) or Ultrasonic devices (highly precise and expensive). The ultrasonic devices again are not specifically designed for underwater applications and hence needs to be modified accordingly. Due to these modifications done to the equipment, there comes lot of limitations with reference to the values achieved and it calls for highly experienced bridge inspector for final conclusions. Although certain easy and precise systems like magnetic particle testing are available, these are more often used in the case of measurement of welds which are not that much applicable to bridges. Concrete: Various Non-destructive equipments can be used for checking the concrete structures, but again they need to be modified for the underwater use. The V meter for checking the flaws within the structure like cracks etc., can be used either direct or indirect or semidirect methods, depending on the position of transmitter and receiver. Practically, the bridge piers being very large and length of cable connecting the transmitter / receiver to the display or device can not be very long for precise measurements, mostly semi-direct or indirect methods are adapted. Hence again the trained and experience technician is required for interpretation. The schmidt hammer modified for underwater use by enclosing in water proof enclosure can be used to determine 579

5 the concrete surface strength. The modified R-meter helps in locating the steel within concrete and its cover. However, the modifications being discussed are not so easy and calls for highly experienced and multi disciplinary technicians to carry out the same. This is very difficult and almost impossible. The most convenient and affordable way of testing under this level is taking out cores using pneumatic or hydraulic driven core machines. The cores can be tested and interpreted for necessary data. 5 DEFECTS IN STRUCTURES Concrete The concrete structures can be broadly divided as Plain, Reinforced and Prestressed concrete structures. Most of the old bridges are constructed with plain concrete or have a core material of plain concrete with peripheral wall in masonry. Prestressed concrete more widely used in the super structure and very rarely used in pile foundations. The following are the general defects that are found in concrete structures. Cracks: Concrete basically being strong in compression and weak in tension tends to crack apart from other reasons like shrinkage, temperature etc., Cracks can also be caused due to corrosion of reinforcement, overloading or settlement of structure. It is very important to determine whether the cracks are structural or superficial. Even superficial cracks when left unattended might turn into structural due to penetration of unwanted material into the concrete. When reporting the cracks it is better to define them three dimensional way viz., Length, width and depth of penetration. Also it is important to observe the presence of rust stains, efflorescence or settlement when a crack is found. Cracks may also be raised due to overdriving of piles into the soil. CRACK IN CONCRETE Scaling: Scaling is gradual and continuous loss of surface mortar and aggregate from an area. This is caused by Freeze thaw action that generally occurs during cold climates. The scaling can be classified as under: 1. Light Scale: Loss of surface mortar up to 6mm penetration with coarse aggregate exposed. 2. Medium Scale: Loss of surface mortar with penetration between 6mm to 12mm with added mortar loss between aggregates. 3. Heavy Scale: Loss Surface mortar between aggregates with penetration between 12mm to 25mm. The aggregates are clearly exposed and stand out from the concrete. 4. Severe Scale: Loss of surface mortar and aggregates as well as mortar between aggregates. Depth of penetration is more than 25mm.When reporting the scaling the Height (vertical distance), Width (horizontal distance) and Depth (to be referred as penetration) in standard units shall be mentioned. Spalling: Spalling is a depression in the surface of the concrete exposing the reinforcing steel. This is usually caused by the corrosion of reinforcement, with corrosion products exerting enormous pressure due to increase in volume on the cover concrete. This is generally occurred at the water line (splash zone) due to abrasion and constant wet dry cycles. Salt water or water with acidic pollutants corrodes the steel and wave and tidal action removes the corrosion products making way for fresh corrosion resulting loss of cross section. At times spalling can occur at large areas due to development of internal fracture planes covered by surface concrete. 580

6 Hence while reporting spalling a simple testing of the surrounding concrete by tapping with hammer to find out extents of spalling. The spalling shall also be reported in three dimensional way with additional details about the reinforcement corrosion. In general unattended cracks & scaling lead to spalling. Also protruded rebars used for fixing the formwork or wire ropes for lifting piles lead to spalling. CONCRETE CAVITY Chemical Attack: Substructures located in water are often subjected to chemical attack that may be present naturally or due to man made pollution. The chlorides due to de-icing salts or salt water and sulfates from salt water often attack the concrete. The effects of pollution varies depending upon the chemical present but where chemical attack is suspected it is observed that uniform scaling is formed in submerged portions. Abrasion: Abrasion is caused by the action of external forces on the structure. Minor abrasion damage causes scaling and major abrasion damage causes cracks, voids and spalls. In some rivers, scaling is found at the mudline due to abrasive action of bottom material being carried along in a swift current. Ferry vessels which repeatedly and rapidly start and reverse their propellers can effectively sand blast the underwater elements causing severe damage in long term. Steel:Steel is used as structural material or protective cladding around the concrete element. Most of the damages in steel occur due to corrosion. Corrosion is most prevalent in splash zone but also present in above and underwater. Corrosion: The most important factors affecting corrosion are oxygen, moisture, stray currents and water velocity. In bridges generally light weight piles are driven into massive soil channel bottom and support massive concrete decks. These two massive conditions act as cathodes and exposed slender metal pile acts as anode giving up electrons. Corrosion is the conversion of metallic ion, through electrochemical processes into a compound form (rust). The current flow is caused by external forces or due to difference in potentials of different metals. Bridges located in industrial areas experience severe corrosion due to presence of stray currents. The water acts as an electrolyte and sea water containing sulfides and chlorides is more conducive for this action. In splash zones and areas of high velocities the corrosion rate is much high than in still waters. The high water velocities and tidal waves remove the top corrosion products which are acting as inhibitors and increase the corrosion rate. Corrosion is also found due to bacteria present in water, and this can noted by the presence of brown orange nodules. Connections: Connections such bolts, nuts, splices and welds are potential areas of corrosion. Since the material for these connections is not similar to that of parent material, the dissimilarity in metals leads to formation of corrosion cells. Also coated structures shall be checked on welded joints since coating generally is thinner on welded areas. Masonry: Most of the old bridges have piers and abutments in brick on stone masonry. Problems generally encountered in masonry include cracking, scaling and deteriorated pointing. Masonry being porous is highly susceptible to freeze thaw action and might have cracks developed. The man made pointing mortar is susceptible to deterioration like concrete. It is not unusual to find stone masonry in good conditions with no filling mortar present in the joints. Where high velocities of water are present chances of scooping out of masonry units leading to formation of big cavities is very high. In structures which are protected with reinforced concrete cladding, the same needs to check for deterioration. The abrasive action of sand in water may cause deterioration of masonry and mortar in water. The length, width and depth of penetration need to be measured while doing inspection. 581

7 PEELED JOINTS IN MASONRY 6 REPAIR METHODOLOGIES & MATERIALS Almost any repair carried out above water can be carried in underwater, but it is more time consuming and very expensive. Underwater repairs are carried out in two ways either by dry or wet methods. In dry method a coffer dam is constructed around the element and water is drawn out. The repair procedure is similar to that of surface repair. Though the repair is very effective sometimes the cost of cofferdam is much higher than repair itself. In wet methods the repairs are carried out using divers and diving equipment. This requires special materials and equipments. Also the economics of the project is highly dependant on the design of methodology of repair. The less cumbersome and time consuming job for diver lesser would be the cost. Concrete Most of the distresses in concrete occur due to the corrosion of reinforcement steel. The corrosion is caused by ingress of moisture into the concrete. Hence even cracks which appear like minor defects should not be neglected to avoid further deterioration of concrete. The same is the case with scaling and obviously spalling or cavities. Cracks: Cracks in the concrete shall be repaired with the combination of epoxy putty and epoxy grout. Cracks are to be opened using pneumatic tools and cleaned of any impure deposits using high pressure water jet. The cracks need to be filled with epoxy putty using specialized putty guns. Whilst filling the epoxy putty at regular intervals injection ports shall be installed by drilling holes of required diameter along the length of the crack using pneumatic drill. After the full cure of the epoxy putty the ports shall be grouted with moisture insensitive, low viscous epoxy grouts using specialized grouting guns. The materials used for this application are specially designed for use in underwater applications. The materials need to be tested for bonding with concrete when applied in underwater. The major drawback to these applications is lower pot life of materials. Scaling: The scaling of concrete can be repaired by re-profiling the area with either cementitious or epoxy paste. For Small & Medium scales where the depth of penetration is less than 12mm, it is advisable to repair with epoxy putty. Also in places where the scaling is caused by abrasion of surface by high currents it is better to repair with epoxy material which has higher abrasion resistance than cement. Before carrying out repair, the area needs to be given proper surface preparation. It is essential to clear the area from marine growth and impure deposits. This can be efficiently achieved by hydro blasting of surfaces. The loose concrete shall be chipped off the area using pneumatic concrete chippers. The edges of the concrete shall be chipped to a minimum depth of 12mm to avoid formation of failure zones in the form of feather edges. Where depth of penetration is around 25mm and large areas are involved, it would be helpful to provide shear connectors by drilling holes at required locations within the scaled area. The shear connectors can be anchored into the damaged area using polyester anchor materials. To the shear connectors a weld mesh or wire mesh can be tied which would act as additional reinforcement for abrasion resistance. The material should have the following characteristics: Early setting facilitating quick, efficient repairs. Resistant to wave action & tides during application. Excellent adhesion in submerged conditions. Good Consistency for easy spread. Resistant to sulphurous ground waters. 582

8 Voids / Cavities: The voids or cavities that are smaller in area can be repaired with either hand applied cementitious or epoxy mortar as described above. But for larger areas, it will be economical to fix a formwork around the damaged area and fill the same with free flow, anti-wash out cementitious grout or fillerised moisture insensitive epoxy grout. The formwork can be rigid or flexible type, but practically flexible formworks are easier and less cumbersome to fix than rigid formwork. In case of flexible formworks bracings needs to be given for support and spacer bars are to be provided to maintain required cover. The surrounding water tends to push the formwork towards the element and also helps in preventing too much bulging. The periphery of the formwork shall be sealed with fast setting cementitious repair mortar. The mortar is allowed to harden before filling the formwork. While sealing the formwork an inlet and outlet for grout shall be fixed. The most effective way of filling the formwork is to fill it from bottom of the formwork which requires specialized pumping equipment. This would ensure proper displacement of water from the damaged area and avoid water droplets getting stuck between the repair mortar and parent substrate, which may effect the bonding of repair material with parent substrate. The material to be used for this purpose should have following properties. Anti-Washout, Effective water displacement. Free flow to enable easy and complete filling. Non-Shrink to compensate shrinkage. Shall set in water. Achieves High early strength underwater. Excellent Bond strength with substrate underwater. Steel The repair of steel structures is accomplished generally by jacketing the deteriorated area with anti wash out cementitious grout. Repair of steel sections by bolted or welded replacement is not cost effective but can be carried out if necessary. The quality control of underwater welding is very tough. The procedure to be adapted for jacketing is same as for that of concrete. Generally jacketing is done from the top till several feet into the water, considering the deterioration is more in the splash zone. However, Experiences suggest that the area between mudline and waterline is also susceptible for deterioration and may require similar treatment. One of the disadvantages of jacketing with cementitious grouting is its rigidness and can develop cracks due to bridge movement or vessel impact. Masonry The major defects that are found in masonry are open joints leading to further deterioration of bonding material. The same can be filled with rapid, setting, anti-wash out cementitious repair mortar. However where voids or cavities are formed due to scooping out of masonry units from the pier, the same needs to repaired by fixing formwork and fillng the same with grout. As a protective measure, the masonry piers can be jacketed with anti-wash out, cementitious grout. Before jacketing the same, dowel bars of torsteel shall be fixed into the masonry at regular intervals by drilling holes and grouting with polyester anchor material. The dowel bars can also be used for fixing longitudinal as well as secondary steel to make the jacket act as confinement additionally. New Technologies In spite of extensive research and development carried out, corrosion is still a phenomenon which remained as an enigma. However, every new development has some additional advantages which will take care of the shortcomings of the existing solutions. One such shortcoming observed irrespective of extensive work carried out is deterioration of piles in the splash zone area. It was found that most of the available solutions like cementitious jacketing or protective coatings or liners that were being applied in this are having one are other problems which are affecting the durability of repair carried out. To overcome the shortcomings of the above mentioned solutions for splash zone area, a state of the art solution was developed and was being used successively in USA called as 583

9 Advanced Pile Encapsulation In this method, the splash zone area of the pile shall be given an FRP encapsulation that is filled with epoxy grout. The encapsulation system consists of translucent FRP jacket customized to the pile dimensions. The FRP jacket shall be sealed at the bottom and top with Underwater Epoxy paste and shall be grouted with epoxy grout from the bottom. The set epoxy achieves excellent adhesion with concrete and FRP to make it highly durable. 584