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    Distress Diagnosis, Repair and Retrofitting of Bridges

    Dr. A.K.Pandey, Scientist and Dr. Rajesh Deoliya, Scientist, Structural Engg. Division, Central Building Research Institute, Roorkee, Uttaranchal

    Introduction

    Rapid industrialization in India after independence required infrastructure for transport of goods and raw materials and that has led to construction of wide network of roads with inbuilt bridges and flyovers. A number of existing bridges are showing sign of distress within their designed service life in many cases as early as 10-20 years of construction in India and in several other countries. Collapse of I-35W bridge at Mississippi river in United States of America in the month of August 2007 has drawn attention of international media. In India, collapse of Mandovi Bridge at Goa in 1986 (after 16 years of service life) and serious distress in Zuari bridge (rehabilitated in 2000 with the held of international consultant) at Goa and several other bridges like Khalghat and Borad Bridge on NH3 (Agra to Mumbai route) due to environmental loads is drawing the attention of transport authorities and scientific community to evolve an efficient bridge management system. Identification of the cause of distress, level of distress and reduction in load carrying capacity of the bridge may be ascertained in order to proceed with necessary repair to ensure safety of the bridge during their service life. The evaluation of state of stress and load carrying capacity of the existing bridges and enhancement of the load carrying capacity is required during upgradation of the highways. India has already implemented strengthening and four lanning of 7000 Km of highways under Golden Quadrilateral scheme where strengthening/retrofitting of many existing bridges was required. The various causes of distress of reinforced concrete bridges are poor quality of construction, corrosion of steel in reinforced concrete bridges and pre-stressing cables in prestressed concrete bridges, improper design and detailing for normal loads and seismic forces, improper functioning of bearings etc. A number of case studies have been reported in literature on damage of reinforced concrete and prestressed concrete bridges during earthquake [1-6] and distress of reinforced and prestressed concrete bridges [7-10] because of environmental and other loads.

    In this paper, an attempt has been made to critical review of the literature of distress diagnosis of reinforced concrete and prestressed concrete bridges and repair/ retrofitting strategy based on the experience from rehabilitation projects for bridges in which the authors have been associated.

    Case Studies and Type of Distress

    1. Soil / foundation related factors
    2. Corrosion of reinforcement in abutment, pier, super structure
    3. Bearing Failure
    4. Structural distress exceeding design loads

    Soil / foundation related factors

    Bridges are built on spread footings or on deep foundations (pile and well). Bridges on spread footings are supported on firm soil layers or on rock close to the surface. Normally, bridge failure because of settlement of foundation for normal loads has been rarely reported however damage to superstructure because of foundation failure has been reported during earthquakes.

    Bridges built on sites with soft clay or silt, loose saturated sand or improperly compacted fills have been damaged by amplification of ground motion or by soil failure during earthquake. Bridges supported on soft or sensitive clay have been particularly vulnerable to earthquakes. When close to epicenter, weak clay will undergo large displacements that can drag the foundations and pier out from bridge super structure [6]. Saturated loose sand and silty layers of soil are potentially liquefiable. Soil liquefaction can cause a loss of bearing capacity and that may result into lateral movement of the sub structure. Another common problem is settlement of bridge approaches during earthquakes. Even well compacted structural backfill may be vulnerable to this type of damage.

    Corrosion of reinforcement in abutment, pier & superstructure

    The corrosion of reinforcement in concrete in adverse environment is cause of concern in many reinforced concrete and composite bridges. Two of the most common causes of deterioration are due to chloride penetration and carbonation of concrete leading to pitting and general corrosion of steel respectively. Corrosion of steel results in cracking and spalling of the concrete and this in turn leads to secondary damage caused by enhanced ingress of aggressive agents. As shown in Figure 1 (a, b) corrosion of reinforcement in the pier and slab has resulted in spalling of concrete.

    Bearing Failure

    The function of bridge bearing is to transmit the dead load of the superstructure and live load to the substructure while accommodating the necessary movement (including the thermal movement) of the superstructure. A defective bearing may induce overstresses at the bridge seat and restrain movement of super structure which may induce stress in the super structure. Therefore, bearings that fail to perform as designed have to be replaced so as not to damage the bridge. During earthquakes, many examples of bearing failure has been reported in India. There was failure of bearings in Silchar in Cachar earthquake of 1984 [1]. There was failure of elastomeric bearings in Surjabari Bridge in Bhuj earthquake of 2001 [5].

    Structural distress of abutment/ pier and super structure exceeding designed load

    Distress in abutment/pier and super structure of reinforced and prestressed concrete during earthquake are widely reported [6]. There are cases where distress in abutment/ pier and superstructure has occurred because of increased loading than the designed load. In July 2004, Bailey bridge on NH-37 at Deosila, Assam collapsed because 60 tonne heavy truck was allowed to pass which much more than the designed axle load. The first author has been associated in a project [8] in which load carrying capacity of an arch bridge was required for transporting heavier equipment for BHEL.

    Distress Diagnosis

    Sometimes the cause of distress is known i.e. distress during earthquake, impact and blasts is known probably the exerted loadings exceeding the designed load however many times it is difficult to ascertain the actual cause of distress where many variables are involved e.g. poor quality of construction (faulty detailing, improper materials and workmanship), shrinkage, creep, reinforcement corrosion etc., The quantification of the level of distress is required to succeed the repair/ strengthening strategy. Following test may be required to ascertain the cause and level of distress.
    1. Determination of Compressive Strength from NDT and concrete cores
    2. Crack detection techniques
    3. Signature/Vibration Analysis
    4. Load tests to find out deflections/strains and load distribution
    5. Corrosion Studies (pH, carbonation depth, chloride)
    6. Deflections/slopes strains from Instrumented Bridge

    Determination of Compressive Strength from NDT and Cores

    Non destructive tests like Rebound Hammer, Ultra Sound Pulse Velocity are used to assess the compressive strength and quality of concrete on bridges and other structures. Rebound hammer test is based on the principle of dynamic impact in which when a moving object hits a body, the rebounding energy is proportional to the surface stiffness, strength, and static inertia of the body. This test gives fair idea about the strength of concrete, especially near the surface. The accuracy of the test results depends upon the following factors.
    1. The reading of the hammer on the standard steel anvil should be 79 ± 2, however the springs used in the rebound hammer show fatigue after certain number of blows and reading on the steel tester reduces. The author has observed after long use the hammer develops permanent deformation in springs and linearity of the spring gets affected.
    2. Rebound number is more if the impact is made on an aggregate (normally the impact is to be made in the mortar between the aggregates) and therefore sufficient number of readings are required to be taken and offshoots has to be taken out.
    3. For old concrete structures the surface concrete becomes harder because of carbonation however the inside concrete strength is relatively less therefore the strength reported from rebound hammer test is to be corrected for old concretes.
    To know about the quality of concrete, a method based on ultrasonic technique is used. In this technique, sound-waves of ultrasonic frequencies are transmitted through the concrete members under test to measure the velocity of ultrasonic pulses. The velocity of ultrasonic pulses traveling through the concrete mainly depends on the elastic properties of concrete which indirectly indicate the strength of concrete. Since, there is no unique correlation between the velocity and strength of concrete for different mix proportions, this technique is used more as a qualitative test rather than a quantitative one. Determination of compressive strength by extracting cylindrical cores from the structure is the most reliable method however it is partially destructive test therefore a limited number of cores are only taken from the bridge sub and super structure.

    Crack detection technique

    Damage due to deterioration process, over load, bad design, poor quality of material including durability aspects may grow with passage of time until collapse of the structure. Acoustic emission technique [11] and infrared thermography, impact echo and ground penetrating radar techniques [12] have been used for damage detection and progression in laboratory and field in some cases.

    Signature/Vibration Analysis

    Distress in the superstructure/sub structure of the bridges changes the strength and stiffness and this may be reflected in the form of change in the frequency and mode shape of the structure.
    Various studies have indicated that increase in damage reflects decreased natural frequency of the structure. The author has been associated in two projects (repair and rehabilitation of Khalghat and Borad Bridge on NH-3 in Madhya Pradesh) in which for damage identification signature/vibration analysis was also used. Both the bridges were simply supported composite bridges (Reinforced Concrete Diaphragm and Deck Slab and Prestressed Concrete Main Girders). The analysis indicated change in the frequencies is not appreciable with the decrease in the stiffness (damage). Several laboratory studies also has conformed this finding. However, through the vibration tests structural integrity (main girder, diaphragm and deck) of the bridge may be ascertained.

    Load test to find out deflections/strains and load distribution

    Load test is a reliable method for assessing the level of damage in the bridge. The static load equivalent to design load is placed on the deck in stages and measurement of deflection in the girders is recorded from dial gauges in stages. Total deflection as well as recovery of deflection is important when fully loaded and when load is removed. As per IS- 456-2000 under imposed load 75% recovery is must for satisfactory performance. The sharing of the load in girders can be back calculated based on deflections in each girder and may be compared with the designed load sharing in the girders which indicate relative stiffness of the girders of the bridge.

    Corrosion Studies (pH, carbonation depth, chloride)

    If a risk of reinforcement corrosion is detected early during routine inspection, damage can be avoided or reduced significantly by relatively simple protective measures. On the other hand, if corrosion of the reinforcement has proceeded to the point where it has caused cracking and spalling of the concrete, repair measures are normally more complicated and expensive. The condition assessment of the structure for the corrosion involves determination of pH of concrete samples taken from the structure, measurement of carbonation depth in the actual structure and chloride content and sometimes even the corrosion rate.

    Concrete is alkaline in nature as the pH of concrete is more than 12.5 and due to the alkaline nature of concrete steel is protected from corrosion. Calcium present in concrete reacts with moisture and lowers the alkalinity of concrete. The outer zone is affected first but with the passage of time, carbonation penetrates deeper into the mass and once it reaches to the reinforcement the corrosion is initiated. The carbonation depth is determined at various location using Phenolpthlein indicator. Chloride ions react with Ca(OH)2 present in cement matrix thereby reducing the alkalinity of the cement paste. The two parameters responsible which influence the ingress of chloride ion into concrete are diffusion resistance of the concrete (impermeability of concrete) and the other binding capacity of the concrete with respect to the chloride ions. The chloride ion concentration is also determined for the samples collected from the field. The pH of the concrete samples collected from site is tested from the laboratory will indicated the severity of the corrosion process.

    Deflections/slopes strains from Instrumented Bridge

    Scientific monitoring of health and performance of major concrete structures is no longer in dispute in view of the large number of distressed structures. The author has been associated with long term performance monitoring of bridges and nuclear power structures. The sensors are embedded in the concrete during the construction of the major long span bridges and these are monitored for change in values in an interval of six months. Creep and shrinkage specimens of specific dimensions (to simulate the condition in the actual structure) are cast during the construction and values obtained in these will be used to separate the stresses, strains and deflections.

    Repair/Retrofitting Techniques

    1. Superstructure
      1. External Pre-stressing
      2. Epoxy and Polymer Mortar for crack repair
      3. Composite Materials
    2. Bearings Replacement
    3. Substructure
      1. Steel jacketing for columns/ piers
      2. Concrete jacketing for columns/ piers

    Super-structure

    Normally, during earthquake loading, super structure is not critical because of its high rigidity and large flexural capacity, super structure deficiency is associated with their unseating in expansion joints or on bearing supports due to relative displacements. Distress in slabs and diaphragms has been observed in composite bridges (Prestressed concrete longitudinal girders and Reinforced concrete Diaphragms and Slabs) due to corrosion of reinforcement. The author has been associated with the rehabilitation of two composite bridges (Khalghat, Borad Bridge in on NH3 in MP), where major distress in the form of cracking and spalling of concrete because of corrosion of reinforcement reducing the strength and stiffness.

    (a) External Pre-stressing

    Pre-stressed concrete bridges are also subjected to environmental effects like corrosion of reinforcement in aggressive sea environments. The stress corrosion in pre-stressing cables of Juari bridge at Goa has lead to distress in super structure and the structure was rehabilitated by external prestressing.

    (b) Epoxy and polymer mortar for crack repair

    Distress due to corrosion of steel in concrete may result in cracks in concrete and if the distress is less than 30% of the concrete surface, patch repair using epoxy or polymer modified mortar may be done.

    (c) Composite Materials

    Fiber reinforced polymer laminates (both glass and carbon) have been used for repair and strengthening of reinforced concrete bridges. The fiber reinforced polymer laminates are used for increasing the shear as well as flexural capacity of reinforced concrete girder and flexural capacity of slabs. The cost of these laminates is much more than the conventional techniques and use of these fibers has been reported some structures in India

    Bearing Replacement

    In many cases, distress in the superstructure is caused because of defective bearings and replacement of defective bearing is required. Bridge bearings are required to be periodically inspected and damaged bearings as well as bearings for which service life is over has to be replaced. Replacement of bearings below pre-stressed girders has to be cautiously done. Normally, limitation of space requires proper selection of jacks and jacking operations. The end diaphragm for long span girders should have sufficient flexural and shear capacity for jacking operations.

    Substructure

    The distress in abutments and piers has been found because of reinforcement corrosion. Enhancement in load carrying capacity (axial/shear) of piers/ columns may be required during upgradation of highways. The columns/piers of bridges which have not been designed for earthquake loading may need retrofitting for increasing ductility, flexural and shear resistance of the piers/columns. Steel or concrete jacketing of piers/columns has been used for increasing the load carrying capacity.

    (a) Concrete Jacketing forColumns/Piers

    Concrete jacketing around the bride piers and columns for enhancing the load carrying capacity as well as ductility has been widely used. The footing has also to be ensured that it will be able to take the increased load and the vertical reinforcement should be dowelled with adequate anchorage with footing. The distressed surface because of corrosion of reinforcement has to be cleaned properly and bond improving chemicals has to be applied for bonding between old and new concrete. Normally, a thick layer of reinforced concrete (vertical bars and vertical and horizontal reinforcement) is provided. In Figure 3, concrete jacketing is being used to increase the load carrying capacity of the bridge.

    (b) Steel Jacketing for columns/piers

    The repair/strengthening of columns/piers of the bridge is done using steel sheets circular/elliptic in shape [4]. The steel sheets provide a continuous tube around the concrete with an annular gap. The gap is grouted with cement grout or epoxy resin. The jacket primarily increases shear strength and confinement.

    Conclusions

    A large number of highway bridges and flyovers are showing sign of distress all over the globe. Identification of the cause and level of distress/damage is a very important step for successful repair and to achieve the designed service life of the bridge. Several new techniques new radar mapping, infrared thermography and impact echo to detect flaws in concrete bridge decks has been tried which are intended to look into invisible defects in concrete in early stages. Various steps in distress diagnosis include determination of concrete compressive strength form non destructive tests, signature/vibration analysis, load tests, tests for state of corrosion etc. Distress diagnosis is important step for implementing proper repair scheme to achieve the designed life of the bridge. In view of large number of distressed bridges monitoring of state of health of important bridges through instrumentation during the construction stage will help in early detection of flaws and suggesting suitable strengthening scheme.

    References

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    • Elkin, SJ. Nacamuli, AM. Lehman, DE. and Moehle JP., “Repair of moderately and severely damaged bridge columns,” Proceedings of Sixth US national conference on Earthquake Engineering, Earthquake Engineering Research Institute, 1998, Oakland CA.
    • Park, R. and Rodriguez, ME., “Assessment and retrofit of a reinforced concrete bridge pier for seismic resistance,” Earthquake Spectra, Vol. 9(4), 1993.
    • Thakkar SK., “Retrofitting techniques for bridges and flyovers,” New Building Materials and Construction World, Vol. 13(9), 2008, pp. 180-192.
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