RETROFITTING OF CORROSION-EFFECTED STEEL BRIDGE MEMBERS USING STEEL - CFRP BONDING SYSTEMS

Abstract

Most steel bridges and structures often need strengthening or retrofitting after a set period of time either due to damage/ loss of cross-section caused by prolonged exposure to severe corrosive environments or due to a need of upgradation because of increased live loads in due period of time or due to changes in the kind use of structure or to improve fatigue performance by reducing stress level for a given loading condition. The current techniques of retrofit of steel structures available have several short comings including requirement of heavy equipments for installation and skilled labour. Thus, retrofitting/strengthening of structures using advanced composites have become a very popular method in the past couple of decades. Recent research has emphasized on strengthening and rehabilitation of steel structures and bridges using Carbon Fibre Reinforced Polymer (CFRP) materials. The technique of retrofitting with Carbon Fibre Reinforced Polymer (CFRP) has drawn growing attention in research field and in practice and also its environmental durability is of high importance. This report majorly encompasses the use of adhesively bonded Carbon Fibre Reinforced Polymers (CFRP) sheets in retrofitting steel bridge members affected by corrosion. While the use of fibre reinforced polymer sheets for the repair and strengthening of reinforced concrete structures is well recognised, research work on the relevance of FRP composites to steel structures has been limited. The use of FRP material for the repair and rehabilitation of steel members has abundant benefits over the traditional methods of retrofit by bolting or welding of steel plates. Carbon FRP’S (CFRP’S) have been preferred over other FRP material for strengthening of steel structures since CFRP’S tend to posses higher stiffness. The emergence of high modulus CFRP plates, with an elastic modulus higher than/comparable to that of steel, enables researchers to achieve considerable load transfer in steel beams prior to the yielding of steel. In such CFRP strengthened structures, the behaviour of CFRP-to-parent material bonded joints play a very important role. Existing work on CFRP-to-steel bonded joints is very limited. In a CFRP-to-steel bonded joint, the weak link has been found to be the epoxy adhesive, while in a CFRP-to-concrete bonded joint, the concrete is the weak link. Thus it can be said that the bond behaviour of CFRP materials to steel structures is quite unparalleled from that of concrete structures. Preliminary test results confirmed indispensable amount of very high bond stresses for most strengthening applications due to the tune of strengthening required for steel structures and bridges. Bond stresses might be much more critical for steel structures/members than for concrete structures since more strengthening material is required for steel structures to achieve a similar ascend in strength due to the high intrinsic strength of steel. There is, however, a concern in relation to possible galvanic corrosion when carbon and steel are bonded together. In this report, surface preparation methods of steel and CFRP for effective bonding and means of preventing galvanic corrosion have been discussed. The results obtained from an experimental program verifying the efficiency of method of retrofitting the steel bridge members using CFRPepoxy strengthening systems with or without end-anchorages have been presented. Also the effectiveness of the retrofitted members in resisting further corrosion in extreme exposures has been tested and the results comparison documented.