Long term performance of concrete repair in highway structures.

In recent years, the number of reinforced concrete structures requiring repair and rehabilitation has increased considerably. This is due mainly to the corrosion of the reinforcing steel within the concrete which has become contaminated with a highly corrosive environment containing chlorides and carbon dioxide. Specification of repair materials at present, in accordance with BD 27/86, is based on an inadequate understanding of the interaction between the substrate concrete and repair material. Cracking may occur in the early ages after application of the repair material due to the restraint to shrinkage provided by the substrate concrete and steel reinforcement. This would have serious durability consequences which would aid the onset of further corrosion. Furthermore, it is not clear at present if an efficient composite interaction is developed between the repair patch and substrate concrete in the long term to enable the repair patch to carry external load, or if the repair patch just acts as a barrier to protect the steel reinforcement from further corrosion.This research relates the basic properties of repair materials (elastic modulus, shrinkage and creep) to the long term performance of repair materials applied to actual concrete highway structures and simulated laboratory repairs. The research forms part of a LINK project sponsored by three companies and the Department of Transport. Three deteriorating highway structures were selected for concrete repairs. The repairs were monitored over the long term to gain an understanding of the interaction between the repair material, substrate concrete and steel reinforcement. These were Gunthorpe Bridge, a three span reinforced concrete arch bridge, crossing the River Trent to the east of Nottingham, Lawns Lane Bridge, carrying part of the Ml near Wakefield in West Yorkshire, and finally, Sutherland Street Bridge, carrying the B6080 in Sheffield, South Yorkshire. In order to gain information on the effectiveness of the repair materials in developing an efficient composite action with the substrate concrete, vibrating wire strain gauges were attached to the substrate concrete and steel reinforcement after the deteriorated concrete was removed. Embedment and surface gauges were installed in the repair patches either during or after repair material was applied. Laboratory investigations consisted of repairs to simply supported reinforced concrete beams. Repair patches were applied centrally at mid-span to beams which were subjected to two point loading at one-third span points. The interaction between the repair materials, steel reinforcement and concrete substrates were monitored by a combination of a demec extensometer and electrical resistance strain gauges.A theory is presented which predicts the long term interaction between the concrete substrate and repair materials to unpropped compression members. The analysis is divided into two stages. The first part deals with the distribution of shrinkage strain within the repair patch; the second part deals with the redistribution of external load from the substrate concrete into the repair patch. The distribution of shrinkage strain in the repair patch is based on the analogy of a bi-metallic strip contracting due to adrop in temperature (similar to a repair material contracting due to shrinkage). It has been observed that a repair material with an elastic modulus greater than the elastic modulus of the substrate concrete is able to transfer some shrinkage strain to the substrate concrete. This analysis, therefore, enables the tensile stress in the repair material, due to the partial restraint to shrinkage provided by the substrate concrete, to be predicted. Similarly, the restrained shrinkage strain that is transferred to the substrate concrete is also predicted as an increase in compressive stress in the substrate concrete. The transfer of external load into the repair patch is based on analysing the distribution of stress in a compound member when subjected to an axial force. The compound member consists of two different materials - the substrate concrete and repair material. The analytical expressions require basic properties of the repair material (elastic modulus, shrinkage and creep) and substrate concrete (elastic modulus) to enable a prediction of the stress across the repair patch in the long term to be made. The validity of the model is verified on the basis of the field data from the spray applied repair materials on two of the highway structures (Lawns Lane Bridge and Gunthorpe Bridge).