Chloride transport and reinforcement corrosion in the vicinity of the transition zone between substrate and repair concrete

Sammanfattning: One of the major degradation problems in concrete structures is chloride initiated reinforcement corrosion resulting in cracked, spalled and delaminated concrete. Thesedamages are repaired for large amounts of money and because of high repair costs it is important to perform durable repairs.This thesis work has included interviews, laboratory and field studies in the area of concrete repairs. The interviews were held mainly with bridge engineers at the Swedish Road Administration and Banverket (the Swedish Railway Administration). The questions asked tothese people were about their experience in the field of concrete repairs. The objective withthe interview study was to collect knowledge and experience about concrete repair at these two governmental authorities.The main objective of the laboratory study was to investigate chloride transport in the transition zone between a chloride contaminated substrate concrete and an initially chloride free repair concrete by establishing chloride profiles. The second objective was to detect any reinforcement corrosion in relation to the transition zone. The reinforced laboratory specimens with premixed chlorides (1 or 4 wt % per cement) in the substrate concrete have been exposed during 13 years either outdoors or in a climate chamber with relative humidity of 80 %. The main results show that reinforcement corrosion occurred in and near the transition zone in local active areas with passive areas between, macrocell corrosion, and that the chlorides are transported from the contaminated substrate concrete into the repair concrete.Three different repaired concrete structures, one road bridge, one pedestrian bridge and one parking structure, were investigated in the field study. All three structures are treated with deicing agents in the winter time. The objective with the field study was to investigate if the same phenomenon can be observed as in the laboratory study. The main conclusions, from the investigation on drilled cores from each structure, are that chlorides in contaminated substrate concrete can be transported into a repair concrete and reinforcement corrosion may occur in the transition zone between the two different materials. The results are in accordance with the results in the laboratory study. It was also seen that a repair concrete with relative low w/cratio may reduce the ingress of external chlorides.Finally, the results in this thesis indicate that there is a risk for reinforcement corrosion in the vicinity of the transition zone between repair concrete and a substrate concrete which must be considered in concrete repair work. For example the chloride content around a repair must not exceed a certain level when a repair is performed. The chloride transport into the repair concrete can be useful to transport chlorides and reduce the content of chlorides in the substrate concrete. The advantages with this are that the amount of removed contaminated substrate concrete can be minimized and the risk for corrosion in the substrate concrete is lower. If the repair concrete is reinforced with conventional black steel there may on the other hand be a risk for corrosion in the repair concrete instead if high levels of chlorides are transported. Also external exposure of chlorides must be considered for the risk of corrosion in the repair material.Further research is needed about when different corrosion mechanisms and chloride diffusion mechanisms occur in reinforced concrete repairs. The experience from the used analyzing methods in the laboratory investigations are possible to use in coming research projects in the field of concrete repair. The next step after this thesis work should be in order to increase the understanding of the repaired concrete system including the important compatibility issues, the author believes that the most fruitful way consists of further laboratory investigations, field studies and optimisation of the repair material.