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Title: Assessment and prediction by mathematical modelling of electrochemical chloride removal from concrete
Author: Sa'id-Shawqi, Qaisar Husam
ISNI:       0000 0001 3543 9249
Awarding Body: South Bank University
Current Institution: London South Bank University
Date of Award: 1998
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This project was carried out to develop a better understanding of "Electrochemical Chloride Removal (also known as Desalination), a relatively new technique for rehabilitating reinforced concrete structures suffering from chloride induced corrosion. The factors influencing chloride removal were investigated and a mathematical model for predicting the spatial distribution of chloride in the concrete at the end of the treatment was developed. To investigate the factors influencing chloride removal, concrete prisms containing varying levels of admixed sodium chloride and number of steel bars were tested. The amount of chloride removed during the treatment was assessed by analysing the anolyte. It was found that chloride removal increased with increasing applied potential, number of reinforcing bars at a particular depth, water/cement ratio, chloride contamination depth and initial chloride content. Chloride removal was unaffected by concrete strength but decreased with increasing percentage of cement replacement material. A greater percentage of chloride was removed from prisms where the thickness of the chloride bearing layer of concrete was less than the depth of cover to the reinforcement. Where the thickness of the chloride bearing layer exceeded the cover to the reinforcement, the use of an external cathode significantly increased the total amount of chloride removed. Chloride removal from a face remote from the source of the chloride contamination (soffit desalination) was shown to be feasible. In prisms containing 2, 3 and 4% Cl (by weight of cement), it was found that the amount of chloride remaining in the prisms reached a limiting value irrespective of the initial admixed chloride content. This value was believed to be approximately equal to the amount of bound chloride in the concrete. Analysis of ground samples of concrete showed, however, that in a few cases bound chloride was removed locally to the reinforcing bars. Even though significant amounts of chloride remained in the concrete after completion of the treatment, the reinforcement remained passive for periods exceeding 30 months. Towards the second aim of predicting the spatial chloride distribution in the concrete, a mathematical model for electrochemical chloride removal from concrete based on the Nernst-Planck and Laplace equations was developed. The model relies on experimentally derived chloride transport number profiles. The predicted quantities of chloride removed into the anolyte and chloride remaining in concrete correlate well with data obtained by testing prisms containing one bar and dosed with 2, 3 and 4% Cl. The model shows that all parts of the concrete prisms undergo chloride removal albeit at different rates. Concrete directly between the anode and cathode undergoes the highest rate of chloride removal while concrete below the cathode, the lowest. It would appear, therefore, that there are no 'dark spots' in the concrete where chloride removal is prevented.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Desalination; Cathodic protection