Development of a new test protocol for the permit ion migration test

• Main Author: Nanukuttan, Sreejith V.
• Other Authors: Basheer, P. A. M. ; Robinson, D. J.
• Published: Queen's University Belfast 2007
• Subjects: 620.136990287
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.479417

Chloride induced corrosion of steel in concrete is one of the most common reasons for the deterioration of reinforced concrete in both marine and transportation structures. In order to assess the chloride penetration resistance of concrete, the common practice is to remove cores from the structure and test them in a laboratory to determine the chloride diffusion coefficient using the steady state diffusion test. This test is not popular due to its long test duration to achieve a steady state of flow of chlorides through the test specimen, which is used to calculate the diffusion coefficient. Therefore, applied voltage tests (known as migration tests) have become quite common, in which the transport of chlorides ions through the test specimens is accelerated by applying a potential difference across them. The measurements during either the non-steady state condition or the steady state condition are used to calculate a chloride migration coefficient, which has been reported to correlate well with the corresponding coefficient from the diffusion based tests. The chloride diffusion coefficient of concrete can also be predicted more rapidly using other indirect methods, such as the electrical resistivity test. By following the principle of the migration test, a new in situ migration test (called the Permit ion migration test) was developed at Queen's University Belfast in the late 90s. The validity of this test was established for concretes containing normal Portland cement, by comparing the in situ migration coefficient with both the coefficient of diffusion (from both steady state and non-steady state diffusion tests) and the migration coefficient from the steady state migration test. However, it was considered to be necessary to broaden its applicability for testing concretes containing supplementary cementitious materials, by repeating the validation study on concretes containing such materials. Furthermore, there was a need to redesign the apparatus to make it more reliable and user-friendly for site applications. Therefore, a detailed investigation was carried out, initially as part of a European Round Robin Test programme (viz. EU FP5 Growth Programme - Chlortest) to identify the most reliable laboratory-based methods for assessing the chloride diffusivity of concretes which are commonly used in practice. This was followed by a detailed laboratory study on concretes containing supplementary cementitious materials, such as microsilica (ms), pulverised fuel ash (pfa) and ground granulated blast furnace slag (ggbs), in addition to normal Portland cement (ope) as a control. In this investigation, not only the tests identified in the initial investigation were used, but also were additional tests such as the new Permit ion migration test and the Wenner four probe resistivity test. The results from these investigations were used to establish the validity of the Permit ion migration test for testing concretes containing supplementary cementitious materials and to improve its test protocol. As part of the Chlortest programme, a non-steady state diffusion test (to act as a reference method), a non-steady state migration test, a steady state migration test and a resistivity test were selected and a comparative (reliability) study was carried out using concretes, manufactured by four different EU countries, containing ope, pfa,ggbs and ms as binders. The results indicated that both the non-steady state migration test and the bulk resistivity test are the most reliable tests in assessing the chloride diffusivity of these. The results from the steady state migration test were found to be affected by the use of a thickness of the test sample less than the maximum size of the coarse aggregate. In the validation study that was carried out using the Permit ion migration test, the insitu migration coefficient correlated well with the non-steady state migration coefficient, the steady state migration coefficient and the bulk resistivity for a range of concrete mixes containing different types of binders, such as ope, ms, pfa andggbs. For the determination of the onset of the steady state condition and the estimation of the steady state chloride flux, it was found that the conductivity of the anolyte could effectively be used, which in turn could eliminate the need for sampling chloride solutions from the anolyte periodically. Further, there existed an excellent degree of correlation between the peak current and the steady state migration coefficient from both the steady state migration test and the Permit ion migration test, which indicated that the former could be used to predict the latter, with much lesser effort and complexity of the test protocol. On the basis of the findings from both sets of investigation, a new test protocol was developed for the Permit ion migration test and the Permit was redesigned. The new test protocol used conductivity of the anolyte instead of the chloride concentration to identify the onset of the steady state condition and there is the option to calculate the chloride migration coefficient from either the peak current or the steady state of chloride flux. The new Permit was designed to work as a stand-alone instrument onsite, with little interference from the operation once the test had been started, but at any stage a computer could be connected to view the progress of the test.