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The presence of chloride in concrete is a critical issue raising concerns in the construction industry as they promote corrosion of the steel reinforcements, drastically reducing the strength of the structure. The aim of this study is to compare the performance of a neutron-based nondestructive testing method, Prompt Gamma Activation Analysis (PGAA) against the destructive wet chemistry method ASTM C-1152 currently used to determine the chloride concentration in concrete. Two modes of PGAA operation were tested. One was to use PGAA with a slit collimator to measure the chlorides at 2 mm thick cross-section in intact samples. The other was a direct comparison with C-1152 to analyze powdered concrete samples. Concrete was prepared in four batches, in which three batches had added chloride -at nominally 0.2%, 0.1%, 0.01% by weight of cement and the fourth (control) batch has zero added. The PGAA analysis was done at the Cold Neutron PGAA station at NIST and the C1152 testing was done at the National Ready Mixed Concrete Association (NRMCA) laboratory. The intact samples were scanned at three different vertical positions.
The PGAA method is capable of detecting Cl at levels corresponding to the corrosion threshold of 0.1-0.2% Cl by weight of cement. The minimum detectable limit for PGAA is below 0.02% Cl by weight of cement and approaches the Cl background contributed by the raw materials, in this case, the cement. The PGAA- measured chlorides concentrations showed excellent linearity after correction for the chloride content in the concrete raw materials, mainly the cement. For the powdered samples, the C1152 and PGAA results were in very good agreement. However, the PGAA data showed much less scatter with an uncertainty as low as 0.3%. The findings of this study indicate that PGAA is a feasible replacement for the C1152 method and since it can be done on intact specimens, it avoids the time-consuming steps of crushing, sieving and nitric acid extraction and can be more cost-effective. |
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