|Abstract or Summary
- The first goal of this master’s thesis was to investigate the linkage between a non-destructive testing method, surface resistivity, and premature concrete deterioration brought on by chloride ingress and or freeze-thaw cycling. The experimental method for this project consists of taking surface resistivity measurements of reinforced concrete slabs after they were ponded with a magnesium chloride de-icing solution, containing a corrosion inhibitor. Reinforced concrete slabs and companion concrete cylinders also underwent freeze thaw cycling to observe its effect on surface resistivity in conjunction with exposure to chlorides. Many factors such as internal and ambient temperature and relative humidity affect surface resistivity measurements so this data was also collected. To compare the surface resistivity to the rate of chloride ingress, cores were taken from the slabs. As expected, the surface resistivity of the concrete slabs initially increased while the concrete cured and lost moisture due to the hydration process and to the environment. Once ponding of the chloride solution started, however, there began to be a decrease in surface resistivity. This decrease in surface resistivity is believed to be caused by the increase of free chlorides present in the slabs after being saturated with the de-icing solution. The surface resistivity of the corresponding slabs that were ponded with tap water remained constant. These results show that surface resistivity measurements are a potential indicator of chloride ingress and, therefore, may correlate to risk of corrosion in reinforced concrete. The effect of freeze thaw cycling on surface resistivity, according to this study, is that it increases the surface resistivity. This is due to the micro-cracking in the concrete matrix, caused by the freeze-thaw damage. In the second part of this master’s thesis the corrosion susceptibility of ductile iron pipe (DIP) was investigated. Ductile iron pipe has been used mostly in the water transmission field since the 1950s. At that time, ductile iron pipe replaced traditional cast iron pipe, due to ductile iron’s superior strength properties. More recently, ductile iron pipe has been considered as a replacement for traditional timber poles for power transmission. While there are benefits to using ductile iron such as its strength and sustainability, ductile iron suffers from pitting corrosion due to the nodular structure of its free graphite. This type of corrosion can lead to severe deterioration of the pipe in localized areas. Due to this, researchers have been studying ductile iron durability and developing solutions for its durability issues. In this research study ductile iron was tested for its susceptibility to corrosion. Samples were subjected to an accelerated corrosion process inside a salt fog chamber. After the samples had corroded, mass loss measurements were taken. Tensile strength tests were also done before and after corrosion. Stainless steel samples were tested alongside the ductile iron for comparison. In addition to the corrosion chamber and strength tests, electrochemical tests were done to characterize the corrosion kinetics of ductile iron. Stainless steel samples showed almost no change in mass loss and tensile strength after corrosion. Ductile iron samples, on the other hand, had a corrosion rate of approximately 52.6 mpy (1.3 mmy), and a 10 ksi (69 MPa) decrease in ultimate tensile strength after corrosion. Open circuit potential results showed that the ductile iron samples had a higher potential, and thus, lower corrosion resistance than the stainless steel samples. Similarly, electrochemical impedance spectroscopy showed that stainless steel had a greater impedance than ductile iron, which correlates to increased corrosion resistance. Analysis of cyclic polarization curves showed that the ductile iron samples had much higher corrosion rates than stainless steel samples. However, the corrosion products which formed on the ductile iron samples appeared to be protective in nature. This indicates that ductile iron may have good field performance.
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