Microwave Reflection Properties of Mortar Possessing a Cyclically Ingressed Sodium Chloride Profile

摘要

Reinforced concrete structures account for a large percentage of the infrastructure in the United States and worldwide. The alkaline nature of concrete provides adequate protection from corrosion for reinforcing steel bars through a thin protective film tint forms at the steel/concrete interface. However, once aggressive chloride ions permeate inside concrete pores, they destroy this protective film. Subsequently, in the presence of moisture and oxygen the steel reinforcing bars corrode through an electrochemical process. Corrosion byproducts occupy a larger volume than concrete and result in adverse internal stresses that cause spatting and significant reduction in the structural integrity of the structure. Thus, nondestructive detection of chloride ions (for example free salt) in cement based structures is an important practical issue, in addition to testing the profile of salt ingress. Microwave nondestructive testing (NDT) techniques are well suited for testing properties of materials. To this end, several mortar samples were produced which subsequently were periodically exposed to sodium chloride solution and then dried in an oven. The exposure process was such that the samples obtained a profile of salt ingress along their lengths. The microwave reflection properties of these samples were measured at the S-band (2.6 to 3.95 GHz) using a short circuited rectangular waveguide sample holder. The effective reflection properties of these samples were then measured, indicating material changes after each soaking and drying cycle. Additionally, an electromagnetic model for simulating the measured reflection properties of these samples was also developed. Using this electromagnetic model along with a dielectric mixing formula, the measured reflection properties of these samples were simulated. The simulation and measured reflection property results were found to be in good agreement. Finally, using this approach the profile of salt ingress in the samples was obtained as well.