Remote moisture quantification of concrete using SAR images and the K-R-I transform

Abstract

Moisture content in Portland cement concrete structures is commonly associated with many durability problems (e.g., steel corrosion) and can be used in structural health monitoring and nondestructive evaluation/testing (NDE/T) of concrete structures. While several intrusive techniques are available for quantifying moisture content inside concrete structures, it is a challenging task to estimate the moisture content of concrete in the field without using embedded moisture sensors. In this paper, synthetic aperture radar (SAR) imaging as a nonintrusive technique and the K-R-I (curvature-area-amplitude) transform are applied to concrete panel specimens made of two water-to-cement (w/c) ratios (w/c = 0.4 and 0.5) for moisture determination. Concrete panel specimens with dimensions of 0.3 m-by-0.3 m-by-0.05 m were manufactured and conditioned in a laboratory environment by air-drying for three months. Its time-dependent moisture variation was simultaneously monitored by a 10.5- GHz center frequency SAR imaging sensor inside an anechoic chamber. Quantitative analysis of SAR images was carried out by using the K-R-I transform to understand the simultaneous change of SAR amplitude and distribution (contour shape) at different moisture contents. It was found that SAR amplitude and its distribution increase with the increase of moisture content inside concrete panel specimens. Spatial distribution of SAR amplitudes can be used to indicate subsurface moisture distribution inside concrete. The area-amplitude (R-I) curve of SAR images can be used to quantify the relationship between moisture content and its distribution.