Application of X-Ray CT to Study Diffusivity in Cracked Concrete Through the Observation of Tracer Transport

Abstract

This paper demonstrates the application of microfocus X-ray computed tomography (CT) to study solute transport in cracked concrete. Cracks in a cylindrical specimen of ordinary Portland cement (OPC) and fly ash mortar were induced using a splitting tensile test. Cesium Carbonate (Cs 2 CO 3 ) was then used as a tracer in the in-situ diffusion test with the aid of X-ray CT. Image analysis was also employed to measure the 3D crack geometry and tracer diffusivity from these CT images. The geometric tortuosity of the crack was approximately 1.25 irrespective of the crack opening width and whether fly ash was added or not. On the other hand, the constrictivity increased for the fly ash mortar having roughly the equivalent crack opening width. The measured diffusivity in the crack was controlled by both crack opening width and constrictivity. Results obtained from microtomographic images suggest that the entire crack space may not always be filled with the tracer. The diffusive transport of solute in cracks thus can be restricted from microstructure's point of view. Smaller crack opening would increase such restricted diffusion. Indications also suggest that the addition of fly ash would lead to the reduction of diffusivity through uncracked body of the mortar.

Visualization and Quantification of Solute Diffusivity in Cracked Concrete by X-Ray CT

“…Cesium carbonate can serve as an excellent tracer in concrete because it has a high X-ray absorption in contrast to concrete and its constituent materials. X-ray absorption (represented by the CT number in the case of the microfocus CT system used in this study) has a linear relationship with the tracer concentration (Darma et al, 2013). The solution was prepared by mixing cesium carbonate powder (ρ=4.07 g/cm 3 ) with water at a concentration of 40% by weight (1.84 mol/L), based on previous experimental results, which showed good contrast during image analyses at that concentration level (Ikeda et al, 2012).…”

“…This process was repeated for the entire slice stack at selected heights, and at 10 regions of interest per image where the crack was filled with the solution. The average of the maximum observed CT number after isolating the cesium solution profile was then converted into the cesium solution concentration using the calibration data provided by previous research (Darma et al, 2013). Finally, the diffusion coefficients were calculated using a solution of Fick's second law under the proper boundary conditions.…”

“…In the concrete field, X-ray CT has been applied to a variety of research areas, including pore structure characterization, freeze-thaw and fire damage, and diffusivity in cracks (Darma, Sugiyama, & Promentilla, 2013;Henry, Darma, & Sugiyama, 2014;Sugiyama, Promentilla, Hitomi, & Takeda, 2010). This technology is uniquely suited to analyzing crack characteristics in particular, as air voids inside concrete-both connected (cracks) and isolated-have little to no X-ray absorption properties, and thus, contrast highly with the cement paste and aggregates.…”

“…In this paper, the results of two studies on solute diffusivity that used a cesium carbonate solution as a tracer to simulate liquid ingress in cracked concrete are introduced and summarized (Darma et al, 2013;Ikeda, Sugiyama, Kikkawa, & Shimura, 2012;). These studies examined the solute diffusion under a variety of conditions using X-ray CT to both visualize and quantify the diffusion behavior, and the results provide valuable information for better understanding the transport phenomenon of a solution in cracked concrete.…”

Evaluation of Micro-damage of Concrete Specimens under Cyclic Uniaxial Loading by X-ray CT Method

“…applied micro-focus X-ray CT to characterization of internal structure of mortal exposed by freezing-thawing action, then made clear void function, air void size distribution, crack width and tortuosity of connected crack network in three dimensions. Application of X-ray CT to estimating diffusivity in cracked concrete was performed by Darma et al (2013). In the research, cesium carbonate solution was used as a tracer, the diffusion from a fracture to concrete was discussed.…”