Citation: Behravan, A.; Ley, M.T.; Cook, D.; Hu, Q.; Rywelski, A.; Brorsen, R. Measuring the Diffusion Coefficient of Paste and Concrete by Using Dental X-ray Equipment. CivilEng 2023, 4, 224–247. https://doi.org/10.3390/ civileng4010014 Academic Editors: Angelo Luongo and Francesco D’Annibale Received: 28 December 2022 Revised: 20 February 2023 Accepted: 21 February 2023 Published: 24 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Measuring the Diffusion Coefficient of Paste and Concrete by Using Dental X-ray Equipment Amir Behravan 1,2, *, M. Tyler Ley 2 , Dan Cook 2 , Qinang Hu 2 , Anna Rywelski 2 and Russell Brorsen 3 1 Virginia Transportation Research Council (VTRC), Charlottesville, VA 22903, USA 2 School of Civil & Environmental Engineering, Oklahoma State University, 248 Engineering North, Stillwater, OK 74078, USA 3 Brorsen Family Dental Clinic, Stillwater, OK 74075, USA * Correspondence: amir.behravan@vdot.virginia.gov Abstract: The penetration of ions plays an important role in the durability of concrete structures. This study aims to establish the feasibility of using dental X-ray equipment to measure the concentration and penetration of iodide within cementitious systems. This technique is known as checking ion penetration (CHIP). This test uses iodide as a tracer because it has a high electron density, and so it can be observed with X-ray imaging as it penetrates the concrete. Concentration profiles from CHIP are used to calculate the apparent diffusion coefficient (D ac ). These results are similar to measurements from bulk chloride ponding tests. The D ac is used to predict the service life or evaluate the quality of an as-built concrete structure or concrete mixture. Because of the wide availability of dental equipment, CHIP shows promise to be used as a method to measure the in-place quality control of the concrete. Keywords: X-ray microscopy; TXM; chloride intrusion; service life; durability; corrosion; CHIP; dental X-ray 1. Introduction Concrete has been used for over a century as the building material of choice for long service life. The durability of concrete is often threatened by the transport of outside ions that cause deterioration of the concrete or the reinforcing [1]. The most common way to quantify the resistance to outside chemical penetration is by using electrical methods to indirectly investigate the microstructure. The most common electrical tests are the rapid chloride permeability test (ASTM C1202) and the electrical resistivity tests (AASHTO TP95 and ASTM C 1760). These tests have challenges predicting the resistance to outside chemi- cals when supplementary cementing materials (SCMs), certain chemical admixtures, steel fibers, and some particular aggregates are used in the mixtures, or when the tempera- ture/moisture or amount of carbonation content varies within the mixtures [2–10]. The most trusted method to investigate ion ingress into cement-based materials is the bulk diffusion test. This test is completed by placing a salt solution on top of the sample as described in ASTM C 1556 and AASHTO T259, followed by a titration test on powder taken at different depths within the sample [11–14]. Because the powder from the concrete contains about 75% aggregate and 25% paste by volume, a large amount of powder is needed to be sure that the chlorides in the paste are well characterized. These tests are time-consuming and labor-intensive, and so they are not widely used. Several studies have used X-ray imaging techniques to simplify the ASTM C 1556 test. Some of these include using X-ray fluorescence (XRF) and X-ray microtomography [15–26]. The equipment needed for these measurements is expensive, time-consuming, and needs significant sample preparation. Recently, transmission X-ray microscopy (TXM) using μCT equipment has been used to place iodide in water and observe the change in X-ray absorption. TXM is useful because it is non-destructive, fast, requires minimal sample CivilEng 2023, 4, 224–247. https://doi.org/10.3390/civileng4010014 https://www.mdpi.com/journal/civileng