APPLICATION OF IMAGE PROCESSING AND FINITE ELEMENT ANALYSIS IN MODELING CHLORIDE DIFFUSION IN CONCRETE

Utilizing numerical simulation models to predict the long-term mechanical and transport behavior of concrete structures is becoming increasingly popular. The majority of these models have been developed using laboratory test data that consider concrete as a homogeneous material with spherical aggregates. These models could not be a represented of real concrete because it has no primitive shaped aggregate besides that the porosity size and distribution varies from point to point. In this study a novel method for more accurate prediction of the chloride diffusion in concrete was developed. A general framework of the quantitative computed tomography (QCT) and finite element analysis was used to construct 3D images of concrete cylinders. A computer code was developed using Matlab to analyze images and to measure the amount and distribution of coarse aggregates and voids in the concrete cylinders. The rapid performance and independency from personnel, as well as the capability of inspecting the internal structure and possible damages within the cylinders, make this method very applicable for quality control and quality assurance applications as well as for forensic investigations. During this study, it was realized that the shape and distribution of aggregates as well as Interfacial Transition Zones (ITZs) have significant impact on the chloride diffusion into the concrete. Therefore, it was imperative to construct a predictive model which was closer to reality, considering the distribution of aggregate particles (coarse and fine), voids, and ITZs (around both coarse and fine aggregates). Thus, a numerical method for the prediction of the chloride penetration into concrete was developed using a scanned copy of the concrete internal structure. The results obtained from this study showed that, QCT along with image analysis techniques used to study the air void content and distribution as well as coarse aggregate content in concrete in 3D had a good agreement with the microscopic analysis. The major advantage of QCT technique is much short time required for analysis with the QCT method compared to that with the conventional microscopic studies. The result from the chloride diffusion in concrete showed that chloride concentration gradient when ITZ is considered around aggregates is much higher compared to that in concrete without considering the ITZ. The positions and shapes of the coarse aggregates can also affect the diffusion process and the chloride ion diffusivity. The experimental and simulation results indicated that closer aggregates to the steel bar can increase the rate of the chloride diffusion as well as the rate of corrosion

Finite Element Modeling of Concrete Based on Quantitative Computed Tomography (QCT)

Models have been used before to predict the mechanical and transport behavior of concrete. In most of these studies, aggregates were considered either circle or sphere and the impact of the aggregates geometry and in-homogeneities in concrete structure is ignored. The objective of this study is to develop a novel method for accurate prediction of the mechanical behavior of concrete using quantitative computed tomography (QCT)-based finite element analysis. Concrete cylinders were cast and cured for 28 days. The QCT scans were carried out on the samples using a clinical CT scanner. An image processing method was applied to detect aggregates, paste content and the air voids. The distribution of each phase then calculated in each image slice (2D) and in the bulk material (3D). The processed QCT images were directly converted into voxel-based 3D FE models for linear and nonlinear analyses. The FE models were generated by conversion of each voxel into an 8-noded brick element. Air void content of the cylinders (2D and 3D) was determined. In addition, the aggregates content was estimated using the image analysis. In both cases, the results obtained by the image analysis and the actual measurement and ASTM method are in very good agreement