Modeling alkali-silica reaction using image analysis and finite element method

Concrete deterioration due to alkali-silica reaction (ASR) is difficult to model because concrete itself has a highly complex structure that includes random heterogeneous components In addition, modeling ASR is hampered by imperfect knowledge of the mechanisms and requires a technique that bridges concrete science and structural engineering and that goes up and down across multiple scales of length. Thus ASR modeling remains a difficult task and relatively little work has been done to simulate expansion and structural damage resulting from ASR.;The objective of this study was to develop a numerical model of mortar tested according to ASTM C1260. The model employed two main strategies -- image analysis and finite element modeling. Macroscopic expansion of the mortar bar was estimated on the basis of numerical results at microscopic level.;The modeling work includes acquisition of microstructural images of specimens using scanning electron microscopy (SEM), qualitative analysis of the images for identification of features using SEM and energy-dispersive X-ray analysis, image processing and analysis using image analysis program, and finite element modeling of the microstructural images using an object-oriented finite element program. To select images for modeling, a simple probabilistic approach was introduced. After three images were chosen for the model, individual features in the images were identified and quantitative information was extracted to get the initial length of the microstructure that is necessary to compute the strain of the sample. The processed images were also used for finite element modeling, which aims to determine the value of the final length of specimen in the wet condition. Swelling pressure caused by the ASR product and mechanical properties of constituents in mortar were input for the finite element modeling. Finally, the strain was obtained by dividing the elongation of microstructure by its initial length. The numerical results were seen to be valid by comparing calculated strain with the measured expansion of mortar.;Based on the results of the numerical model herein, the mechanism of ASR expansion was quantitatively proposed as follows. In the beginning, new ASR gel forms and swells somewhere in concrete. With continuation of the reaction, more gel accumulates and swells, and the total volume of gel increases. The new crack formation was seen to cause the majority of expansion of mortar specimens. On the other hand, elastic elongation of aggregate and paste due to gel swelling was seen to be much less important.;The interrelations among factors like the amount of each component and their influence on the strain of mortar specimen were comprehensively analyzed. No parameters that favor the formation and development of new phases were found. The amounts of Type 2 (gel-filled cracks) and Type 3 cracks (cracks without gel, but always connected with Type 2 crack) have the direct influence on the behavior of specimen.;A sensitivity test was conducted to clearly understand the effect of elastic modulus, Poisson's ratio, and swelling pressure of gel on the strain due to gel swelling and the total strain. The results show that the strain due to gel swelling is inversely proportional to the elastic modulus and Poisson's ratio of gel. When the elastic modulus of gel exceeded 100 MPa, elastic modulus and Poisson's ratio of gel were not a critical factor. However, strain is linearly proportional to gel swelling and therefore the total strain is very sensitive to swelling pressure of gel.;The microstructure-based model developed in the present study appears to be unique and promising in spite of some difference because the numerical results are in a good agreement with experimental data. It is considered that image analysis and FEM are powerful tools to understand the overall behavior of concrete affected by ASR.