Application of the Deformation Mapping Technique to Assess Deterioration in Cement-Based Materials

A revealing fact concerning the sophistication of modern concrete durability research is that, for the most part, structures still are categorized as sufficiently intact, irreparably damaged, or usually somewhere subjectively in between. Standard compression tests may indicate loss of serviceability in the advanced stages of deterioration when macroscopic damage is clearly visible and when future repair is inevitable. If there were a way, however, to quantify the progression of early microscopic damage before macroscopic damage occurs, structures conceivably could be identified for which early repair or isolation could provide costefficient alternatives to removal and replacement. With this goal in mind, the image analysis–based deformation mapping technique (DMT) becomes an attractive candidate for a quantitative forensic tool. DMT is a two-dimensional image correlation technique that determines microscopic deformations by computing the relative pixel displacements between an initial and a deformed image. The unique ability of DMT to translate these displacements into principal deformations allows for more accurate quantification of heterogeneous deformation at the microscopic scale, where the central deforming axis is not necessarily coincident with the loading axis. In the current study, the time progression of microstructural changes is shown in a mature ordinary portland cement paste dried at −30 percent relative humidity. Also, DMT illustrates the ability to quantify early microstructural changes by comparing the mechanical deformation response of a sound mortar and a mortar pre-exposed for 7 days in a saturated Na₂SO₄ solution. Isolation of the deformations specific to the paste phase of the mortars was obtained from a combined gray-scale thresholding–DMT procedure, from which high deformation concentrations could be visualized at the paste-aggregate interface of the sulfate-exposed specimen.