Microscopic analysis of 50% axially strained cementitious materials

The microscopic behavior of 50% axially strained cementitious materials is studied. The microscopic behavior of concrete under the triaxial stress condition must be fully understood to explain the additional concrete ductility that comes from lateral confinement and a microstructural mechanism which has been subjected to extreme strain. The so-called "tube-squash" test was used to achieve 50% axial strain and over 70° of shear angle change in a concrete under extremely high pressure, without visible cracks. Then, microscopic analysis focusing on hydration and the microstructure of a 50% axially strained cement paste were performed on cored-out deformed and virgin (undeformed) cement paste specimens; the first specimen was 40 days old and the second one was 1 year old. Field emission scanning electronic microscope (FESEM) analysis was performed to compare the microstructures of the 40-day-old and 1-year-old specimens applied with finite strain. On the 1-year-old specimens, X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDS), and differential thermal analysis/thermo-gravity (DTA/TG) analyses were also performed to study the change of hydration and microstructures due to 50% axial strain. The results and implications from these various analyses are discussed.