International Concrete Abstracts Portal

In this study, various fresh cement paste microstructure evolutions were observed during setting in a conventional SEM by employing a new sample technique called Quantomix capsuling system. Individual cement particle and different phases growths were studied quantitatively by using image analysis techniques on observed micrographs. The ASTM C191 method was used to determine the cement pastes setting times and further the developments of different phases in the cement pastes microstructure were studied at various stages of setting. It was observed that irrespective of the water to cement (w/c) ratio, cement particle connectivity plays a major role in Vicat needle settlement (or in cement paste setting); however, it was observed that w/c ratio influences the hydration rate in mixtures. It was also observed that during setting water phase depletion wholly depends on richness of the mixture, whereas solid phase growth is a combined effect of cement particle growth and connectivity.

Electrical impedance based methods are frequently used to monitor the microstructural development in cement-based materials due to their non-invasive nature and the ability to make continuous measurements. This paper discusses two other applications of electrical impedance for two different classes of concretes—micro-porous conventional concretes and macro-porous pervious concretes. The effectiveness of a microstructural parameter derived from electrical impedance data to relate to the rapid chloride transport parameters of conventional concretes is described. The influence of the non-steady state migration test in changing the microstructure of concretes due to formation of new solid products through chloride binding of cement hydrates is also evaluated in detail through a combination of measured electrical impedance parameters and an equivalent electrical circuit model. The values of the components of the model determined before and after the migration test provide confirmation of the formation of binding products along the pore walls in concrete. Electrical impedance observations along with a modified Bergman equation are used to predict the porosity of pervious concretes. The experimentally determined and predicted porosities match adequately. The effective electrical conductivity can also be used in well known permeability prediction equations such as the Kozeny-Carman or Katz-Thompson equations for performance prediction of pervious concretes.

Uniaxially passive restraint experiments (for example, “cracking frames”) provide enough information to extract useful viscoelastic constitutive properties when combined with free deformation and mass loss experiments (in the case of drying shrinkage). In this paper, analytical techniques are described for deriving a closed-form solution to extract the viscoelastic Young’s modulus from solidifying concrete in a uniaxial passive restraint test. In addition, for the particular case where drying shrinkage is restrained in the test, approximate closed-form solutions are derived for the non-uniform internal relative humidity (RH), free drying shrinkage, and stress gradients. An example problem demonstrates the utility of the derived solutions.

X-ray nanotomography was explored for investigation of the microstructure of cement paste in various ages, including 1-day and 28-day. 2D and 3D images were obtained for quantitative analysis and morphological reconstruction. The technique also provided a prospect of viewing interfacial transition zone (ITZ), of which the microstructures were simulated and the evolution of components with respect to the distance from the interface was calculated.

Characterizing mechanical properties of cement-based materials is a basic task to be performed before using it as a structural material. An array of different mechanical tests has been developed and applied to measure the stiffness or strength of materials. A recent addition to this field concerns nano-mechanical characterization. It is an extension of our interests: how rigid is a certain volume of materials in nanoscale space? Measuring the properties of interfacial transition zone (ITZ) locating between an inert aggregate and bulk paste is a key application example for nanoscale characterization. In order to measure the properties of ITZ, spatial resolution should be enhanced less than the size of ITZ (a few tens of micrometers), while the compressive strength test, a conventional macroscale measurement, usually uses a specimen of 150 mm diameter 300 mm height (6 in by 12 in) cylinder. This paper discusses two nanoscale techniques, that is, nanoindentation and ultrasonic atomic force microscopy (AFM) used at the center for Advanced Cement-Based Materials (ACBM). The principle of both techniques measuring the elastic modulus at the nanoscale is clarified with an application to a cement paste sample having 50% water-binder ratio. The measurements by both techniques are not exactly the same due to their different mechanism. However, both techniques identically find that the peak probability of the measured elastic modulus of the cement paste is distributed between 10 and 20 GPa (1450 and 2900 ksi).