International Concrete Abstracts Portal

This CD-ROM consists of ten papers that were presented by ACI Committee 236 at the ACI Spring convention in Chicago, IL, in 2010. Topics include Characterization of the Nanostructure and Microstructure of Concrete, Cracking Reduction, Incorporating Nanofibers into Concrete Mixtures,and Material Behavior Reinforced with Examinations at the Microstructural Level. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-270

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.

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.

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.

Concrete is a composite of aggregates in a cement paste matrix. Dissimilar volume changes in these constituent materials may result in localized stress development. This is particularly problematic when the aggregate expands more than the surrounding paste. This expansion results in tensile stress development in the cement paste matrix which can lead to micro-cracking in the cement paste matrix. These micro-cracks can eventually coalesce and localize in visible cracking. Quantifying this type of damage can be difficult. This paper describes a conceptual model and physical simulation of this damage considering the expansion of polymeric inclusions (i.e., aggregates) in cement paste matrix subjected to temperature changes. Thermal loading (i.e., temperature change) was selected since it provides a method to control the expansion. Physical experiments were performed where continuous length change measurement and acoustic emission measurements were carried out. These experimental methods are used to better understand the mechanics of the damage. The experimental results indicate that a deviation from classical composite behavior occurs when damage develops which can be seen in the length change measurements. This deviation can be used to quantify the extent of damage. A numerical model is used to interpret the experimental results. An Eshelby misfit approach was used to determine the pressure created by the expanding aggregate. This enables the stresses that develop in a composite material to be determined. A linear fracture mechanics failure criterion is used to calculate the onset of damage formation. Results are in agreement with length change measurements and acoustic emission measurements. A composite damage model for direct calculation of the extent of damage from length change measurements is proposed.