International Concrete Abstracts Portal

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.

There have been numerous studies that have aimed at improving the low tensile strength, stiffness, and toughness of cementitious materials. This study aims to show that all of these characteristics can be greatly improved by the addition of ladder scale reinforcement at the nano and micro scale. Carbon nanofibers (CNFs) as well as polyvinyl alcohol (PVA) microfibers were used as reinforcement. The mechanical properties of the nanocomposites were investigated by fracture mechanics three-point bending test. The microstructure and the morphology of nanocomposite samples were studied using an ultra high resolution scanning electron microscope (SEM). The results clearly illustrate that the incorporation of nanofibers and microfibers greatly improves the flexural strength, Young’s modulus, and toughness of the cement matrix.

This paper discusses the design and development of an automated test device for measuring the chemical shrinkage of hydrating cements. The device uses a very sensitive pressure sensor to determine water level drop in an open capillary tube, from which the volume change due to chemical shrinkage can be calculated continuously. The repeatability of the measurements and the stability of the measuring device have been demonstrated. The comparison of the result produced by the new device with that of a standard ASTM test showed excellent matching. The range of applicability of this device was demonstrated by obtaining shrinkage measurements for different types of cements, such as ordinary portland cement, Class H oil well cement and white portland cements, and for different temperatures.

Internal curing can be used to counteract the effects of self-desiccation in low water-cement ratio mixtures. Internal curing uses prewetted lightweight aggregate (LWA) as a reservoir that can release water as it is needed to keep the hydrating cement paste structure saturated. While many developments have been made in advancing the field of internal curing, this work examines the composition of the early-age pore solution. The effect of the pore solution composition on the relative humidity is also discussed. The paper performs desorption measurements of the lightweight aggregate to better understand how fluid is released from the lightweight aggregate. An approach is suggested to use solutions rather than pure distilled water to improve the relevance of the measurement of desorption at high relative humidities.

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