International Concrete Abstracts Portal

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).

Although clays such as metakaolin are typically incorporated into cement-based materials in order to improve hardened-state properties, recent findings at the Center for Advanced Cement-Based Materials (ACBM) have shown a large influence on the fresh-state. Furthermore, these benefits come at a much lower dosage, (~1% by mass of binder). A recent focus of ACBM has been the use of small dosages of clays in order to improve fresh-state properties such as extrudability and green strength. This paper summarizes several current projects at ACBM which demonstrate the effects of different commercial clays on relevant properties for three different scenarios: extrusion, formwork pressure of self-consolidating concrete (SCC) and slipform paving SCC. In each case, small additions of clays have shown marked improvements in the rheology of the cement composite. The use of two experimental methods to characterize the microstructural changes that occur with additions of clays is also reviewed.

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 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.

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.