International Concrete Abstracts Portal

The electrical properties of nanophase carbon black-filled cement-based composites are sensitive to moisture content. Previous studies indicate that cementbased composites filled with 120 nm carbon black (CB) in the amounts of 15% (A-15) and 25% (A-25) by weight of cement have promising strain self-sensing properties (that is, piezoresistance properties), thus, this study investigated the effects of moisture on the electrical properties of A-15 and A-25. The results indicate that the initial resistance of composites increases with moisture content. Additionally, the resistance of specimens with certain moisture content increases with measurement time. These two phenomena are mainly attributed to a polarization effect. A waterproof measurement (that is, a specimen encapsulated by epoxy) was developed to insulate the composites from ambient moisture for the composites as strain self-sensing materials. The initial resistance of the specimens encapsulated with epoxy and dipped into water stayed constant during measurement time, and their piezoresistance properties were almost the same as those of the specimens exposed to ambient moisture.

The latest research on the reactivity toward cross-linking of inorganic matrixes formed by cement-based materials, and the associated grafting at the nanoscopic and atomic levels, is highly promising. Macro-defect-free (MDF) materials and technologies were originally comprised of high-alumina cements cross-linked to poly(vinyl alcohol/acetate) or of portland cement with poly(acrylamide). Although the high-alumina system has shown promising results and is the more efficient system, it suffers from economic disadvantages; modern efforts to identify MDF systems focus on portland cement and a variety of polymer additives. We report recent findings regarding the potential and limitations of portland cement-based MDF materials, considering aspects of the associated chemistry (at the nanoscopic and atomic levels), of the mechanism linking polymer to the surface of cement grains, and of technologically relevant attributes such as moisture resistance of the formed MDF material.

Nanotechnology has changed our vision, expectations, and abilities to control the material world. The developments in nanoscience can also have a great impact on the field of construction materials. Portland cement, one of the largest commodities consumed by mankind, is obviously the product with great, but not completely explored, potential. Better understanding and engineering of complex structure of cement-based materials at nanolevel will definitely result in a new generation of concrete, stronger and more durable, with desired stressstrain behavior and, possibly, with the whole range of newly introduced "smart" properties. The reported research examined the mechanical properties of mortars with nano-SiO2 synthesized by sol-gel method. Experimental results demonstrate an increase in compressive strength of mortars with developed nanoparticles at early stages of hardening followed by the strength reduction at later age (versus the reference). Addition of superplasticizer was proposed to overcome this obstacle. Superplasticized mortars with selected nano-SiO2 demonstrated a 15-20% increase of compressive strength, reaching up to 144.8 MPa (21 ksi) at 90-day age. Mechanochemical activation was found to be effective method to improve the strength of cement-based materials. It was proposed that this process is governed by the solid state interaction between the organic modifiers and cement. During this process, the surface of cement particles attaches the functional groups introduced from the modifiers; so the organo-mineral nano-layers or nano-grids are formed on the surface of cement. The developed high-performance cements demonstrate the 28-day compressive strength at the range of 93 - 115 MPa (13.5-16.7 ksi), which is higher than 72 - 89 Mpa (10.4-12.9 ksi), the strength of reference cements.

This paper describes the development of a high-strength, high-density cementitious rock-matching grout mixture for use in a field project. This portland cement-based grout incorporated silica nanoparticles in the form of an ultra-fine amorphous colloidal silica (UFACS) along with hematite fine aggregate, silica fine aggregate, silica fume, water, and other chemical admixtures to match as closely as possible the given in-situ high-strength rock properties at a given field site. The new admixture, UFACS, was used as a viscosity-modifying agent to prevent aggregate segregation and was used to replace sodium bentonite clay, which frequently leads to a decrease in unconfined compressive strength. The resultant rock-matching grout (RMG) mixture was then placed in the field with over 65 individual batches to fill the annular space surrounding instrumentation packages previously placed in drilled boreholes. The resultant mechanical properties of this field-placed RMG included unconfined compressive strengths of 91.2 MPa (13,230 psi), hardened densities of 2680 kg/m3 (4,520 lb/yd3), and ultrasonic pulse velocities of 4.40 km/s (14,500 ft/s).

This volume consists of ten papers that were presented at a technical session, sponsored by ACI Committee 236, Materials of Science of Concrete, at the ACI Fall 2007 Convention in Denver, CO. Papers include “ Nanotechnology in Construction: A Roadmap for Development”, “ Nanotechnology and Concrete: Research Opportunities”, “Molecular Engineering of the Cohesion in Neat and Hybrid Cement Hydrates”, and “Use of Atomic Force Microscopy and Nanoindentation for Characterization of Cementitious Materials at the Nanoscale”.