Many of the papers presented in this volume were included in the two-part session Nanotechnology for Improved Concrete Performance, sponsored by ACI Committee 241, Nanotechnology of Concrete at the ACI Convention in Philadelphia, PA, on October 26, 2016. In line with the practice and requirements of the American Concrete Institute, peer review, followed by appropriate response and revision by authors, has been implemented.

In cold regions, freezing temperatures limit the construction season to few months, usually between May and September. The use of nanoparticles, which have high specific surface and vigorous reactivity, may potentially enhance the performance of concrete placed at low temperatures. Therefore, this study focused on developing concrete mixtures incorporating nano-silica which were mixed, placed and cured at -5°C (23°F) without any insulation or protection targeting field applications in late fall and early spring periods. Eight mixtures incorporating general use (GU) cement, fly ash (up to 25%), and nano-silica (up to 4%) were tested for this purpose, with water-to-binder ratios of 0.32 and 0.4. All mixtures contained a combination of calcium nitrate and calcium nitrite as an antifreeze admixture. Testing involved concrete setting time (placement), 7 and 28 days compressive strengths (hardened properties) and resistance to freezing-thawing cycles (durability). Moreover, mercury intrusion porosimetry, thermal analysis and scanning electron microscopy were performed to corroborate the trends from the macro-scale tests. It was found that nano-silica significantly improved the overall performance of concrete placed and cured at -5°C (23°F), which implicates its promising use for construction applications under low temperatures.

Fiber additions in portland cement composites is a regular practice for crack prevention and for increasing the flexural strength. In this research, fiber-reinforced composites (FRC) with polyvinyl alcohol (PVA) fibers and carbon nanofibers (CNF) or carbon nanotubes (CNT) were investigated. Specimens were tested to measure their flexural strength, water absorption and electrical conductivity in water or sodium chloride solution. It was found that the developed composites, depending on applied stress and exposure to chloride solutions, exhibit some electrical conductivity. These dependencies can be characterized by piezoresistive and chemo-resistive coefficients demonstrating that the material possesses self-sensing capabilities. The sensitivity to strain,  crack formation, and chloride solutions can be enhanced by incorporating small amounts of CNF or CNT into a composite structure. Conducted research has demonstrated a strong dependency of electrical properties of the composite on crack formation in moist environments. The developed procedure is scalable for industrial application in concrete structures that require nondestructive stress monitoring, integrity under high service loads and stability in harsh environments.

In this work, effects of nanosilica (NS), nanolimestone (NL), and nanoclay (NC) additions on hydration and strength of cement pastes were studied. The pastes were made with Type I ordinary Portland cement (OPC), 0 and 30% Class F fly ash (FA), and 0 or 1% nanomaterials. All pastes had a water-to-binder ratio of 0.5. Chemical shrinkage was monitored as an indication of cement hydration process. X-ray diffraction (XRD) was conducted to identify crystalline hydration products. Thermogravimetric analysis (TGA) was used to quantify calcium hydroxide (CH) and chemically bound water. The results indicate that the rate of chemical shrinkage curve can be divided into five stages, similar to that observed from the rate of cement hydration curve measured from a calorimetry test. All nanomaterials increased the rate of chemical shrinkage associated with C3S and C2S reactions; but different types of nanomaterials had different effects on the rate of chemical shrinkage associated with secondary C3A reaction. All nanomaterials improved strength of OPC paste at ages up to 28 days; but the improvement was not clear for OPCFA pastes. Through reaction with OPC and FA, NL stabilized voluminous ettringite and produced hemicarbonate (Hc) instead of less voluminous monosulfate (Ms).

There is significant potential for the use of nanoparticles in cementitious materials, especially in ultra-high performance concrete. These nanoparticles can further increase packing density, accelerate the pozzolanic reaction or can be used to induce new properties to the material, such as air purification or self-cleaning. Little is known about the interaction mechanisms between nanoparticles in cementitious materials, including their dispersion quality. The characterization of these nanoparticles can be challenging, especially when these nanoparticles interact with cementitious materials and their reaction products during hydration. Thorough characterization of the nanoparticle system is essential to understand how to optimize mixing constituents, procedures, and parameters.