Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.
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FEATURE BY TOM KUENNEN Test shafts with intentional voids were created using sand bags and Styrofoam to investigate further the ability of other nondestructive test methods. In the test shafts, the CSL and sonic echo/impulse response (SE/IR) methods were used to determine integrity. In the laboratory, the acousto-ultrasonic (AU) method was evaluated to determine the cover depth. Ozyildirim and Sharp found: • Conventional and SCC mixtures with satisfactory workability and strength can be produced, but SCC mixtures have higher workability and are easier to place. • Temporary casings are easier to remove from drilled shafts with SCC. • There is higher variability in the sonic velocity measurements of SCC mixtures as compared to the conventional concretes. However, the average value of SCC mixtures is higher (12,881 ft./s) when compared to the average value of the conventional mixtures (12,592 ft./s). • CSL testing is easy to conduct and detects large voids attributable to honeycombing, but must be conducted early before the placement of the concrete cap. In addition, metal tubes provide much clearer signals than plastic tubes, especially at later ages. • SE/IR can detect voids inside drilled shafts. • Acousto-ultrasonic (AU) testing needs additional development for field use on actual drilled shafts. NANOSILICA NANOSILICA BOOSTS FRICTION, LOWERS CONCRETE PAVEMENT NOISE Nanotechnology can increase friction and sound absorption of concrete pavements, note Marcelo Gonzalez, M.Sc., Md. Safiuddin, Ph.D., P.Eng., Jingwen Cao, and Susan L. Tighe, Ph.D., P.Eng., Department of Civil and Environmental Engineering, University of Waterloo, Ont., in their paper, Sound Absorption and Friction Responses of Nano-Concrete for Rigid Pavements. Friction qualities of pavements are influenced by their microtexture, macrotexture, materials properties and environmental conditions, such temperature, water and snow, the authors say, while attempts to increase pavement friction have focused on creating different surface textures or macrotexture. Similarly, tire/pavement noise predominates over other sources of roadway noise in many circumstances, and is produced at the interface between the two. Research is underway to evaluate the effects of two nano-materials on the friction and sound absorption of concrete pavements. Specific objectives are: • Evaluate the effect of nanosilica on the key properties of fresh concrete. • Examine the effect of nanosilica on the strength of concrete. • Evaluate whether nanosilica can improve the friction and sound absorption of concrete pavement. • Study the mechanisms of how nanosilica improves the microstructure/microtexture of concrete. PROPERTY NATURE/VALUE Color Form Apparent density Particle size Silica (SiO2) content (dry basis) Dispersity (CC14) Free water content White Amorphous 0.3696 g/cm3 10-20 nm 99.8% 98.6% <=3% SOURCE: Gonzalez, Safiuddin, Cao, and Tighe 34 | MARCH 2013 • Evaluate whether material emulating the nano-surface properties of the lotus leaf can be applied as a coating to improve the friction and sound absorption of concrete pavement under wet conditions. "The effect of nanosilica on friction property and sound absorption was evaluated through a microtexture modification," Gonzalez, Safiuddin, Cao, and Tighe say. "The physical effect of the nano-lotus leaf on the concrete surface was also visually observed. The preliminary results revealed that suitable nanomaterials can increase friction and sound absorption of concrete pavement." More research already is underway. Microtexture can be improved using nano-materials, the authors write. "Nanomaterials consist of particles at scales below 100 nm (a nanometer is a billionth of a meter (10-9 m))," they say. "The use of nano-materials could be a promising and revolutionary tool to engineering applications. [A] literature survey reveals that nanosilica, nano-lotus leaf and nanofibers can be applied in concrete pavements." Nanosilica improves the compressive strength of portland cement mortar, they say, with the main beneficial effects of nanosilica on the microstructure and performance being an improvement in aggregate/mortar contact zone, resulting in a better bond; a reduction in segregation; an acceleration of the hydration process; and the creation of small-size crystal and clusters of calcium silicate hydrate (C-SH) during pozzolanic reaction. And because silica fume can improve the sound absorption ability of concrete due to increased vibration damping capacity, nanosilica may show a similar effect. Their literature survey revealed that as the lotus leaf exhibits an exceptional ability to keep itself clean and dry due to the "lotus effect" at the nanolevel— which creates very high water repellence—nano-lotus leaf can be used to create a self-cleaning coating on concrete pavements. This self-cleaning coating is a superhydrophobic, nanoscopic surface based on a physio-chemical effect. "Moreover, carbon nanofibers are excellent candidates for cement reinforcement because they provide high specific strength, chemical resistance, and electrical and thermal conductivity," the authors affirm. "They can be used to produce concrete pavements with strong and durable surface." WWW.CONCRETEPRODUCTS.COM