Concrete Products

FEB 2019

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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54 • February 2019 TECHNICAL TALK BY TOM KUENNEN Precast concrete products were on the agenda of the 99th annual Transportation Research Board meeting in January in Washington, D.C. Over 12,000 delegates heard presentations in 800-plus sessions addressing multimodal transportation policy and construction prac- tice, including materials and design involving precast/prestressed products. Concrete Products was there and we present this report on new research in precast structures and methods. We will look at TRB 2019 research in cast-in-place concrete later this year. The 100th meeting of TRB will take place January 12-16, 2020. For more information, visit NANOSILICAS IMPROVE BOND STRENGTH OF GROUTS Colloidal nanosilica suspensions as a surface preparation technique increase the bond strength between precast components and the field-cast grout, say Igor De la Varga, José Muñoz, Robert Spragg, and Caleb Nickel, SES Group and Associates at Turner-Fairbank Highway Research Center; Lucas Bohn, Wiss, Janney, Elstner Associates Inc.; Ariana Fay, University of Buffalo; and, Benjamin Graybeal, FHWA at TFHRC, in their peer-reviewed paper, Nanosilica Coatings to Improve the Tensile Bond Strength of Cementitious Grouts. Accelerated bridge construction techniques benefit from the use of prefabricated bridge elements (PBEs), the authors write, adding "For concrete PBEs, the connections between elements are key to the success of this system, and, depending on the design, are often completed using field-cast cementitious grouts." Yet prior research shows such grouts used for field-cast connec- tions can vary widely in performance. "Many grouts demonstrate volume instabilities, which result in shrinkage cracking and a poten- tial loss of bond to the precast element," write De la Varga, et al. Bond performance improvements include use of rougher surfaces of the concrete PBE achieved by sandblasting, hydrodemolition or in-form retarder application; pre-moistening of the concrete PBE closure pour surface with water spray or wet burlap cover; and, use of bonding agents. "Epoxy-based bonding agents are commonly preferred due to their reliable performance," the authors write. "However, their high cost and the difficulty of applying them to congested concrete substrate surfaces in PBE connections make their use challenging." The idea of using nanosilica suspensions as bonding agents to improve the bond between cementitious grouts and concrete PBEs opens the possibility of transitioning the critical point in the struc- tural system away from the interface region of these two materials, they say, adding "The topical application of nanoparticles to form TRB 2019: Nanotechnology boosts performance of cementitious grouts After sandblasting of concrete slab, colloidal nanosilica suspension is applied by brush, followed by grout pour. coatings has been shown to improve the performance of different type of interfaces in cementitious materials, such as paste-aggregate bond, and bond between precast concrete surfaces." The application of nanoparticles, mainly nanosilica, as coatings in interfaces has several advantages: • The nanosilica coating promotes the formation of a hydration-product rich layer right at the interface that could benefit tensile strength and durability; • Nanomaterial coating application techniques, e.g., paint-on or spray-on, have a lower equipment cost and person-hour effort compared to other preparation techniques; and, • The application of the nanoparticles as coatings, rather than adding the nanosilica to the bulk of the material as done in other concrete applications, reduces the amount of materials needed, thus lowering material cost. Their study used nanosilica coatings as bonding agents to improve hydration at targeted locations, that is, at the interface between the cementitious grout and the concrete PBE, with the goal of improv- ing the bond performance. "The nanosilica suspension [was] applied directly to the concrete PBE surface before casting the cementitious grout," the authors write." The impact of nanosilica on the bond performance was observed using microscopy to evaluate the micro- structure at the interface and demonstrated with strength testing. De la Varga, Muñoz, Spragg, Nickel, Bohn, Fay and Graybeal conclude: • The tensile bond strength of concrete-grout interfaces where nanosilica coatings were added was considerably enhanced. • The coating resulting from applying a 0.5 percent concentration of nanosilica suspension fully reacted after seven days of grout hydration. The reaction of the coating improved the contact between hydration products of the grout and the surface of the concrete, thus resulting in increased tensile bond strength at the interface. • The increase in concentration of the nanosilica suspension from 0.5 to 5 percent used to create the coatings disrupted the contact between the grout and the concrete surface in specific areas of the interface. In these specific areas, both materials were separated by the presence of a 5 μm-thick gap caused by an excessive deposition of the nanosilica coating, emphasizing the need to optimize the thickness of the coating to enhance the contact between both materials at the interface. • The dissolution and reaction of the nanosilica coating caused a reduc- tion in the amount of unhydrated particles and porosity, and an increase in the amount of hydration products when compared to the control. This effect was observed in the grout paste located up to 100 μm away from the surface of the concrete. PHOTOS: De la Varga, et al.

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