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The exposure of reinforced concrete structures to freezing temperatures during winters can create internal stresses and surface microcracks in concrete owing to tiny ice crystal formations in the pores of concrete structures. Thus, in the present study, superhydrophobic surfaces were created on concrete by spraying and admixing superhydrophobic materials, comprising 1H,1H,1H,2H-perfluorodecyltriethoxysilane (PFDTS) with nano TiO2 and SiO2 inclusions, with cement mortar. TiO2 nanomaterials were synthesized using ginger (Zingiber officinale) as a bio-template. The PFDTS enriched with nanomaterials (PFDTS-NM) was admixed with cement mortar during casting and was spray-coated on the cement mortar surface after casting. The spray-coated and admixed cement mortar sample surfaces were characterized by XRD, SEM/EDAX, and UV–vis spectroscopy, and their water-repellent properties were evaluated by measuring the contact angles (CA) and conducting water absorption and freeze-thawing tests. The pore size distributions of the samples before and after freeze-thawing were evaluated by mercury intrusion porosimetry (MIP). The results confirm the presence of PFDTS with inclusions of rod-shaped TiO2 nanoparticles on the concrete surface. Further, the CAs of normal, coated, and admixed cement mortar surfaces were found to be 45.5°, 162.3°, and 162.0°, respectively. The spray-coated and admixed concrete can be used to form superhydrophobic surfaces on concrete structures to reduce water absorption and increase the durability of cement mortar.

Shear performance of plain UHPC (ultrahigh-performance concrete) construction joints is studied in both experimental and analytical ways. In push-off tests, three different contact surfaces of the construction joint were considered, while the case without any joint was provided for the reference. Test results indicate that the geometry of contact surfaces greatly affects shear performance of the construction joint. With simplifying structural behavior of contact surfaces and UHPC substrate, the finite-element analysis model is developed for every case studied by utilizing the ABAQUS software and validated against the test results. Agreement between experimental and numerical simulation results is excellent especially in terms of displacement, strength, and failure mechanism. It is expected that the present work provides a basis for further study on reinforced UHPC construction joints.

As a part of an ongoing research to develop construction joints using ultra-high performance concrete (UHPC) and reinforcements, shear performance of plain concrete construction joints is experimentally examined with push-off tests. Two different sets, namely, a combined set of UHPC (180 MPa) and a set of UHPC (180 MPa) and NSC (normal strength concrete, 30 MPa) were considered, each of which had five cases of interface treatment including use of water jet, vertical joint, and three cases of a grooved joint. In addition, cases without any construction joint were also considered as a reference. Key factors affecting shear performance of individual construction joint were investigated in terms of shear strength, failure mechanism, and displacement responses. Experimental results show that steel fibers and grooved geometry greatly affect shear performance for the combined set of UHPC, while interlocking mechanism of coarse aggregates is crucial for the set of UHPC and NSC.

This study set out to derive the optimal conditions for ensuring the monolithicity of ultra-high-performance concrete (UHPC). Direct shear tests were performed to examine the influence on the bonding shear performance. The experimental variables included tamping and delay, which were set to 0, 15, 30, and 60 min. SEM and XRD analyses of the microstructure and composition were performed. The direct shear tests showed that the bonding shear strength was enhanced by the addition of tamping. For the normal-strength concrete (NSC), it is thought that a monolithicity of around 95% can be attained with a cold joint formation delay up to 60 min. In contrast, while the normalized bonding shear strength reduction of UHPC with a delay of 15 min was the lowest at around 8%, a dramatic degradation in the bonding shear performance was observed after 15 min. XRD analyses of the middle and surface sections revealed the composition of the thin film formed at the surface of the UHPC and, as a result, the main component appeared to be SiO2, which is believed to be a result of the rising of the SiO2-based filler, used as an admixture in this study, towards the surface, due to its low specific gravity.

The purpose of this study was to evaluate the flexural performance of recycled aggregate RC beam reinforced with aramid fiber sheets. Compressive strength of concrete using recycled aggregate is generally similar or slightly lower than normal concrete. To improve the compressive strength, aramid fiber sheets have been used in this study. This study examines the structural behavior of concrete beams prepared with recycled aggregate and strengthened aramid fiber sheets at varying locations. One concrete beam as a control specimen, that is prepared with 30 percent recycled aggregate and 70 percent natural aggregate, has been tested, and 3 more strengthened beams (bottom, bottom and sides, bottom and both ends with U-shaped strengthened beams) are tested. The ultimate loads have increased by 38.01%, 39.88%, and 100.79% for bottom, bottom and sides, bottom and both ends with U-shaped strengthened beams. The ductility ratios are 2.75~6.20 for strengthened beams. The experimental results showed that the strengthening system with U-shaped band controls the premature debonding and provides a more ductile failure mode than the strengthening system without U-shaped bands. It can be found that the ultimate strength of H40-RGA30-BS specimen based on load-deflection curves shows most promising result. The experimental results are compared with the analytical results of nonlinear flexural behaviors for strengthened reinforced recycled aggregate concrete beam.