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Prefabricated internal structures of road tunnels, consisting of precast elements and the connections between them, provide advantages in terms of quality control and manufacturing costs. However, the limited construction space in tunnels creates challenges for on-site assembly. To identify feasible connecting joints, flexural tests of precast straight beams connected by welding-spliced or lap-spliced reinforcements embedded in normal concrete or ultra-high-performance fiber-reinforced concrete (UHPFRC) are first performed and analyzed. With an improvement in the strength grade of the closure concrete for the lap-spliced joint, the failure of the beam transforms from a brittle splitting mode to a ductile flexural mode. The beam connected by UHPFRC100 with short lap-spliced reinforcements can achieve almost equivalent mechanical performance in terms of the bearing capacity, ductility, and stiffness as the beam connected by normal concrete with welding-spliced reinforcements. This favorable solution is then applied to the connection of neighboring updeck slabs resting on columns in a double-deck tunnel. The applicability is validated by flexural tests of T-shaped joints, which, fail in a ductile fashion dominated by the ultimate bearing capacity of the precast elements, similar to the corresponding straight beam. The utilization of UHPFRC significantly reduces the required lap-splice length of reinforcements owing to its strong bonding strength. [ABSTRACT FROM AUTHOR]

In this work, the effects of intra-layer hybridization on the flexural properties and failure mechanisms of composite laminates were experimentally and numerically investigated. Test specimens were prepared via the thermo-forming technique with plain woven fabrics reinforced using net carbon, glass, and their intra-layer hybrids. The force–displacement curves, failure modes, and flexural properties were analyzed and compared. The experimental results revealed that the catastrophic failure modes of carbon fiber-reinforced composite laminates can be improved after hybridization with glass fibers. However, the flexural properties of the composite laminates, such as flexural strength and modulus, depended on the type of fiber along the longitudinal (i.e., load-carrying) direction. A higher flexural strain can be obtained by replacing the longitudinal carbon fiber yarns with glass fiber ones. Subsequently, finite element models that considered the inter-layer and intra-layer deformation behaviors were established and validated with the tested results. The maximum error between the experimental and predicted flexural properties was 6.15%. The fracture process and stress distribution of the specimens were predicted and analyzed to understand the failure mechanisms. Notably, it was observed that intra-layer hybridization critically affected the transverse stress distribution state across the thickness of the composites, which resulted in complicated failure behaviors among adjacent fiber yarns. Finally, the effects of interfacial properties on the flexural responses were simulated and analyzed. [ABSTRACT FROM AUTHOR]

The impact of a thin layer of a ductile fiber-reinforced concrete referred to as engineered cementitious composites (ECC) on unreinforced masonry (URM) prisms and beams has been evaluated. The objective of the research was to characterize the performance and potential benefits of using ECC to retrofit URM with eventual application to masonry infill walls in non-ductile reinforced concrete frames. Compression tests of masonry prisms and flexural tests of masonry beams with different ECC retrofit schemes were conducted. The variables studied were the use of wall anchors to improve the ECC-masonry bond and alternate steel reinforcement ratios within the ECC layer in the form of welded wire fabric. The ECC retrofit was found to increase the strength and stiffness of URM prisms by 45 and 53 %, respectively compared to those of a plain specimen. When wall anchors were installed on the masonry specimens, the bond between the ECC layer and the masonry surface was improved. Four-point bending tests indicated that the strength and more importantly the ductility of an ECC retrofitted brick beam are increased significantly, especially when light reinforcement is added to the ECC layer, relative to an URM beam. Analytical models for estimating the strength and stiffness of ECC retrofitted masonry specimens are proposed and evaluated. [ABSTRACT FROM AUTHOR]

The effect of different amine/epoxy ratios on different properties of an epoxy resin was evaluated. Also, different amounts of carbon nanotubes (CNT) were added in order to analyse which of these two factors is more relevant. Dynamic thermomechanical analysis and flexural tests were carried out. The results obtained for the epoxy resin are in agreement with that reported by several researchers. The glass transition temperature of the resin and composites is maximal for the amine/epoxy stoichiometric ratio, while the highest glassy storage and flexural modulus correspond to the epoxy-rich systems, showing that the most crosslinked network is not the stiffest one. The effect of changing the stoichiometry is more relevant than adding CNT to the epoxy resin used in this work. However, the addition of CNT causes more remarkable changes in the epoxy-rich resin, promoting an increase of the glass transition temperature and the elastic modulus. [ABSTRACT FROM AUTHOR]

Polyester resin based composite materials are widely used in the manufacture of fiberglass boats. Production time of fiberglass laminate components could be strongly reduced by using an intense energy source as well as microwaves. In this work a polyester resin was used with 2% by weight of catalyst and reinforced with chopped or woven glass fabric. Pure resin and composite samples were cured by microwaves exposition for different radiation times. A three point bending test was performed on all the cured samples by using an universal testing machine and the resulting fracture surfaces were observed by means of scanning electron microscopy (SEM). The results of mechanical and microscopy analyses evidenced that microwave activation lowers curing time of the composite while good mechanical properties were retained. Microwaves exposition time is crucial for mechanical performance of the composite. It was evidenced that short exposition times suffice for resin activation while long exposure times cause fast cross linking and premature matrix fracture. Furthermore high-radiation times induce bubbles growth or defects nucleation within the sample, decreasing composite performance. On the basis of such results microwave curing activation of polyester resin based composites could be proposed as a valid alternative method for faster processing of laminated materials employed for large-scale applications. [ABSTRACT FROM AUTHOR]