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Yin H.-Y.,Sichuan University | Wei X.-F.,Sichuan University | Bao R.-Y.,Sichuan University | Dong Q.-X.,Beijing Building Construction Research Institute | And 4 more authors.

Even though self-nucleation is considered to be the ideal case for polymer nucleation, it has rarely been used to enhance polymer crystallization in practical processing techniques. Inspired by self-nucleation theory and by utilizing the large difference in melting points of various poly(l-lactide) (PLLA) resins, we introduced high-melting-point PLLA (hPLLA) crystallites into a low-melting-point PLLA (lPLLA) matrix via melt blending at a processing temperature between the two melting points of the selected PLLA resins. The hPLLA crystallites turn out to be efficient nucleating agents (NAs) for lPLLA and high crystallinity (>40%) PLLA samples with a greatly accelerated crystallization rate can be easily obtained. The results of non-isothermal crystallization show that the crystallization process is remarkably accelerated with a small amount (0.1 wt%) of hPLLA. With a further increase of hPLLA content, the crystallization temperature of the blends continues to shift to higher temperature. This crystallization promoting effect results from the excellent nucleation ability of the hPLLA crystallites, as revealed by in situ optical microscopy observation. Furthermore, an incredibly high nucleation efficiency of 103.0% (exceeding 100%) was obtained for the PLLA sample with 5.0 wt% hPLLA. The nucleation mechanism for hPLLA was studied systematically. It was found that the lPLLA matrix and hPLLA crystallites possess an absolutely identical crystal structure of α-form crystals and an excellent interfacial interaction between the nucleating agent, i.e., hPLLA crystallite, and the lPLLA matrix is achieved, resulting in the reduction of the energy barrier for heterogeneous nucleation and acceleration of crystallization kinetics. Therefore, by using high-melting-point polymer crystals as NAs, the crystallization rate of their low-melting-point polymer matrix can be improved largely, thus providing a simple way to obtain high crystallinity products for semicrystalline polymers with very low crystallization rates. This journal is © The Royal Society of Chemistry 2015. Source

Liu H.,Beijing Building Construction Research Institute | Lan C.,Beijing Building Construction Research Institute | Lan C.,Beijing Jiaotong University | Yang X.,Beijing Building Construction Research Institute
Jianzhu Jiegou Xuebao/Journal of Building Structures

The seismic retrofitting method of clay brick masonry structure using vertical post-tensioning tendons is developed to improve the poor seismic performance of the structure. By arranging the post-tensioning tendons uniformly along the wall and applying vertical prestressing, the brick wall is under uniform compression, thus the failure pattern of the masonry walls is improved and the shear capacity and the energy dissipating ability are also improved. The effectiveness of this method was investigated experimentally through quasi-static test of nine unreinforced masonry walls, including five walls without opening and four walls with opening. The failure pattern and the mechanical behavior of the unretrofitted walls and the retrofitted walls were analyzed and compared. The results show that post-tensioning can increase the shear capacity and the deformability substantially, and the seismic energy dissipating ability is also significantly increased. And the improvement of the shear capacity and the ductility of the walls without opening is more significant than that of the walls with opening.However, the axial force due to the addition of prestressing force and vertical load shouldn't exceed a certain limit, otherwise the ductility will decrease in some degree. Moreover, the lateral stiffness degradation speed of the wall retrofitted with post-tensioning tendons can be slowed down and after cracking stiffness can be improved, which may be very helpful to the mechanical behavior of the wall under a severe earthquake. In addition, the elastic stiffness and the weight of the brick wall are hardly changed due to the retrofitting method, which shows that the retrofitting method using vertical post-tensioning tendons is an effective and reliable strengthening alternative for unreinforced masonry structures. ©, 2015, Science Press. All right reserved. Source

Lan C.,Beijing Building Construction Research Institute | Lan C.,Beijing Jiaotong University | Liu H.,Beijing Building Construction Research Institute | Zhou Z.,Dalian University of Technology
Tumu Gongcheng Xuebao/China Civil Engineering Journal

Prestress loss is critical to the safety of prestressed structures. Unfortunately, nowadays, there are no qualified techniques to treat with this issue due to the fact that monitoring sensors may not survive the harsh construction environments and the time-dependent service life of the large-span prestressed structures. In this study, a novel smart steel strand based on the technique of integrating the Brillouin with fiber Bragg gratings (FBG) signal was introduced. Two kinds of prestressed PC beams were used to verify the idea of monitoring prestress loss by using smart steel strands. The prestress loss data may be achieved by Brillouin and FBG sensors, respectively, and then the data were compared with those from conventional sensors. It was shown that the monitored data can reveal the entire process of prestress loss in the construction stage and service life, and the data agree well with those from the structural analysis and provide higher accuracy than the data from conventional sensors, indicating that such kind of smart steel strands may be conveniently used to monitor the prestressed reinforcement in prestressed concrete structures. Source

Shi G.,Tsinghua University | Fan H.,CITIC General Institute of Architectural Design and Research Co. | Bai Y.,Monash University | Zheng J.,Beijing Building Construction Research Institute
Proceedings of the Institution of Civil Engineers: Structures and Buildings

Damage such as cable pre-stress loss and anchorage failure may occur in a glass curtain wall structure supported by single-layer plane cable net but the influence on overall structural responses has not yet been identified. A finiteelement model considering the stiffness contribution from glass panels is established to study the effects of two forms of damage: pre-stress loss and anchorage failure. The accuracy and reliability of the numerical model are validated experimentally, and the effects of stiffness of the glass panel on stress distributions are discussed. Compared to experimental results, the finite-element model has sufficient accuracy to quantify the damage effects. The contribution from bending stiffness of glass panels to overall structural stiffness is identified as about 10.6% and that from the glass panel membrane effect is about 12.2%. The modelling approach is extended to describe a realistic 15 × 3 15 m glass curtain wall structure. The corresponding parametric analysis demonstrates that the stiffness contribution from glass panels depends on certain design parameters (e.g. number of glass grids and overall span), but can be ignored for structures with more than ten grids. In the investigated structures with 6~15 grids, almost no difference was found between the effects of damage on the glass curtain wall structure and on the pure supporting cable net structure. © ICE Publishing: All rights reserved. Source

Shi G.,Tsinghua University | Zuo Y.,Beijing Building Construction Research Institute | Zheng J.,Beijing Building Construction Research Institute | Huo D.,Beijing University of Technology
Jianzhu Jiegou Xuebao/Journal of Building Structures

During the service of single-layer plane cable net supported glass buildings, as the supporting structure the single-layer plane cable net will inevitably be subjected to different types of damages. In order to study the influence of such damages on the static behavior of the single-layer plane cable net, a full-scale model with the dimension of 4.85 m × 4.85 m and 4 × 4 grids was designed and constructed. Static tests were conducted after introducing three types of damages including failure of connectors between horizontal and vertical cables, cable prestress loss and cable anchorage failure. The results indicate that, failure of connectors between horizontal and vertical cables almost has no influence on the static behavior of the single-layer plane cable net; the change of nodal displacements induced by the cable prestress loss does not exceed 15%; the cable anchorage failure affects the local part of the single-layer plane cable net significantly, and the induced change of nodal displacements is between 18% and 36%; damages have stronger effect on nodal displacements than cable tension forces. The intensity of the influence of damages depends on the damage degree and location, and decreases with the increase of load. The research results provide a sound foundation for establishing the safety evaluating method of in-service single-layer plane cable net structures. Source

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