Topic 1: Concrete-Reinforcement Interactions: Constitutive Modelling and Finite Element Implementation
Topic 2: Packing and Film Thickness Theories for Production of High-Performance Concrete
时间：2016年4月7日 9:00 a.m.
地点： 安中大楼B328
Speaker: Ir Dr P.L. NG (吴沛林)
Biography: P.L. Ng (BEng(CivE), PhD, MBA, BEAM Pro, FAIIB, FHKCI, FHKIUS, MHKIE, MICE, APEC Engineer, CEng, Eur Ing, IntPE, RPE) is a visiting adjunct professor at Vilnius Gediminas Technical University in Lithuania. He obtained his Bachelor of Civil Engineering and Doctor of Philosophy degrees at the University of Hong Kong. After graduation, he has been involved in the design and construction of a number of major infrastructure projects in Hong Kong and he is chartered civil engineer in Hong Kong, United Kingdom, and Europe. He is an Executive Board member and an honorary secretary of the Hong Kong Concrete Institute. He has more than 15 years of experience in fundamental and applied research, and his research interests are in the areas of sustainable concrete materials and concrete structures. He is serving in the editorial board of 3 international journals, and he has published more than 75 technical papers. He was the recipient of Certificate of Merit of The HKIE Innovation Awards for Young Members in 2013 and The HKIE Outstanding Paper Award for Young Engineers/Researchers in 2012.
Abstract 1: In reinforced concrete structures, the steel bars embedded into concrete primarily resist axial tension or compression. In addition, the steel bars also interact with concrete in manifold ways. These interactions include the bond action between concrete and reinforcement which stiffens the concrete through the bond force transferred; dowel action of reinforcing bars intercepting cracks which restrains shear sliding of concrete across the crack surfaces; and confinement effect by confining reinforcement which enhances the load-carrying capacity of concrete in compression. The constitutive modelling of these concrete-reinforcement interactions and the finite element implementation in conjunction with the discrete representation of reinforcing bars are presented. The Goodman interface element, which originated from the modelling of joints in rock mass, is adapted for modelling concrete-to-reinforcement bond with the implementation of nonlinear bond stress-slip relation. The dowel action for discrete reinforcing bars is modelled with the dowel stiffness assembled into concrete elements in accordance with the energy principle. The dowel force- displacement relation is idealised in linearly elastic-perfectly plastic form, in which the elastic portion is derived from the theory of beam on elastic foundation. The confinement effect is accounted for by means of modified stress-strain relationship and adjusted multi-axial strength envelope of concrete. The serviceability and ultimate responses of reinforced concrete beams are analysed to illustrate the applicability and accuracy of the numerical techniques.
Abstract 2: High-performance concrete (HPC) are generally required to have all round high performance in various attributes such as strength, workability, durability, dimensional stability, cohesiveness and passing ability. The mix design of HPC is often complicated for the following reasons. Firstly, the numbers of ingredients in HPC are usually more than those in conventional concrete. Secondly, the requirements of HPC may be conflicting in the sense that improvement in one property may at the same time cause impairment of another property. Thirdly, there is a lack of understanding regarding how the various mix parameters could be optimised for best performance of concrete. Most concrete producers are conducting mix design primarily by trial concrete mixing, which is laborious, ineffective and often too slow in responding to sudden changes in the properties of ingredients. To address the above issues, research on packing theory and particuology of concrete materials has been conducted, in order to revamp the mix design philosophy of HPC to base on the water film thickness (WFT), paste film thickness (PFT), and mortar film thickness (MFT). From the research findings, it is proven that the major factors affecting the performance of concrete are the WFT, PFT and MFT. The development of a three-tier concrete mix design method for production of HPC is advocated. It features the design of concrete mix in three tiers, first the paste, then the mortar and finally the concrete, by selecting suitable values of WFT, PFT and MFT.
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