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CONTENTS
Volume 8, Number 4, August 2011
 

Abstract
In this paper, time-continuous constitutive equations for strain rate-dependent materials are presented first, among which those for the overstress and the consistency viscoplastic models are considered. By allowing the stress states to be outside the yield surface, the overstress viscoplastic model directly defines the flow rule for viscoplastic strain rate. In comparison, a rate-dependent yield surface is defined in the consistency viscoplastic model, so that the standard Kuhn-Tucker loading/unloading condition still remains true for rate-dependent plasticity. Based on the formulation of the consistency viscoplasticity, a computational elasto-viscoplastic constitutive model is proposed for the short fiber-reinforced fresh cementitious paste for extrusion purpose. The proposed constitutive model adopts the von-Mises yield criterion, the associated flow rule and nonlinear strain rate-hardening law. It is found that the predicted flow stresses of the extrudable fresh cementitious paste agree well with experimental results. The rate-form constitutive equations are then integrated into an incremental formulation, which is implemented into a numerical framework based on ANSYS/LS-DYNA finite element code. Then, a series of upsetting and ram extrusion processes are simulated. It is found that the predicted forming load-time data are in good agreement with experimental results, suggesting that the proposed constitutive model could describe the elasto-viscoplastic behavior of the short fiber-reinforced extrudable fresh cementitious paste.

Key Words
viscoplastic; cement paste; extrusion; rheology; flow stress; constitutive modeling.

Address
Xiangming Zhou:School of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom
Zongjin Li: Department of Civil and Environmental Engineering, The Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong

Abstract
The increasing concern for reinforced concrete structure durability has been justifying in many ways in the last few decades. However, there is no perfect durability test method till now. In this research an alternative method, which is based on the cumulative moisture loss percent of the concrete specimens after oven-drying, was proposed to estimate the durability performance of the concrete. Two temperatures were considered for the oven-drying tests: 100

Key Words
concrete; durability; drying.

Address
How-Ji Chen: Department of Civil Engineering, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C.
Chao-Wei Tang: Department of Civil Engineering & Engineering Informatics, Cheng-Shiu University,
No. 840, Chengcing Road, Niaosong District, Kaohsiung City, Taiwan, R.O.C.
Hsien-Sheng Peng: Department of Civil Engineering, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C.

Abstract
In this paper, a mesoscale model of concrete is presented, which considers particles, matrix material and the interfacial transition zone (ITZ) as separate constituents. Particles are represented as ellipsoides, generated according to a prescribed grading curve and placed randomly into the specimen. In this context, an efficient separation procedure is used. The nonlinear behavior is simulated with a cohesive interface model for the ITZ and a combined damage/plasticity model for the matrix material. The mesoscale model is used to simulate a compression and a tensile test. Furthermore, the influence of the particle distribution on the loaddisplacement curve is investigated.

Key Words
mesoscale; concrete.

Address
Jorg F. Unger, Stefan Eckardt and Carsten Konke: Institute of Structural Mechanics, Bauhaus University Weimar,
Marienstr. 15, D-99423 Weimar, Germany

Abstract
Most previous studies have generally overlooked the contribution of thermal stresses generated within the concrete mass when subjected to microwave heating and reported on pore-pressure as being the dominant cause of surface spalling. Also, the variation in electromagnetic properties of concrete and its effects on the microwave heating process have not been studied in detail. In this paper, finite element modeling is used to examine the simultaneous development of compressive thermal stresses and porepressure arising from the microwave heating of concrete. A modified Lambert

Key Words
concrete; microwaves; contamination; heating; thermal stress; pore pressure.

Address
A. Akbarnezhad and K.C.G. Ong: Department of Civil Engineering, National University of Singapore, # 04-03E,
Block E2, 5 Engineering drive 2, Singapore 117576

Abstract
In the design of reinforced concrete (RC) beams, apart from providing adequate strength, it is also necessary to provide a minimum deformability even for beams not located in seismic regions. In most RC design codes, this is achieved by restricting the maximum tension steel ratio or neutral axis depth. However, this empirical deemed-to-satisfy method, which was developed based on beams made of normal-strength concrete (NSC) and normal-strength steel (NSS), would not provide a consistent deformability to beams made of high-strength concrete (HSC) and/or high-strength steel (HSS). More critically, HSC beams would have much lower deformability than that provided previously to NSC beams. To ensure that a consistent deformability is provided to all RC beams, it is proposed herein to set an absolute minimum rotation capacity to all RC beams in the design. Based on this requirement, the respective maximum limits of tension steel ratio and neutral axis depth for different concrete and steel yield strengths are derived based on a formula developed by the authors. Finally for incorporation into design codes, simplified guidelines for designing RC beams having the proposed minimum deformability are developed.

Key Words
beams; curvature; deformability; design aids; high-strength concrete; high-strength steel; reinforced concrete; rotation capacity.

Address
J.C.M. Ho and K.J.H. Zhou: Department of Civil Engineering, The University of Hong Kong, Hong Kong

Abstract
This paper investigates bond behavior of structural lightweight concrete (SLWC) and ordinary concrete (OC) comparatively using bending test called Standard Belgium Hinged Beam Test (SBHBT). For this purpose the experiments were carried out as three series on 36 beam specimens (12 specimens of SLWC and OC with 20

Key Words
bond strength; bending; structural lightweight concrete; ordinary concrete.

Address
Mehmet Emin Arslan and Ahmet Durmu: Department of Civil Engineering, Karadeniz Technical University, Trabzon 61080, Turkey

Abstract
In designing a flexural member for structural safety, both the flexural strength and ductility have to be considered. For this purpose, the flexural ductility of reinforced concrete sections has been studied quite extensively. As there have been relatively few studies on the flexural ductility of prestressed concrete sections, it is not well understood how various structural parameters affect the flexural ductility. In the present study, the full-range flexural responses of reinforced and prestressed concrete sections are analyzed taking into account the nonlinearity and stress-path dependence of constitutive materials. From the numerical results, the effects of steel content, yield strength and degree of prestressing on the yield curvature and ultimate curvature are evaluated. It is found that whilst the concept of flexural ductility in terms of the ductility factor works well for reinforced sections, it can be misleading when applied to prestressed concrete sections. For prestressed concrete sections, the concept of flexural deformability in terms of ultimate curvature times overall depth of section may be more appropriate.

Key Words
deformability; flexural ductility; prestressed concrete; reinforced concrete.

Address
Francis T.K. Au, Cliff C.Y. Leung and Albert K.H. Kwan:
Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong


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