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CONTENTS
Volume 13, Number 3, March 2014
 

Abstract
Based on the compositions and structures of cement-based materials, the geometrical models of the tortuosity of transport paths in hardened cement pastes, mortar and concrete, which are associated with the capillary porosity, cement hydration degree, mixture particle shape, aggregate volume fraction and water-cement ratio, are established by using a geometric approach. Numerical simulations are carried out to investigate the effects of material parameters such as water-cement ratio, volume fraction of the mixtures, shape and size of aggregates and cement hydration degree, on the tortuosity of transport paths in hardened cement pastes, mortar and concrete. Results indicate that the transport tortuosity in cement-based materials decreases with the increasing of water-cement ratio, and increases with the cement hydration degree, the volume fraction of cement and aggregate, the shape factor and diameter of aggregates, and the material parameters related to cement pastes, such as the water-cement ratio, cement hydration degree and cement volume fraction, are the primary factors that influence the transport tortuosity of cement-based materials.

Key Words
tortuosity; cement-based materials; model; geometric approach; numerical simulation

Address
Xiao-Bao Zuo and Yu-Juan Tang: Department of Civil Engineering, Nanjing University Of Science & Technology, Nanjing, 210094, P.R.China
Wei Sun: Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing, 211189, P.R.China
Zhi-Yong Liu: College of Civil Engineering, Yantai University, Yantai, 264005, P.R.China

Abstract
The use of lightweight aggregate (LWA) instead of ordinary aggregate may make lightweight aggregate concrete, which possesses many advantages such as lightweight, lower thermal conductivity, and better fire and seismic resistance. Recently the developments of LWA have been focused on using industrial wastes as raw materials to reduce the use of limited natural resources. In view of this, the intent of this study was to apply Taguchi optimization technique in determining process condition for producing synthetic LWA by incorporating waste thin film transition liquid crystal displays (TFT-LCD) glass powder with reservoir sediments. In the study the waste TFT-LCD glass cullet was used as an additive. It was incorporated with reservoir sediments to produce LWA. Taguchi method with an orthogonal array L16(45) and five controllable 4-level factors (i.e., cullet content, preheat temperature, preheat time, sintering temperature, and sintering time) was adopted. Then, in order to optimize the selected parameters, the analysis of variance method was used to explore the effects of the experimental factors on the performances (particle density, water absorption, bloating ratio, and loss of ignition) of the produced LWA. The results showed that it is possible to produce high performance LWA by incorporating waste TFT-LCD glass cullet with reservoir sediments. Moreover, Taguchi method is a promising approach for optimizing process condition of synthetic LWA using recycled glass cullet and reservoir sediments and it significantly reduces the number of tests.

Key Words
lightweight aggregate; cullet; taguchi experimental design method

Address
Chao-Wei Tang: Department of Civil Engineering and Geomatics, Cheng Shiu University, 840 Chengcing Road, Niaosong District, Kaohsiung City 833, Taiwan

Abstract
In this work a numerical method to simulate the response of reinforced concrete structures subject to cyclically imposed displacements is proposed. The method consists of a combination of a displacement and load controlled version of the Newton–Raphson iterative technique, used for the loading and the unloading part of the cycles respectively. The whole procedure is combined with a relatively simple concrete model whose only material parameter is its uniaxial compressive strength. The proposed methodology may realistically simulate, in an easy way, the physical process of any experimentally tested RC structure under imposed displacements cycles. The efficiency of the approach is demonstrated through a series of analyses of experimentally tested specimens reported in the literature.

Key Words
reinforced concrete, nonlinear analysis, cyclic loading, smeared crack model, 3D solid finite element

Address
Georgios Ch. Lykidis and Konstantinos V. Spiliopoulos: Institute of Structural Analysis and Antiseismic Research, National Technical University of Athens,
Department of Civil Engineering, Zografou Campus, Zografou 157–73, Athens, Greece

Abstract
The moisture transport in underground concrete was experimentally investigated and the nonlinear model of moisture transport considering the effects of water diffusion, hydration of cementicious materials and water permeability was proposed. The consumed moisture content by self-desiccation could be firstly calculated according to evolved hydration degree of cement and mineral admixtures. Furthermore, the finite differential method was adopted to solve the moisture transport model by linearizing the nonlinear moisture diffusion coefficient. The comparison between experimental and calculated results showed a good agreement, which indicated that the proposed moisture model could be used to predict moisture content evolution in underground concrete members with drying–wetting boundaries.

Key Words
nonlinear moisture model; underground concrete; hydration degree evolution

Address
M.F. Ba: Faculty of Architectural Civil Engineering and Environment, Ningbo University, Ningbo, 315211, China
C.X. Qian: School of Materials Science and Engineering, Southeast University, Nanjing, 211189, China
G.B.Gao: Shandong Provincial Academy of Building Research, Jinan 250031, China


Abstract
This paper presents an experimental study on socket base connections of precast reinforced concrete columns. The main purpose of this study is to determine socket base connection which has the closest behavior to monolithic casted column-base joints. For this purpose, six specimens having different column-socket base connection details were tested under cyclic loading. For each test, strength, stiffness, ductility and drift ratios of the specimens were determined. Test results indicated that a suggested connection type is 10% - 30% stronger than the other type of connections under lateral loading. The welded connection (PC-5) had better lateral load carrying capacity and ductility. On the other hand, performance of standard connection (PC-1) which is commonly used in construction was weaker than other connections. Thus, decision of connection type should be referred not only performance but also applicability.

Key Words
precast columns; socket base connection; cyclic loading; energy dissipations; stiffness degradation; displacement ductility

Address
Selim Pul, Metin Hüsem, Mehmet Emin Arslan and Sertaç Hamzaçebi: Karadeniz Technical University, Department of Civil Engineering, 61080, Trabzon, Turkey

Abstract
Precast hollow core slabs (HCS) are technically advanced products in the precast concrete industry, widely used in the last years due to their versatility, their multipurpose potential and their low cost. Using three dimensional FEM (Finite Element Method) elements, this study focuses on the stresses induced by the prestressing of steel. In particular the investigation of the spalling crack formation that takes place during prestressing is carried out, since it is important to assure the appropriate necessary margins concerning such stresses. In fact, spalling cracks may spread rapidly towards the web, leading to the detachment of the lower part of the slab. A parametric study takes place, capable of evaluating the influence of the tendon position and of the web width on the spalling stress. Consequently, after an extensive literature review on the topic of soft computing, an optimization of the HCS is performed by means of Genetic Algorithms coupled with 3-D FEM models.

Key Words
hollow core slabs; genetic algorithms; finite element method; optimization; soft computing; spalling; rotating crack model; prestressed reinforced concrete structures

Address
Luca Sgambi: Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy
Konstantinos Gkoumas and Franco Bontempi: Department of Structural and Geotechnical Engineering, Sapienza University of Rome, Via Eudossiana 18, 00184 Rome, Italy

Abstract
A central pullout test was conducted to investigate the bonding properties between high strength rebar and reactive powder concrete (RPC), which covered ultimate pullout load, ultimate bonding stress, free end initial slip, free end slip at peak load, and load-slip curve characteristics. The effects of varying rebar buried length, thickness of protective layer and diameter of rebars on the bonding properties were studied, and how to determine the minimum thickness of protective layer and critical anchorage length was suggested according the test results. The results prove that: 1) Ultimate pull out load and free end initial slip load increases with increase in buried length, while ultimate bonding stress and slip corresponding to the peak load reduces. When buried length is increased from 3d to 4d(d is the diameter of rebar), after peak load, the load-slip curve descending segment declines faster, but later the load rises again exceeding the first peak load. When buried length reaches 5d, rebar pull fracture occurs. 2) As thickness of protective layer increases, the ultimate pull out load, ultimate bond stress, free end initial slip load and the slip corresponding to the peak load increase, and the descending section of the curve becomes gentle. The recommended minimum thickness of protective layer for plate type members should be the greater value between d and 10 mm, and for beams or columns the greater value between d and 15 mm. 3) Increasing the diameter of HRB500 rebars leads to a gentle slope in the descending segment of the pullout curve. 4) The bonding properties between high strength steel HRB500 and RPC is very good. The suggested buried length for test determining bonding strength between high strength rebars and RPC is 4d and a formula to calculate the critical anchorage length is established. The relationships between ultimate bonding stress and thickness of protective layer or the buried length was obtained.

Key Words
HRB500 rebar; reactive powder concrete; bonding; critical anchorage length

Address
Deng Zong-cai, Jumbe R.Daud and Yuan Chang-xing: The Key Laboratory of Urban Security and Disaster Engineering, Ministry of Education, Beijing University of Technology, Beijing 100124 China


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