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CONTENTS
Volume 27, Number 5, May 2021
 

Abstract
This study experimentally and analytically investigates the shear behavior of corroded reinforced concrete (RC) beams repaired using steel fiber-reinforced concrete (SFRC) in the flexural zone. The experimental parameters are the corrosion degree (0%, 12%, and 17%) and the steel fiber volume in the SFRC (1.0%, 1.5%, and 2.0%). The test results reveal that corrosion degree significantly affects the shear resistance of the beams. The shear capacity of the beam with the corrosion degree of 17% was higher than that of the uncorroded beam, whereas the shear capacity of the beam with the corrosion degree of 12% was lower than that of the uncorroded beam. The shear efficiency of damaged beams can be recovered by repairing them using SFRC that contains a reasonable amount of steel fibers. In addition, two methods to estimate the shear capacity of the repaired beams are developed using the modified truss analogy and strut-and-tie models. The estimated shear capacity of the beam using the modified truss analogy model agrees well with the experimental data.

Key Words
fiber-reinforced concrete; steel fibers; repair; corrosion; shear behavior

Address
Pitcha Jongvivatsakul: Innovative Construction Materials Research Unit, Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand
Phattarakan Laopaitoon: Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand
Yen T. H. Nguyen: Faculty of Civil Engineering, Industrial University of Ho Chi Minh City, 12 Nguyen Van Bao, Ward 4, Go Vap District, Ho Chi Minh City 700000, Vietnam
Phuoc T. Nguyen: Faculty of Civil Engineering, Ho Chi Minh City Open University, 97 Vo Van Tan, District 3, Ho Chi Minh City 700000, Vietnam
Linh V. H. Bui: Faculty of Civil Engineering, Ho Chi Minh City Open University, 97 Vo Van Tan, District 3, Ho Chi Minh City 700000, Vietnam

Abstract
The checking stresses in the Chinese codes for reinforced concrete (RC) or prestressed concrete (PC) bridges are aimed for the thin-web beam, which cannot reflect the actual behavior of the modern structures. The incompleteness of the checking stresses could give rise to the deficiency in the design and calculation, and unable to reveal the reason of some common cracks in the structure. In this paper, the complete stress checklist for RC or PC girder bridges are listed, as well as the corresponding crack shapes. The expression of the complete checking stresses is proposed in details. Spatial Grid Model can reflect all the concerned stresses in the structure. Through the comparison of the calculation results from the spatial grid model and the solid model, it is seen that the spatial grid model can reflect load effects such as shear lag effect, thin-wall effect and local effect. The stresses obtained from the spatial grid model could help engineers to have a good understanding of the structural behavior. Meanwhile, the stress checklist provides the information for analyzing and solving the deficiency in the structure.

Key Words
stress checklist; crack shape; spatial grid model; refined reinforcement design

Address
Ying-sheng Ni: Research Institute of Highway Ministry of Transport, M.O.T, Beijing, 100088, P.R. China
Ming Li: China-Road Transportation Verification & Inspection Hi-Tech Co., Ltd., Beijing, 100088, P.R. China
Dong Xu: Department of Bridge Engineering, Tongji Univ., Shanghai, 200092, P.R. China

Abstract
The optimal distribution of steel fibers over different layers of concrete can be considered as an appropriate method in improving the structural performance and reducing the cost of fiber-reinforced concrete members. In addition, the use of waste tire rubber in concrete mixes, as one of the practical ways to address environmental problems, is highly significant. Thus, this study aimed to evaluate the flexural behavior of functionally graded steel fiber-reinforced concrete containing recycled tire crumb rubber, as a volume replacement of sand, after exposure to elevated temperatures. Little information is available in the literature regarding this subject. To achieve this goal, a set of 54 one-, two-, and three-layer concrete beam specimens with different fiber volume fractions (0, 0.25, 0.5, 1, and 1.25%), but the same overall fiber content, and different volume percentages of the waste tire rubber (0, 5, and 10%) were exposed to different temperatures (23, 300, and 600oC). Afterward, the parameters affecting the post-heating flexural performance of concrete, including flexural strength and stiffness, toughness, fracture energy, and load-deflection diagrams, along with the compressive strength and weight loss of concrete specimens, were evaluated. The results indicated that the flexural strength and stiffness of the three-layer concrete beams respectively increased by 10 and 7%, compared to the one-layer beam specimens with the same fiber content. However, the flexural performance of the two-layer beams was reduced relative to those with one layer and equal fiber content. Besides, the flexural strength, toughness, fracture energy, and stiffness were reduced by approximately 10% when a 10% of natural sand was replaced with tire rubber in the threelayer specimens compared to the corresponding beams without crumb rubber. Although the flexural properties of concrete specimens increased with increasing the temperature up to 300oC, these properties degraded significantly with elevating the temperature up to 600oC, leading to a sharp increase in the deflection at peak load.

Key Words
elevated temperature; functionally graded fiber-reinforced concrete; flexural behavior; fracture energy; steel fiber; recycled tire rubber

Address
Mahdi Nematzadeh and Reza Mousavi: Department of Civil Engineering, University of Mazandaran, Babolsar, Iran

Abstract
Retrofitting of structures has gained importance over the recent years. Particularly, Reinforced Cement Concrete (RCC) column strengthening has become a challenge to the structural engineers, owing to the risks and complexities involved in it. There are several methods of RCC column strengthening viz. RCC jacketing, steel jacketing and Fiber Reinforced Polymer (FRP) wrapping etc., FRP wrapping is the most promising alternative when compared to the others. The large research database shows FRP wrapping, through lateral confinement, improves the axial load carrying capacity of the columns under concentric loading. However, its confining efficiency reduces under eccentric loading. Hence a relative newer technique called Near Surface Mounting (NSM), in which Carbon FRP (CFRP) strips are epoxy grouted to the precut grooves in the cover concrete of the columns, has been thrust domain of research. NSM technique strengthens the column nominally under concentric load case while significantly under eccentric case. A novel configuration of NSM in which the vertical NSM (VNSM) strips are being connected by horizontal NSM (HNSM) strips was numerically investigated under both concentric and eccentric loading. It was found that the configuration with 6 HNSM strips performed better under eccentric loading than under concentric loading, while the configuration with 3 HNSM strips performed better under concentric loading than under eccentric loading. Hence an optimum of 4 HNSM strips is recommended as strengthening measure for the given column specifications. It was also found that Aluminum alloy cannot be used instead of CFRP in NSM applications owing to its lower mechanical properties.

Key Words
retrofitting; FRP wrapping; near surface mounting; aluminum alloy

Address
M. Gurunandan: Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, 400076, Maharashtra, India
T. Raghavendra: Department of Civil Engineering, R V College of Engineering, Visvesvaraya Technological University, Bengaluru, 560059, Karnataka, India

Abstract
Research on damage detection methods in structures began a few decades ago with the introduction of methods based on structural vibration frequencies, which, of course, continues to this day. The value of important structures, on the one hand, and the countless maintenance costs on the other hand, have led researchers to always try to identify more accurate methods to diagnose damage to structures in the early stages. Among these, one of the most important and widely used methods in damage detection is the use of time-frequency representations. By using time-frequency representations, it is possible to process signals simultaneously in the time and frequency domains. In this research, the Short-Time Fourier transform, a known time-frequency function, has been used to process signals and identify the system. Besides, a new damage index has been introduced to identify damages in concrete piers of bridges. The proposed method has relatively simple calculations. To evaluate the method, the finite element model of an existing concrete bridge was created using as-built details. Based on the results, the method identifies the damages with high accuracy.

Key Words
damage detection; bridge piers; short time fourier transform; new damage index

Address
Hamid Reza Ahmadi: Department of Civil Engineering, Faculty of Engineering, University of Maragheh, Maragheh 55136-553, Iran
Navideh Mahdavi: Department of Civil Engineering, Marand Branch, Islamic Azad University, Marand, Iran
Mahmoud Bayat: Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC, USA

Abstract
Reinforced concrete (RC) buildings in Taiwan have suffered failure from strong earthquakes, which was magnified by the non-ductile detailing frames. Inadequate reinforcement as a consequence of the design philosophy prior to the introduction of current standards resulted in severe damage in the column and beam-column joint (BCJ). This study establishes a finite element analysis (FEA) of the non-ductile detailing RC column, BCJ, and three-story building that was previously tested through a tri-axial shaking table test. The results were then validated to laboratory specimens having the exact same dimensions and properties. FEA simulation integrates the concrete damage plasticity model and the elastic-perfectly plastic model for steel. The load-displacement responses of the column and BCJ specimens obtained from FEA were in a reasonable agreement with the experimental curves. The resulting initial stiffness and maximum base shear were found to be a close approximation to the experimental results. Also, the findings of a dynamic analysis of the three-story building showed that the time-history data of acceleration and displacement correlated well with the shaking table test results. This indicates the FEA implementation can be effectively used to predict the RC frame performance and failure mode under seismic loads.

Key Words
finite element analysis; concrete damage plasticity; non-ductile detailing; acceleration; displacement

Address
Banu A. Hidayat: Department of Civil Engineering, College of Engineering, National Cheng Kung University, No. 1 University Road, Tainan, 701, Taiwan R.O.C.; Department of Civil Engineering, Faculty of Engineering, Universitas Diponegoro, Jalan Prof. Soedarto, Tembalang, Semarang, 50275, Indonesia
Hsuan-Teh Hu: Department of Civil Engineering, College of Engineering, National Cheng Kung University, No. 1 University Road, Tainan, 701, Taiwan R.O.C.; Department of Civil and Disaster Prevention Engineering, College of Engineering and Science, National United University, No. 2, Lien Da, Nan Shih Li, Miaoli, 36063, Taiwan R.O.C.
Fu-Pei Hsiao: Department of Civil Engineering, College of Engineering, National Cheng Kung University, No. 1 University Road, Tainan, 701, Taiwan R.O.C.;
Ay Lie Han: National Center for Research on Earthquake Engineering, 200 Sec. 3, Xinhai Road, Taipei, 10668, Taiwan R.O.C.
Lisha Sosa: Department of Civil Engineering, Faculty of Engineering, Universitas Diponegoro, Jalan Prof. Soedarto, Tembalang, Semarang, 50275, Indonesia
Li-Yin Chan: Department of Civil Engineering, College of Engineering, National Cheng Kung University, No. 1 University Road, Tainan, 701, Taiwan R.O.C.
Yanuar Haryanto: Department of Civil Engineering, College of Engineering, National Cheng Kung University, No. 1 University Road, Tainan, 701, Taiwan R.O.C.; Department of Civil Engineering, Faculty of Engineering, Jenderal Soedirman University, Jalan Mayjen. Sungkono KM 5, Blater, Purbalingga, 53371, Indonesia

Abstract
The review of the literature and design guidelines indicates a lack of design codes governing the shear strength of reinforced concrete (RC) beams strengthened with ultrahigh-performance fiber-reinforced concrete (UHPFRC). This study uses the results of a 3D finite element model constructed previously by the authors and verified against an experimental programme to gain a clear understanding of the shear strength of RC beams strengthened with UHPFRC by using different schemes. Experimental results found in the literature along with the numerical results for shear capacities of normal-strength RC and UHPFRC beams without stirrups are compared with available code design guidelines and empirical models found in the literature. The results show variance between the empirical models and the experimental results. Accordingly, proposed equations derived based on empirical models found in the literature were set to estimate the shear capacity of normal-strength RC beams without stirrups. In addition, the term 'shear span-to-depth ratio' is not considered in the equations for design guidelines found in the literature regarding the shear capacity of UHPFRC beams without stirrups. Consequently, a formula estimating the shear strength of UHPFRC and RC beams strengthened with UHPFRC plates and considering the effect of shear span-to-depth ratio is proposed and validated against an experimental programme previously conducted by the authors.

Key Words
RC beams; UHPFRC beams; shear; strengthening; empirical models; shear span-to-depth ratio; reinforcement ratio; concrete compressive strength

Address
Walid Mansour: Department of Civil Engineering, Kafrelsheikh University, Kafrelsheikh, Egypt
Mohammed Sakr: Department of Structural Engineering, Tanta University, Tanta, Egypt
Ayman Seleemah: Department of Structural Engineering, Tanta University, Tanta, Egypt
Bassam A. Tayeh: Department of Civil Engineering, Faculty of Engineering, Islamic University of Gaza, Palestine
Tarek Khalifa: Department of Structural Engineering, Tanta University, Tanta, Egypt

Abstract
In this study, two powerful techniques, namely particle swarm optimization (PSO) and imperialist competitive algorithm (ICA) were selected and combined with a pre-developed ANN model aiming at improving its performance prediction of the compressive strength of concrete modified with fly ash. To achieve this study's aims, a comprehensive database with 379 data samples was collected from the available literature. The output of the database is the compressive strength (CS) of concrete samples, which are influenced by 9 parameters as model inputs, namely those related to mix composition. The modeling steps related to ICA-ANN (or neuro-imperialism) and PSO-ANN (or neuro-swarm) were conducted through the use of several parametric studies to design the most influential parameters on these hybrid models. A comparison of the CS values predicted by hybrid intelligence techniques with the experimental CS values confirmed that the neuro-swarm model could provide a higher degree of accuracy than another proposed hybrid model (i.e., neuro-imperialism). The train and test correlation coefficient values of (0.9042 and 0.9137) and (0.8383 and 0.8777) for neuro-swarm and neuro-imperialism models, respectively revealed that although both techniques are capable enough in prediction tasks, the developed neuro-swarm model can be considered as a better alternative technique in mapping the concrete strength behavior.

Key Words
artificial neural networks; fly ash; compressive strength; statistical analysis; intelligent computing

Address
Ahmed Mohammed: College of Engineering, Civil Engineering Department, University of Sulaimani, Kurdistan Region, Iraq
Rawaz Kurda: Department of Highway Engineering Techniques, Technical Engineering College, Erbil Polytechnic University, Erbil, Kurdistan-Region, Iraq; CERIS, Civil Engineering, Architecture, and Georresources Department, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001, Lisbon, Portugal
Danial Jahed Armaghani: Department of Urban Planning, Engineering Networks and Systems, Institute of Architecture and Construction, South Ural State University, 76, Lenin Prospect, Chelyabinsk 454080, Russia
Mahdi Hasanipanah: Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam

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