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CONTENTS
Volume 34, Number 4, October 2024
 

Abstract
The ultra-high performance concrete (UHPC) mixed with hybrid fibers has excellent mechanical properties and durability, and the hybrid fibers have a certain impact on the bearing capacity, deformation capacity, and crack propagation of beams. Many scholars have conducted a series of studies on the bending performance of prestressed UHPC beams, but there are few studies on prestressed UHPC beams mixed with hybrid fibers. In this study, five bonded post-tensioned partially prestressed UHPC beams mixed with steel fibers and macro-polyolefin fibers were poured and subjected to four-points symmetric loading bending tests. The effects of different prestressing degrees and prestressing levels on the load-deflection curves, crack propagation, failure modes and ultimate bearing capacity of beams were discussed. The results showed that flexural failure occurred in the prestressed UHPC beams with hybrid fibers, and the integrity of specimens was good. When the prestressing degree was the same, the higher the prestressing level, the better the crack resistance capacity of UHPC beams; When the prestressing level was 90%, increasing the prestressing degree was beneficial to improve the crack resistance and ultimate bearing capacity of UHPC beams. When the prestressing degree increased from 0.41 to 0.59, the cracking load and ultimate load increased by 66.0% and 41.4%, respectively, but the ductility decreased by 61.2%. Based on the plane section assumption and considering the bridging effect of short fibers, the cracking moment and ultimate bearing moment were calculated, with good agreement between the test and calculated values.

Key Words
flexural behavior; hybrid fibers; prestressing degrees; prestressing levels; UHPC beams

Address
Zongcai Deng and Qian Li: The Key Laboratory of Urban Security and Disaster Engineering, Ministry of Education, Beijing University of Technology, 100 Pingyuan Park, Chaoyang District, Beijing 100124, China
Rabin Tuladhar: College of Science & Engineering, James Cook University, 383 Flinders Street, Townsville, QLD 4814, Australia
Feng Shi: Ningbo Shike New Material Technology Co. Ltd, 518 Xinmei Road, Ningbo 315000, China

Abstract
In recent decades the strengthening of reinforced concrete (RC) structural elements using Fiber-reinforced polymer (FRP) has received much attention. The behavior of RC elements can vary from axial compression to pure bending, depending on their loading. When the compressive behavior is dominant, the FRP jacket application is common, but when the flexural behavior is prevalent, the codes consider the FRP jacket ineffective. Codes suggest applying FRP bars or strips as Near-surface Mounted (NSM) or Externally Bonded (EB) in the tensile face to strengthen the beams under flexure. To strengthen the columns in tension-control mode, some researchers have suggested NSM FRP bars in both tension and compression faces alone or with the FRP jacket (hybrid). However, the number of tests that evaluate the pure bending of the strengthened columns as one of the pivotal points of the axial force-moment interaction curve is limited. In this paper, 11 RC elements strengthened using the NSM (in both tension and compression faces) or hybrid method were subjected to bending to assess the effect of the amount and material type of the FRP bar and jacket and the dimensions of the groove. The test results revealed that the NSM method increased the flexural capacity of the members between 10% to 50%. Furthermore, using the hybrid method increased the capacity between 51% to 91%. Finally, an analytical model was presented considering the effect of the NSM FRP bond in different circumstances, and its results were in good agreement with the experimental results.

Key Words
fiber-reinforced polymer (FRP); near-surface-mounted (NSM) FRP; reinforced concrete (RC) member; strengthening

Address
Mohsen A. Shayanfar and Solmaz Afzali: School of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran
Mohammad Ghanooni-Bagha: Department of Civil Engineering, East Tehran Branch, Islamic Azad University, Tehran, P.O. Box 18735-136, Iran

Abstract
This paper utilizes LS-DYNA software to numerically investigate impact response and damage evaluation of fiber reinforced polymer (FRP) bars-reinforced ultra-high-performance concrete (UHPC) composite beams (FRP-UHPC beams). Three-dimensional finite element (FE) models are established and calibrated by using literature-based static and impact tests, demonstrating high accuracy in simulating FRP-UHPC beams under impact loading. Parametric analyses explore the effects of impact mass, impactor height, FRP bar type and diameter, and clear span length on dynamic response and damage modes. Two failure modes emerge: tensile failure with bottom longitudinal reinforcement fracture and compression failure with local concrete compression near the impact region. Impact mass or height variation under the same impact energy significantly affects the first peak impact force, but minimally influences peak midspan displacement with a difference of no more than 5% and damage patterns. Increasing static flexural load-carrying capacity enhances FRP-UHPC beam impact resistance, reducing displacement deformation by up to 30%. Despite similar static load-carrying capacities, different FRP bars result in varied impact resistance. The paper proposes a damage assessment index based on impact energy, static load-carrying capacity, and clear span length, correlating well with beam end rotation. Their linearly-fitting coefficient was 1.285, 1.512, and 1.709 for the cases with CFRP, GFRP, and BFRP bars, respectively. This index establishes a foundation for an impact-resistant design method, including a simplified formula for peak midspan displacement assessment.

Key Words
beam; damage assessment; failure mode; finite element analysis; FRP; impact loading; UHPC

Address
Tao Liu, Qi M. Zhu, Rong Ge and Lin Chen: School of Civil Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Seongwon Hong: Department of Safety Engineering, Korea National University of Transportation, Chungbuk 27469, Republic of Korea

Abstract
The I-shaped steel reinforced recycled aggregate concrete (SRRC) composite structure has the advantages of high bearing capacity and environmental protection, and the interfacial bond strength is an important theory. To this end, the I-shaped SRRC bond strength and its calculation based on artificial neural network (ANN) will be studied. Firstly, 39 push out tests of I-shaped SRRC were conducted, the load-slip curve has obvious regularity, which is divided into 4 segments by 3 regular points. Three bond strengths were defined based on these three rule points, and the approximate ranges of their values and the laws of influence of each factor on them were found. Secondly, the Elman ANN model used for the prediction of bond strength was established, and the parameters of Elman ANN predicting I-shaped SRRC bond strength were studied, and the effects of detailed parameters on the prediction results were revealed. Finally, the bond strength of SRRC was predicted using Elman and BP (back propagation) neural network models, both of which showed good prediction results. This study is a theoretical basis for the design and fine simulation of I-shaped SRRC composite structures.

Key Words
artificial neural network; bond strength; I-shaped steel; recycled concrete

Address
Biao Liu: 1) College of Water Conservancy and Civil Engineering, Northwest A&F University, Yangling,712100, China,
2) Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A & F University, Yangling, 712100, China
Feng Xue, Yu-Ting Wu and Zheng-Zhong Wang: College of Water Conservancy and Civil Engineering, Northwest A&F University, Yangling,712100, China
Guo-Liang Bai: School of Civil Engineering, Xi'an 710055, China

Abstract
The current research proposes an innovative finite element model established within the context of higher-order beam theory to examine the bending and buckling behaviors of functionally graded carbon nanotube-reinforced composite (FG CNTRC) beams resting on Winkler-Pasternak elastic foundations. This two-node beam element includes four degrees of freedom per node and achieves inter-element continuity with both C1 and C0 continuities for kinematic variables. The isoparametric coordinate system is implemented to generate the elementary stiffness and geometric matrices as a way to enhance the existing model formulation. The weak variational equilibrium equations are derived from the principle of virtual work. The mechanical properties of FG-CNTRC beams are considered to vary gradually and smoothly over the beam thickness. The current investigation highlights the influence of porosity dispersions through the beam cross-section, which is frequently omitted in previous studies. For this reason, this analysis offers an enhanced comprehension of the mechanical behavior of FG-CNTRC beams under various boundary conditions. Through the comparison of the current results with those published previously, the proposed finite element model demonstrates a high rate of efficiency and accuracy. The estimated results not only refine the precision in the mechanical analysis of FG-CNTRC beams but also offer a comprehensive conceptual model for analyzing the performance of porous composite structures. Moreover, the current results are crucial in various sectors that depend on structural integrity in specific environments.

Key Words
bending; carbon-nanotube reinforcement; elastic stability; finite element beam model; porosity distribution patterns

Address
Zakaria Belabed: 1) Artificial Intelligence Laboratory for Mechanical and Civil Structures, and Soil, Institute of Technology, University Center of Naama, BP 66, 45000 Naama, Algeria, 2) Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria
Abdeldjebbar Tounsi: 1) Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria, 2) Industrial Engineering and Sustainable Development Laboratory, University of Rélizane, Faculty of Science & Technology, Mechanical Engineering Department, Algeria
Abdelmoumen Anis Bousahla: Laboratoire de Modélisation et Simulation Multi-échelle, Université de Sidi Bel Abbés, Algeria
Abdelouahed Tounsi: 1) Material and Hydrology Laboratory, University of Sidi Bel Abbes, Faculty of Technology, Civil Engineering Department, Algeria, 2) Department of Civil and Environmental Engineering, Lebanese American University, 309 Bassil Building, Byblos, Lebanon, 3) Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Eastern Province, Saudi Arabia, 4) YFL (Yonsei Frontier Lab), Yonsei University, Seoul, Korea
Khaled Mohamed Khedher: Department of Civil Engineering, College of Engineering, King Khalid University, Abha 61421, Saudi Arabia
Mohamed Abdelaziz Salem: Department of Mechanical Engineering, College of Engineering, King Khalid University, Abha 61421, Saudi Arabia


Abstract
The in-situ stresses of concrete are an essential index for assessing the safety performance of concrete structures. Conventional methods for pore pressure release often face challenges in selecting drilling ring parameters, uncontrollable stress release, and unstable detection accuracy. In this paper, the parameters affecting the results of the concrete ring hole stress release method are cross-combined, and finite elements are used to simulate the combined parameters and extract the stress release values to establish a training set. The GridSearchCV function is utilized to determine the optimal hyperparameters. The mean absolute error (MAE), root mean square error (RMSE), and coefficient of determination (R2) are used as evaluation indexes to train the gradient boosting decision tree (GBDT) algorithm, and the other three common algorithms are compared. The RMSE of the GBDT algorithm for the test set is 4.499, and the R2 of the GBDT algorithm for the test set is 0.962, which is 9.66% higher than the R2 of the best-performing comparison algorithm. The model generated by the GBDT algorithm can accurately calculate the concrete in-situ stresses based on the drilling ring parameters and the corresponding stress release values and has a high accuracy and generalization ability.

Key Words
finite element; GBDT algorithm; grid search; ring hole method; stress release

Address
School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an 710000, China

Abstract
Geopolymers are part of a class of materials characterized by properties combining polymers, ceramics, and cement. These include exceptionally high thermal and chemical stability, excellent mechanical strength and durability in aggressive environments. This work deals with the synthesis, characterization, and sustainability evaluation of GPGBFS-MK geopolymers by alkaline activation of a granulated blast furnace slag-metakaolin mixture. In the first step, elemental and oxide analyses by XRF and EDS showed that the main constituents of GPGBFS-MK geopolymers are silicon, sodium, and aluminium oxides. The structural analyses by XRD and FTIR confirmed that the geopolymerization for GPGBFS-MK geopolymers did occur, accompanied by the formation of disordered networks from the blends and a modification to the microstructure by the geopolymerization process. Similarly, the microstructural study made by SEM showed that the GPGBFS-MK geopolymers are constituted by aluminosilicates in the form of dense clusters on which are adsorbed particles of unreacted GBFS in the form of spheroids and white residues of the alkaline activating solution. In addition, the study of the sustainability evaluation of GPGBFS-MK geopolymers showed that the water absorption of geopolymeric materials is lower than that of OPC cement. As for the elevated temperature resistance, the analyses indicated an excellent elevated temperature resistance of GPGBFS-MK. In the same way, the study of the resistance to chemical aggressions showed that the GPGBFS-MK geopolymeric materials are unattackable, contrary to the OPC cement-based materials which are strongly altered.

Key Words
geopolymers; granulated blast furnace slag; metakaolin; physicochemical properties; sustainability

Address
Anas Driouich, Safae El Alami El Hassani, Slimane El Harfaoui and Hassan Chaair: Laboratory of Process Engineering and Environment, Faculty of Sciences and Technology, University Hassan II 28806, Mohammedia, Morocco
Zakia Zmirli: Laboratory of Advanced Materials and Process Engineering, Faculty of Sciences, University Ibn Tofail 242, Kenitra, Morocco
Nadhim Hamah Sor: 1) Civil Engineering Department, University of Garmian, Kurdistan Region, Kalar 46021, Iraq, 2) Department of Civil Engineering, Harran University, Sanliurfa 63510, Turkey
Ayoub Aziz: Geo-Biodiversity and Natural Patrimony Laboratory (GEOBIO), Scientific Institute, "Geophysics, Natural Patrimony and Green Chemistry" Research Center (GEOPAC), Mohammed V University in Rabat, Avenue Ibn Batouta, P.B. 703, 10106 Rabat-Agdal, Morocco
Jong Wan Hu: 1) Department of Civil and Environmental Engineering, Incheon National University, Incheon 22012, South Korea, 2) Incheon Disaster Prevention Research Center, Incheon National University, Incheon 22012, South Korea
Haytham F. Isleem: Department of Construction Management, Qujing Normal University, Qujing 655011, Yunnan, China
Hadee Mohammed Najm: Department of Civil Engineering, Zakir Husain Engineering College, Aligarh Muslim University, Aligarh, India

Abstract
This study presents an analysis model to estimate the shear strength of a reinforced concrete (RC) member with corroded tensile reinforcements. The thick-walled cylinder theory was modified to fit the dual potential capacity model to reflect interdependent failure mechanisms, including the degradation effect of bonds in corroded tensile reinforcement. In the proposed model, it is considered that the shear failure of corroded RC members with no proper anchorage detail is primarily dominated by the flexural-bond mechanism, where insufficient bond strength is provided owing to corrosion damage. However, when tensile reinforcements are properly anchored in the end regions using end hooks or mechanical devices, it is assumed that the tied-arch action can be developed as a secondary shear transfer mechanism, even under severe corrosion damage. The proposed model was verified by comparison with shear test results of corroded RC members collected from the literature, and it appeared that the proposed model can estimate their shear strengths with a good level of accuracy, regardless of various anchorage details and corrosion rates in tensile reinforcements.

Key Words
anchorage detail; bond performance; corrosion; dual potential capacity model; shear capacity

Address
Sun-Jin Han: Department of Architectural Engineering, Jeonju University, 303, Cheonjam-ro, Wansan-gu, Jeonju-si, Jeonbuk-do, 55069, Korea
Deuckhang Lee: Department of Architectural Engineering, Chunbuk National University, Chungdae-ro 1, Seowon-gu, Cheongju, Chungbuk, 28644, Korea
Hyo-Eun Joo: Department of Civil Engineering, The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyo City, Tokyo, 113-8654, Japan
Kang Su Kim: Department of Architectural Engineering and Smart City Interdisciplinary Major Program, University of Seoul, 163 Siripdaero, Dongdaemun-gu, Seoul, 02504, Korea

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