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CONTENTS
Volume 31, Number 6, June 2023
 

Abstract
The response mechanism of simply supported two-way reinforced concrete (RC) slabs under fire was numerically studied from the view of stress redistribution using the finite element software ABAQUS. Results show that: (1) Simply supported two-way RC slabs undergo intense stress redistribution, and their responses show four stages, namely elastic, elasticplastic, plastic and tensile membrane stages. There is no cracking in the fire area of the slabs until the tensile membrane stage. (2) The inverted arch effect and tensile membrane effect improve the fire resistance of the two-way slabs. When the deflection is L/20, the slab is in an inverted arch effect state, and the slab still has a good deflection reserve. The deformation rate of the slab in the tensile membrane stage is smaller than that in the elastic-plastic and plastic stages. (3) Fire resistance of square slabs is better than that of rectangular slabs. Besides, increasing the reinforcement ratio or slab thickness improves the fire resistance of the slabs. However, an increase of cover thickness has little effect on the fire resistance of two-way slabs. (4) Compared with one-way slabs, the time for two-way slabs to enter the plastic and tensile cracking stage is postponed, and the deformation rate in the plastic and tensile cracking stage is also slowed down. (5) The simply supported two-way RC slabs can satisfy with the requirements of a class I fire resistance rating of 90 min without additional fire protection.

Key Words
finite element (FE); inverted arch effect; mechanical response; stress redistribution; two-way reinforced concrete slab

Address
Wenjun Wang, Binhui Jiang and Zhiwu Yu: School of Civil Engineering, Central South University, Changsha, Hunan Province, 410075, P.R. China
Fa-xing Ding: 1) School of Civil Engineering, Central South University, Changsha, Hunan Province, 410075, P.R. China, 2) Engineering Technology Research Center for Prefabricated Construction Industrialization of Hunan Province, 410075, P.R. China

Abstract
Both damage and unsaturated conditions accelerate the transport of erosive media inside concrete. However, their combined effects have not been fully investigated. A multiscale mortar model using representative volume elements is developed, capturing the number and distribution in each phase. Afterwards, mortar damage microstructure evolution is simulated in the tensile process. Finally, the unsaturated mortar transport is predicted and analysed. The results indicate that damage significantly affects the diffusion process in the early stage, while the transport performance is weakened due to the obstruction of the nontransport phase in the later stage. The higher the saturation and the more connected pores, the faster the diffusion rate of chloride ions. Chloride ions spread around the cracks in a tree-like manner along. The model can very well predict the chloride ion transport performance of unsaturated and damaged mortar.

Key Words
damage; load; microstructure; transport; unsaturated mortar

Address
Zhiyong Liu, Jinyang Jiang, Tongning Cao and Yuncheng Wang: School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
Yunsheng Zhang: School of Civil Engineering, Lanzhou University of Technology, Lanzhou 730050, China
Rusheng Qian: College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310023, China
Guowen Sun: School of Materials Science and Engineering, Shijiazhuang TieDao University, Shijiazhuang 050043, China

Abstract
The strengthening efficiency of biopolymer treated soil depends on biopolymer type, concentration ratio, soil type, initial water content, curing time and mixing method. In this study, the physical and mechanical properties of xanthan gum (XG) treated kaolinite were investigated through compaction test, Atterberg limit test, triaxial test and unconfined compression test. The results indicated that the optimum water content (OWC) increased from 30.3% of untreated clay to 33.5% of 5% XG treated clay, while the maximum dry density has a slight increase from 13.96 kg/m3 to 14 kg/m3 of 0.2% XG treated clay and decrease to 2.7 kg/m3 of 5% XG treated clay. Meanwhile, the plastic limit of XG treated clay increased with the increase of XG concentration, while 0.5% XG treated clay can be observed the maximum liquid limit with 79.5%. Moreover, there are the ideal water content about 1.3-1.5 times of the optimum water content achieving the maximum dry density and curing time to obtain the maximum compressive strength for different XG contents, which the UCS is 1.52 and 2.07 times of the maximum UCS of untreated soil for 0.5% and 1% XG treated clay, respectively. In addition, hot-dry mixing can achieve highest UCS than other mixing methods (e.g., dry mixing, wet mixing and hot-wet mixing).

Key Words
ground improvement; initial water content; unconfined compressive strength; xanthan gum

Address
Zhanbo Cheng: 1) School of Engineering, University of Warwick, Coventry CV47AL, UK, 2) School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang 639798, Singapore
Xueyu Geng: School of Engineering, University of Warwick, Coventry CV47AL, UK

Abstract
This research presents the experimental and theoretical evaluations on circular steel-fiber-reinforced-concrete (SFRC) columns reinforced by glass-fiber-reinforced-polymer (GFRP) rebar under the axial compressive loading. Test programs were designed to investigate and compare the effect of different parameters on the structural behavior of columns by performing tests. Theses variables included conventional concrete (CC), fiber concrete (FC), steel/GFRP longitudinal rebars, and transversal rebars configurations. A total of 16 specimens were constructed and categorized into four groups in terms of different rebarconcrete configurations, including GFRP-rebar-reinforced-CC columns (GRCC), GFRP-rebar-reinforced-FC columns (GRFC), steel-rebar-reinforced-CC columns (SRCC) and steel-rebar- reinforced-FC columns (SRFC). Experimental observations displayed that failure modes and cracking patterns of four groups of columns were similar, especially in pre-peak branches of load-deflection curves. Although the average ultimate axial load of columns with longitudinal GFRP rebars was obtained by 17.9% less than the average ultimate axial load of columns with longitudinal steel rebars, the average axial ductility index (DI) of them was gained by 10.2% higher than their counterpart columns. Adding steel fibers (SFs) into concrete led to the increases of 7.7% and 6.7% of the axial peak load and the DI of columns than their counterpart columns with CC. The volumetric ratio had greater efficiency on peak loads and DIs of columns than the type of transversal reinforcement. A simple analytical equation was proposed to predict the axial compressive capacity of columns by considering the axial involvement of longitudinal GFRP rebars, volumetric ratio, and steel spiral/hoop rebar. There was a good correlation between test results and predictions of the proposed equation.

Key Words
axial compressive load; ductility index (DI); GFRP rebars; load-strain curves; theoretical equations

Address
Iman Saffarian and Gholam Reza Atefatdoost: Department of Civil Engineering, Estahban Branch, Islamic Azad University, Estahban, Iran
Seyed Abbas Hosseini: Faculty of Technology and Mining, Yasouj University, Choram, Iran
Leila Shahryari: Department of Civil Engineering, Shiraz Branch, Islamic Azad University, Shiraz, Iran

Abstract
Fiber Reinforced Polymer (FRP) bar has emerged as a viable and sustainable replacement to steel in reinforced concrete (RC) under severe corrosive environment. The behavior of concrete columns reinforced with FRP bars, spirals, and hoops is an ongoing area of research. In this study, 3D nonlinear numerical modelling of circular concrete columns reinforced with Glass Fiber Reinforced Polymer (GFRP) bars and transversely confined with GFRP spirals were conducted using fractureplastic model. The numerical models and experimental results are found to be in good agreement. The effectiveness of confinement was accessed through von-mises stresses, and it was found that the stresses in the concrete's core are higher with a 30 mm pitch (46 MPa) compared to a 60 mm pitch (36 MPa). The validated models are used to conduct parametric studies. In terms of axial load carrying capacity and member ductility, the effect of concrete strength, spiral pitch, and longitudinal reinforcement ratio are thoroughly investigated. The confinement effect and member ductility of a GFRP RC column increases as the spiral pitch decreases. It is also found that the confinement effect and member ductility decreased with increase in strength of concrete.

Key Words
circular column; confinement; FEM; fracture-plastic model; GFRP RC column; noncorrosive rebar; numerical modelling

Address
Department of Civil Engineering, NED University of Engineering and Technology, Karachi, Pakistan

Abstract
To study the seismic behavior of recycled aggregate concrete filled circular steel tube (RACFCST) frames, the seismic behavior experiment of RACFCST frame was carried out to measure the hysteresis curve, skeleton curve and other seismic behavior indexes. Moreover, based on the experimental study, a feasible numerical analysis model was established to analyze the finite element parameters of 8 RACFCST frame specimens, and the influence of different variation parameters on the seismic behavior index for RACFCST frame was revealed. The results showed that the skeleton curve of specimens under different axial compression ratios were divided into three stages: elastic stage, elastic-plastic stage and descending stage, and the descending stage was relatively stable, indicating that the specimen had stronger deformation capacity in the descending stage. With the increase of axial compression ratio, the peak bearing capacity of all specimens reduced gradually, and the reduction was less than 5%. With the decrease of beam-column linear stiffness ratio, the peak bearing capacity decreased gradually. With the decrease of yield bending moment ratio of beam-column, the peak bearing capacity decreased gradually, and the decreasing rate of peak bearing capacity gradually accelerated. In addition, compared with the axial compression ratio, the beam-column linear stiffness ratio and the yield bending moment ratio of beam-column had a more significant influence on the peak bearing capacity of RACFCST frame.

Key Words
experimental study; hysteretic curve; numerical analysis; recycled aggregate concrete filled circular steel tube frame; seismic behavior; skeleton curve

Address
Xianggang Zhang: 1) School of Civil Engineering, Henan Polytechnic University, Jiaozuo 454000, China, 2) School of Intelligent Construction, Wuchang University of Technology, Wuhan 430223, China
Gaoqiang Zhou, Xuyan Liu, Yuhui Fan and Junna Yang: School of Civil Engineering, Henan Polytechnic University, Jiaozuo 454000, China
Yajun Huang: School of Intelligent Construction, Wuchang University of Technology, Wuhan 430223, China
Ercong Meng: School of Engineering and Technology, Southwest University, Chongqing 400715, China

Abstract
In this study, an experimental investigation was conducted to assess the influence of ground granulated blast furnace slag (GGBS) and silica fume (SF) on the fresh and hardened properties of grout specimens and preplaced aggregate concrete (PAC). Grout proportions were optimized statistically using a factorial design and were applied to 10 mm and 20 mm coarse aggregates to produce PAC. The results demonstrate that GGBS has a more significant effect on the compressive strength of grout compared to SF, with a small increase or decrease in the GGBS content having a greater influence on the compressive strength of grout than SF. The water to binder ratio had the most significant effect on the compressive strength of PAC, followed by the coarse aggregate size and sand to binder ratio. An empirical relationship to predict the compressive strength of PAC was proposed through an experimentally derived factorial design along with a statistical analysis of collectively obtained data and a deep literature review. The results predicted by the empirical relationship were in good agreement with those of PAC produced for verification.

Key Words
compressive strength; empirical relationship; ground granulated blast furnace slag; preplaced aggregate concrete; silica fume

Address
Kunal Krishna Das: 1)Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, 2) Division of Architecture and Urban Design, Urban Sciences Institute, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea
Eddie Siu-Shu Lam: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong
Jeong Gook Jang: Division of Architecture and Urban Design, Urban Sciences Institute, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea

Abstract
In this paper, the structural performance of RC and HPFRCC slabs exposed to a TNT explosion were numerically investigated. A finite element model was established using the MM-ALE method in the LS-DYNA program to simulate a nearground TNT explosion at a scaled distance of 1.08 m/kg3. The K&C model was calibrated to exactly reflect the material properties of HPFRCCs that were developed in KICT and KNU. Numerical and experimental results were compared for the damage distribution and failure shape of the slabs. Based on the verified numerical model, a parametric study was carried out to demonstrate the effects of compressive strength and thickness of the slab on the blast resistance. In particular, the spallation failure on the back side of the slab is greatly dependent on the thickness. Finally, additional numerical simulations were conducted to explore the variation in blast pressure characteristics according to the scaled distance and explosive shape. It was confirmed that the pressure induced by cubic TNT was more destructive to the slab than cylindrical and spherical TNT in a nearfield explosion.

Key Words
ALE method; HPFRCC; parametric study; TNT explosion

Address
MinJoo Lee and Hyo-Gyoung Kwak: Department of Civil and Environmental Engineering, KAIST, Daejeon 34141, Republic of Korea
Sung-Wook Kim and Gang-Kyu Park: Department of Structural Engineering Research, Korea Institute of Civil Engineering and Building Technology, 283 Goyangdae-Ro, Ilsanseo-Gu, Goyang-Si 10223, Korea

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