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Volume 15, Number 6, June 2023

Ultra-high-performance concrete (UHPC) has become an attractive cast-in-place repairing material for existing engineering structures. The present study aims to investigate age-dependent high-early-strength UHPC (HESUHPC) material properties (i.e., compressive strength, elastic modulus, flexural strength, and tensile strength) as well as interfacial shear properties of HESUHPC-normal strength concrete (NSC) composites cured at different season temperatures (i.e., summer, autumn, and winter). The typical temperatures were kept for at least seven days in different seasons from weather forecasting to guarantee an approximately consistent curing and testing condition (i.e., temperature and relative humidity) for specimens at different ages. The HESUHPC material properties are tested through standardized testing methods, and the interfacial bond performance is tested through a bi-surface shear testing method. The test results quantify the positive development of HESUHPC material properties at the early age, and the increasing amplitude decreases from summer to winter. Three-day mechanical properties in winter (with the lowest curing temperature) still gain more than 60% of the 28-day mechanical properties, and the impact of season temperatures becomes small at the later age. The HESUHPC shrinkage mainly occurs at the early age, and the final shrinkage value is not significant. The HESUHPC-NSC interface exhibits sound shear performance, the interface in most specimens does not fail, and most interfacial shear strengths are higher than the NSC-NSC composite. The HESUHPC-NSC composites at the shear failure do not exhibit a large relative slip and present a significant brittleness at the failure. The typical failures are characterized by thin-layer NSC debonding near the interface, and NSC pure shear failure. Two load-slip development patterns, and two types of main crack location are identified for the HESUHPC-NSC composites tested in different ages and seasons. In addition, shear capacity of the HESUHPC-NSC composite develops rapidly at the early age, and the increasing amplitude decreases as the season temperature decreases. This study will promote the HESUHPC application in practical engineering as a cast-in-place repairing material subjected to different natural environments.

Key Words
bi-surface shear test; HESUHPC-NSC composite; high-early-strength ultra-high-performance concrete; season temperatures; shear performance

"(1) Yang Zhang, Yanping Zhu, Shuilong He, Xudong Shao:
College of Civil Engineering, Hunan University, Changsha, China;
(2) Yanping Zhu, Pengfei Ma:
Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, Rolla, United States."

In this study, an improved modular arch system with the lower arch space composed of a precast arch block and an outrigger was proposed as an underground culvert, and its applicability and structural behaviors were confirmed. This modular arch culvert structure with vertical walls was designed using precast blocks and by adjusting the placement spacing of concrete blocks to the upper part form an arch shape and the lower part form a vertical wall shape, based on previously researched modular arch systems. Owing to the vertical wall of the proposed modular arch system, it is possible to secure a load-carrying capacity and an arch space that can sufficiently resist the earth pressure generated from the backfill soil and loading on the arch system. To verify the structural characteristics, and applicability of the proposed modular precast arch culvert structure, a fullscale modular culvert specimen was fabricated, and a loading test was conducted. By examining its construction process and loading test results, the applicability and constructability of the proposed structure were analyzed along with its structural characteristics. In addition, its the structural predictability and safety for the applicability were evaluated by comparing the construction process and loading test results with the FE analysis results.

Key Words
applicability; construction stage; load-resistance performance; modular underground arch culvert; realscale field test

"(1) Tae-Yun Kwon, Jin-Hee Ahn, Hong-duk Moon:
Department of Civil and Infrastructure Engineering, Gyeongsang National University, Jinju 52725, Korea;
(2) Kwang-Il Cho:
Research Team, Tekhan Inc., Uiwang 16006, Korea;
(3) Jungwon Huh:
Department of Civil Engineering, Chonnam National University, Gwangju 61186, Korea."

In this study, the modeling of the transverse connection of fully precast steel-UHPC (Ultra-High-Performance Concrete) lightweight composite bridges were conducted. The transverse connection between precast components plays a critical role in the overall performance and safety of the bridge. To achieve an accurate and reliable simulation of the interface behavior, the cohesive model in ABAQUS was employed, considering both bending-tension and compression-shear behaviors. The parameters of the cohesive model are obtained through interface bending and oblique shear tests on UHPC samples with different surface roughness. By validating the numerical simulation against actual joint tests, the effectiveness and accuracy of the proposed model in capturing the interface behavior of the fully precast steel-UHPC lightweight composite bridge were demonstrated.

Key Words
bridge connections; cohesive model; interfacial properties; numerical simulation method; ultra-high-performance concrete (UHPC)

"(1) Shuwen Deng:
College of Civil Engineering, Hunan University, Changsha 410082, China;
(2) Shuwen Deng, Zhiming Huang:
College of Water Resource & Civil Engineering, Hunan Agriculture University, Changsha 410025, China;
(3) Guangqing Xiao:
Guangdong Shengxiang Traffic Engineering Testing Co., Ltd., Guangzhou 510000, China;
(4) Lian Shen:
Department of Civil Engineering, Changsha University, Changsha 410022, China."

While ultra-high performance concrete (UHPC) is commonly reinforced with micro straight steel fibers in existing applications, studies have indicated that the use of deformed steel macro-fibers leads to enhanced ductility and post-peak responses for UHPC structural elements, which is of particular importance for earthquake-resistant structures. However, there are potential concerns regarding the use of UHPC reinforced with macro-fibers due to the issues of workability and fiber distribution. The objective of this study was to address these issues by extensively investigating the restricted and non-restricted deformability, filling ability, horizontal and vertical velocities, and passing ability of UHPC containing macro hooked-end steel fibers. A new approach is suggested to examine the homogeneity of fiber distribution in UHPC. The influences of ultra-fine fillers and steel macro-fibers on the workability of fresh UHPC and the mechanics of hardened UHPC were examined. It was found that although increasing the ratio of quartz powder to cement led to an improvement in the workability and tensile strain hardening behavior of UHPC, it reduced the fiber distribution homogeneity. The addition of 1% volume fraction of macro-fibers in UHPC improved workability, but reduced its compressive strength, which is contrary to the effect of micro-fiber inclusion in UHPC.

Key Words
fiber dispersion; fine fillers; flowability; macro deformed steel fibers; strain hardening; UHPC

Department of Civil Engineering, National Cheng Kung University, 1, University Rd, Tainan City, 701, Taiwan.

In this study, the effect of two types of aggregates (fly ash aggregate and shale aggregate) on the density, strength, and durability of preplaced lightweight aggregate concrete (PLWAC) was studied. The results showed that the 7 - 28 days strength of concrete prepared with fly ash aggregates (high water absorption rate) significantly increased, which could attribute to the long-term water release of fly ash aggregates by the refined pore structure. In contrast, the strength increase of concrete prepared with shale aggregates (low water absorption rate) is not apparent. Although PLWAC prepared with fly ash aggregates has a lower density and higher strength (56.8 MPa @ 1600 kg/m3), the chloride diffusion coefficient is relatively high, which could attribute to the diffusion paths established by connected porous aggregates and the negative over-curing effect. Compared to the control group, the partial replacement of fly ash aggregates (30%) with asphalt emulsion (20% solid content) coated aggregates can reduce the chloride diffusion coefficient of concrete by 53.6% while increasing the peak load obtained in a threepoint bending test by 107.3%, fracture energy by 30.3% and characteristic length by 103.5%. The improvement in concrete performance could be attributed to the reduction in the water absorption rate of aggregates and increased energy absorption by polymer during crack propagation.

Key Words
durability; lightweight aggregate concrete; mechanical properties; polymer

"(1) Bo Peng, Jiantao Wang, Xianzheng Dong, Chuming Sheng, Yunpeng Liu:
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, China;
(2) Jiantao Wang:
Huarun Cement Research & Development Co.Ltd, Guangzhou, 410460, China;
(3) Feihua Yang:
Beijing Building Materials Academy of Sciences Research, Beijing, 100041, China."

This study investigated the use of latex-modified sand concrete reinforced with sisal fibers (LMSC) as a repair material. Notably, no prior research has explored the application of LMSC for this purpose. This paper examines the interface bond strength and the type of failure between LMSC as a repair material and the normal concrete (NC) substrate utilising four different surfaces: without surface preparation as a reference (SR), hand hammer (HA), sandblasted (SB), and grooved (GR). The bond strength was measured by bi-surface shear, splitting tensile, and pull-off strength tests at 7, 28, and 90 days. Scanning electron microscopy analysis was also performed to study the microstructure of the interface between the normal concrete substrate and the latex-modified sand concrete reinforced with sisal fibers. The results of this study indicate that LMSC has bonding strength with NC, especially for HR and SB surfaces with high roughness. Therefore, substrate NC surface roughness is essential in increasing the bonding strength and adhesion. Eventually, The LMSC has the potential to repair and rehabilitate concrete structures.

Key Words
bond strength; durability; latex; mechanical; microstructural; repair material sand concrete; sisal fibres; substrate

(1) Oday Z. Jaradat, Karima Gadri, Abdelhamid Guettala:
Laboratory of Research in Civil Engineering, Mohamed Khider University of Biskra, Algeria;
(2) Bassam A. Tayeh:
Civil Engineering Department, Faculty of Engineering, Islamic University of Gaza, Palestine;
(3) Ahmed M. Maglad:
Department of Civil Engineering, Najran University, Najran, Saudi Arabia.

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