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Shear behavior of precast ultrahigh-performance concrete (UHPC) segmental beams with external tendons and dry joints

Ye Meng, Li Lifeng, Yoo Doo-Yeol, Wang Lianhua, Li Huihui, Shao Xudong

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 3

art. no. e143, 2023

EN

In this study, ultrahigh-performance concrete (UHPC) was utilized in precast segmental beams to reduce the self-weight, shorten the construction time, and improve the performance and durability of bridges. Owing to the discontinuity in the joints, shear behavior plays a critical role in the overall structural performance of precast UHPC segmental beams (PUSBs). Therefore, four dry-jointed segmental specimens along with one monolithic specimen were designed and tested under a two-point concentrated load with various joint types, shear span-to-depth ratios (λ), and numbers of shear keys. Two types of shear failure modes were observed in the tests: shear compression failure of the web (λ = 1.44 and 2.56) and local shear failure of the flanges at the joint (λ = 3.67). The shear capacity, stiffness, and cracking load of the dry-jointed segmental specimens were lower than those of the monolithic specimen, and the single-keyed specimen exhibited better shear behavior than the three-keyed specimen. Increasing λ decreased the shear strength and stiffness of the segmental beams and considerably affected their failure modes and crack distributions. Additionally, four UHPC design codes were evaluated for their accuracy in estimating the shear strength of the specimens, and a simplified strut-and-tie model was developed to predict the shear strength of externally pre-stressed PUSBs. Finally, several design recommendations were proposed. This study is expected to facilitate the research and application of PUSBs.

2

Experimental and theoretical investigation of the shear lag effect in the novel non‑prismatic prestressed CSW-UHPC composite box girders

Li Huihui, Li Lifeng, Yoo Doo-Yeol, Ye Meng, Zhou Cong, Shao Xudong

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 2

art. no. e132, 2023

EN

Due to their featured mechanical and structural merits, such as the light self-weight, excellent prestressing efficiency, appealing appearance, and optimal shear force and bending moment distributions in corrugated steel webs (CSWs) and concrete flanges, the prestressed concrete composite box girders with CSWs (CCBGCSWs) are popularly applied in highway bridges nowadays. To further enhance the cracking resistance of the concrete flanges of conventional prestressed CCBGCSWs, by replacing the regular concrete flanges with that made by ultra-high-performance concrete (UHPC) (i.e., with the much superior mechanical properties), this paper proposed a novel non-prismatic prestressed CSW-UHPC composite box girder to achieve the lighter dead weight, superior spanning capacity, and more rapid and cost-efficient construction for highway bridges. Owing to the differences in both the geometric dimensions and material properties, shear lag behavior of the proposed novel non-prismatic prestressed CSW-UHPC composite box girder could significantly differ from that of conventional prestressed CCBGCSWs. The shear lag effect refers to the non-uniform distributions of the longitudinal bending normal stress within the flanges caused by shear interaction between the webs and flanges, and the improper consideration of the shear lag behavior would impair the safety of thin-walled CCBGCSWs, especially the proposed novel non-prismatic prestressed CSW-UHPC composite box girder. Therefore, to investigate the shear lag behavior of the proposed novel nonprismatic prestressed CSW-UHPC composite box girder during different construction stages, a representative test specimen (5.55 m in length) with different boundary conditions (e.g., simply supported and cantilever) was designed and investigated under five different loading cases using the experimental tests and finite element (FE) analyses. In addition, a modified bar simulation method was proposed for the theoretical analysis of the shear lag behavior of the girder, and its feasibility and effectiveness were demonstrated through the comparisons to the experimental and numerical results. Finally, the results indicated that (i) the shear lag effect of UHPC flanges in the stress concentration region of the proposed novel non-prismatic CSW-UHPC composite box girder was more pronounced than that in the non-stress concentration region under different loading conditions; (ii) the shear lag coefficient (λ) of UHPC flanges in the non-stress and stress concentrated regions of the girder could be conservatively recommended not less than 1.1 and 1.25, respectively; and (iii) the boundary conditions and loading forms had significant influences on the shear lag behavior of the girder. The results of this study could serve as the experimental, numerical, and theoretical references for the shear lag behavior of the novel non-prismatic prestressed CSW-UHPC composite box girders.

3

Electromagnetic interference shielding effectiveness of multi-cracked strain-hardening cementitious composites (SHCC)

Kim Soonho, Jang Yun Sik, Oh Taegeun, Lee Seung Kyun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 3

art. no. e179, 2023

EN

With the rapid increase in the use of wireless electronic devices, electromagnetic pollution has been recognized as a serious threat. There has been an increasing demand for the use of cement composites as electromagnetic shielding materials. Thus, this study investigated the advantages of adding a small dosage of carbon fibers to enhance the mechanical and electrical properties of strain-hardening cementitious composites (SHCCs) containing steel fibers. In addition, the effect of microcrack formation on the electromagnetic interference (EMI) shielding effectiveness of the SHCCs was analyzed. For this purpose, four different residual tensile strains were applied in preloading tests in the range of 0.015–0.1%. The test results suggested that the tensile performance of the SHCCs was improved by adding 0.2 vol% carbon fibers. Moreover, the rate of increase of the energy absorption capacity was higher (50%) than those of the tensile strength and strain capacity. The electrical conductivity and EMI shielding effectiveness of the SHCCs were noticeably increased by the addition of carbon fibers. The highest shielding effectiveness of 45.6 dB, at 1 GHz, was achieved for the SHCC containing 2% steel fibers and 0.2% carbon fibers, which was approximately 6% higher than that of the corresponding plain SHCC with only steel fibers. An approximately 44–47% lower shielding effectiveness was observed with the formation of through microcracks; however, the number of cracks and the residual tensile strain did not significantly influence the shielding effectiveness. This study can be a basis for evaluating EMI shielding effectiveness of damaged structures.

4

Mechanical performance of ultra-high-performance strain-hardening cementitious composites according to binder composition and curing conditions

Kim Min-Jae, Oh Taekgeun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 2

art. no. e63, 1--12.

EN

This study investigated the mechanical properties and microstructures of three ultra-high-performance strain-hardening cementitious composites (UHP-SHCCs) with different mix proportions and curing conditions. The binders comprised ordinary Portland cement (OPC), silica fume, and ground granulated blast furnace slag (GGBS); the specimens were cured under air and wet curing conditions for 28 and 91 days, respectively. Compressive and direct tensile tests were performed, along with subsequent microstructural analyses using the particle packing theory and scanning electron microscopy, on the composite matrix and reinforcing polyethylene (PE) fibers. The test results indicate that the inclusion of GGBS, more than 50% (by weight of OPC), leads to a decrease in compressive and tensile strength by up to 35.7% but an increase in ductility by up to 55.9%. In addition, a higher content of GGBS resulted in larger deviations based on the curing conditions. The wet curing condition was more effective for the development of a higher energy absorption capacity than the air curing condition at a curing age of 28 d. By contrast, 91 d of wet curing resulted in the lowest strain energy in this study, mainly because of the considerably reduced strain capacity.

5

Influence of curing conditions on the mechanical performance of ultra-high-performance strain-hardening cementitious composites

Kim Min-Jae, Oh Taekgeun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 3

607--617

EN

This study investigated the influence of curing conditions and the inclusion of ground granulated blast furnace slag (GGBS) on the mechanical performance of ultra-high-performance strain-hardening cementitious composites (UHP-SHCC). Air- and wet-curing conditions were applied for 28 and 91 days, respectively. Compressive strength and direct tensile tests were performed, and the microstructure of the tested cementitious matrix and surface of the polyethylene (PE) fibers were inspected using scanning electron microscopy. The results showed that 3 months of wet-curing notably deteriorated the tensile performance of UHP-SHCC with or without GGBS as compared to those at the curing age of 1 month, whereas the 3 months of air-curing further enhanced the tensile performance. Therefore, the 3 months air-cured specimens, using binders consisting only of ordinary portland cement (OPC) or OPC with GGBS, could develop the highest tensile strength and strain capacity of up to 12.1 MPa and 9.1% or 13.6 MPa and 9.1%, respectively. The inclusion of GGBS led to a higher rate of stress development as well as tensile strength at the air-curing age of 3 months, resulting in the highest energy absorption capacity of 985 kJ/m3 measured in this study.

6

Dynamic compressive and flexural behaviors of ultra-rapid-hardening mortar containing polyethylene fibers

Chun Booki, Shin Wonsik, Oh Taekgeun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 2

576--592

EN

In this study, the dynamic, compressive, and flexural behaviors of ultra-rapid-hardening mortar (URHM) containing 2% polyethylene fiber are investigated. The results confirm the robust strain-hardening behavior of URHM at an early age of 4 h. Its tensile strength, strain capacity, and g value at 4 h were found to be 7.3 MPa, 5.1%, and 297.5 kJ/m3, respectively. The compressive and flexural strength and toughness of URHM increased with the strain rate. A higher loading rate led to a greater increase in the strength; the rate sensitivity was higher during flexure compared to that during compression. The highest dynamic increase factor (DIF) of the compressive strength was 1.75 up to a strain rate of 115/s; the highest DIF of the flexural strength was 3.34 up to a strain rate of 96/s. Its deflection-hardening behavior was converted to deflection-softening behavior under impact loads having a potential energy of 392 J or greater. Furthermore, the greater potential energy led to a lower energy dissipation rate, and more energy remained in the system. The rate sensitivity of the URHM under compression was similar to that of other fiber-reinforced concretes; however, its flexural strength was less sensitive to the strain rate than that of the others.

7

Spacing and bundling effects on rate-dependent pullout behavior of various steel fibers embedded in ultra-high-performance concrete

Kim Jae-Jin, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 2

218--235

EN

This study examines the effects of fiber geometry, spacing, and loading rate on the pullout resistance of steel fibers in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, four different fiber spacings, and three different loading rates ranging from 0.018 to 740 mm/s were considered. Test results indicated that the single straight fiber in UHPC was most rate sensitive for pullout resistance, followed by the single twisted and then hooked fibers. The bond strengths and pullout energy of specimens with multiple straight fibers were improved by increasing the loading rate but were not affected by fiber spacing. Closer fiber spacing had a detrimental effect on the dynamic pullout resistance of multiple hooked steel fibers in UHPC, while no enhancement of average bond strength of multiple twisted fibers was observed as fiber spacing and loading rate varied. The average bond strengths of single and bundled hooked and twisted steel fibers in UHPC were clearly improved by increasing the loading rate. Bundling of fibers enhanced the impact pullout resistance of all the steel fibers in UHPC. The highest dynamic increase factors for the bundled straight, hooked, and twisted fibers were approximately 3.78, 1.57, and 1.41, respectively, at the impact loads.

8

Influence of chemically treated carbon fibers on the electromagnetic shielding of ultra-high-performance fiber-reinforced concrete

Yoo Doo-Yeol, Kang Min-Chang, Choi Hong-Joon, Shin Wonsik, Kim Soonho

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 4

367--381

EN

The effects of carbon fiber and its surface treatment through chemical solutions on the mechanical properties and electromagnetic (EM) shielding of ultra-high-performance fiber-reinforced concrete (UHPFRC) were analyzed. Three types of carbon fibers chemically treated with sodium hydroxide, nitric acid, and ammonia solutions were evaluated, along with a plain carbon fiber control sample, at two different concentrations of 0.1% and 0.3% by weight. The surface of carbon fiber was oxidized by chemical solutions. The conductivity of UHPFRC increased with increasing the carbon fiber content, and slightly better conductivity was obtained using the chemically treated carbon fibers than plain fibers at the lower content of 0.1 wt%. Both steel and carbon fibers were effective at improving the shielding effectiveness of ultra-high-performance concrete, and a higher shielding effectiveness was achieved for higher carbon fiber content. Surface treatment using the nitric acid solution was the most effective at enhancing the tensile performance and EM shielding effectiveness, and the best shielding effectiveness (49.0 dB at 1 GHz) was achieved for UHPFRC with 0.1 wt% nitric acid treated carbon fibers. The shielding effectiveness was found to be generally proportional to the electrical conductivity, although its increase was minor relative to that of the conductivity.