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1

Utilizing ensemble learning in the classifications of ductile and brittle failure modes of UHPC strengthened RC members

Taffese Woubishet Zewdu, Zhu Yanping, Chen Genda

Archives of Civil and Mechanical Engineering

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2024

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Vol. 24, no. 2

art. no. e86, 2024

EN

This study aims to achieve the swift and precise classification of ductile and brittle failure modes in flexural reinforced concrete (RC) members, specifically those with tension sides strengthened by ultrahigh performance concrete (UHPC). Employing six ensemble learning techniques - Bagging, Random Forest, AdaBoost, Gradient Boosting, XGBoost, and LightGBM - the authors utilize a comprehensive dataset comprising 14 features, which include manually labeled failure modes obtain from load-deflection curves. The model training spans four scenarios, varying in the inclusion or exclusion of features describing the cross-sectional area of RC members and moment resistance. XGBoost emerges as the most effective classifier, achieving an impressive 84% accuracy with high confidence. Additionally, the study employs the Shapley Additive Explanation (SHAP) technique on the best-performing model to illuminate the significance and impacts of various features in UHPC-strengthened flexural members’ failure modes. Notably, moment resistance and UHPC tensile strength surface as the most influential factors in predicting failure modes. Increased rebar yield strength, UHPC compressive strength, UHPC reinforcement ratio, and steel fiber volume in UHPC contribute to enhanced ductility in flexural members, while heightened moment resistance and UHPC layer thickness, along with a robust RC-UHPC interface, tend to induce brittleness. The introduction of such an effective failure modes classification model, coupled with the model’s explainability, instills trust in its predictions and facilitates seamless integration into real-world applications, particularly in seismic areas. The model’s ability to operate without the need for pre-experimental tests marks a significant advancement in the field.

2

Study on preparation and mechanical properties of a new type of high-temperature resistant ultra-high performance concrete

Wang Xingchen, Zhang Wenhua, Chen Ruixing, Chen Yuan, Zhang Yunsheng, Liu Yanjun

Archives of Civil and Mechanical Engineering

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2024

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Vol. 24, no. 2

art. no. e115, 2024

EN

Ultra-high performance concrete (UHPC) tends to crack and flake at high temperature, which causes the structure to lose its overall stability and collapse, posing a serious threat to people’s lives and property safety. In this study, by selecting the cementitious material and the high-temperature resistant aggregate, a new type of high-temperature resistant ultra-high performance concrete (HTR-UHPC) was successfully developed. The high temperature resistance tests of HTR-UHPC were systematically carried out at 5 different temperatures (20 ℃, 250 ℃, 500 ℃, 750 ℃, 1000 ℃). The compressive and axial tensile properties of the specimens were investigated after high temperatures, and the compressive and tensile stress-strain curves were obtained. High-temperature products and internal structures were tested by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively, and the microscopic mechanisms were revealed. The results showed that the mechanical properties of the HTR-UHPC had been significantly improved after heating at 500 ℃, 750 ℃, and 1000 ℃. When heated to 500 ℃, the compressive and tensile strength of HTR-UHPC retained 106 and 88% of the unheated status, respectively. While the structure of traditional UHPC was damaged, the compressive and tensile strength retained only 90 and 76%. Moreover, when heated to 750 ℃, the HTR-UHPC still maintained structural integrity. The residual compressive and tensile strength could still reach 115.83 and 6.17 MPa. During the hydration process, the HTR-UHPC did not generate calcium hydroxide, and the failure stress caused by high-temperature dehydration was small. At the same time, the ability of late crystal transition to reduce strength was suppressed by the addition of mineral admixtures, which enabled the formation of cracks and pores to be effectively suppressed. The stability and mechanical properties of the structure were maintained. Overall, these findings offer valuable insights into the influence of cementitious material system and high-temperature resistant aggregate on the mechanical properties of HTR-UHPC.

3

Experimental investigation on flexural performance of UHPC beams reinforced with steel-FRP bars

Yan Weihua, Zhang Rui, Sushant Subedi, Ashour Ashraf, Fu Shihu, Qiu Linfeng, Zhang Zhiwen, Ge Wenjie

Archives of Civil and Mechanical Engineering

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2024

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Vol. 24, no. 2

art. no. e132, 2024

EN

To investigate the flexural performance of Steel-FRP Composite Bars (SFCBs) reinforced Ultra-High Performance Concrete (UHPC) beams, eight concrete beams with different reinforcement ratio, types of concrete were designed and fabricated. Flexural performance tests were conducted to examine the effect of various parameters on bearing capacity, deflections, crack patterns, ductility, and failure modes. The results indicate a significant enhancement in the flexural capacity of tested beams with UHPC. The bearing capacity of SFCB-UHPC beam is higher than that of steel-reinforced UHPC beams, but less than that of FRP (Fiber-Reinforced Polymer) reinforced UHPC beams. The deformation and crack resistance ability of SFCB-UHPC beams fall between those of steel-reinforced UHPC beam and BFRP-reinforced UHPC beam. Increasing the concrete strength and SFCB reinforcement ratio can significantly enhance the deformation and crack resistance ability of SFCB-UHPC beam. All tested specimens exhibited ductile failure. At the serviceability limit state controlled by deflection/crack, the steel-reinforced UHPC beams and BFRP-reinforced UHPC beams exhibit the highest and lowest utilization factors of flexural capacity, respectively, and that of SFCB-UHPC beams falling in between. High-ductility UHPC enhances energy absorption, ductility, initial and secant stiffness. The reinforcement type has a minor impact on the energy dissipation of flexural beams. SFCB, on the other hand, enhances the ductility, initial and secant stiffness of specimens. Based on a simplified material constitutive model and fundamental assumptions, three failure modes for the SFCB-UHPC beam under bending were defined, along with their respective criteria. This enables the establishment of a simplified load capacity calculation formula. With reference to ACI440.1R-03, a stiffness calculation formula was developed to predict the deformation of SFCB-UHPC beams. This research can provide a technological reference for the design and analysis of SFCB-reinforced UHPC beams.

4

Experimental and numerical investigation on shear and bond-slip behaviors of FSK‑reinforced FRP-UHPC composite beams

Ge Wenjie, Zhang Zhiwen, Ashour Ashraf, Jiang Hongbo, Li Shengcai, Cao Dafu

Archives of Civil and Mechanical Engineering

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2024

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Vol. 24, no. 2

art. no. e87, 2024

EN

The shear performance of fiber-reinforced polymer (FRP)-ultra-high-performance concrete (UHPC) composite beams with FRP shear keys (FSK) was investigated through a four-point loading test and refined finite element (FE) analysis. In total, five test specimens having different concrete strength, concrete slab width and height as well as FSK spacings were experimentally tested. The test specimens were simulated using a refined FE model in ABAQUS. The concrete damaged plasticity model (CDPM) and the Puck failure criterion were adopted to simulate the progressive damage of concrete and FRP profiles, respectively. The mechanical behavior of the interface was captured using a bilinear cohesive zone model (CZM). The comparison between the FE analysis and experimental results demonstrated a good agreement. Based on the validated model, a parametric analysis was conducted on the shear performance of FRP-UHPC composite beams with FSK, focusing on parameters such as concrete slab strength, height and width, FRP web shear strength, shear modulus, height and thickness, and FSK spacing. The results indicate that the maximum local slip beam is less than 4 mm, which verifies that FSK has good interfacial shear resistance. Increasing the strength and section size of the concrete slab can improve the flexural stiffness and the shear capacity of composite beams. The use of UHPC for concrete slabs can also effectively inhibit interface slip. Increasing the shear strength and thickness of FRP web can result in improved load-carrying capacity and reduced deformation of composite beams. This can also lead to a shift in the failure mode from shear failure to bending failure. The reduction of FSK spacing can effectively enhance the shear performance of the interface, thereby improving the composite action and increasing the bearing capacity and deformation resistance of composite beams.

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A close look at fire-induced explosive spalling of ultra-high performance concrete: from materials to structures

Liu Jin-Cheng, Du Lin-Pu, Yao Yao, Beaucour Anne-Lise, Wang Jing-Quan, Zhao Xin-Yu

Archives of Civil and Mechanical Engineering

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2024

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Vol. 24, no. 2

art. no. e124, 2024

EN

The advent of ultra-high performance concrete (UHPC) represents a significant leap in concrete technology. Yet, the material’s vulnerability to fire-induced explosive spalling, characterized by concrete fragments being forcefully dislodged from the mass in fire scenarios, is the Achilles’ heel that could severely jeopardize UHPC’s integrity and hence structural safety. In response to this risk, there has been a growing interest in studying the explosive spalling of UHPC under fire exposure. This paper provides a critical review of the state-of-the-art research in this area. It looks into different experimental approaches for observing and demystifying fire-induced explosive spalling, then assesses how various factors (e.g., fiber type) affect UHPC’s propensity to such unfavorable events. Moving forward, the paper discusses numerical predictions of this phenomenon and, further, explains the consequences of explosive spalling on the fire resistance of UHPC components. Thus, the paper brings to light key insights from a large body of published literature. It also puts forward strategies to tackle this risk, with a focus on structural-level interventions, which have been largely overlooked in previous studies. The paper concludes by summarizing critical findings, highlighting ongoing challenges, pinpointing current knowledge gaps, and charting future research pathways.