Research

27 pages, 19764 KiB

Open AccessArticle

Theoretical Model for the Stress–Strain Curve of CNT-Reinforced Concrete under Uniaxial Compression

by Peng Zhu, Qihao Jia, Zhuoxuan Li, Yuching Wu and Zhongguo John Ma

Buildings 2024, 14(2), 418; https://doi.org/10.3390/buildings14020418 - 03 Feb 2024

Viewed by 470

The incorporation of carbon nanotubes (CNTs) can enhance the mechanical properties of concrete. The stress–strain curves of CNT-reinforced concrete under uniaxial compression are investigated through an experimental program with different CNT and steel fiber proportions considered. The test results demonstrate that CNTs can [...] Read more.

The incorporation of carbon nanotubes (CNTs) can enhance the mechanical properties of concrete. The stress–strain curves of CNT-reinforced concrete under uniaxial compression are investigated through an experimental program with different CNT and steel fiber proportions considered. The test results demonstrate that CNTs can increase both peak stress and peak strain, and steel fibers can further enhance the effect of CNTs. Additionally, steel fibers can effectively enhance both the strength and ductility. Theoretical models for the peak strain, initial elastic modulus, toughness index and relative absorbed energy are established. A theoretical model for the uniaxial compressive constitutive relationship of CNT-reinforced concrete considering CNT and steel fiber content is developed. Finite element (FE) modelling is developed to simulate the axial compression behavior of CNT-reinforced concrete. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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13 pages, 2800 KiB

Open AccessArticle

Transfer Length and Prestress Losses of a Prestressed Concrete Box Girder with 18 mm Straight Strands

by Xin Jiang, Haoxuan Chen, Yongjun Zhou, Lin Ma, Jianqun Du, Wei Zhang and Yunli Li

Buildings 2023, 13(8), 1939; https://doi.org/10.3390/buildings13081939 - 30 Jul 2023

Viewed by 710

Abstract

Despite the potential advantages of 18 mm strands, the limited research on the behavior of girders with larger-diameter strands hinders the application in bridges. Transfer length and prestress losses are two important indicators. In this research, a 32.6 m long prestressed concrete box [...] Read more.

Despite the potential advantages of 18 mm strands, the limited research on the behavior of girders with larger-diameter strands hinders the application in bridges. Transfer length and prestress losses are two important indicators. In this research, a 32.6 m long prestressed concrete box girder with 18 mm straight strands and 15 mm harped strands was produced, and the transfer length and the prestress losses were studied. The transfer length was calculated based on the existing equations in codes and previous research. Three beam specimens were fabricated, and strain gauges were pasted on the concrete surface to measure the transfer length of 18 mm strands. It indicated that the average measured transfer length was 700 mm. This value was smaller than the transfer lengths predicted by AASHTO LRFD 2017 and ACI 318-19, while Mitchell’s equation offered the closest prediction to the average measured transfer length. Additionally, the prestress losses at different stages were evaluated. A one-end stressing test was conducted to analyze the effect of strand harping on the loss of tensile force. In comparison with the actual measured loss based on the concrete strain and the longitudinal shortening, the instantaneous prestress loss calculated using the AASTHO LRFD 2017 alternative equation was appropriate. The time-dependent prestress losses due to shrinkage, creep, and relaxation were predicted using two different methods addressed in AASHTO LRFD 2017. The time-dependent predicted losses of 69.2 MPa at 28 d using the refined method were 37% higher than the measured losses 47.4 MPa at 28 d, indicating an overestimation of AASHTO LRFD 2017. The accumulation of the total losses over time revealed that the prestress losses developed in the first two months occupied the majority of the total losses in the long term. The research may provide guidelines for the design of a pretensioned concrete box girder with 18 mm strands. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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16 pages, 6340 KiB

Open AccessArticle

Low-Carbon Emission Demolition of an Existing Urban Bridge Based on SPMT Technology and Full Procedure Monitoring

by Wenqi Hou, Shiyang Liang, Tao Zhang, Tianzhu Ma and Yanqun Han

Buildings 2023, 13(6), 1379; https://doi.org/10.3390/buildings13061379 - 25 May 2023

Cited by 1 | Viewed by 1228

Abstract

Due to the need for comprehensive transportation hub construction, an existing bridge in a bustling urban area with an operation duration of 25 years was required to be demolished. Based on Life Cycle Assessment (LCA), this paper proposes a scheme of “Self-propelled modular [...] Read more.

Due to the need for comprehensive transportation hub construction, an existing bridge in a bustling urban area with an operation duration of 25 years was required to be demolished. Based on Life Cycle Assessment (LCA), this paper proposes a scheme of “Self-propelled modular transporter (SPMT) technology + large segment cutting” to compare the carbon emissions of demolition schemes qualitatively and quantitatively. To ensure structural safety during demolition, the finite element analysis was used to simulate the entire demolishing process, and measuring points were set up to monitor the deformation of the main girder in real time under various demolition conditions. The results indicate that the scheme of SPMT has the lowest carbon emissions during the demolition stage. Additionally, the long-term prestress loss shall be considered when demolishing existing bridges; the suggested 25% proportional value for the long-term prestress loss of the Caitian Bridge is appropriate, which is determined by comparing the calculated results from various formulas. The values of the calculated and measured deformations of the main girder under different working conditions are in good agreement, with errors mostly within 10% and a maximum of no more than 14.7%. The demolition of the entire bridge was completed in a total of 28 h with little noise and pollution, and the impact on daily traffic was avoided, proving that the proposed “SPMT technology + large segment cutting” scheme is safe, efficient, and achieves the goal of green, environmentally friendly, and rapid demolition. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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18 pages, 9131 KiB

Open AccessArticle

Research on Hybrid FRP–Steel-Reinforced Concrete Slabs under Blast Load

by Zebin Han, Wenjun Qu and Peng Zhu

Buildings 2023, 13(4), 1058; https://doi.org/10.3390/buildings13041058 - 18 Apr 2023

Cited by 2 | Viewed by 1224

Abstract

The service environment of civil air defense engineering structures is relatively harsh, and the corrosion of steel bars is the main reason for reducing the durability of concrete structures in civil air defense engineering. A hybrid FRP–steel-reinforced concrete (hybrid-RC) structure has excellent durability. [...] Read more.

The service environment of civil air defense engineering structures is relatively harsh, and the corrosion of steel bars is the main reason for reducing the durability of concrete structures in civil air defense engineering. A hybrid FRP–steel-reinforced concrete (hybrid-RC) structure has excellent durability. Therefore, it is a good choice to apply hybrid-RC to civil air defense engineering structures. In order to study the blast resistance of hybrid-RC structures, close blast and contact blast experiments were carried out on hybrid-RC slabs, steel-reinforced concrete (SRC) slabs and GFRP-reinforced concrete (GRC) slabs. For the close blast experiment, the steel reinforcement in the SRC slab first entered the plasticity stage, whereas the GFRP reinforcement in the hybrid-RC slab was in the elastic stage under the close blast. Therefore, the capacity to dissipate energy through the vibration in the hybrid-RC slab was better than that of the SRC slab. The residual deformation in the hybrid-RC slab after the close blast experiment was smaller than that of the SRC slab. The Blast Recovery Index (BRI) was introduced to evaluate the recovery capacity of the concrete slab after the close blast, and damage assessment criteria for the hybrid-RC slabs were proposed according to the maximum support rotation θ_m and BRI. There was little difference in the size of the local damage in the hybrid-RC slab and the SRC slab under the contact blast. However, since the GFRP reinforcement was still in the elastic stage and the steel reinforcement was plastic after the contact blast, the ratio of the residual bearing capacity to the original bearing capacity in the hybrid-RC concrete slab would be larger than that of the SRC slab. The prediction formula for the top face diameter D and blasting depth L of the hybrid-RC slab was obtained through dimensionless analysis. This research can provide a reference for the anti-blast design of hybrid-RC slabs. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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23 pages, 13890 KiB

Open AccessArticle

Prediction of the Long-Term Tensile Strength of GFRP Bars in Concrete

by Peng Zhu, Zongyang Li, Yunming Zhu, Yuching Wu and Wenjun Qu

Buildings 2023, 13(4), 1035; https://doi.org/10.3390/buildings13041035 - 14 Apr 2023

Viewed by 1188

Abstract

The durability of two types of widely used glass fiber reinforced polymer (GFRP) bars, one without coating (G1) and one with slightly surface sand-coating (G2), were studied through accelerated aging. Concrete cylinders reinforced with GFRP bars were immersed in tap water in temperature-controlled [...] Read more.

The durability of two types of widely used glass fiber reinforced polymer (GFRP) bars, one without coating (G1) and one with slightly surface sand-coating (G2), were studied through accelerated aging. Concrete cylinders reinforced with GFRP bars were immersed in tap water in temperature-controlled tanks. The influence of different exposure temperatures, 20, 40, and 60 °C, and also different exposure times, 30, 60, 90, 120, and 180 days, on the degradation of the two types of GFRP bars was investigated. The tensile strengths of GFRP bars after different exposure times were evaluated with tensile tests, and the variation of the microstructure and elemental compositions of conditioned specimens was evaluated with scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS), respectively. The degradation rate of the tensile strength retentions of two types of GFRP bars decreased with an increase in the exposure time at all exposure temperatures. The tensile strength retentions of the GFRP bars were studied by three commonly used prediction models. Based on the degradation mechanism of fiber-matrix debonding, a new model was proposed. These four models were evaluated with the test results and a new model proposed was suggested as the best model to predict the residual tensile strength of the GFRP bars. The durability parameters of the GFRP bars were discussed. The tensile strength retention tended to converge to a constant value (52%) with the increase of exposure time, which contributes to the determination of the environmental reduction factor in relevant design guides, and the fiber-matrix debonding was found to be the main degradation mechanism due to the surrounding concrete environment. The sand-coating had some effect on the activation energy of the GFRP bars. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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20 pages, 4930 KiB

Open AccessArticle

Stress–Strain Model of High-Strength Concrete Confined by Lateral Ties under Axial Compression

by Lei Wang, Xiaokun Huang and Fuquan Xu

Buildings 2023, 13(4), 870; https://doi.org/10.3390/buildings13040870 - 26 Mar 2023

Viewed by 1721

Abstract

High-strength concrete can effectively reduce the cross-sectional size, increase space usage, and cut material costs. To analyze the mechanical properties of high-strength concrete vertical members, various confinement models have been proposed to define the ties-confined concrete stress–strain relationship. However, most existing models are [...] Read more.

High-strength concrete can effectively reduce the cross-sectional size, increase space usage, and cut material costs. To analyze the mechanical properties of high-strength concrete vertical members, various confinement models have been proposed to define the ties-confined concrete stress–strain relationship. However, most existing models are divided into ascending and descending segments. These are continuous but not derivable at the peak point, which does not facilitate numerical calculations. Moreover, these models have a large number of parameters that are mostly obtained based on the fitting of experimental data, which may also lead to the limited applicability of the models. In this study, existing confinement models for high-strength concrete under axial compression are reviewed, and the differences between the models are discussed. Based on the results of normal triaxial experiments on high-strength concrete and the test data from other studies on ties-confined concrete columns, the effective confinement coefficient and empirical formula of ties strain at the peak stress of confined concrete are proposed. A confinement model is proposed based on the continuous derivable function, and it is validated based on the available experimental data. Results show that the proposed model can reflect the stress–strain relationship of the test specimens more simply while keeping the basic accuracy with other models. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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18 pages, 10932 KiB

Open AccessArticle

Flexural Tensile Behavior of Interface between Precast and Cast-in-Place UHPC Members Based on Four-Point Bending Test

by Hong Cai, Zhifeng Liu, Ziyi Xu, Ziyang Zhang and Tengfei Xu

Buildings 2023, 13(3), 745; https://doi.org/10.3390/buildings13030745 - 12 Mar 2023

Cited by 3 | Viewed by 1214

Abstract

This paper proposed a new method of exposing fibers using expandable polyethylene (EPE) foam to improve the interface performance. The flexural tensile behavior of the interface between precast steam-cured and cast-in-place non-steam-cured ultra-high performance concrete (UHPC) members was examined under four-point bending. The [...] Read more.

This paper proposed a new method of exposing fibers using expandable polyethylene (EPE) foam to improve the interface performance. The flexural tensile behavior of the interface between precast steam-cured and cast-in-place non-steam-cured ultra-high performance concrete (UHPC) members was examined under four-point bending. The improvement effects of four non-destructive interface treatments—e.g., coating with epoxy glue and coating with a cement-based interface agent—were experimentally studied for comparison. The load-deflection curves of the specimens were measured, and the flexural tensile behavior of the interfaces was analyzed, including the failure mode, flexural tensile strength, ductility, fracture energy, and residual strength. The results indicate that the flexural tensile strength of an interface without any treatment is low and that coating with the cement epoxy glue or the cement-based interface agent has a limited effect on improving the flexural tensile strength. As a result of the bridging effect, the exposed steel fibers can transfer the tensile force across the interface. Therefore, exposing fibers using EPE foam can significantly improve the bond behavior of an interface. This enhancement effect depends on the number of exposed steel fibers and can be simulated based on the pullout load–slip relationship of the steel fiber embedded in the UHPC matrix. The proposed numerical model can be used to predict the bond behavior of an interface with the fiber-exposing treatment. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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17 pages, 4788 KiB

Open AccessArticle

Study on the Performance of Active Embedded Steel Wire Knot Form in Silicone Graphene Composite Thermal Insulation Structure Integrated System

by Xiaoxia Zhao, Dajiang Geng, Zhiqiang Cheng, Zhicheng Bai, Mingjian Long, Yang Chen, Qingqing Lu and Wei Ying

Buildings 2023, 13(3), 705; https://doi.org/10.3390/buildings13030705 - 07 Mar 2023

Cited by 4 | Viewed by 1106

Abstract

Based on the advantages of the silicone graphene composite thermal insulation board, it was used to replace traditional plywood in the external wall formwork system, and the active embedded steel wire knot form in silicone graphene composite thermal insulation structure integrated system was [...] Read more.

Based on the advantages of the silicone graphene composite thermal insulation board, it was used to replace traditional plywood in the external wall formwork system, and the active embedded steel wire knot form in silicone graphene composite thermal insulation structure integrated system was designed. Firstly, the theoretical model of steel wire drawing resistance was established by theoretical analysis method, and the rationality of the theoretical model was verified by combining relevant experimental data. The relationship between multiple variables and steel wire pull-out resistance was analyzed. Then, combined with the theory of wind pressure strength of the exterior wall of a building structure, the layout form and the corresponding number of embedded steel wires of thermal insulation board under different building heights were analyzed. Finally, the silicone graphene composite thermal insulation board and ordinary plywood were compared and analyzed from the force of perspective of external wall formwork. The results showed that the pull-out resistance of steel wire was directly proportional to the diameter of steel wire, embedded depth, and embedded deflection angle. With the increase of building height, the number of steel wires to be arranged also increased. When the thickness of the silicone graphene composite thermal insulation board is not less than 80 mm, the anti-deformation effect is close to that of the ordinary plywood, which can meet the construction requirements of the external wall formwork. It can ensure the energy conservation and thermal insulation of the external wall, integrate the building’s exterior wall and thermal insulation structure of the building, and achieve the purpose of exemption from formwork removal. Full article

(This article belongs to the Special Issue Latest Advances of Green Concrete Technology in Civil Engineering)

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