To explore the feasibility of using ultra-high performance concrete (UHPC) to strengthen reinforced concrete (RC) beams in torsion, pure torsion tests on 9 specimens have been conducted in this study, including one RC beam without strengthening (referred as control beam) and 8 UHPC-strengthened beams. The variables considered for investigation included the wrapping form of UHPC layer, thickness of UHPC layer, shape of steel fibers added in UHPC layer, UHPC-RC interfacial treatment, and reinforcement ratio of UHPC layer. The overall torsional behavior, torque–twist curves, torque-principal strain curves, cracking pattern and UHPC-RC interfacial slippage for all specimens were obtained based on the torsion tests. Results show that the cracking and ultimate torsional moments of all strengthened beams were higher than those of the control beam, with maximum increases of 488.5% and 593.2%, respectively. When the RC beam cannot be fully wrapped, which is the preferred strengthening scheme, 3-sided wrapping form is also an acceptable choice, but the 2-sided strengthening scheme should be avoided. The determination of the thickness of UHPC layer should comprehensively consider the strengthening effect, cost for strengthening and requirements for RC beam's dimensions after strengthening. RC beam surfaces should be fully roughened before casting UHPC to ensure that the UHPC layers and the RC beam persistently act together as a whole to resist torsional loads. The UHPC layers are recommended to be equipped with steel bars, which could substantially improve the torsional capacity and ductility of the strengthened beam on the premise of limited increase in the cost. Finally, the effectiveness of torsional strengthening using UHPC was compared with that using fiber reinforced polymer (FRP) in terms of increase in cross-sectional area, increases in cracking and ultimate torsional moments, increase in torsional stiffness, failure mode and long-term performance. 

The feasibility of using ultra-high performance concrete (UHPC) to strengthen RC beams in torsion

This paper attempts to acquire fundamental knowledge on the mechanical properties of high strength steels and the corresponding high strength weld metals at arctic low temperatures. Two types of high strength steels (Q890, Q960) and three types of high strength weld metals (made of ER100S-G, ER110S-G, ER120S-G feedstock wires) were tested at arctic low and ambient temperatures ranging from −75 °C to 25 °C. Tensile coupon specimens for steel materials were directly extracted from high strength steel plates, whilst robotic gas metal arc welding was employed to fabricate the tensile specimens of weld metals. The tensile coupon specimens were designed as per ASTM E8M and wire-cut into shapes. Twenty-three tensile coupon tests on high strength steels and eighteen tensile coupon tests on high strength weld metals were carried out. Coupon specimens were tested in liquid nitrogen cooling chamber to mimic the arctic low temperature environment. The stress–strain responses and key mechanical properties of high strength steels and weld metals at both ambient and arctic low temperatures are presented and discussed. Prediction equations for key mechanical properties, including the Young’s modulus, yield stress and ultimate tensile strength, of high strength steels and weld metals at arctic low temperatures were proposed. 

Mechanical properties of high strength steels and weld metals at arctic low temperatures

Effect of bar diameter on bond performance of helically ribbed GFRP bar to UHPC

Flexural and compressive performance of BFRP-reinforced geopolymer sea-sand concrete beams and columns: Experimental and analytical investigation

The bond behavior of carbon fiber-reinforced polymer (CFRP) bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) is a pivotal factor affecting the performance of CFRP bar-reinforced UHPFRC structures. The present investigation delves into the bond performance of CFRP bars in UHPFRC by conducting tests on thirty-six pullout specimens. In these tests, the effects of steel fiber orientation, steel fiber content, and CFRP bar diameter were considered. The results revealed that specimens with 12 mm-diameter CFRP bars primarily failed in a splitting-pullout mode, whereas those with 10 mm- and 8 mm-diameter CFRP bars exhibited pullout failure. It was observed that specimens containing 45° oriented steel fibers have the maximum bond strength (∼35.30 MPa), as the cracks within UHPFRC were more effectively constrained. The bond strength was found to increase with an increase in steel fiber content, attributed to the improved cracking resistance of UHPFRC. The average bond strengths of specimens with fiber contents of 2% and 2.5% are respectively increased by 10.66% and 14.52% compared to those with a fiber content of 1.5%. The bond strength increases with the increase in CFRP bar diameter, owing to the greater rib height and rib height-to-inner diameter ratio of larger-diameter CFRP bars. Finally, a theoretical model was developed to predict the bond strength, with an average ratio of predicted results to experimental results of 0.93 and a standard deviation of 0.176. 

Bond performance of CFRP bars embedded in UHPFRC incorporating orientation and content of steel fibers

Fatigue failure mechanisms and probabilistic S-N curves for CFRP–steel adhesively bonded joints

This study develops an interpretable machine learning model for predicting the bond strength of CFRP-steel epoxy bonding interfaces and reveals key bond parameters. A total of 302 sets of experimental data were collected from the existing literature, including 16 influencing factors (i.e. the “features”) for the bond strength of the CFRP-steel bonded interface. Firstly, the sequential backward selection algorithms based on random forest (RF), maximum information coefficient (MIC), and distance coefficient (DC) were assessed. Then, Catboost and RF-based models were optimized and evaluated to determine the best prediction model. Finally, Shapley Additive Explanation (SHAP) was used to interpret the Catboost-based model. The results showed that the average Coefficient of Determination (R2) of the prediction for the test set by the Catboost and RF-based models was 95.5 % and 91.5 %, respectively, indicating the Catboost-based model has a higher prediction accuracy. The SHAP analysis indicates that the bond length, bond width, and the elastic modulus of the adhesive are the critical bonding parameters governing the bond strength of CFRP-steel epoxy-bonded interfaces. The Pareto diagram shows that the effective bond length is about 120 mm. This study provides valuable guidance for the design of bond performance and process optimization of CFRP-steel epoxy bonding interfaces. 

An interpretable machine learning model for predicting bond strength of CFRP-steel epoxy-bonded interface

This paper proposed a Si IGBT and SiC MOSFET hybrid switch (Si/SiC HyS)-based solid-state circuit breaker (SSCB) for the DC applications. At nominal current, the proposed Si/SiC HyS based SSCB has lower semiconductor device losses compared to the pure Si IGBT based SSCB. Compared to the pure SiC MOSFET solution, the proposed Si/SiC HyS SSCB solution has lower device cost. Moreover, benefit from the high overload capability of Si IGBT, the proposed Si/SiC HyS SSCB has higher upper limit of the current-time response profile. The experimental results show that compared to the pure SiC MOSFET-based SSCB, the proposed Si/SiC HyS-based SSCB has a lower surge voltage at the turn-off transient of semiconductor devices. The lower surge voltage helps to reduce the gate voltage oscillation and avoid the device false turn-on. The overload testing results show that the proposed Si/SiC HyS-based SSCB has higher overload capability than the pure SiC MOSFET-based solution. The proposed Si/SiC HyS-based SSCB can be a cost-effective protection device to improve electrical safety.

Si IGBT and SiC MOSFET Hybrid Switch-Based Solid State Circuit Breaker for DC Applications

Glued-laminated timber arches are widely used in gymnasiums, bridges, and roof trusses. However, studies on their mechanical behaviours and design methods are still insufficient. This paper investigates the in-plane loading capacity of circular glued-laminated timber arches made of Douglas fir. Experiments were conducted on four timber-arch models with different rise-to-span ratios under concentrated loads at mid-span and quarter-point locations. The structural responses, failure modes, and loading capacity of the timber arch specimens were obtained. The results show that the timber arches presented symmetric and antisymmetric deformation under mid-point and quarter-point loading conditions, respectively. The downward shifting of the neutral axis of the cross section was observed under mid-point loading condition, which contributes to higher loading capacity compared to that under quarter-point loading condition. The loading condition significantly affects the ultimate loads and the strain distribution in the cross section. Based on the design formula in current standards for timber structures, an equivalent beam-column method was introduced to estimate the loading capacity of the laminated timber arches under vertical concentrated loads. The moment amplification factor in the formula was compared and discussed, and the value provided in the National Design Specification for Wood Construction was recommended with acceptable accuracy.

/pdf/experimental-study-on-loading-capacity-of-glued-laminated-529prz4j2l.pdf

Experimental Study on Loading Capacity of Glued-Laminated Timber Arches Subjected to Vertical Concentrated Loads

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