Search Results (189)

The study presents a comprehensive analysis of the effects of high temperatures (500 °C and 700 °C) on the microstructure, mechanical properties, and acoustic emission (AE) parameters of cold-drawn prestressing steel. The investigations included mechanical testing, AE signal acquisition, and numerical verification using the finite element method (FEM). It was demonstrated that increasing temperature leads to significant microstructural changes (pearlite spheroidisation, carbide coarsening), resulting in strength degradation and a shift in the failure mechanism from quasi-brittle (initial state) to transitional (500 °C), and finally to ductile (700 °C). For the first time, AE parameters (Counts to Peak and RA-value) were correlated with local axial strains ε₂₂ and von Mises equivalent stress, enabling the identification of the moment of onset load-bearing capacity loss and the determination of critical material damage thresholds. A multi-criteria diagnostic indicator was proposed to assess the condition of prestressing steel after fire exposure. The results confirm the high potential of AE as a non-invasive tool for evaluating the safety of prestressing tendons and cables in reinforced concrete structures subjected to overheating or fire. Full article

The SBPDN (Steel Bar Prestressed Deformed Normal relaxation) bar, which has ultra-high yield strength yet much lower bond resistance than conventional deformed bars, has been recently proposed to be used as the longitudinal rebar instead of a normal-strength deformed bar to simply realize strong earthquake-resilient concrete components. To facilitate and promote the application of concrete components reinforced with SBPDN rebars to the structures located in earthquake-prone regions, it is indispensable to develop reliable and effective anchoring means and clarify the bond strength of SBPDN bars embedded in concrete and/or grout mortar. This paper presents experimental information on the pull-out tests of fifteen SBPDN bars embedded in grout mortar, along with a discussion on the effective anchorage details and the bond strength of SBPDN bars. The tested SBPDN bars have a nominal diameter of 22.2 mm, the maximum diameter currently available on the market. All SBPDN bars were embedded in high-strength grout mortar with a targeted compressive strength of 60 MPa. The primary experimental variables included the end anchorage details, the diameter of sheath ducts, and the embedded length of the bars. Test results demonstrated that either screwing two nuts and a washer at the end of SBPDN bars or providing a rolling-threaded end region was effective in preventing them from premature slip from grout mortar. If the embedment length was 20 times the bar diameter or longer, the proposed two anchorages could ensure the SBPDN bar to fully develop its specific yielding strength as high as 1275 MPa. In addition, it has also been experimentally revealed that the bond strength of SBPDN bars embedded in grout mortar was much lower than that of conventional deformed bars and varied between 2.84 MPa and 3.98 MPa. Full article

The paper presents two innovative measurement methods for assessing the flexural performance of precast concrete beams that are prestressed with Carbon Fiber-Reinforced Polymer (CFRP) bars. Strains, displacements, crack development, and failure modes were recorded until failure occurred, using a combination of Distributed Fiber Optic Sensing (DFOS) and Digital Image Correlation (DIC) techniques. This approach provides a more comprehensive understanding of the behavior of CFRP-prestressed beams under load, allowing for more accurate predictions than traditional measurement systems. By integrating both techniques, it is possible to validate results and establish an effective monitoring system. Based on tests conducted on three CFRP-prestressed beams subjected to three-point bending, general recommendations are made for both DFOS and DIC measurement techniques for reinforced concrete (RC) members. DFOS is more effective at detecting minor strains, while DIC excels in measuring large strains in RC structures. Both DFOS and DIC techniques facilitated the monitoring of crack development in RC structures, providing detailed analyses of the location, number, spacing, and width of the cracks. However, beyond the cracking load, the DFOS results can become unreliable due to the impact of cracks on the fiber optic readings. Therefore, DFOS and DIC measurement techniques can be used complementarily, taking into account their respective limitations. Full article

This study presents an amplitude-based non-destructive testing (NDT) approach for estimating reinforcement bar diameter in reinforced concrete members using ground-penetrating radar (GPR). The novelty of this work lies in the use of normalized amplitude-diameter-depth (NADD) relationships, which link the reflected electromagnetic wave amplitude to both rebar diameter and cover depth through an exponential attenuation model. Normalization was applied to remove the influence of varying signal energy and antenna coupling, thereby allowing consistent comparison of amplitudes across different depths and improving the reliability of amplitude-depth interpretation. The NADD equation was developed from GPR measurements obtained on a reinforced concrete slab containing bars with diameters ranging from 9.5 mm (#3 bar) to 25.4 mm (#8 bar) and then validated using data from three prestressed concrete box beams recovered from a decommissioned bridge managed by the West Virginia Department of Highways. The normalized amplitude prediction error (E_a) was calculated to quantify model performance. The minimum mean error of approximately 4.7% corresponded to the 12.7 mm (#4 bar), which matched the actual reinforcement used in the beams. The results demonstrate that the proposed normalization-based approach effectively captures the amplitude-depth-diameter relationship, offering a quantitative framework for interpreting GPR data and improving the evaluation of reinforcement characteristics in existing concrete structures. Full article

This study quantifies shear and flexural stiffnesses and their changes over time to support structural health monitoring of in-service bridge superstructures across four girder types: reinforced concrete (RC) beams, prestressed concrete (PSC) girders, steel girders, and ultra-high-performance concrete (UHPC) sections, using field ambient vibration testing. A total of 20 bridges across Georgia and Iowa are assessed, involving over 100 hours of on-site data collection and traffic control strategies. Results show that field-measured natural frequencies differ from theoretical predictions by average of 30–35% for RC, and 20–25% for PSC, 15–25% for steel and 2% for UHPC, reflecting the complexity of in situ structural dynamics and challenges in estimating material properties. Site-placed RC beams showed stiffness reduction due to deterioration, whereas prefabricated PSC girders maintained consistent stiffness with predictable variations. UHPC sections exhibited the highest stiffness, reflecting superior performance. Steel girders matched theoretical values, but a span-level test revealed that deck damage can reduce frequencies undetected by localized measurements. Importantly, vibration-based measurements revealed reductions in structural stiffness that were not apparent through conventional visual inspection, particularly in RC beams. The research significance of this work lies in establishing a portfolio-based framework that enables cross-comparison of stiffness behavior across multiple girder types, providing a scalable and field-validated approach for system-level bridge health monitoring and serving as a quantitative metric to support bridge inspections and decision-making. Full article

Incorporating Nitinol (NiTi) shape memory alloy (SMA) into concrete structures has gained significant attention in recent years due to its ability to enhance the properties of concrete. This review paper illustrates the history of NiTi SMA and its use in various civil engineering structural applications. A detailed analysis of the existing literature and case studies offers perspectives on the possible applications, benefits, and prospects of utilizing NiTi SMA to reinforce and strengthen elements in concrete structures. The study examined publications on the internal usage of NiTi SMA in concrete and cement-based matrices as an embedded element, including fibers, bars, cables, wires, powder, and strands. In addition, superelastic and shape memory forms of NiTi were considered. It was concluded that the superelasticity of NiTi aided in energy dissipation from impact or seismic events. It also improved the re-centering performance and deformation capacity and reduced residual stresses, strains, and cracks. Conversely, the SMA effect of NiTi helped bridge cracks, recover the original shape, and induced prestressing forces under thermal activation. Full article

Prestressed concrete structures are facing serviceability challenges due to rising live loads, material degradation, and seismic demands. Retrofitting with carbon fiber-reinforced polymer (CFRP) offers a cost-effective alternative to full replacement. This study presents a finite element (FE) modeling framework to simulate the seismic performance of prestressed concrete T-beams retrofitted in the negative moment region using near-surface-mounted (NSM) CFRP rods and sheets. The model incorporates nonlinear material behavior and cohesive interaction at the CFRP–concrete interface and is validated against experimental benchmarks, with ultimate load prediction errors of 4.41% for RC T-beams, 0.49% for prestressed I-beams, and 1.30% for prestressed slabs. A parametric investigation was conducted to examine the influence of CFRP embedment depth and initial prestressing level under three seismic conditions. The results showed that fully embedded CFRP rods consistently improved the beams’ ultimate load capacity, with gains of up to 10.84%, 16.84%, and 14.91% under cyclic loading, near-fault ground motion, and far-field ground motion, respectively. Half-embedded CFRP rods also prove effective and offer comparable improvements where full-depth installation is impractical. The cyclic load–displacement histories, the time–load histories under near-fault and far-field excitations, stiffness degradation, and damage contour analysis further confirm that the synergy between full-depth CFRP retrofitting and optimized prestressing enhances structural resilience and energy dissipation under seismic excitation. Full article

The first concrete structures post-tensioned with unbonded tendons were constructed in the 1950s. Despite the popularity of such a type of construction solution, the theory describing the behavior of members with unbonded prestress remains relatively unknown. Different standards, provisions, and theories described by scientists can be found in the literature. The main problem is related to determining the value of the prestressing force and its increments because it is dependent upon the member rather than the section due to a lack of bond between the concrete and the tendons. Both theoretical and experimental studies enable the definition of parameters that have an influence on stress increase. Three of the most important of these parameters were investigated in tests conducted by the authors. This paper presents the findings of an experimental study conducted on twelve simply supported RC beams that were prestressed with unbonded tendons. A total of twelve elements were grouped according to various criteria, including their span-to-depth ratio, prestressed reinforcement ratio, and type of loading. All beams had a low reinforcing bars index, which met the Eurocode 2 requirements. The aim of this research was to check if such a level of ordinary reinforcement ratio will enable the achievement of a satisfactory crack pattern and also a high stress increase in unbonded tendons. The members were tested to investigate their behavior and the stress increment in tendons in terms of their load-carrying capacity. Full article

This study examines the prestress transmission behavior in near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP)-strengthened reinforced concrete structures, with particular emphasis on the effects of temperature. Experimental tests were conducted to evaluate the tensile and shear properties of epoxy adhesives under a range of curing temperatures (20–100 °C) and ambient service temperatures (0–80 °C). The results reveal an inverse exponential relationship between curing time and temperature. Notably, adhesive strength declines significantly above 60 °C and the adhesive loses functionality at 80 °C. Building on these findings, an analytical model was developed to predict prestress transfer length, CFRP strain distribution, and interfacial shear stress. The model incorporates effective bond stiffness and a prestress reduction coefficient to account for varying prestress levels (10–50%). Parametric analyses identify the CFRP elastic modulus, cross-sectional geometry, adhesive thickness, and degree of curing as critical factors influencing prestress transmission. The model’s predictions were validated against experimental data, demonstrating its reliability. Overall, this work provides a theoretical foundation for optimizing the design of NSM CFRP-strengthened structures under complex thermal conditions. Full article

Conventional strengthening measures for existing structures are usually not effective for the self-weight, which accounts for around 70% of the total load in reinforced concrete structures. Therefore, their effect on the overall load-bearing capacity is low. A self-weight-effective alternative for flexural strengthening is the thermal prestressing of additional reinforcement installed on the structure. In this method, reinforcing bars are slotted into the tensile zone, embedded in filler material, and tempered from the outside. They are thermally stretched, and once cooling starts, the bond with the hardened filler prevents re-deformation. The induced prestressing force counteracts dead loads and relieves the tensile zone, making the additional bars effective for the self-weight. In this paper, the effectiveness of the strengthening method is experimentally investigated in the serviceability and the ultimate limit states. Experiments involve strengthening a reinforced concrete beam under load by a thermally prestressed additional bar. Moreover, two reference tests are made to evaluate the method. An unstrengthened beam characterizes the lower capacity limit. Another beam with the same reinforcement amount as the strengthened one, but completely installed at casting, serves as the upper benchmark. All beams are loaded until bending failure. The strengthening method is assessed by means of the load-bearing behavior, deflection, crack development, and the strains in the initial as well as the added reinforcement. The results demonstrate the effectiveness of the strengthening method. The thermally prestressed bar achieves an effective pre-strain of approximately. 0.4‰ by heating at about 70 °C. The induced prestressing force and associated compression reduce tensile cracks by approx. 45% and increase stiffness. The strengthened beam reaches the maximum load of the upper benchmark, but with about 33% less deflection. The filler, which also expands thermally, generates an additional prestressing force that is effective up to about 20% of the load capacity. Beyond this, the filler begins to crack and its effect decreases, but the pre-strain in the reinforcing bar remains until maximum load. Full article

This paper presents an analysis of the mechanical characteristics of bridge structures during both construction and operation phases, with a focus on stress distribution patterns and the impact of vehicle loads on structural safety. The monitoring during the construction phase indicates that the compressive stress of the main beam segments is mainly controlled by prestress, and the maximum compressive stress meets the specification requirements; the maximum tensile stress of the main beam occurs in the stage when the tension reinforcement of the top pier is under stress, and the tensile stress value is within the allowable range of the specification. Under the negative bending moment of the pier top, the tensile stress at the upper edge reaches the peak simultaneously with the pre-pressurization stress. In contrast, the tensile stress at the mid-span joint transfers to the lower edge, and the corresponding bending moment significantly decreases. Based on the maximum tensile stress theory, when the stress of the structure caused by the earthquake wave reaches the ultimate tensile strength of the concrete, it is prone to cause structural damage. Therefore, it is necessary to limit the vehicle weight and driving speed to reduce the vibration impact. According to the “Regulations on the Management of Over-limit Transport Vehicles on Highways” issued by the Ministry of Transport (the total designed load shall not exceed 55 tons), after calculation, it is known that the maximum allowable driving speed of a 60-ton vehicle is 81.4 km per hour, which exceeds the safety limit of the specification. The research shows that in actual operation, the driving speed needs to be dynamically controlled according to the vehicle weight to ensure the long-term safety and durability of the bridge structure. Full article

In response to issues of long construction cycles, high pollution, and labor shortages in traditional cast in situ subway station construction, a refined 3D model of a large-segment prefabricated subway station was established using ABAQUS software 2024, with mechanical behavior throughout the construction process studied based on the Shenzhen Huaxia Station project case. The model incorporates a concrete inelastic damage constitutive model and a steel elastic–plastic model, accurately simulates key components, including dry joints of mortise–tenon grooves, prestressed reinforcement, and bolted connections, and implements a seven-phase construction sequence. Research findings indicate the following: (1) During component assembly, the roof vault settlement remains ≤3.8 mm, but backfilling significantly increases displacements (roof settlement reaches 45 mm, middle slab deflection measures 66.91 mm). (2) Longitudinal mortise–tenon joints develop stress concentrations due to stiffness disparities, with mid-column installation slots identified as vulnerable zones exhibiting maximum Von Mises stress of 32 MPa. (3) Mid-column eccentricity induces structural asymmetry, causing increased deflection in longer-span middle slabs, corbel contact stress differentials up to 6 MPa, and bolt tensile stresses exceeding 1.1 GPa. (4) The arched roof effectively transfers loads via three-hinged arch mechanisms, though spandrel horizontal displacement triggers 5 cm rebound in diaphragm wall displacement. Conclusions confirm overall the stability of the prefabricated structure while recommending the optimization of member stiffness matching, avoidance of asymmetric designs, and localized reinforcement for mortise–tenon edges and mid-column joints. Results provide valuable references for analogous projects. Full article

Strain monitoring during the service life of a nuclear containment structure is an effective means to evaluate whether the structure is operating safely. Due to the failure of embedded strain sensors, surface-mounted strain sensors should be installed on the outer wall of the structure. However, whether the data from these substitute sensors can reasonably reflect the internal deformation behavior requires further investigation. To ensure the feasibility of the added strain sensors, a refined 3D model of a Chinese Pressurized Reactor (CPR1000) nuclear containment structure was developed in ANSYS 19.1 to study the internal and external deformation laws during a containment test (CTT). Solid reinforcement and cooling methods were employed to simulate prestressed cables and pre-tension application. The influence of ordinary steel bars in concrete was modeled using the smeared model, while interactions between the steel liner and concrete were simulated through coupled nodes. The model’s validity was verified against embedded strain sensor data recorded during a CTT. Furthermore, concrete and prestressed material parameters were refined through a sensitivity analysis. Finally, the variation law between the internal and external deformation of the containment structure was investigated under typical CTT loading conditions. Strain values in the wall thickness direction exhibited an essentially linear relationship. Near the equipment hatch, however, the strain distribution pattern was significantly influenced by the spatial arrangement of prestressed cables. Refined FEM and sensor systems are vital containment monitoring tools. Critically, surface-mounted strain sensors offer a feasible approach for inferring internal stress states and deformation behavior. This study provides theoretical support and a technical foundation for the safe assessment and maintenance of nuclear containment structures during operational service. Full article

Ensuring the stability of monolithic concrete slabs during construction represents a crucial safety challenge in monolithic reinforced concrete buildings. Theoretical models and structural analyses often assume ideal conditions of supporting props. However, significant deviations occur in practice due to variations in technical condition and installation methods. This study investigates the magnitude of prestressing forces generated in adjustable telescopic steel props depending on manual tightening and hammer blows. Experimental measurements were conducted on different types of props compliant with EN 1065, including both new and worn specimens, to simulate real on-site conditions. The influence of worker body weight was also analyzed. The results confirmed that the technical condition of the prop is the decisive factor affecting the level of prestress. Props in poor condition achieved substantially lower and inconsistent prestressing forces, while new props subjected to five hammer blows reached maximum values up to 13.16 kN. This difference can significantly influence static calculations for slab construction. Contrary to expectations, the influence of worker body weight was not statistically significant; instead, the dominant role was played by installation technique and the accuracy of hammer blows. The findings contribute to the optimization of safety guidelines and the improvement of calculation models for temporary support systems in monolithic construction. Full article

Current research primarily focuses on using CFRP materials to strengthen small or medium-sized test specimens. To address this, our study employed ABAQUS software to analyze the axial compression behavior of large-scale reinforced concrete (RC) columns strengthened with prestressed carbon fiber reinforced polymer (CFRP) sheets. We conducted comparative analyses on key parameters: the prestress level applied to the CFRP, the width of CFRP strips, the spacing between strips, the confinement ratio, and the overall load–displacement curves of the columns. The results demonstrate that applying prestress significantly improves the efficiency of stress transfer in the CFRP sheet, effectively mitigating the stress lag phenomenon common in traditional CFRP strengthening, leading to a substantially enhanced strengthening effect. The CFRP wrapping method critically impacts performance: increasing the confinement ratio enhanced ultimate load capacity by 21.8–59.9%; reducing the strip spacing increased capacity by 21.8–50.4%; and widening the strips boosted capacity by 38.7–58%. Although full wrapping achieved the highest capacity increase (up to 73.2%), it also incurred significantly higher costs. To ensure the required strengthening effect while optimizing economic efficiency and CFRP material utilization, the strip wrapping technique is recommended. For designing optimal reinforcement, priority should be given to optimizing the confinement ratio first, followed by adjusting strip width and spacing. Proper optimization of these parameters significantly enhances the strengthened member’s ultimate load capacity, ductility, and energy dissipation capacity. This study enriches the theoretical foundation for prestressed CFRP strengthening and provides an essential basis for rationally selecting prestress levels and layout parameters in engineering practice, thereby aiding the efficient design of strengthening projects for structures like bridges, with significant engineering and scientific value. Full article