Search Results (652)

This study investigates the behaviour of normal-strength concrete (NSC)–ultra-high-performance concrete (UHPC) composite slabs under four-point bending through experimental tests, finite element simulations, and analytical modelling. A total of 12 slabs were tested under static loading to examine the effects of UHPC layer position, UHPC thickness, reinforcement ratio in the UHPC layer, and shear span ratio on load-deflection behaviour, crack development, strain evolution, and failure mode. The test results showed that, when UHPC was placed in the tension zone, the slabs generally exhibited higher post-cracking stiffness and load capacity. The specimens with the longitudinal reinforcement spacing in the UHPC layer reduced from 100 mm to 50 mm exhibited an approximately 16% increase in ultimate load, but they were more prone to develop localised cracks and shear failure with a high reinforcement ratio in the UHPC layer and a small shear span ratio. When UHPC was placed in the compression zone, the specimens showed greater deformation capacities. With the same UHPC thickness, reinforcement ratio, and shear span ratio, their failure deflections were approximately 1.5–4.8 times those of the corresponding specimens with UHPC in the tension zone, whereas their failure was mainly governed by interface debonding. Finite element models were also developed and validated against the test results, followed by parametric analyses. The numerical results indicate that interface bond strength plays a dominant role in failure mode and deformation capacity, particularly for slabs with UHPC cast in the compression zone. Increasing the UHPC thickness generally improves the stiffness and load capacity, whereas insufficient thickness increases the risk of shear failure or interface debonding. Finally, an analytical model was proposed for composite slabs with different UHPC layer positions, and the model was shown to provide a reasonable prediction of the load-deflection curve of flexure-dominated slabs. Full article

Surface-breaking cracks in concrete structures can accelerate deterioration by facilitating the ingress of moisture, chlorides, and other aggressive agents. Reliable characterization of crack depth is therefore essential for structural health monitoring and maintenance of concrete infrastructure. This study presents an ultrasonic common midpoint (CMP)-based approach for crack-tip localization and crack-depth characterization in concrete. Ultrasonic measurements were acquired using a pitch-catch configuration in which the transmitter and receiver were positioned symmetrically on both sides of surface crack while maintaining a fixed midpoint. Measurements obtained at multiple transmitter–receiver separations were processed to extract the time-of-arrival (ToA) associated with crack-tip diffraction. The measured ToAs were subsequently used within a travel-time-based localization framework to generate crack-tip images and estimate crack-tip coordinates. The proposed methodology was evaluated on concrete slabs containing vertical and inclined surface-breaking cracks of varying depths. In addition, the approach was applied to a reinforced concrete beam specimen containing thin cracks caused by flexural loading. The localized crack-tip positions from ultrasonic imaging are in good agreement with the observed crack depths and geometries. The proposed method offers a non-destructive approach for crack-tip localization and crack-depth characterization in concrete and may support condition assessment of concrete infrastructure. Full article

This study proposes a deep-notched precast concrete slab system with post-cast connections for modular construction, aiming to improve on-site assembly efficiency while maintaining flexural continuity. A total of 22 full-scale slab specimens, including monolithic cast-in-place slabs and precast slabs with different notch depths, anchorage lengths, reinforcement diameters, and notch geometries, were tested under positive and negative bending conditions. The experimental results showed that, for the investigated reinforcement layout and loading conditions, the connected slabs with adequate anchorage achieved approximately 85% and 95% of the flexural capacity of the corresponding cast-in-place slabs under positive and negative bending, respectively. Anchorage length was found to be a key factor governing failure mode and load-transfer efficiency, while reinforcement diameter had a significant influence on ultimate capacity. Within the tested range, notch depth and notch geometry showed comparatively limited effects on flexural capacity, although their influence should be interpreted in relation to the adopted reinforcement arrangement and anchorage conditions. Finite element models based on the concrete damage plasticity approach were developed to interpret the load-transfer mechanism and parameter sensitivity. The models predicted the ultimate load with acceptable accuracy, while larger discrepancies were observed in some displacement estimates because of the simplification of interface behavior, bond-slip response, and local cracking. The experimental and numerical results indicate that load transfer across the joint was mainly achieved through reinforcement bridging, whereas the contribution of concrete bonding at the joint was limited under the investigated conditions. Based on these findings, a modified analytical model was proposed to predict the ultimate flexural capacity of the connected slabs. The model showed reasonable agreement with the test results for specimens with adequate anchorage. The proposed connection provides a feasible detailing solution for precast concrete slab connections in modular construction, but its application should be limited to configurations with comparable reinforcement layout, anchorage conditions, material properties, and monotonic bending behavior. Full article

Protective reinforced-concrete (RC) elements designed to resist contact blast loading must reconcile high energy dissipation with material and weight efficiency. This study examines HollowHex, an RC slab architecture in which periodic hexagonal cellular voids redistribute blast-induced stresses along inclined web-walls through a Vierendeel-type framing action. A full-factorial design of experiments across web thickness, charge mass, and hexagonal cell radius was carried out with Abaqus/Explicit using a concrete-damaged-plasticity model and mass-dependent Friedlander overpressure histories calibrated to UFC 3-340-02 scaled-distance relations. A six-level mesh-convergence study with three independent fine-mesh verification runs established the residual mesh effect as regime-dependent, bounded within approximately 13% in the elastic and severe-damage regimes and approximately 18% in the transition regime. Ten surrogate-model families—linear, polynomial, kernel, ensemble, and multilayer-perceptron—were benchmarked under leave-one-out, 5-fold, and 7-fold cross-validation. The best models achieved out-of-sample R² = 0.96 for peak displacement and R² = 0.93 for a continuous damage volume ratio (DVR), with train-to-validation gaps of only 0.03 and 0.06, indicating genuine generalization on the small dataset. A direct identical-condition comparison against circular-hollow slabs of matched void area shows blast-equivalent performance across the elastic, transition, and severe damage regimes (peak displacements within 2%, damage volume ratios within 7%), positioning the hexagonal architecture as a blast penalty-free alternative whose selection can be driven by non-blast criteria. A cross-validated parametric design heatmap is provided as a screening tool within the verified envelope. The uniform loading idealization is cross-checked against the spatially resolved CONWEP model, conservative on peak displacement by a factor of approximately 3.5, while approximately damage-equivalent and the constitutive model is validated at the damage level against documented contact-explosion tests through coupled FEM–SPH simulation. The findings position HollowHex not as a universally superior geometry but as a quantitatively beneficial alternative within the service/transition design range of greatest practical interest for blast protection. Full article

The connection between the space frame and its supporting columns is a critical aspect of structural design. The way these two elements are joined significantly influences the overall stability, load transfer, and behavior of the entire structure. This study investigates the seismic behavior of composite double-layer space frames with reinforced concrete (RC) slabs under three alternative support configurations: simple point supports, inverted-pyramid supports, and crosshead-beam supports. A detailed finite element (FE) model was developed in ABAQUS to conduct a nonlinear time-history analysis under recorded earthquake excitation. The selected record captures the temporal variation in ground motion and enables an accurate assessment of structural response under dynamic loading. The Concrete Damage Plasticity model was incorporated into the analysis of the concrete slab. The steel was modeled using an elastic–plastic material behavior to mimic the formation of plastic hinges. Maximum lateral displacements, base shear forces, and hysteretic behavior were studied to evaluate the comparative performance of the systems. The results show that the inverted-pyramid configuration reduced vertical (Y-direction) displacement by 33.6% compared to the simple point support. At the same time, the crosshead-beam system exhibited similar levels of vertical displacement. However, it exhibited significantly enhanced energy-dissipation capacity, with hysteretic force ranges up to 1250 kN, compared to approximately 922 kN and 860 kN for the simple point and inverted configurations, respectively. In the X-direction (lateral displacement), the simple support and inverted-pyramid cases showed comparable responses, whereas the crosshead-beam configuration exhibited greater displacement, indicating reduced lateral stiffness. Overall, the results of this study indicate that the inverted-pyramid system improves displacement control, and the crosshead-beam configuration provides superior hysteretic energy dissipation. These findings highlight the importance of selecting appropriate support configurations in the seismic design of composite space frame systems. Full article

Reinforced concrete (RC) slabs in buildings and protective structures are vulnerable to repeated low-velocity impacts caused by falling objects, vehicle collisions, industrial accidents, and successive debris strikes. Such repeated impacts can result in cumulative damage, progressive stiffness degradation, and eventual structural failure. Although externally bonded carbon fiber-reinforced polymer (CFRP) sheets have been widely adopted to improve the impact resistance of RC members, their effectiveness under repeated impact loading is often limited by premature debonding, while the contribution of mechanical anchorage systems to mitigating debonding and improving structural performance remains insufficiently understood. Accordingly, this study experimentally and numerically investigates the repeated low-velocity impact behavior of RC two-way slabs strengthened with externally bonded CFRP sheets incorporating boundary and distributed mechanical anchorage configurations. Four slab groups were investigated: unstrengthened control slabs (SL1), CFRP-strengthened slabs (SL2), CFRP-strengthened slabs with boundary anchors only (SL3), and CFRP-strengthened slabs with distributed anchors across the entire slab area (SL4). Repeated impact tests were conducted using a 92 kg drop weight released from progressively increasing heights until failure. The outcomes showed that strengthening of RC slab with CFRP sheets significantly improved the impact resistance at a 1.50 m drop height by reducing the residual displacement, crater diameter, and indentation depth by up to 67%, 55%, and 70%, respectively, compared with the control slabs. The incorporation of mechanical anchors further delayed premature CFRP debonding, maintained the CFRP–concrete bond, and enhanced the structural response, achieving maximum reductions of 77%, 63%, and 85%, respectively. Furthermore, the anchored slabs withstood repeated impacts from a 2.50 m drop height, whereas both the control and unanchored CFRP-strengthened slabs failed at a 2 m drop height. The developed finite element model accurately captured the structural response, CFRP debonding, anchorage failure, and damage evolution of the RC slabs, with good agreement between the numerical predictions and the experimental observations in terms of failure patterns, damage characteristics, and residual displacements. The proposed strengthening strategy and validated numerical model provide a reliable framework for assessing the effectiveness of different mechanical anchorage configurations and predicting the progressive failure behavior of CFRP-strengthened RC slabs subjected to repeated low-velocity impacts. Full article

An experimental morphology-based information extraction methodology is presented for locating reinforcing bars and estimating their effective ultrasonic scattering diameters in reinforced concrete (RC) members using ultrasonic pitch–catch (UPC) non-destructive testing combined with synthetic aperture focusing technique (SAFT) reconstruction. Reinforced concrete slab specimens with known reinforcement layouts were constructed and tested using a commercial ultrasonic device equipped with dry point contact shear wave transducers in a pitch–catch configuration. The collected ultrasonic data were post-processed using an in-house software package to reconstruct two-dimensional (2D) SAFT images of the specimens. In the reconstructed images, embedded rebars consistently produced characteristic bipolar scattering responses composed of dominant trough–crest waveform pairs. Based on these repeatable scattering response features, an empirical morphology-based procedure was developed in which rebar depth was estimated from the midpoint of the dominant trough–crest pair, while the effective ultrasonic scattering diameter was characterized using their vertical separation. The experimental results obtained from twelve reinforced concrete slab specimens demonstrated reliable depth estimation and a strong monotonic relationship between reconstructed scattering response width and nominal rebar diameter under the investigated testing conditions. Regression analysis indicated that larger rebars generally produced broader reconstructed bipolar scattering responses due to cylindrical wave interaction, diffraction, interference, finite transducer aperture effects, and SAFT reconstruction characteristics. The proposed methodology does not attempt to reconstruct the exact physical boundary of the reinforcement or solve the full elastodynamic inverse problem. Instead, the proposed methodology provides a practical and computationally efficient morphology-based information extraction framework that transforms reconstructed ultrasonic scattering responses into quantitative morphological descriptors for embedded reinforcement characterization. By extracting repeatable scattering response features from SAFT-reconstructed images and establishing an empirical mapping between these descriptors and reinforcement characteristics, the proposed approach enables the quantitative interpretation of ultrasonic images without requiring computationally intensive full waveform inversion. Full article

Construction-stage bridge fires are seldom documented in detail, although they can change the behavior of an incomplete structural system. This paper records a 35 m span of a six-cell RC box girder at the Al-Sadreen intersection in Samawa, Iraq, damaged after the bottom slab and webs had been cast and before the top slab was completed. Burning timber formwork locally removed the temporary soffit support. The open-top section, therefore, shifted from the intended fixed–pin construction-stage response toward a pin–pin-like response, with sagging and vertical web cracks near the intended fixed support. A closed-form check showed that the required negative-restraint moment was about 5.3–5.9 times the cracking moment of the incomplete section. Visual inspection, Schmidt hammer, UPV, cores, and steel tests showed localized damage; 28 MPa was used as a representative residual concrete strength for the affected cast components. CSiBridge was used only for completed rehabilitated-state verification. The strengthened model gave maximum shear D/C ≈ 0.529 and flexural D/C < 1.0. Spreadsheet-guided jacking, top-slab reinforcement upgrading, sensitivity checks, and a 350-ton five-lane load test confirmed satisfactory service behavior and negligible residual response. Full article

Water leakage in underground construction works is a prominent and persistent quality defect, particularly for the joint between the foundation slab and prestressed anti-floating anchors or piles. Previous studies have focused on optimizing the structural details to improve impermeability, while overlooking water seepage caused by insufficient adhesion at the interface between the polyethylene (PE) sheath and concrete. Therefore, this study aimed to enhance this interfacial adhesion through the hydrophilic modification of PE, thereby improving the impermeability of anti-floating anchors or piles prestressed with retarded-bond tendons. Physical blending modification was adopted in which hydrophilic PE granules were incorporated into ordinary PE. The variations in the water contact angle and mechanical properties of PE were analyzed at contents ranging from 0% to 8%. Adhesion strength tests were conducted to evaluate the changes in the interfacial adhesion strength between ordinary PE, modified PE, and concrete with different cement grades. Water impermeability tests were performed to measure the impermeability grades of concrete specimens reinforced with unbonded, ordinary retarded-bond, and modified retarded-bond prestressing tendons. The results showed that with increasing hydrophilic PE granule content, the hydrophilicity of PE improved markedly, while its mechanical properties improved slightly. A content of 8% hydrophilic PE granules is recommended. Debonding occurs between ordinary PE and concrete, whereas the adhesion strength of hydrophilic PE to concrete gradually increases with the cement grade. The impermeability grade of concrete with modified retarded-bond prestressing tendons is six grades higher than that with ordinary retarded-bond prestressing tendons, reaching P8. This indicates that the incorporation of hydrophilic PE granules significantly improves the impermeability of anti-floating anchors or piles prestressed with retarded-bond tendons. Full article

The anti-floating punching failure of basement slabs subjected to groundwater uplift remains insufficiently understood due to the complex stress state and lack of applicable design guidance. This study investigates the punching behavior of a damaged basement slab in Shenzhen, China, using a three-dimensional finite element model developed in LS-DYNA with the Concrete Damage Plasticity (CDP) model. The model was validated against field observations and experimental data, with a prediction error of less than 8%. The results show that anti-floating punching failure evolves from crack initiation in the anchorage zone to damage propagation and final penetration. Increasing the slab thickness from 400 mm to 600 mm significantly alleviated tensile damage concentration and improved stress redistribution. Increasing the concrete compressive strength from 20 MPa to 60 MPa enhanced punching resistance and delayed crack development, but promoted localized brittle failure. Enlarging the foundation pad from CT-6 to CT-9 effectively reduced stress concentration and improved the overall anti-punching performance, whereas the influence of column size was limited. A comparative assessment of three reinforcement measures further revealed their respective applicability under different engineering conditions. The study clarifies the anti-floating punching mechanism of basement slabs and provides a theoretical basis for the anti-floating design and reinforcement optimization of underground structures. Full article

This study investigates the blast-protective performance of RC slab strengthened on the blast face with various concrete protective layers under contact-detonation loading. The research focuses on analyzing shock wave propagation characteristics, peak pressures at measurement points, energy absorption capacities of the protective layers, the development of damage, and the governing failure mechanisms of the RC slab. The protective layers used for structural reinforcement include Steel Fiber-Reinforced Cellular Concrete (SFR-CC), Asphalt Concrete (AC), Rubberized Concrete (RBC), and Foamed Concrete (FC). Among these, the maximum support rotation angle of the structure strengthened with the SFR-CC concrete layer (T-1) is 0.20°, indicating significantly less damage and deformation compared to other protective schemes. Based on the damage coefficient calculated from the remaining sectional moment of inertia of the protected RC slabs, the destruction grades of the structures at different concrete protective schemes were classified. Among these, the SFR-CC layer exhibits the most effective attenuation of shock wave peak pressure. Additionally, the maximum support rotation angle of the structure strengthened with the SFR-CC concrete layer is 0.20°, indicating significantly less damage and deformation compared to other protective schemes. Damage grades were assigned according to a coefficient derived from the residual sectional moment of inertia of the protected RC slabs. The SFR-CC configuration (T-1) gives the lowest damage index, 0.178, approximately 64.5% below that of the NC scheme, and is classified as slight damage. In contrast to the severe damage sustained by the protected RC slabs strengthened with the NC concrete scheme, those strengthened with the AC, RBC, and FC protective layer schemes exhibit only a moderate damage grade. Empirical formulas predicting the damage index of protected structures under the combined effects of varying blast charges and concrete layer thicknesses were further developed for rapid damage assessment. Full article

Early-stage structural design requires rapid exploration of large design spaces, where the initial sizing of reinforced concrete members shapes downstream material use, cost, and the number of design iterations. Conventional predimensioning relies on experience and simplified formulae, while finite element analysis remains too slow for iterative use. This study presents a multi-task graph neural network surrogate that predicts per-element Eurocode compliance together with the required reinforcement for reinforced concrete slab-and-column buildings in one pass. A shared GraphSAGE encoder, trained on 2562 synthetic building graphs from automated finite element simulations, feeds one head for a compliance probability and another for reinforcement quantities. Because the rule-based Eurocode check is a hard pass-or-fail decision that does not vary smoothly with the design, the surrogate learns a continuous, differentiable compliance probability in its place, demonstrated for two representative criteria, one per element type, namely the $l/250$ deflection limit for slabs and the 4% reinforcement-ratio limit for columns. Across five random seeds, cost-sensitive focal-loss training that weights missed non-compliance above false alarms reached 90.9% balanced accuracy and held the share of non-compliant elements wrongly passed as compliant at 6.1% for columns and 1.6% for slabs, with a mean reinforcement error near 2% of the normalised target range. Inference averaged approximately 0.5 ms per building, between five and six orders of magnitude faster than the finite element analyses. A differentiable, multi-task graph surrogate therefore supports fast, cost-sensitive compliance screening for early-stage predimensioning, serving as a seed for gradient-based design exploration and a starting point for finite element verification. Full article

Reinforced concrete (RC) slabs, as the core load-bearing components in construction engineering, are prone to internal damage induced by impact loads, and accurate positioning of impact locations is a key task in structural health monitoring. The proposed method was developed for typical RC slabs such as building floors, bridge decks, and road slabs. Traditional acoustic emission (AE) positioning methods suffer from low positioning accuracy and a tendency to fall into local optimum when applied to RC slabs, which is attributed to the material’s heterogeneity, the complex propagation characteristics of stress waves and ambient noise interference. In this study, a GA-PSO hybrid algorithm is proposed, which integrates the global search capability of the Genetic Algorithm (GA) with the superior local convergence performance of the Particle Swarm Optimization (PSO) algorithm. The premature convergence issue of the traditional PSO algorithm is alleviated by adopting strategies including tournament selection, α hybrid crossover, boundary-constrained mutation, and linearly decreasing inertia weight. Based on the Time Difference of Arrival (TDOA) principle, the root mean square error between the theoretical and measured time differences is taken as the fitness function, and a boundary penalty mechanism is incorporated to ensure the physical validity of positioning results. AE data were acquired through drop weight impact tests to verify the performance of the proposed algorithm. Compared with traditional TDOA grid search, pure GA, and pure PSO methods under the same conditions, the proposed GA-PSO algorithm achieves an average localization error of only 54.95 mm, which is 61.0% lower than that of pure GA, while reducing the error standard deviation from approximately 114 mm to 24.87 mm. The average positioning error for all impact sources on the RC slab is within 100 mm, with the error in the central area as low as 42.97 mm. These results demonstrate that the GA-PSO algorithm significantly outperforms existing methods in terms of accuracy, stability, and maximum error control, verifying its high potential for impact source localization in complex heterogeneous materials. Full article

To balance load-bearing capacity and acoustic insulation, this study proposes a sandwich reinforced concrete (RC) slab with W-shaped interlayer inclined rebar connectors and investigates their influence on acoustic bridging. A three-dimensional vibro-acoustic finite element modeling approach was adapted to analyze airborne sound insulation and impact sound pressure levels in the frequency domain and was validated against experimental results. Parametric analyses were then conducted to evaluate the effects of connector number, connector geometry, anchorage-end treatment, and core-layer parameters. The results revealed distinct frequency-dependent behavior. The introduction of connectors produced a stable dip in airborne sound insulation near 400 Hz and a pronounced impact-sound peak within 200–400 Hz, both associated with connector-controlled coupled characteristic frequencies. Increasing the number of connectors strengthened interlayer stiffness coupling and intensified acoustic bridging in these frequency ranges. By contrast, optimizing the connector structure or introducing a compliant end layer reduced coupling and improved acoustic insulation. Overall, acoustic performance can be improved by reducing the equivalent coupling stiffness of the connectors, enhancing energy dissipation at the connector ends, and appropriately selecting the core-layer parameters. These measures help suppress the characteristic-frequency response and improve mid- to high-frequency sound insulation. Full article

Punching shear failure in reinforced concrete RC slabs is one of the most significant and detrimental failure modes due to its sudden nature and its dependence on a complex interaction between concrete strength, the reinforcement, and the loading conditions. In recent years, there has been increasing interest in utilizing sustainable cementitious materials and steel fibers as a way of enhancing structural performance and improving the durability of concrete. The study aims to assess the structural behavior of RC slabs utilizing a partial cement substitution with limestone powder (LP) and granulated blast-furnace slag (GBFS), with the addition of steel fibers. Twelve RC slabs were examined under uniform concentric loading to analyze cracking behavior, load–deflection relationship, stiffness variation, and ultimate punching shear strength. The results demonstrated that using limestone powder (LP) had a significant impact on the crack distribution pattern and resulted in a slight reduction in initial stiffness, with the load-bearing capacity decreasing to approximately 55.8% of the control mixture at high replacement ratios. Due to a slower hydraulic reaction than with other mixtures, increasing additional granulated blast-furnace slag resulted in a decrease in crack resistance and relative deformation. With a load-bearing capacity of approximately 92.9% of the control mixture, a tertiary mixture of limestone powder and granulated blast-furnace slag (GBFS) demonstrated a better balance in structural behavior, leading to improved crack control while maintaining a sufficient level of load-bearing capacity. The steel fibers also significantly contributed to enhanced post-cracking behavior by decreasing crack width and improving the stress redistribution mechanism within the RC slab. This led to increased punching shear resistance and enhanced energy absorption, with the ultimate load increased to 119 kN compared to the control mixture. Overall, the findings show that combining sustainable cementitious materials with steel fibers can effectively improve punching shear performance and enhance the efficiency and durability of reinforced concrete. Full article