Search Results (159)

Reinforced concrete (RC) structures are susceptible to crack initiation and propagation during service, making accurate numerical simulation of crack behavior essential for assessing structural durability and safety. Current numerical approaches for simulating concrete cracking include smeared/continuum approaches, extended finite element method (XFEM), phase-field methods, and meso-mechanical models. In particular, smeared/continuum approaches (e.g., smeared crack and plastic-damage models such as CDP) indirectly reflect cracking through diffusive damage fields without providing explicit geometric information on crack locations and propagation paths. The XFEM module in commercial software is further restricted to first-order elements and encounters difficulties in simulating multi-crack propagation. These limitations indicate that further development of complementary crack-simulation frameworks is warranted. To this end, this paper presents a cracking simulation framework for RC members within the theoretical framework of the Specified Stress Method, adopting an adaptive degree-of-freedom strategy to balance computational accuracy and efficiency. The method introduces inelastic strain as an additional unknown and establishes a variational principle and the corresponding virtual work equation. Concrete cracking is described by specifying the stress on the crack plane to zero, so that the crack-surface stress remains zero after cracking, thereby avoiding the issue of damage reversibility and improving computational convergence. The method requires neither a predefined crack path nor remeshing after cracking. Unlike smeared/continuum approaches that rely on diffusive damage fields, the crack propagation paths, distribution characteristics, and evolution of multiple cracks are characterized through the spatial distribution of cracked integration points within the finite element mesh. In the present implementation, crack initiation is governed by the maximum tensile stress criterion, and a linear elastic constitutive model is adopted for concrete as a deliberate simplification to establish and verify the core computational mechanism of the framework. The proposed method was examined through three numerical examples. First, comparison with theoretical solutions confirmed the algorithm’s correctness in simulating cracking in heterogeneous RC tension members. Second, comparison with experimental results demonstrated qualitatively consistent crack propagation trends and load–displacement responses for RC beams under mixed-mode cracking; the calculated ultimate load of the plain concrete beam is lower than the experimental value, which is attributable to the use of the maximum tensile stress criterion without fracture energy considerations, and certain crack morphology deviations are observed due to the neglect of reinforcement–concrete bond-slip. Third, a multi-crack simulation of an under-reinforced RC beam showed that, whereas the XFEM module in ABAQUS captures only a single dominant crack near the mid-span, the proposed algorithm predicts multiple distributed cracking zones on both sides of the mid-span, qualitatively consistent with the typical flexural cracking behavior of under-reinforced RC beams; the algorithm also supports second-order elements (e.g., C3D20R) unavailable in the ABAQUS XFEM implementation. While the method is still in an exploratory stage, these results confirm the feasibility and potential of the Specified Stress Method as a complementary framework for RC cracking simulation, providing a basis for further development. Full article

With the continuous advancement of urban renewal and the renovation and utilization of existing buildings, a large number of existing reinforced concrete slender columns face challenges in capacity evaluation. Quick and accurate calculation of their stability capacity, which represents the upper limit of member capacity, holds significant importance. These columns often exhibit plastic characteristics such as concrete cracking and steel yielding. Moreover, the bracing restraint provided by adjacent columns typically falls between that of a sway frame and a non-sway frame, classifying them as elastically restrained sway frame columns. Current design codes lack appropriate effective length factor tables for such columns, while the stiffness degradation induced by material nonlinearity is difficult to quantify accurately. To address these issues, the frame column is isolated from the overall structure and modeled as a rigid compression member system with three springs. The influence of bracing stiffness on the column’s critical load is revealed, leading to a formula for the elastic critical load of elastically restrained sway frame columns. Based on tests of reinforced concrete columns under compression, the influence mechanisms of eccentricity ratio and longitudinal reinforcement ratio on flexural stiffness degradation are elucidated. The obtained elastoplastic stiffness is then integrated into the stability calculation framework for elastically restrained sway frame columns, resulting in a method for determining the elastoplastic stability capacity of reinforced concrete columns that accounts for both geometric and material nonlinearities. This method avoids solving complex transcendental equations and offers a straightforward calculation process, providing a simple and practical hand-calculation tool for evaluating the stability capacity of reinforced concrete columns in existing buildings. Full article

Existing spillway piers constructed more than 60 years ago in Japan are usually reinforced with round rebars and have very low rebar ratios; consequently, their seismic response may be strongly influenced by post-cracking bond–slip. However, the applicability of nonlinear analytical models to such piers has not yet been systematically clarified. In this study, practical modeling strategies for existing spillway piers were investigated by performing Incremental Dynamic Analysis (IDA) using both a beam model based on nonlinear moment–curvature (M-φ) relationships and a 3D finite element analysis that explicitly accounts for bond–slip between concrete and rebar. An actual spillway pier was analyzed at multiple seismic intensity levels, and the effects of bar diameter, rebar ratio, and bond condition were examined via 3D finite element analysis. The results showed that the beam model is useful for global screening but may misclassify the damage mode because it cannot explicitly represent bond–slip. By contrast, the 3D finite element analysis reproduced flexure-dominant damage patterns and quantified the influence of bond–slip on maximum displacement, residual displacement, rebar strain, tensile damage distribution, and compression damage distribution. These differences became more pronounced under stronger ground motions and for larger bar diameters and lower rebar ratios. The findings support a staged strategy for seismic performance evaluation that combines beam models for global screening with 3D finite element analysis for detailed member-level assessment. Full article

The deflection of prestressed (pretensioned) concrete members strengthened with FRP requires a comprehensive evaluation. An extensive parametric study is performed using a rigorous analysis procedure based on a trilinear moment-curvature approach. There are 8100 pretensioned concrete beams analyzed by varying the cross-section dimensions, span length-to-depth ratio, shear span-to-span ratio, concrete compressive strength, prestressing reinforcement ratio, FRP strengthening ratio and FRP material properties. It was determined that the normalized effective moment of inertia at first yielding is statistically correlated with the normalized cracked moment of inertia, with an almost-perfect regression (R² = 0.9886). It was further found that when postulating the inverse of the effective moment of inertia in terms of a parabolic function of the beam maximum moment, the deflections of the cracked beam agree closely with experimental deflections. Boundary conditions for that equation are applied at the cracking and prestress-yielding points. Ultimately, it was realized that the immediate deflection predictions based on the modified beam effective moment of inertia expression proposed yield reliable deflection estimates for cracked prestressed members externally strengthened with FRP, compared with experimental results and other analytical predictions. Full article

Geopolymeric concrete beams are gaining increasing attention as sustainable structural members. The paper presents an experimental investigation on the deflection and cracking behavior of geopolymeric recycled aggregate concrete (GRAC) beams, with emphasis on effects of the longitudinal reinforcement ratio and the recycled aggregate (RA) replacement ratio. Using digital image correlation (DIC) technology, the failure modes, load–deflection curves, deflection characteristics, stiffness, and cracking behavior were systematically analyzed. The results indicated that increasing the reinforcement ratio leads to the same trend in GRAC beams as that observed in ordinary reinforced concrete beams. At 50% RA replacement, GRAC beams exhibit improved cracking resistance, 13.41% higher cracking stiffness, 6.93% lower deflection, and enhanced ductility compared to specimens without RA, attributed to the enhanced RA–matrix interface. However, a further increase in the RA replacement ratio leads to poorer flexural performance of the GRAC beams. In addition, predictive models for cracking moment, stiffness, deflection, and maximum crack width of GRAC beams were proposed based on the experimental results, incorporating the plastic influence coefficient, the comprehensive coefficient for the average strain at the extreme compression zone of concrete and the maximum crack width correction factor. The calculated values agreed well with the test data, offering a basis for structural design and engineering application. Full article

Real-time monitoring of surface cracks in reinforced concrete (RC) beams is critical to structural safety and service performance evaluation. Current structural crack monitoring still faces prominent scientific and technical bottlenecks: conventional unidirectional sensors cannot achieve multi-directional collaborative sensing, rigid piezoelectric materials exhibit poor compatibility with the large deformation of concrete, and there is a lack of quantitative mapping relationships from sensing signals to crack parameters, making it difficult to simultaneously measure crack width, angle, and morphology. This paper presents a novel ring-shaped piezoelectric sensor based on polyvinylidene fluoride (PVDF) and an annular piezoelectric sensing mechanism for real-time monitoring of crack angle, width, and morphology. The sensor incorporates a laminated structure with four strip sensing units for multi-directional strain detection. Experiments were conducted on RC beams under various loading conditions, and finite element analysis was performed using COMSOL Multiphysics. An innovative crack damage index (B) was introduced to assess structural damage quantitatively. Results demonstrate high sensor sensitivity and stable output. Voltage signals increase both with crack width and crack angle, showing responses of 0.045 mV, 0.041 mV, and 0.023 mV for crack angles of 60°, 45°, and 30°, respectively, at a crack width of 9 mm. Strong consistency between experimental and simulation data validates the effectiveness of the mechanism in monitoring the direction, width, and types of cracks. The crack damage index B exhibits a positive correlation with the structural stress response, enabling a quantitative assessment of damage. This study is applicable to the prestressed concrete box girders and T-beams commonly used in large-span bridges, which are typically with a main span of 20–50 m, a beam length of 6–30 m, a section height of 1.2–2.5 m, and designed for Grade C35–C50 concrete. The findings provide a practical foundation for real-time crack monitoring in large-scale bridge beam members. Full article

One-dimensional (1-D) tension stiffening is a fundamental behavior of structural concrete. It refers to the composite uniaxial behavior of a slender, symmetric concrete member of constant cross-section, bonded to a single reinforcing bar (rebar) along its axis. The rebar is subjected to tension by a pair of axial tensile forces applied at its ends. Despite the apparent simplicity of this configuration, the problem represents a cornerstone in RC mechanics. During the loading process, cracks are formed at different cross-sections along the structural member at stages where the tensile stress in the concrete at these cross-sections reaches the concrete tensile strength level. Each crack formation reduces the overall axial stiffness of the RC member, while inducing stress and strain redistributions in both the concrete and the rebar. The interaction between the concrete and the rebar is governed by the bond–slip relationship along their interface, which plays a critical role in controlling the transfer of stresses, the development of strains and the evolution of cracking. Most existing analytical and numerical models addressing this problem are based on simplifying assumptions assuming constant (deterministic) material properties and are denoted herein as “deterministic models”. Comparisons between analysis results of such models and experimental observations reveal substantial discrepancies in terms of the number of cracks, their spatial distribution, crack spacing, and the order of crack formation. Considering these inconsistencies, the present study postulates that the inherent variability of concrete properties, particularly its tensile strength, has a decisive influence on the structural response. To address this issue, the tensile strength of concrete is treated as a random variable characterized by the prescribed mean tensile strength and the coefficient of variation (CoV). The “stochastic analyses” with the variable tensile strength are conducted using an exact one-dimensional finite element formulation that explicitly accounts for discrete crack formation within the structural domain. These analyses yield results that differ markedly from those predicted by the deterministic approaches and exhibit characteristics that are in closer agreement with experimental evidence. These analyses indicate a more complex behavior of real structural members. It demonstrates that the CoV significantly influences the magnitude of cracking loads, crack locations, crack spacing, and the order of crack formation. The findings highlight the critical role of even slight material variability in tension stiffening behavior and justify the incorporation of concrete strength variability in tension stiffening modeling. Full article

Glass fiber–reinforced polymer (GFRP) reinforcement provides an effective alternative to conventional steel in concrete structures due to its corrosion resistance. Nevertheless, the lower elastic modulus of GFRP necessitates careful consideration of serviceability behavior in GFRP-reinforced concrete members. This study presents a numerical sectional analysis model for predicting the flexural response and ultimate capacity of hybrid reinforced concrete beams incorporating embedded GFRP profiles in combination with either mild steel or GFRP reinforcement bars under monotonic static loading. The proposed model employs realistic nonlinear stress–strain relationships for concrete and steel, together with secant moduli of elasticity evaluated at different loading stages. Particular emphasis is placed on detailed stress distribution in flexural sections, including the contribution of tension stiffening in the post-cracking regime. The formulation integrates nonlinear constitutive material behavior with theoretical sectional equilibrium to evaluate the effective flexural secant stiffness. For practical serviceability assessment and to reduce dependence on complex analytical procedures, strain vectors and stiffness matrix components are derived using elasticity coefficients that reflect modulus degradation obtained from numerical analysis. The accuracy of the model is verified through comparison with experimental results, including ultimate flexural capacity and moment–deflection responses. Many crucial parameters were studied, such as the longitudinal reinforcement ratio, type of reinforcement, concrete compressive strength, position of the I-GFRP profile, and rotation of the I-GFRP profile. The results of this study demonstrated that both the longitudinal reinforcement ratio and the rotation of the I-GFRP profile have a significant influence on the ultimate load capacity and deflection behavior. The close agreement between numerical predictions and experimental observations demonstrates the reliability and applicability of the proposed model for structural engineering analysis and design. Full article

Textile-reinforced concrete (TRC) beams have attracted widespread interest in recent years as an alternative to fiber-reinforced polymer (FRP) techniques. However, despite their effectiveness, they are often associated with high material cost, sensitivity to elevated temperatures, and limitations in bonding performance under certain environmental and surface conditions. This research examines incorporating textile reinforcement internally (INT) by supplementing steel bars with glass fiber grids, as well as externally (EXT) by retrofitting existing members. The experimental work evaluates five RC beams: a control (CTR), two INT beams strengthened with alkali-resistant glass fabric textile (AR-GFT), one using one layer (INT1L) and the other three layers (INT3L), and two EXT beams where AR-GFT is bonded with mortar, again with one layer (EXT1L) and three layers (EXT3L). Altogether, 10 beams were tested, with duplicate specimens for every configuration. Observing load-deflection responses, cracking behavior, and the strengthening system’s performance revealed that AR-GFT contributes to enhanced load-bearing resistance in the RC beams. The INT1L beams exhibited negligible improvement compared with the CTR specimen, suggesting that internal strengthening alone does not meaningfully increase strength. Conversely, the INT3L beams demonstrated a 45% rise in strength for one sample, although the second performed similarly to the CTR specimen owing to slippage between the textile and adjacent matrix. EXT3L beams achieved up to a 90% increase in load-bearing capacity in one specimen. Nevertheless, the second specimen exhibited textile layer debonding and performed similarly to the CTR beam, underlining the necessity for correct textile positioning and sufficient mortar impregnation during application. Moreover, a three-dimensional (3D) nonlinear finite-element analysis (FEA) was performed to replicate beam responses, showing strong correlation with experimental observations. Overall, the results indicate that textile-based strengthening systems can successfully retrofit and upgrade RC structures, provided meticulous attention is paid to the quality and execution of the installation process. The study provides new insights into the flexural behavior of textile-strengthened RC beams, particularly in terms of the interaction between internal and external textile reinforcement with conventional steel. Full article

This study examines the flexural behavior of slender ultra-high-performance fiber-reinforced concrete (UHPC) beams with cross-sections intended for scalable precast production. The members are prestressed only, with no passive reinforcement. An experimental program on eighteen beams combined three cross-sectional typologies (rectangular as a reference, I-shaped, and H-shaped), three UHPC mixes with fiber contents of 130, 160, and hybrid 130 + 60 kg/m³, and two prestressing layouts (bottom-only and symmetric top-and-bottom). Prestress was indirectly controlled by evaluating effective tendon stress, with time-dependent prestress losses quantified using vibrating-wire strain gauges. Four-point bending tests provided material characterization and structural response, enabling assessment of stiffness and ultimate capacity. The results highlight the coupled influence of cross-section, fiber dosage, and prestress configuration on global response. Post-cracking residual strength in UHPC promoted stable multiple cracking, while prestressing governed deflection control. Residual equivalent flexural tensile stresses above 35 MPa at deflections over 50 mm, span/70, were achieved in I- and H-shaped sections, exceeding those of rectangular sections. Overall, the study substantiates the feasibility of lightweight, durable, prestressed UHPC members that deliver significant self-weight reductions without compromising reliability. Full article

Steel reinforcement corrosion induced by chloride ingress in coastal environments is the dominant factor leading to the durability degradation of concrete structures. In this study, Ordinary Portland Cement (OPC) concrete beams and Portland Pozzolana Cement (PPC) concrete beams were used as test specimens, subjected to sustained loads to induce cracks, and exposed to accelerated reinforcement corrosion through 10 wet–dry cycles using a 3% NaCl solution. Testing methods including half-cell potential, corrosion current, and acoustic emission signals were employed to quantify the likelihood and progression of reinforcement corrosion. The results show that the half-cell potential of the loaded PPC beams remained below −350 mV, with a corrosion current density exceeding 0.5 μA/cm², indicating a significantly higher corrosion risk than that of the OPC beams; under unloaded conditions, the half-cell potential of the PPC beams remained consistently above −200 mV, with a corrosion current density below 0.2 μA/cm², exhibiting superior corrosion resistance. The event counts in the acoustic emission tests additionally revealed the progression of chloride ions gradually penetrating and corroding the steel reinforcement. Although PPC beams exhibit lower early-stage crack resistance under loading conditions and are prone to forming more cracks, their advantage in resisting chloride ingress becomes significant after appropriate mitigation measures are implemented to reduce early crack formation, making them remain a preferred material for reinforced concrete members in coastal environments. Full article

Alkali-activated concrete can reduce reliance on Portland cement by valorizing industrial by-products. This study evaluates slag-based alkali-activated concretes incorporating natural diatomaceous earth (M2, M3) and residual diatomaceous earth from industrial filtration (V6–V7), benchmarked against an OPC reference. The experimental program measures compressive, tensile and flexural strengths and elastic modulus, and examines steel–concrete bond behavior through bond stress–slip response at multiple slip levels. Member-level performance is assessed using reinforced beams tested under four-point bending, and cracking is compared in the constant-moment region using crack number and average spacing derived from post-test observations. Results show that diatom-based alkali-activated mixtures can achieve mechanical performance comparable to OPC concrete, with clear dependence on diatom source and mixture design. Bond response is markedly mixture-dependent and cannot be inferred from compressive strength alone. All beams exhibited flexural behavior suitable for structural applications, with the RV6 mixture providing the most favorable overall response among the tested members. These findings support the feasibility of residual diatomaceous earth as a viable component in structural alkali-activated concretes. Full article

The effect of steel fiber on the maximum crack width of steel fiber reinforced concrete (SFRC) members under normal service conditions was studied through tests on a series of 19 flexural beams, and the applicability of the crack spacing reduction coefficient of steel fiber in SFRC beams from Modified Rilem model that using for calculating maximum crack width was discussed, of which the reduction coefficient value was used to determine the influence of steel fibers on the crack width. Results show that the maximum crack width of the RC beam under bending conditions clearly decreased with the addition of fibers, and the reduction coefficient value of the crack width of the SFRC beam is greater when compared with that specified in the Modified Rilem model. The effect of steel fiber on reducing crack width was overrated, while the reduction coefficient proposed in the Modified Rilem model was used to calculate the maximum crack width. The probability of the reduction coefficient was analyzed based on experimental data collected from the relevant literature, and the reduction coefficients for different steel fiber types with various guarantee rates were obtained. Finally, the suggestion for the value of the reduction coefficient in the Modified Rilem model was proposed to be 1.19, which could be referred to for the code revision. Full article

This study addresses the limited availability of unified experimental datasets comparing ribbed steel and smooth FRP bars embedded in the same hybrid-fiber high-performance concrete (HPC) matrix under identical conditions. It investigates the mechanical and bond behavior of a triple-fiber HPC combining hooked-end steel (ST), basalt (BA), and polypropylene (PP) fibers and reinforced with steel, GFRP, and CFRP bars of identical diameter and embedment. Under a uniform curing regime, the HFRC reached a compressive strength of approximately 82 MPa and exhibited a high fracture energy G_f approximately 3.7 kJ/m² with a stable post-peak response in a notched-beam test, demonstrating effective multi-scale crack bridging within a dense hybrid fiber network. Pull-out tests on 200 mm embedment revealed distinct interfacial mechanisms: ribbed steel developed a pronounced peak bond stress (τ_max = 13.05 MPa) and the largest bond energy (G_b = 146 N/mm) due to mechanical interlock, whereas smooth GFRP and CFRP showed low τ_max (=1.46 and 0.78 MPa) and smoothly decaying τ–s governed by adhesion–friction with G_b = 3–4 N/mm. A consistent experimental framework enabled direct mechanistic comparison of bond–slip behavior across reinforcement types without confounding matrix or curing variables. Simple constitutive laws calibrated to the experimental τ–s curves (ramp–softening for steel and ramp–plateau or exponential for FRP) captured the stiffness, strength, and energy hierarchy with low error. The main contribution of this study lies in providing a configuration-consistent reference dataset and calibrated bond–slip descriptions for hybrid-fiber HPC members reinforced with both steel and FRP bars. The results highlight the role of the hybrid fiber network in improving crack stability and provide design-oriented parameters for anchorage assessment and nonlinear bond–slip modeling. Although the results are based on a limited experimental program, they establish a mechanistically coherent basis for further optimization of hybrid HPC matrices and development of performance-based anchorage formulations in high-performance structural applications. Full article

Sustainable materials are increasingly being incorporated into high-strength concrete (HSC) to reduce environmental impact while maintaining structural performance. This study experimentally investigates the combined use of steel scale waste (SSW) as a replacement for natural fine aggregates and graphite nano/micro platelets (GNMPs) as a nano-modifying additive in HSC. Natural sand was replaced with SSW at levels of 0%, 50%, and 100%, while GNMPs were incorporated at dosages of 0%, 0.1%, 0.3%, and 0.5% by weight of cement. The results indicate that partial replacement of sand with SSW significantly improves concrete density and mechanical performance due to enhanced particle packing and the high specific gravity of steel scale particles. At the nanoscale, GNMPs contribute to pore refinement, improved nucleation of hydration products, and crack-bridging within the cement matrix, thereby strengthening the interfacial transition zone and delaying crack propagation. The combined effect of these mechanisms produces a synergistic enhancement in concrete performance. The optimum mixture containing 50% SSW and 0.3% GNMPs achieved a compressive strength of 68.2 MPa and splitting tensile strength of 7.6 MPa, representing improvements of approximately 54% and 52%, respectively, compared with the control mix. Durability-related properties such as water absorption and sorptivity were also significantly improved due to matrix densification and pore structure refinement. Although the incorporation of SSW and GNMPs reduced workability, all mixtures remained within a practical range for casting. The developed concrete is particularly suitable for structural applications requiring high strength and durability, such as high-rise building components, bridge elements, and precast structural members. The findings demonstrate that the combined use of industrial steel waste and nano-reinforcement offers a promising pathway toward sustainable and high-performance concrete. Full article