Concrete Members Research Articles

Experimental and FEM evaluation of the influence of interlayer bonding strength in 3D printed concrete members under compressive and flexural loadings

The use of 3D concrete printing has remarkably increased in the building industry recently. However, the buildability and mechanical qualities of the printed concrete still depends on various influential elements. The purpose of this research was to investigate the impact of the interlayer bond on the anisotropic mechanical behavior of 3D printed concrete after 7 and 28 days of curing, under compressive and flexural loading of in and out of plane laminated surfaces while varying the concrete materials. The mechanical properties of the 3D printed concrete specimens were tested, and the results revealed that the compressive and flexural strengths were strongly dependent on the loading direction of the laminated surface. Analytical modeling of the specimens was performed using the LS-DYNA tool to further validate the experimental results. The effects of the interfacial friction bond and nozzle diameters on compressive and flexural strengths were investigated. Four alternative interlocking configurations, such as “I,” “T,” “S,” and “V”-shaped forms, were examined to increase strength by improving mechanical interlocking in the layers and the laminated surface, as well as interface responses.

FRP strip wraps for enhancing the fire resistance of RC beams strengthened with CFRP sheets bonded using the EBROG method: An experimental study

In recent decades, the application of fiber-reinforced polymers (FRP) composites in strengthening structural members has increased due to their low weight, high strength, and easy implementation. However, one of the main weaknesses of these materials is that as the temperature rises, the resin resistance and, consequently, the bond strength between the FRP and concrete member rapidly diminish. This causes the FRP to separate from the concrete substrate and prematurely fail the member. Therefore, previous studies have widely suggested fire protection materials to mitigate this weakness. However, it increases costs and exacerbates architectural problems. To eliminate the need for fire protection systems or reduce their thickness, this paper evaluates the combined effect of using FRP mechanical anchors and utilizing the EBROG Grooving method on reinforced concrete beams for flexural strengthening FRP. A total of six rectangular control beams were tested under flexural loads at ambient temperature, and four beams were examined under simultaneous flexural loading and fire conditions. The proposed anchorage system included two-layer wrapped CFRP strips along the beam axis at specified intervals and fan anchorage with holes with 120 mm spacing. To assess the effectiveness of the proposed anchorage systems and determine the maximum capacity of the beams, they were first examined at ambient temperatures. The results showed that under standard fire conditions, due to the formation of a tangential cable mechanism, RC beams strengthened with one and two layers of flexural CFRP and anchored with two-layer wrapped CFRP strips withstood constant loads of 7,85 KN and 9.8 KN for 77 and 66 min, respectively. Also, the RC beam resisted fire for up to 106 min under the applied load.

Neighborhood component analysis-based feature selection in machine learning to predict tendon ultimate stress of unbonded prestressed concrete beams

Precisely estimating tendon ultimate stress in prestressed concrete beam members while accounting for the impact of various structural parameters holds practical significance in Structural Engineering. Existing approaches in design codes often rely on simplified empirical formulas, frequently falling in accurately representing outcomes across diverse conditions. This paper proposes a novel application of ensemble learning to predict tendon ultimate stress, leveraging relevant parameters identified through Neighborhood Component Analysis (NCA). NCA is strategically utilized for feature selection, enhancing prediction performance and reducing computational cost. An innovative integration of Gradient Boosted Regression Trees (GBRT) with Bayesian optimization for hyper-parameter tuning is introduced, trained, and validated using a robust database of 251 experimental results from simply supported prestressed concrete beams, encompassing a wide range of conditions and structural configurations. The study employs Individual Conditional Expectation (ICE) analysis to elucidate the correlation between input variables and predictions, providing deeper insights into parameter influence. Additionally, the GBRT model is benchmarked against state-of-the-art machine learning algorithms, including Artificial Neural Networks (ANN), Support Vector Machine (SVM), Decision Trees (DT), and Random Forest (RF), demonstrating a good prediction accuracy with R2 of 0.9330. Detailed correlation analysis and benchmarking results highlight the robustness and reliability of the proposed model, underscoring its potential to improve the design of prestressed concrete beam members and offering a significant improvement over traditional empirical approaches.

Multi-Dimensional Iterative Constitutive Model of Concrete under Complex Stress Conditions in Composite Structures

In composite structures or complex concrete members, some concrete bears multiple forces, called core concrete. The properties of the core concrete are variable under complex stress conditions, which will influence the structure performance analysis. Therefore, it is necessary to establish an accurate and theoretical constitutive model of concrete under complex stress conditions. The elastic–plastic properties of concrete in complex stress conditions were analyzed first. Then, the failure criterion of concrete in complex stress conditions was discussed to identify the key parameters. And the relationship between the stress–strain curve and failure criterion was analyzed through mathematical derivation. Finally, the multi-dimensional iterative constitutive model of concrete under complex stress conditions was established and verified. Based on the analysis results, the concrete under multi-axial stress conditions shows a spindle-shape stress envelope diagram. The failure criterion should be established by the analysis of concrete under high multi-axial compression conditions, tension–compression conditions, and shear–compression conditions. The plastic modulus is the key to reflecting the plastic strain development trend and the stress–strain relationship.

Full-scale experimental investigation of prestressed concrete bridge girders strengthened with aluminium channels

The bridge network in South Carolina encompasses more than 9,000 structures, many of which were originally constructed to accommodate H10 (89 kN) or H15 (134 kN) truck loads and thus require strengthening to carry current loading standard (HL-93). The South Carolina Department of Transportation (SCDOT) has recognized that prestressed concrete channel bridge girders, a specific type of bridge superstructure, often face challenges in achieving adequate load ratings, based on flexural strength. Hence, it is imperative to explore strengthening methods for these girders. Aluminium alloys possess desirable properties that make them attractive as external reinforcement materials. While previous studies have investigated the use of aluminium alloys on small-scale reinforced concrete members, there is a lack of investigation on their use on full-scale prestressed concrete bridge girders. To address this gap, this study investigates the feasibility of utilizing aluminium alloy channels as an external reinforcement material on full-scale prestressed concrete channel bridge girders. The main goal of this paper is to propose a strengthening method for prestressed concrete channel girders that is cost effective and easily implemented in the field to reduce the number of load-posted bridges in the state of South Carolina. Nine girders from recently decommissioned bridges in South Carolina were tested under flexural loading conditions to failure. The test program consisted of six unstrengthened girders, one strengthened with bonded aluminium channels (SE), one strengthened with bonded and bolted aluminium channels (SEB), and one strengthened with bolted aluminium channels (SB). Before testing, a visual inspection of the girders was conducted to identify any existing deterioration, and each girder was given a condition rating based on the Specifications for the National Bridge Inventory (SNBI). Theflexuralicate that externally anchored aluminium alloy channels with bolts was the most effective strengthening method in terms of practicality and increase in the flexural strength. Moreover, the results also revealed that the measured flexural strength of the girders varied based on the extent of the observed girder deterioration.

Machine-Learning Methods for Estimating Performance of Structural Concrete Members Reinforced with Fiber-Reinforced Polymers

AbstractIn recent years, fiber-reinforced polymers (FRP) in reinforced concrete (RC) members have gained significant attention due to their exceptional properties, including lightweight construction, high specific strength, and stiffness. These attributes have found application in structures, infrastructures, wind power equipment, and various advanced civil products. However, the production process and the extensive testing required for assessing their suitability incur significant time and cost. The emergence of Industry 4.0 has presented opportunities to address these drawbacks by leveraging machine learning (ML) methods. ML techniques have recently been used to forecast the properties and assess the importance of process parameters for efficient structural design and their broad applications. Given their wide range of applications, this work aims to perform a comprehensive analysis of ML algorithms used for predicting the mechanical properties of FRPs. The performance evaluation of various models was discussed, and a detailed analysis of their pros and cons was provided. Finally, the limitations that currently exist in these techniques were pinpointed, and suggestions were given to improve their prediction precision suitable for evaluating the mechanical properties of FRP components.

Out-of-plane impact behavior and post-impact damage evaluation of GFRP bar-reinforced concrete shear walls in fire

Fiber-reinforced polymers (FRP) have been a prospective material in engineering. However, the attenuation of FRP bar-reinforced concrete members in fire is unclear. This paper studies the impact resistance of glass fiber-reinforced polymer bar-reinforced concrete (GFRP-RC) shear walls under high temperatures. Considering the impacts of strain rate enhancement and high temperature softening on GFRP bars and concrete, a numerical model with finite elements in three dimensions was created. To verify the model, three cases were simulated and compared with the test records, including the seismic performance of FRP-RC shear walls under axial compression, the seismic behavior of reinforced concrete (RC) shear walls under high temperatures, and RC shear walls' impact response. Based on the calibrated numerical model, the displacement, impact force, reaction force, failure mode, energy distribution, internal force and the residual shear bearing capacity of GFRP-RC shear wall subjected to impact under different fire times were analyzed. The influence of fire time on the failure mechanism of GFRP-RC shear walls was studied. Taking the mid-span displacement as the damage index, the evaluation criteria for the four damage levels were determined. The findings indicate that the type of reinforcement has little effect on the shape of impact force, reaction force and displacement time history curve. The peak values of impact force and reaction force of RC walls are larger than those of GFRP-RC shear walls, while the former has smaller peak displacements and residual displacements. The damage of shear walls becomes more severe as the fire time increases, as does the displacement. Also, the impact force, reaction force, shear force and bending moment decrease gradually. The energy absorbed by shear walls is primarily dissipated by concrete, the part by GFRP bars accounting for a small proportion. The post-impact residual bearing capacity of GFRP-RC shear walls is linearly related to the mid-span displacement under impact loads, primarily influenced by high-temperature exposure.

Long-Term Behavior of Timber–Concrete Composite Structures: A Literature Review on Experimental and Numerical Investigations

Timber–concrete composite structure is a type of efficient combination form composed of concrete floors and timber beams or floors through shear connectors, and shows good application potential in the floor system of timber buildings. The long-term performance of the timber–concrete composite structures is complex and is affected by the creep of timber and concrete, as well as the long-term slip of the shear connectors. This article presents a comprehensive overview of the research status on the long-term behavior of timber–concrete composite members and different shear connectors. For the shear connectors, the effects of loading levels, environments, and component materials on their creep coefficients are summarized. As to the timber–concrete composite members, both the experimental and numerical investigations are gathered into discussions: the connection types, component materials, loading conditions, and durations in the long-term tests are also discussed; various models for describing long-term behavior of timber, concrete, and connection systems are provided, and then a comprehensive description of the progress of numerical investigations over the last decades is made. In addition, the suggestions for future research are proposed to reach a clearer understanding of the bending mechanisms and mechanical characteristics of timber–concrete composite structures.

Aggregate interlock and effective strain of FRP-strengthened flanged RC members subjected to torsion: Experimental evaluation and analytical modeling

The effective strain of externally bonded FRP strengthening systems and the loss of concrete aggregate interlock are critical factors for predicting strength and developing design models for structural strengthening. Current research on FRP strengthening of reinforced concrete (RC) members primarily focuses on flexural, shear, and axial loading, neglecting torsional systems. This study introduces an innovative analytical-experimental model/formulation predicting FRP effective strain and concrete strain, based on Digital Image Correlation (DIC) analysis outcomes of an experimental study on RC beams strengthened with FRP sheets under torsion. Eight flanged RC beams, strengthened with various FRP ratios, configurations, and installation methods were also systematically tested. The findings emphasize the need for reevaluation of strain limits in existing guidelines. It points out that the shear integrity of concrete is influenced by concrete compressive strength and strengthening scheme. The DIC results suggest that the strain limit for concrete surfaces between FRP strips should be in the range of 0.0055–0.008, as opposed to the limit of 0.004 set by ACI 440.2R-17. The use of the grooving method in certain systems also requires an increase factor of 1.65 and 1.45 for the effective strain of transverse and longitudinal FRP sheets.

Experimental and numerical assessment of GFRP and synthetic fiber reinforced waste aggregate concrete members

Improper disposal of plastic waste (P-waste) poses significant pollution and health risks to the environment. Additionally, steel rebar corrosion in concrete structures reduces their strength and serviceability. To address these issues, this study explores using glass fiber-reinforced polymer (GFRP) rebars and replacing natural coarse aggregates with P-waste aggregates for sustainable and eco-friendly construction. This work investigates the axial performance of polypropylene structural fiber-reinforced P-waste aggregate concrete (SFPC) compressive components having GFRP-reinforcement (GSFPC) under various loading conditions. For comparison, steel-reinforced SFPC compressive components (SSFPC) were also fabricated. Eighteen circular components with 1200 mm height and 300 mm diameter were tested and compared under different loading conditions. SSFPC components exhibited up to 21.9 % higher axial strength but up to 27.4 % lower ductility compared to GSFPC components. Eccentric loading similarly reduced axial strength in both GSFPC and SSFPC components. A 3-D finite element analysis (FEA) of GSFPC components was proposed using a modified damaged plastic model for SFPC which showed deviations of 2.3 % in axial strength and 7.7 % in equivalent axial shortening, demonstrating a good accuracy of the FEA model.

A refined model for the flexural behavior of reinforced UHPC members under sustained loading

The long-term behavior of reinforced Ultra-High Performance Concrete (UHPC) members is significantly influenced by UHPC’s creep properties. To predict the long-term performance of structural members, it is essential to have both a material-level creep model and a constitutive model for structural analysis. This paper presents a refined model for the long-term performance of UHPC, incorporating critical nonlinear deformation mechanisms: instantaneous damage, plastic strain, creep strain, and time-dependent damage. The constitutive model for instantaneous damage and plastic strain mechanisms is based on experimental data from short-term cyclic loading tests on UHPC specimens. Additionally, the creep strain components and time-dependent damage evolution laws are derived from well-established linear and nonlinear creep models, validated for cementitious materials through extensive research. The model parameters were meticulously calibrated using creep damage test results from UHPC. The validation process included linear and nonlinear creep tests on UHPC cylinders under sustained stress levels ranging from 0.2fc to 0.66fc, creep failure tests on UHPC cylinders under sustained stress levels from 0.85fc to 0.95fc, and long-term bending tests of reinforced UHPC beams under sustained load. In the linear and nonlinear creep tests, comparison between the calculated ultimate creep coefficients and the experimental results demonstrates the model’s validity under low and moderate stress levels. In the creep failure test, a comparative analysis of strain evolution and creep lifetime between experimental and model results confirms the model’s validity under high sustained stress levels. Similarly, during the long-term bending test, simulated time-dependent displacement and compressive strain show good agreement with the experimental data, confirming the model’s accuracy in predicting the long-term performance of reinforced UHPC members. Furthermore, to streamline the constitutive model, the instantaneous damage effect was simplified and ultimately neglected. A comparison of simulation results between the original model and the simplified version demonstrates that disregarding the instantaneous damage effect is a safe approach for practical engineering applications.

Investigating the efficiency and reliability of different lap-splice configurations in NSM BFRP rods for strengthening RC beams

The use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement has shown great promise for flexural and shear strengthening of reinforced concrete (RC) members. However, the continuity of NSM FRP rods is often interrupted due to length constraints, necessitating lap splices to transfer forces between spliced rods. The performance of lap splices depends on design configuration factors, including anchorage shape, splice length, and connection method. This research describes an experimental investigation into the bond behaviour and failure mechanisms of various lap splice designs for NSM basalt FRP (BFRP) rods in RC beams. The parameters investigated were end anchorage shape (straight or Embedded Through-Section (ETS) anchors), and connection type (overlay rods or face-to-face with FRP sheet wrap). A total of 8 simply supported RC beams strengthened with different NSM BFRP lap splice configurations were tested under four-point bending. The load-deflection response, strain distribution, and failure modes were evaluated. Test results showed that beams with NSM FRP rods with ETS anchors exhibited higher bond strength and ductility than face-to-face connections. The outcomes of this study can help optimize lap splice design and detail provisions for NSM FRP systems in future strengthening applications.

Bond Properties of CFRP Externally Bonded Reinforcement on Groove in Concrete

Externally bonded reinforcement on groove (EBROG) is a significant reinforcement technology proposed by researchers to enhance the bond properties of reinforced concrete structural members. To understand the influence of groove size on concrete specimens of different strength, a total of 60 concrete specimens with 4 different strengths were cast with the single shear test in this paper, among which 48 EBROG specimens and 12 specimens with externally bonded reinforcement method (EBR) were used as the control group. The failure modes and failure mechanisms of specimens with various sizes and reinforcement methods were analyzed. Additionally, the test results of ultimate load, load–displacement curves, and bond-slip curves for specimens with different groove sizes were compared. The effectiveness of EBROG method in enhancing the ultimate load capacity at the bond interface of the specimens is proved. Furthermore, in situations where the volume of the groove was kept constant, the specimens with lower concrete strength and deeper groove exhibited superior bond properties. Also, the influence of groove width on bond properties was better than that of groove depth. Finally, the test results in this paper were compared with the prediction of the existing EBR and EBROG models to evaluate the performance of different models, and based on the original model, a prediction model for EBROG method in the groove region with higher accuracy was proposed.

Implantable sensing technology for civil engineering structures

Providing crack location and stiffness variation information using practical structural health monitoring technology is highly significant for in-service structures. Inspired by the fast development of implantable devices, this paper introduced an implantable sensing technology (IST) for health monitoring of a typical concrete member known as shear wall undergoing loading progression. Two collaborative monitoring steps were developed to acquire targeted damage information at different loading stages, the one was, a cylindrical concrete implantable module (CCIM) sensor-enabled IST was applied for monitoring and imaging the early cracks of the shear wall at the initial loading stage; and the other was, a concrete implantable bar (CIB) sensor-enabled IST was employed for monitoring and assessing the damage evolution at the mid-late loading stage. Experimental results showed that these two collaborative ISTs can accurately identify the early cracking regions of the shear wall and reveal its stiffness degradation processes throughout the loading.

Impact-Echo for Crack Detection in Concrete with Artificial Intelligence based on Supervised Deep Learning

Aging concrete infrastructure such as bridges and tunnels need to be appropriately maintained in their service life because these structures must be employed with ensured safety level. Inspection and maintenance with reasonable measures are typically operated for the reinforced concrete before severe damage and failure are developed. Especially, the crack propagation in concrete can significantly affect the structural integrity, durability and overall performance of structural elements. Effective maintenance strategies are required for material and structural assessment with detection and repair of cracks to prevent further deterioration. Identification of cracks in concrete is effectively operated to extend the service life of structures for mitigating the potential safety hazards and minimize the repair costs. In order to detect the cracks in the concrete structure member, non-destructive testing (NDT) system can be often introduced to satisfy the technical issues in terms of damage evaluation and estimation of repair condition. Currently, various NDT methods have been utilized for quantitative analysis of internal damage/defects using elastic wave method such as impact-echo method, focusing on reflection of P-wave in concrete as wave propagation behavior. In experimental laboratory tests, concrete specimens are usually prepared with interior damages (cracks) which are simulated as embedded plastic plate or styrofoam. In this study, starch- type polysaccharide sheet is used to form the simulated cracks in concrete specimen. This study aims at applying impact-echo method with AI (Artificial Intelligence) to identify the internal crack information based on the waveform characteristics obtained from the elastic wave propagation behavior. The waveform results are converted to frequency spectrum by FFT analysis. Consequently, it is found in this study that AI, which is employed by supervised deep learning models, successfully evaluates the data and displays the probability of crack existence in concrete. Since FFT is a powerful algorithm that quickly converts a signal from the time domain to the frequency domain, it gives efficient analysis to the impact echo method for crack detection with the application of AI system.

Theoretical Analysis of Behavior of FRP-Strengthened Reinforced Concrete Members Subjected to Combined Torsion and Biaxial Bending

Theoretical Analysis of Behavior of FRP-Strengthened Reinforced Concrete Members Subjected to Combined Torsion and Biaxial Bending

Cementitious composites for structural strengthening of reinforced concrete members: A review

Abstract Existing concrete buildings are always in need of repair or renovation as they are getting older, carrying more weight, or needing to be redesigned. The most common materials used to strengthen concrete structures are ferrocement, fiber-reinforced polymer, engineered cementitious composite, and ultra-high-performance concrete. However, there are some other methods and materials that can be used as well. There are pros and cons to every composite material that is unique to it. Each material works very well in certain situations and is known around the world for being able to be used in a wide range of situations. This paper gives a brief overview of several cement-based strengthening materials that have shown very promising results when used to repair older structures. This review looks at certain factors, such as how to retrofit a structure for bending, shear, and confinement. In conclusion, more studies should be done to come up with flexible industry standards for the use of cementitious materials to strengthen structures made of reinforced concrete.

Flexural and Shear Strengthening of Reinforced-Concrete Beams with Ultra-High-Performance Concrete (UHPC)

Ultra-high-performance concrete (UHPC) is considered to be a promising material for the strengthening of damaged reinforced concrete (RC) members due to its high mechanical strength and low permeability. However, its high material cost, limited code provisions, and scattered material properties limit its wide application. There is a great need to review existing articles and create a database to assist different technical committees for future code provisions on UHPC. This study presents a comprehensive overview focusing on the effect of the UHPC layer on the flexural and shear strengthening of RC beams. From this review, it was evident that (1) different retrofitting configurations have a remarkable effect on the cracking moment compared to the maximum moment in the case of flexural strengthening; (2) the ratios of the shear span and UHPC layer thickness have a notable effect on shear strengthening and the failure mode; and (3) different bonding techniques have insignificant effects on shear strengthening but a positive impact on flexural strengthening. Overall, it can be concluded that three-side strengthening has a higher increment range for flexural (maximum, 81%–120%; cracking, 300%–500%) and shear (maximum, 51%–80%; cracking, 121%–180%) strengthening. From this literature review, an experimental database was established, and different failure modes were identified. Finally, this research highlights current issues with UHPC and recommends some future works.

Anchorage Zone Stress Mapping using ANSYS

Abstract: This paper presents a comprehensive analysis of stress development in the anchorage zone of prestressed posttensioned concrete beams using the finite element method with ANSYS. Focusing on both concentric and eccentric loading scenarios, the study varies the loaded area ratio (k) to investigate its effect on stress distribution. The obtained results are meticulously compared with existing literature, enhancing understanding of the behaviour of anchorage blocks within prestressed concrete members. Beyond stress assessment, the study evaluates the performance and structural integrity of the anchorage zone, providing valuable insights for design optimization and construction practices.

Flexural loading test of a tunnel lining concrete model strengthened with carbon-fiber composite cables

In general, incorporating fiber-reinforced polymer reinforcements into arch-shaped tunnel lining concrete (TLC) is challenging. To overcome this, the TLC can be strengthened by combining a flexible carbon-fiber composite cable (CFCC) with the near-surface mounted (NSM) technique. Thus, this study evaluated the load-carrying capacity of arched concrete members strengthened by NSM–CFCC and investigated the effectiveness of the strengthening system for TLC. To measure the load-carrying capacity of arched concrete members, a specialized loading system was fabricated to conduct flexural loading tests on TLC models. The primary test models were the reference TLC models without reinforcement. TLC models bonded with a carbon-fiber sheet (CFS), and NSM–CFCC-strengthened TLC models, and two types of NSM–CFCC TLC models were tested. Eight TLC models were constructed using the conventional NSM technique in which the CFCC was embedded into a cementitious material-filled groove. The results revealed that the TLC member was not perforated by a flexural crack, even in non-strengthened concrete, owing to the arch effect. The load-carrying capacity of the NSM–CFCC TLC was remarkably higher than that of the reference and CFS strengthening models. The ultimate failure of all TLC members was caused by the crushing of the upper concrete. A finite difference analysis (FDA) for the loading test was also performed using commercial software FLAC3D. The numerical simulation results were found to be in good agreement with the cracking behavior of the strengthened TLC members observed in the experiment. These results suggest that the strengthening system using NSM–CFCC is effective for the TLC.