Cracked Reinforced Research Articles

Effect of strain rate on tensile characteristics and failure behavior of Al/steel welded joint producing by a laser-MIG composite heating source

Studies on dynamic tensile characteristics for Al/steel dissimilar materials are crucial to ensure the integrity and dependability of light-weight structures. In this study, the tensile characteristics and failure behaviors of Al/steel laser-MIG welded-brazed joints were studied. The dynamic fracture mechanisms of the joints are discussed. With the increase in strain rate, the fracture mode and tensile properties of Al/steel joints with reinforcement changed. The joints exhibited two failure modes of heat-affected zone failure at lower strain rates and interface failure at higher strain rates. When the strain rate was 1000 s−1, the increase in dislocation density in the weld reinforcement and diversion of cracks in the intermetallic compound layer significantly enhanced the joint’s properties. The tensile and yield strengths reached 222.6 and 166.6 MPa, 6.5 % and 17.9 % higher than those at a stain rate of 1 s−1, respectively. For Al/steel joints without reinforcement, as the strain rate increased, the crack gradually grew to the Al8Fe2Si phase, leading to a notable enhancement in the joint’s performance. At a strain rate of 500 s−1, the maximum strength of the joint reached 238.3 MPa, representing an increase of 58.9 % in comparison to the strain rate of 1 s−1.

The development and performance evaluation of diagonal tension cracks control devices

ABSTRACT The cause of diagonal tension cracks is the drying shrinkage of the initial concrete, which causes them to appear at the corners of square openings, which are the most vulnerable areas to stress. Looking specifically at the cause from a structural aspect, the concentrated stress exceeds the tensile strength of the concrete, causing cracks to occur at the corners. Typically, diagonal tension reinforcement bars are arranged to prevent diagonal tension cracks, however, they have often failed to be practical solutions because they do not entirely prevent the continued occurrence of cracks. Consequently, the present study developed diagonal tension cracks control devices that are capable of preventing diagonal tension cracks. Next, comparative experiments on existing diagonal tension reinforcement bars with respect to drying shrinkage were conducted to verify the utility of the diagonal tension crack reinforcement materials through performance evaluation. According to the results, the maximum elastic strain at the top and bottom opening edges was 29.2% and 35.8%, respectively, which was a large reduction compared to the 6.7% and 4.9% of the existing diagonal tension reinforcement. Considering the above points, it is expected that the developed control device will be very practical in terms of industry.

A Study on Fatigue Crack Reinforcement of Bridge Deck and U-Rib Weld after Considering Residual Stress

In order to study the fatigue performance of the initial crack between the CFRP reinforced deck and the U-rib weld after considering the residual stress, the welding residual stress of the deck and the U-rib weld was analyzed by the welding subprogram of ABAQUS, and then the segment model of the deck and the U-rib of the steel bridge deck was established by using the interaction technology of FRANC 3D-ABAQUS, and the stress intensity factor of the crack between the deck and the U-rib weld under the vehicle-only, vehicle-residual stress combined loads, and CFRP-strengthened combined loads were analyzed. The growth analysis of cracks before and after reinforcement was carried out. The results show that the amplitude of the stress intensity factor of the crack tip is less than the threshold when only the vehicle is applied, and the crack will not spread. After considering the residual stress, the maximum KI value of the crack at the welding toe is increased by 6.9 times compared with the maximum value of the vehicle only, and the maximum KI value at the welding root is increased by 10.7 times; the reinforcement method of CFRP pasted on the lower surface has a reinforcement effect on the fatigue crack at the welding toe, but the fatigue crack at the welding root has no reinforcement effect; the reinforcement effect of the welded toe crack increases with the increase of the number of CFRP layers, length, and width. When the weld toe crack after reinforcement extends to half the thickness of the deck, the number of car-residual stress combined load cycles increases by 43% compared to when it is not reinforced.

Flexural Performance of Cracked Reinforced Concrete Beams Strengthened with Prestressed CFRP Sheets under Repeated Loads

Because researchers are aiming to restore the deformation and minimize the crack width of existing concrete structures, the strengthening technology of prestressed carbon-fiber-reinforced plastic (CFRP) is currently the focus of many studies and applications. In terms of the strengthening of a prestressed CFRP sheet on the flexural performance of cracked reinforced concrete beams under repeated loads, a four-point bending test of 12 beams was conducted considering the prestress degree reflected by the amount and the prestress force of the CFRP sheet. The longitudinal strengthened CFRP sheet was bonded on the bottom surface of the test beam and fixed by U-jacket CFRP sheets at the ends after tensioning. The strains of concrete, longitudinal tensile steel bars and CFRP sheets were measured at the pure bending segment of test beams, while the cracks, midspan deflection and failure pattern were recorded. The results show that the normal strain on the mid-span section of the strengthened beams by the prestress CFRP sheets was fitted for the assumption of plane section, the cracks and mid-span deflection decreased with the prestress degree of the CFRP sheets to provide better serviceability for the strengthened beams, the load capacity could be increased by 41.0–88.8% at the yield of longitudinal tensile steel bars and increased by 41.9–74.8% at the ultimate state and the ductility at the failure state was sharply reduced by 54.9–186%. The peeling off of broken CFRP sheets played a role in controlling the failure pattern of the strengthened beams under repeated loads. Finally, methods for predicting the bending performance of reinforced concrete beams strengthened by prestressed CFRP sheets were proposed. This study enriches the knowledge about damaged reinforced concrete beams that were strengthened with prestressed CFRP sheets.

Analysis of Concrete Crack Treatment Technologies in Buildings

With the continuous development of civil engineering, concrete crack treatment technology has become an important research field. This paper proposes treatment techniques for different types of cracks, including the prevention and repair of surface cracks, the reinforcement and grouting of structural cracks, and the design and construction of controlled cracks through the analysis of the causes and classification of concrete cracks. The methods and suggestions proposed in this paper are practical and can improve the quality and safety of buildings.

Novel crack repair method of steel bridge diaphragm employing Fe-SMA

To repair cracks at the arc-shape cutouts of the diaphragm in the orthotropic steel deck (OSD) bridges, the Fe-SMA plates covering crack-stop holes method is proposed. Bonding the activated Fe-SMA plate can effectively reduce the stress level of the cracked region by increasing the local stiffness and introducing precompression. The recovery stresses of Fe-SMA with the pretension strain of 4% and activation temperatures of 150 °C, 200 °C, and 250 °C are 192.3, 226.4, and 270.8 MPa respectively, which can meet the requirements for crack reinforcement of engineering structures. To reinforce the diaphragms, compression stresses of 44.2–63.9 MPa are introduced at the hole edge after activating Fe-SMA, considerably reducing the local stress amplitude and stress concentration. The activating operation by heating has little effect on the bonding performance between the Fe-SMA plate and the diaphragm. The fatigue notch factor is reduced by 66.36% by bonding the Fe-SMA plate, and can be further reduced by about 14% after activating Fe-SMA. Additionally, the practical reinforcement application on the cracked diaphragm cutouts of the Sutong Bridge has confirmed the feasibility and efficacy of the novel Fe-SMA plates covering crack-stop holes method.

Simplified Evaluation of Shear Stiffness Degradation of Diagonally Cracked Reinforced Concrete Beams.

Shear cracking in concrete box-girder bridges, which could cause excessive deflection during the serviceability limit state, cannot be effectively avoided by code-guided design. While elastic shear deformation only accounts for a small proportion of total deformation for un-cracked reinforced concrete (RC) beams, the magnitude of after-cracking shear deformation becomes comparable to flexural deformation for RC beams. However, there is still a lack of practical models to predict the after-cracking shear deformation of RC beams. First, six thin-webbed I beams were tested to investigate the shear stiffness degradation mechanism and the decrease ratio. Then, a very simple truss strut angle formula, which is the crucial parameter for shear stiffness, was established. Furthermore, a stiffness degradation rule for partially cracked beams was proposed considering the influence of concrete tension stiffening, which is essential for predicting the development process of after-cracking shear deformation. Finally, directly measured shear strains were used to validate the proposed shear stiffness model. The results showed that the shear stiffness drops to about 30~40% of the original stiffness after the first diagonal crack, and the remaining shear stiffness is only about 10% of the original one when the stirrup yields. Increasing the stirrup ratio is a more effective method to control shear stiffness degradation for diagonally cracked RC beams. Also, the proposed shear stiffness model well captures the main features of the shear stiffness degradation, and it provides a relatively accurate prediction of the equivalent shear stiffness at the post-cracking stage.

Preparation of a carbon-based nanomaterial and its influence on construction engineering

Carbon nanotube cement-based grouting materials currently mostly stay in the pure slurry research stage, and there is little research in the field of rock crack reinforcement combined with the engineering application background. Therefore, on the basis of existing research, it is important to expand the dispersion of carbon nanotubes in cement paste, and to carry out research on the reinforcement of fractured rocks in combination with the engineering background. This paper combines the actual needs of construction engineering to prepare carbon-based nanomaterials, and analyzes its impact on construction engineering in combination with experiments. Moreover, this paper analyzes the influence of carbon nanotubes on the macro-mechanical properties of cement-based materials, and studies the influence of modified carbon nanotubes on the macro-mechanical properties of cement-based materials. In addition, this paper explores the micromechanical properties of carbon nanotube cement materials at the nanometer scale. The research results verify the effectiveness of carbon-based nanomaterials.

Study on Crack Propagation Parameters of Tunnel Lining Structure Based on Peridynamics

The numerical simulation results utilizing the Peridynamics (PD) method reveal that the initial crack and crack propagation of the tunnel concrete lining structure agree with the experimental data compared to the Japanese prototype lining test. The load structure model takes into account the cracking process and distribution of the lining segment under the influence of local bias pressure and lining thickness. In addition, the influence of preset cracks and lining section form on the crack propagation of the concrete lining model is studied. This study evaluates the stability and sustainability of tunnel structure by the Peridynamics method, which provides a reference for the analysis of the causes of lining cracks, and also lays a foundation for the prevention, reinforcement and repair of tunnel lining cracks.

Refined Simulation of Cracked Reinforced Concrete Beams Based on Enhanced Bond-Based Peridynamics

Peridynamics (PD) has been widely used in simulating the crack behaviors of brittle material due to its extraordinary capacity in analyzing deformations with discontinuities. In this paper, an enhanced bond-based peridynamics (BPD) model is proposed to study the crack behaviors of reinforced concrete (RC) beams. A modified nonlinear concrete bond model is presented to simulate the tensile-compressive softening and improve the model’s convergence. A novel steel bond model is developed considering the nonlinear behaviors in both axial and shear directions. A coupled axial–shear interaction (ASI) model is adopted to simulate the bond-slip behaviors between concrete and steel. Furthermore, reformulated BPD equilibrium equations are modified for implicit static analyses. A gradually weakening fictitious element (GWFE) approach is presented to improve the stability in implicitly solving the BPD equations. The algorithms are implemented in an open-source finite element (FE) software, OpenSees, for BPD analysis. Experiments are conducted for three RC beams with different shear span to depth ratios under concentrated vertical loads to verify the enhanced BPD model. The comparative studies are performed between experimental and simulated results, and nonlinear responses of RC beam are investigated, e.g. the responses of deflection-shear force, strain distribution on stirrups, the shear resistance, and the propagation of cracks. The results show that as the shear span-to-effective depth ratio increases, the capacity provided by RC beam decreases while stirrups gradually provide more capacity instead. It is also found that the yield deflection increases significantly with the growing shear span-to-effective depth ratio.

Long-Term Flexural Behaviour of Cracked Reinforced Concrete Beams with Recycled Aggregate

This paper presents the results of an investigation into the long-term flexural behaviour of cracked reinforced recycled aggregate concrete (RAC) beams. Washed construction and demolition wastes (CDW) with a maximum size of 20 mm were used as the coarse recycled aggregate. The main variable in the research was the replacement ratio of recycled aggregate. Specimens with 0%, 50% and 100% recycled aggregate were cast and tested. The experimental results showed that samples with an increased amount of recycled aggregate had significantly reduced strength and a noticeable increase in both short-term and long-term deflection of RAC beams over equivalent normal concrete (NC) beams. Increased levels of RA resulted in greater creep and shrinkage of RAC and greater long-term loss of tension stiffening in RAC reinforced tension specimens. Prediction of long-term deflections using Eurocode 2, even after incorporating the experimental concrete properties within the Code method, underestimated the experimental deflections of the RAC beams. However, by modifying the tension stiffening factor, β used in Eurocode 2, deflections were predicted to within approximately 1%. From this investigation, it is recommended that the factor β be reduced from 0.5 (for NC) to 0.4 (for RAC @50% replacement) and 0.3 (for RAC @100% replacement).

Brief Talking about the Main Factors and Reinforcement of Building Wall Cracks

Brief Talking about the Main Factors and Reinforcement of Building Wall Cracks

Bearing characteristics and failure mechanism of a novel plate-short anchor structure for tunnel crack reinforcement

In order to solve the problem of tunnel diseases caused by lining cracking in mountain tunnels, this paper studies the crack repair technology of mountain tunnels. Based on this, this paper innovatively puts forward a new repair method of tunnel cracks—plate-short anchor assembly structure. Through literature investigation and theoretical derivation, the mechanical mechanism of the plate-short anchor assembly structure is analyzed, and the calculation formula of bearing capacity is proposed. The loading device for the bias pressure test of prototype components of tunnel lining is independently designed, and the destruction test is successfully carried out by simulating the stress mode of tunnel lining. By simulating the state changes of the plate-short anchor assembly structure under different cracking states, the ultimate bearing capacity under different working conditions is obtained, and the evolution process of the failure mode of the new structure is revealed.

Tension-Stiffening Effect Consideration for Modeling Deflection of Cracked Reinforced UHPC Beams

Tension-stiffening effects can significantly influence the flexural performance of cracked reinforced concrete specimens. Such effect is amplified for fiber-reinforced concrete, given the fact that fibers can bridge the cracks. The objective of this study was to develop a model to predict the deflection of cracked reinforced ultra-high performance concrete (R-UHPC) beam elements. The modeling approach characterized the average bending moment of inertia by combining the existing model used for conventional reinforced concrete and the analytical model of stress distribution of UHPC along the cross-section. The finite element analysis (FEA) was employed to evaluate the flexural deflection based on the average bending moment of inertia. The calculated load-deflection relationships have been compared to experimental results. The results indicated that the relative errors of deflection between predicted and experimental results can be controlled within 15%, compared to values ranging from 5% to 50% calculated by neglecting the tensile properties of cracked UHPC and values ranging from 5% to 30% calculated by effective inertia of bending moment of ACI code. Therefore, the developed model can be used in practice because it can secure the accuracy of deflection prediction of the R-UHPC beams. Such a simplified model also has higher sustainability compared to FEA using solid elements since it is easier and time-saving to be established and calculated.

Integration of eddy current sensors into repair patches for fatigue reinforcement at rivet holes

AbstractFatigue cracks at rivet holes occur at advanced service life of aircrafts due to cyclic loading. As a repair method, adhesively bonded stiffener patches enhance the fatigue life of the structure by delaying crack initiation and reducing crack growth. Combining a crack sensor with a repair patch to a sensor‐based stiffener patch allows crack growth reduction and monitoring at the same time. This paper presents a feasibility study on the integration of eddy current sensors into carbon fibre reinforced plastic (CFRP) repair patches. To this end, a specific patch design is developed, and samples are manufactured by ultrasonic fabrication. The performance of the patches is investigated in fatigue tests in terms of ‘crack reinforcement’ and ‘crack detection.’ Both requirements are met. The undertaking of future efforts to tailor the sensor‐based repair patch concept to an individual application in aircraft maintenance seems reasonable.

Study on the Force Transfer in Cracked Reinforced Concrete Elements

Both theoretical and experimental studies on strength resistance of cracked RC concrete elements become complicated due to significant material anisotropy and changes in “stress – deformation” relationship for cracked concrete. Despite the fact that cracks are usually formed normally to principal tensile stresses in concrete, their further opening may not always coincide with their direction, resulting in additional shear stresses and transmission of the forces across the cracks through mechanisms of aggregate interlock in concrete and dowel/tension effects in the reinforcement crossing the crack. A better understanding of these mechanisms may be obtained through the reliable data on the relationship between stresses and displacements in sections containing the cracks.

Stiffness Assessment of Cracked Reinforced Concrete Beams Based on a Fictitious Crack Model

A fictitious crack model is introduced into cracked reinforced concrete beams to assess the changing beam stiffness under loads. Firstly, nonlinear concrete stress distributions near cracks are built based on the model. Then the stress of the steel bar at the cracked section is considered as cohesive stress. The concrete and steel stresses are substituted into the equilibrium equations of forces to solve the concrete stress. Based on the solution, the section inertias are estimated by iterating the calculation of the cracking open displacement, and finally the beam stiffness is assessed. Experimental data from seven concrete beams after cracking are adopted to validate the effectiveness of the proposed method, and the results show that the fictitious cracks ahead of actual cracks increase their depth with the load, which will raise the neutral axis and change the inertias of cracked sections and their neighboring sections. These changes are taken into account in the stiffness assessment, so the results predicted by the proposed method are shown to coincide well with the nonlinear deflections measured in the experiments.

Study on Physical Properties of Mortar for Section Restoration Using Calcium Nitrite and CO2 Nano-Bubble Water.

This study investigated the physical properties of section-restoration mortar with calcium nitrite (Ca(NO2)2) and carbon dioxide (CO2) nanobubble mixing water to develop materials and methods for the repair and reinforcement of cracks in reinforced concrete (RC) structures. As the calcium nitrite content increased, the generation rate and generated amount of nitrite-based hydration products also increased, owing to the rapid reaction between NO2− ions in calcium nitrite and C3A(Al2O3). Further, the reaction with C3S and C2S was accelerated, thereby increasing the generation rates of Ca(OH)2 and C-S-H. The large amount of Ca2+ ions in these hydration products reacted with CO32− ions in CO2 nanobubble water, thereby increasing the generation of calcite-based CaCO3 in the cement matrix. This appears to have affected strength development and durability improvement via the densification of the structure. These results suggest that the performance of polymer cement mortar for repairing concrete structures can be improved if calcium nitrite and CO2 nanobubble water are properly combined and applied.

Experimental Behavior of Cracked Reinforced Concrete Columns Strengthened with Reinforced Concrete Jacketing.

Reinforced concrete (RC) columns often need to be strengthened or rehabilitated to allow them to carry the loads applied to them. In previous studies, RC columns have been strengthened by jacketing, without considering the occurrence of cracking. In this study, the behavior of RC columns strengthened externally by jacketing after cracking is analyzed. The accuracy of the existing models was verified by analyzing the performance of fifteen RC columns with different cross-sections to determine the effect of new variables, such as the column size, amount of steel reinforcement, and whether the column was cracked or not, on the effectiveness of strengthening. The analysis demonstrated that this strengthening technique could effectively improve both the ductility and strength of RC column cross-sections. The results indicate that the model suggested by the ACI-318 code can predict the ultimate load capacity of RC columns without strengthening, or strengthened by RC jacketing before or after cracking, with higher accuracy and material efficiency. The RC columns without strengthening met the safety limit of the ACI-318 model. However, for strengthened columns, a reduction coefficient must be used to enable the columns to meet the safety limit, with values of 94% and 76% for columns strengthened before and after cracking, respectively. Furthermore, strengthening after cracking affects the ultimate load capacity of the column, with 15.7%, 14.1%, and 13.5% lower loads for square, rectangular, and circular columns than those strengthened before cracking, respectively.

Assessing the Instantaneous Stiffness of Cracked Reinforced Concrete Beams Based on a Gradual Change in Strain Distributions

Concrete cracking causes a gradual change in strain distributions along the cross section height of reinforced concrete beams, which will finally affect their instantaneous stiffness. A method for assessing the stiffness is proposed based on the gradual change, which is considered through modeling different strain distributions for key sections in cracked regions. Internal force equilibria are adopted to find a solution to top strains and neutral axes in the models, and then the inertias of the key sections are calculated to assess the beam stiffness. The proposed method has been validated using experimental results obtained from tests on five reinforced concrete beams. The predicted stiffness and displacements are shown to provide a good agreement with experimental data. The instantaneous stiffness is proven to greatly depend on the crack number and depth. This dependence can be exactly reflected by the proposed method through simulating the gradual change in concrete strain distributions.