Concrete Zone Research Articles

Nonlinear finite element investigations on different configurations of exterior beam-column connections with different concrete strengths in column and floor

The fast development of quality concrete production has increased the usage of high-strength concrete (HSC) in the columns of high-rise buildings where high axial column loads exist. In contrast, normal-strength concrete (NSC) is used in the floor system for economic reasons. This produces a weak concrete zone at the intersection of columns and floors. Studies focused on evaluating the performance of such connections are still limited. The current numerical study investigates the performance of such connections. The developed numerical model has been verified against the experimental test for connections with different column-to-beam strengths. The numerical model has demonstrated acceptable accuracy compared to the experimental results. Moreover, the validated model has been employed to investigate the influence of different column-to-floor strengths on connections with and without transverse beams and floor slabs. It has been found that the strength and stiffness of connections with different concrete strengths have been improved compared to monolithic connections. Furthermore, the increase in joint strength and stiffness for connections with different column-to-floor strengths is more pronounced for connections with transverse beams and floor slabs due to their contribution to achieving proper confinement conditions for the weak concrete in the joint zone. In addition, the presence of a transverse beam affects the induced stress of the beam bars, resulting in a change in the mode of failure to beam flexural failure, whereas the presence of a slab floor increases both the stiffness and flexural strength of the beam. However, connections with the floor slab suffered high damage due to the transferred equilibrium torsion. Moreover, connections with different column-to-floor strengths exhibited larger stains in their joint stirrups; hence, additional joint stirrups are recommended to avoid the brittle shear failure of the joint. The study ended by evaluating the response of such connections under high axial column loads. It has been observed that planer connections perform poorly in terms of strength and ductility, while adequate performance has been observed for connections with transverse beams or slabs. Therefore, the role of the transverse beam and slab should be taken into account in the design of the connections with different column-to-floor strengths.

Characterization of interfacial transition zone of fly ash concrete with different coarse-aggregates by optical microscopy and digital image correlation coupled method

Characterization of interfacial transition zone of fly ash concrete with different coarse-aggregates by optical microscopy and digital image correlation coupled method

An Artificial Network-Based Prediction of Key Reference Zones on Axial Stress–Strain Curves of FRP-Confined Concrete

The accurate prediction of reference points on the axial stress–axial strain relationship of fiber-reinforced polymer (FRP)-confined concrete is vital to pre-design structures made with this system. This study uses an artificial neural network (ANN) for predicting hoop rupture strain (εh,rup) and transition zone, namely transition strain (εc1) and stress (f’c1), on axial stress–strain curves of FRP-confined concrete. These are key parameters for estimating a transition zone of stress–strain curves. In this study, accompanied with these parameters, ultimate condition parameters, including compressive strength and ultimate axial strain, were predicted using a comprehensive database. Various combinations of input data and ANN parameters were used to increase the accuracy of the predictions. A sensitivity analysis and a model validation assessment were performed to evaluate the predictability of the developed models. At the end, a comparison between the proposed models in this study and existing ANN and design-oriented models was presented. It is shown that the accuracy of the developed ANN models in this study is higher or comparable to that of existing ANN models. Additionally, the developed models in this study to predict f’c1 and εc1 exhibit a higher accuracy compared to existing design-oriented models. These results indicate that the proposed ANN models capture the lateral confinement effect on ultimate and transition zones of FRP-confined concrete with a more robust performance compared to existing models.

Performance-based experimental study into quality zones of lightweight concrete using pumice aggregates

The quality and performance of concrete depends on the material used. Lightweight concrete is usually formed from aggregates with lightweight specific gravity, such as pumice. This study aims to identify the zone of quality and performance of concrete when partially replacing coarse aggregate (NA) with pumice coarse aggregate (PA). The performance parameters studied include unit weight, compressive strength, elastic modulus, tensile strength, and rupture modulus. Geochemical analysis and porosity of aggregate were carried out using SEM and X-RD. The percentage variation of pumice aggregate is 0 %PA, 25 %PA, 50 %PA, 75 %PA, and 100 %PA. Quality zones based on observations of the behaviour and mechanical properties of concrete are determined. The results show that concrete using PA consists of two quality zones, namely the brittle zone for concrete with PA above 50 % and the ductile zone for concrete with PA below 50%. Formulas to calculate the elastic modulus and the rupture modulus are proposed. Utilisation of pumice as lightweight concrete aggregate to get better lightweight concrete performance must be selective.

The process of optimizing the interfacial transition zone in ultra-high performance recycled aggregate concrete through immersion in a water glass solution

In this study, the use of water glass to immerse silicomanganese slag (SS) was investigated as a method for optimizing the interfacial transition zone (ITZ) in ultra-high performance concrete (UHPC). The length of the interfacial transition zone, microstructure of the composite, and mechanical performance of the UHPC were all analyzed. The results showed that the appropriate concentration of water glass can significantly improve the interfacial transition zone, leading to improved durability and strength of the UHPC.

Enhancing the performance of alkali-activated material based coral concrete through microbubble aeration clean technology

The porous nature of coral aggregate challenges the manufacturing of high performance coral concrete, which obstacles its development and application. By using microbubble aeration clean technology, this study reports an ecofriendly and economical method to modify coral aggregates. This improvement can overcome the technical bottleneck of manufacturing high performance coral concrete. Innovatively, phosphoric acid (which is also a retarder) was used to clean the original coral aggregate particles, and alkali-activated materials (AAMs) were used to coat coral aggregates. Subsequently, AAMs were used as binder to manufacture the high performance coral concrete. The microbubble aeration cleaning removed loose attachments and soft layer on the surface of coral aggregates. It improved the filling effect of AAM coating into pores and enhanced the interfacial bonding between the aggregate and the AAM coating. The microbubble aeration clean technology modified the interfacial transition zone of coral concrete, successfully solved the problem of poor interface between coral aggregates and mortar. The residual phosphoric acid and phosphate on the aggregates could prolong the setting time of AAM based coral concrete. Comparing to the concrete prepared with original coral aggregates, the concrete prepared using the proposed new method extended initial and final setting times by 34 min and 54 min, improved flexural strength by 32.9% and compressive strength by 35.1%, meeting the performance requirements of the most ocean reef construction requirements.

Feasibility of using Fe-SMA rebar as cracking resistance spiral stirrup in the anchorage zone of post-tensioned prestressed concrete

To alleviate the problem of cracking in the anchorage zone of post-tensioned prestressed concrete. The novel iron-based shape memory alloy (Fe-SMA) spiral stirrups with self-prestressing properties were used in this study, which can provide active circumferential confinement to the anchorage zone. The compressive behavior of the anchorage zone was experimentally tested in terms of the damage modes, crack patterns, and load–displacement curves. Additionally, piezoelectric ceramics (PZT)-based smart aggregate (SA) was utilized to monitor the development of internal damages in the anchorage zone. The test results showed that the crack resistance of the anchorage zone was improved by Fe-SMA spiral stirrups. Since higher heating temperature and longer heating time resulted in temperature diffusion which aggravated the initial damage in the anchorage zone, the activation temperature and time needed to be limited. And numerical simulations were carried out based on the experimental results. Overall, the use of Fe-SMA spiral stirrups is found to be helpful in retarding the internal cracking damage of anchorage concrete at a reasonable activation temperature and time.

Behavior of two-way reinforced concrete voided slabs enhanced by steel fibers and GFRP sheets under repeated loading

Moderately thick voided slab systems are used in the construction of long-span slab buildings to incorporate lower weight on foundations and enhance the thermal and sound insulation of the slab. In some cases, these slabs required an enhancement if there is an increase in the applied loads or a defect in their construction properties. This research is focused on giving better enhancement techniques for such cases with keeping the flexural ductile failure of the tested slabs. Eight slab specimens of (1000 × 1000 mm2) were cast and tested as two-way simply supported slabs. The tested specimens consist of one solid slab and seven voided slabs. The study variables comprised the nature of the slab (solid and voided), the thickness of the slab (100 and 125 mm), the presence of steel fibers (0% and 1%), and the number of GFRP layers. The voids in slabs were made using high-density polystyrene of dimensions (200 × 200 × 50 mm) with a central hole of dimensions (50 × 50 × 50 mm) to give the shape of donat. These voids are made at the ineffective concrete zones to give a reduction in weight by (34%–38%). The slabs were tested as simply supported slabs under partial uniform repeated loading. The results of tested specimens showed that the enhancement with a combination of steel fibers and GFRP sheets gave the least deflection (4.2 mm), higher ultimate loading capacity (150 kN), larger stiffness at cracking, and at ultimate load (52.5, and 35.7 kN/mm) respectively, more ductility index (1.35), and larger energy absorption (1098.7 kN mm)..At the same stage of loading, the effect of adding steel fibers by (1%) for voided slab leads to a decrease in the deflection by (30%) and increase the ultimate loads by (31%). Therefore the strengthening technique adopted in this research enhances the behavior of moderately thick voided slabs effectively and preserves a ductile flexural behavior.

Possibilities of using alizarin and neutral red indicators to determine the neutralized layer of concrete in the field

The article concerns with the prospects of using the phenolphthalein test solution in the practice of surveying concrete and reinforced concrete building structures in planta. The problem of the study is the limits of application of the phenolphthalein test solution on the pH level of the determined concrete (the indicator works only at 8≤rn≤10). There is a need of expanding the phenolphthalein test solution application. In order to determine the different pH zones of concrete we have to modernize the method by adding other indicators to phenolphthalein. The standard phenolphthalein test solution does not allow a high degree of accuracy in determining the most sensitive to carbonation boundary zones of concrete. We proposed to modernize the phenolphthalein sample solution by using additional acid-base indicators alizarin and neutral red. We presented the results of experiments on measuring the surface neutralized layer of concrete by alcoholic solutions of acid-base indicators alizarin and neutral red on different age and size concrete samples. As a result, one of the proposed indicators (neutral red) allows to expand the phenolphthalein test solution application. We compared the results obtained by the traditional and modified methods. According to it, the proposed modified method is more accurate one.

Modelagem do concreto usando modelos micromecânicos de múltiplas fases e análise multiescala

abstract: Concrete in its macrostructure is a multiphase cementitious composite material, however, by reducing its scale, it is possible to identify the phases that compose it, among the phases are those embedded in the microscale: the hydrated silicates, in the mesoscale: the cement paste, transition zones and aggregates and in the macro phase: the composite itself. Modeling this type of material with two-phase micromechanical models is common in the literature, but there are already proven limitations that two-phase models can provide high modeling errors and are not recommended for this type of study. Faced with this problem, an alternative would be to use multiple-phase models, combined with a multiscale perspective in an attempt to minimize the error in modeling this material. The present paper models the concrete in two different constructions: without an interfacial transition zone and with the inclusion of the interfacial transition zone, verifying the modeling error when neglecting this important phase. The entire homogenization process is performed using the decoupled multiscale technique, obtaining results that rule out the use of two-phase models and methodologies that do not evaluate the interfacial transition zone in conventional concrete. The results obtained with the use of multiple-phase models reduced the relative error to practically zero (compared to experimental tests), demonstrating that micromechanics can be a concrete modeling tool provided that the multiscale process considers as many as possible phases and robust models that take this nature into account.

Understanding the role of interfacial transition zone in cement paste and concrete

Understanding the role of interfacial transition zone in cement paste and concrete

The analysis of numerical self-compacting concrete wall panel models with variations of shear reinforcement

Reinforced concrete wall critical zones are the responsive areas of dissipated earthquake loads. They are formed in the connection of the wall panels and the fixed restraints. The longitudinal and transversal steel reinforcements with certain spacing are designed according to the required nominal strength at the connections. Under certain conditions, the reinforcement distance becomes very tight, making working on castings using normal concrete difficult. This condition also occurs in boundary elements consisting of longitudinal and transversal reinforcements in tight spaces. A concrete material that flows easily and solidifies itself is required to avoid segregation. One type of this material is Self-Compacting Concrete (SCC). The SCC performance as a wall panel material that withstands gravity and cyclic lateral loads still require further research. This study aimed to analyze the hysteretic performance of reinforced SCC wall panels with variations of shear reinforcement in resisting cyclic lateral loads. The analysis used software based on numerical analysis. The drift ratios, hysteretic curves, stress patterns, ductility, and stiffness of the wall panels were analyzed. The SCC wall panel with ordinary shear reinforcement resisted lateral positive and negative loads of 152.32 kN and 143.09 kN, respectively. In comparison, the wall panel with boundary elements and tighter shear reinforcements could withstand the positive and negative lateral loads of 187.62 kN and 145.98 kN, respectively. The SCC wall panel reached the best ductility of 21.38 with ordinary shear reinforcement because the yield occurred faster than in other wall panels. The results showed that the boundary elements and shear reinforcements of reinforced SCC wall panels affected the performance in resisting cyclic lateral loads.

Crossbars of frame buildings when changing the sign of forces

A large number of reinforced concrete frame buildings are located in seismically dangerous regions. In case of seismic impacts plastic strains and local destructions occur in elements of such buildings, which influence sufficiently on their bearing capacity and stress-strain state when changing the sign of internal forces. This fact is not taken into account in current Russian norms on earthquake-resistant construction. The numerical research of reinforced concrete beams behavior under action of alternating load with regard to elastic-plastic properties of concrete and reinforcement has been conducted. The evaluation of influence of plastic strain maximum value in tensed reinforcement on the relative bearing capacity of the beam when changing the force sign and on the limit value of coefficient of plasticity corresponding to the destruction of compressed zone of concrete has been made. The comparison of results of numerical simulation with the results of experimental tests of beams with the same parameters has been made, which has demonstrated good convergence of numerical calculations with experimental data both in terms of bearing capacity and deformations for plasticity coefficients less than 2.5. During the experiment it has been found that when changing the sign of internal forces the through cracks form in bending reinforced concrete elements throughout the entire section height, which close afterwards if the coefficient of plasticity for reinforcement deformations does not exceed 2.5. For greater values of coefficient of plasticity the through cracks closing doesn’t take place until the destruction of the specimens due to the rupture of reinforcement bars. According to the results of the study it was concluded, that the decrease by 40% of relative bearing capacity of beams compared with reference samples as the increase of coefficient of plasticity in the first semi cycle of loading takes place. Also with the increase of the coefficient of plasticity the three times decrease of the limit values of coefficients of plasticity, corresponding to destruction of compressed concrete, when changing the sign of forces is observed. The numerical calculations of beams models for 10 and 50 alternating load cycles have been conducted with cycle asymmetry coefficient equal to -1. The conclusion has been made that under the action of alternating load with the number of loading cycles not exceeding 50 and for maximum values of coefficient of plasticity not exceeding 1.62 the insignificant decrease of relative bearing capacity of beams, not exceeding 10%, takes place.

Numerical modeling of combined reinforcement concrete beam

Because polymer-composite reinforcements are a new material in construction, the possibilities of their use in load-bearing structures, including concrete beams, are somewhat limited by existing regulations. The research work implemented in this article is to study their strength and stiffness in cases where steel reinforcement is in the tensile zone and composite polymer reinforcement is in the compressive zone of concrete. Concrete beams with combined reinforcement are the object of the study, and the study of the stress-deformation state is its subject. The behavior of concrete beams with combined reinforcement under static load was studied. Considering the nonlinear properties of materials in the finite element method, their stress-strain states were investigated. A 3D beam model was created using the ANSYS Workbench 2022R1, and 3 series of samples were chosen and compared with hand calculations. The behavior of concrete beams with metal and composite reinforcement was carried out using numerical analysis. Also, the study’s results show that the role of the reinforcement installed in the compressive zone of the beam is better than the performance of the beam without the reinforcement installed in the compressive zone. Although the failure starts with the rebar in the tensile zone, the rebar installation in the compression zone shows an increase in the bearing capacity and stiffness of the beam.

Bilinear Model of Behavior Reinforced Concrete Column under High-intensity Lateral Loads after Fire

The experience of destructive earthquakes shows that the problem of determining the response of RC frames under seismic loads after a fire is relevant. Calculation models for individual elements and buildings as a whole should reflect nonlinear behavior in this case. A brief review of models for describing reinforced concrete elements under low-cycle loads is given. It is noted that such models for elements damaged by fire do not exist at present. A model based on a bilinear diagram for calculating an eccentrically compressed RC column damaged by fire is proposed. Only three parameters are required to describe the model: the ultimate moment, the ultimate curvature, and the effective initial stiffness. The ultimate moment and curvature are determined by analyzing the distribution of stresses and strains in the column cross-section during the fracture stage. The column cross section is represented by a set of separate layers heated to different temperatures. The effective stiffness is defined by a straight line passing through the point in the curvilinear diagram where the initial yielding in the reinforcement or concrete occurs. The model takes into account different levels of axial loading, indirect reinforcement by transverse hoops, second-order effects and non-uniform distribution of stresses in the compressed zone of concrete. On the basis of the proposed model, bilinear deformation diagrams of RC columns, which are subjected to standard fire of different duration, are compared. The calculation results have shown a significant decrease in bearing capacity and stiffness of the damaged columns and an increase in their plasticity. The resulting model is simple enough to be used and suitable for most engineering calculations. This model can be used as a basis for constructing a hysteresis diagram for low-cycle impacts after a fire, which is necessary for seismic analysis of structures in the time domain.

Punching shear strength under static and dynamic loads

Modern domestic calculation methods and developed countries for determining the bearing capacity of monolithic reinforced concrete slabs for punching do not fully take into account all factors of design solutions and operating conditions. The available design provisions are made for the static operation of structures and there are no recommendations for taking into account the features of the dynamic impact on the overlap and the nature of the work of the node interfaces. The accepted empirical assumptions of the calculation, based on numerous experimental data, do not take into account the features of the stress-strain state of the coupling of the overlap with the column during destruction according to the punching scheme. This is due to the lack of computational models in which all the acting internal forces ensuring the resistance of the interface to penetration would be considered comprehensively. The complexity of the problem is due to the fact that the sections of the nodal interface are in an inhomogeneous stressed state. The stress-strain state of plates for punching under dynamic load is currently little studied. This article proposes a method for determining the bearing capacity of a symmetrical nodal coupling of a column with an overlap for punching under static and short-term dynamic loading. The proposed design model of the punching strength is based on the following prerequisites: the resistance to punching of a monolithic reinforced floor consists of the shear resistance along the surface of the reduced punching pyramid formed by the height of the compressed concrete zone; the strength of the concrete shear resistance increases due to volumetric compressive forces on the surface of the reduced punching pyramid; the angle of inclination of the faces of the punching pyramid depends on the loading speed. The obtained theoretical dependences are applicable under static and dynamic loading and are in satisfactory agreement with experimental data.

Experimental studies of non-centrally compressed corrosion-damaged reinforced concrete elements under dynamic loading

In the scientific literature, there is practically no analysis of the effect of corrosion damage on the operation of compressed reinforced concrete elements, especially on the stress-strain state of such structures under dynamic loading. For experimental studies, 37 reinforced concrete samples were made – columns of square cross-section with dimensions of 100×100mm, height of 700mm, extensions of 100×200mm were made in the supporting parts to create off-center compression. In the manufactured samples of reinforced concrete, local corrosion damage of concrete and reinforcement was created for accelerated corrosion of elements, a concentrated solution (37%) of hydrochloric acid (HCl) was used as an aggressor. The article describes experimental studies of non-centrally compressed reinforced concrete elements damaged by corrosion under dynamic loading. According to the load cell readings, it was recorded that corrosion damage leads to a decrease in the height of the compressed concrete zone, due to a decrease in the cross-section of the stretched reinforcement, as well as the lack of joint reinforcement with concrete. The obtained deformation diagrams of transverse, non-centrally compressed, corroded and undamaged samples based on glued strain gages on concrete and stretched reinforcement showed that these deformation diagrams fundamentally differ in shape. The deformations of reinforcement and concrete obtained as a result of a full-scale study made it possible to assess the stress-strain state of damaged and uncorroded structures according to the parameter Ne-1/r (curvature). The effect of corrosion damage on the nature of destruction of non-centrally compressed elements has been established.

THE INFLUENCE OF CONCRETE COMPRESSION ZONE ON THE SHEAR RESISTANCE OF REINFORCED CONCRETE BEAMS

The paper analyzes the effect of the compressed zone of concrete on the shear resistance of reinforced concrete flexible beams. The calculation models are considered, by which the contribution of the compressed zone of concrete to the total value of the shear resistance is further determined. The features of each model are described, and schemes and dependencies for calculating the transverse force in the compressed zone of concrete are presented. The experimental database of specimens is presented and the analysis by the considered calculation models is carried out. Varying parameters in the database are marked and the analysis by the obtained calculated values of transverse force in the compressed concrete strip by models is performed. The contribution of the transverse force in the compressed zone to the quantitative contribution to the shear resistance is indicated and appropriate conclusions are drawn on the models presented and the parameters considered.

On the localisation of the FPZ under a pure mode II load identified by a hybrid method

In this paper, the fracture process zone (FPZ) of a high-performance concrete (HPC) is studied. The analysis is done under a pure mode II load, making use of a digital image correlation (DIC) technique. The experimental test was performed on Brazilian disc with central notch (BDCN) specimens with a relative notch ratio a/R of 0.267. The main focus of this experimental study was the traction-free crack ahead of the notch tip, together with the FPZ extent ahead of the crack. This is done by adjusting the current method for mode I crack localisation to pure mode II cracks. Additionally, a traditional analytical formulation to predict FPZ length was substituted with the fracture toughness for mode II to cover mode II load cases. To verify the adjustments to the mode I crack identification, crack tip opening (CTOD) and crack tip sliding (CTSD) displacements were measured by DIC, which showed a mode II dominance in the crack tip displacement components.

Experimental case study for practical implementation of real pre-stressed two layer reinforced concrete beams

The present study summarizes a long-term 14 - year research program, focused on investigation of reinforced concrete (RC) two - layer beams (TLB). In the frame of this program the TLB idea was proposed in 2007. The idea was based on using high strength steel fibered concrete (SFHSC) in the compressed zone of RC bending elements and normal strength concrete (NSC) in the tensile one. Further development of the above-mentioned idea included testing SFHSC at material level, experimental investigation of non – pre - stressed simple supported 3 m and continuous 4 m (2 × 2 m) two-span TLBs and testing pre-stressed simple supported 3 m TLBs (PTLBs) that represented a model of long - span prestressed concrete beams. The present paper presents a case study, aimed at investigating real 8 m long PTLB behavior at four-point bending and its comparison with pre-stressed single – layer beam with the same span. Like in the previous research stages, interaction of concrete layers in a PTLB was examined. No de-bonding between the layers was observed that demonstrates proper interaction between the layers, like in the 3 m long PTLBs. Results, obtained in the present study, enable to recommend PTLBs for practical implementation as effective real pre-stressed concrete elements for new structures and also for repairing existing ones.