Passive Confinement Research Articles

Bearing capacity of concentric anchorage zones in post-tensioned members: A stress field solution

The post-tensioned anchorage zones are generally subdivided into local and general zones, which are treated with semi-empirical approaches and strut-and-tie models separately in many design codes. Even with their functions that look different, the two adjacent zones are intrinsically related in terms of the compression dispersion mechanism. Holding this view, this paper intends to present a mechanical model to integrate their combined actions. Firstly, two confinement effects in anchorage zones are studied: the local confinement under the bearing plate and the passive confinement by bursting reinforcement. Then, a discontinuous stress field model for concentric anchorage zones is proposed, which allows for a triaxial state of compression stresses in the local zone and the contribution of bursting reinforcement in the general zone simultaneously. After that, an explicit bearing capacity formula for anchorage zones is derived, in which the ultimate failure can be attributed to either the yielding of the distributed tension subfield or the crushing of fan subfields. The prediction of bearing capacity is in fairly good agreement with the collected test results.

Seismic Retrofitting of Nonseismically Detailed Exterior Reinforced Concrete Beam-Column Joint by Active Confinement Using Shape Memory Alloy Wires

It is widely accepted that the seismic retrofitting of nonseismically detailed reinforced concrete beam-column joints (BCJs) in reinforced concrete (RC) frame buildings is an urgent necessity due to its high vulnerability and potential consequences in seismic events. As a result, considerable effort has already been put into developing efficient and practical retrofitting solutions for such BCJs. Most of the existing techniques, however, are based on either passive confinement techniques, for example, fiber reinforced polymer (FRP) wrappings, or involve a considerable joint enlargement, which, in many cases, is undesirable. In this study a new technique of retrofitting BCJs is proposed by employing a more effective method of confinement (i.e., active confinement), utilizing the shape recovery feature of shape memory alloys (SMAs). To evaluate the performance of the proposed retrofitting scheme, experimental tests were conducted on full-scale BCJ specimens. The efficacy of the proposed retrofitting scheme is evaluated in terms of enhancement in strength, ductility, energy dissipation capacity, damage reduction in the specimens, and ease of application. The results from this study suggest that the proposed retrofitting scheme could be effectively used in achieving the full capacity of the joints corresponding to beam yielding and consequently enhances the energy dissipation capacity of the system significantly. The test results demonstrated that the proposed retrofitting scheme performs excellently in reducing the joint shear strain (core damage) (to almost zero or negligible) and also in retaining the full axial load carrying capacity of the column, even at very large drift values. The proposed retrofitted scheme could also be conveniently used in cases where the capacity ratio of column-to-beam needs to be improved.

Damage evolution and full-field 3D strain distribution in passively confined concrete

The damage evolution of concrete under multiple stress states is complicated because the triaxial stress on the heterogeneous concrete affects mesoscale interactions between aggregates and mortar, which further alters the internal cracking of concrete and its load bearing mechanism. Hence, probing the internal cracking evolution of concrete under a complex stress state is the key to better understanding the characteristics and failure mechanism of concrete material. This study conducted a series of X-ray computed tomography (X-CT) tests on fiber-reinforced polymer (FRP) confined concrete with varied in situ axial load levels. Different cross-sections of concrete specimens—circular, square with round corners, and square with sharp corners—were designed to achieve uniform or nonuniform passive confinement conditions. The obtained volumetric images from X-CT clearly exhibited the internal damage evolution of concrete, where crack growth, shear banding, and failure localization were displayed for the concrete under varying triaxial stress states. The development of damage to concrete due to cracking with increasing load was also quantified by image segmentation. In addition, the digital volumetric correlation technique (DVC) was employed to produce the volumetric displacement field and strain distribution in the confined concrete according to the image variation of natural speckles. A three-dimensional internal strain analysis revealed that strain localization was mainly caused by mesoscale concrete heterogeneity and nonuniform confinement. More importantly, this study provides the first database of full-field strain tensors for FRP-confined concrete under different axial load levels. These data are valuable for the development of concrete mechanics, failure criteria of materials, and computational modeling of concrete materials and their structures, especially for concrete under complex stress states.

Finite-Element Modeling of FRP-Confined Noncircular Concrete Columns Using the Evolutionary Potential-Surface Trace Plasticity Constitutive Model for Concrete

The compressive behavior of fiber-reinforced polymer (FRP)-confined concrete columns with a noncircular cross section has been investigated through extensive experimental, analytical, and numerical research, but a unified theoretical/numerical approach that can accurately predict both their section-average behavior and local concrete behavior is not yet available. In noncircular columns under axial compression, the concrete is typically under a nonuniform stress state of three-dimensional (3D) compression, with the lateral compressive stresses being the reactive stresses from the confining device (i.e., passive confinement). The authors of the present paper recently developed a plasticity constitutive model for concrete under general 3D compressive stresses, which possesses a potential surface with an evolutionary deviatoric trace that can accurately capture the results of existing compression tests of concrete cubes under nonuniform, passive confinement. This paper explores the application and capability of this evolutionary potential-surface trace (EPT) plasticity constitutive model in finite-element (FE) analysis of FRP-confined square, rectangular, and elliptical plain-concrete columns under concentric compression. The section-average behavior of all the selected noncircular columns predicted by these FE analyses was close to the existing experimental data. The numerical results obtained with the EPT plasticity constitutive model were then examined in detail to achieve an improved understanding of local concrete behavior in FRP-confined noncircular columns.

Pull‐out strength of perforated steel plates with oval‐shaped perforation

AbstractThe use of Perfobond Leisten (PBL) shear connectors in building structures has attracted increasing interest. A new approach for using perforated steel plates in concrete‐filled steel tube (CFST) column splices was implemented in this study. The shape of the perforation opening in perforated steel plates is generally a circle, similar to that of PBL shear connectors. However, in this study, an oval‐shaped perforation was used to increase the double shear area of the concrete. A pull‐out experiment was conducted on perforated steel plates with oval‐shaped perforations in a CFST block to evaluate the tensile performance of the perforated steel plates. A total of 29 specimens were tested, and the pull‐out strengths of the perforated steel plates were determined and analyzed under passive confinement from the steel tube. The contribution of the stress transfer was discussed in terms of the double shear of the concrete in the perforation and the bond and/or friction between the surface of the steel plate and concrete. Formulas for determining the maximum pull‐out strength were also developed.

Performance of recycled aggregate concrete-filled high-strength steel tubes under axial compression, tension and torsion

This paper reports the performance of recycled aggregate concrete-filled high-strength steel tubes (RAC-FHST) under axial compression, tension and torsion. A total of 50 specimens, including 22 compressive specimens, 16 tensile ones and 12 torsional ones, were tested. The main parameters varied in the tests were the sectional type, the steel yield strength and the replacement ratio of recycled coarse aggregate (RCA). Analysis on the full-range load–deformation response, the typical failure modes, the ultimate strength, the passive confinement effect and the influence of key parameters were conducted. Results reveal that the use of RCA affected the failure modes especially when subjected to tension or torsion. Meanwhile, the performance of RAC-FHST is improved due to the excellent confinement effect of high-strength steel tube. The finite element modelling method was used to investigate the mechanical property and the effects of important parameters. Simplified models for the calculation of ultimate compressive, tensile and torsional strengths of RAC-FHST were proposed. Based on reliability analysis, the effects of the steel yield strength, the concrete strength and the confinement factor on the reliability index of RAC-FHST were evaluated.

Experimental Investigation of Sand Subjected to High Stress Levels in Wet and Dry Conditions.

This work aimed to understand the mechanical behavior of siliceous and calcareous sand materials under uniaxial confined compression loading at high stress levels. For this purpose, a series of quasi-oedometric compression tests were conducted on sand materials, to examine the effects of grain size, nature, and moisture contents on the soil crushability and the compression behavior, using an upgraded thick pressure vessel device that can reach mean stress up to 500 MPa. All samples were prepared using an aspect ratio of 1:1 (diameter: height), placed inside a high strength steel vessel, and compressed at a uniform axial displacement rate of 5 µm/s. The vessel is instrumented with multiple strain gauges allowing for the characterization of the hydrostatic and deviatoric behavior of each test. The results of quasi-oedometric tests, conducted on these types of sand, up to a passive confinement of 500 MPa, show that particle breakage is enhanced by the presence of water. It was noticed that, for siliceous sand, smaller particles break more than larger particles, and that the calcareous grains manifest a rapid response to axial stress compared to siliceous sand. Testing various soil properties shows a great potential to better characterize the sensitivity to breakage of soils. Lastly, a post-mortem analysis of samples before and after testing, using the X-ray micro-tomography technique, was applied to study the mechanical damage of sand specimens.

Effect of Aggregate Size on the Axial Compressive Behavior of FRP-Confined Coral Aggregate Concrete.

Using locally available raw materials for preparing concrete, such as coral reefs, seawater, and sea sand, is conducive to compensating for the shortage of construction materials used on remote islands. Jacketing fiber-reinforced polymer (FRP), as passive confinement, is a practical approach to enhance the strength, ductility, and durability of such coral aggregate concrete (CAC). Rational and economical CAC structural design requires understanding the interactions between the CAC fracture process and FRP confinement. The coral aggregate size is the critical parameter of their interaction since it affects the crack propagation of CAC and FRP confinement efficiency. This study conducted axial compression tests on FRP-confined CAC cylinders with varying coral aggregate sizes and FRP confinement levels. The test results indicate that the coral aggregate sizes affected the unconfined CAC strength. In addition, the dilation behavior of FRP-confined CAC varied with aggregate sizes, showing that CAC with smaller coral aggregate featured a more uniform hoop strain distribution and larger FRP rupture strain. These coupling effects are epitomized by the variation in the transition stress on the stress–strain curve, which makes the existing stress–strain models not applicable for FRP-confined CAC. A modified stress–strain model is subsequently proposed. Finally, the practical and environmental implications of the present study are discussed.

Experimental and numerical investigation of the uniaxial compression behavior of SRG-Jacketed R/C columns

This paper presents an experimental and numerical investigation of Reinforced Concrete (R/C) short columns strengthened with Steel Reinforced Grout (SRG) jackets. Nine specimens were constructed and tested under concentric monotonic uniaxial compression, measuring compressive force, axial and lateral displacements. Parameters of investigation were the influence of internal steel reinforcement (i.e. no reinforcement, sparse or dense stirrups) and the number of the externally applied SRG jackets (i.e. one or two layers). Subsequently, a three-dimensional finite element methodology was developed, aiming to predict the response of those specimens. Experimental results show that the strength and deformation capacity of the specimens was significantly enhanced with the addition of the SRG jackets, since the strengthening schemes provided sufficient passive confinement. The comparison between experimental and numerical results, in terms of failure mode, axial and lateral stress–strain curves, demonstrated satisfying correlation, proving the efficiency of the proposed numerical model.

Axial Performance of Cementitious Grouted Cable Bolts Under Rotation Constraint Scenarios

With the increased use of cable bolts in mining and civil projects worldwide, it is important to understanding the performance of these support structures in different scenarios. The axial load transfer mechanisms of six cable bolts were investigated experimentally in this research study. A large-scale pull-out testing apparatus with a capacity of 1000 kN was designed and manufactured as part of the investigation. A common commercial cementitious grout with a uniaxial compressive strength of 60 MPa was used to encapsulate 300 mm length samples. Passive confinement encapsulated each sample to ensure the concrete’s integrity during the testing. The encapsulated cable bolts were then subjected to axial loads using a hydraulic ram, with the axial load and displacement measured using a load cell and LVDTs. Two scenarios were investigated: free rotation and restricted rotation. The results demonstrated that the apparatus effectively recorded the behaviour of the cable during pull-out and identified that the radial stiffness of the samples had a significant impact on the overall load of the system. The results also indicated that rotation had a limited influence on the load behaviour of the samples tested.

Interfacial Shear Performance of Epoxy Adhesive Joints of Prefabricated Elements Made of Ultra-High-Performance Concrete.

Application of ultra-high-performance concrete (UHPC) in joints can improve the impact resistance, crack resistance, and durability of structures. In this paper, the direct shear performance of ultra-high-performance concrete (UHPC) adhesive joints was experimentally studied. Twenty-four direct shear loading tests of UHPC adhesive joints were carried out considering different interface types and constraint states. The failure modes and load-slip curves of different interfaces were studied. Results indicated that passive confinement could enhance the strength and ductility of the interface; the average ultimate bearing capacity of the smooth, rough, grooved, and keyway specimens with passive restraint were, respectively, increased by 11.92%, 8.91%, 11.93%, and 17.766% compared with the unrestrained ones. The passive constraint force changes with the loading and finally tends to be stable. The epoxy adhesive has high reliability as a coating for the UHPC interface. The adhesive layer is not cracked before the failure of the specimen, which is also different from the common failure mode of adhesive joints. Failure of all specimens occurred in the UHPC layer, and the convex part of the groove interface shows the UHPC matrix peeling failure; the keyway interface is the shear damage of the key-tooth root, and the rest of the keyway showed UHPC surface peeling failure. According to the failure mode, the shear capacity of UHPC keyway adhesive joints under passive restraint is mainly provided by the shear resistance of key teeth, the friction force of the joint surface, and the bonding force of the UHPC surface. The friction coefficient was determined based on the test results, and the high-precision fitting formula between the shear strength of the UHPC surface and the passive constraint force was established. According to the Mohr stress circle theory, the proposed formula for direct shear strength of UHPC bonded joints under passive constraint was established. The average ratio of the proposed UHPC adhesive joint calculation formula to the test results was 0.99, and the standard deviation was 0.027.

Studi Pengaruh Radius Kelengkungan terhadap Efektifitas Kekangan pada Kolom Persegi dengan Perkuatan FRP

Revised code of earthquake-resistant building design from SNI 1726:2012 to SNI 1726:2019 as a result of changes in the earthquake magnitude that occurred in recent years and developments in earthquake engineering research resulted in many building structural elements should be strengthened, especially on column elements. One of the most adopted strengthening techniques for enhancing the performance of existing RC columns consists of wrapping a member with CFRP (Carbon Fiber Reinforcement Polymer) sheets, inducing a passive confinement action capable of increasing the columns axial strength and displacement capacity. The use of CRFP as strengthening on square columns in existing buildings should consider various factors, including the column corner radius and the width of the strip of FRP required if not using full wrapping. This paper presents the confinement stresses analysis at the different radius of curvature of the square column corner and analyzes the correlation of stress-strain in the cross-section using data from various existing studies. An experimental study will also be done by axial loading test FRP column (100mm x 100mm x 200mm) with several different curvature radii. The results of the axial compression test to the specimens found that the effect of curvature is quite significant for confinement effectiveness at r/b < 0.15. Based on this experiment, a slight increase of the axial capacity on the column specimens with radius curvature r=0mm and r=10mm ( below r=13mm, the smallest radius allowed by the code). Rectangular concrete columns strengthened by FRP with r/b < 0.15 could not be effective to the confined concrete core due to stress concentration at the corner.

Behavior of Confined Concrete using Prestressing Skew Bars

This paper presents the results of an experimental study on the biaxial compression behavior of concrete prism confined using pre-stressed bars. The pre-stressed bars could provide passive confinement stress, that preventing the lateral strain of the prism from increasing leading to an increase in both the initial modulus of elasticity and prism compressive strength. The confined concrete had a higher compressive strength that was directly proportional to the confinement bar pressing force and lower ductility than the plain prisms. The concrete initial modulus of elasticity is directly proportioned to the confinement lateral pressure of the prestressing bar and inversely proportion with the spacing between prestressing bars. It was simple to find out that the best pre-stressing stress was 10 N/mm2, also the compressive strength of the confined concrete with pre-stressed skew bars was greater than the compressive strength of the unconfined concrete by more 3.3 times.

Eccentrically Loaded Concrete under Nonuniform Passive Confinement

AbstractThere is no doubt that the strength and ductility of concrete structures can be enhanced significantly by lateral confining with steel or fiber reinforced polymer (FRP). Nevertheless, study...

Investigation on GFRP Pultruded tube confined double skin tubular columns under axial loading

Composite construction with concrete filled steel tubes (CFT) as structural columns gained prominence in the recent construction scenario to make use of the benefits of both steel and concrete. Double skinned tubular (CFDST) filled with concrete between tubes is subjected to passive confinement pressure increasing the load carrying capacity of columns. This paper delineates the effect of confinement by outer steel and GFRP tube on the axial load capacity on CFDST short as well as slender columns made of steel inner tube. The experimental study was carried out on twenty composite columns consisting of ten numbers of double skin tubular short columns and ten numbers of double skin slender columns (with slenderness ratio 12.5). Ten short columns consist two pairs of columns with outer tube material as steel tube, two pairs of columns with outer tube materials as GFRP tube and one pair of reinforced concrete column. Diameter of the outer tube to thickness ratio (Do/t) of 21 and 30 is considered for all the columns irrespective of the length. The finite element analysis was undergone by modelling the columns in the Abaqus software, simulated the experimental work in the analytical study and the results are compared in terms of load deflection curves and failure modes of the columns. The SCFDST short columns failed by local buckling of outer tube whereas the failure of FCFDST short columns failure is characterized by rupture of the outer tube. The column with Do/t ratio 21 confines the concrete significantly than the tubes with Do/t ratio 30 irrespective of the outer tube material. The slender columns failed by global buckling and the ultimate load taken by the slender columns is reduced by 23% compared to short columns in SCFDST and 28% in FCFDST columns.

Bond behaviour of deformed steel bars in steel fibre high-strength self-compacting concrete

Research on the bond behaviour between deformed steel bars and the Self-Compacting Concrete (SCC) has been conducted extensively. However, bond study using the high-strength SCC (HSSCC) added with steel fibres is still rare and has not been fully explored. Therefore, in this study, experimental works were carried out to investigate the effects of adding 1% hooked-end steel fibres on the bond behaviour between deformed steel bars and the Steel Fibre High-Strength Self-Compacting Concrete (SFHSSCC) using the direct pullout test method. A total of 39 cubes of HSSCC and SFHSSCC pullout specimens with a dimension of 200 mm were prepared and tested. High-yield deformed steel bars were used as reinforcement bars. The effects of bar sizes (12, 16 and 20 mm diameter), concrete cover (94, 92, 90 and 35 mm thickness), embedment length (3 and 5 times bar diameter) and concrete age (30 ± 2 days and 6 months) towards the mode of failures and bond stress-slip curves were investigated. Coring samples were also taken from the pullout specimen to examine the distribution of the steel fibres. The results showed that size effects in rebars and the concrete cover thickness did not affect the mode of failures for a well-confined SFHSSCC as all bar sizes exhibit the same bond failure, which is pullout failure. More importantly, the addition of steel fibres enhanced the bond ductility in SFHSSCC through the passive confinement effects. The addition of steel fibres to SFHSSCC specimens bonded with 20 mm diameter bars also changed the failure mode from premature splitting to pullout. Meanwhile, the self-flowable SFHSSCC mixture effectively flowed and distributed the steel fibres within the concrete matrix.

A plasticity constitutive model for concrete under multiaxial compression

Structural members with confined concrete are becoming increasingly popular in civil engineering applications because of their superior strength and ductility. In these structural members, the concrete is subjected to dilation-induced (passive) lateral compressive stresses from the confining device (e.g., a steel tube). Existing research has led to theoretical models that predict closely the stress–strain behavior of concrete under uniform confinement (e.g., concrete in circular steel tubes under concentric axial compression), but theoretical models with a similar capability have not been achieved for the more common situation of concrete under non-uniform confinement (e.g., concrete in rectangular steel tubes). This paper presents a three-dimensional (3D) plasticity constitutive model that is accurate in predicting the stress–strain behavior of concrete in various scenarios of confinement. In the proposed model, a well-established open strength surface with associated open yield surfaces is combined with a hardening/softening rule compatible with both plastic volumetric compaction and dilation. In addition, a novel potential surface with a triangle-like deviatoric trace is proposed and calibrated with available experimental data of non-uniformly confined concrete. The implementation of the constitutive model in finite element analysis with an enhanced stress-return algorithm suitable for the novel potential surface is explained. While the focus of the present work is on monotonic compression-dominated loading, the model can be combined with fracture and damage theories to depict the behavior of concrete under tension-dominated and cyclic loading conditions. The performance of the proposed model is evaluated by comparing its predictions with a wide range of experimental data covering uniform active, uniform passive, and non-uniform passive confinement conditions, which demonstrates the capability and high accuracy of the proposed model.

A modified method for estimating the stress state of granular materials in the passive confined pressure SHPB tests

The Split Hopkinson Pressure Bar (SHPB) is the common testing method used to investigate the high strain rates behavior of engineering materials. In this paper, a theoretical modified method for the determination of stress state in the passive confinement SHPB tests of granular materials is proposed to overcome the disadvantages in the previous works. To validate the modified method, the dynamical response of calcareous sand with two moisture contents, namely, 25% and 30%, were tested by SHPB at high strain rates around 1000 s−1. The predictions provided by modified method were also compared with the experimental results, indicating that the proposed modified method in this paper is applicable in estimating the stress state in sand specimen.

Effect of active and passive concrete confinement on the bond stress-slip response of steel bars in tension

This paper summarizes the results of an experimental investigation undertaken to evaluate the effect of different types of concrete confinement on the bond strength and the local bond stress-slip behavior of steel bars in tension. Thirty-two pullout specimens were tested. The investigated parameters included type and level of confinement, and ratio of concrete cover to steel bar diameter (c/db). Two categories covering four different types of confinement were evaluated: (1) active confinement applied as external transverse pressure and (2) passive confinement using: (i) conventional transverse steel ties, (ii) carbon fiber reinforced polymer (CFRP) jackets, (iii) a combination of CFRP jackets and transverse steel ties, or (iv) a combination of CFRP jackets and CFRP anchors. The specimens developed three distinct modes of bond failure: splitting mode associated with splitting of the concrete cover, pullout mode associated with pullout of the steel bar from the concrete, and a combination of pullout and splitting modes. Confining the concrete with any of the confinement methods used increased significantly the local bond strength, altered the failure mode from splitting to pullout or mixed splitting/pullout, and improved the ductility of local bond stress-slip response when compared to unconfined concrete. These improvements were highly dependent on the type and level of confinement used as well as the ratio of concrete cover to bar diameter c/db. The failure modes, the maximum local bond strength, and the local bond stress-slip behavior corresponding to the different types of confinement were compared and discussed vis-à-vis unconfined concrete. The intrinsic shapes of the experimentally measured local bond stress-slip responses were compared with the prediction of available analytical bond constitutive relationships.

Efficacy of prestressed SMA diagonal loops in seismic retrofitting of non-seismically detailed RC beam-column joints

In this study, a detailed experimental and numerical investigation is carried out to study the efficacy of prestressed SMA diagonal compression loops in seismic retrofitting of non-seismically detailed RC beam-column joints. Prestressed diagonal compression loops exert active confinement to the joint core which is found to perform better than the conventional passive confinement in terms of improving the shear strength, ductility, and energy dissipation capacity. In this study, active confinement of the joint core is achieved by applying in-plane diagonal compression forces using externally fastened prestressed NiTiNb shape memory alloy (SMA) diagonal loops. In the experimental part, two different techniques are investigated to prestress the SMA loops, which are: 1) rapid heat-activated prestressing technique utilizing the shape memory feature of SMAs; and 2) conventional prestressing of SMA loops by mechanical means. In the finite element analysis, a wide range of prestress values (post-tension force) is studied, keeping the basic arrangement same. The efficacy of the retrofitting technique is evaluated in terms of enhancement in strength, ductility, energy dissipation capacity, damage reduction in the specimens, and the ease of application. The results from this study suggest that depending on the applied confinement level, the retrofitting scheme can increase the ultimate strength in the range of 20–30% and the energy dissipation capacity in the range of 60–70% however, no significant enhancement in ductility of the retrofitted specimen may be achieved using this approach. The study also highlights the complexity involved in controlling the material behaviour of NiTiNb SMA wires and the risks associated with the application of SMAs in practical problems.