Experimental Investigation of Bonded Fiber Reinforced Polymer-Concrete Joints under Cyclic Loading

The strengthening of reinforced concrete structures by means of externally bonded fiber reinforced polymers (FRPs) is becoming an attractive technique for upgrading existing structures. Although previous laboratory investigations have shown that the bending capacities of beams can be increased considerably with this strengthening technique, premature failure by debonding of the FRP reinforcement can often limit its effectiveness. To gain insight into debonding phenomena, various experimental and analytical investigations of the behavior of bonded FRP-to-concrete joints have been carried out. However, such studies have generally been limited to monotonic (“static”) loading conditions. In this paper, we present results from an experimental investigation of bonded FRP-to-concrete joints under cyclic loading. First, we describe the experimental setup and test parameters. Next experimental results for the effects of cyclic loading on slip at the FRP–concrete interface, crack opening, and strain profiles along the bonded FRP joint are presented and discussed. A power-law expression for the so-called “S–N” curves (cyclic stress ranges versus numbers of cycles to failure) is proposed, and the parameters in this expression are determined from the experimental data. The influence of various parameters such as bond length, bond width, and cyclic bond stress levels on fatigue behavior are discussed.     

Contact Interface Model for Shallow Foundations Subjected to Combined Cyclic Loading

It has been recognized that the ductility demands on a superstructure might be reduced by allowing rocking behavior and mobilization of the ultimate capacity of shallow foundations during seismic loading. However, the absence of practical reliable foundation modeling techniques to accurately design foundations with the desired capacity and energy dissipation characteristics and concerns about permanent deformations have hindered the use of nonlinear soil–foundation–structure interaction as a designed mechanism for improving performance of structural systems. This paper presents a new “contact interface model” that has been developed to provide nonlinear relations between cyclic loads and displacements of the footing–soil system during combined cyclic loading (vertical, shear, and moment). The rigid footing and the soil beneath the footing in the zone of influence, considered as a macroelement, are modeled by keeping track of the geometry of the soil surface beneath the footing, along with the kinematics of the footing–soil system, interaction diagrams in vertical, shear, and moment space, and the introduction of a parameter, critical contact area ratio (A/Ac); the ratio of footing area (A) to the footing contact area required to support vertical and shear loads (Ac). Several contact interface model simulations were carried out and the model simulations are compared with centrifuge model test results. Using only six user-defined model input parameters, the contact interface model is capable of capturing the essential features (load capacities, stiffness degradation, energy dissipation, and deformations) of shallow foundations subjected to combined cyclic loading.     

Simple Model for the Cyclic Behavior of Smooth Sand-Steel Interfaces

The object of this note is the formulation of a simple elastoplastic model for the behavior of smooth sand-steel interfaces. The model is derived from a series of constant normal stiffness direct shear tests between a siliceous sand and a smooth steel plate. These tests highlight the importance of the shear stress degradation on the final value of the shear resistance and can be seen as the elementary mechanism that models in the laboratory the pile shaft-soil interaction. The goal of the presented model is to represent the observed behavior in a very simple way by using a reduced number of constitutive parameters.     

Assessment of Subballast Filtration under Cyclic Loading

This paper presents an investigation into the seepage hydraulics of a layer of subballast filter subjected to cyclic loading in a fully saturated environment. A multilayer mathematical approach was used to predict the time-dependent permeability of this filter, with a reduction in porosity as a function of compression under cyclic loading, and the amount of base soil (<150?μm) trapped within the filter voids being the two main aspects of this proposed model. Laboratory test results conducted on a novel cyclic loading permeameter apparatus were used to validate the proposed model. The family of equations that are an integral part of the proposed model are then presented in the form of compact visual guidelines anticipated to provide a more practical tool for railway design practitioners.     

Analytical Model for CFRP-Strengthened Prestressed Concrete Girders Subject to Cyclic Loading

An analytical model is presented to simulate the behavior of prestressed concrete girders strengthened with various carbon fiber-reinforced polymer systems and subjected to static and cyclic loading. The initial concrete strains owing to prestressing and girder self weight load at the moment of the application of the strengthening system and the concrete cyclic creep as a result of the cyclic loading are considered in the model. Experimental results are used to validate the analytical model. Additionally, deflection and concrete strain increases on account of the cyclic loading are compared to the values provided by Comité Euro-International du Béton and Fédération Internationale de la Précontrainte (CEB-FIP) and Asso?i?o Brasileira de Normas Técnicas (NBR) 6118 codes. Deflection and concrete strain obtained from the analytical model were above those observed in the tested girders, especially after 100,000?cycles, owing to the logarithmic function used to express the fatigue behavior of concrete. In general, deflection provided by CEB-FIP was above experimental and analytical deflection, but otherwise concrete strain values provided by NBR 6118 were close to experimental results.     

Stress Deformation and Fluid Pressure of Bone Specimens under Cyclic Loading

Cyclic loading has been known to induce fluid flow and thus mechanotransduction in bones. In the past, four-point bending tests have been used exclusively in studying fluid flow in bones. In order to better understand the mechanism of deformation and fluid flow under loading, compression tests were done on trabecular bone specimens under drained and undrained conditions. In the drained tests, the volume change was observed, whereas in the undrained tests, excess pore fluid pressure was measured. Cyclic loading tests were conducted in addition to monotonic loading tests to observe the permanent volume change or excess pore fluid pressure with loading cycles. A fast loading rate gave a sharp rise in the excess fluid pressure compared to a slow loading rate. The strength and stiffness of the specimens appeared to deteriorate with an increased speed of loadings, but there was no appreciable difference between the results obtained from drained and undrained tests. The drained and undrained tests as described allowed a better understanding of bone behavior under loadings for a coupled stress-flow analysis.     

Behavior of a Semiintegral Bridge Abutment under Static and Temperature-Induced Cyclic Loading

This paper presents the results of an experimental study of semiintegral bridge abutments. Primary interests were to investigate (1) potential problems with the particular detail tested; (2) rotational characteristics of the semiintegral abutments; and (3) ability of the specimens to withstand cyclic loading induced by temperature variations during the expected life of the bridge. Sixteen experiments were conducted on three large-scale specimens. The results of the tests have shown that semiintegral abutments can significantly reduce the moments transferred from the superstructure to the foundation piles. Test results have also shown that semiintegral abutments can tolerate the number of displacement cycles that a bridge will experience during the course of its economic life.     

Response of Multilayer Foundation System beneath Railway Track under Cyclic Loading

For an efficient and economical design of a railway track system, it is necessary to understand the behavior of each track component with special reference to ballast and subgrade, which play a pivotal role in distributing the large, cyclic wheel loads longitudinally, laterally, and vertically away from the wheel contact area on the rail surface to the underlying soil strata. This paper presents an analytical model of a track-ballast-subgrade system with different formation soils such as dense uniform sand, stiff clay, loose sand, and soft clay modeled by using a mass-spring dashpot system with two degrees of freedom. This represents the varying energy distribution through ballast and subgrade in the vertical direction. Results are presented in the form of time-displacement response profiles for both the ballast and subgrade layers. In addition, the magnification factors for displacements with variation in subgrade soils for cyclic loading frequencies are reported. It is observed that the results obtained from the present analysis follow the experimentally observed trends already available in the literature.     

A Robust Macroelement Model for Soil–Pile Interaction under Cyclic Loads

The principal focus of this study is the development of a robust macroelement model for soil–pile interaction under cyclic loads. The model incorporates frictional forces and formation of gaps at the soil–pile interface as well as hysteretic behavior of the soil. The plastic envelope of the soil behavior is modeled via the so-called p–y approach, outlined in American Petroleum Institute’s guidelines for design of foundation piles for offshore platforms. The macroelement is an intuitive assembly of various basic elements, each of which incorporating a particular aspect of the soil–pile interaction. The modular structure of this macroelement allows straightforward adaptation of improved constitutive models for its building blocks. Herein, we focus on large-diameter, cast-in-drilled-hole reinforced concrete piles (piers) that are partially or fully embedded in soil. These types of piles are frequently used as support structures in highway construction. Consequently, the numerical robustness of the interaction model is assessed with parametric studies on pile systems and soil types relevant to this type of construction. Both elastic and inelastic pile behaviors are considered in the parametric studies. The results indicate that the proposed interaction element is numerically robust, and thus, amenable to routine structural analysis.     

Multicomponent Model of Reinforced Concrete Joints for Cyclic Loading

Efficient simplified models are available for reinforced concrete beams and columns, with the assumption of a perfectly rigid connection. However, interior beam∕column joints have been shown to be the area of strong deformation and degradation mechanisms, involving mainly the cyclic behaviors of concrete under shear and of bond under push-pull loading. In this paper, a global component-based model is proposed for the beam∕column connections of reinforced concrete frame structures that can be directly connected to beam elements. This model incorporates explicitly the modeling of concrete, steel, and steel∕concrete bond. Thus, the key mechanisms of deformation and degradation of the connection, as well as various interactions can be taken into account naturally. In particular, the effect of push-pull-type loading on the flexural steels can be captured. On the basis of a local finite-element modeling of the connection, simplifying assumptions are proposed and implemented leading to the component-based model. Both approaches (local and component-based) are compared on an application example.     

Uplift Capacity of Suction Caissons under Sustained and Cyclic Loading in Soft Clay

A series of centrifuge tests were conducted on instrumented model suction caissons in normally consolidated, lightly overconsolidated, and sensitive clays, to investigate the uplift capacity and external radial stress changes for sealed caissons subjected to sustained loading and cyclic loading. The external shaft friction ratio during vertical pullout for these two types of loading was analyzed from the radial stress measured at failure, then the corresponding reverse end-bearing capacity factor was derived from the pullout capacity. Tests results were compared with those under monotonic undrained pullout. For caissons under sustained loading, the holding capacity was found to be 72–85% of that under monotonic undrained loading. Radial stress reduction around the caisson shaft reduced the external shaft friction ratio to 0.67–0.75, while dissipation of the “passive” suction at the caisson tip reduced the reverse end-bearing capacity factor to 7.5–9.4. Under cyclic loading, the uplift capacity of the caisson was found to be 72–86% of the monotonic capacity. Repeated loading reduced the external shaft friction ratio to 0.65–0.80, while the reverse end-bearing capacity factor reduced to 6.4–9.0.     

Experimental and Numerical Study of Railway Ballast Behavior under Cyclic Loading

This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10–40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20?Hz ? f ? 30?Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel approach was used to model a two-dimensional (2D) projection of real ballast particles. Clusters of bonded circular particles were used to model a 2D projection of angular ballast particles. Degradation of the bonds within a cluster was considered to represent particle breakage. The results of DEM simulations captured the ballast behavior under cyclic loading in accordance with the experimental observations. Moreover, the evolution of micromechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading was presented to explain the mechanism of particle breakage. It has been revealed that particle breakage is mainly due to the tensile stress developed during cyclic loading and is located mainly in the direction of the movement of ballast particles.     

Bond Behavior of Near-Surface Mounted CFRP Laminate Strips under Monotonic and Cyclic Loading

Near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminate strips are used to increase the load-carrying capacity of concrete structures. This is done by inserting the CFRP strips into slits made in the concrete cover of the elements to be strengthened and gluing the strips to the concrete with an epoxy adhesive. In several cases the NSM technique has substantial advantages when compared with externally bonded laminates. To assess the bond behavior between the CFRP and concrete under monotonic and cyclic loading, an experimental program was carried out based on a series of pullout-bending tests. The influence of the bond length and loading history on the bond behavior was investigated. In this work the details of the tests are described and the obtained results discussed. Using the experimental data and an analytical-numerical strategy, a local bond stress-slip relationship was determined. A finite-element analysis was performed to evaluate the influence of the adhesive on the global response observed in the pullout-bending tests.     

Increasing the Resistance of Piles Subject to Cyclic Lateral Loading

Small-scale tests were carried out on a monopile and fin piles to determine the effect the length of fins had upon the lateral displacement of cyclically loaded piles. A variety of loading conditions were applied to model piles in a dense sand by using a mechanical loading system. Ten thousand cycles were used in each test to represent 20 years of environmental loading on offshore structures. Variables included the magnitude, frequency, and direction of the load; the type of pile tip; and the length of the fins. The reduction in pile head displacement was used as a measure of the efficiency of the fins. The tests show that the fins reduced the lateral displacement by at least 50% after 10,000 cycles.     

Instability and Complete Failure of Steel Columns Subjected to Cyclic Loading

In a testing system design for large deformations, structural columns were loaded to complete failure, defined as either complete separation of the column or inability to sustain the prescribed axial load. The test system consists of very large stroke quasistatic jacks, digital displacement transducers that can ensure accurate measurement of large deformations, hydraulic pump units capable of controlling the oil flow, controllers that control the jack motion, and separate personal computers for operating the jack controllers and for supervising and measuring data. These components are connected on-line for data and signal operations, which enables automatic and accurate load control for tests that lead specimens to complete failure. Six columns having a square tube cross section are tested in cyclic loading condition, with the axial load and column length as major parameters. The load–deformation relationships obtained from the tests are presented in detail, and relationships among the deformation capacity, failure mode, slenderness, and axial load are discussed. Intermediate axial load (30% of the yield axial load) is effective in retarding the occurrence and growth of cracks, resulting in larger deformation capacity to complete failure. Finite element analysis accurately duplicates the experimental behavior up to a large inelastic range including material yielding, strain hardening, and local buckling. It fails to simulate the experimental behavior in a very large deformation range where the column surfaces crashed and contacted each other. More experimental data is strongly needed for the behavior of structural systems and elements at and near complete failure.     

Possible Stress Path of HCA for Cyclic Principal Stress Rotation under Constant Confining Pressures

Nowadays most hollow cylinder apparatus (HCA) in the world can only apply two dynamic loads, namely the axial load (W) and torque (MT), during cyclic principal stress rotation. The limitation of this loading mode is presented, and based on such limitation other possible and applicable loading modes are put forward: (1) On the premise of setting W a special constant related to the inner (pi) and outer (po) cell pressure, the stress path at which shear stress (q) regularly changes with rotation angle (α) under the constant b (Bishop parameter) and the constant mean principal stress p can be achieved by adjusting MT; (2) setting b at a constant (not equal to 0.5), the linear relation between p and q can be realized under a regular changing α. In this case W and MT should satisfy a certain equation and meanwhile the peak value of q should be restricted within the p?q critical state line; (3) without regarding the effect of intermediate principal stress, a stress path of (σ1+σ3)/2 monotonously changing with α can be achieved under constant q. This implies that W and MT should satisfy a combination of a number of simple trigonometric functions; and (4) the stress path of constant b (with a random value between 0 and 0.5) and monotonous relation between p and R can be available under a regular changing α. In realizing this stress path, W and MT should satisfy certain equations and the maximum R should also be restricted by the critical state line. Results of this research can not only validate possible stress paths in cyclic principal stress rotation tests but can also provide a basis for further HCA improvement.     

Inelastic Cyclic Model for Steel Braces

A beam–column element that can accurately model the inelastic cyclic behavior of steel braces is presented. A bounding surface plasticity model in stress-resultant space coupled with a backward Euler algorithm is used to keep track of spread of plasticity through the cross section. Deterioration of cross-section stiffness due to local buckling is accounted for through a damage model. The proposed formulation has been implemented in a large deformation analysis program and is shown to be capable of predicting with reasonable accuracy the experimentally observed inelastic behavior of a variety of members subjected to reversed cyclic loading and a subassemblage under simulated seismic conditions.     

Passive Earth Pressure Mobilization during Cyclic Loading

The passive resistance measured in a series of full-scale tests on a pile cap is compared with existing theories. Four different soils were selected as backfill in front of the pile cap and the load-deflection relationships under cyclic loading were investigated. The log spiral theory provided the best agreement with the measured passive resistance. The Rankine theory significantly underestimated the passive force, while the Coulomb theory generally overestimated the resistance. The displacement necessary to mobilize the maximum passive force was compared with previous model and full-scale tests and ranged from 3.0 to 5.2% of the cap height. A hyperbolic model provided the best agreement with the measured backbone passive resistance curve compared with recommendations given by Caltrans and the U.S. Navy. However, this model overestimated the passive resistance for cyclic loading conditions due to the formation of a gap between the pile cap and backfill soil and backfill stiffness reduction. Based on the test results, the cyclic-hyperbolic model is developed to define load-deflection relationships for both virgin and cyclic loading conditions with the presence of a gap.     

Two-Surface Plasticity Model for Cyclic Undrained Behavior of Clays

Based on a new type of kinematic hardening and the theory of critical state soil mechanics, a two-surface model is herein developed for predicting the undrained behavior of saturated cohesive soils under cyclic loads. The anisotropic hardening rule works in two steps: (1) introducing a new concept, memory center, to take into account the memory of particular loading history; and (2) regulating the movement of the bounding and loading surfaces according to the direction of loading paths in stress space. Conventional triaxial tests have been performed on reconstituted clay samples in the laboratory. The proposed model is verified with respect to the observed behavior of soil samples. It is shown that like a multisurface model, this model can realistically describe some important responses of clays subjected to both monotonic and cyclic loading, while incorporating the memory of particular loading events.     

Seismic Behavior of As-Built, ACI-Complying, and CFRP-Repaired Exterior RC Beam-Column Joints

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.