Freeze-thaw degradation of FRP-concrete interface: Impact on cohesive fracture response

Results from an experimental investigation into the influence of freeze-thaw action on the FRP-concrete interface fracture properties are presented. The FRP-concrete bond behavior is investigated using a direct shear test. The cohesive stress transfer between FRP and concrete during debonding is determined from spatially continuous measurements of surface strains obtained at different stages of the debonding load response. The non-linear material law for the interface shear fracture, which provides a relation between the interface shear stress as a function of relative slip between the FRP and concrete, is established for specimens subjected to different levels of damage associated with freezing and thawing action. The influence of freeze-thaw action on the cohesive stress transfer during crack propagation, and on the cohesive interface fracture parameters is evaluated using a statistical hypothesis testing method. A larger percentage decrease in the interface fracture energy due to freeze-thaw cycles compared to the corresponding decrease in the ultimate nominal stress at debonding was noted. A decrease in the length of the cohesive stress transfer zone and the maximum interface cohesive stress were also observed with freeze-thaw cycling.     

Width effect in the interface fracture during shear debonding of FRP sheets from concrete

The increasing use of carbon fiber reinforced polymer (FRP) sheets for strengthening existing reinforced concrete beams has generated considerable research interest in understanding the debonding mechanism of failure in such systems. The influence of the width of the FRP on the load-carrying capacity is investigated in this paper. The interfacial crack propagation and strain distribution during shear debonding are studied using a full-field optical technique known as digital image correlation. The results indicate the development of high stress/strain gradients at the interface as a consequence of the relative slip between the FRP and the concrete. The interface stress transfer between the FRP and concrete produces axial strain gradients in the FRP along its length. In the vicinity of the edges along the width of the FRP, edge regions comprising of both FRP and concrete are established. The edge region is characterized by high strain gradients in a direction perpendicular to the length and is of fixed width throughout the debonding process. The size of the edge regions is also found to be quite independent of the width of the FRP. Mode-II fracture condition exists in the interface directly below the FRP away from the edge regions. The interfacial crack is shown to be associated with a cohesive stress transfer zone of fixed length. During debonding, the stress transfer zone is shown to propagate in a self-similar manner at a fixed load. The interface fracture properties obtained from the portion of FRP away from the edge regions are shown to be independent of the FRP width. It is shown that when the width of concrete is larger than that required for establishing the edge regions, the nominal stress at debonding increases with an increase in the width of FRP. The scaling in the load carrying capacity during shear debonding is shown to be the result of the edge regions which do not scale with the width of the FRP.     

Experimental determination of FRP-concrete cohesive interface properties under fatigue loading

An experimental investigation of the fatigue behavior of externally strengthened concrete with fiber-reinforced polymers (FRP) is conducted. In the experimental program, strain patterns along the bonded length and the surrounding concrete are determined using digital image correlation. The results herein presented indicate that debonding occurs during fatigue and it is related to the load range. The post-fatigue bond capacity of the interface is not affected by the previous cyclic loading if the remaining bonded part is enough to fully establish the stress transfer zone associated with quasi-static crack growth. Although further research is needed, this work points out several new and interesting aspects of the fatigue behavior of the FRP-concrete interface: (1) during fatigue loading the length of stress transfer zone is smaller than the stress transfer zone associated with the cohesive crack under quasi-static loading; (2) post-fatigue results suggest the possibility of a different debonding mechanism during fatigue loading; (3) fatigue life is dominated by crack initiation for fatigue loading with high amplitude and by crack propagation on decreasing the amplitude of fatigue load cycle.     

Interfacial debonding of pipe joints under torsion loads: a model for arbitrary nonlinear cohesive laws

Adhesively bonded pipe joints are extensively used in pipelines. In the present work, Cohesive Zone Model (CZM) based analytical solutions are obtained for the bonded pipe joints under torsion. An integral form based general expression is derived which is suitable for arbitrary type of nonlinear cohesive laws. The concept of the minimum interfacial cohesive shear slip δ _m is introduced and used in the fundamental expression of the external torsion load. It is found that, when the bond length of the pipe joint is large enough, the torsion load capacity is indeed independent of the shape of cohesive laws and the bond length. It is interesting to note that the maximum torsion load capacity is achieved when the torsion stiffness of the pipe and coupler are identical. A good agreement with finite element analysis (FEA) result indicates that the current model works well. The formulation to develop a simple test method for determining the τ–δ constitutive relationship in pipe joints under torsional loads is suggested. Parametric studies of various cohesive laws are conducted. This model deepens the understanding of the interfacial debonding problem of bonded joints. The fracture energy based formulas of the torsion load capacity derived in the present work can be directly used in the design of adhesively bonded pipe joints.     

The cohesive zone model: advantages, limitations and challenges

This paper reviews the cohesive process zone model, a general model which can deal with the nonlinear zone ahead of the crack tip--due to plasticity or microcracking--present in many materials. Furthermore, the cohesive zone model is able to adequately predict the behaviour of uncracked structures, including those with blunt notches, and not only the response of bodies with cracks--a usual drawback of most fracture models. The cohesive zone model, originally applied to concrete and cementitious composites, can be used with success for other materials. More powerful computer programs and better knowledge of material properties may widen its potential field of application. In this paper, the cohesive zone model is shown to provide good predictions for concrete and for different notched samples of a glassy polymer (PMMA) and some steels. The paper is structured in two main sections: First, the cohesive model is reviewed and emphasis is on determination of the softening function, an essential ingredient of the cohesive model, by inverse analysis procedures. The second section is devoted to some examples of the predictive capability of the cohesive zone model when applied to different materials; concrete, PMMA and steel.     

A fracture-based model for FRP debonding in strengthened beams

This paper presents an experimental and analytical research study aimed at understanding and modeling of debonding failures in fiber reinforced polymer (FRP) strengthened reinforced concrete (RC) beams. The experimental program investigated debonding failure modes and mechanisms in beams strengthened in shear and/or flexure and tested under monotonic loading. A newly developed fracture mechanics based model considers the global energy balance of the system and predicts the FRP debonding failure load by characterizing the dominant mechanisms of energy dissipation during debonding. Validation of the model is performed using experimental data from several independent research studies and a design procedure is outlined.     

An irreversible cohesive zone model for interface fatigue crack growth simulation

Fatigue crack growth (FCG) along an interface is studied. Instead of using the Paris equation, the actual process of material separation during FCG is described by the use of an irreversible constitutive equation for the cyclic interface traction-separation behavior within the cohesive zone model (CZM) approach. In contrast to past development of CZMs, the traction-separation behavior does not follows a predefined path. The model definition, its predicted cyclic material separation behavior and application to a numerical study of interface FCG in double-cantilever beam, end-loaded split and mixed-mode beam specimens are reported.     

Direct determination of cohesive stress transfer during debonding of FRP from concrete

Interface cohesive stress transfer between FRP and concrete during debonding is typically obtained using measured surface strains on the FRP, along the direction of the fibers. The cohesive material law is derived under a set of assumptions which include: (a) the bending stiffness of the FRP laminate is insignificant with respect to that of the concrete test block; (b) the strains in the bulk concrete produced by debonding are negligible, thus concrete substrate can be considered rigid; (c) there is stress transfer between FRP and concrete through the FRP–concrete interface which is of zero thickness; and (d) the axial strain in the FRP composite is uniform across its thickness. In this paper, a test procedure for directly obtaining the through-thickness strains in the FRP and the concrete substrate during cohesive stress transfer associated with debonding is presented. The displacement and strain fields are measured on the side of a direct-shear specimen with the FRP strip attached on the edge. Based on the experimental results, the influence of the assumptions which have been introduced to determine the cohesive law is discussed. Within the stress transfer zone there is a sharp gradient in the shear strain. The location of the interface crack within the stress transfer zone and the cohesive stress transfer during the propagation of the interface crack are determined.     

Reinforced concrete beams strengthened with carbon fiber reinforced polymer by friction hybrid bond technique: Experimental investigation

Carbon fiber reinforced polymer (CFRP) can be used to strengthen the reinforced concrete (RC) beams. But premature debonding is the main failure model in ordinary bond technique, and the strengthening effect is limited. In order to improve bonding and restricting sliding displacement, Friction Hybrid Bonded FRP Technique (FHB-FRP) is developed. Six simple-span RC specimen beams with different strengthened methods were tested in four-point bending. The experiment results indicate that FRP debonding can be effectively prevented by the FHB-FRP strengthened beam. The ultimate load-carrying capacity of the specimen strengthened by FHB-FRP technique is able to increase by a factor of 2.13 times compared with the beam strengthened with ordinary bond technique (U-jacketing technique). In addition, the cracking and yielding loads are improved more significantly by FHB-FRP technique than U-jacketing technique. Specimens strengthened with FHB-FRP technique have cracks with a more limited distribution and width. Finally, the finite element method (FEM) is conducted to simulate the behavior of the test specimens. The results obtained from the finite element method are compared with experiment. Excellent agreements have been achieved in the comparison of results.     

Prediction of interfacial fracture between concrete and fiber reinforced polymer (FRP) by using cohesive zone modeling

Interfacial debonding between concrete and fiber reinforced polymer (FRP) is investigated through integrating experiments and computations. An experimental program is designed to evaluate interfacial fracture parameters of mode-I through cutting and bonding specimens with an FRP sheet. The evaluated fracture parameters, i.e. the fracture energy and the bonding strength, are confirmed by predicting FRP debonding failure with the cohesive zone modeling approach. In the cohesive zone model, a traction-separation relation for FRP debonding is proposed with a shape index while providing various initial descending slopes. Computational results of the cohesive zone model agree well with three-point bending test results for both FRP debonding and plain concrete fracture. Furthermore, both experimental and computational results demonstrate that the fracture energy and the cohesive strength are essential fracture parameters for the prediction of FRP debonding behavior.     

Prediction of intermediate crack debonding failure in FRP-strengthened reinforced concrete beams

The present paper addresses with intermediate crack (IC) debonding failure modes in FRP-strengthened reinforced concrete beams; a non-linear local deformation model, derived from a cracking analysis based on slip and bond stress, is adopted to predict the stresses and strains distribution at failure. Local bond-slip laws at the longitudinal steel-to-concrete and FRP-to-concrete interfaces, as well as the tension stiffening effect of the reinforcement (steel and FRP) to the concrete, are considered. Model predictions are compared to experimental results available in the literature together with predictions of other models. Reasonable agreement with experimentally measured IC debonding loads and FRP strains is observed for all examined strengthened beams. Results of a parametric analysis, varying geometrical and mechanical parameters involved in the physical problem are also presented and discussed.     

Cohesive zone model for the prediction of interfacial shear stresses in a composite-plate RC beam with an intermediate flexural crack

In this paper, an analytical method is developed to predict the distribution of interfacial shear stresses in concrete beams strengthened by composite plates. Accurate predictions of such stresses are necessary when designing to prevent debonding induced by a central flexural crack in a FRP-plated reinforced concrete (RC) beam. In the present analysis, a new theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth. Adherent shear deformations have been included in the present theoretical analyses by assuming a parabolic shear stress through the thickness of the adherents, verifying the cubic variation of the longitudinal displacement function, whereas all existing solutions neglect this effect. The results obtained for interfacial shear stress distribution near the crack are compared to the Jialai Wang analytical model and the numerical solutions are based on finite element analysis. Parametric studies are carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the softening zone size is considerably larger than that obtained by other models which neglect adherent shear deformations. However, loads at the limit of the softening and debonding stages are larger than those calculated without the thickness effect. Consequently, debonding at the interface becomes less apparent and the lifespan of our structure is greater.     

Cohesive zone modeling of interface fracture in elastic bi-materials

Some basic issues regarding the cohesive zone modeling of interface fracture between two dissimilar elastic materials are studied. The dependence of the cohesive energy density on the phase angle is first discussed under small scale cohesive zone conditions. It is then shown that in general the stress singularities in tension and shear cannot be simultaneously removed at the cohesive zone tip if a single cohesive zone length is adopted for both tensile and shear fracture modes. Finally, the energy dissipation at the tip of a prescribed cohesive zone is examined using a bilinear cohesive zone model under the uncoupled tension/shear conditions.     

Triaxiality dependent cohesive zone model

A versatile cohesive zone model to predict ductile fracture at different states of stresses is proposed. The formulation developed for mode-I plane strain accounts for triaxiality of the stress-state explicitly by using basic elastic-plastic constitutive relations combined with two stress-state independent new model parameters. Comparison with available predictions of cohesive models based on porous plasticity damage models is used to demonstrate the efficacy of the proposed model. The effect of triaxiality on conventional cohesive parameters is well predicted as peak stresses are shown to increase while the cohesive energy decreases with triaxiality.     

A single-domain dual-boundary-element formulation incorporating a cohesive zone model for elastostatic cracks

A cracked elastostatic structure is artificially divided into subdomains of simpler topology such that the well-developed classic dual integral equations can be applied appropriately to each domain. Applying the continuity and equilibrium conditions along artificial boundaries and properties of the integral kernels a single-domain dual-boundary-integral equation formulation is derived for a cracked elastic structure. A cohesive zone model is used to model the crack tip processes and is coupled with the single-domain dual-boundary-integral equation formulation; the resulting nonlinear equations are solved using the iterative method of successive-over-relaxation. The constitutive law used for a crack includes three parts: a law relating cohesive force to crack displacement difference when a crack is opening, a characterization of tangential interaction between crack surfaces when the crack surfaces are in contact, and a maximum principal stress criterion of crack advance. Incorporation of local unloading effect of the cohesive zone material has enabled a simulation of fracture with initial damage, partial development of the failure process zone at structural instability and multiple crack interaction. Some of the features of the method are demonstrated by considering three examples. The first problem is a single-edge-cracked specimen that exhibits a snap-back instability. The second example is the development of wing cracks from an angled crack under compression. The last example demonstrates the capability to consider mixed-mode crack growth and interaction of cracks. Thus, the problem of crack growth has been reduced to the determination of the cohesive model for the fracture process. This revised version was published online in July 2006 with corrections to the Cover Date.     

A superimposed cohesive zone model for investigating the fracture properties of concrete–asphalt interface debonding

A cohesive zone model is proposed to simulate the interface debonding, a preponderant cause of failure for bonded concrete overlay of asphalt (BCOA). The model is constructed by superimposing four root models, each representing the mechanism of one subcritical failure at the interface zone observed in laboratory experiments. The model parameters are established through an inverse analysis of wedge splitting tests performed on BCOA specimens. These inputs are mainly a function of the materials at the interface zone, such as microtexture and macrotexture, and thus can be expected to be applicable to the numerical simulation of a full scale BCOA slab. For modeling across scales, the impact of specimen size, milling depth and initial flaw size on the model, in terms of peak traction and fracture energy, is also discussed.     

Debonding analysis of thin plates from curved substrates

This paper is focused on the analytical modeling of the interface between a rigid substrate with constant curvature and a thin bonded plate. In a previous investigation, an analytical cohesive zone (CZ) model capable of predicting the interfacial behavior prior to the onset of debonding has been proposed by the authors. As a follow-up, this paper presents a new analytical model, which is able to predict both the load at onset of debonding and the subsequent evolution of the debonding process. The study clarifies the essential differences in behavior between joints with flat and curved substrates during debonding. The new analytical model is based on linear-elastic fracture mechanics (LEFM) and is developed under two distinct assumptions: a mode-independent fracture energy, and a fracture energy dependent on the mode mixity, according to a simple mixed-mode fracture criterion. The model is applied to the determination of the load at onset of debonding, and to the prediction of the load-displacement behavior during the entire debonding process. The relationship between the predictions of the CZ and LEFM models regarding the load at initiation of debonding is also established.     

Cohesive zone simulations of crack growth along a rough interface between two elastic-plastic solids

It has been known for many years that crack propagation along interfaces is influenced by interface topography or roughness profile. This has given rise to a small body of literature in which interface toughening with stochastic surface finishes, produced by grinding, rolling, or grit blasting, has been the primary focus. However, there is very little information currently available on the effect of patterned interfaces that are characterized by a minimal number of geometric parameters. In the present article, roughness-enhanced toughening of a cohesive interface between two identical materials is explored with a pure sinusoidal interface morphology that is characterized by its aspect ratio or ratio of amplitude to wavelength. Sixteen finite element meshes, each with a different aspect ratio, were constructed to study initiation and growth of a semi-infinite interface crack due to remote mode-I loading. The cohesive interface was modeled with a viscosity-modified Xu-Needleman cohesive zone law, and the solids were characterized with continuum elastic and elastic-plastic constitutive models. Predicted relationships between the aspect ratio and the macroscopic toughness point to key differences in the material models. A set of critical parameters which include the aspect ratio, material and cohesive properties is predicted such that catastrophic crack growth is inhibited due to crack blunting. A clear boundary between brittle and ductile fracture behavior is thus identified. The results suggest some guidelines for practical design of failure resistant interfaces through appropriate choice of geometric, material, and cohesive parameters.