Bond Fracture Energy Research Articles

Influence of Fiber Dimensions on Bridging Performance of Polyvinyl Alcohol Fiber-Reinforced Cementitious Composite (PVA-FRCC)

This study investigates the influence of fiber dimensions on the bridging performance of polyvinyl alcohol fiber-reinforced cementitious composite (PVA-FRCC) through an experimental and analytical program. Bending tests, bridging law calculations, and section analysis are conducted. Bending tests of notched specimens of PVA-FRCC with six different PVA fiber dimensions are performed to determine the load–deflection (LPD) and bending moment–crack mouth opening displacement (CMOD) relationships. The fiber volume fraction for all PVA-FRCCs is set to 2%. It is found that the load capacity of PVA-FRCC with a 27 μm diameter fiber is much higher than that of the other fibers, and the load capacity decreases as the fiber diameter increases. The study proposes parameters for the characteristic points of the tri-linear model for the single-fiber pullout model as functions of diameter, bond fracture energy, elastic modulus, cross-sectional area, and perimeter of the fiber. These findings provide valuable insights into the behavior of PVA-FRCC under different fiber dimensions. Bridging law calculations are conducted to obtain tensile stress–crack width relationships using the developed single-fiber pullout models. The Popovics model for the complete tensile stress–crack width relationship is adopted to obtain a better fit with the bridging law calculation, and then section analysis is conducted. The bridging law calculation results show that the maximum tensile stress decreases as the fiber diameter increases. It is also determined that most of the smaller-diameter fibers ruptured, whereas the larger fiber diameters pulled out from the matrix. The section analysis results show good agreement with the maximum bending moments obtained from the bending test.

Experimental characterisation of flexural bond behaviour in brick masonry

The brick-to-mortar bond often represents the weakest link leading to cracking and failure of masonry structures. For this reason, the in-situ characterization of masonry’s flexural bond behaviour (here defined as flexural bond strength and flexural bond fracture energy), is essential for the assessment of existing buildings. Among masonry bond properties, the flexural bond strength is commonly determined on-site, given the minimal invasiveness of the so-called bond wrench test. However, often the reliability of the results is questioned inputting their large variability to the operator. The present study discharges this assumption by comparing the accuracy of various testing set-ups (manually-operated vs computer-controlled set-ups). Additionally, the influence of the specimen’s type (with/without head joints and couplets vs wallet) on the flexural bond strength assessment is studied providing preliminary correlation factors that can be of help for the in-situ measurement on single-wythe masonry. In addition, to obtain a complete description of the bond behaviour, a new test set-up able to determine the post-peak response is presented. Considerations regarding the dissipated bond fracture energy and its relation to the tensile fracture energy are provided with the support of literature data.

Experimental study on the bonding behavior of post-embedded steel bar and brick masonry

Post-embedding of steel bars into mortar joints of unreinforced masonry walls is a widely used strengthening technique in historic masonry construction. Despite its widespread use, limited research is available on the effectiveness of this technique for improving the bonding performance of post-embedded steel bars. In this study, the bond-slip relationship for bars embedded in mortar joints was investigated. The masonry specimens were built using mortar with a strength of 5 MPa, and then reinforced with 5, 10, 15, and 20 MPa mortar and plain steel bars 6 and 8 mm in diameter, and a series of pull-out tests were performed on these specimens. By analyzing the bond strength between the post-embedded steel bars and brick masonry, the bond failure modes, and the bond-slip curves, a typical bond-slip curve and bonding parameters were obtained. The effects of the mortar strength, steel bar diameter, and bond length on the bond strength were discussed, and a mathematical expression for the bond strength between the post-embedded steel bars and brick masonry was presented. In addition, the relationship between the bond strength and the slip distance of the steel bars was obtained, and the interfacial bond fracture energy was analyzed.

Experimental and numerical investigations of the tensile behaviour of steel-TRM prepared with recycled glass sands and lime

Due to the decline in the material strength and increase in natural loading, the need to strengthen historical masonry buildings is vital, and different techniques have been used to enhance the structural performance of those buildings. Textile reinforced mortar (TRM) composite systems are one of the newly developed techniques that has attracted increasing interest from researchers. To investigate the feasibility of the use of more eco-friendly materials in mortars, recycled glass sand (RGS) is substituted for natural river sand in the preparation of mortar used in the TRM system, as well as lime as a partial cement replacement. RGS was substituted for river sand at 50% and 100%, and lime was substituted for cement at 50%. A total of ten mix proportions were prepared. The mechanical properties of the mortar were analysed in terms of the flexural strength and compressive strength at 7 and 28 days. The results showed that the strengths tend to decrease as the lime and RGS proportion increases. There are the maximum average reductions of 40.3% in flexural strength and 60.4% in compressive strength for the samples with 50% lime replacement, and 38.3% in flexural strength and 33.8% in compressive strength for the samples with 100% RGS replacement. The discrepancies in the strength increase rate from 7 to 28 days were observed in some samples which may due to the different surface area and grading of RGS particles change the rate of hydration and porous in mortar.Then, the tensile behaviour of steel-TRM prepared with recycled glass sand with four different mix proportions were experimentally studied. The tensile behaviours of the tested coupons agreed well with the three-phase tensile behaviour. The TRM composite coupons prepared with 100% RGS and 50% lime replacement showed the maximum reductions (73.4%) of average tensile strength compare to the TRM coupons with 50% RGS replacement, which follow the trend of the mechanical behaviour of mortar. Finally, analytical and numerical models considering varying parameters related to the material properties were developed to understand the stress–strain responses and crack patterns of different TRM coupons in tension. Both analytical and macromodelling approaches are able to capture the key global tensile behaviour of TRM as a reasonable fitting was observed with regard to predicting the change in the stiffness in the three stages. The results obtained by micromodelling approach show that TRM coupons with 100% RGS show a larger reduction in the mortar tensile strength and bond fracture energy of the mortar-textile interface. The reason for these reductions might be that higher RGS replacement can be more porous in mortar and the angular edges and rough texture of the RGS, respectively.

FRP-concrete bond performance under accelerated hygrothermal conditions

Drawing on information obtained via double lap shear tests, this paper discusses FRP-concrete bond behavior following five weeks of exposure to several accelerated hygrothermal conditions. Material tests were also carried out for both dry carbon fibers and the epoxy used in the bond study under similar hygrothermal conditions. The bond-slip relationship for the hygrothermal conditions were developed, and fracture energy calculated for the FRP-concrete bond. The use of degraded material properties obtained from material test coupons under the same exposure conditions prevented overestimation of the FRP-concrete bond fracture energy. Testing revealed that the FRP-concrete bond strength increased as temperature increased at low humidity (dry heat) — a potential consequence of alteration in the epoxy glass transition temperature. The bond strength of concrete with steel slag incorporated was superior at all temperatures and low humidity. In response to continuous or periodic submergence in water, specimen failure mode transitioned from substrate failure to adhesive-concrete interface failure. Alternating two-day wet/dry cycles resulted in a 12% drop of FRP-concrete bond strength. Under low humidity, the FRP dry fiber tensile stress increased as temperature increased (dry heat), while the opposite dynamic was observed in the epoxy coupons.

Confinement effects on fiber pullout forces for ultra-high-performance concrete

Fiber reinforcement of ultra-high-performance concrete (UHPC) is necessary for adding toughness and ductility to an otherwise very brittle matrix. The action of fibers pulling out of the concrete matrix is one of the primary mechanisms of energy dissipation. Of interest in this work is the effect of a compressive stress in the concrete matrix on the fiber pullout behavior. An experimental fixture was developed to apply confining forces to the UHPC matrix while a fiber is pulled out of the matrix. Approximately 90 pullout tests were conducted with specimens subjected to confinements of 2, 38, and 76 MPa. Complete force-pullout length data were recorded and analyzed. The results showed that confinement stresses had a positive correlation with both peak force and work of the pullout force, but the latter was only valid for the first few millimeters of pullout. When the entire response was considered, the correlation with work of pullout disappeared. An analytical pullout model was employed to isolate the effects of bond fracture energy and friction. Fitting the model to the data showed that bond fracture energy was not affected, but friction stress increased up to 60% for high confining stresses.

Effect of Pressure Cycling on Fracture Energy of Polyurethane/Aluminum Adhesive Bonds

An experimental study was conducted to investigate the effect of pressure-cycling on adhesive bond fracture energy of polyurethane/aluminum adhesive bond joints. Initially, two types of peel tests were conducted to characterize adhesive bond strength and challenges associated with pre-mature polyurethane cracking and failure during these tests are discussed. A modified double cantilever beam (MDCB) specimen configuration was specially designed and opening-mode loading conditions were employed to determine the interfacial adhesive bond energy (GC). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The GC of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased GC values to those of the control and water-soaked samples: hence, prolonged pressure-cycling could be problematic to polymer/metal adhesive bonds of hardware installed outboard of submarine pressure hulls.

Nonlinear Deterioration Model for Bond Interfacial Fracture Energy of FRP-Concrete Joints in Moist Environments

This study proposed a bond mechanism based deterioration model of bond interfacial fracture energy for fiber-reinforced polymer (FRP)-concrete joints in moist environments. The bond interface region relative humidity (IRRH) in a moist environment was correlated to the bond fracture energy in this deterioration model. The IRRH-dependent interface separation tractions were derived in the frame of a cohesive zone model. Such an IRRH-dependent interface separation-traction law was simulated by a series of nonlinear interface elements attached to the bond interface to calculate the macroscopic load-displacement curves for the modified double cantilever beam (MDCB) specimens. Through moisture diffusion analysis, IRRH was determined as a function of the moisture exposure time for given specimen dimension and environmental RH. Using IRRH as the bridge, the time-dependent load-displacement curves of the MDCB specimens were obtained. The good agreement with the experimental data indicated that the model worked well. The approach developed in this study can be used to simulate and predict the durability of the bond between the FRP and concrete in moist environments.

STUDY ON BOND SPLITTING BEHAVIOR OF REINFORCED CONCRETE MEMBERS

This study presents the theoretical solution of bond stress distribution solving fundamental differential equation on bond problem with modeled bond stress - slippage relationship by parabolic curve. The modeled relationship has almost the same shape with previously proposed by the authors considering the equilibrium conditions of bond stress and splitting stress of concrete. By the theoretical bond stress solution, the bond strength is solved by the integration of the bond stress distribution. The previously proposed calculation method using the equivalent bond stress block (EBSB) is verified by the theoretical solution, and the calculated values of bond strength show a good agreement with theoretical ones. The relationship between bond strength and bond fracture energy can be also led by the theoretical solution.

Experimental and Numerical Study of Moisture Effects on the Bond Fracture Energy of FRP/Concrete Joints

This study experimentally investigates the mechanism of deterioration for the bond between CFRP plate and concrete due to moisture attack. The debonding failure in the concrete structures with externally bonded FRP has two major modes: cohesive failure in the concrete surface layer and adhesion failure at the adhesive/concrete interface. A modified double cantilever beam (MDCB) test was used to measure the interfacial fracture energy for the CFRP plate debonding from concrete substrate under Mode I loading. A simple method was developed to measure the value of interface region relative humidity (IRRH). The relation between IRRH and the bond fracture energy was then obtained. By quantitatively measuring the residual thickness of concrete (RTC) on the detached plate, it was found that RTC was directly related to the value of IRRH. Based on the experimental data, a constitutive relation between RTC and IRRH was obtained. Using this relation and virtual crack closure technique (VCCT), the bond fracture energy was numerically determined as a function of IRRH for the FRP/concrete bond joints. It was found that when the IRRH value was greater than 55%, the deterioration of fracture energy was accelerated. When the IRRH was greater than 75%, the fracture energy tended to become steady, and did not change significantly with the increases of IRRH.

Effects of FRP-Concrete Interface Bond Properties on the Performance of RC Beams Strengthened in Flexure with Externally Bonded FRP Sheets

Fiber reinforced polymer (FRP) sheets have been increasingly used as externally bonded reinforcements in the rehabilitation of concrete structures. The efficacy of the FRP bonding technology highly depends on the bond integrity between the FRP sheets and the concrete. The bond performance may directly influence the cracking of the concrete, whereas the presence of concrete cracks would impair the bond between the FRP sheets and the concrete. This paper aims to clarify the effect of interface bond properties on the performance of FRP-strengthened reinforced concrete (RC) beams in terms of concrete cracking, interface stress transfer, and failure mechanisms using nonlinear fracture mechanics based finite element analyses. To represent the typical crack patterns and capture the local interaction between FRP debonding and concrete cracking, a specially designed structural model with uniformly distributed cracking is used within the frame of the discrete crack approach. A detailed parametric study is performed to investigate the effects of interface bond properties in terms of stiffness, strength, fracture energy (or toughness), and bond curve shape. It is concluded that bond fracture energy (or toughness) is the main parameter influencing the structural strength and ductility. This study may serve as a valuable reference for optimization of the FRP-concrete bond interface in practical applications.

Surface Energy and Adhesion in Composite–Composite Adhesive Bonds

In the absence of weak boundary layers, surface energy can be an excellent indicator of the suitability of a fiber-reinforced composite surface for adhesive bonding. Mechanical surface treatments such as grit blasting are effective and commonly used to prepare composite surfaces, but the roughness introduced by these treatments makes quantification of the surface energy by contact angle methods difficult. This paper shows that the diameter of a small drop of a low-viscosity fluid chosen to have surface tension characteristics very similar to the adhesive can be used as an effective predictor of adhesive bond fracture energy. This technique could form the basis of a sensitive quality assurance tool for manufacturing.