Baolin Wan

The Effect of the Presence of Water on the Durability of Bond between CFRP and Concrete

This research assesses the effects of the presence of water during the carbon fiber reinforced polymer (CFRP) application and after the CFRP cure on the bond between the CFRP and the concrete. Modified double cantilever beam (MDCB) specimens are used to determine the interfacial energy release rate, G, of the CFRP—concrete bond. A CFRP fabric is applied to the specimens with three different initial levels of water/moisture presence. Other specimens, with a CFRP fabric applied in dry conditions and allowed to cure, are conditioned in a saturated environment for different lengths of time. The test results indicate that the presence of water during the CFRP application decreases the bond quality significantly and most of the resulting failures are adhesive failures along the primer/concrete interface. The use of a specially formulated primer results in a slightly higher bond capacity but the same undesirable failure. High quality CFRP applications, conditioned in a saturated environment for a relatively short period of time, from 3 to 8 weeks, demonstrate that exposure to water degrades the bond between CFRP and concrete after the epoxy has cured.

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.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...

INFLUENCE OF MORTAR RHEOLOGY ON AGGREGATE SETTLEMENT

This study examined the influence of the rheology of fresh concrete on the settlement of aggregate. Fresh concrete exhibits a yield stress that, under certain conditions, prevents the settlement of coarse aggregate, although its density is larger than that of the suspending mortar. Calculations, based on estimates of the yield stress obtained from slump tests, predict that aggregate normally used in concrete should not sink. To test this prediction, the settlement of a stone in fresh mortar is monitored. The stone does not sink in the undisturbed mortar (which has a high yield stress), but sinks when the mortar is vibrated, presumably because of a large reduction in its yield stress. This implies that during placement of concrete, the aggregate settles only while the concrete is being vibrated. A unique experimental method for measuring aggregate settlement is also introduced and demonstrated.

Fracture Criterion for Carbon Fiber Reinforced Polymer Sheet to Concrete Interface Subjected to Coupled Pull-Out and Push-Off Actions

Flexural strengthening of reinforced concrete (RC) beams using externally bonded fiber reinforced polymer (FRP) sheet is a popular application nowadays. In this application, the FRP to concrete bond interface is typically exposed to a mixed-mode loading condition at the vicinity of a critical flexural/shear crack, which includes a pull-out action parallel to the interface and introduced by the opening displacement of the flexural/shear crack together with a push-off action perpendicular to the interface and introduced by the shear sliding displacement of the flexural/shear crack. This paper presents an experimental program to evaluate the fracture properties of the FRP to concrete interface under the coupled pull-out (mode II) and push-off (mode I) loading condition. An analytical model was developed to evaluate the mode I and mode II strain energy release rates and the corresponding fracture mechanisms of the FRP to concrete interface under a pure bending action, a pure dowel action, and the coupled action of both, respectively. The reliability of the proposed analytical model was verified in comparison with the experimental results.

TOP BAR EFFECTS IN PRESTRESSED CONCRETE PILES

The top bar effect in reinforced concrete is a widely recognized phenomenon. Currently, the ACI Building Code prescribes a 30% increase in the development length of top cast reinforcing bars. No such provision is required for strands in prestressed concrete members. In this paper, the top bar effect for prestressing strands is introduced. Parameters affecting top bar phenomena in prestressed concrete piles are identified, and strategies for reducing this effect are presented. Finally, the application of a top bar effect factor for prestressed concrete development length calculations, similar to the one applied in reinforced concrete structural elements, is proposed.

TRANSFER LENGTH OF STRANDS IN PRESTRESSED CONCRETE PILES

A top bar effect was identified in prestressed concrete piles, and the effects developmental effects on the prestressing strand were investigated. Strand transfer length was found to be proportional to the observed end slip. While the average transfer length of all strands in a section may satisfy the assumptions inherent in the ACI transfer length equation, due to the top bar effect, top-cast strand transfer lengths are considerably in excess of the ACI-calculated value. The flexural behavior of the pile, accounting for varying transfer lengths through its section, is also studied. Lastly, recommendations for in-plant testing and acceptance criteria for prestressed strand bond quality are proposed.

Bond Behavior of FRP–Concrete in Presence of Intermediate Crack Debonding Failure

AbstractAn experimental and numerical program to investigate the behavior of intermediate crack (IC) debonding failure and the bond-slip relationship between the fiber-reinforced polymer (FRP) plat...

An Analytical Model of FRP-Concrete Bond Deterioration in Moist Environment:

Debonding of FRP from concrete substrate can lead to premature failure of the FRP repaired or retrofitted concrete structures. Therefore, FRP-concrete bond is a major factor affecting the performance of FRP strengthened concrete members. Experimental studies show that moisture plays an important role in the reliability of the bond between FRP and concrete. In this study, a cohesive zone model (CZM) of debonding along the FRP–concrete interface under peel load was analytically established by using a linear elastic separation-stress law. The interface region relative humidity (IRRH) was selected as the primary factor to study the bond durability of FRP/concrete bond. Based on a nonlinear deterioration model, the analytical solution of the CZM was used to determine critical load vs. crack length relations (P-a curves) of Modified Double Cantilever Beam (MDCB) specimens at different moisture exposure durations. The good agreement with experimental data indicates that the CZM for FRP-concrete joint is an efficient way to analyze bond behavior in moist environments.

Excessive Strand End Slip in Prestressed Piles

Recently, inspectors of the South Carolina Department of Transportation observed a strand end slip problem involving 610 mm (24 in.) octagonal piles being cast at the Socastee Bridge location near Conway, South Carolina. From measurements taken in the field, it is apparent that the problem of excessive initial strand slip is independent of pile shape and size. Excessive strand end slip was found in both the top and bottom of the cross section of the piles, although the top portion of the cross section generally exhibited much higher initial slip. Several preventive measures can be adopted to reduce the excessive strand end slip. These measures include proper concrete mixture proportioning to reduce top bar effect, use of higher-strength concrete with the lowest possible slump and setting time, assessment of the condition of the strands prior to installation to insure excellent bond characteristics, gradual release of prestress with an optimal release sequence, and use of adequate vibration to ensure consolidation. The strand end slip measured at five prestressing plants in the Southeast was considerably higher than the allowable end slip and is expected to affect the pile performance. If the strand slip theory is adopted, the strand development length increases substantially because of the excessive strand end slip. A top bar effect factor similar to the one used in reinforced concrete design is recommended. To maintain the excellent quality of precast and prestressed concrete products, manufacturers should adopt a dynamic quality control process that follows the rapid changes in the industry. More tests are necessary to ensure quality, such as the Moustafa or an equivalent test, to assess the bond capabilities of the strands, end slip measurements, and direct measurement of the transfer length. Installation of piles should proceed in a manner to alleviate the top bar effects by placing piles alternately in their best and worst directions.

Sensitivity Analysis of Stress State and Bond Strength of Fiber-reinforced Polymer/Concrete Interface to Boundary Conditions in Single Shear Pull-out Test

The bond between fiber-reinforced polymer and concrete substrate plays a key role in the performance of concrete structures after strengthened by externally bonded fiber-reinforced polymer composite materials. The single shear pull-out test is generally used to determine the interface characteristics, and various bond–slip models have been proposed based on the results of this test. However, the sensitivity of the bond strength to the boundary conditions has not yet been considered in the available models in the literatures. This article presents an experimental and numerical study targeted at understanding the influence of the boundary conditions on the bond strength of the fiber-reinforced polymer/concrete interface in the single shear pull-out test. The validated finite element analysis by experimental results is used for the sensitivity study of the bond strength and stress state of the interface to the boundary conditions of the concrete block. It is found that the constraint height of the concrete block at the loaded side is an influential parameter on the stress state of the interface and the bond strength.