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

Finite Element Analysis of FRP Debonding Failure at the Tip of Flexural/Shear Crack in Concrete Beam

One of the most common failure modes of strengthened RC beams with externally bonded FRP is intermediate crack (IC) debonding of FRP initiated at the tip of flexural/shear cracks. This study presents a method, using extended finite element method (XFEM), to model IC debonding in an FRP-strengthened concrete beam. In XFEM, as soon as a damage initiation criterion is reached in an element, additional degrees of element freedom are added to model crack initiation. Crack propagation is then modeled using fracture energy criterion. This method can be used to simulate debonding failure along an arbitrary, solution-dependent path without the requirement of remeshing. The numerical results are validated against experimental data and good agreement is found. A sensitivity analysis is conducted to study the effects of damage band properties and geometry on FRP debonding failure. This verifies that shear strength and critical mode II fracture energy are the parameters most affecting the FRP debonding model when the crack tip is subjected to mode II loading

The Role of Shrinkage Strains Causing Early-Age Cracking in Cast-in-Place Concrete Bridge Decks

Early-age cracking in cast-in-place reinforced concrete bridge decks is occurring more frequently now than three decades ago and principle factors that lead to early-age deck cracking are not fully understood. A finite element (FE) simulation methodology for assessing the role of shrinkage-induced strains in generating early-age bridge deck cracking is described. The simulations conducted indicate that drying shrinkage appears to be capable of causing transverse (and possibly longitudinal) bridge deck cracks as early as 9 to II days after bridge deck placement. The drying-shrinkage induced stresses would result in transverse cracking over interior pier supports in a typical bridge superstructure considered in the finite element simulations conducted

Bond–slip Behavior of Fiber-reinforced Polymer/concrete Interface in Single Shear Pull-out and Beam Tests

It has been assumed that the fiber-reinforced polymer/concrete interface is subjected to in-plane shear condition when intermediate crack debonding failure occurs. Therefore, the single shear pull-out test results are often used to predict the intermediate crack debonding failure in beams. In this study, the behavior of fiber-reinforced polymer-strengthened concrete beams and single shear pull-out specimens were studied experimentally and numerically. The bond–slip behavior of the fiber-reinforced polymer/concrete interface was obtained by single shear pull-out and beam tests. In all beam specimens, a concrete wedge located at the edge of the notch detached with the fiber-reinforced polymer debonding failure. This phenomenon shows that the initiation of debonding is due to a diagonal crack formation close to the major flexural/shear crack inside the concrete. The diagonal crack formation is due to a local moment at the tip of the notch. This causes the different stress state and slip of the fiber-reinforced polymer/concrete interface of beam specimens from that of the pull-out specimens. It is found that the bond–slip relation obtained from the pull-out test does not represent the bond–slip relation of the fiber-reinforced polymer/concrete interface in the fiber-reinforced polymer-strengthened concrete beams, and it cannot be directly used for predicting the load capacity of the fiber-reinforced polymer-strengthened concrete beams

Fatigue Risks in the Connections of Sign Support Structures

This research effort develops a reliability-based approach for prescribing inspection intervals for mast-arm sign support structures corresponding to user-specified levels of fatigue-induced fracture risk. The resulting level of risk for a particular structure is dependent upon its geographical location, the type of connection it contains, the orientation of its mast-arm relative to north and the number of years it has been in service. The results of this research effort indicate that implementation of state-of-the-art reliability-based assessment procedures can contribute very valuable procedures for assigning inspection protocols (i.e. inspection intervals) that are based upon probabilities of finding fatigue-induced cracking in these structures. The engineering community can use the results of this research effort to design inspection intervals based upon risk and thereby better align inspection needs with limited fiscal and human resources

Transfer Length of Strands in Prestressed Concrete Piles

A top bar effect has been identified in prestressed concrete piles. The effect that this top bar effect has on the development of the prestressing strand is investigated. Strand transfer length is 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 investigated. Finally, recommendations for in-plant testing and acceptance criteria for prestressed strand bond quality are proposed

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, for the first time, 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

Enhancing FRP-to-concrete Bond Behavior by Epoxy Ribs

The bond between external bonding (EB) of fiber reinforced polymer (FRP) composite materials to concrete is the weakest link in the strengthened concrete flexural members. There are three mechanisms to transfer the interfacial shear between FRP and the concrete substrate, i.e., adhesion, interlocking and friction. This paper proposes a new approach by grooving on the concrete surface before applying epoxy to make epoxy ribs to increase interlocking. An experimental program was conducted to verify the effectiveness of the proposed epoxy ribs. Six grooves perpendicular to the fiber direction were cut on the bonding surface of the concrete blocks. The grooves were filled by wax in the unfilled specimens and with epoxy primer in the epoxy filled specimens before CFRP plate was installed. The experimental results show that epoxy-filled grooves can significantly improve the bond between FRP and concrete

Excessive Strand End Slip in Prestressed Piles

This paper presents the results of a research project that investigated excessive strand end slip observed recently in some prestressed piles. From measurements taken in the field, it is apparent that the problem o excessive initial strand slip is independent of pile shape and size. Strand end slip is evident in piles of different manufacturers in different states in the Southeast. 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 preventive measures include: a) proper concrete mixture proportioning to reduce top bar effect; b) use of higher-strength concrete with the lowest possible slump and setting time; c) assessment of the condition of the strands prior to installation to insure excellent bond characteristics; d) gradual release of prestress, with an optimal release sequence; and e) use of adequate vibration to ensure consolidation. The strand end slip measured at five prestressing plants in the Southeast is 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 due to 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 excellent 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