Direct Pull-out Tests Research Articles

The bond and flexural strengthening of reinforced concrete elements strengthened with near surface mounted prestressing steel (PS) bars

The main objective of this work is to study the performance of prestressing steel (PS) bars as reinforcements in the reinforced concrete (RC) elements strengthened by the near-surface mounted method (NSM). The work includes two parts. In the first part, direct pull-out tests are performed in order to study the bond performance between PS reinforcement and concrete. The influences of groove sizes and PS surface conditions (smooth and sand coated) are evaluated. The results show that the sand coated PS (PS-Sc) reinforcement has the best adhesion behavior compared with the smooth bar, and its pull-out force is increased by 48%. For this reason, the PS-Sc bars are used in the second part of this work as NSM reinforcement to strengthen RC beams subjected to bending forces. Then, four-point bending tests are carried out to understand the flexural behavior of strengthened RC beams with PS-Sc reinforcements of different lengths and ratios. The obtained results demonstrate that the use of NSM-PS-Sc bars strengthening technique leads to important enhancement in the load carrying capacity of the RC beams. The first crack load and ultimate load of the strengthened RC beams attain 71.41 and 65.67%, respectively, which are higher than those of the control beam. Furthermore, the experimental values show a good agreement with the analytical values in both the ultimate deflection and the ultimate load. This proves that the NSM-PS-Sc bars studied in this work are promising reinforcement of the RC beams.

Bond behaviour of NSM CFRP laminate strip systems in concrete using stiff and flexible adhesives

The influence of the adhesive type on bond behaviour between concrete and Carbon Fibre Reinforced Polymers (CFRP) laminate strips, in the context of Near Surface Mounted (NSM) strengthening technique, is considered as crucial for an efficient design. Thus, direct pullout tests were carried out to assess the influence of i) type of adhesive ii)CFRP cross-section and (iii) bond length on behaviour of NSM-CFRP system. Two types of stiff adhesives and one type of flexible adhesive were studied. For similar bond lengths, significantly higher maximum pullout force and bond stiffness were observed in the case of stiff adhesives, while noticeably higher slip at maximum force was achieved with the flexible adhesive. Analytical and numerical investigations were carried out in order to determine the local bond stress-slip relationships for both stiff and flexible adhesives. After demonstrating its good predictive performance, the analytical approach was used to design curves of the required anchorage lengths of NSM-CFRP system at ultimate limit state conditions. For the analytically calibrated mechanical parameters of NSM bond-slip law a two-dimensional numerical model of the direct pullout tests was worked out.

Bond behavior of reinforcing bars embedded in concrete made with crushed clay bricks as coarse aggregates

This study investigated the bond behavior between reinforcing bars and brick aggregate concrete (BAC) by testing cylindrical and cubic BAC specimens with design compressive strength values of 20, 25 and 30 MPa. Forty-six direct pull-out tests were conducted to assess the influence of several parameters: bar diameter, bond length, concrete confinement, specimen shape and concrete compressive strength. The effects of these parameters were presented as force-slip relations. The bond strength measurements were compared with predictions from design equations used for stone aggregate concrete (SAC). The results showed that the bond strength measurements were larger than the bond strength predictions from ACI 318 and very close to those from CEB-FIP and AS 3600. Moreover, cubic specimens exhibited larger bond strength than cylindrical specimens due to shorter bar embedment lengths. Furthermore, a comparison between SAC and BAC revealed significant mechanical property differences. Finally, a model equation was proposed using the test results to evaluate the bond strength of reinforcing bars in BAC.

Bond load-slip behaviour of FRP bars in recycled-aggregate concrete

This study presents an experimental program conducted to investigate the bond strength of FRP bars in recycled-aggregate concrete compared to the one in normal-aggregate concrete for the improved evaluation of results. The experimental program contains thirty six specimens tested using direct pull-out test. In this study, glass, carbon and basalt FRP bars are used with 12 mm diameter and bar bond lengths of 5d, where d is the bar diameter. The FRP bars are casted in different recycled-aggregate concrete strengths of 30, 45 and 60 MPa. The behaviour of bars in normal-aggregate concrete strength of 30 MPa is used as a benchmark and its behaviour is compared with the ones in the recycled-aggregate concrete. The impact of the concrete strength considered is identified based on the gain in the bond behaviour. The experimental results demonstrate the prospect of the recycled aggregates applied as an alternative to normal aggregates in the FRP reinforced concrete. In addition, the use of the recycled aggregate increases the bearing friction behaviour between the FRP bars and concrete.

The bonding strength of GFRP bars embedded within concrete under direct pull-out test

Most of the reinforced concrete (RC) structures in the marine environment attacked with corrosion of steel reinforcements that required costly repairs or replacements. The utilization of FRP bars as a non-corrosion material is one of the solutions to increase the durability of RC structures in the marine environment. The innovation of using FRP bars should be developed to improve the quality and reduce the construction cost such as by removing the concrete cover and shear rebar. By removing the concrete cover, the depth of the beam can be minimized without reducing the effective depth of the beam. To substitute the conventional shear rebar, the GFRP sheet strip will be attached to the shear span after concrete hardened. However, the problem is that the bonding area between FRP bars and concrete reduced due to the removal of concrete cover. Therefore, before applying this innovation to the beams’ structure, the pull-out test should be conducted in order to clarify this behavior. The influence of the location of FRP bars and the effect of the U-Wrap GFRP sheet on the bond-slip curves will be analyzed. It is expected from this study that the bonding strength between FRP bars and concrete does not reduce significantly and hence, the innovation for constructing RC beams without a concrete cover on the tensile zone can be applied.

Effect of silica fume and calcium carbonate whisker on interfacial behavior of near-surface-mounted FRP with magnesium phosphate cement

Magnesium phosphate cement (MPC) is a potential substitute of epoxy as an adhesive material due to its advantages in setting time, early strength, and good fire and corrosion resistance. In this study, silica fume (SF) and calcium carbonate whisker (CCW) were employed to improve bond capacity of MPC which were used instead of epoxy in near-surface-mounted (NSM) fiber-reinforced polymer (FRP) systems. A direct pull-out test (DPT) was carried out to investigate bond performances of FRP–concrete interface after incorporating SF and CCW. According to the mix proportion of the MPC, a total of twenty-seven specimens in nine sets were divided into four groups: one group without additive, one group with SF alone, one group with CCW alone, and the last group with SF and CCW combination. Results showed that SF or CCW alone could improve MPC bond capacity, but their excess application would reduce it. SF and CCW combination, however, did not improve bond capacity as effectively as SF or CCW alone. Moreover, the addition of CCW would improve MPC’s bond ductility, with or without the adding of SF, but with the increase in CCW concentration, this improvement effect would decrease. Meanwhile, SF alone lowered its bond ductility.

Bond behavior between multi-strand tendons and surrounding grout: Interface equivalent modeling method

An interface equivalent modeling method is proposed in this study to describe the bond behavior between multi-strand tendons and surrounding grout. The interface equivalent principle is firstly proposed to determine the effective bond interface between tendons and grout considering the effects of the quantity, the arrangement and the space of strands. Then, the maximum bond stress of the multi-strand tendons provided by the interface is derived and incorporated into the existing bond stress-slip relationship to characterize the bond of multi-strand tendons. Subsequently, a pull-out numerical model incorporating the considered effects is developed to predict pullout forces-slip response for tendons. Finally, a direct pull-out test on three strands tendons is designed to verify the interface equivalent modeling method by comparing with the numerical and experimental results. The proposed method is also validated against previous experiment on seven strand tendons. Results indicate that the proposed interface equivalent modeling method can reasonably predict the bond behavior between multi-strand tendons and surrounding grout. The applicability and feasibility of the proposed equivalent modeling method is also investigated.

Bond behavior between steel bar and engineered cementitious composite (ECC) considering lateral FRP confinement: Test and modeling

Engineered cementitious composite (ECC) exhibits good post-cracking resistance and ductility and its unique properties make it a viable material for increasing structural performance under severe loading conditions. To understand the bond behavior between the ECC and steel bar, direct pullout tests were conducted in this paper. A comprehensive analysis was carried out on effects of ECC strength, steel bar diameter, and fiber reinforced polymer (FRP) confinement on steel bar-ECC bonding behavior. Based on the experimental data, performances of existing bond slip models for steel bar-ECC are evaluated and a new model with improved accuracy is proposed. The proposed model considers the influence of FRP confinement on bond-slip behavior.

Methodology for Testing Rebar-Concrete Bond in Specimens from Decommissioned Structures

Mimicking natural deterioration in accelerated tests is challenging; a highly relevant alternative option is to use deteriorated specimens from decommissioned structures. This paper describes a methodology to select and design tests of the bond and anchorage between reinforcement and concrete in such specimens, with the aim of providing general information, needed when developing methods for assessing structures in general. The methodology includes the following steps: (1) choice of existing structure for samples, (2) choice of test method, (3) design of test setup, and (4) design of test programme. Each step is discussed in detail and comments are made on considerations and challenges arising specifically due to the use of specimens from existing structures. As the scatter of test results is typically large, a suitable test method should enable a large number of tests by being robust, quick and affordable. It is recommended to keep track of the position of the specimens in the original structure, to document cracks, and to take samples also of uncorroded bars. These can then be used for reference in quantifying the corrosion level of corroded bars. This methodology is exemplified in the design of three test series on edge beams from two bridges; two series resulted in beam test setups and one in direct pull-out tests. The methodology described strongly highlights that careful investigations are required to design experiments which generate reliable data. Acquiring data from decommissioned structures will improve our understanding of the structural behaviour of existing structures and thus enable improved assessment methods.

Investigation of Bonding Behavior of FRP and Steel Bars in Self-Compacting Concrete Structures Using Acoustic Emission Method.

To extend understanding of the bonding behavior of fiber reinforced polymer (FRP) and steel bars in self-compacting concrete (SCC), an experimental series consisting of 36 direct pull-out tests monitored by acoustic emission (AE) were performed in this paper. The test variables involved rebar type, bar diameter, embedded length, and polypropylene (PP) fiber volume content. For each test, the pull-out force and free end slip were continuously measured and compared with the corresponding AE signals. It was found that the proposed AE method was effective in detecting the debonding process between the FRP/steel bars and the hosting concrete. The AE signal strength exhibited a good correlation with the actual bond stress-slip relationship measured in each specimen. Based on the AE location technique, the invisible non-uniform distribution of bonding stress along the bar was further revealed, the initial location of damage and the debonding process were captured. Additionally, the contribution of bar-to-concrete load-bearing mechanism (chemical adhesion, friction, and mechanical interlocking) to sustain the pull-out force was effectively clarified by studying the collected signals in the frequency domain of AE methods. The experimental results demonstrate that the proposed AE method has potential to detect the debonding damage of FRP/steel bar reinforced SCC structures accurately.

Effect of casting direction on bond of reinforcement in High Performance Self-Compacting Concrete (HPSCC)

This paper presents the results of an investigation into the steel-to-concrete bond in high-performance self-compacting concrete (HPSCC) based on direct pull-out tests. Specifically, the effect of the casting direction on bond properties is examined. Two variants of concrete mixture casting were considered in this work: from the top and from the bottom of a mould with a single casting point at one edge. Horizontal specimens with transverse rebars distributed over their heights (480 mm) and lengths (1600 mm) were cast. The experimental program has shown that the direction of concreting plays an important role in the formation of the bond condition. In the case of casting a mixture from the bottom of a mould, in the lower part of a specimen there is no significant change of the bond stress. However, there is a strong improvement in the upper part. The phenomenon observed eliminates the top-bar effect. What is more, for the rebars situated in the upper part of a specimen, improvements in bond characteristics, such as the bond stiffness, bond strength and the effect of the rebar distance from the casting point are all observed in the case of casting a mixture from the bottom of a mould.

On bond-slip response and development length of steel bars in pre-cracked concrete

Previous research on steel bar-concrete bond behaviour has been concentrated mostly on the intact concrete without considering initial cracks induced by transverse tensile loading called pre-cracking phenomenon. There is no accurate model for evaluating bond behaviour and development length of steel bar in pre-cracked concrete. This paper aims to characterise the bond-slip behaviour of steel bars in pre-cracked concrete by direct pull-out tests, and proposes a constitutive law as a function of the crack width. Results show that induced cracks, notably cracks wider than 0.15 mm, cause a significant reduction in maximum and residual bond strength. Also, results indicate that larger crack widths result in considerably lower dissipated energy by the bond mechanism. The results obtained from both the experimental tests and referenced database demonstrate that the pre-cracking phenomenon has a higher impact on the residual bond strength compared to the maximum bond strength. Unlike existing equations, the proposed model accurately considers cracking effects on the steel bar-concrete bond properties and shows a satisfactory fit with the experimental database. A predictive equation is also proposed for calculation of the development length in pre-cracked concrete, which is more conservative and prudent compared to existing regulations in design codes.

Influence of silica fume content on bond behaviour of reinforcement in HPSCC

The steel-to-concrete bond in high-performance self-compacting concrete (HPSCC) was investigated based on direct pull-out tests. The analysis was performed on specimens made from four different HPSCC mixes with varying contents of silica fume (SF) (0, 5, 10 and 15% by mass of cement). The specimens used allowed the determination of changes in bond at individual levels of elements with a total height of 160, 480, 800 and 1600 mm. The rebars in the elements were placed perpendicularly to the direction of concreting. A reference element, characterised by a parallel orientation to the direction of concreting, was also prepared. It was found that the modification of HPSCC using SF improved the quality of bond conditions, especially in the case of elements with substantial height. There was, however, no effect of rebar orientation with respect to this casting direction and to the change in bond stiffness.

Fuzzy logic model for pullout capacity of near-surface-mounted FRP reinforcement bonded to concrete

The application of fiber-reinforced polymer (FRP) strips or rods in the form of near-surface-mounted (NSM) reinforcement has become an attractive solution to strengthen the existing buildings and bridges. It is of interest to engineers to have an accurate estimate of the bond capacity of this technique. In this paper, fuzzy logic approach is utilized to propose an alternative method of determining the pullout strength of NSM FRP strips/rods which are bonded to the concrete block. Two types of fuzzy logic models, namely Mamdani and Takagi–Sugeno, are developed. With the aim of enhancing the interpretability of the fuzzy model, the rule base of Mamdani model is extracted from the classification decision tree, and the membership functions corresponding to the linguistic concepts are built by uniform partitioning the range of variables. On the other hand, in order to arrive at closed-form equations for pullout capacity, the subtractive clustering algorithm is employed to deduce the rule base and membership functions of Takagi–Sugeno model (first order), and its consequent part is tuned by the least square optimization using training dataset. Several fuzzy logic models of both types with different numbers of rules are developed and compared in terms of different error measures. To train and validate the fuzzy models, a large database of 384 direct pullout tests on NSM FRP bonded to concrete is assembled from the literature. The results reveal that both of the proposed Mamdani and Takagi–Sugeno models demonstrate good accuracy against the experimental data and outperform the published models. A parametric study indicates that the proposed fuzzy models can predict the maximum effective bond length, and thus, they are able to capture the underlying mechanics of the problem.

Bond behavior of steel bar embedded in Engineered Cementitious Composites under pullout load

Composite action of steel bar and Engineered Cementitious Composites (ECC) plays an important role in reinforced ECC structures, which depends on the bond performance of two materials. To evaluate the bond behavior of steel bar embedded in ECC, direct pullout tests were conducted on 10 groups of ECC specimens and 2 groups of control specimens in this paper. The parameters investigated were bar shape, bar diameter, cover thickness, ECC strength and fiber volume content. Experimental results indicated that the ultimate bond strength of the plain bar embedded in ECC increased 1.71 times than that in concrete. For the deformed bar, the bond failure mode of ECC specimens exhibited an obvious ductility due to fiber bridging effect preventing the opening and propagation of internal cracks, whereas the brittle splitting failure occurred for concrete specimens. Furthermore, the ultimate bond strength increased 2.14 times as ECC strength increased from 27.7 to 57.2 MPa. Besides, a critical relative cover thickness was confirmed as 3.41. Finally, based on the distribution of bond stress along the embedded length, the anchorage position function was ascertained and then an accurate bond slip relationship was built, which was significant in structural design and numerical simulation.

Parametric study on a novel grouted rolling pipe splice for precast concrete construction

Grout-filled mechanical splice system, with its remarkable ability to connect precast elements, is one of the most important technologies for precast concrete construction. However, for the time being, its high cost has hindered the implementation of precast concrete structures to a certain degree in developing countries. This paper presents a new type of grouted sleeve which can be easily and economically fabricated with a standard seamless steel pipe through cold rolling techniques. Utilizing this sleeve, 69 sets of coupler specimens were prepared considering parameters of inner cavity structure and cross-sectional dimension of sleeve, grout strength, and diameter and embedded length of steel bar. Through direct pullout test, it is shown that the proposed splice is capable of developing of 100% of ultimate strength of the bar providing an embedded length of 6 times bar diameter. Increasing the number and height of inner concentric rib which rolled on the sleeve only in inelastic segment is an effective technique to improve the bond strength of the splice. With the increment of the ribs, clearly different effects of sleeve dimension and grout cylinder thickness on the bond strength can be observed due to different confining mechanism. Finally, a design method for the novel deformed grouted splice sleeve is proposed.

Bond performance of NSM CFRP strips embedded in concrete using direct pull-out testing with cementitious adhesive made with graphene oxide

Concrete structures are normally strengthened using fibre reinforced polymer (FRP) with epoxy adhesives and polymer cementitious mortars. Epoxy adhesives have significant issues, such as the release of toxic fumes throughout curing, loss of strength and stiffness when exposed to hot temperatures, and low permeability and weakness to UV radiation. In the case of polymer cementitious adhesives, their properties are adversely affected by hydrothermal conditions. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) has recently been developed by the authors which uses graphene oxide and cementitious materials. This paper presents the bond response of carbon FRP strips bonded to concrete substrate using near-surface mounted (NSM) technique with IHSSC-CA, epoxy and polymer cement-based adhesives using direct pull-out tests. The behaviour of each adhesive is presented and compared and the local bond-slip relationship is calculated. Finally, an analytical model is proposed to predict the ultimate pull-out force (bond strength). The results of this study confirm the effectiveness of using IHSSC-CA to improve the bond strength, stiffness, CFRP strip utilisation, ductility and residual strength of NSM CFRP system. Moreover, the proposed analytical model can simulate experimental conditions reasonably well.

Durability of bond in NSM CFRP-concrete systems under different environmental conditions

Abstract This paper addresses the durability of bond between concrete and carbon fibre reinforced polymer (CFRP) strips installed according to the near-surface mounted (NSM) technique (NSM CFRP-concrete systems) under the effects of two main groups of environmental conditions: (i) laboratory-based ageing conditions; (ii) real outdoor ageing conditions. The bond degradation was evaluated by carrying out direct pullout tests on aged specimens that were previously subjected to distinct environmental conditions for different periods of exposure. Moreover, the degradation of the mechanical properties of the involved materials was investigated. The digital image correlation (DIC) method was used to document the evolution of the deformation fields at the surface over the whole region of interest consisting of concrete and epoxy adhesive at the ligament region. This information supported the discussion about the evolution of the bond resistant mechanism developed in NSM CFRP-concrete specimens during testing, as well as the assessment of the bond quality of the system. In general, the results obtained from the durability tests conducted have shown that the different exposure environments, which may be considered as quite severe, did not result in significant damage on NSM CFRP-concrete system. The maximum decrease of about 12% on bond strength was obtained for real outdoor environments. Conversely, a maximum increase of 8% on bond strength was obtained on the specimens exposed to the temperature cycles between −15 °C and +60 °C. DIC allowed to document the stress transfer mechanisms established between the CFRP and the concrete substrate, revealing the crack patterns and the influence widths of the CFRP reinforcement strips, which were shown to be important for avoiding group effect when using multiple parallel strengthening CFRP strips.

CFRP-ECC hybrid for strengthening of the concrete structures

The strengthening of concrete structures using FRP composite and polymer adhesive has been challenged considering structural fire. This technology is limited to situations with low fire hazards. Though the cementitious adhesive showed improved fire resistance than the polymer counterpart, the former faces challenges of brittleness and high temperature spalling. This paper proposes a CFRP-ECC hybrid system for strengthening of the concrete structures. This hybrid consists of CFRP composite embedded in the ECC matrix. CFRP functions as the main load-carrying element, while ECC acts as the adhesive layer for protecting the CFRP and transferring the load between the concrete structure and CFRP. ECC is ductile in tension and has been shown to have improved fire performance without spalling over normal cementitious materials. In this paper, ECC was developed with a tensile strain capacity of 3%. Then direct pull-out tests were conducted to quantify the interface behavior between CFRP and ECC, and to obtain the effective bond length of CFRP. Finally a concrete beam with the CFRP-ECC hybrid strengthening was tested under four-point bending. The experimental results showed that the effective bond length of CFRP in ECC was longer than 170mm. When the CFRP-ECC hybrid was used for strengthening of the concrete beam, premature debonding occurred at the interface between CFRP-ECC hybrid layer and the surface of the concrete beam, thus the flexural behavior of the concrete beam was barely improved. Therefore, it is recommended that proper surface treatment of the concrete beam should be prepared or innovative anchoring techniques should be developed before the CFRP-ECC hybrid can be used for the strengthening of the concrete structures.

Bond performance of ribbed steel reinforcing bars in vibrated and self-compacting concrete

The bond between steel reinforcing bars and concrete plays a vital role in the effective performance of reinforced concrete as a composite member. Failure of this bond can lead to catastrophic structural failure of buildings and infrastructure. Factors influencing the steel–concrete bond include concrete strength, bar properties such as embedment length, ribs and diameter, and confinement in concrete. This paper reports on an experimental study to evaluate the bond performance of ribbed reinforcing bars in self-compacting concrete and to compare it with that in normal vibrated concrete. Different characteristic concrete strengths (20, 40 and 60 MPa), bar diameters (12 mm and 16 mm) and embedment lengths (2·5 and 5 times bar diameter and full depth of specimen) were used. The bond strength measurements were based on direct pull-out tests in accordance with IS 2770-1 and a comparison was made with the test procedure specified by Rilem TC 9-RC. The experimental bond stress values obtained were compared with those obtained from various models available in the literature. An analytical model was then proposed to predict the bond strength of ribbed reinforcing bars in both vibrated and self-compacting concretes.