Performance of Polyester Grouted Connector for Splicing of Reinforcement Bars

: The research aims to investigate the viability of employing polyester grout as a bonding agent for the splicing of reinforcement bars. 0.43 and 1 binder-to-filler ratio were examined. The process of splicing reinforcement bars under tension using a polyester grouted splice connector is elucidated. Through direct tension tests, the optimal mix design and required length of the splicing sleeve were identified. The findings indicate that the polyester grouted connector with a 0.43 binder-to-filler ratio performs satisfactorily for the splicing of tensile reinforcement bars. Specifically, a spiral length of 250 mm with an embedded length of 125 mm is demonstrated to achieve the full strength of the spliced bar.


Introduction
A spliced grouted connector is a construction device, particularly prevalent in civil engineering, employed to link two structural elements.This type of connector is utilized to join reinforcing bars, commonly known as rebar, within concrete structures.Its purpose is to ensure the stability and overall integrity of the structure by establishing a robust and reliable connection between the bars.The history of grouted connectors for splicing dates back to the late 19 th and early 20 th centuries, coinciding with the development of reinforced concrete.As construction methods progressed, the efficient and dependable connection of reinforcing bars became increasingly crucial.This necessity led to the development of various types of splicing connectors, including grouted connectors, which have now become a standard in modern construction practices.
These connectors have found widespread application in diverse construction projects, such as bridges, buildings and various infrastructures, offering a range of benefits; including strengthening the integrity of the structure, facilitating improved load transfer and enhancing overall durability.Moreover, they are engineered to withstand diverse environmental conditions like moisture and corrosion, making them adaptable to a broad spectrum of applications.
In recent years, advancements in the design and manufacturing of grouted connectors for splicing have yielded new and improved products boasting enhanced performance and reliability.These developments are propelled by ongoing research and development initiatives in the field of civil engineering, spurred by the demand for more efficient and sustainable construction practices.Overall, grouted connectors for splicing have played a pivotal role in advancing construction technology and continue to be an essential component in the development of safe and durable structures (ACI, 2014;ASTM, 2019).
Various connection types, including beam splice, column splice and beam-to-column connections, rely on grouted connections to maintain and augment their structural integrity and stability.The effectiveness of these grouted connections is contingent upon both confinement action and the strength of the grout.The specification of high-strength grout serves a dual purpose: Firstly, to enhance the bond between the grout and the bar, ensuring that stresses in the reinforcement bars can be transferred effectively to the adjacent material, thus favoring the desired failure mode of bar fracture over bar pull-out; and secondly, to minimize the required embedment lengths for grouted-in bars (Ling et al., 2012).
Traditional cement grouts, typically employed in grouted splice connectors, come with several drawbacks, such as extended curing periods and a gradual progression in strength.An emerging alternative is Polymer grout (PM) or resin mortar, a modern composite formed by blending inorganic aggregates (such as sand) with a resin binder (Raymond and Sauntson, 1987;Mohammed et al., 2015).Moreover, crucial additives in polymer formulations include microfillers like fly ash, which serve to diminish the rate of the curing reaction and reduce the exothermic degree (Petrie, 2006;Fadiel et al., 2023).PM exhibits superior strength and durability compared to cement grout.Its most notable advantage lies in its rapid curing, typically taking only a few hours, while cement grouting mix requires more time sometimes reaching weeks to achieve desired strength (Rebeiz et al., 1996).One outcome of the effort to lessen the harmful effects of byproducts and solid waste materials is the use of fly ash as filler material.There is a lack of research on the utilization of polymer grout as a bonding material in grouted splice connectors.In 1970, markested and Johansen proposed the utilization of polymer mortar as a bonding material for a steel pipe sleeve.Various resin types, including polyester and epoxy, were combined with fine sand to create the filler in the polymer paste.The conducted research demonstrated the successful use of sleeve spliced with polymer mortar for splicing steel bars.However, it's worth noting that the temperature range for utilizing epoxy mortar in splicing reinforcement steel bars under tensile stresses could be constrained due to an observed increase in creep with the change in temperature (Navaratnarajah, 1983).The heightened creep effects observed in the markested and Johansen study may be attributed to the creep occurring within the resin mortar situated between the sleeve and the deformed steel bars.This phenomenon is associated with both a high reaction rate and a significant exotherm, factors that depend on the properties of the filler and the quantity of resin present in the polymer mortar.
In this experimental study, a bonding material for grouted splice connectors was created using polyester grout containing 30 and 50% polyester resin, along with the inclusion of fly ash.This formulation aimed to minimize the degree of exotherm and the rate of cure response.The polyester grouted splice connectors underwent testing under tensile load to evaluate their effectiveness.Additionally, the goal of the investigation was to ascertain the ideal sleeve length required to achieve the highest possible strength for the spliced bar in the polyester grout.

Materials and Methods
Six specimens of polyester grouted splices were tested under increasing tensile loads.Additionally, three highstrength steel bars (Y16) were evaluated to serve as a reference for the behavior of an ideal grouted splice, providing a basis for comparison with the proposed polyester grouted splice specimens.

Materials and Test Specimens
Two different mixtures of polyester grout, identified as mix A and B, were employed as bonding materials.These polyester grout mixes were produced using a combination of polyester binder, sand and fly ash as fillers.The proportions for mixes A and B involved a mix of polyester and filler in the ratio of 30:70 and 50:50 parts by volume, respectively.Fly ash made up 14% of the mixture's total volume.The polyester grouted splice connectors were made by joining high-strength deformed bars measuring 16 mm with a steel spiral reinforcing bar that had an external reinforcement bar welded to it.The dimensional configurations and specifications of the connector are illustrated in Fig. 1 and Table 1.

Test Setup
Steel bars are first embedded into the spiral from both ends before applying the polyester grout which serves as the bonding material.Before testing the specimens, strain gauges were attached to the spliced bars at a distance of 16 mm, extruding from the grouted spiral connections.Figure 2 illustrates the setup for the tensile test.The collected data from the testing encompassed the load applied, displacement and strain in the steel bar.Additionally, the laboratory test determined the compressive strength of the polyester grout.

Results and Discussion
Figures 3-4 present the load-displacement curves and modes failure observed in the tested polyester grouted connectors.Table 2 illustrates the ultimate tensile load, ultimate stress and ultimate strain for all tested connectors.
According to Figs. 3-4, it is evident that for connectors using Mix A as the bonding material, two distinct behaviors are observed.Firstly, specimens D33-L200-A and D33-L150-A exhibit insufficient bonds and fail in a brittle mode, with bars slipping out of the sleeve before the spliced bars yield.Secondly, specimen D33-L250-A, with a bar-embedded length of 125 mm, demonstrates an acceptable bond and fails in a ductile manner, with the spliced bar fracturing outside the sleeve.The spliced bars within the sleeve yielded and lengthened before fracturing the outer side of the sleeve.In contrast, grouted splice connectors D43-L250-B, D43-L200-B and D43-L150-B, which utilize mix B as the bonding material, fail due to the splitting of the surrounding grout, resulting in a complete loss of load-carrying capacity.Additionally, the maximum strains recorded in these bars, 355010⁻⁶ and 293010⁻⁶ mm/mm, significantly exceeded the yield strain of 230010⁻⁶ mm/mm.This suggests that specimen D33-L250-A provided the full tensile strength of the connected steel bars.The behavior of this specimen closely resembles that of the control specimens representing an effective grouted splice connector, demonstrating stages of elasticity, yielding and bar fracture failure.Therefore, the optimal sleeve length essential to achieve the spliced bar's ultimate strength by using polyester grout as the bonding material was determined to be 250 mm, with an anchorage length of 125 mm.Additionally, despite Mix A having a compressive strength of 61.4 MPa, lower than the compression strength of Mix B at 75.5 MPa, specimen D33-L250-A exhibited an undesirable failure mechanism, specifically splitting failure due to an increase in creep associated with the higher amount of polyester (Fig. 6).
With an increase in bar embedded length, the polyester grouted spiral connections produce more bond capacity.To withstand the tensile load inside the spiral, the spiral connections need a minimum bar embedded length that is around six times the bar diameter.The bond capacity of polyester grouted spiral connections improves as the spiral diameter decreases.A decrease in spiral diameter enhances the confinement stress acting on the grouted connection, hence increasing bond capacity.

Conclusion
In this investigation, polyester grouted connectors underwent testing with increasing tensile loads to evaluate the viability of using polyester grout as a bonding material for such connectors.Based on the test results and the subsequent discussion, it can be concluded that fastsetting polyester grout holds promise as a potential material for filling joints and connections.Moreover, by making use of the spiral's confinement and the high strength of polymer grout, the necessary development length can be drastically shortened to about eight times the diameter of the bar.This is around 22.3% of the anchorage length that BS8110 recommends, which is 35 times the bar's diameter (BS8110, 1997).

Table 1 :
Details of specimens

Table 2 :
Results of tensile load test