Mechanical properties of full-grouted sleeves with grouting defects

ABSTRACT Prefabricated concrete structure is an important aspect of the building structure development in China, while the grouted sleeve connection represents a key tool for connection on prefabricated concrete structure parts. Various grouting defects may arise during the prefabrication and actual construction processes, thereby causing unsafe connection among components. Here, we report determination of the mechanical properties of a full-grouted sleeve with grouting defects，using the defect position (end defect, middle defect, uniform defect, unilateral defect, and horizontal defect) and the defect length as test variables. We generated a total of 11 groups of full-grouted sleeve specimens, with one group as a full-grouted standard specimen. All specimens were subjected to a unidirectional tensile test, and failure mode, ultimate bearing capacity, load-slip curve, load-displacement curve as well as load-stress curve obtained. Meanwhile, the ANSYS software was used for numerical simulation. Notably, our numerical results corroborated experimental findings. The detailed profiles of mechanical properties of full-grouted sleeve connection with defects, described in this paper, are expected to serve both as a baseline and reference for further investigations.


Introduction
The prefabricated concrete structure has become a crucial aspect in the development of building structures in China, owing to the associated advantages such as high production efficiency, low levels or environmental pollution and energy saving (Li, Z, and Sun 2017).Grouted sleeve connection is the main approach for reinforcing bars in the precast components, with its connection performance reportedly directly affecting safety of the structure (Liu, Li, and Xu 2014).The full-grouted sleeves connection is to set the reserved rebar and the connected rebar into the sleeves, followed by injection of grouting material into the sleeves.Studies have shown that connection between rebars can be realized through the grouting material (Guo et al. 2022).
Many scholars have studied full-grouted sleeves.For instance, Anon et al (Anon 1985).prefabricated entire components with the rebars connected by grouting.Notably, grouting material was filled into the sleeves to provide mechanical bonding between the steel bar and that inside of the sleeves.In another study, Zheng et al (Zheng et al. 2020).tested 24 sleeve joints with reserved vertical grouting defects and found that fracture of reinforcement occurred at the defective joints when the anchorage length of steel bars was>4d (d is the diameter of reinforcement).However, defective joint fractures occurred under cyclic loading when the anchorage length of steel bars was>5d.They concluded that repeated grouting is an effective approach to repair defects in grouted sleeves during engineering applications.Furthermore, Guo et al (Guo et al. 2022).tested 42 full-grouted sleeve connectors in order to consider various grouted defects and found that joint performance deteriorated when the end defect was longer than 42 mm.Moreover, their results showed that uniform defect specimens had the lowest ultimate anchorage strength, whereas that of the medium and end defect specimens were the same.Notably, the longitudinal defect had little influence on the bearing capacity of the joint when the defect length was less than 6 mm.In another study, Liu et al (Liu et al. 2020).subjected 15 groups of grouted sleeve specimens to uniaxial tensile test and finite element analysis.Test results revealed that the sliding failure of reinforcement and grout material could be prevented when the anchorage diameter ratio was greater than 7. Interestingly, the authors noted rapid development of rebar slips and grouted damage, and the damaged area was mainly distributed around the rebar.When the rebar was broken, the damage developed slowly, and the damaged area was mainly concentrated in the port of the sleeves.Espoir et al (Kahama and Fuzhe 2021).employed ABAQUS to analyze the parameters of 50 model Settings and found that addition of local grouting could not only improve bond strength but also increase tensile strength of the connector by 70%.Notably, reducing the sleeve diameter was associated with improved connector performance, and the safety anchorage of rebar was 8d.In a separate study, Zheng et al (Zheng et al. 2017).evaluated the effect of various grouted defects on bearing capacity and deformation performance of 70 grout reinforced sleeve joints and found that the middle and horizontal defects had more influence on the performance than the end and vertical grouted defects.In addition, eccentric specimens were not only more prone to slip but also exhibited worse overall deformation capacity.Meanwhile, Zheng et al (Zheng et al. 2021).tested dynamic performance of fulland half-grouted sleeve joints and observed that half-grouted sleeves connection were easily damaged when the rebar diameter was small.The authors also found that dynamic load had little effect on semi-grouting sleeves connection.In addition, bearing capacity of full-grouted sleeves specimens increased with increase in the loading rate, and the dynamic bearing capacity was higher than the static bearing capacity.A study by Huang et al (Huang et al. 2016).reported analysis of 14 halfgrouted sleeve connectors Results revealed occurrence of three types of failure in the specimens, namely steel bars pulling out, steel bars pulling out and sleeves sliding.Notably, deflection of the steel bar had little effect on the bearing capacity of specimens when the steel bar was broken.Wu et al (Wu et al. 2019).studied the effects of sleeve types, anchorage length and steel bar diameter on sleeve wall strain, and found that under similar conditions, sleeve wall strain decreased with increase in rebar anchorage length, but increased with increase in rebar diameter.Notably, the steel sleeve wall strain was less than that recorded in cast iron sleeve wall, although it gradually increased from the grouting to the anchorage end, reaching a maximum at the anchorage end.Furthermore, Kuang et al (Kuang, Zheng, and Jiao 2019).found that the failure mode of specimens mainly depended on both bonding capacity and ultimate tensile strength of steel bars between rebars and grouting materials.
High stress and large deformation repeated tension and compression loading did not cause significant degradation of bearing capacity.
Several problems and defects are encountered during the process of prefabrication and site construction, including leakage of grouting mouth and outlet, poor quality of grouting material or decreased fluidity, rebar tilting, corrosion on the surface of steel bars, and defects caused by foreign matter pollution, among others (Wenjie, Tao, and Qinglin 2018;Yong-Feng, Tian-Yun, and Hu 2022;Li and Liu 2019).To date, only a handful of studies have described the types and locations of defects in the full-grouted sleeve.In the present study, we explored the influence of different positions and degrees of grouting defects on the mechanical properties of fully-grouted reinforced sleeves connectors.To this end, we designed 11 groups (alongside 1 standard group) of full-grouted sleeves specimens, with 3 specimens in each group.Three types of defects were set in the upper, middle and lower part of the vertical grouted sleeve, with horizontal defects generated in the horizontal grouted sleeve.The specimens were subjected to a uniaxial tensile test and numerical simulation.Experimental results revealed failure mode, ultimate bearing capacity, load-slip curve, load-displacement curve and loadstress curve of the specimens.The effects and lengths of different defects on the specimens' mechanical properties were analyzed and compared to those in the standard group.Collectively, these findings provide a reference for future studies on mechanical properties of full-grouted sleeve connectors with defects.

Materials
This experiment employed a full-grouted sleeve made of ductile iron.The sleeve's mechanical properties met the requirements of "Grouted Sleeve for Reinforcement Connection" (JG/T398-2019) (2019).The sleeve's dimensions are shown in Figure 1, whereas its material properties are outlined in Table 1.
Grouting material comprises a high strength material as the aggregate, cement as a binder, and is supplemented by high flow state, micro expansion, antisegregation as well as other material ratios.The grouting material has several advantages, such as good gravity, early and high strength, and no shrinkage.A grouting test block, measuring 40 mm × 40 mm × 160 mm, was fabricated and cured for 28 days under standard conditions.Next, the block was subjected to compressive and flexural strength tests as shown in Figure 2. The block had average flexural and compressive strengths of 23.6MPa and 88.1MPa, respectively (Table 2).These test results indicated that the material met the requirements of Sleeve Grouting Material for Steel Bar Connection (JGT398-2019) (2019).
Fluidity test results of the grouting material are shown in Figure 3. Summarily, the material's initial fluidity should be more than 300 mm, awhile that within half an hour should be more than 260 mm.Test results outlined in Table 3 indicated that and the fluidity meets the requirements.
The HRB400 ribbed rebar, with a diameter of 14 mm, represents the connecting rebar.Three rebars, with a length of 420 mm, were randomly selected from the same batch.The average yield and tensile strengths of the three rebars, as obtained by uniaxial tension, were 436.33MPa and 608MPa, respectively (Figure 4).Properties of the rebar material are outlined in Table 4, and indicated that all specimens met the required specifications (JGJ107-2016) (2016).

Specimen preparation
T/CCIAT0004-2019 (T/CCIAT 2019) describes the specific requirements for construction of a grouted sleeve connector.Notably, non-standard operation by workers, lack of timely grouting and a dirty construction environment should be avoided as these lead to development of various grouting defects during the construction process.In the present study, we prepared 11 fullgrouted sleeve groups of specimens (ten defect specimens and a control group), with three specimens in each group.Details on the specimens were as follows: (1) Control group (BM): Sleeves were fully grouted without defects.
(2) End defects (DB): The vertical sleeves were first grouted completely, with the bottom grouting hole either not blocked in time or not tightly sealed.This resulted in leakage of slurry and end defects at the top.Three end defects were set    with a thickness of 5 mm, length of 3D, 2.5D and 2D (D denotes rebar diameter).
(3) Central defect (ZB): After the completion of grouting, the air was not blocked up in time, resulting in the formation of central defects.
Thickness and length of the central defect were set at 5 mm and 2.5D, respectively.(4) Uniform defect (JB): Corrosion or presence of impurities on the surface the rebar reduces bonding force between grout material and rebars with uniform distribution defects.Here, 2 types of uniform distribution defects were set.
The rebar inserted into the sleeve was divided into an average of five sections, with a defect thickness of 5 mm, and lengths of 2 × 1.2D and 3 × 0.8D.(5) Unilateral defect (DC): The horizontal sleeves were first completely grouted, and the internal air not emptied, a phenomenon that resulted in unilateral defects.Two types of unilateral defects were set, and the steel bar inserted inside the sleeve.Defect thickness was 5 mm, whereas the lengths were 2 × 1.2D, and 3 × 0.8D.(6) Level defects (SP): The grouting of the horizontal sleeves was not filled.The gap in the sleeve wall generated a horizontal defect.Two kinds of horizontal defects were set.The defect height was the distance from the upper surface of the rebar to the inner diameter of the sleeve, set at 5 mm and 7 mm.
In all specimens, 5 mm thick foam tapes were pasted on the surface of the rebar, and the length shall be as required by various defects.D denotes a diameter of rebar of 14 mm.The numbers and parameters of all specimens are outlined in Table 5, whereas defect construction diagrams are shown in Figure 5.
gauge.These are illustrated in Figure 6.After processing the specimens, they were bound and fixed onto a wooden frame to ensure normal force transmission of the connecting piece.The water-cement ratio of the grouting material was controlled at 0.13.After grouting, the specimens were cured for 28 days under standard conditions.

Experimental device and loading pattern
The rebar is connected at both ends of the sleeve and clamped by the two collets of the universal machine.
The specimen is unidirectionally stretched.The rebar is loaded according to 30kN/min force control before yielding, then loaded according to 15 mm/min displacement control until it is destroyed.The instrument automatically records the load-displacement curve, and the automated data acquisition systems monitor the strain.The extensometer is directly bound to the specimen, one end is bound to the sleeve, and the other end is clamped to the port rebar (Figure 7).

Failure modes
All the failure modes between the rebar and grouting material were either rebar tensile or slip failure (Figure 8 and 9).Specimens BM, DB-2D, SP-A and SP-B suffered the rebar tensile failure, and there was no obvious change in the specimens before rebar yielding.Moreover, we observed large deformations on the rebar, and the grouting material exhibited cracks prior to rebar reinforcement.The grouting material began to fall off as the displacement increased.Notably, the deformation was exacerbated even after rebar reinforcement, although the rebar's ultimate strength was less than the joint's bearing capacity.Moreover, the rebar experienced tensile failure following necking.Rebar slip failure occurred in DB-2.5D,DB-3D, DC-2-2.5D,DC-3-2.5D,JB-2-2.5D,JB-3-2.5D and ZB-2.5D specimens.
There was no visible change in the specimens prior to rebar yielding.Grouting material exhibited more cracks while the amount of slip gradually increased before rebar reinforcement.These defects, which reduced the friction resistance, resulted mechanical bite force and cementation force among grouting material as well as rebar and sleeve.The displacement of the sleeve end increased obviously with the rebar reinforcement.Upon reaching peak load, the joint bearing capacity was lower than the ultimate strength of the rebar, then the rebar happened slip failure (Zheng et al. 2017).Profiles of the ultimate bearing capacity, maximum displacement, and failure modes as well as results from other performance tests in each specimen are outlined in Table 6.The maximum displacement represented the displacement at the end of the test loading.

Load -displacement curves
Load-displacement curves for all specimens under this study are shown in Figure 10.The displacement was monitored by the universal testing machine and represented the total displacement of the specimen.Summarily, increase in the load did not cause a significant change in displacement across all specimens at the elastic stage.However, deformation and displacement of the rebar increased in the yield stage.At the same time, small cracks appeared on grouting material, and the specimens began to fail.Interestingly, obvious failures were observed on the specimens after strengthening, and the displacement increased with increase in the load.In addition, tensile or pull-out failure of the rebar occurred as the material neared maximum bearing capacity (Li et al. 2018).
From Table 6 and Figure 10, we concluded that: end defect (DB) reduced the specimen's ultimate bearing capacity, which decreased with increase in defect length.The ultimate bearing capacity of GT14-DB-2D, GT14-DB-2.5Dand GT14-DB-3D reduced by 0.48%, 2.23% and 7.02%, respectively, compared to that of GT14-BM, indicating that a defect length greater than 2.5D had a reducing effect on the specimens' ultimate bearing capacity.Compared with GT14-BM, the ultimate bearing capacity reduced by about 10.2%, and the ultimate displacement reduced about 50.48%, indicating that the central derect (ZB) had a higher influence on the specimens than that exerted by the end defect (DB).On the other hand, the ultimate bearing capacities of GT14-JB-2-2.5Dand GT14-JB-2-3D reduced by about 12.5% and 4.6%, respectively, compared to that of GT14-BM, while the ultimate displacement of GT14-JB-2-2.5Dand GT14-JB-2-3D reduced by about 53.8% and 34.4%, respectively.Notably, the ultimate bearing capacity decreased with increase in the number of uniform defect (JB), while the effect of the unilateral defect (DC) on specimens was less than that of the uniform defect (JB).The average limit displacements for GT14-SP-A and GT14-SP-B were about 5.69% and 16.04%, respectively, less than that recorded in GT14-BM, while the specimens' bearing capacities decreased with increase in horizontal defect height.

Load -slip curves
Profiles of load-slip curves across various specimens are presented in Figure 11.The slip was monitored by an extensometer and represented the amount of slip between the rebar and the sleeve.The curves exhibited a varying trend, which was similar to that observed in the load-displacement curves.All slip values across specimens were less than 1 mm and the curves changed linearly prior to yielding.After rebar yielding, the specimen slip rapidly increased with increase in load, while the curve exhibited a downward trend after the ultimate load had been reached.The rate of curve decline varied across different defects (Wu et al. 2019).Notably, yield and limit values across all defective specimens were lower than those recorded in standard specimens.

Load-stress curves
Load-stress curves for all specimen groups are shown in Figures 12a,b.Summarily, axial stress is the tensile stress generated by the longitudinal force on the sleeve surface during the stretching process, whereas circumferential stress denotes compressive stress caused by circumferential force on the sleeve surface.Both circumferential and axial stresses exhibited a linear change at the beginning of loading.The grouting material began to crack and the stress rapidly  increased with increase with increase in the load.The sleeve's surface stress was determined by the expansion effect of the grouting material, as well as the restraint effect of the sleeve on the grouting material.
The maximum axial tensile stress and maximum circumferential compressive stress of specimens were shown as Table 7.
We observed a similar trend in axial and the circumferential stresses, with both exhibiting a linear increase during the initial loading stage.Notably, the stress gradually decreased with increase in the load during the late loading stage, but gradually increased from the middle to both ends without obvious stress concentration (Guo, Zhang, and Wang 2020;Wu, Liu, and Du 2020).Circumferential stress at H1 of GT14-DB-2D, GT14-DB-2.5D,GT14-DB-3D, GT14-ZB-2.5D,GT14-JB-2.5D-3and GT14-DC-2.5D-3changed from compressive to tensile modes.The upper anchorage end reinforcement was divided into three equal parts, and strain gauge was pasted, named S1-S3.The lower anchorage end rebar was divided into five equal parts, and strain gauge was pasted, named X1-X5.Three annular strain gauges, named H1-H3, were pasted on the sleeve.Loadstress curves.(Continued).

Finite element analysis
Technological detection of grouted sleeves has advanced in the recent past.In the present study, we explored failure modes and mechanical properties of full-grouted sleeves using finite element modeling and analysis.Specifically, we employed finite element software for numerical simulation analysis of full-grouted sleeve connector.Next, we compared load-slip, load-displacement and loadstress curves obtained from the tests to validate the simulation results.

Material constitutive model
The sleeve had a tensile strength of 600 N/mm 2 , elastic modulus of 2.0 × 10 5 MPa, and a Poisson's ratio of 0.3.The stress-strain relationship of the sleeve is based on the BISO model of bilinear equidirectional strengthening.In addition, the rebar had a yield strength of 436 MPa, as well as ultimate tensile strength and elastic modulus of 608 MPa, and 2.0 × 10 5 MPa, respectively.The rebar's constitutive relation obeyed the Mises yield criterion, according to the bilinear isotropic strengthening model, as shown in Figures 13.Since research on the constitutive relation of grouting material is in the premature stages, we calculated the associated constitutive relationship using the code (GB50010-2010) (GB 2010).Results are shown in Figure 14.

Finite element model
The simulation comprises three joints of contacts as, namely reserved rebar and grouting material, connected rebar and grouting material, as well as sleeve and grouting material.The rebar and the sleeve in the model adopt the SOLID185 unit.The concrete SOLID65 element is used for the grouting model.The rebar and sleeve were set as the target surface, while the grouting material was set as the contact surface.The target surface names TARGE170, the contact surface names CONTA172.We employed a 3D solid model, owing to the symmetrical nature of the simulation, with half of the model taken for analysis.The grouting sleeve's unidirectional tensile test was similar to the pull test of the rebar, and the fixed constraints were added at one end of the rebar to limit its displacement and tension (Gao and Zhao 0000).The numerical model parameters were shown as Table 8. Results are shown in Figure 15.

Comparison of load-displacement curves
Failure modes for the specimens included tensile and slip failure.The GT14-BM group was chosen for         the elastic stage.Test results revealed obvious yield at the end of the yielding stage.Notably, the curves did not reflect the yield stage and raised in a straight line, as evidenced by the double broken line constitutive model used in the simulation.The simulated ultimate tensile strength was consistent with experimental results under the ultimate load, thereby validating our model, although there were some errors between them.

Global stress distribution in the rebar
The strain diagram for GT14-BM is shown in Figure 17.Summarily, the rebar anchorage section of GT14-BM had uniform stress, which gradually increased from the middle to the end of the sleeve.Stress mutation occurred at the sleeve end, while rebar stress increased rapidly, reaching the maximum value at the loading end.This was consistent with test results.
The strain diagram for GT14-DB-2.5D is shown in Figure 18.In briefly, anchoring rebars at the side of the full end of the upper grouting exhibited uniform stress, which gradually increased from the middle to the end of the sleeve.The rebar stress at the central defect of the lower sleeve was zero, and the stress gradually increased from the anchorage point to the end of the sleeve.This finding was also consistent with test results.
The strain diagram for GT14-DC-3-2.5D is shown in Figure 19.Summarily, the rebar force was uniform in the upper defect, and gradually increased from the middle to the end of the sleeve.Rebar stress at the defect of the lower side was consistent with that recorded at the adjacent anchorage end, was consistent with test results.
The strain diagram for GT14-JB-3-2.5D is illustrated in Figure 20.Summarily, the overall stress of the anchoring rebar at the upper full grouting end was uniform, and gradually increased from the middle to the end of the sleeve.The stress of the lower defect was similar to that recorded in GT14-DC-2-2.5Dand GT14-DC-2-3D, a finding that was consistent with test results.
The overall stress for GT14-SP-A rebar was similar to that recorded in GT14-BM (Figure 21).In addition, the upper and lower steel bars exhibited uniform stress, which gradually increased from the middle to the end of the sleeve.This too was consistent with the test results.
Uniform overall stress was recorded in the anchoring rebar at the upper full grouting end (Figure 22).Notably, this stress gradually increased from the middle to the end of the sleeve, and the rebar force in the lower sleeve remained unchanged at the central defect.The stress of rebars at other anchoring ends gradually increased from the middle to the end of the sleeve, which was consistent with the test results.

Overall stress distribution of the sleeve
According to Figure 23, the simulated stress distribution of the sleeve was consistent with the experimental results.The overall force of the sleeve was not affected by defects, and gradually decreased from the middle of the sleeve to the end.Moreover, the stress on the upper and lower ends of the sleeve was symmetrically distributed.

Conclusion
In the present study, we performed unidirectional tensile tests on full-grouted sleeves connectors with defects, then analyzed the resultant test phenomena, failure modes, bearing capacity, load-slip curves, loaddisplacement curves and load-stress curves.We also employed finite element software used for modeling and analysis, then compared simulation and test results.From our findings, the following conclusions are drawn: (1) Two failure modes, namely tensile failure of rebar and bond slip failure between rebar and grouting material, occurred.(2) Various defects reduce the maximum bearing capacity of the connectors relative to the benchmark specimen GT14-BM.Under DB and JB defects, the maximum bearing capacity of the specimens decreased with increase in defect length.Notably, the length of DC defects had little effect on the maximum bearing capacity of specimens.Under SP defects, GT14-SP-A exhibited a larger maximum bearing capacity than GT14-SP-B.the maximum bearing capacity of the specimen markedly reduced when the ZB defect length was 2.5d.(3) The defects caused a marked decrease in rebar strain at the defect site.Notably, the defects have no effect on the overall strain of the sleeve, which decreased from the middle to the end of the sleeve.The defects affected stress distribution of the rebar under a similar load, and the stress concentration phenomenon occurs at the defect site.The defects have no influence on stress distribution of the sleeve, which decreased from the middle to the two ends of the sleeve.(4) Results from finite element software analysis revealed consistent trends in load-displacement curves between experimental and simulation activities, which validated our simulation model.
Housing and Urban-Rural Construction (Grant No: 2020-YF47)and BIM Engineering Center of Anhui Province (Grant No: AHBIM2021KF02).

Figure 1 .
Figure 1.Dimensions of the full-grouted sleeve used in this study.
(a) Upper rebar Mises strain diagram (b) Lower rebar Mises strain diagram

Table 1 .
Material properties of the sleeve.

Table 2 .
Mechanical properties of the fabricated grouting material at different curing ages.

Table 3 .
Fluidity profiles of the grouting material.

Table 4 .
Test results of rebar properties.

Table 6 .
Ultimate bearing capacity and failure mode.

Table 7 .
Maximum axial tensile stress and circumferential compressive stress of specimens.
Hongmei Ren, PhD, is an associate professor at School of Digital Construction, Shanghai Urban Construction Vocational College.She received Ph.D in College of Civil Engineering, Tongji University.She has published more than 20 papers in domestic and international academic journals and conferences.Yalong Liu is a professor and dean of School of Digital Construction, Shanghai Urban Construction Vocational College.His research interests include structural engineering and intelligent construction.PhD, is a professor at School of Digital Construction, Shanghai Urban Construction Vocational College.He received Ph.D in College of Civil Engineering, Tongji University.He is currently researching the field of structural engineering.PhD, is a lecturer at School of Digital Construction, Shanghai Urban Construction Vocational College.She received Ph.D in College of Civil Engineering, Tongji University.She is currently researching the field of structural engineering.PhD, is a professor at BIM Engineering Center of Anhui Province, Anhui Jianzhu University.He is currently researching the field of structural engineering and prefabricated construction.