# Experimental Study on the Effect of Grouting Defects on Mechanical Properties of the Rebar Connected by Full-Grouted Sleeves

Authors: Fei Guo; Junhua Li; Sicong He; Chunheng Zhou
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

In this article, the mechanical properties of the steel bars connected by the full-grouted sleeve with grouting defects were investigated experimentally. The study focused on two defect factors: the insufficiency of grouting material strength and the lack of grouting fullness. The HRB400 steel bar with a diameter of 20 mm and the full-grouted sleeve with the model of GTQ4J 16 were applied in the experiment. The mechanical properties of the steel bar connected by grouted sleeve under different conditions were tested under tensile load, and these mechanical parameters were compared with the standard values in the specification. The results show that both kinds of grouting defects have a significant impact on the mechanical properties of reinforcement. The tensile strength and elongation of the joint were decreased with the decrease of the strength of the grouting material. When the water-binder ratio reaches 0.14, the strength of the grouting material is 66.4 MPa, and the mechanical properties of the joint cannot meet the requirements. At the same time, the mechanical properties of the joints were also reduced with the decrease of the grouting fullness. When the grouting fullness is below 80%, the tensile strength decreased more obviously than before, and the mechanical properties cannot reach the relevant standard.


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## Body

## 1. Introduction

Nowadays, prefabricated construction has become a new building system that China vigorously promote. Since the “Guiding Opinions of the General Office of the State Council on Vigorously Developing Prefab Buildings” released in 2016, Chinese government have selected many demonstration cities of prefabricated construction in various provinces, and set standards about design construction as well as acceptance of prefabricated construction to promote the development of this industry [1]. Besides, the government had set several phased goals and nowadays gained remarkable effects in many cities [2]. The main reason why China vigorously promotes this building mode is that compared with the traditional building mode, the prefabricated building has the advantages of high efficiency, low pollution, good seismic performance, and so on. It was found that a steel-structured prefabricated system resulted in reduced material consumption of up to 78% by comparing conventional concrete construction [3]. Cao et al. [4] conducted a fair comparison between two typical residential projects by various measuring methods and concluded that prefabrication technology was more environmentally friendly because of its advantages in reducing damage to the environment compared with traditional cast-in-situ construction technology. It is generally considered to be an actual sustainable method in the building sector [5].Developing prefabricated structures conforms to the trend of green building mode and industrialization of construction that is not just popular in China. The market share of prefabricated building systems in whole building industry was more than 80% in Sweden [6]. El-Abidi et al. [7] proposed that the application of prefabricated building systems can offer considerable advantages in cases where house problems are immediately required in Libya.For prefabricated construction, the connection behavior among the prefabricated elements can make a direct difference to the reliability of the structure, so ensuring the connection performance of the joints is also a critical task. Since the technology of sleeve grouting connection invented by American engineer Dr. Alfred A Yee in the late 1960s, it has developed into a major way of reinforcing bar connection [8]. At present, the vertical and horizontal reinforced connection mainly uses the grouted sleeve splice in our country. It is widely applied in the project and has two main categories: full-grouted sleeve and half-grouted sleeve. The connection performance of grouted sleeve has been studied by many scholars. In 1995, Einea et al. [9] conducted uniaxial tensile tests on the joint connected by various sleeves to study their mechanical properties. Experimental results of Goh [10] show that with the age of grouting material increasing (1 d, 3 d, 9 d), its strength also increases significantly. However, the bonding strength between grouting material and rebar and bearing capacity of the joint can also be improved. Zheng and Guo [11] concluded that the ultimate bonding strength between steel bar and grouting material under cyclic loading (tension and compression) was about 10% lower than that under uniaxial tension. On this basis, many scholars even improved the sleeve structure and material to do further research. Zheng et al. [12] made finite element analysis on a new kind of sleeve and found that the internal structure exercised a crucial influence on its constraint capability and strain distribution. Yu et al. [13] put forward a new grouted sleeve lapping connector and conducted tensile tests of 16 groups specimens with different lap lengths to investigate failure modes, force-displacement curves, bearing capacity, ductility, reinforcement strain, and circumferential strain of the sleeve. Grouting is a crucially important step of the whole technique due to its function is connect steel bars with the sleeve via the grouting material’s adhesion stress and friction. Past research revealed that the bonding strength of reinforcement can be improved obviously by restricting the splitting deformation of grout material [14]. However, the grouting defects are often caused by the wrong operation, such as the lack of grouting fullness, the insufficient of the grout’s strength, the deviation of the bar’s position, and so forth. These factors will have a great impact on the connection behavior of the sleeve. The defect of grout can be measured by various method such as surface hardness method [15] and stress wave measurement [16]. Li et al. [17] had investigated the effects of the size of grout defect on a half-grouted sleeve, and Chen et al. [18] carried out an experimental study of mechanical behavior of rebar in steel half-grouted sleeve connections with defects in water-to-binder ratio.This article will discuss several main factors of grouting defect and analyze concretely, including the following aspect: the defect caused by the lack of grout’s fullness and the insufficient of the grout’s strength. The author performed several sets of tensile tests and analyzed the influence of two factors on the grouted sleeve’s connection property by means of specific experimental data.


## 2. Materials and Methods

### 2.1. Materials

(1)
This experiment used mechanical extruded full-grouted sleeves as shown in Figure1, and the size of applied GTQ4J sleeve is φ42 × 320.(2)
According to the standard of the matching rebar’s dimension inthe grouting sleeve for rebars splicing (JG/T 398—2012) [19], the HRB400 rebar with the diameter of 20 mm was applied in the test. The basic tensile test can get mechanical properties of reinforcement, which are shown in the Table 1.(3)
The grouting material adopted the type of CGMJM-VI produced by “Beijing Sida Jianmao Science& Technology Development,” the design compressive strength is 85 MPa. Main composition of this grouting material is Portland cement, aggregate, and admixture.Figure 1
Full-grouted sleeve.Table 1
Reinforcement properties.
Strength gradeYield strength (MPa)Tensile strength (MPa)Elasticity modulus (GPa)HRB400456605196


### 2.2. Specimen Preparation and Test Setup

#### 2.2.1. The Insufficiency of the Grout’s Strength

This test’s objective is investigating the effect of the grouting strength defect on the sleeve’s connection behavior. As known that the setting and hardening process will volatilize the water of grouting material and then pores will be formed in the interior. The larger porosity will cause the lower density of the grouting material. It is obvious that the larger water:material ratio is, the lower strength of grouting material will be. However, too low water:material ratio will also lead to poor workability, which makes it difficult to mix and vibrate. Thus, every grouting material has its own optimal water:bind ratio in general. Based on the product specification, the appropriate water:binder ratio of CGMJM-VI is 12%.Table 2
Results of block’s compressive strength.
NumberABCDCompressive strength (MPa)91.977.366.451.6To sum up, the defect of strength was simply simulated by increasing different water:bind ratio, which are 12%,14%,16%, and 18% (experiment represent byA, B, C, and D, respectively), to control the strength of the grout in this test. Meanwhile, the filling degree of the grout in all groups is 100%. Various destruction forms in different conditions have presented after the tensile test and maximum tension elongation and ultimate bearing capacity of the jointing element were measured too. Based on the analysis and comparison of these parameters, the conclusion can be summarized. In order to ensure the accuracy of the data, each experiment was done in three pairs (such as A-1, A-2, A-3). The specific steps are as follows:(1)
According to the standard, the anchorage length of the jointed steel bar should be eight times longer than its diameter. Inserted two steel bars into both end of the sleeve as prescribed and fixed them in the center with a rubber sealing ring. It is necessary to ensure that the position of the steel bar in the sleeve is in the middle so that it can weaken eccentricity error. Finally bolted sleeves into a shelf and kept vertical, which is shown in Figure2.(2)
Mixed required amount of grouting material with water in four designed water:material ratios described above strictly. During the mixing process, the most significant point is to stir well by the agitator and it maybe take about 5 minutes. After work, it should be left standing for 2 minutes until the bubbles are removed.(3)
After standing, the grouting material poured into 40 mm × 40 mm × 160 mm mold to make a standard specimen, which needs cure for 28 days under the standard conditions of room temperature 20 ± 2°C and humidity greater than 50%. The final cured sample blocks were tested for their compressive strength and flexural strength according to the method in GB/T 17671 “Method of testing cements—Determination of strength (ISO)” [20]. The electrohydraulic servo universal testing machine (GMT5205) of the Ningbo University Structure Laboratory (see Figure 3) was used to load, and controlled the loading rate of 50 N/s until the standard specimens were fractured. Then, the broken specimens were subjected to compressive strength loading, which was carried out by the electrohydraulic servo universal testing machine (WAW-600C) (see Figure 3(b)) at a loading rate of 1.5 MPa/s. The compressive strength of each sample block can be calculated via the following formula:(1)fcuc=FA.fcuc is the the compressive strength of the sample block (MPa). F is the damage load of the sample block (N). A is the bearing area of the sample block (mm).The data of each water:material ratio specimen in three groups are shown in Table2:(4)
Poured the rest grouting material into the grouting gun (a tool used for grouting) and injected it into the sleeve until the material spilled from the grout outlet at the upper part of the sleeve, which represented that the sleeve is full of grouting. Plugged up two orifices with rubber stopper then the grouting is done. The most remarkable thing was making sure the sleeve was always vertical and there was nothing impurity in grout material during the whole process to prevent loss in final strength of the material. The grouted sleeves were fixed to the rack and maintains 28 ds under standard conditions.(5)
The device applied in this tensile test was the microcomputer-controlled electrohydraulic servo universal testing machine, which model of WAW-1000C in the structural laboratory of Ningbo University, as shown in Figure3(c), the loading framework in force-control mode. Keep the specimen under loading until the reinforcement pulled out from sleeve or broken and recorded the relevant data output by the testing machine such as load and displacement. Based on the definition of maximum elongation in “Technical specification for mechanical splicing of steel reinforcing bars” JGJ107-2010 [21], the maximum elongation can be calculated by the following formula (the measuring points arrangement sketch on the surface of the rebar is shown as Figure 4):(2)Asgt=L02−L01L01+fmst0E×100%.L01 is the measured length of AB or CD before loading the specimen; L02 is the measured length of AB or CD after loading the specimen; E is the elastic modulus of steel bar; fmst0 is the stress of the reinforcement when the load reaches the maximum value.Figure 2
The sleeves with different grouting material strength bolted into the shelf.Figure 3
Test equipment. (a) GMT5205. (b) WAW-600C. (c) WAW-1000C.
(a)(b)(c)Figure 4
Measured gauge point.The failure mode of the specimen in this test including rebar pullout and rebar tension fracture are shown in Figure5. According to the observation of raster images at room temperature by Zheng et al. [22], the strain of the steel bar increased linearly with the increasing load and decreased with the distance from anchor side. Therefore, the fracture position of the steel bar is largely out of the sleeve. The bond failure between rebar and grout material depends on the bond strength, which may occur in two failure form: bond slip failure between grout material and sleeve as well as bond slip failure between grout material and steel bar [23], the latter form was the same as that in this article. The results of the test are as follows in Table 3:Figure 5
The failure mode of the specimen. (a) Rebar pullout. (b) Rebar tension fracture.
(a)(b)Table 3
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeA-1224.32714.0314.56Rebar tension fractureA-2202.07643.2113.22Rebar tension fractureA-3196.87626.6612.81Rebar tension fractureB-1199.47622.5513.42Rebar pulloutB-2210.48669.9811.84Rebar tension fractureB-3201.42641.1411.52Rebar tension fractureC-1189.53603.299.80Rebar pulloutC-2177.29564.3310.48Rebar pulloutC-3185.86591.618.50Rebar pulloutD-1174.88556.663.18Rebar pulloutD-2143.87457.954.93Rebar pulloutD-3167.66533.685.77Rebar pullout


#### 2.2.2. The Lack of the Grouting Fullness

In this experiment, in order to verify the influence of grouting fullness on the sleeve’s connection performance, the variable was designed as the percentage of grouting material. According to the product instructions of grouting material, the water:binder ratio was unified at 12%, and the grout filling degrees were controlled at 100%, 90%, 80%, 70%, and 60% (the five groups of tests are represented by “I,“ “II,” “III,” “IV,” “V,” respectively). Finally, the tensile strength and elongation of the steel bar with each condition were tested by experiment, and the data were recorded. In order to exclude the contingency of the test data, three groups of repeated tests were conducted for every test of various filling degrees (denoted by “I-1,” “I-2” and “I-3”). All the equipment requested in this test is shown in Figure3. The specific test step was roughly same as that in Section 2.2.1. The following are some attention points that need to take into consideration:(1)
The water ratio of the mixture should be in strict accordance with the standard at 12%. It was crucial to mix thoroughly and prevent impurities.(2)
Syringe was used for grouting in this experiment, and the amount of grouting material was controlled by its measurements to meet the requirements of different filling degrees.(3)
The required amount of grouting material should be poured into each specimen according to the designed percentage. The grouting material should be set aside for making standard specimens during the grouting of the test.(4)
The grouting process should be quick to prevent the material’s fluidity changing over time that may be affecting the strength of the grouting material.(5)
The whole process of grouting and maintenance should make sure that no eccentricity of the steel bar.According to the compressive test and calculation, the compressive strength of the standard specimens was 88.6 MPa.After the tensile test and calculation, the experimental results are shown in Table4.Table 4
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeI-1192.58612.8113.69Rebar tension fractureI-2203.89649.0013.31Rebar tension fractureI-3193.21615.0012.66Rebar tension fractureII-1188.65600.4910.31Rebar tension fractureII-2190.38606.0011.45Rebar tension fractureII-3186.30593.0112.81Rebar tension fractureIII-1195.06620.908.32Rebar tension fractureIII-2188.70600.6512.77Rebar tension fractureIII-3185.04589.0011.56Rebar tension fractureIV-1179.66571.884.30Rebar pulloutIV-2193.52615.997.42Rebar tension fractureIV-3175.47558.544.29Rebar pulloutV-1158.65505.003.14Rebar pulloutV-2151.42481.994.26Rebar pulloutV-3152.90486.702.25Rebar pulloutThere are anomalies in the data of IV-2, which may be caused by misoperation during the experiment. Therefore, this group of data should be discarded when analyzing the results.Figure 6
Water ratio-compressive strength curve.Figure 7
Displacement-load curve. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 8
Comparison of tensile strength with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 9
Comparison of elongation with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)


## 2.1. Materials

(1)
This experiment used mechanical extruded full-grouted sleeves as shown in Figure1, and the size of applied GTQ4J sleeve is φ42 × 320.(2)
According to the standard of the matching rebar’s dimension inthe grouting sleeve for rebars splicing (JG/T 398—2012) [19], the HRB400 rebar with the diameter of 20 mm was applied in the test. The basic tensile test can get mechanical properties of reinforcement, which are shown in the Table 1.(3)
The grouting material adopted the type of CGMJM-VI produced by “Beijing Sida Jianmao Science& Technology Development,” the design compressive strength is 85 MPa. Main composition of this grouting material is Portland cement, aggregate, and admixture.Figure 1
Full-grouted sleeve.Table 1
Reinforcement properties.
Strength gradeYield strength (MPa)Tensile strength (MPa)Elasticity modulus (GPa)HRB400456605196


## 2.2. Specimen Preparation and Test Setup

### 2.2.1. The Insufficiency of the Grout’s Strength

This test’s objective is investigating the effect of the grouting strength defect on the sleeve’s connection behavior. As known that the setting and hardening process will volatilize the water of grouting material and then pores will be formed in the interior. The larger porosity will cause the lower density of the grouting material. It is obvious that the larger water:material ratio is, the lower strength of grouting material will be. However, too low water:material ratio will also lead to poor workability, which makes it difficult to mix and vibrate. Thus, every grouting material has its own optimal water:bind ratio in general. Based on the product specification, the appropriate water:binder ratio of CGMJM-VI is 12%.Table 2
Results of block’s compressive strength.
NumberABCDCompressive strength (MPa)91.977.366.451.6To sum up, the defect of strength was simply simulated by increasing different water:bind ratio, which are 12%,14%,16%, and 18% (experiment represent byA, B, C, and D, respectively), to control the strength of the grout in this test. Meanwhile, the filling degree of the grout in all groups is 100%. Various destruction forms in different conditions have presented after the tensile test and maximum tension elongation and ultimate bearing capacity of the jointing element were measured too. Based on the analysis and comparison of these parameters, the conclusion can be summarized. In order to ensure the accuracy of the data, each experiment was done in three pairs (such as A-1, A-2, A-3). The specific steps are as follows:(1)
According to the standard, the anchorage length of the jointed steel bar should be eight times longer than its diameter. Inserted two steel bars into both end of the sleeve as prescribed and fixed them in the center with a rubber sealing ring. It is necessary to ensure that the position of the steel bar in the sleeve is in the middle so that it can weaken eccentricity error. Finally bolted sleeves into a shelf and kept vertical, which is shown in Figure2.(2)
Mixed required amount of grouting material with water in four designed water:material ratios described above strictly. During the mixing process, the most significant point is to stir well by the agitator and it maybe take about 5 minutes. After work, it should be left standing for 2 minutes until the bubbles are removed.(3)
After standing, the grouting material poured into 40 mm × 40 mm × 160 mm mold to make a standard specimen, which needs cure for 28 days under the standard conditions of room temperature 20 ± 2°C and humidity greater than 50%. The final cured sample blocks were tested for their compressive strength and flexural strength according to the method in GB/T 17671 “Method of testing cements—Determination of strength (ISO)” [20]. The electrohydraulic servo universal testing machine (GMT5205) of the Ningbo University Structure Laboratory (see Figure 3) was used to load, and controlled the loading rate of 50 N/s until the standard specimens were fractured. Then, the broken specimens were subjected to compressive strength loading, which was carried out by the electrohydraulic servo universal testing machine (WAW-600C) (see Figure 3(b)) at a loading rate of 1.5 MPa/s. The compressive strength of each sample block can be calculated via the following formula:(1)fcuc=FA.fcuc is the the compressive strength of the sample block (MPa). F is the damage load of the sample block (N). A is the bearing area of the sample block (mm).The data of each water:material ratio specimen in three groups are shown in Table2:(4)
Poured the rest grouting material into the grouting gun (a tool used for grouting) and injected it into the sleeve until the material spilled from the grout outlet at the upper part of the sleeve, which represented that the sleeve is full of grouting. Plugged up two orifices with rubber stopper then the grouting is done. The most remarkable thing was making sure the sleeve was always vertical and there was nothing impurity in grout material during the whole process to prevent loss in final strength of the material. The grouted sleeves were fixed to the rack and maintains 28 ds under standard conditions.(5)
The device applied in this tensile test was the microcomputer-controlled electrohydraulic servo universal testing machine, which model of WAW-1000C in the structural laboratory of Ningbo University, as shown in Figure3(c), the loading framework in force-control mode. Keep the specimen under loading until the reinforcement pulled out from sleeve or broken and recorded the relevant data output by the testing machine such as load and displacement. Based on the definition of maximum elongation in “Technical specification for mechanical splicing of steel reinforcing bars” JGJ107-2010 [21], the maximum elongation can be calculated by the following formula (the measuring points arrangement sketch on the surface of the rebar is shown as Figure 4):(2)Asgt=L02−L01L01+fmst0E×100%.L01 is the measured length of AB or CD before loading the specimen; L02 is the measured length of AB or CD after loading the specimen; E is the elastic modulus of steel bar; fmst0 is the stress of the reinforcement when the load reaches the maximum value.Figure 2
The sleeves with different grouting material strength bolted into the shelf.Figure 3
Test equipment. (a) GMT5205. (b) WAW-600C. (c) WAW-1000C.
(a)(b)(c)Figure 4
Measured gauge point.The failure mode of the specimen in this test including rebar pullout and rebar tension fracture are shown in Figure5. According to the observation of raster images at room temperature by Zheng et al. [22], the strain of the steel bar increased linearly with the increasing load and decreased with the distance from anchor side. Therefore, the fracture position of the steel bar is largely out of the sleeve. The bond failure between rebar and grout material depends on the bond strength, which may occur in two failure form: bond slip failure between grout material and sleeve as well as bond slip failure between grout material and steel bar [23], the latter form was the same as that in this article. The results of the test are as follows in Table 3:Figure 5
The failure mode of the specimen. (a) Rebar pullout. (b) Rebar tension fracture.
(a)(b)Table 3
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeA-1224.32714.0314.56Rebar tension fractureA-2202.07643.2113.22Rebar tension fractureA-3196.87626.6612.81Rebar tension fractureB-1199.47622.5513.42Rebar pulloutB-2210.48669.9811.84Rebar tension fractureB-3201.42641.1411.52Rebar tension fractureC-1189.53603.299.80Rebar pulloutC-2177.29564.3310.48Rebar pulloutC-3185.86591.618.50Rebar pulloutD-1174.88556.663.18Rebar pulloutD-2143.87457.954.93Rebar pulloutD-3167.66533.685.77Rebar pullout


### 2.2.2. The Lack of the Grouting Fullness

In this experiment, in order to verify the influence of grouting fullness on the sleeve’s connection performance, the variable was designed as the percentage of grouting material. According to the product instructions of grouting material, the water:binder ratio was unified at 12%, and the grout filling degrees were controlled at 100%, 90%, 80%, 70%, and 60% (the five groups of tests are represented by “I,“ “II,” “III,” “IV,” “V,” respectively). Finally, the tensile strength and elongation of the steel bar with each condition were tested by experiment, and the data were recorded. In order to exclude the contingency of the test data, three groups of repeated tests were conducted for every test of various filling degrees (denoted by “I-1,” “I-2” and “I-3”). All the equipment requested in this test is shown in Figure3. The specific test step was roughly same as that in Section 2.2.1. The following are some attention points that need to take into consideration:(1)
The water ratio of the mixture should be in strict accordance with the standard at 12%. It was crucial to mix thoroughly and prevent impurities.(2)
Syringe was used for grouting in this experiment, and the amount of grouting material was controlled by its measurements to meet the requirements of different filling degrees.(3)
The required amount of grouting material should be poured into each specimen according to the designed percentage. The grouting material should be set aside for making standard specimens during the grouting of the test.(4)
The grouting process should be quick to prevent the material’s fluidity changing over time that may be affecting the strength of the grouting material.(5)
The whole process of grouting and maintenance should make sure that no eccentricity of the steel bar.According to the compressive test and calculation, the compressive strength of the standard specimens was 88.6 MPa.After the tensile test and calculation, the experimental results are shown in Table4.Table 4
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeI-1192.58612.8113.69Rebar tension fractureI-2203.89649.0013.31Rebar tension fractureI-3193.21615.0012.66Rebar tension fractureII-1188.65600.4910.31Rebar tension fractureII-2190.38606.0011.45Rebar tension fractureII-3186.30593.0112.81Rebar tension fractureIII-1195.06620.908.32Rebar tension fractureIII-2188.70600.6512.77Rebar tension fractureIII-3185.04589.0011.56Rebar tension fractureIV-1179.66571.884.30Rebar pulloutIV-2193.52615.997.42Rebar tension fractureIV-3175.47558.544.29Rebar pulloutV-1158.65505.003.14Rebar pulloutV-2151.42481.994.26Rebar pulloutV-3152.90486.702.25Rebar pulloutThere are anomalies in the data of IV-2, which may be caused by misoperation during the experiment. Therefore, this group of data should be discarded when analyzing the results.Figure 6
Water ratio-compressive strength curve.Figure 7
Displacement-load curve. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 8
Comparison of tensile strength with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 9
Comparison of elongation with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)


## 2.2.1. The Insufficiency of the Grout’s Strength

This test’s objective is investigating the effect of the grouting strength defect on the sleeve’s connection behavior. As known that the setting and hardening process will volatilize the water of grouting material and then pores will be formed in the interior. The larger porosity will cause the lower density of the grouting material. It is obvious that the larger water:material ratio is, the lower strength of grouting material will be. However, too low water:material ratio will also lead to poor workability, which makes it difficult to mix and vibrate. Thus, every grouting material has its own optimal water:bind ratio in general. Based on the product specification, the appropriate water:binder ratio of CGMJM-VI is 12%.Table 2
Results of block’s compressive strength.
NumberABCDCompressive strength (MPa)91.977.366.451.6To sum up, the defect of strength was simply simulated by increasing different water:bind ratio, which are 12%,14%,16%, and 18% (experiment represent byA, B, C, and D, respectively), to control the strength of the grout in this test. Meanwhile, the filling degree of the grout in all groups is 100%. Various destruction forms in different conditions have presented after the tensile test and maximum tension elongation and ultimate bearing capacity of the jointing element were measured too. Based on the analysis and comparison of these parameters, the conclusion can be summarized. In order to ensure the accuracy of the data, each experiment was done in three pairs (such as A-1, A-2, A-3). The specific steps are as follows:(1)
According to the standard, the anchorage length of the jointed steel bar should be eight times longer than its diameter. Inserted two steel bars into both end of the sleeve as prescribed and fixed them in the center with a rubber sealing ring. It is necessary to ensure that the position of the steel bar in the sleeve is in the middle so that it can weaken eccentricity error. Finally bolted sleeves into a shelf and kept vertical, which is shown in Figure2.(2)
Mixed required amount of grouting material with water in four designed water:material ratios described above strictly. During the mixing process, the most significant point is to stir well by the agitator and it maybe take about 5 minutes. After work, it should be left standing for 2 minutes until the bubbles are removed.(3)
After standing, the grouting material poured into 40 mm × 40 mm × 160 mm mold to make a standard specimen, which needs cure for 28 days under the standard conditions of room temperature 20 ± 2°C and humidity greater than 50%. The final cured sample blocks were tested for their compressive strength and flexural strength according to the method in GB/T 17671 “Method of testing cements—Determination of strength (ISO)” [20]. The electrohydraulic servo universal testing machine (GMT5205) of the Ningbo University Structure Laboratory (see Figure 3) was used to load, and controlled the loading rate of 50 N/s until the standard specimens were fractured. Then, the broken specimens were subjected to compressive strength loading, which was carried out by the electrohydraulic servo universal testing machine (WAW-600C) (see Figure 3(b)) at a loading rate of 1.5 MPa/s. The compressive strength of each sample block can be calculated via the following formula:(1)fcuc=FA.fcuc is the the compressive strength of the sample block (MPa). F is the damage load of the sample block (N). A is the bearing area of the sample block (mm).The data of each water:material ratio specimen in three groups are shown in Table2:(4)
Poured the rest grouting material into the grouting gun (a tool used for grouting) and injected it into the sleeve until the material spilled from the grout outlet at the upper part of the sleeve, which represented that the sleeve is full of grouting. Plugged up two orifices with rubber stopper then the grouting is done. The most remarkable thing was making sure the sleeve was always vertical and there was nothing impurity in grout material during the whole process to prevent loss in final strength of the material. The grouted sleeves were fixed to the rack and maintains 28 ds under standard conditions.(5)
The device applied in this tensile test was the microcomputer-controlled electrohydraulic servo universal testing machine, which model of WAW-1000C in the structural laboratory of Ningbo University, as shown in Figure3(c), the loading framework in force-control mode. Keep the specimen under loading until the reinforcement pulled out from sleeve or broken and recorded the relevant data output by the testing machine such as load and displacement. Based on the definition of maximum elongation in “Technical specification for mechanical splicing of steel reinforcing bars” JGJ107-2010 [21], the maximum elongation can be calculated by the following formula (the measuring points arrangement sketch on the surface of the rebar is shown as Figure 4):(2)Asgt=L02−L01L01+fmst0E×100%.L01 is the measured length of AB or CD before loading the specimen; L02 is the measured length of AB or CD after loading the specimen; E is the elastic modulus of steel bar; fmst0 is the stress of the reinforcement when the load reaches the maximum value.Figure 2
The sleeves with different grouting material strength bolted into the shelf.Figure 3
Test equipment. (a) GMT5205. (b) WAW-600C. (c) WAW-1000C.
(a)(b)(c)Figure 4
Measured gauge point.The failure mode of the specimen in this test including rebar pullout and rebar tension fracture are shown in Figure5. According to the observation of raster images at room temperature by Zheng et al. [22], the strain of the steel bar increased linearly with the increasing load and decreased with the distance from anchor side. Therefore, the fracture position of the steel bar is largely out of the sleeve. The bond failure between rebar and grout material depends on the bond strength, which may occur in two failure form: bond slip failure between grout material and sleeve as well as bond slip failure between grout material and steel bar [23], the latter form was the same as that in this article. The results of the test are as follows in Table 3:Figure 5
The failure mode of the specimen. (a) Rebar pullout. (b) Rebar tension fracture.
(a)(b)Table 3
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeA-1224.32714.0314.56Rebar tension fractureA-2202.07643.2113.22Rebar tension fractureA-3196.87626.6612.81Rebar tension fractureB-1199.47622.5513.42Rebar pulloutB-2210.48669.9811.84Rebar tension fractureB-3201.42641.1411.52Rebar tension fractureC-1189.53603.299.80Rebar pulloutC-2177.29564.3310.48Rebar pulloutC-3185.86591.618.50Rebar pulloutD-1174.88556.663.18Rebar pulloutD-2143.87457.954.93Rebar pulloutD-3167.66533.685.77Rebar pullout


## 2.2.2. The Lack of the Grouting Fullness

In this experiment, in order to verify the influence of grouting fullness on the sleeve’s connection performance, the variable was designed as the percentage of grouting material. According to the product instructions of grouting material, the water:binder ratio was unified at 12%, and the grout filling degrees were controlled at 100%, 90%, 80%, 70%, and 60% (the five groups of tests are represented by “I,“ “II,” “III,” “IV,” “V,” respectively). Finally, the tensile strength and elongation of the steel bar with each condition were tested by experiment, and the data were recorded. In order to exclude the contingency of the test data, three groups of repeated tests were conducted for every test of various filling degrees (denoted by “I-1,” “I-2” and “I-3”). All the equipment requested in this test is shown in Figure3. The specific test step was roughly same as that in Section 2.2.1. The following are some attention points that need to take into consideration:(1)
The water ratio of the mixture should be in strict accordance with the standard at 12%. It was crucial to mix thoroughly and prevent impurities.(2)
Syringe was used for grouting in this experiment, and the amount of grouting material was controlled by its measurements to meet the requirements of different filling degrees.(3)
The required amount of grouting material should be poured into each specimen according to the designed percentage. The grouting material should be set aside for making standard specimens during the grouting of the test.(4)
The grouting process should be quick to prevent the material’s fluidity changing over time that may be affecting the strength of the grouting material.(5)
The whole process of grouting and maintenance should make sure that no eccentricity of the steel bar.According to the compressive test and calculation, the compressive strength of the standard specimens was 88.6 MPa.After the tensile test and calculation, the experimental results are shown in Table4.Table 4
Specimen loading test results.
NumberUltimate bearing capacity (kN)Stress strength (MPa)Maximum elongation (%)Failure modeI-1192.58612.8113.69Rebar tension fractureI-2203.89649.0013.31Rebar tension fractureI-3193.21615.0012.66Rebar tension fractureII-1188.65600.4910.31Rebar tension fractureII-2190.38606.0011.45Rebar tension fractureII-3186.30593.0112.81Rebar tension fractureIII-1195.06620.908.32Rebar tension fractureIII-2188.70600.6512.77Rebar tension fractureIII-3185.04589.0011.56Rebar tension fractureIV-1179.66571.884.30Rebar pulloutIV-2193.52615.997.42Rebar tension fractureIV-3175.47558.544.29Rebar pulloutV-1158.65505.003.14Rebar pulloutV-2151.42481.994.26Rebar pulloutV-3152.90486.702.25Rebar pulloutThere are anomalies in the data of IV-2, which may be caused by misoperation during the experiment. Therefore, this group of data should be discarded when analyzing the results.Figure 6
Water ratio-compressive strength curve.Figure 7
Displacement-load curve. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 8
Comparison of tensile strength with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)Figure 9
Comparison of elongation with different condition. (a) Different grout’s strength. (b) Different grouting fullness.
(a)(b)


## 3. Results and Discussion

(1)
According to the compressive strength of standard blocks in Table2, a line graph of water:binder ratio and compressive strength is drawn in Figure 6. It can be observed that the strength of grouting blocks was decreased with the increase in water ratio of the grouting material, and it is an almost linear correlation between the water:material ratio and the strength of the grouting. The most typical group of test results from the parallel test of each grouting material strength were selected for comparison, and the load-displacement curve of the steel bar connected by the grouted sleeve with different grouting material strength can be drawn by the data of Table 3 (shown as Figure 7(a)). It can be seen from the figure that the load-displacement curves of the four groups are similar in the elastic deformation range and yield range. When the specimen entered the strain-hardening range, the maximum load that the steel bars can bear was different due to the different strength of the grouting material. The failure form of the test group with water percentage of 12% and 14% were rebar tension fracture, and the ultimate load of them was the largest among the four groups. However, when the water:material ratio increased to 16%, the failure form of reinforcement changed to pullout failure, the corresponding ultimate bearing capacity also began to decrease, and the ultimate load of 18% was the minimum. This phenomenon indicates that when the strength defect of the grouting material has existed, the lower strength of the grouting material is the less the maximum load it can bear and the displacement will decrease before load reaching the peak value.Based on the experiment results of Table4 in Section 2.2.2 to draw the displacement-load curve, as shown in Figure 7(b), the rebar of five different conditions have all reached the yield point and the shape of the curve was roughly same before the steel bar entered the strain-hardened stage. However, the displacement and the maximum load which joint can bear were divergent due to different grouting fullness. The figure gave the outline of the relationship of two variable: with the decrease of grouting degree, the ultimate load that the reinforcement can bear decreased. The reason for this phenomenon is that grouted sleeves connect two rebars via mechanical interaction force, friction, and bond force. When the grouting is insufficient, the anchorage length of the steel bar will reduce correspondingly. If the failure mode is rebar pulls out, the bonding force can be approximately equal to the maximum tension. Based on the computational formula of bonding stress (2) and the formula of bonding strength (3), which improved by Kim and Ahn [24], bonding strength is relate to confining pressure and grouting material strength. Due to the bonding strength is all the same in this test, the bonding force will decrease with the decrease in anchorage length, and even make influence of the mechanical properties. Ling et al. [25] mentioned that bonding force can increase by increasing anchorage length and reducing the diameter of the sleeve.(3)Pb=τb⋅π⋅d⋅L,(4)τb=1.49+0.45fn⋅fg.Pb is the bond force; τb is the bond strength; d is the inner diameter of the sleeve; L is the anchorage length of the rebar; Pbfnis the lateral confining pressure, and fg is the compressive strength of grouting material.(2)
According to the standard of the joint’s mechanical properties under uniaxial tension test in “Technical specification for grout sleeve splicing of rebar” JG/J 355-2015 [26], the damage position should be outside the joint, and the yield strength of the joint should not be less than the standard value of the yield strength as well as the tensile strength should not be less than the corresponding standard value of tensile strength. If specimens are not damaged until the joint’s tensile strength is greater than 1.15 times of the connected steel bar’s standard value of the tensile strength, they are recognized as qualified. Table 3 records the relevant parameters obtained from the tensile test of the grout strength defect. Nearly all test groups with four water:binder ratios reached the yield point, which meets the standard. Base on test data, a line graph of tensile strength corresponding to four strength of grouting materials is drawn as below (Figure 8(a) (group1, group2, group3 in Figure 8 represent three repeat tests of each condition). It brings about a discovery that the tensile strength of the reinforcement decreased with the decrease in the strength of the grouting materials. The standard value of HRB400 reinforcement tensile strength is 540 MPa, and the average value of test tensile strength with the water:binder ratio of 12% and 14% was 661.3 MPa and 644.56 MPa, respectively, which all pass muster. However, the counterpart value with the water:binder ratio of 16% and 18% (586.41 MPa and 516.1 Mpa, respectively) were both less than 1.15 times of the tensile strength standard value of reinforcement, so these two groups of specimens were unqualified. Meanwhile, the comparison in regard to the test of lack of grouting fullness is depicted as in Figure 8(b). As the fullness degree decreasing, the line in the chart shows a declining trend. The failure mode of specimens with more than 80% grouting were rebar failure, and the tensile strength was all greater than the standard value, the decline of this part was flat. On the contrary, the specimens with 70% and 60% grouting destroyed as rebar pullout and the tensile strength were all less than 1.15 times standard value (621 MPa), so they were not qualified.The specification also has certain requirements for the deformation performance of the rebar connected with the grout sleeve: total elongation of the steel bar under the maximum force should be ≥6%. Therefore, the specimens with the water:binder ratio of 12%, 14%, and 16% in 2.2.1 all met the requirements, while the maximum elongation of the three parallel tests with 18% water:binder ratio was 3.18%, 4.93%, and 5.77% that all did not meet the requirements. In Section2.2.2 we found that the degree of 80% was a threshold value, the elongation almost not reached the standard when the degree below it. Comparisons of total elongation with two grouting defects are sketched in Figures 9(a) and 9(b), respectively (group1, group2, group3 in Figure 9 represent three repeat tests of each condition).(3)
In addition, grouting sleeves are also widely used in seismic structures as Ameli et al. [27] pointed out that grouted sleeves can transfer force and cause a smaller displacement in prefabricated components. The steel bars applied in aseismic structure has special specification in 5.2.3 of “code for acceptance of the constructional quality of concrete construction” GB 50204-2002(2011) [28]. The ratio of rebar’s tensile strength to yield strength should not be less than 1.25 as a consequence of the structure should have a certain ductility to satisfy the deformation performance. Kim [29] has conducted an experimental study on the connection performance of the grouted sleeve under cyclic loading. The result showed that when the rebar fracture outside the sleeve, its tensile strength could reach 1.34–1.52 times of the yield strength. In this test, the calculation and analysis showed that specimens with the water:binder ratio of 12% and 14% measured up. The tensile-yield ratio of C-2 and C-3 were 1.23 and 1.24 which did not satisf the standard, and all three parallel groups with the water:binder ratio of 18% were not up to the standard.


## 4. Conclusions

In this study, 27 sleeve specimens were manufactured in the experiment to ensure the effect of grouting defects on the mechanical properties of connected rebar. The experiment divided into two parts to investigate different factors. The main content of the research was testing mechanical properties of specimens under different conditions by means of tensile tests, then compared them with the standard in the specifications and summarizes the rules. The following conclusions can be drawn from this article:(1)
Because of different grouting conditions, there are two damage forms of steel reinforcement: rebar tension fracture and rebar pullout. The load carried by steel bar that connected by grouted sleeve is much lower than the ultimate load of itself when rebar pulls out, and the mechanical performance of the reinforcement is not fully utilized in this condition. So this damage must be avoided in the project.(2)
Base on the data provided, the strength of the grout is reduced with the increase in the water:binder ratio of grout, whereas the tensile strength of the joint is decreased at a similar tendency. The compression strength of the grout’s water:ratio at 14% is 77.3 MPa, and the mechanical properties of the joint satisfied the specifications, but when the water ratio increases to more than 16% and the corresponding compression reduces to less than 66.4 MPa, the tensile strength and the elongation are no longer meet the standard. From the above discussion, one may roughly conclude that the threshold of the water:binder ratio of CGMJM-VI grout material is between 14% and 16%. Thus, it is critical to control the water:binder ratio no more than 14%, at optimal value of 12% as possible, to ensure that the mechanical properties of steel bars are fully realized.(3)
For the sleeve with a deficiency in grouting fullness, the mechanical properties of the joint will decrease with the decrease of the fulling degree. When grouting material fullness is above 80%, the strength properties and deformation properties of the steel bar will decrease smoothly and both can meet the specification requirements. When the degree is below 80%, the tensile strength and maximum elongation of the joint begins to decrease significantly, and the mechanical properties of the steel bar are also unqualified. Therefore, it becomes obvious that the lack of grouting caused by improper operation in the project must be controlled fullness degree greater than 80% which hardly make a conspicuous difference on joint’s properties in this range.



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*Source: 5036505-2022-03-25.xml*