Keywords

1 Introduction

Sleeve grouting materials, as an important part of prefabricated buildings, have been widely used with the rapid development of prefabricated buildings in recent years. Concurrently, the widespread construction of high-rise and super high-rise buildings also imposes higher requirements on the performance of grouting materials. However, the existing sleeve grouting material cannot fully meet the construction needs of high-rise prefabricated buildings, and there are still certain performance defects in terms of fluidity, early strength and expansion. Therefore, it is of great significance to develop sleeve grouting material that meets the construction requirements and has good performance to improve the quality of construction, save resources, protect the environment and promote the development of prefabricated buildings.

By the present juncture, an extensive body of research has been dedicated to the investigation of sleeve grouting materials. In order to improve the performances of sleeve grouting materials at low temperatures, Lu et al. [1] studied the effects of different types of early strength agents on sleeve grouting materials; Sun et al. [2] studied the effects of five different components on the compressive strength, fluidity, and expansion rate of the sleeve grouting materials, and analyzed their microscopic appearance with an electron microscope; Li et al. [3] prepared a grouting material with low cost and good performance based on ordinary Portland cement through an orthogonal test; Hu et al. developed a cement-based grouting material [4] to improve the performance of the current grouting materials. The results show that the 28d compressive strength and 30-min fluidity of the grouting materials have been improved compared with the ordinary ones. Xiong et al. [5] designed 402 grouting material specimens in the study of the compressive strength of sleeve grouting materials, analyzing the relationship between their shape, size, and water-material ratio and the compressive strength, and establishing the conversion formula between the compressive strength of the standard specimen and the test one; Huang et al. [6] developed a sleeve grouting material with a good performance by mixing sulphoaluminate cement, Portland cement, and gypsum ternary cementitious materials, and analyzed the effects of water-binder ratio, silica fume content, and other factors on the performance of the grouting materials; When studying the composition of grouting materials for prefabricated buildings; Yazan Alrefaei et al. [7] studied the effect of capsules on the flowability, compressive strength, and porosity of grouting materials through experiments. The results showed that adding no more than 3% of capsules to grouting materials can meet the requirements of engineering for flowability and compressive strength; Kiarash Koushfar et al. [8] explore the feasibility of grouted joint sleeve connections (GSSC) using carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) sheets; Xu et al. [9] considered the effects of quartz sand gradation, sand-cement ratio, water-cement ratio, admixtures and other factors on the performance of grouting materials, determining the ideal mix proportion; Yang et al. [10] analyzed the influence of water-binder ratio, sand-cement ratio, defoamer, silica fume, and expansion agent on the fluidity and compressive strength of sleeve grouting materials through experimental methods. The results indicate that the sand-cement ratio has a greater impact on the performance of grouting materials, and the addition of silica fume could improve the overall compressive strength. However, the performance of sleeve grouting material is the result of the interaction of multiple components, and the best mix proportion cannot be accurately obtained only through the analysis of a single component. Zhu Wen et al. [11] established the optimal ratio ternary composite material system of sulfonated aluminate cement, Portland cement, and gypsum through orthogonal experiments.

Consequently, based on the existing research results, a progressive test is adopted in this study to analyze the influence of cement sand ratio, water reducer, silica fume, sulphoaluminate, and lithium carbonate content on the performance of sleeve grouting materials. The best admixture content obtained from the former group of tests is used as the initial mix proportion of the next group of tests. The orthogonal test method can determine the influence of different materials on the fluidity, compressive strength and vertical expansion rate of sleeve grouting material. By employing this method, the optimal mixture ratio of the grouting materials can be continuously corrected, and the grouting material with excellent properties can be found. This study introduces novel ideas and methods for the preparation of new sleeve grouting materials for prefabricated construction.

2 Properties and Composition of Sleeve Grouting Materials

2.1 Technical Index of Sleeve Grouting Materials

The properties of sleeve grouting materials for steel bar connection have a great influence on the connection effect of prefabricated components. Reasonable preparation and excellent performance of grouting materials can not only ensure the stability and reliability of the connection, but also ensure the safety of the whole structure of prefabricated buildings, and at the same time, it can also assist in reducing the difficulty of construction. The technical requirements for the property indexes of sleeve grouting material are specified in Sleeve Grouting Materials for Steel Bar Connections [12], mainly including fluidity, compressive strength, and vertical expansion rate. The specific index requirements are shown in Table 1.

Table 1. Technical indexes of grouting material for steel bar sleeve connection.
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2.2 Selection of the Components of Grouting Materials

The different components and contents of the sleeve grouting materials directly affect their properties. Therefore, it becomes crucial to select raw materials that have a positive impact on the performance of the grouting materials while being easily accessible and preparable. Traditional grouting materials are mainly composed of cementitious materials, aggregates, and some admixtures, among which, the common admixtures are water reducers, expansion agents, and defoamers. Considering the strength of quartz sand is better than that of river sand, the former is used as a fine aggregate in the experimental preparation process. In order to address the problem of bleeding and segregation caused by water reducers, cellulose ether is added to increase viscosity and thickening.

3 Study on the Preparation of Base Grout

The main components of the grouting materials are cementitious materials and sand, and the base grout with excellent performance directly affects the overall characteristics of the grouting material, so it is necessary to determine the appropriate cement-sand ratio before the test, and on this basis, to study the effects of admixtures and their contents on the performance of grouting materials by adding different contents of admixtures.

3.1 Mix Proportion Design of Base Grout of Grouting Materials

When preparing the base grout of the grouting material, the content of the admixture shall be kept unchanged, and the change law of the base grout performance with the cement-sand ratio shall be studied by constantly adjusting the proportion of cementitious materials and sand, and then the appropriate cement-sand ratio shall be selected. The design scheme for preparing the base grout in this experiment is shown in Table 2 below.

Table 2. The design scheme of base slurry preparation
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3.2 Properties of Base Grout of Grouting Materials

The fluidity, compressive strength, and expansion rate of the sleeve grouting materials obtained through the test are shown in Fig. 1 according to the above base grout preparation and design method.

Fig. 1.
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Test value of basic slurry performance of sleeve grouting materials

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The figure above demonstrates a similarity between the changes in fluidity and compressive strength of the base grout in sleeve grouting materials with respect to the cement-sand ratio. Specifically, both properties exhibit a pattern of initial increase followed by subsequent decrease. However, from the experimental results, the fluidity reaches the peak value when the cement-sand ratio is about 0.9, while the compressive strength reaches the peak value when the cement-sand ratio is about 1.0. The 3 h vertical expansion rate increases with the cement-sand ratio, but the change range is limited. Overall, the fluidity and vertical expansion rate of the base grout cannot meet the requirements of the specification, and the 1 d compressive strength under the conditions of 28 d and Mix ratio 1 is also lower than the strength requirements. Therefore, considering the three indicators, the cement-sand ratio of 1.0 is selected as the mix proportion of the base grout.

4 Study on Preparation Technology of Sleeve Grouting Materials

4.1 Influence of the Content of Water Reducers on Sleeve Grouting Materials

Water reducer can be used as an important additive in sleeve grouting materials because it can enhance the hydration reaction of cement, and improve the fluidity and workability of cement stone materials.

Based on the mixing scheme of the base grout of sleeve grouting materials under the condition of the cement-sand ratio of 1.0, the variation law of the performance of sleeve grouting materials with the content of water reducer is studied by changing the percentage of water reducer in the total mass of cementitious materials. In this test, the dosages of water reducer are 10.5 g, 13.6 g, 15.7 g, 17.8 g, and 20 g respectively, accounting for 1.0%, 1.3%, 1.5%, 1.7%, and 1.9%. The experimental results are shown in Fig. 2 below.

Fig. 2.
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Effect of the content of water reducer on properties of sleeve grouting materials

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The experimental results reveal that the effect of the water reducer on the vertical expansion rate of grouting material is not obvious, which cannot meet the requirements of the specification but can improve its fluidity and compressive strength. As the content of water reducer increases, the initial values of fluidity and compressive strength exhibit an initial ascent followed by a subsequent decline. The peak values are attained at approximately 1.5% water reducer content. Meanwhile, under this condition, the vertical expansion rate of 3 h and 24 h is also the maximum. Therefore, based on the above conditions, the optimal dosage of water reducer in this test is 15.7 g.

4.2 Influence of Expansion Agent Content on Sleeve Grouting Material

Based on the test results of the water reducer, the influence of the content of the expansion agent on the fluidity, compressive strength, and vertical expansion rate of sleeve grouting materials can be studied by adjusting the content of the expansion agent. During the test, the contents of other components remain unchanged, and the contents of the expansion agent are set to 0.4 g, 0.8 g, 1.2 g, 1.6 g, and 2.0 g. The test results are shown in Fig. 3 below.

Fig. 3.
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Influence of expansion agent content on the performance of sleeve grouting materials

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It can be seen from the above figure that the expansion agent can remarkably improve the performance of the sleeve grouting materials. The vertical expansion rates of 3 h and 24 h demonstrate a significant increase as the content of the expansion agent increases. However, when the content of the expansion agent increases to 1.6 g, the difference between the vertical expansion rate of 24 h and 3 h exceeds the technical specification and does not meet the requirements. The fluidity and compressive strength indicators meet the technical requirements, and the overall fluctuation range is small. Therefore, the above data reveals that the dosage of the expansion agent is 1.2 g, that is, the dosage of 0.114% is the best.

4.3 Influence of Silica Fume Content on Sleeve Grouting Material

Given the lightweight nature, high fineness, and low porosity of silica fume, the addition of a suitable quantity of this material to the sleeve grouting mixture is advantageous for enhancing both the overall strength and compactness of the slurry. Moreover, the active substances on the surface of the silica fume will make the electrostatic repulsion between particles greater than the cohesion, which can disperse cement particles, play a role in lubrication, and then improve the fluidity of slurry. In this test, the content of the expansion agent is kept at 1.2 g, and the content of the silica fume was set at 0 g, 21 g, 42 g, 63 g, and 84 g respectively, to explore the change in the properties of grouting materials. The experimental results are in Fig. 4 below.

Fig. 4.
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Effect of the content of silica fume on properties of sleeve grouting materials

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The results show that with the increase of the content of silica fume, the fluidity and compressive strength of the sleeve grouting material increase first and then decrease, and reach the maximum when the silica fume content is about 42g. It is also found that the fluidity is sensitive to the content of silica fume. When the content of silica fume is low, the fluidity of the slurry can be improved by increasing content, which is related to the characteristics of the surface active substances of silica fume. However, with the further increase of the silica fume content, the fluidity of the slurry will be significantly reduced. Through this experiment, the optimum silica fume content is determined to be 42 g.

4.4 Effect of Sulphoaluminate and Lithium Carbonate Content on Sleeve Grouting Material

Based on the above experiments, the sleeve grouting material can be obtained when the cement-sand ratio is 1.0, the content of the water reducer is 15.7 g, the content of the expansion agent is 1.2 g and the content of silica fume is 42 g. Moreover, the fluidity, compressive strength, and vertical expansion rate of the prepared slurry can meet the technical requirements. To further develop new materials with better performance, based on the above basic experimental ratio, a better sleeve grouting material is obtained by mixing ordinary Portland cement with sulphoaluminate cement and using lithium carbonate as an early strength agent. The ratio of sulphoaluminate to lithium carbonate and the corresponding experimental results are shown in Table 3 and Fig. 5, respectively.

Table 3. Experimental proportioning of high-performance sleeve grouting material.
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Fig. 5.
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Effect of the content of sulphoaluminate and lithium carbonate on sleeve grouting material

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Based on the experimental results of the first and second groups, when the sulphoaluminate cement is directly mixed with ordinary Portland cement without adding an early strength agent, the fluidity of the grouting material will be reduced. Moreover, the reduction will be more obvious with the increased sulphoaluminate content while the early strength demonstrates an improvement. Therefore, based on the experimental ratio of the second group, the performance of the grouting material can be further optimized by adding the early strength agent lithium carbonate.

It can be seen from the test results of the third, fourth, and fifth groups that the fluidity of the grouting material gradually decreases with the increased content of lithium carbonate. Moreover, the fluidity of the fifth group does not meet the requirements of Sleeve Grouting Materials for Steel Bar Connection (JG/T408–2019). In contrast, the fluidity of the third group satisfies the requirements, but its 3 h compressive strength fails to exceed 40 MPa. Generally speaking, the compressive strength is improved after adding an early strength agent, but the change of early strength tends to be stable when the content of the early strength agent is more than 0.04%. Based on the above analysis, the best proportion of sulphoaluminate and lithium carbonate can be determined as the fourth group.

5 Conclusion

In this paper, the effects of cement-sand ratio, superplasticizer, silica fume, sulfoaluminate and lithium carbonate contents on the fluidity, compressive strength and vertical expansion rate of sleeve grouting material are analyzed sequentially by using the orthogonal test method, which can not only improve the performance of grouting material while reducing the amount of material, but also save costs, and provide a reference for the preparation of high-performance grouting material, which is of great significance for promoting the development of prefabricated buildings. Based on the experimental results and the research of this paper, the following conclusions can be drawn:

1. With the increase of the cement-sand ratio, the fluidity and strength of the sleeve grouting materials increase first and then decrease, and the corresponding cement-sand ratio is close when they reach their peak values; water reducer can play a role in improving the compressive strength and fluidity of the sleeve grouting materials, but has little effect on the expansion. Notably, the dosage of the water reducer should be within the appropriate range, and excessive addition of water reducer will lead to a decrease in compressive strength and poor fluidity of grout. The optimal cement-sand ratio is 1.0, and the dosage of water reducer is 1.5%.

2. The expansion agent can improve the compressive strength, fluidity, and expansibility of the sleeve grouting material to a certain extent. However, when its content exceeds 0.114%, the properties of the grouting material no longer meet the requirements of the standard specification. Additionally, within a certain range of content, silica fume plays a role in lubricating the sleeve grouting material, and the compressive strength reaches a peak value when the silica fume content is 4%.

3. Both lithium carbonate and sulphoaluminate cement can accelerate the reaction rate of ordinary Portland cement and improve its early strength. Based on ordinary sleeve grouting materials, 10% of sulphoaluminate is added, and the content of lithium carbonate in sulphoaluminate is set to 0.04%. Based on the above ratio, the high-performance sleeve grouting material with a 3 h compressive strength of more than 40 MPa is obtained.