Study on the structure and properties of non-water reacted polyurethane grouting materials

In this study, non-water reacted polyurethane grouting materials were prepared using three different molecular chain lengths of polyether polyols. The influence of polyols and -NCO/-OH index (Rx) on the initial viscosity, mechanical properties, and concrete adhesion performance was investigated. Additionally, the damping performance of the cured materials was evaluated using dynamic mechanical analysis (DMA). The research results showed that with an increase in the molecular chain length of the polyether polyols, the strength of the cured material significantly decreased, while the elongation at break increased noticeably. Compared to the molecular chain length of the polyether polyols, the effect of the Rx on the mechanical properties of the cured material was not significant, especially in the low molecular weight polyether polyol system with shorter molecular chains (G3). The adhesion strength of the non-water reacted polyurethane grouting materials to concrete was highly correlated with the gelation time, and a slow gelation time could result in the formation of foam-like structures and reduce the adhesion strength. Grouting materials with shorter molecular chain lengths combined with low Rx (G3-1.0) or medium molecular chain lengths combined with moderate Rx (G5-1.4) exhibited excellent mechanical and damping properties.


Introduction
Statistics have shown that the leakage rate of construction projects in China exceeds 80%, and prolonged water leakage can lead to structural damage and compromise structural safety.Chemical grouting has become a common construction technique for repairing building leakage issues [1][2][3][4][5].Among the various chemical grouting materials, polyurethane grouting material is the most widely applicable and extensively used.The feasibility and effectiveness of using polyurethane foam materials for sealing concrete cracks were investigated by Parkash et al. [6].The results indicated that the use of polyurethane foam materials is a viable approach for sealing concrete cracks.This method enables rapid and effective sealing of concrete cracks, enhancing the durability and impermeability of concrete structures.
Water-reacted polyurethane grouting material, in the form of prepolymer, has gained significant application in practical engineering due to its simple construction process.However, water-reacted polyurethane grouting materials typically have disadvantages such as low strength, poor environmental friendliness (high viscosity of the prepolymer necessitating the addition of a large amount of solvent to reduce viscosity), and inability to self-cure (curing process involves the reaction between -NCO and water).On the other hand, non-water reacted polyurethane grouting material, composed primarily of polyols, additives, and isocyanate or prepolymers terminated with -NCO groups, offers advantages such as high strength, self-curing, low shrinkage, and environmental friendliness [7][8][9].Consequently, it has been extensively used in the treatment of engineering issues in coal mines, underground projects, nonexcavation repairs, railways, and other fields [10].
To better guide the application of non-water-reacted polyurethane grouting materials in concrete crack repair projects, this study prepared non-water reacted polyurethane grouting materials using three different molecular chain lengths of polyether polyols.The effects of polyol molecular weight and -NCO/-OH index (Rx) on the initial viscosity, mechanical properties, and concrete bonding performance were investigated.Additionally, dynamic mechanical analysis (DMA) was employed to evaluate the damping properties of the solidified material.

Sample preparation 2.2.1. Grouting material preparation
Weigh 80 parts by mass of polyether polyol and 20 parts by mass of plasticizer into a three-necked flask.Attach a thermometer, stirrer, and vacuum dehydration device.Dehydrate the mixture at 120°C under a vacuum of -0.1 MPa for 2 hours.Cool the mixture to below 60°C, then add 2 parts by mass of foam stabilizer and 0.16 parts by mass of catalyst.Stir the mixture evenly to obtain Component A. Component B consists of isocyanates.

Sample preparation
Weigh the required amount of Component A, and calculate the amount of Component B based on different Rx values (1.0, 1.2, 1.4, 1.6, 1.8).Add Component B to Component A and mix rapidly for 10-15s.Pour the mixture into molds and let it cure at room temperature for 7 days before testing.
In this paper, the grouting materials prepared using tri-functional polyether polyols with hydroxyl equivalent weights of 125, 166, and 233 are denoted as G3, G5, and G7, respectively.The corresponding solidified grouting materials are represented as G3-Rx, G5-Rx, and G7-Rx.

Characterization
The initial viscosity was measured according to GB/T 2794-2013 (Chinese standards) to determine the initial viscosity of the mixed A and B components.Compression strength testing was conducted in accordance with JC/T 2041-2020 (Chinese standards).Tensile strength and bonding strength were tested according to JC/T 1041-2007 (Chinese standards).The damping performance was evaluated using Dynamic Mechanical Analysis with a Mettler-Toledo DMA1 instrument.The test was performed in the dual cantilever mode with a scanning frequency of 5 Hz and a temperature range of -50 to 150°C.The heating rate was 3°C/min, and the sample dimensions were 25 mm × 5 mm × 3 mm.

Initial viscosity
The low initial viscosity of the grouting material allows for effective penetration into fine cracks, facilitating concrete crack repair [11].Some standards specify that the viscosity of polyurethane grouting materials should be less than or equal to 1000 mPa• s.The influence of different polyol molecular weights and Rx on the initial viscosity of non-water reacted polyurethane grouting material is shown in Figure 1.It can be observed that the initial viscosity of all tested grouting materials was less than 1000 mPa• s.Among them, G3 had the highest initial viscosity, exceeding 400 mPa• s, and even reaching 569 mPa• s at Rx=1.6.This is mainly due to the smaller molecular weight of the polyether polyol, which exhibits a stronger intermolecular hydrogen bonding, resulting in a higher apparent viscosity (actual measured viscosity values are given in Section 2.1).The initial viscosities of G5 and G7 showed relatively small variations with Rx, remaining at approximately 200-300 mPa• s.The compressive strength of water-reacted polyurethane grouting materials is mostly in the range of several megapascals because their curing process is heavily influenced by water in the environment.In contrast to water-reactive polyurethane grouting materials, non-water reacted polyurethane grouting materials can achieve self-curing, where the A and B components react with each other to form a solidified mass without the involvement of water.Figure 2a shows the compression performance of the grouted specimens under different polyether polyol and isocyanate index conditions.It can be observed that the grouted specimens formed with low molecular weight polyols exhibit a higher cross-linking density, resulting in higher compressive strength: G3 > G5 > G7.Specifically, the compressive strength of the G3 specimens is consistently above 60 MPa, with G3-1.2 reaching an impressive strength of 80 MPa.The compressive strength of the G5 specimens is noticeably lower than that of G3 but still remains above 20 MPa, with G5-1.2 achieving a strength of 58.8 MPa.Due to the longer molecular chains, the overall compressive strength of the G7 specimens is lower, below 10 MPa.With an increase in Rx, the compressive strength of all the grouting materials initially increases and then decreases.This may be attributed to the rapid increase in the hard segment content as Rx increases, which enhances the strength of the solidified mass [12].However, as Rx further increases, excess isocyanate reacts with the already formed urethane and urea groups, leading to the formation of more brittle structural materials such as biuret and urea-aldehyde condensates, thus reducing the compressive strength of the solidified mass [13].2b-c.It can be observed that as Rx increases, the tensile strength of the specimens gradually increases while the elongation at break decreases significantly.This is mainly attributed to the increase in the hard segment content in the grouting material components.Typical tensile curves of the specimens are shown in Figure 3.

Mechanical properties
As Rx increases from 1.0 to 1.8, the tensile strength of the solidified mass formed by G3 remains consistently between 40-50 MPa, while the elongation at break decreases from 6.4% to 3.2%.For G5, when Rx=1.0, its tensile strength is only 8.2 MPa, but the elongation at break is as high as 91%.With an increase in Rx, its tensile strength continues to improve while the elongation at break decreases significantly.After Rx≥1.2, its tensile strength remains between 30-40 MPa, and the elongation at break decreases to 6-10%.In contrast to G3 and G5, G7 is significantly influenced by Rx.When Rx = 1.0, it does not form an effective cross-linked structure, resulting in a tensile strength of less than 1 MPa.As Rx increases, both the tensile strength and elongation at break improve.At Rx = 1.4, the tensile strength reaches 6.5 MPa, and the elongation at break is as high as 102%, indicating good toughness.At Rx=1.8, the tensile strength reaches 20 MPa, and the elongation at break is 43%, showing excellent overall performance.
The results of the tensile and compressive performance tests indicate that G3, with shorter molecular chains, exhibits better rigidity.G5, with moderate molecular chain length, demonstrates a certain level of strength and appropriate toughness.G7, due to its longer molecular chains, has lower strength but better toughness.These characteristics make non-water reacted polyurethane grouting materials suitable for various application scenarios.

Concrete bonding performance
The adhesion strength between the non-water reactive polyurethane grouting material and the concrete interface is crucial in determining its repair performance [14].In this study, the adhesion performance of concrete specimens was tested using an "8" shaped concrete block, and the results are shown in Figure 4a.It can be observed that the adhesion strength of the grouting material increases with the increase in Rx.This is mainly attributed to the excess isocyanate, which can react with the hydroxyl groups on the surface of the adhered substrate, forming chemical bonds that enhance the adhesion strength.
It is worth noting that the bond strength of all grouting materials is less than 1 MPa.This may be due to the long gelation time of the grouting material (≥100s), where the moisture present in the air or on the concrete surface participates in the curing reaction, resulting in the formation of foam within the solidified material.The strength of the grouting material significantly decreases after foaming, and fracture occurs at the original crack position when subjected to external forces.This hypothesis was confirmed by accelerating the gelation time.Figure 4b-c provide cross-sectional images of the grouting samples at two different gelation times.It is evident that the grouting material with a faster gelation speed (28s) forms a resinous body between the concrete blocks, and fracture occurs in other parts of the concrete under external force, resulting in a bonding strength of 2.3MPa.On the other hand, the grouting material with a slower gelation time (112s) forms a foam-like structure between the concrete blocks, and fracture occurs at the original crack location, with a strength not exceeding 1MPa.

Dynamic Mechanical Analysis(DMA)
. Tanδ curves of three grouting material solidified bodies.The excellent damping performance of the grouting material can prevent secondary cracking of the concrete structure when subjected to external vibrations after repair.DMA is a commonly used method to study the damping properties of resin-based materials.By analyzing the tanδ (loss factor) curve, the glass transition temperature (Tg) of the material can be accurately determined.The damping performance of the material can also be evaluated based on the temperature range (TR> 0.3) and the area under the damping peak (TA> 0.3) where tanδ > 0.3 [15,16].
In this study, three grouting materials with excellent comprehensive performance (G3-1.0,G5-1.4,G7-1.8) were selected for DMA testing, and the obtained tanδ curves are shown in Figure 5.All three solidified samples showed significant secondary relaxation peaks (β peak) in the tanδ curves.The secondary relaxation motion units can move at relatively low temperatures, allowing the material to exhibit toughness at lower temperatures [17].
Table 1 presents the mechanical and DMA curve data for the three grouting materials.It can be observed that G3-1.0 and G5-1.4 exhibit comparable mechanical properties and glass transition temperature.G5-1.4 has a larger TR > 0.3 than G3-1.0,indicating better damping performance over a wider temperature range, which is correlated with its moderate intermolecular cross-linking density.Although G7-1.8 has a higher Rx, the cross-linking density in its solidified material does not show a significant increase, resulting in a lower Tg (65.0°C).Additionally, an excess of isocyanate forms a large

Figure 1 .
Figure 1.Initial viscosity test tata of grouting materials.It can be observed that the initial viscosity of all tested grouting materials was less than 1000 mPa• s.Among them, G3 had the highest initial viscosity, exceeding 400 mPa• s, and even reaching 569 mPa• s at Rx=1.6.This is mainly due to the smaller molecular weight of the polyether polyol, which exhibits a stronger intermolecular hydrogen bonding, resulting in a higher apparent viscosity (actual measured viscosity values are given in Section 2.1).The initial viscosities of G5 and G7 showed relatively small variations with Rx, remaining at approximately 200-300 mPa• s.

Figure 2 .
Figure 2. Mechanical performance test data of grouting material solidified bodies.The compressive strength of water-reacted polyurethane grouting materials is mostly in the range of several megapascals because their curing process is heavily influenced by water in the environment.In contrast to water-reactive polyurethane grouting materials, non-water reacted polyurethane grouting materials can achieve self-curing, where the A and B components react with each other to form a solidified mass without the involvement of water.Figure2ashows the compression performance of the grouted specimens under different polyether polyol and isocyanate index conditions.It can be observed that the grouted specimens formed with low molecular weight polyols exhibit a higher cross-linking density, resulting in higher compressive strength: G3 > G5 > G7.Specifically, the compressive strength of the G3 specimens is consistently above 60 MPa, with G3-1.2 reaching an impressive strength of 80 MPa.The compressive strength of the G5 specimens is noticeably lower than that of G3 but still remains above 20 MPa, with G5-1.2 achieving a strength of 58.8 MPa.Due to the longer molecular chains, the overall compressive strength of the G7 specimens is lower, below 10 MPa.With an increase in Rx, the compressive strength of all the grouting materials initially increases and then decreases.This may be

Figure 3 .
Figure 3.Typical tensile curves of grouting material solidified bodies.The results of the tensile performance tests for non-water reactive polyurethane grouting materials under different polyol and Rx are shown in Figure2b-c.It can be observed that as Rx increases, the tensile strength of the specimens gradually increases while the elongation at break decreases significantly.This is mainly attributed to the increase in the hard segment content in the grouting material components.Typical tensile curves of the specimens are shown in Figure3.As Rx increases from 1.0 to 1.8, the tensile strength of the solidified mass formed by G3 remains consistently between 40-50 MPa, while the elongation at break decreases from 6.4% to 3.2%.For G5, when Rx=1.0, its tensile strength is only 8.2 MPa, but the elongation at break is as high as 91%.With an increase in Rx, its tensile strength continues to improve while the elongation at break decreases significantly.After Rx≥1.2, its tensile strength remains between 30-40 MPa, and the elongation at break decreases to 6-10%.In contrast to G3 and G5, G7 is significantly influenced by Rx.When Rx = 1.0, it does not form an effective cross-linked structure, resulting in a tensile strength of less than 1 MPa.As Rx increases, both the tensile strength and elongation at break improve.At Rx = 1.4, the tensile strength reaches 6.5 MPa, and the elongation at break is as high as 102%, indicating good toughness.At Rx=1.8, the tensile strength reaches 20 MPa, and the elongation at break is 43%, showing excellent overall performance.The results of the tensile and compressive performance tests indicate that G3, with shorter molecular chains, exhibits better rigidity.G5, with moderate molecular chain length, demonstrates a certain level of strength and appropriate toughness.G7, due to its longer molecular chains, has lower strength but better toughness.These characteristics make non-water reacted polyurethane grouting materials suitable for various application scenarios.

Figure 4 .
Figure 4. Adhesion performance test results of grouting materials.( a: Adhesive strength test data.bc: Cross-sectional images of the bonded specimens under different gelation times.)