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Article

Experimental Study on the Mechanical Properties of Nano-Silicon-Modified Polyurethane Crack Repair Materials

by
Bingsen Fan
,
Xiaolong Li
,
Shengjie Xu
,
Yanhui Zhong
,
Bei Zhang
and
Xiaofeng Liu
*
School of Water Conservancy and Transportation, Zhengzhou University, Zhengzhou 450001, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(8), 1735; https://doi.org/10.3390/pr12081735
Submission received: 3 June 2024 / Revised: 8 August 2024 / Accepted: 13 August 2024 / Published: 18 August 2024
(This article belongs to the Section Materials Processes)
Browse Figures
Versions Notes

Abstract

:
This study aims to solve the problem of dynamic crack repair in concrete. Although conventional polyurethane has good strength, its tensile and shear properties are poor. It was found that nano-silicon had an overall enhancing effect on the mechanical properties of polyurethane; therefore, five sets of tests with different dosages (0%, 2%, 5%, 7.5%, and 10%) were designed. The compressive, tensile, and shear mechanical properties of nano-silicon-modified polyurethanes were tested by compression, tensile, and straight shear tests, and the microscopic appearance of the materials was observed by scanning electron microscopy. The results showed that nano-silicon could enhance the mechanical properties of polyurethane. The best filling effect on polyurethane was achieved at a dosage of 5%, which increased the compressive, tensile, and shear strengths by 29.4%, 257.6%, and 202.1%, respectively, compared with the substrate. The compressive and tensile moduli in the small strain range were enhanced by 268.5% and 511.8%, respectively. After exceeding 5%, the mechanical properties of the materials decreased due to the enhanced nanoparticle agglomeration effect, which led to the appearance of voids inside the materials. The comprehensive analysis shows that nano-silicon can better enhance the mechanical properties of polyurethane with an optimal dosage of 5%, which is stronger relative to other repair materials and does not require time maintenance.

1. Introduction

China is a large agricultural country with a significant amount of water conservancy infrastructure, and diversion channels are mainly constructed with concrete. According to statistics, the annual water loss due to cracks in the channels is 173.46 billion m3, accounting for 45.9% of the water used in agriculture [1,2,3,4]. Cracks not repaired in time at the initial stage gradually become wider and penetrate under the influence of hydraulic scour and freeze–thaw cycles, which aggravates water loss [5,6]. At the same time, cracks also weaken the integrity and functionality of the concrete structure, seriously affecting the functionality and safety of the channel. Therefore, it is very important to repair and reinforce the cracks in the concrete slabs of the channels [7]. Due to its high brittleness and poor deformation resistance, concrete, coupled with drying shrinkage, uneven settlement, and other factors, can easily develop cracks in the channel slab. Additionally, the cracked structure becomes unstable with seasonal environmental changes and undergoes relative displacement. This requires repair materials with good toughness, tensile, and shear resistance. Hence, the research and development of new materials and technologies for crack repair has been of concern to many scholars.
Currently, crack repair is broadly categorized into two types: self-repair and artificial repair [8]. Self-repair is currently a hot topic of research, mainly focusing on the encapsulation of microorganisms or polymers in capsules [9,10,11]. This method takes a long time, requires a high level of encapsulation technology, and adds an additional cost of 7–28% [12]. There is also uncertainty regarding the location of the cracks, which may avoid all capsules, further increasing the cost of repair. Even if the capsule is at the location of the rupture, channel water flow can carry away microorganisms or healing agents, leading to repair failure. The main artificial repair materials are polymer cement mortar, epoxy resin, and polyurethane [13]. Polymer cement mortar is a commonly used repair material [14], typically applied by surface spray coverage, but this is not suitable for dynamic cracks. Polymer cement mortar can also be used to repair cracks by grouting, but the large particle size of the cement mortar makes it unsuitable for microcracks with openings less than 0.5 mm [15]. Epoxy resins have high strength, adhesive properties, and abrasion resistance due to the presence of special reactive and polar groups, but they also exhibit high brittleness and poor impact, deformation, and crack resistance [16]. Therefore, it is clear that cementitious and epoxy-based brittle materials are not suitable for repairing dynamic cracks. Polyurethane is widely used in construction, transportation, water conservancy, and other fields due to its unique soft and hard structure, making it highly processable [17]. When the content of the soft segment in polyurethane is high, the elasticity of the material is high, and the hard segment content increases the strength of the material accordingly [18]. In response to different needs, different materials can be prepared by adjusting the content of soft and hard segments, with flexible processability. Currently, widely used polyurethane has better compressive properties and toughness, but weak tensile and shear properties. With the increasing complexity of the environment in which projects are located, the performance requirements for repair materials are also increasing. In the face of a complex engineering environment, how to improve tensile and shear properties while also maintaining high compressive strength and toughness has become a problem that must be solved. For this reason, some scholars try to improve polyurethane by adjusting the chemical composition. Bourbigot et al. [19] prepared polyurethanes with different ratios of hard and soft segments and found that the material has higher compressive strength when the content of hard segments is high, but its tensile strength is greatly reduced, while the opposite result is found when the content of soft segments is high. To improve compressive strength while also enhancing or retaining tensile and shear properties, some scholars have tried to mix and match the two kinds of polyols, polyether and polyester, to prepare polyurethane [20]. However, the reactivity of polyether and polyester is different, making the ratio difficult to control stably, and thus the expected effect cannot be achieved. Polyurethanes are also prepared by directly synthesizing polyether and polyester block polyols, but this method is still in its infancy and difficult to apply practically due to the extremely complicated preparation process and high cost [20]. Therefore, improving polyurethane by adjusting the chemical composition is not feasible for the time being. Instead, using nanofillers to improve the properties of the material is a simple, feasible, and effective method [21].
Numerous studies have shown that doping nanomaterials into polyurethanes can confer excellent properties to the materials and is a widely used method for improvement [22,23,24,25]. Shi et al. [26] doped titanium carbide/nano-copper into polyurethane to enhance the high-temperature stability of the material. Song et al. [27] doped nano-SiO2 into polyurethane and significantly improved the wear resistance of the material. Xu et al. [28] doped carbon nanotubes into polyurethane and obtained ultralight composites with conductivity and very low density. Choi et al. [29] incorporated graphene into polyurethane, which enhanced the shear strength of the material, but the tensile strength was greatly reduced. Liu and Charpentier et al. [30,31] found that polyurethane/nano-TiO2 composites have good surface self-cleaning and photocatalytic properties. Gao [32] added zinc borate nanoparticles to polyurethane and improved the flame retardancy of the material. He et al. [33] added nano montmorillonite nanoparticles to polyurethane, greatly improving the material’s thermal stability. Ye et al. [34]. prepared Ni/PU composites, which substantially improved the abrasion resistance of polyurethane. Hsu et al. [35] found that incorporating nanogold into polyurethane resulted in materials with excellent cellular response and bacterial inhibition. Li et al. [36] compared the effects of three types of nanomaterials, Cr2O3, ITO, and Mg(OH)2, on the infrared properties of polyurethanes, and found that coatings containing Mg(OH)2 had the lowest infrared reflectance, the highest absorbance, and the highest number of scrubs. Akbarian et al. [37] incorporated nanosilver into polyurethanes and found that nanosilver conferred the material with good antimicrobial properties, and its thermal stability was also significantly improved.
In summary, metal nanoparticles such as Au, Ag, and Ni, nano-oxides such as TiO2, Cr2O3, ITO, as well as montmorillonite, graphene, and carbon nanotubes, have been widely used to modify polyurethane materials and have significantly improved the material properties. For example, Au and Ag enable the materials to have excellent antimicrobial properties; Cu, TiC, Zn(BO2)2, and montmorillonite enhance the thermal stability of the materials; and SiO2, Ni, and Mg(OH)2 enhance the wear resistance of the materials. Although graphene enhanced the shear resistance of the material, the tensile strength was greatly reduced. Currently, the modification of polyurethane is mainly focused on biological, chemical, and mechanical fields. With the development of society, the requirements for material performance in civil engineering are becoming higher and higher. How to effectively and comprehensively improve the performance of materials to adapt to the complex environment of the project is also key in engineering repair. This study addresses the dynamic crack repair needs of channel slabs based on engineering practice. During their service life, crack-filling materials are exposed to various environmental factors and must endure forces such as compression, tension, and shear. Therefore, these materials must exhibit both high toughness as well as strong compressive, tensile, and shear strengths. At present, the tensile and shear properties of polyurethane in engineering applications are weak, and research on nano-silicon-modified polyurethane has not been reported. Therefore, in this study, nano-silicon will be used to modify polyurethane, and the effect of nano-silicon on the mechanical properties of polyurethane at different dosages will be investigated to satisfy the high requirements for comprehensive mechanical properties of materials in engineering applications.

2. Materials and Methods

2.1. Materials

The polyurethane grouting material used in the test consists of components A and B. Component A is transparent and component B is dark brown in color. The composition is shown in Table 1, and the materials were obtained from Wanhua Chemical Group Co. (Yantai, China). The nano materials were selected as nano-silicon.
In component A, compared with polyester polyol, polyether polyol has no easily hydrolyzable groups, while the ether bond is more flexible; therefore, it has better water resistance, flexibility, and temperature resistance [38]. The catalysts triethylamine and diethylenetriamine reduce the activation energy of the reaction, regulate the reaction rate, promote the self-polymerization reaction of isocyanate, and enhance the mechanical properties of the material [39]. Polyoxyethylene-based nonionic surfactants improve components’ compatibility and stability, as well as the lubricity of the contact surface between the material and the restoration. This enhancement facilitates the penetration of the material into cracks [40]. Dibutyl phthalate helps to increase the fluidity and decrease the viscosity of the material, allowing the slurry to diffuse to a wider and deeper extent [41].
Component B is mainly isocyanate, which reacts with polyether polyol to form urethane. At the end of the reaction, the urethane is further cross-linked with the isocyanate to form higher-strength urethane and diurethane groups. In the reaction with isocyanates, the urethane together with the polyether polyol provides more “soft satin”, which gives the material good toughness [41]. The flame retardant gives the material excellent flame retardant properties and, together with the diluent, reduces the viscosity of the system and enhances the fluidity of the material. This also helps to reduce bubbles generated by the heat of the reaction and improves the compactness of the solid.

2.2. Methods

At present, there is no test specification for polyurethane materials to refer to. In view of the rapid molding characteristics of the selected materials, this study will refer to GB/T 20219-2015 [42] and GB/T 2567-2021 [43] for testing. In order to investigate the feasibility and optimum dosage of nano-silicon for modification of polyurethane, five sets of control tests with different dosages were designed. An equal volume of 100 mL of each AB material was taken separately, and the total mass was 220 g. MacLachlan et al. [44] showed that the dosage should not exceed 10% when nanomaterials are used to prepare composites. Therefore, nano-silicon was weighed out at 0%, 2%, 5%, 7.5%, and 10% of the total mass of AB material. Material B was poured into the mixing bucket, and then nano-silicon was mixed into material B and stirred with a glass rod for not less than 30 s. Subsequently, material A was poured into the mixing bucket and stirred for 15 s to ensure that material A and material B were fully mixed. The slurry was then poured into the mold, and the ambient temperature during the specimen production process was kept at 25 °C. Before demolding, the specimens were fully rested in the same environment for at least 1 h. Three specimens were taken as a group for mechanical property testing. We took the middle value of each group of data for stress–strain full process analysis, and we took the average value of each group of data for quantitative change analysis. The modeling and testing are shown in Figure 1.
The compression property test was conducted using 50 mm × 50 mm × 50 mm cube specimens. The axial unconfined compression test was carried out using a WHY-1000 universal testing machine at a loading rate of 5.0 mm/min. The test ended when the strain of the specimen reached 80% or the stress was reduced to 80% of the maximum value.
Both tensile and shear properties were tested using a WDW-50 universal testing machine. The tensile test was performed using dumbbell-type specimens, with axial tension applied at a loading rate of 2.0 mm/min. The test was terminated when the specimen broke. Shear tests were conducted using 50 mm concrete cube specimens. Artificially penetration slits of 0.3 mm and 0.5 mm were created in concrete channel slabs to simulate cracks, which were then filled with polyurethane infused with varying dosages of nano-silicon. A JCT989 shear fixture was used to apply longitudinal shear force to the specimens at a loading rate of 0.5 mm/min. The test was terminated when the stress drop approached 0 or the specimen was completely peeled off.
For scanning electron microscopy tests, the cured material with different dosages was cut into 1 cm × 1 cm × 0.5 cm blocks using a razor blade. The samples were coated with gold using an SBC-12 sputtering apparatus before observation. The surface of the samples was then observed under vacuum conditions using a KYKY-EM6200 electron microscope, and representative images were selected and saved for analysis.

3. Test Results

3.1. SEM Images

Figure 2 shows SEM images of the surface of the materials with different nano-silicon contents. Polyurethane is formed from alternating periodic soft and hard chain segments. The darker regions in Figure 2a show the soft chain segments of polyols forming the continuous phase, and the brighter regions show the hard chain segments of urethanes forming the dispersed phase [45]. The hard chain segments have a larger spatial volume and are attracted to each other through hydrogen bonding to form microregions. The soft and hard chain segments are entangled with each other and connected by hydrogen bonds to form block copolymers [46]. From the morphology images of the specimens with different dosages in Figure 2a–c, it can be seen that the bright-colored regions are becoming fewer, and nano-silicon has a filling effect on the structure of the material. With a smaller dosage (Figure 2b), the structure still has bright-colored regions, indicating that a small amount of nano-silicon can only fill part of the space of the material, providing limited improvement to the structure. At a dosage of 5%, almost no brightly colored regions appear, as seen in Figure 2c. This indicates that the filling effect of nano-silicon is optimal at this point, making microregions no longer appear. When the dosage was increased to 7.5%, brightly colored regions reappeared (Figure 2d), and the distribution of these regions was also uneven compared to Figure 2a. This indicates that excessive addition leads to uneven filling due to weakened dispersion and increased agglomeration of nanoparticles. At a dosage of 10%, the agglomeration effect of nano-silicon is enhanced, the dispersion is further weakened, and the nanoparticles exist in clusters and are unevenly distributed. This leads to the emergence of stress concentration, and bubbles cannot be completely discharged, resulting in voids inside the material, as shown in Figure 2e. This also shows that excessive addition worsens the filling effect and reduces the denseness of the material. Therefore, a moderate amount of nano-silicon can better improve the material matrix and make the internal structure denser. When the dosage is too high, the agglomeration effect of nano-silicon is enhanced and the dispersion becomes worse, which leads to the appearance of cavity regions inside the matrix and reduces the material’s density.

3.2. Compression Properties

Figure 3 shows the compressive stress–strain curves of the modified polyurethane at different dosages. The results show that the stress–strain curves of the modified polyurethane prepared in this study are nonlinear. The compression process can be roughly divided into three stages: the elastic stage (Stage I), the elastic-plastic stage (Stage II), and the destructive stage (Stage III). The initial slope of the stress–strain curve of the material increases gradually with the increase in nano-silicon dosage and starts to decrease when it exceeds 5%, as shown in Figure 3b. In the first stage, the stress–strain curve of the material is approximately linear. The material undergoes elastic deformation when it is subjected to an instantaneous force (strain less than 0.04), and the internal structure is compressed to become dense. The strain in this process is very small. The elastic-plastic stage (Stage II) begins after passing point a. The slope of the stress–strain curve becomes smaller and subsequently becomes more linear. The slope of the stress–strain curve then gradually increases. During the elastic-plastic phase, the overall trend of the ratio of the stress increment to the strain increment becomes gradually larger, and the stress–strain curve transforms from roughly linear to nonlinear. In this stage, the material is subjected to vertical loading force, and the volume begins to spread around, making the vertical stress disperse in all directions. At this time, the material absorbs a large amount of energy through deformation. When the stress reaches point b, it reaches the limit compressive strength, and the material enters the destruction stage (Stage III). At this time, the material suddenly emits a crisp fracture sound, and instantaneous vertical cracks appear on the material’s surface, rapidly expanding in a rugby ball shape to both sides. This results in a rapid decline in the wall’s cracking stress, leading to the specimen’s destruction. With continuous loading, the material will continue to be compressed, and the stress will increase again after the drop. At this point, the material is pressed into a shape resembling a cake and no longer holds any significance, so the test is ended when the stress reaches 80% of the peak value. It is worth noting that the composites with a nano-silicon dosage of 5% exhibited higher strength. This is due to the fact that the right amount of nano-silicon is better dispersed homogeneously and rubs against the polyurethane material [47]. This not only increases the energy dissipation and damping ratio but also stores more deformation energy than the matrix material [48]. Therefore, after unloading at the damage stage, the material still has some deformation recovery ability, as shown in Figure 4. All other dosages are completely destroyed at this point and cannot recover from a certain amount of deformation as the 5% dosage can.
As shown in Figure 5, the ultimate compressive stress–strain of the materials with different dosages is shown. The ultimate stress increases gradually with the addition of nano-silicon. The ultimate stress is slightly enhanced when a small amount of nano-silicon (2%) is added compared to the substrate. The ultimate stress reaches 92.5 MPa when the dosage is 5%, which is 29.4% higher than the 71.5 MPa of the substrate. However, when the dosage was increased to 7.5% and 10%, the ultimate stress started to decrease to as low as 32.1 MPa, which is a 55.1% decrease compared to the substrate. This phenomenon is due to the fact that the addition of nanoparticles in the right amount enhances the cross-linking density within the matrix, thus improving the mechanical properties of the material. When an excessive amount of nanoparticles is added, increased agglomeration occurs, creating voids within the material and significantly diminishing its mechanical properties. It can also be seen from Figure 5 that the ultimate strain value of the material shows an overall decreasing trend with the addition of nano-silicon. The decrease in ultimate strain can be attributed to the nanoparticles’ increase in the hardness of the material [49], and the increase in hardness results in a loss of toughness. Although the polyurethane substrate showed extreme deformability with ultimate strains as high as 71%, the ultimate strain of the material at 5% dosage is still as high as 67% with a relative loss of only 5.6%. Since the overall compressive stress–strain curve of the material shows nonlinear behavior, the data with strain less than 0.03 within the first stage were selected to analyze the change in compressive modulus of the material at small strains. As can be seen from Figure 6, the compressive modulus of the material increases gradually with the addition of nano-silicon, peaks at 91.4 MPa at 5%, which is 268.5% higher than the substrate. It begins to decrease after exceeding 5%, with the overall trend of change being the same as that of stress. Although the ultimate compressive strength of the material at 7.5% and 10% is less than that of the substrate, the compressive modulus of the material is greater than that of the substrate in the range of small strains. This suggests that nano-silicon can enhance the stiffness of the material, even in excess, resulting in better stiffness in the small strain range.
In water facilities, concrete channel slabs are in the field and subject to a variety of factors that can cause cracks. Subsequently, the structure on both sides of the crack is in an active state, influenced by external factors such as daily temperature differences and hydraulic scour. Modified polyurethane has a strain limit close to 70% and a high compressive strength. It shows good recovery ability at less than the ultimate strain. After the crack-filling material is squeezed and deformed and the crack reopens, the modified polyurethane is able to recover to refill the crack to some extent due to its good deformation recovery ability. This is very favorable for the repair of dynamic cracks under expansion, contraction, and hydraulic influence.

3.3. Tensile Properties

Figure 7 shows the tensile stress–strain curves of materials with different nano-silicon dosages. The stress–strain curves of the modified polyurethanes can be divided into three stages: dense deformation (Stage I), elastic deformation (Stage II), and plastic deformation (Stage III). As nano-silicon is added, the initial slope of the stress–strain curve increases. However, it decreases when the dosage exceeds 5%. The material undergoes elastic deformation under low stress when the strain is less than 0.01. In this phase, the tensile stress–strain curve (section 0–a) is linear. This minimal deformation occurs due to the transient forces applied, causing changes in bond lengths and angles within the molecular chains of the material [46]. After point a, the specimen enters a plateau period (segment a–b). The plateau length decreases with the addition of nano-silicon but increases again when the dosage exceeds 5%. This change occurs because the hardness of the material increases with more nano-silicon, which fills the matrix space, turning many soft segments into “hard segments”. During this plateau period, the stress increment/strain increment ratio remains constant. The tensile force makes the internal structure denser. Beyond point b, the stress–strain curve becomes roughly linear elastic. Here, a small strain increment causes a larger stress increment. As the nano-silicon dosage increases, the slope in the linear elastic phase also increases. Beyond point c, the strain continues to increase. The ratio of stress increment to strain increment decreases, and the specimen enters the plastic deformation stage (Stage III). At this stage, the molecular chain deforms due to the movement of the chain segments [39]. Macroscopically, this is observed as the specimen elongating under increased tensile force. In the plastic deformation stage (Stage c–d), the slope of the stress–strain curve gradually decreases, making the curve more linear. This indicates a transition from elastic to plastic behavior. Near point d, the strain reaches the ultimate bearing capacity of the molecular chain, leading to a sudden fracture of the specimen as it reaches the ultimate stress. As shown in Figure 8, at 2% and 5% dosages, the tensile fracture surface of the material is smooth. The appropriate amount of nano-silicon enhances the material’s hardness and tensile properties. However, as the dosage increases, the fracture surface becomes rough and uneven. Figure 8d,e show that vesicles inside the material gradually increase, leading to a decrease in tensile properties.
Figure 9 presents the ultimate tensile stress–strain comparison of nano-silicon polyurethane materials with different doping levels. With the addition of nano-silicon, the ultimate tensile stress of the material increases. At 5% dosage, the ultimate tensile stress reaches 11.8 MPa, which is 257.6% higher than the 3.3 MPa of the substrate. However, the increase in tensile strength leads to a loss of toughness in the material, resulting in a significant decrease in ultimate strain. From the SEM and compressive tests, it can be seen that 5% dosage of nano-silicon has the best filling effect on the material, and the increase in compactness and hardness reduces the deformation properties of the material. Thus, the ultimate strain of the material at 5% dosage is 8.9%, which is 32.6% lower than the 13.2% of the substrate. Excessive dosage (7.5%, 10%) results in a significant decrease in the ultimate tensile strength due to the lack of compactness in the internal structure of the material. Although the strength is increased by 28.4% and 26.5% over the substrate, the ultimate strain is reduced by 83.3% and 79.5%, respectively. Since the stress-strain curves of the materials exhibit varying nonlinear behavior with different nano-silicon dosages, but consistently display a brief linear elastic phase in the initial stage, data with strain values less than 0.001 from this phase were selected to analyze the changes in Young’s modulus at small strains. Figure 10 shows that the Young’s modulus of the material increases gradually with the addition of nano-silicon. It reaches a peak value of 774.6 MPa at 5%, which is 511.8% higher than the substrate. It begins to decrease after exceeding 5%, with the overall trend of change being the same as that of the stress. Similar to the compressive test, although the tensile strength of 7.5% and 10% materials is close to that of the original substrate, the Young’s modulus of the materials is greater than that of the substrate in both small strain ranges. This indicates that nano-silicon enhances the stiffness of the material. Even in excess, it can make the material exhibit better stiffness in the low-strain range and possess strong resistance to deformation.
Tensile test results show that at a dosage of 5%, the structure of the material becomes exceptionally strong and is able to withstand extremely high tensile forces. However, the filling of space by nanoparticles and the increase in hardness cause the material to lose a certain degree of ductility and deformability, exhibiting a certain degree of brittleness. This also explains the phenomenon observed in the compression testing of the material: when the ultimate stress is reached, cracks suddenly appear on the surface of the material and expand rapidly, leading to the rupture of the specimen and a rapid decrease in stress.

3.4. Shear Properties

Figure 11 shows the shear stress–strain curves for materials with different dosages. The whole shear stress–strain curve can be divided into four stages: the stress growth stage (I), the yielding stage (II), the damage development stage (III), and the complete damage stage (IV). With the increase in nano-silicon dosage, the initial slope of the stress–strain curve increases first and then decreases slowly. In the first stage (0–a), the stress–strain curve increases rapidly with increasing stress, and the slope gradually becomes larger and more linear. As the stress increases, the specimen enters the second stage (stress yielding, a–b) after point a. Within this stage, the slope of the stress–strain curve decreases very slowly and is almost stable until the shear force reaches the maximum value τ at point b. When the shear force reaches τ, it has basically canceled out the chaining force between the material particles. The first and second stages account for about 20% of the entire stress–strain curve, and τ is the ultimate shear strength of the interface. When the strain passes point b, the stress decreases into the destruction stage (b–c), and the absolute value of the slope of the stress–strain curve becomes smaller slowly. As the strain increases, the specimen slip distance becomes larger to the naked eye. After the strain passes point c, it rapidly enters the complete destruction stage (c–f), at which time the specimen slides completely and is destroyed, and the concrete structures on both sides are completely separated.
As shown in Figure 12a, the shear strength of the material gradually increases with the increase in nano-silicon dosage. When the dosage is low, the filling degree of the raw material is low, which leads to the uneven performance of the material injected into the cracks and limited enhancement of the shear strength. The shear strength is maximized when the dosage is 5%. Above 5%, due to the agglomeration effect of nanoparticles, the material contains air bubbles. This results in a gradual decrease in shear strength. From Figure 12b, it can be seen that the trend of the ultimate shear strain of the material is basically the same as that of the shear strength. At a small amount (2%) of addition, it leads to a small decrease in the ultimate shear strain. At a dosage of 5%, the ultimate shear strain reaches a maximum. Above 5%, the ultimate shear strain begins to decrease substantially.
It can be seen from Figure 12 that as the crack opening increases from 0.3 mm to 0.5 mm, the general trend of the shear strength and ultimate strain of the specimens showed an increase. This may be due to the fact that as the crack width increases, it makes the permeability in the crack gradually increase. At the same time, the amount of polyurethane infusion becomes more, and the material is able to fully contact the concrete wall, which makes the bond stronger. It was also noted that at a dosage of 10%, the shear strength of the 0.3 mm crack specimens was slightly higher than that of the raw material, and the shear strength of the 0.5 mm crack specimens was almost the same as that of the 0% material. This indicates that although the agglomeration effect occurs when nanomaterials are overdosed, the incorporation of nano-silicon still gives the material a certain bonding force.
When the crack width increases from 0.3 mm to 0.5 mm, the shear resistance of the material gradually increases at dosages of 2% and 5%, with the ultimate destructive strain rate increasing accordingly. Taking 5% as an example, the ultimate strains of the specimens are all greater than 0.015. In the test with a 0.5 mm crack, the shear strength of the material is as high as 1.266 MPa, which is 202.1% higher compared with the 0.419 MPa of the substrate. The ultimate strain rate is 2.1%, which is 10.5% higher than the 1.9% rate of the substrate. The shear strength and ultimate strain rate of the material at a crack opening of 0.3 mm were 0.874 MPa and 1.6%, respectively. Compared with the 0.308 MPa and 1.3% of the substrate, they increased by 183.8% and 23.1%, respectively, and the enhancement effect was remarkable.
The results of the shear test show that nano-silicon can enhance the bond between the interface of polyurethane and concrete and improve the structural shear strength of concrete cracks. The best enhancement effect was observed at a dosage of 5%. The shear strength and ultimate strain of the material decreased to different degrees when the dosage exceeded 5%. This is because an excessive dosage of nano-silicon leads to a smaller structural compactness of the material and reduces the bond area at the interface between the material and concrete. This results in lower bond strength and lower shear resistance.
In Figure 13, the surface images of the left and right sides of the cracked specimen after the damage of the shear specimen are shown. The type of damage at the interface is mainly categorized into two types: A. Damage occurs along the bonding surface of polyurethane and concrete, as shown in Figure 13a. B. Damage occurs along the internal interface of the polyurethane material, as shown in Figure 13b. Type A damage mainly occurs in the substrate, because there is no nano-silicon incorporated, the bonding force at the interface between the material and the concrete is lower than the shear resistance of the material, and the damage interface occurs along the contact surface between the material and the concrete, as in Figure 13a. Type B damage mainly occurs in the modified polyurethane material, and the incorporation of nano-silicon increases the adhesion of the modified polyurethane. In this case, the bond force at the interface between the material and the concrete is greater than the shear resistance of the material, and the damage interface occurs along the internal part of the material, as shown in Figure 13b.
In addition, the nano-silicon-modified polyurethane in this study has more excellent mechanical properties than other crack repair materials such as cement-based polymers and epoxy resins. The optimal dosage of nano-silicon for the modified polyurethane is 5%, which is when the filling effect of nano-silicon on the polyurethane reaches the optimal degree, and the comprehensive mechanical properties of the material are enhanced. As shown in Table 2, the nano-silicon-modified polyurethane material in this study has more excellent mechanical properties compared with other crack repair materials. Additionally, it requires no periodic maintenance, resulting in time and cost savings, making it a more practical solution for engineering applications.

4. Conclusions

To address the dynamic cracking of concrete, this study used nano-silicon to modify polyurethane by a physical blending method. Using microscopic samples and mechanical property tests, it revealed the mechanism of nano-silicon’s enhancement of the mechanical properties of polyurethane. The effects of different dosages of nano-silicon on compressive, tensile, and shear strengths were also systematically analyzed. The main research results are summarized as follows:
In the micro-sample test, different dosages of nano-silicon had different filling effects on polyurethane. When added in small amounts, nano-silicon was able to fill some space in the polyurethane with a good filling effect. At a 5% dosage, the filling effect of nano-silicon on the material was optimal. However, when the dosage was further increased, the agglomeration of nanoparticles was enhanced and the dispersion became poor. This led to the appearance of voids within the structure, which caused a decrease in mechanical properties.
In the compression test, the addition of nano-silicon significantly enhanced the compressive strength and modulus of polyurethane, particularly at lower strain levels. At dosages of 2% and 5%, the compressive strength of the modified polyurethane gradually increased. At 5%, the strength of the modified polyurethane increased by 29.4% compared to the substrate, while the strain loss was only 5.6%. When the dosage was increased to 7.5% and 10%, the agglomeration effect of nanoparticles gradually increased and the dispersion deteriorated. Voids were created within the material, leading to a significant reduction in compressive strength. However, the compressive modulus of the modified polyurethane was still higher than that of the substrate in the small strain range.
In the tensile test, the addition of nano-silicon enhanced the tensile strength and Young’s modulus of polyurethane. The tensile strength of the modified material was gradually enhanced at dosages of 2% and 5%. The tensile strain decreased with increasing dosage due to the ability of nanoparticles to enhance the hardness of the material. At a 5% dosage, the tensile strength increased by 257.6%, while the strain loss was only 32.6%. The increase in dosage to 7.5% and 10% resulted in a drastic decrease in tensile strength due to the excessive incorporation of nano-silicon, which resulted in a much lower tensile strain, although it was higher than that of the substrate. Despite the decrease in tensile strength, the Young’s modulus of the material remained higher than that of the substrate in the low-strain range.
In the shear test, the addition of nano-silicon significantly increased the bonding of the polyurethane–concrete interface, thus enhancing the shear strength of the cracked concrete specimens. At dosages of 2% and 5%, the bonding of polyurethane gradually increased, resulting in a gradual increase in the shear strength of the cracked specimens. At 5%, the strength was increased by 202.1% and the strain by 10.5%. The bonding performance of the modified material was greatly improved, which was very favorable for dynamic crack repair. When the dosage was increased to 7.5% and 10%, the shear strength and strain of the modified material decreased.
In conclusion, nano-silicon incorporated into polyurethane can significantly enhance compressive, tensile, and shear properties with an optimal dosage of 5%. However, the agglomeration effect of nanoparticles needs to be emphasized in large-scale applications. The small scale of this study allowed this issue to be temporarily overlooked; however, it becomes unavoidable when large dosages are applied. Nevertheless, this study provides a reference for the modification of polyurethane in engineering applications. Finally, the combination of materials science and artificial intelligence is becoming the key to promote the development of innovation. The test data on the comprehensive mechanical properties of the materials in this study provide data support for the application of machine learning in materials R&D, which can help to optimize the structure design and further identify the optimal dosage of nanocomposites.

Author Contributions

B.F.: Conceptualization, Methodology, Validation, Investigation, Data Curation, Writing—Original Draft. X.L. (Xiaolong Li): Conceptualization, Funding Acquisition, Project Administration, Supervision, Writing—Review and Editing. S.X.: Conceptualization, Methodology, Writing—Review and Editing. Y.Z.: Conceptualization, Writing—Review and Editing. B.Z.: Writing—Review and Editing. X.L. (Xiaofeng Liu): Data Curation, Methodology, Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 52178401), the Science and Technology Innovation Team Support Program for Henan Universities (Grant No. 23IRTSTHN014), the Central Plains Talent Program in China (Grant No. 234200510014), and the Water Conservancy Science and Technology Research Projects in Henan Province (Grant No. 72).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Specimen fabrication and test methods.
Figure 1. Specimen fabrication and test methods.
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Figure 2. SEM images of materials with different dosages. (a) w(Si) = 0%; (b) w(Si) = 2%; (c) w(Si) = 5%; (d) w(Si) = 7.5%; (e) w(Si) = 10%.
Figure 2. SEM images of materials with different dosages. (a) w(Si) = 0%; (b) w(Si) = 2%; (c) w(Si) = 5%; (d) w(Si) = 7.5%; (e) w(Si) = 10%.
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Figure 3. Compressive stress–strain curve for materials with different dosages. (a) Measurement curve; (b) Stage I; (c) schematic curve.
Figure 3. Compressive stress–strain curve for materials with different dosages. (a) Measurement curve; (b) Stage I; (c) schematic curve.
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Figure 4. Deformation recovery after unloading of 5% dosage of material.
Figure 4. Deformation recovery after unloading of 5% dosage of material.
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Figure 5. Ultimate compressive stress–strain of materials with different dosages.
Figure 5. Ultimate compressive stress–strain of materials with different dosages.
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Figure 6. Compression modulus of materials at different dosages.
Figure 6. Compression modulus of materials at different dosages.
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Figure 7. Tensile stress–strain curve. (a) Measurement curve; (b) Stage I; (c) schematic curve.
Figure 7. Tensile stress–strain curve. (a) Measurement curve; (b) Stage I; (c) schematic curve.
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Figure 8. Tensile fracture surfaces of materials with different dosages.
Figure 8. Tensile fracture surfaces of materials with different dosages.
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Figure 9. Ultimate tensile stress–strain of materials at different dosages.
Figure 9. Ultimate tensile stress–strain of materials at different dosages.
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Figure 10. Young’s modulus of materials at different dosages.
Figure 10. Young’s modulus of materials at different dosages.
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Figure 11. Shear stress–strain curves of (a) 0.3 mm and (b) 0.5 mm specimens; (c) schematic curve.
Figure 11. Shear stress–strain curves of (a) 0.3 mm and (b) 0.5 mm specimens; (c) schematic curve.
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Figure 12. Shear properties of materials at different dosage. (a) Shear Stress; (b) Ultimate strain rate.
Figure 12. Shear properties of materials at different dosage. (a) Shear Stress; (b) Ultimate strain rate.
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Figure 13. Specimens with type A and type B damage surfaces. (a) Type A damage; (b) Type B damage.
Figure 13. Specimens with type A and type B damage surfaces. (a) Type A damage; (b) Type B damage.
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Table 1. Polyurethane components.
Table 1. Polyurethane components.
Main IngredientsCatalystsSurfactantDiluentFlame Retardant
Component APolyether polyolTriethylamine, DiethylenetriaminePolyoxyethylene-based nonionic active agentDibutyl phthalate\
Component BIsocyanate\\Dichloromethane, Dimethyl carbonateTrimethyl phosphate
Table 2. Modified polyurethane properties compared to other crack repair materials.
Table 2. Modified polyurethane properties compared to other crack repair materials.
Properties/UnitsPolyurethaneECA [16]Carbon Nanotube Cement [50]CIS [51]AWG [52]Hi-FA [53]Nano-Polyurethane
Compressive (MPa)71.585.66 (28 d)65 (10 d)15 (28 d)17 (90 d)35 (28 d)92.5
Tensile (MPa)3.36.18 (3 d)\\\\11.8
Shear (MPa)0.42\\\\\1.27
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Fan, B.; Li, X.; Xu, S.; Zhong, Y.; Zhang, B.; Liu, X. Experimental Study on the Mechanical Properties of Nano-Silicon-Modified Polyurethane Crack Repair Materials. Processes 2024, 12, 1735. https://doi.org/10.3390/pr12081735

AMA Style

Fan B, Li X, Xu S, Zhong Y, Zhang B, Liu X. Experimental Study on the Mechanical Properties of Nano-Silicon-Modified Polyurethane Crack Repair Materials. Processes. 2024; 12(8):1735. https://doi.org/10.3390/pr12081735

Chicago/Turabian Style

Fan, Bingsen, Xiaolong Li, Shengjie Xu, Yanhui Zhong, Bei Zhang, and Xiaofeng Liu. 2024. "Experimental Study on the Mechanical Properties of Nano-Silicon-Modified Polyurethane Crack Repair Materials" Processes 12, no. 8: 1735. https://doi.org/10.3390/pr12081735

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