Water diffusion in polyurethane grouting materials and its effect on compressive strength

Abstract

The mechanical properties of polyurethane (PU) grouting materials are susceptible to the influence of water during underground grouting. To elucidate the diffusion mechanism of water within the material and the variation in compressive strength after water immersion. Scanning Electron Microscopy (SEM) was employed to analyze the water diffusion mechanism in polyurethane grouting materials from a microscopic perspective. Furthermore, a series of laboratory experiments were conducted to investigate the effects of factors such as density, surface condition, immersion duration, water pressure, saline environment, and ambient temperature and humidity on the water absorption. Additionally, the compressive strengths before and after immersion were compared and analyzed. The results indicate that the water absorption decreases as a quadratic polynomial function of density (R² = 0.98). Specifically, at densities of 0.30 g/cm³ and 0.42 g/cm³, the water absorption was merely 4.38% and 1.28%, respectively. Compared with the original surface (water absorption of 15.77%), sealing treatment reduces the rate by 44.32%, whereas surface peeling treatment increases it by 151.11%. Moreover, as the water pressure increases by 2 MPa, the water absorption rises gradually with a diminishing marginal increase. The most drastic variation in water absorption occurs within the initial 24 h; thereafter, for every additional 24 h of immersion, the increment remains below 0.5% and progressively decelerates. In saline environments, the water absorption of specimens with various densities increases by 22%, and the water absorption exhibit an upward trend with elevated environmental temperature and humidity. Notably, compressive strength both before and after absorption increases with density (R²>0.88). In the density range of 0.070–0.112 g/cm³, the material exhibits a relatively higher water absorption. Due to the supporting effect of the internal water, the compressive strength experiences a marginal increase of approximately 4.56%. These findings can provide theoretical support for the design and durability evaluation of underwater grouting reinforcement in underground engineering.

Keywords

polyurethane SEM water absorption compressive strength

Highlights

• The water absorption decreases as a quadratic polynomial function of increasing density, with a coefficient of determination of 0.98.

• The surface condition directly influence the water absorption, which is highest when the skin is removed.

• The water absorption increases with increasing water pressure and immersion time.

• The compressive strength both before and after water immersion increased linearly with increasing density, with a coefficient of determination greater than 0.88.

Introduction

PU grouting material is a low-density microcellular foam characterized by a closed-cell structure, exhibiting advantages such as early strength, rapid formation, high expansibility, and excellent impermeability. It has been widely applied in engineering practices, including the treatment of hidden road defects, tunnel leakage remediation, anti-seepage reinforcement of dams, leveling of high-speed railway ballastless tracks, and mitigation of contact leakage in pipelines.^1–5 However, the grouting zones are frequently situated in complex groundwater environments. The continuous hydrostatic pressure exerted by groundwater, coupled with its subsequent infiltration into the material, alters the mechanical properties of the material. This can lead to reinforcement failure or even trigger secondary engineering disasters. Therefore, systematically elucidating the water diffusion mechanism within PU grouting materials and the resulting evolution of their mechanical properties is an urgent requirement to ensure the long-term safety of underground grouting engineering.

Regarding the water absorption characteristics of PU grouting materials, Wang⁶ measured the volumetric water absorption of rigid PU foams with various foaming systems, processes, and applications using a simplified method and the traditional national standard method, proposing that the simplified method can serve as a rapid, in-situ measurement approach for scientific research. Furthermore, experimental studies⁷ revealed that the mass water absorption of PU materials decreases with an increase in density. Wang et al.⁸ investigated the effects of water content on the properties of PU foams and the underlying mechanisms in aqueous environments; additionally, they comprehensively summarized the broader impacts of water environments on the overall performance of PU foam materials. When applied as a repair material in deep-water engineering, PU is subjected to significant water pressure, which facilitates water penetration into the material interior along micropores.⁹ Microstructural studies have revealed that two-component PU grouting materials possess a continuous self-closed skin and high-strength interconnected closed-pore walls with a closure rate exceeding 95%¹⁰; these materials exhibit a permeability coefficient of approximately 10^-8 cm/s, indicating good impermeability. Sabbahi et al.^11–14 experimentally investigated the water absorption of PU foams using the gravimetric method and derived the time-dependent variation of water diffusion. Mei⁹ and Li et al.¹⁵ investigated the water absorption of reinforced PU foams, and their results indicated that water diffusion within the material is rapid in the initial stage, while the diffusion rate decreases over time. Reference 16 summarized relevant literature regarding the water absorption of PU^17–20 and plotted curves of water absorption versus immersion time. The study demonstrates that the water absorption gradually increases with immersion time; the growth rate is relatively high in the initial stage, while the amplitude of increase diminishes as the immersion duration extends. Chen²¹ employed the buoyancy and weight-loss methods to determine the water absorption and desiccation rate in dry environments of rigid PU foam with a density of 0.5 g/cm³. The results indicated that water absorption is closely related to surface morphology and cell structure, whereas the desorption rate and equilibrium time are governed by environmental temperature and humidity. Shi et al.^22,23 investigated the water absorption characteristics of PU grouting materials and the effect of temperature variations on material volume, and determined the initial water seepage pressure of specimens with different densities. The results revealed that both the water absorption and the volume change rate were inversely proportional to density, and the impermeability of the material improved with increasing density. Zhang²⁴ utilized NaCl tracing, electrochemical impedance spectroscopy (EIS), and other electrochemical techniques to elucidate the water diffusion processes on the surface and within the interior of rigid PU foams, further establishing a dynamic model for water permeation, cell collapse, and diffusion. Dai et al.²⁵ employed TG, SEM, and FTIR to examine the effect of KH-550 dosage on the water resistance of rigid PU foams, demonstrating that KH-550 significantly reduces the material’s water absorption. Lu²⁶ utilized a weighing method to measure the mass variations of PU specimens and PU composite mixtures before and after water absorption, finding an inverse relationship between density and water absorption for both materials, which fitted quadratic polynomial and linear models, respectively. However, the aforementioned studies predominantly remain at the macroscopic level of water absorption measurement, neglecting the potential microscopic damage to the closed-cell structure under long-term hydrostatic pressure and complex hydrochemical environments, as well as the dynamic diffusion process of water within the cell structure.

Research on the mechanical properties of PU grouting materials has encompassed various strength parameters, including flexural,²⁷ compressive, and tensile strengths,²⁸ as well as shear strength and static/dynamic shear moduli.^29,30 Additionally, studies have established the correlation between mechanical and dielectric properties.³¹ At the microscopic level, Gao et al.³² utilized scanning electron microscopy (SEM) to observe the microstructures of polymers with varying densities, analyzing the characteristics and evolution of their stress-strain curves under uniaxial compression. Similarly, Fang et al.³³ employed SEM to investigate the fracture surface characteristics and failure mechanisms of specimens under compression, tension, and torsional shear.

However, the aforementioned studies on mechanical properties failed to consider the influence of aqueous environments. Current research on the mechanical behavior of materials in complex aqueous environments primarily focuses on factors such as pH levels, hygrothermal conditions, and saline environments. Regarding general water environments, Tagliavia³⁴ examined the influence of a water environment on the flexural properties of PU, finding that the flexural modulus of specimens decreased after water immersion. Marsavina et al.³⁵ evaluated the effect of immersion at room temperature on the mechanical properties of PU foams, discovering that the immersed layer increased the flexural modulus and altered the foam’s behavior under impact. Li³⁶ analyzed the frequency spectra of PU materials, investigated the differences in viscoelasticity between immersed and non-immersed conditions, and proposed functional relationships describing the storage modulus, loss factor, and density as functions of frequency under immersion conditions. Regarding the effect of immersion duration, the variation in the mechanical strength of PU with water absorption duration can be divided into two stages: initially, the mechanical strength increases with immersion time as water penetrates the open pores of the PU, filling the voids and thereby increasing the strength; after the material reaches water saturation, the mechanical strength gradually decreases with prolonged immersion time, at which point the water has damaged the pore walls.³⁷ In studies on saline environments, Yu et al. and Xue et al.^18,38,39 conducted a comparative analysis of the effects of immersion in distilled water versus seawater on the flexural properties of PU materials using three-point bending tests. The research demonstrated that mechanical properties, such as flexural strength and elastic modulus, were inferior to those of non-immersed specimens, with specimens corroded by seawater exhibiting the weakest flexural performance. In contrast, Zhang⁴⁰ investigated the combined effects of a saline environment and freeze-thaw cycles. Two methods, namely salt solution immersion (S-I) and salt-freeze-thaw (S-F-T) cycles, were employed to explore the impact of deicing salt on polymer grouting materials of different densities. The results indicated that the S-F-T cycles exerted a more severe destructive effect on the materials. The superimposition of liquid immersion further exacerbated the freeze-thaw damage, leading to declines in the mass, compressive strength, and tensile strength of the materials. Furthermore, after 200 S-F-T cycles, the material exhibited the highest compressive strength loss rate in the CaCl₂ solution, followed by the CH₃COOK solution, and the lowest in pure water. Additionally, materials with higher densities demonstrated a greater resistance to freeze-thaw damage. In studies on acidic and alkaline aqueous environments, Wang et al.⁴¹ immersed PU grouting materials of varying densities in water and solutions of H₂SO₄, HCl and NaOH, and subsequently conducted uniaxial compression tests. The results indicated a reduction in the compressive strength of the materials, with the water environment exhibiting a relatively minor impact. Mohamed et al.⁴² drew similar conclusions through water absorption tests on PU. In studies on hygrothermal environments, Yi et al.¹⁷ investigated the effects of a hydrothermal environment on the compressive and flexural properties of PU materials by studying their durability under hydrothermal aging. They found that both compressive and flexural strengths decreased after hydrothermal treatment. Both Mourad⁴³ and Chou⁴⁴ assessed the influence of water absorption on the elastic modulus and strength of PU composites. In a saltwater environment, changes in the compressive properties of PU were negligible, whereas both flexural modulus and flexural strength declined.

In summary, although extensive experimental studies have been conducted on the water absorption and mechanical properties of PU grouting materials, systematic research and comprehensive summaries regarding their water absorption mechanisms and characteristics remain lacking. Particularly in grouting projects subjected to complex groundwater environments, there is a notable absence of quantitative analyses concerning the compressive strength of these materials post-absorption. As underground grouting operations extend to greater depths, the hydrogeological environments become increasingly complicated. Consequently, the ability to quantitatively evaluate the mechanical properties of water-immersed PU grouting materials will significantly mitigate the empirical nature and potential safety hazards in the design of anti-seepage and reinforcement engineering. Therefore, it is of paramount importance to investigate the water absorption mechanisms of PU grouting materials and to elucidate the variation patterns of their compressive strength as a function of water absorption.

To elucidate the diffusion mechanism of water in PU grouting materials and its influence on compressive strength, this study employs a combined micro- and macroscopic approach. SEM was utilized to investigate variations in the pore structure of PU grouting materials before and after water immersion. Furthermore, the diffusion process and mechanism of water within the material were dynamically observed from a microscopic perspective. Subsequently, water absorption tests were conducted to examine the variations in moisture content under various experimental conditions, including density, surface condition, immersion duration, water pressure, saline environment, and ambient temperature and humidity. Based on these experiments, the underlying mechanisms were analyzed in conjunction with microscopic SEM images. Finally, the impact of moisture content on compressive strength was evaluated. The research results comprehensively demonstrate the service state of underground PU materials.

Physical and mechanical testing

Raw materials

The raw materials used in the water absorption test were polyols and isocyanates. Both produced by Wanhua Chemical (Shandong, China). The polyol, commonly referred to as the “white material”, is a liquid polymer containing multiple hydroxyl groups (-OH) in its molecular structure. It appears as a brownish-yellow liquid with a commercial grade of WANOL^®R2305. The isocyanate, commonly known as the “black material”, is a macromolecular organic compound composed of isocyanate groups (-NCO). The isocyanate group is chemically reactive, and the material exhibits a brownish-red color, with a commercial grade of WANNATE^®CW20. The physical properties of the polyol and isocyanate are presented in Table 1.

Table 1.

The physical properties of the polyol and isocyanate.

Category	Polyols	Isocyanates
Physical property	Amber liquid	Umber liquid
Chemical formula	HO─(R─O─)n─H	R─N═C═O
Solid content (%)	>99	>99
Density (25°C)/g·cm^-3	1.01	1.23
Viscosity (25°C)/MPa·s	720 ± 50	350 ± 50
-OH/mgKOH·g^-1	330 ± 15	——
-NCO/wt%	——	21.7 ± 0.5%
Moisture content/%	<0.2	0
Flash point/°C	>200	>200

Specimen preparation

The specimens required for the experiment were prepared by casting in accordance with the standards ‘Test Method for Dimensional Stability of Rigid Cellular Plastics’ (GB/T 8811-2008)⁴⁵ and ‘Cellular Plastics and Rubber-Determination of Apparent Density’ (GB/T 6343-2009).⁴⁶ The specimen casting process is illustrated in Figure 1, which primarily comprises four stages: mold fabrication, raw material mixing and pouring, curing, and demolding. First, the polyol and isocyanate were conditioned separately at (23 ± 2) °C for 24 h. Subsequently, they were mixed at a volumetric ratio of 1:1 using an electric stirrer at a speed of 500 r/min for 15 s, followed immediately by pouring. To facilitate demolding, a layer of lubricant was brushed onto the inner walls of the mold to form a thin film prior to casting. The pouring process was then initiated using injection equipment, whereby the mixture was injected uniformly and rapidly into the mold via a pouring gun under pressure. Upon mixing, the two raw materials underwent a vigorous chemical reaction, causing rapid volume expansion and foaming to form a PU foam structure, which eventually filled the entire mold cavity. After pouring, the mixture was allowed to cool for 30 min once the reaction was fully complete before demolding. During the casting process, the density of the specimens was controlled by regulating the volume of the injected mixture.

Figure 1.

Specimen casting and chemical reactions.

After casting, the specimens with uneven surfaces or defects were cut and leveled using a cutting machine to remove bottom defects or excess material. The dimensional error of each specimen was controlled within ±0.1 mm. The densities of the cast specimens ranged from 0.02 to 0.5 g/cm³.

Experimental procedures

To comprehensively and systematically analyze the diffusion mechanism of water in PU grouting materials and its influence on compressive properties, this paper conducts quantitative and qualitative analyses via macroscopic water absorption tests and SEM. The specific experimental scheme is illustrated in Figure 2.

Figure 2.

Experimental procedure.

The experiments were conducted along three main lines:

(1) At the micro-scale, the microstructural evolution of the material before and after water immersion was dynamically observed using SEM to reveal the intrinsic mechanisms of water diffusion within closed-cell, low-density microcellular foam materials. The statistical characteristics of the pore structure, such as cell size and wall thickness, were analyzed in terms of their influence on water diffusion. Furthermore, the effects of density and surface condition on water absorption were investigated. To this end, water absorption tests were performed on specimens with varying densities and different surface conditions

(2) At the macro-scale, systematic water absorption experiments were conducted in the laboratory. By establishing distinct experimental conditions, the influence of environmental factors—such as immersion time, water pressure, saline environment, and ambient temperature and humidity—on water absorption was observed. These investigations aimed to elucidate the external factors governing water diffusion in low-density microcellular foam grouting materials.

(3) Uniaxial compression tests were carried out on both dried and moist specimens to analyze the impact of water absorption on the compressive strength of the material.

The water absorption experiment was decomposed into seven sub-experiments, and test specimens were randomly selected for each sub-experiment to investigate: (1) the effect of density on water absorption; (2) the effects of surface condition on water absorption; (3) the effect of immersion duration on water absorption; (4) the effect of water pressure on water absorption; (5) the effect of salt content in water on water absorption; and (6) the effects of ambient temperature and humidity on water absorption.

Based on the research objectives, microscopic electron microscopy tests, water absorption tests, and compression tests were conducted, respectively.

Scanning electron microscopy analysis

To investigate the variation in the internal structure of PU grouting materials with different densities, SEM was employed. The specific procedure is illustrated in Figure 3.

Figure 3.

SEM test procedure.

First, the cast specimens were cut into 10 mm × 10 mm×10 mm cubes using a small cutting saw. To enhance electrical conductivity and ensure high-quality SEM images, the samples were sputter-coated with gold for 300 s using an ion sputter coater and the sputtering process commenced when the current reached 5–8 mA. Finally, the coated specimens were placed in a sample holder for SEM observation, as shown in Figure 4.

Figure 4.

SEM images at different densities and magnifications.

It can be seen from Figure 4 that the internal structure of the PU material exhibits the following characteristics:

(1) On a microscopic scale, it presents a highly porous pore structure consisting of both open and closed pores, with a high proportion of closed pores.

(2) At low densities, as illustrated in Figure 4(a), the pore morphology ranges from circular to polygonal, featuring relatively large pore sizes and large contact areas between adjacent pores. With increasing density, the pore shape gradually transitions from irregular polygons to ellipses and circles; concurrently, the pore size decreases, the integrity of the pores improves, and the closed-pore content increases.

(3) The matrix pore walls between pores are continuous, and the wall thickness between adjacent pores increases as the density increases.

The water absorption of PU grouting materials is directly correlated with their density and pore structure. SEM images reveal that as density increases, porosity decreases, cell diameter reduces, and the number of open pores diminishes. Consequently, the hydrophobic area decreases, and there are fewer pathways for water ingress. This phenomenon can be attributed to a high closed-pore content and gradually thickening continuous cell walls, which collectively form a barrier that impedes water penetration, thereby leading to a gradual reduction in water absorption.

Water absorption test

In the water absorption test, the effects of PU grouting material density, immersion time, saline water environment, water pressure, specimen surface condition, and environmental temperature and humidity on the water absorption were considered. To minimize the randomness of the experimental data, each test was repeated three times, and multiple groups of tests were conducted for specimens with the same density. The test conditions and specimens used to investigate the factors influencing the water absorption are presented in Table 2. Specifically, for the study on surface conditions, specimens with a density of 0.32 g/cm³ were prepared in three states: sealed (with the original skin retained and sealed using 703 silicone rubber and artillery grease), unprocessed skin, and skin removed.

Table 2.

Test conditions and specimen dimensions.

Variables	Water temperature(°C)	Immersion time(d)	Test conditions	Specimen size (number/Group)	Specimen density range
Density	20 ± 2	4 days	20 ± 2°C	Φ70.6 mm × 100 mm (40)	0.30 – 0.42 g/cm³
Time	20 ± 2	4 days	20 ± 2°C	Φ70.6 mm × 100 mm (40)	0.30 – 0.42 g/cm³
Saline environment	20 ± 2	30 days	20 ± 2°C, 8% CaCl₂ solution	Φ25 mm × 50 mm (3)	0.2, 0.3, 0.43 g/cm³
Water press	20 ± 2	4 days	20 ± 2°C, applied incrementally to 0, 2, and 4 MP	71.1 mm × 71.1 mm × 71.1 mm/15 (5/3)	0.32 g/cm³
Surface condition	20 ± 2	1 day	20 ± 2°C, 4 MPa water pressure	71.1 mm × 71.1 mm × 71.1 mm (5/3)	0.32 g/cm³
Ambient temperature and humidity	20 ± 2	8 days	Ambient temperature: 25, 50, 70. Humidity:40, 70, 98.	Φ20 mm × 20 mm (3/5)	0.5 g/cm³

The water absorption was determined in accordance with the standard ‘Test Method for Water Absorption of Rigid Cellular Plastics’ (GB/T 8810-2005).⁴⁷ The mass water absorption of the PU grouting material was calculated using the weighing method, as shown in equation (1).

w = (M_{w} - M_{d}) / M_{d} \times 100 %

(1)

where

w

is the water absorption;

M_{d}

and

M_{w}

represent the mass of the specimen before and after immersion, respectively.

The procedure of the water absorption test is illustrated in Figure 5. The specific steps are as follows:

(1) Weighing of dried specimens. The mass of the dried specimens was measured using an electronic balance, recorded, and labeled. The density of the specimens was calculated based on the volume of the cast specimens.

(2) Specimen immersion. The PU grouting material specimens of different densities were placed in a container, ensuring that the water depth was sufficient to completely submerge the tops of all specimens. Since the density of the specimens was lower than that of water, causing them to float on the surface and potentially leading to inadequate water absorption, steel plates were used to weigh down the specimens to ensure that no surface was exposed above the water level. All specimens were soaked at room temperature for 96 h.

(3) Measurement of moist specimen mass. Upon reaching the designed immersion time, the specimens were removed individually according to their numbers. The surface moisture was wiped off with filter paper, and the mass of the absorbed water specimens was weighed using an electronic balance and recorded. The water absorption of each specimen was calculated using equation (1) based on the mass change before and after absorption.

Figure 5.

Water absorption test procedure.

Compression test

To investigate the effect of water on the compressive strength of the specimens, uniaxial compression tests were conducted on specimens after 96 h of water absorption using a WAW-300 computer-controlled electro-hydraulic servo universal testing machine. The degree and pattern of the influence of water on the strength were subsequently analyzed. The specimen dimensions were 50 mm × 50 mm × 50 mm. Since the water absorption of the specimens decreases with increasing density, specimens with relatively low densities (0.07 g/cm³–0.11 g/cm³) were selected for the compression tests to highlight the effect of the material’s water absorption capacity on its compressive strength, as shown in Figure 6.

Figure 6.

The compression test diagram.

Experimental results and analysis

Effects of density, immersion time, and saline environment on water absorption

The water absorption of the specimens under different densities, immersion times, and saline environments is shown in Figure 7(a)–(c), respectively. Specifically, in the study on immersion time, six specimens from different density ranges were selected for analysis; the variation trend of water absorption with immersion time for the remaining specimens follows a similar pattern.

Figure 7.

Variations in water absorption under different densities, immersion times, and saline environments. (a) Density. (b) Immersion time. (c) Saline and distilled water environments.

As shown in Figure 7(a), the water absorption of the material decreases with increasing density, exhibiting a quadratic polynomial relationship with a correlation coefficient of 0.98, which indicates a strong correlation between density and water absorption. Furthermore, the water absorption varies within a narrow range of 1.2% to 4.5%, demonstrating the low water absorption of the PU grouting material and confirming its suitability as an impermeable material in geotechnical engineering. This observed trend is primarily attributed to the internal cellular structure of the material. As depicted in Figure 4(a) higher density corresponds to lower porosity, smaller cell diameters, and fewer open cells. Consequently, the hydrophobic area decreases, and the channels for water ingress and egress are reduced. Essentially, a higher closed-cell content coupled with progressively thickened continuous cell walls forms an effective barrier that hinders water penetration, thereby leading to a gradual decrease in water absorption.

As shown in Figure 7(b), the water absorption increases rapidly within the first 24 h of immersion, and gradually tends to stabilize as the immersion time increases. SEM micrographs reveal that this behavior is related to the water diffusion process within the PU material. The diffusion of water in the material can be divided into surface diffusion and internal diffusion.²⁴ Since the immersion time in this test was limited to 96 h, water diffusion is primarily manifested as surface diffusion, as illustrated in Figure 8.

Figure 8.

The diffusion process of water on the surface of the material.

First, for the defect-free matrix surface, water is predominantly adsorbed at the junction of pores and the matrix due to the edge effect; partially adsorbed at the polar sites of the matrix, spreads as a thin film on a small portion of the pore wall surfaces, while the majority of the water gradually propagates along the matrix channels driven by the concentration potential difference and gravity, eventually covering the entire surface. Second, regarding the open pores formed by the collapse and wall rupture of partial pores during the foaming process, as well as the destruction of the closed cavity structure due to manual cutting during sample preparation, the capillary tension and internal gas pressure cause water to form a film and bubbles on the pore surface. As the internal gas dissolves and the external pressure increases, the water film collapses, resulting in a massive influx of water into the pores. The aforementioned diffusion processes occur continuously and cyclically.

It can be seen from Figure 7(c) that:

(1) Higher density corresponds to a lower water absorption, which is consistent with the trend observed in distilled water.

(2) For specimens of the same density, the water absorption in the deicing salt solution is higher than that in distilled water. This is attributed to the fact that the addition of salt increases the osmotic pressure, leading to an elevated water absorption of the specimens. Furthermore, the salt in the deicing salt solution penetrates the specimens along with the water. Consequently, compared to specimens immersed in distilled water, the mass of specimens in the deicing salt solution includes the mass of the intruded salt. Therefore, the water absorption of specimens in the deicing salt solution is greater than that of specimens in distilled water

Effects of water pressure, specimen surface condition, and environmental temperature and humidity on water absorption

The effects of water pressure, specimen surface condition, and environmental temperature and humidity on the water absorption of the specimens are shown in Figure 9(a)–(c), respectively.

Figure 9.

Water absorption of specimens with different surface conditions. (a) Water pressures. (b) Surface conditions. (c) Environmental temperature and humidity.

As illustrated in Figure 9(a), pressure has a significant influence on the water absorption of the PU grouting material. The water absorption peaks at 40% under a water pressure of 4 MPa (equivalent to a 400 m water head). This is attributed to the fact that as water pressure increases, water more readily penetrates into the material along the interfaces between internal micropores and the matrix where bonding is weak. Furthermore, the high pressure can cause the rupture of fragile pore walls on the specimen surface, leading to increased water absorption.⁹

As shown in Figure 9(b), the surface-sealed specimens exhibit the lowest water absorption, while the skin-removed specimens show the highest. Notably, the water absorption of the skin-removed specimens is more than twice that of the specimens with intact skin. This phenomenon is attributed to the fact that PU grouting materials possess a foam structure, and their water absorption is closely related to the pore morphology. For the skin-removed specimens, the process of removing the skin disrupts a significant number of closed pores, creating pathways that connect the internal structure to the external environment. This results in increased routes for water ingress, thereby leading to higher water absorption. In contrast, the specimens with intact skin and those with surface sealing have fewer surface open pores and significantly improved sealing performance. This prevents water from easily penetrating into the material, resulting in lower water absorption, which is consistent with the findings of Reference 48.

As shown in Figure 9(c), water diffusion within the material is rapid during the initial stage. As time progresses, the water absorption decreases until the material reaches a saturation state. Consequently, the water absorption increases rapidly at the beginning and then gradually slows down over time. This is because higher ambient temperature and humidity result in greater external water vapor pressure and increased molecular activity. These factors intensify the diffusion movement, facilitating the penetration of water molecules into the PU foam, thereby leading to a corresponding increase in its water absorption.

Effect of water on the compressive strength of specimens

The compressive strengths of the specimens before and after immersion at different densities are shown in Figure 10.

Figure 10.

Compressive strength of specimens before and after water immersion.

As shown in Figure 10:

(1) The compressive strength of the saturated specimens showed a slight increase with increasing material density, and both compressive strength and density exhibit a linear correlation. This trend is consistent with the variation in compressive strength observed before water immersion. From a microstructural perspective, the compressive deformation and failure of PU grouting materials are closely related to the bending deformation and compressive collapse of the pore structure. Although the pore structures of wet specimens are partially filled with water, their deformation, failure, and collapse processes under compression are similar to those of dry specimens. For dry specimens, the internal air is expelled when the pore structure fails under compression. Conversely, in wet specimens, the water infiltrating the pores is gradually extruded due to compression. Therefore, dry and wet specimens exhibit similar trends in variation with density.

(2) The compressive strength of the PU grouting material after water immersion is greater than that before water immersion. This indicates that water intrusion within a short duration enhances the strength of the PU material. This is because the permeated water fills some of the internal voids, when subjected to compression, the extrusion of water requires a certain amount of time. Consequently, as the water is continuously expelled from the pores under pressure, both the water and the pore structure provide support, thereby increasing the ultimate stress of the material to a certain extent.

Conclusions

In this paper, the water absorption characteristics of PU materials were investigated from the perspectives of both internal and external factors. Furthermore, the water absorption mechanism was analyzed in conjunction with microscopic SEM observations before and after water immersion. Finally, by comparing the variations in compressive strength before and after water immersion, the influence of water on the compressive strength of PU grouting materials was elucidated. The main conclusions are drawn as follows:

(1) The water absorption exhibits a quadratic polynomial decrease with increasing density, with a coefficient of determination of 0.98. However, the PU grouting material exhibits a relatively low water absorption. At densities of 0.30 g/cm³ and 0.42 g/cm³, the water absorption are merely 4.38% and 1.28%, respectively. This is attributed to the closed-cell structure within the material, which restricts the penetrated water to the outer layer. Furthermore, the surface condition significantly affects the water absorption. Compared with the original surface (with a water absorption of 15.77%), the water absorption of the sealed specimens decreases by 44.32%. In contrast, the water absorption of the skin-removed specimens increases by 151.11%. This is because the removal of the skin damages the internal cellular structure, leading to an increased number of open cells on the surface and, consequently, a higher water absorption.

(2) The water absorption of the material changes significantly when the water pressure is increased at intervals of 2 MPa. Notably, the most substantial variation occurs as the pressure increases from 0 MPa to 2 MPa, where the water absorption increases by 3.4 times. This is attributed to the higher water pressure facilitating a more comprehensive and deeper penetration of water into the material. Furthermore, the water absorption of the specimens gradually increases with prolonged immersion time; however, after 24 h of immersion, the increment becomes marginal, remaining below 0.5%. In a saline water environment, the water absorption of specimens with different densities increases by 22% compared to that in a distilled water environment.

(3) The water absorption exhibits an increasing trend with rising environmental temperature and humidity, which is associated with the intensity and concentration of water molecular motion in the environment. Within the density range of 0.07 g/cm³–0.11 g/cm³, the compressive strength of the water-absorbed PU grouting material increased by approximately 4.56%. This is attributed to the higher water absorption at lower densities, which leads to a greater volume of water penetrating and filling the internal pores. During compression, although the internal water is partially extruded, the water remaining trapped within the pore structure acts as a support, thereby increasing the ultimate stress of the material to a certain extent.

However, the microstructural analysis did not include the skin layer of the specimens. In the compressive strength study, the density range was relatively narrow (0.07 g/cm³–0.11 g/cm³), and only the initial stage of water immersion was considered, neglecting the size effect of the specimens. Future research should broaden both the density range and the water immersion duration for the compressive strength tests. Furthermore, when investigating the water absorption mechanism and compressive strength, the size effect, as well as the influence of the skin layer’s microstructure on the water absorption, should be taken into account.

Footnotes

ORCID iD

Meili Meng

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is supported by National Natural Science Foundation of China (No. 51608197) and Key Research Projects of Henan Higher Education Institutions (No. 23B570001).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. Due to the ongoing nature of the research project, the data are not publicly available at this time.*

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