Development and Performance Analysis of a Modified Polyurea Hydrophobic Coating for Improving Water Conveyance Efficiency in Concrete Channel Linings

Abstract

To overcome the shortcomings of traditional concrete coatings, such as high roughness and poor frost resistance, this study developed and evaluated a new hydrophobic coating—modified polyurea hydrophobic coating (MPHC). MPHC features strong adhesion, high hydrophobicity and excellent durability. The coating performance was evaluated through contact angle measurement, tensile bond strength test, and assessment of environmental durability under several aging conditions including immersion, heat resistance and freeze–thaw cycles. The experimental results showed that the surface contact angle of MPHC reached 131.2°, demonstrating strong hydrophobicity. After durability testing, there was no significant decrease in contact angle and bond strength, confirming the robustness of the coating. The coating combines a “dual structure” formed by polydimethylsiloxane and microsilica powder, thereby creating a hydrophobic rough surface. This structure minimizes the fluid–solid interface area and adhesion, thereby enhancing drag reduction performance. In drag reduction tests on channel model linings, compared with ordinary concrete, MPHC reduced the roughness coefficient by 10.0–11.6%, and by 7.4–7.5% compared with ordinary polyurea coatings. The outstanding hydrophobicity, durability and drag reduction performance of MPHC make it a promising solution for improving the water conveyance efficiency of concrete-lined channels.

1. Introduction

While numerous concrete coatings are available, their performance and suitability for specific applications differ considerably. For channel lining applications, the coating must meet several critical requirements: it must have a relatively low roughness, and it must be environmentally friendly, non-toxic, and odorless, while maintaining reliable adhesion to concrete and robust hydrophobicity under adverse conditions, including immersion, high temperatures, and freeze–thaw cycles. However, most existing research on concrete coatings focuses primarily on their initial bonding and hydrophobic performance under normal environmental conditions, with limited experimental studies addressing their endurance performance under harsh environmental conditions. As a result, while many concrete coating products are commercially available, few are suitable for channel lining applications, often leading to the actual performance of the channel concrete coating falling far short of the expected functionality of the coating, such as cracking upon contact with water and aging due to freeze–thaw cycles.

The roughness of the water surface in the channel is one of the important indicators characterizing the water transport capacity of the channel [1,2,3]. For example, in the Middle Route Project of the South-to-North Water Diversion, reducing n by just 0.001 could save project costs by several hundred million yuan [4]. The surface roughness coefficient of channel concrete is closely associated with factors such as construction technology, workers’ skills, and curing conditions [5,6]. For large channels, achieving a smooth and flat concrete surface requires stringent quality control during construction, posing significant challenges. Concrete surfaces that meet relevant construction specifications typically exhibit a roughness coefficient of approximately 0.0150, with design values for concrete channels in China generally ranging between 0.0150 and 0.0170 [7]. However, under harsh operational conditions—such as regions with severe cold or sections with steep slope gradients—external factors like climate and water flow scouring often lead to surface deterioration, including freeze–thaw damage and abrasion. These adverse conditions can substantially increase the roughness coefficient, greatly reducing the channel’s flow capacity [8,9,10]. In extreme cases, such degradation may even compromise the structural integrity of the channel.

Sealing coatings effectively reduce the roughness coefficient, improve flow capacity, and protect concrete against freeze–thaw damage. This approach not only improves the durability of channel linings but also extends their service life. Currently, concrete coatings used in engineering applications can be categorized into two main types: traditional sealing coatings and advanced hydrophobic coatings. Traditional sealing coatings generally include polyureas [11,12,13], epoxy resins [14,15], and polyurethanes [16,17,18]. These coatings exhibit high bonding strength and excellent waterproofing properties. However, they provide limited reductions in roughness coefficient after curing and demonstrate poor frost resistance. In contrast, advanced hydrophobic coatings incorporate binders such as epoxy resin, acrylic acid, and polyurethane, combined with low-surface-energy reagents like silane and siloxane. These coatings retain the high bonding strength and waterproofing properties of traditional coatings while offering superior hydrophobicity. The water contact angle on the surface of hydrophobic coatings exceeds 110° [19], which reduces the fluid’s shear strain force on the wall, increases the thickness of the laminar boundary layer, and effectively lowers the wall roughness coefficient [20,21]. Additionally, these coatings demonstrate excellent anti-condensation properties and freeze–thaw durability [22,23]. Liu et al. [22] developed a translucent superhydrophobic coating using polydimethylsiloxane (PDMS) and polymethyl methacrylate, achieving a contact angle of 157.5° on a rough glass substrate. Similarly, Zhang et al. [23] prepared a suspension containing silicon micropowder, nano-silica, epoxy resin, and PDMS. When sprayed onto various substrates, this suspension formed superhydrophobic surfaces with rough micro-nano structures and low surface energy.

Among the various concrete coatings, polyurea stands out as a solvent-free, environmentally friendly material with excellent wear resistance, impermeability, frost resistance, corrosion resistance, aging resistance, and mechanical properties. Polyurea is a type of polymer material, formed by the reaction of multiple isocyanates and multiple amines, with urea bonds being formed in its structure. This makes polyurea a high-strength and tough coating. The polyurea coating is not sensitive to environmental humidity and can even be sprayed onto damp concrete surfaces to form a film. The polyurea coating completely breaks the traditional concept of protection, featuring good flexibility, high bonding strength, density, and continuity. These characteristics make polyurea highly suitable for water conservancy projects [24,25,26]. This study developed a new type of modified polyurea hydrophobic coating (MPHC), which was specifically designed according to the performance requirements of the channel lining concrete. The hydrophobicity and bond strength of MPHC were experimentally evaluated under conditions of immersion, high temperatures, and freeze–thaw cycles.

2. Material and Methods

2.1. Materials and Preparation of Modified Polyurea Hydrophobic Coating

The MPHC was prepared using three primary materials: polyurea, PDMS, and silicon dioxide (SiO₂) (Figure 1). The polyurea, sourced from Chinese Research Institute of Water Resources and Hydropower Engineering, is gray and closely resembles cement mortar. The polyurea featuring good flexibility, high bonding strength, density, and continuity. PDMS, a polymer with the formula (C₂H₆Osi)_n, was procured from Guangdong Fengjiang Chemical Reagent Co., Ltd. This colorless, high-viscosity liquid is non-toxic, odorless, and exhibits notable properties such as heat, cold, and water resistance, as well as low surface tension and remarkable chemical stability. The SiO₂ used in this study, also obtained from Guangdong Fengjiang Chemical Reagent Co., Ltd, consists of spherical particles with a particle size of 1 μm. The SiO₂ is a colorless and odorless crystalline solid with high hardness, a large specific surface area and strong surface adsorption capacity.

The MPHC was formulated with polyurea, PDMS, and SiO₂ powder in a mass ratio of 20:5:1, with polyurea serving as the primary component. The preparation process, illustrated in Figure 2, involved several steps. First, polyurea and PDMS were combined and mixed uniformly with a magnetic stirrer for 30 min. SiO₂ powder (1 μm; mass ratio 20:1) was then added and stirred for an additional 60 min to achieve uniform dispersion. The resulting MPHC is gray, resembling the natural color of concrete (Figure 3). The surface of the MPHC is relatively smooth, and it is covered with micro-silica powder particles, forming a micro-rough structure consistent with the Cassie–Baxter theoretical model. Cassie–Baxter theoretical model [27] suggests that for rough micro-structured surfaces, liquids cannot fully penetrate into the microstructures, and there is air between the droplets and the solid. Reducing the contact area between the solid and the liquid (that is, increasing the contact area between the liquid and the air) will cause the contact angle to increase when the contact angle on the same smooth solid is greater than 90°.

2.2. Evaluation Methods for Coating Performance

The performance of the MPHC was evaluated through two key tests: contact angle measurement and bonding strength testing. For the contact angle measurement, the MPHC was applied to mortar specimens measuring 70 mm × 70 mm × 20 mm, which were prepared using a cement/sand/water ratio of 1:2.5:0.45. These specimens were cured for 28 days under conditions of humidity greater than 95% and a temperature of 20 ± 2 °C. The coating thickness was approximately 1 mm (1 m² concrete requires 1 L MPHC). After curing, the specimens were air-dried in a cool place for 60 min before testing. The contact angle was measured using a KRUSS DSA100 (Clues Scientific Instruments Co., Ltd., Shanghai, China) contact angle measuring instrument, and each specimen is tested three times.

The bonding strength of the MPHC was assessed through a tensile bond strength test. The preparation of bonding test specimens involved applying a uniform layer of MPHC to the surfaces. The coated surfaces of the two specimens were brought into contact, pressed gently, and positioned horizontally. A pressure block measuring 40 mm × 40 mm with a mass of 1.600 ± 0.015 kg was placed on top of the smaller specimen for 30 s, and any excess interface agent was scraped off from the sides. The bonded specimens were cured for 14 days under standard conditions. Subsequently, a pull-out joint was affixed to the smaller specimen with a high-strength bonding agent, followed by a 48-h stabilization period (Figure 4).

The tensile bond strength test was conducted by attaching a fixture to the bonded specimen, as shown in Figure 5. The model of the tensile testing machine is WDW-200 (Shanghai Precision Instrument and Metering Co., Ltd., Shanghai, China). Its maximum test force is 200 kN and the accuracy is 1%. The tensile test was performed at a speed of 5 ± 1 mm/min until specimen failure occurred, and the failure load was recorded. The tensile bond strength was calculated using Equation (1):

$$\sigma ={\displaystyle \frac{F_1}{A_1}}$$

(1)

where $\sigma$ is the tensile bond strength (MPa), $F_1$ is the maximum load (N), and $A_1$ is the adhesive area (mm²).

2.3. Aging Treatments of Coated Specimens

The immersion treatment involved specimens coated with the material and cured for seven days under standard test conditions. These specimens were fully immersed in water maintained at a temperature of (23 ± 2) °C. After six days of immersion, the specimens were removed, and surface water stains were gently dried with a cloth to ensure consistency.

For the heat resistance treatment, specimens coated with the material and cured for seven days under standard test conditions were placed in an oven maintained at a temperature of (70 ± 2) °C. After seven days of exposure to these elevated temperatures, the specimens were removed from the oven and allowed to cool for four hours under standard test conditions to stabilize their properties.

In the freeze–thaw treatment test, specimens were initially cured for seven days under standard test conditions and then immersed in water at (23 ± 2) °C for an additional seven days. After the immersion period, the specimens were removed, and surface water stains were dried using a cloth. The specimens were subsequently subjected to 25 freeze–thaw cycles to evaluate their durability under fluctuating thermal conditions. Each freeze–thaw cycle consisted of two phases: first, the specimens were maintained at (−15 ± 3) °C for 2.0 ± 0.3 h; subsequently, they were immersed in water at (23 ± 2) °C for 2.0 ± 0.3 h.

3. Results

3.1. Hydrophobicity of MPHC

The contact angle, formed at the interface of solid, liquid, and gas phases, is a key parameter for surface characterization. Solid surfaces are classified based on their contact angle into superhydrophilic surfaces (contact angle < 10°), hydrophilic surfaces (contact angle 10–90°), hydrophobic surfaces (contact angle 90–150°), and superhydrophobic surfaces (contact angle > 150°) [28]. The contact angle strongly influences both drag reduction and the roughness coefficient of the solid–liquid interface.

The relationship between the contact angle and drag reduction rate has been extensively studied, particularly in the context of pipeline coating materials. Experimental results show that a larger contact angle corresponds to higher drag reduction, ranging from 6.8% to 76.5% [29]. However, research exploring the relationship between contact angle and roughness coefficient remains limited. Liu [30] investigated this relationship using a rectangular channel with a total length of 25 m, a depth of 50 cm, and a slope of 1/400. The study found that ordinary concrete had a contact angle of 67.2° and a roughness coefficient of 0.01491. In contrast, the superhydrophobic coating had a contact angle of 153.4°—2.28 times higher—and a roughness coefficient of 0.01322, 0.89 times lower. These results demonstrate that increasing the contact angle of a hydrophobic material leads to a reduction in its roughness coefficient.

The surface water contact angles of various materials, including cement mortar (saturated), polyurea, and modified polyurea, are illustrated in Figure 6. Cement mortar serves as the baseline, representing the uncoated concrete surface. When dry or subjected to heat treatment, the water contact angle of cement mortar cannot be measured due to its high water absorption capacity. However, when saturated, the contact angle is measured at 59.9°. The polyurea coating exhibits a contact angle of 62.9°, comparable to that of cement mortar, indicating limited enhancement in surface hydrophobicity. In contrast, the MPHC contact angle is 131.2°, 2.2 times that of cement mortar and 2.1 times that of ordinary polyurea. These findings demonstrate the significant improvement in hydrophobicity achieved with the modified polyurea.

Coatings applied to the bottom of channel linings remain immersed in water, where their operational contact angle affects water conveyance. After immersion, the contact angle of the polyurea coating increases slightly to 75.6°, while the contact angle of the MPHC decreases slightly to 127.9°. The slight reduction in the MPHC contact angle is due to minimal PDMS dissolution, which has negligible impact on hydrophobicity.

When the coating is applied to the slope of the channel concrete lining, it is exposed to natural high temperatures during summer when the water level drops. After heat treatment, the contact angle of the polyurea coating remains virtually unchanged at 62.1°. In contrast, the contact angle of the MPHC decreases to 119.0°. Although this value is lower than its pre-heat-treatment contact angle, it remains significantly higher than the contact angles of both ordinary polyurea and cement mortar.

In areas of the channel concrete lining subjected to water level fluctuations, the coating experiences freeze–thaw cycles during winter. After 25 freeze–thaw cycles, the contact angle of the cement mortar coating decreases to 46.6°, representing a 22.2% reduction, while its roughness coefficient increases significantly, severely impairing flow capacity. In contrast, the polyurea coating and MPHC are less affected by freeze–thaw cycles, with contact angles of 62.0° and 130.2°, respectively. This demonstrates the MPHC’s superior hydrophobicity and durability under such conditions.

3.2. Bonding Tensile Strength

As illustrated in Figure 7, the tensile bond strength of the untreated coatings was measured at 1.3 MPa for the ordinary polyurea coating and 1.2 MPa for the MPHC. Although PDMS improves the coating’s contact angle [22], it slightly reduces its bond strength. However, this reduction is minimal, with the bond strength decreasing by only 0.1 MPa. Following immersion treatment, the tensile bond strength of the ordinary polyurea coating decreased by 8.7%, while the MPHC showed no reduction. This demonstrates the superior resistance of the MPHC to water-induced degradation in bond strength. Following heat treatment, the tensile bond strength of both coatings increased. The ordinary polyurea coating exhibited a 15.4% increase, while the MPHC showed an 8.3% increase. These findings suggest that a dry environment enhances the bonding performance of polyurea coatings.

After exposure to freeze–thaw cycles, the tensile bond strength of the ordinary polyurea coating decreased by 30.7%, while the MPHC experienced a reduction of 25.0%. Despite the decline, the MPHC exhibited superior freeze–thaw resistance compared to ordinary polyurea. Figure 8 shows that tensile failures primarily occurred on fresh mortar surfaces, with some mortar-coating bond surfaces also observed. Additionally, clear signs of freeze–thaw damage were observed in the mortar. Therefore, it can be inferred that the reduction in tensile bond strength after freeze–thaw cycles is primarily attributable to freeze–thaw damage in the mortar specimens rather than the coatings themselves.

3.3. Test of Roughness Coefficient of the Coating in the Channel Model

To evaluate the drag reduction performance of the MPHC, a concrete-lined channel model was utilized. The channel model, illustrated in Figure 9, consists of a rectangular channel with a total length of 20 m, a depth of 50 cm, a width of 20 cm, and a longitudinal bottom slope of i = 1/400. The system includes an automatic water circulation mechanism powered by a water pump. Water flows from the inlet pool through a water-stabilizing grid, passes through the rectangular channel, flows over a rectangular weir for measurement, and exits into the retreat pool. The inlet flow rates were set at 0.030 m³/s, 0.040 m³/s, and 0.050 m³/s. The surface roughness coefficients of three types of channel linings—the common concrete lining, the polyurea-coated lining, and the MPHC lining—were calibrated using the Chezy-Manning formula. The experimental results are summarized in

Table 1. The relationship between the flow and the water depth of the channel model.

Flow/m3/s	Common Concrete Lining			Common Polyurea Coated Lining			MPHC Lining
	Water Depth 1/m	Water Depth 2/m	Roughness Coefficient	Water Depth 1/m	Water Depth 2/m	Roughness Coefficient	Water Depth 1/m	Water Depth 2/m	Roughness Coefficient
0.030	0.182	0.185	0.01445	0.178	0.180	0.01404	0.166	0.168	0.01300
0.040	0.326	0.318	0.01462	0.310	0.315	0.01412	0.290	0.295	0.01306
0.050	0.388	0.390	0.01487	0.366	0.370	0.01423	0.343	0.339	0.01315

.

As shown in Table 1, under all tested flow rates, roughness coefficients decreased progressively from the common concrete lining to the polyurea-coated lining and, lastly, the MPHC lining. The MPHC roughness coefficient was 10.0–11.6% lower than that of common concrete and 7.4–7.5% lower than the polyurea-coated lining. This reduction in the roughness coefficient can be explained by the Cassie–Baxter theoretical model. The Cassie–Baxter model explains that an air layer forms between the water flow and the MPHC surface, reducing the flow–solid interface area and adhesion. Consequently, the velocity gradient of the water flow is reduced, leading to a decrease in the shear force between the water flow and the channel sidewall. These factors collectively contribute to the smaller surface roughness coefficient observed for the MPHC.

4. Conclusions

This study addresses the challenge of achieving a low roughness coefficient in channel linings. Using the Cassie–Baxter model and hydrophobic theory, PDMS and microsilica powder were incorporated to modify ordinary polyurea, creating the non-toxic, odorless, and eco-friendly MPHC. Experimental investigations were conducted to evaluate the hydrophobicity and bond strength of the MPHC after exposure to immersion in water, high-temperature conditions, and freeze–thaw cycles. The key findings are summarized as follows:

(1)

Enhanced hydrophobicity and weather resistance: The incorporation of PDMS and microsilica powder creates a micro-rough structure on the surface of the MPHC. Compared to the ordinary polyurea coating, the contact angle of the MPHC increases significantly, indicating improved hydrophobicity. After undergoing immersion, high-temperature, and freeze–thaw treatments, the contact angle of the MPHC exhibits minimal variation, demonstrating excellent weather resistance and stable hydrophobic properties.

(2)

Bond strength performance: The MPHC bond strength is slightly reduced compared to unmodified polyurea due to the surface presence of PDMS, which slightly hinders bonding performance. However, the improvement in hydrophobicity outweighs this minor drawback. Furthermore, the bonding performance of the MPHC remains stable, with no signs of degradation observed after immersion, high-temperature, and freeze–thaw treatments

(3)

Drag reduction performance: Tests on the channel model lining reveal that the MPHC achieves the lowest roughness coefficient among the tested materials. The roughness coefficient of the MPHC is reduced by 10.0–11.6% compared to ordinary concrete and by 7.4–7.5% compared to the ordinary polyurea coating. This improvement is primarily attributed to the micro-rough structure on the MPHC surface, which reduces the flow–solid interface area and adhesion, thereby lowering the shear force between the water flow and the channel sidewall.