An Experimental Study on the Performance of Materials for Repairing Cracks in Tunnel Linings under Erosive Environments

Abstract

Addressing the current lining cracking problem in coastal tunnels, this paper independently introduces a novel type of repair material for tunnel lining cracks—the composite repair material consisting of waterborne epoxy resin and ultrafine cement (referred to as EC composite repair material). Through indoor testing, we have analyzed the change rule of the mass change rate, compressive strength, flexural strength, and chloride ion concentration of the repair material samples in erosive environments, with the dosage of each component in the EC composite repair material being varied. We have also investigated the working performance, mechanical properties, and microstructure of the repair material. The results of this study show that when the proportion of each component of ultrafine cement, waterborne epoxy resin, waterborne epoxy curing agent, waterborne polyurethane, defoamer, and water is 100:50:50:2.5:0.5:30, the performance of the EC composite repair material in a chloride ion-rich environment is optimal in all aspects. When the mixing ratio of each component of the EC composite repair material is as stated above, the repair material exhibits the best performance in a chloride ion erosion environment. With this ratio of components in the EC composite repair material, the fluidity, setting time, compressive strength, flexural strength, and bond strength of the repair material in a chloride ion erosion environment can meet the requirements of relevant specifications, and it is highly effective in repairing tunnel lining cracks. The polymeric film formed by the reaction between the waterborne epoxy resin emulsion and the curing agent fills the pores between the hydration products, resulting in a densely packed internal structure of EC composite repair material with enhanced erosion resistance, making it very suitable for repairing cracks in tunnel linings in erosive environments.

1. Introduction

During the rapid development of the economy and transportation infrastructure in recent decades, China has taken a leading position in the number of tunnels and underground projects, becoming a pillar of global road development [1]. The construction of tunnels is not only conducive to traffic flow and economic development of the surrounding cities but also promotes the development of the national economy and improves people’s living standards and sense of well-being. However, with the accumulation of time, the aging of materials and environmental and climatic factors affect the safety and service life of tunnels, resulting in the cracking of tunnel linings [2,3,4]. These cracks not only shorten the service life of tunnels but also seriously affect the reliability, stability, and safety of tunnels; hinder the smooth flow of traffic; and even threaten the safety of people’s lives and property [5,6,7].

At present, the materials used in the repair of tunnel lining cracks are mainly divided into three categories: inorganic, organic, and inorganic–organic combinations. Inorganic repair materials are mainly cement-based, including silicate cement, alumina sulphate cement, magnesium phosphate cement, ultrafine cement, and so on [8,9,10,11]. Inorganic materials are inexpensive and highly economical, but they have a lower bond strength with old concrete. Organic repair materials are mainly based on epoxy resin, which is easy to process and mold, has good corrosion resistance and flame resistance, and is widely used in the construction material market [12,13,14,15]. Zhang et al. [16] used epoxy resin paste to seal cracks less than 5 mm wide. Ye [17] repaired pipe sheet cracks and pipe sheet joints using imported rigid epoxy resin. Peng et al. [18] selected XH160 epoxy resin to fill the gap between the annular steel plate and pipe sheet. Feng [19] repaired cracks in bridge concrete structures using epoxy resin. Xiao et al. [20] synthesized two low-viscosity and room-temperature curing epoxy structural adhesives (EPA-1, EPA-2) for repairing cracks in vibrating underground tunnels. Cured epoxy resin exhibits high strength, good mechanical properties, strong adhesion to the concrete matrix, and high stability. However, a single epoxy resin system has many drawbacks, such as poor toughness, consistency, poor fluidity, and poor water resistance. Therefore, in order to improve the performance of epoxy resin, it is usually necessary to modify it with different modifying materials. Li et al. [21] modified epoxy resin with calcium carbonate nanoparticles to improve its toughness and mechanical properties. Geng et al. [22] used silica gel to improve the thermal stability, shrinkage, and strength of epoxy resin repair materials, resulting in improved epoxy resin repair properties. Lin et al. [23], by toughening epoxy resin using nano rubber, significantly increased its fracture toughness. Li [24] found that polyetheramine could enhance the toughness of epoxy-based resins at low temperatures through research. Han et al. [25] improved the toughness and penetration properties of epoxy resin repair materials by adding suitable polyurethane as a toughening agent and glycidyl ether as a diluent. Zheng et al. [26] found that the incorporation of waterborne polyurethane significantly reduces the brittleness of the repair material, ensuring enhanced efficiency in the repair process. Through modification, the epoxy resin will improve its toughness and mechanical properties, and it has the advantages of high strength and strong adhesion [27,28,29]. However, epoxy resin organic materials still have the disadvantages of being expensive, having poor durability, and having a large color contrast with concrete. Organic–inorganic combined materials have the advantages of having a dense internal structure, high bonding strength, good mechanical properties, load resistance, impermeability, and shrinkage resistance [30,31,32].

Currently, the majority of research is focused solely on repairing cracks in conventional concrete, with limited attention being given to addressing cracks in tunnel linings exposed to erosive environments [33,34]. After tunnel lining cracks develop, chloride ions can cause rapid diffusion and erosion of concrete at the crack sites. Therefore, it is crucial to develop a suitable material for repairing tunnel lining cracks in erosive environments. Against this backdrop, this paper aims to explore a tunnel lining crack repair material (EC composite repair material) for erosive environments, which combines organic waterborne epoxy resin with inorganic ultrafine cement. This paper investigates the optimal mixing ratio of each component of the EC composite repair material under chloride ion conditions and analyzes its performance.

2. Test Materials and Methods

2.1. Test Materials

EC composite repair material consists of waterborne epoxy resin, waterborne epoxy curing agent, waterborne polyurethane, defoamer, ultrafine cement, and water. The waterborne epoxy resin used in the test was Baling Petrochemical CYDW-100 waterborne resin, a transparent low-viscosity resin produced by Zhengzhou Penghui Chemical Products Co., Ltd. Zhengzhou in China. The waterborne epoxy curing agent was also a Baling Petrochemical CYDHD-220 room-temperature waterborne curing agent, produced by the same company. Its performance indexes are shown in

Table 1. Performance indicators of waterborne epoxy resin and waterborne epoxy curing agent.

Performance Indicators of Waterborne Epoxy Resin		Performance Indicators of Waterborne Epoxy Curing Agent
Epoxy equivalent (g∙mol−1)	Viscosity (mPa∙s)	Amine value (mgKOH∙g−1)	Viscosity (mPa∙s)
190–210	500–1500	500–1500	160–200

. The proportion of the use of the waterborne epoxy resin and the corresponding waterborne epoxy curing agent was 1:1.

The waterborne polyurethane used in the experiments was PU-300C, a waterborne polyurethane produced by Hefei Terak New Material Technology Co., Ltd. Hefei in China. It had the appearance of a milky-white emulsion, a pH value of 7–9, and a solid content of 35%. The defoamer used in the experiment was tributyl phosphate (TBP), with a relative density of 0.9766 (g/mL, 20/4 °C), produced by Shandong Yusuo Chemical Technology Co. The ultrafine cement used in the experiment was P-O42.5 ultrafine cement, produced by Beijing Zhongde Xinya Construction Technology Co., Ltd. Beijing in China. It had a specific surface area of 994 m²/kg and a volume average particle size of D[4,3] = 25.9 μm. The main chemical compositions of the ultrafine cement are shown in

Table 2. Main chemical composition and percentage content of ultrafine cement.

Chemical Composition	CaO	SiO2	Al2O3	Fe2O3	SO3	MgO	Loss
Percentage Content (%)	59.67	23.21	5.83	3.34	2.12	1.18	2.13

.

The sodium chloride solution used to simulate the erosion environment was sodium chloride (NaCl), analytically pure, with a NaCl content higher than 99.5 percent, produced by Tianjin Chemical Industry No. 3 Factory Co., Ltd. Tianjin in China.

2.2. Specimen Preparation and Maintenance

When preparing EC composite repair material, firstly, place the weighed waterborne epoxy resin and waterborne epoxy curing agent into the beaker and mix them evenly, stirring for 5 min. Then, pour the weighed waterborne polyurethane into the beaker and mix it evenly with the waterborne epoxy resin and waterborne epoxy curing agent to form the waterborne epoxy resin component. Next, place the weighed ultrafine cement and water and mix them evenly to form the cement paste component. Finally, add a defoamer to form the EC composite repair material after thorough mixing. Pour the homogeneous repair material into the mold, vibrate and compact it, scrape the surface flat, cover it with cling film, and leave it indoors at room temperature for 24 h for demolding and sampling. In order to reduce the error of the test, according to the different erosion ages and the grouping of different ratios of materials, each group of tests is prepared with 6 parallel specimens. After the specimens are demolded, all the specimens are placed into the chloride ion solution for 7 d and 28 d, respectively.

2.3. Pilot Test Items

(1)

Rate of Mass Change

After the specimens with different ratios are demolded, the mass is weighed by an electronic scale with an accuracy of 0.1 g, which is the initial mass m₀ before erosion; then, the labelled specimens are placed into the chloride solution for erosion; after the specimens are taken out of the molds after the erosion is completed, the mass m₁ is weighed after wiping the surface moisture lightly; the rate of mass change ${\mathsf{\Delta }}_{\mathrm{m}}$ is calculated according to Equation (1).

$${\mathsf{\Delta }}_{\mathrm{m}}=\left({\displaystyle \frac{{\mathrm{m}}_1 –{ \mathrm{m}}_0}{{\mathrm{m}}_0}}\right)\times 100\%$$

(1)

where ${\mathsf{\Delta }}_{\mathrm{m}}$ is rate of mass change; m₀ is the initial mass of the specimen before erosion, g; and m₁ is the mass after the test erosion is completed, g.

(2)

Work Performance

Setting time: refer to GB/T1346-2011 [35] Cement Standard Consistency Water Consumption, Setting Time, Stability Test Method; fluidity: tested in accordance with GB/T8077-2012 [36] Test Method for Homogeneity of Concrete Admixtures.

(3)

Mechanical Properties

Flexural and compressive strength are tested with reference to GB/T17671-1999 [37] Strength Test of Cementitious Mortar.

(4)

Bond Strength

With reference to the standard JC/T2381-2016 [38] Repair Mortar, the bond strength of the repair material is expressed through the interface flexural and tensile strength. Using an ISO standard sand mold, a 40 mm × 40 mm × 160 mm substrate cementitious mortar test block is created. After breaking, the fractured piece is placed into the original test mold, and the other half of the mold is filled with EC composite repair material. After maintaining it for the specified age, the flexural strength test mentioned above is conducted. This flexural strength represents the bond strength. The test model is shown in Figure 1.

(5)

SEM Microstructure Analysis

SEM is used to observe and analyze the hydration products of the repair material of 28 days.

3. Optimal Proportion of EC Composite Repair Material

3.1. Optimum Content of Waterborne Epoxy Resin

The content of waterborne epoxy resin affects the performance of EC composite repair material. To determine the optimal content of waterborne epoxy resin for EC composite repair material in a chloride ion erosion environment, the water/cement ratio is set to an optimal ratio of 0.3; the ratio of ultrafine cement to waterborne epoxy resin is 1:X; and the ratio of waterborne epoxy resin components is set to waterborne epoxy resin/waterborne epoxy curing agent/waterborne polyurethane/defoamer = 1:1:0.2:0.01. Then, the content X of waterborne epoxy resin is set to 20%, 30%, 40%, 50%, 60%, and 70%.

The measured mass change rate, unconfined compressive strength, flexural strength, and chloride ion concentration under different contents of waterborne epoxy resin are shown in Figure 2.

As seen in Figure 2a, in a chloride ion erosion environment, for the 7 d specimens, the minimum mass change rate occurs when the waterborne epoxy resin content is 50%, resulting in a mass increase of 1.8 g. For the 28 d specimens, the minimum mass change rate is achieved with a waterborne epoxy resin content of 70%, leading to a mass increase of 1.95 g. However, at a waterborne epoxy resin content of 50%, the mass increases by 3.1 g. The values of mass change for the specimens under different waterborne epoxy resin contents are presented in

Table 3. Mass change rate and mass change value of different waterborne epoxy resin content.

Waterborne Epoxy Resin Content	Age
	7 d		28 d
	Rate of Mass Change (%)	Value of Mass Change (g)	Rate of Mass Change (%)	Value of Mass Change (g)
20%	2.350	4.55	2.838	5.60
30%	1.731	3.15	3.454	6.45
40%	1.124	2.00	3.082	5.43
50%	1.060	1.80	1.886	3.1
60%	1.389	2.25	1.288	2.16
70%	1.438	2.05	1.312	1.95

. Analyzing the reasons for the mass change, the main components in cement, such as C3S and C2S, undergo hydration reactions by combining with the water absorbed from the solution, producing hydration products like C-S-H gel, ettringite, and calcium hydroxide. This explains the mass increase observed at each content. As the waterborne epoxy resin content increases, it encapsulates and adsorbs a portion of the cement, hindering its hydration reaction and interfering with cement hydration, resulting in a decreasing trend in the mass change rate. When the waterborne epoxy resin content exceeds 50%, excessive doping can lead to the formation of air bubbles during the mixing of EC composite repair material. This results in a slightly larger pore size within the internal structure of the EC composite repair material, allowing chloride ion solutions to penetrate the specimen’s interior more quickly, thereby causing an increase in mass change and an upward trend in the mass change rate.

As evident from Figure 2b, in a chloride ion erosion environment, for specimens cured for 7 days, as the amount of waterborne epoxy resin increases, their unconfined compressive strength generally decreases. However, for specimens cured for 28 days, the unconfined compressive strength first increases and then decreases with increasing waterborne epoxy resin content. At a waterborne epoxy resin content of 50%, the unconfined compressive strength of the 28 d specimens reaches its maximum value of 28.760 MPa. The specific changes in unconfined compressive strength with erosion time are summarized in

Table 4. Compressive strength of different amounts of waterborne epoxy resin.

Age	Compressive Strength (MPa)
	20% Waterborne Epoxy Resin	30% Waterborne Epoxy Resin	40% Waterborne Epoxy Resin	50% Waterborne Epoxy Resin	60% Waterborne Epoxy Resin	70% Waterborne Epoxy Resin
7 d	26.650	21.253	17.982	21.724	15.94	13.971
28 d	22.504	21.199	24.171	28.760	27.540	20.337

. Analyzing the reasons for the change in unconfined compressive strength, we find that as the waterborne epoxy resin content increases, its composition affects the hydration reaction of a portion of the cement, resulting in a downward trend in unconfined compressive strength. Since the 28 d specimens are cured for a longer duration compared to the 7 d specimens, there is less of a downward trend and more of an upward trend. As the waterborne epoxy resin content continues to increase, it provides a partial source of strength, and its good resistance to chloride ions enhances the strength of the EC composite repair material, leading to an upward trend in unconfined compressive strength. However, when the waterborne epoxy resin content exceeds 50%, its inhibitory effect on cement hydration becomes more severe, coupled with the influence of a small number of air bubbles, reducing chloride ion resistance and increasing the mass change rate, ultimately resulting in a downward trend in strength. When the waterborne epoxy resin content is 20% and 30%, the strength of the 7 d specimens is higher than that of the 28 d specimens. This is primarily due to the fact that the unconfined compressive strength is mainly enhanced by the hydration products generated by cement hydration and the reaction and solidification of waterborne epoxy resin components. In the 28 d specimens, erosive ions gradually saturate and produce a large amount of F’s salt adhering to the sample surface, affecting the cement hydration process and reducing strength. Additionally, the cement components are less resistant to chloride ion erosion compared to waterborne epoxy resin, and their strength decreases with a longer curing time in a chloride ion environment. Once the waterborne epoxy resin content exceeds 30%, the specimens’ resistance to chloride ions increases, and the solidification of waterborne epoxy resin promotes the formation of hydrated calcium silicate, a cement hydration product, leading to higher unconfined compressive strength in the 28 d specimens [39].

As depicted in Figure 2c, with the increasing content of waterborne epoxy resin, the flexural strengths of the 7 d and 28 d cured specimens show a trend of decreasing, then increasing, and then decreasing again. The flexural strength of the 7 d specimens reaches a maximum value of 5.244 MPa at 60% waterborne epoxy resin; then, they reach 4.914 MPa at 50%, which is an increase of 98.79% and 86.28% over the minimum value of 2.638 MPa (at 40% waterborne epoxy resin). For the 28 d cured specimens, the flexural strengths at 20%, 50%, and 60% waterborne epoxy resin are 7.983 MPa, 8.188 MPa, and 8.255 MPa, respectively, indicating a minor difference in flexural strengths. The changes in the flexural strengths with the content of waterborne epoxy resin are shown in

Table 5. Flexural strength of waterborne epoxy resin with different content.

Age	Flexural Strength (MPa)
	20% Waterborne Epoxy Resin	30% Waterborne Epoxy Resin	40% Waterborne Epoxy Resin	50% Waterborne Epoxy Resin	60% Waterborne Epoxy Resin	70% Waterborne Epoxy Resin
7 d	4.144	3.303	2.638	4.914	5.244	4.377
28 d	7.983	6.620	6.907	8.188	8.255	7.862

. Compared with a minimum value of 6.62 MPa (when the content of waterborne epoxy resin is 30%), flexural strength increases by 20.59%, 23.69%, and 24.70%. In summary, the specimens achieve their maximum flexural strength at 60% waterborne epoxy resin in a chloride ion environment, followed closely by 50%. The minimum flexural strength varies slightly between the 7 d and 28 d specimens, with the 7 d specimens achieving the lowest value at 40% waterborne epoxy resin, while the 28 d specimens achieve the lowest value at 30% waterborne epoxy resin.

At a 50% content of waterborne epoxy resin, the EC composite repair material exhibits the lowest chloride ion concentration and the strongest resistance to chloride ion erosion. Taking into account the effects of the waterborne epoxy resin’s content on the rate of mass change, unconfined compressive strength, flexural strength, and chloride ion concentration, the optimal content of waterborne epoxy resin under chloride ion erosion conditions is determined to be 50%.

3.2. Optimal Content of Waterborne Polyurethane

Currently, it has been discovered that waterborne epoxy resin exhibits high brittleness. By adding waterborne polyurethane, it can not only enhance the toughness, impact resistance, and load-bearing capacity of waterborne epoxy resin but also improve its resistance to chloride ion erosion, thus enhancing the applicability of EC composite repair material in chloride ion erosion environments. Through experimental studies in Section 3.1, the optimal content of waterborne epoxy resin in ultrafine cement under chloride ion erosion conditions is determined to be 50%. The dosage of defoamer is temporarily controlled at 1% of the waterborne epoxy resin. The ratio of waterborne epoxy resin to waterborne polyurethane is designed as 1:X, where the dosage of X of waterborne polyurethane is set to 0%, 5%, 10%, 15%, 20%, and 25%. The measured mass change rate, compressive strength, flexural strength, and chloride ion concentration at different waterborne polyurethane contents are shown in Figure 3.

From Figure 3a, it can be observed that in a chloride ion erosion environment, the mass change rate of the 7 d specimen peaks at 10% waterborne polyurethane, with a mass increase of 3.6 g. Conversely, the mass change rate of the 28 d specimen peaks at a 5% waterborne polyurethane, with a mass increase of 4.78 g. The mass change rates of the specimens under different waterborne polyurethane contents are presented in

Table 6. Mass change rate and mass change value of different waterborne polyurethane.

Waterborne Polyurethane Content	Age
	7 d		28 d
	Rate of Mass Change (%)	Value of Mass Change (g)	Rate of Mass Change (%)	Value of Mass Change (g)
0%	1.312	2.25	2.142	3.60
5%	1.446	2.50	2.545	4.75
10%	2.076	3.60	1.461	2.55
15%	2.026	3.45	2.337	4.05
20%	1.060	1.80	1.886	3.10
25%	1.751	2.95	1.737	2.90

. Analyzing the reasons for the mass change, the increase in specimen mass is primarily attributed to the hydration products generated by cement hydration and the reaction of waterborne epoxy resin components during curation. As the waterborne polyurethane content increases, it enhances the toughness of the waterborne epoxy resin and improves its resistance to chloride ion erosion, thus initially leading to an upward trend in the mass change rate. However, for the 28 d specimen, when the waterborne polyurethane content exceeds 5%, excessive doping causes the waterborne epoxy resin emulsion to agglomerate, reducing its toughening effect. This allows chloride ion solutions to penetrate the interior of the specimen, damaging its internal cohesion and resulting in a downward trend in the mass change rate. Due to the shorter curing time of the 7 d specimen, its exposure to chloride ion erosion is minimal. Consequently, the mass change rate only begins to decline when the waterborne polyurethane content exceeds 10%. Furthermore, as waterborne polyurethane itself possesses a degree of resistance to chloride ion erosion, an increase in its dosage initially leads to a temporary rise in the mass change rate. However, for the 28 d specimen, when the waterborne polyurethane content exceeds 15%, the polyurethane component absorbs a portion of the cement, hindering its hydration reaction. Coupled with the influence of chloride ion erosion, this results in a decline in the mass change rate. Specifically, the inhibition of cement hydration due to chloride ion erosion and polyurethane doping explains why the mass change rates of 7 d specimens with 10% and 25% polyurethane doping are higher than those of the 28 d specimens.

From Figure 3b, it can be observed that when the waterborne polyurethane content is 5%, the unconfined compressive strength (UCS) reaches its maximum, achieving 26.439 MPa for the 7 d specimens and 32.085 MPa for the 28 d specimens, indicating the highest load-bearing capacity. As the waterborne polyurethane content increases, it enhances the UCS of the EC composite repair material. The specific values of the compressive strengths under different waterborne polyurethane contents are shown in

Table 7. Compressive strength of different amounts of waterborne polyurethane.

Age	Compressive Strength (MPa)
	0% Waterborne Polyurethane	5% Waterborne Polyurethane	10% Waterborne Polyurethane	15% Waterborne Polyurethane	20% Waterborne Polyurethane	25% Waterborne Polyurethane
7 d	18.719	26.439	23.952	21.128	21.724	21.554
28 d	31.435	32.160	30.264	28.781	28.760	28.648

. Analyzing the reason for the change in compressive strength, it is due to the waterborne polyurethane’s resistance to chloride ion erosion, which results in a higher UCS than that of the blank group without the addition of waterborne polyurethane. At a 5% doping amount, the waterborne polyurethane not only improves the toughness and compactness of the EC composite repair material but also exhibits anti-chloride ion properties. Additionally, due to the low dosage of polyurethane, its inhibitory effect on cement hydration is minimal, allowing the UCS to reach its maximum value. However, when the doping amount of waterborne polyurethane exceeds 5%, its impact on cement hydration surpasses its anti-chloride ion performance, leading to a decrease in strength. As the polyurethane content continues to increase and reaches 15%, the anti-chloride ion effect and the inhibition of cement hydration balance each other, resulting in a stable strength with no significant upward or downward trends. Data analysis suggests that excessive doping of waterborne polyurethane can lead to a reduction in the compressive strength of the EC composite repair material; therefore, excessive doping should be avoided.

From Figure 3c, it can be seen that with the increase in waterborne polyurethane content, the flexural strength exhibits a trend of first increasing and then decreasing, followed by another increase and subsequent decrease. For 28 d cured specimens, when the content of waterborne polyurethane is 5%, the flexural strength increases by 20.5% compared to that of the unadulterated polyurethane. However, when the doping of waterborne polyurethane exceeds 5%, the flexural strength begins to decrease and falls below that of the unadulterated polyurethane. The specific values of flexural strength are shown in

Table 8. Flexural strength of waterborne polyurethane with different content.

Age	Flexural Strength (MPa)
	0% Waterborne Polyurethane	5% Waterborne Polyurethane	10% Waterborne Polyurethane	15% Waterborne Polyurethane	20% Waterborne Polyurethane	25% Waterborne Polyurethane
7 d	6.397	7.625	4.785	4.337	5.016	4.767
28 d	8.418	10.144	7.187	6.614	7.779	6.073

. Analyzing the reason for the change in flexural strength, a moderate amount of waterborne polyurethane can enhance the flexural strength of EC composite repair material, but an excessive amount of waterborne polyurethane has a negative impact on the flexural strength of EC composite repair material. Under the same doping level, the flexural strength increases with the increase in erosion time. Adding an appropriate amount of waterborne polyurethane can improve the flexural strength of EC composite repair material. Firstly, this is due to the addition of waterborne polyurethane improving resistance to chloride erosion. Secondly, the reaction between waterborne polyurethane and waterborne epoxy resin forms a polymer emulsion film that fills the pore space of the cement, creating a tightly bonded internal structure. When EC composite repair materials are subjected to external forces, the emulsion film can improve the flexural strength by absorbing part of these external forces [40]. When the doping of waterborne polyurethane is 5%, the flexural strength reaches its optimal level.

The impact of the dosage of waterborne polyurethane on chloride ion concentration exhibits slight variations between the 7 d and 28 d aged specimens. Specifically, at 7 d, the lowest chloride ion concentration is observed with a dosage of 5%. Conversely, at 28 d, the lowest concentration is achieved with a dosage of 25%, followed closely by 5%. After a comprehensive analysis considering the effects on mass change rate, unconfined compressive strength, flexural strength, and chloride ion concentration, it has been determined that the optimal dosage of waterborne polyurethane under chloride ion erosion conditions stands at 5%.

3.3. Optimum Defoamer Dosage

When waterborne epoxy resin is mixed with ultrafine cement to form EC composite repair material, under the influence of the surface activity of the waterborne epoxy emulsion, the EC composite repair material can introduce some gas during the mixing process. This gas can form macro pores after the hardening of the EC composite repair material, damaging the compactness of the repair material and seriously affecting its performance. The addition of a defoamer can effectively eliminate the impact of gas on the performance of the EC composite repair material, enhancing its applicability. As indicated in Section 3.1 and Section 3.2, under chloride ion erosion conditions, the optimal mixing ratio of waterborne epoxy resin to ultrafine cement is 50%, and the optimal mixing ratio of waterborne polyurethane to waterborne epoxy resin is 5%. With the ratio of waterborne epoxy resin/waterborne epoxy curing agent/waterborne polyurethane/ultrafine cement/water = 50:50:2.5:100:30, the dosage of defoamer in the waterborne epoxy resin is designed to be 0%, 0.5%, 1%, 1.5%, and 2%. The mass change rate, compressive strength, flexural strength, and chloride ion concentration at different defoamer dosages are presented in Figure 4.

From Figure 4a, it can be observed that in a chlorine ion erosion environment, when the defoamer dosage is 0%, the mass change rate is the highest, reaching 2.868% at 7 d and 3.015% at 28 d, corresponding to an increase in mass of 3.85 g and 5.55 g, respectively. When the defoamer dosage is 1%, the mass change rate is the lowest, standing at 1.446% at 7 d and 2.545% at 28 d, representing a decrease of 85.75% and 18.47% compared to the 0% dosage, respectively. After the defoamer dosage exceeds 1%, its impact on the mass change rate becomes less significant, leading to a slight upward trend in the mass change rate, which still remains lower than that of the 0% dosage.

From Figure 4b and

Table 9. Compressive strength of different defoamer amounts.

Age	Compressive Strength (MPa)
	0% Defoamer	0.5% Defoamer	1% Defoamer	1.5% Defoamer	2% Defoamer
7 d	20.44	24.85	26.44	23.71	23.44
28 d	27.93	30.15	32.16	29.20	28.64

, it can be observed that when the defoamer content is 1%, the compressive strengths of the 7 d and 28 d specimens are the highest, reaching 26.44 MPa and 32.16 MPa, respectively, representing an increase of 6 MPa and 4.23 MPa compared to the unadulterated blank group. The defoamer can alleviate the issue of reduced EC composite repair material strength caused by the air-entraining property of waterborne epoxy resin. Notably, at the same defoamer content, the improvement in 7 d strength is more significant, resulting in a high early-stage strength of the EC composite repair material, which is beneficial for repairing tunnel lining cracks. The optimal enhancement effect on compressive strength is achieved when the defoamer content is 1%. Further increasing the content leads to a diminishing improvement in compressive strength. This is because within a certain dosage range, the defoamer can effectively reduce the number and size of harmful pores in the EC composite repair material, thereby improving its compactness and pore structure, leading to a substantial increase in compressive strength. However, when the defoamer content exceeds a certain level, its defoaming ability is compromised after reaching the critical concentration of colloidal groups, resulting in no further increase or even a slight decrease in compressive strength.

As shown in Figure 4c and

Table 10. Flexural strength of different defoamer dosages.

Age	Flexural Strength (MPa)
	0% Defoamer	0.5% Defoamer	1% Defoamer	1.5% Defoamer	2% Defoamer
7 d	3.387	5.432	7.625	5.567	4.590
28 d	7.162	8.858	10.144	9.358	8.111

, the flexural strength of the 7 d and 28 d specimens peaks at a 1% dosage of the defoamer, showing an increase of 125.13% and 41.64% over the non-doped counterparts, respectively, and exhibiting the best resistance to bending deformation. This indicates that the defoamer can significantly enhance the flexural strength of EC composite repair material, but the enhancement effect becomes less obvious when the doping content exceeds 1%. TBP can be adsorbed at the water/gas interface, reducing the viscosity of the liquid film to accelerate gas discharge, decreasing the number of detrimental pores, and increasing the internal structural compactness of EC composite repair material, significantly enhancing their flexural strength.

As shown in Figure 4d, the chlorine ion concentration of the specimen reaches its lowest level when the defoamer dosage is 1%, representing a decrease of 81.29% and 48.51% compared to the blank group without defoamer for the 7 d and 28 d specimens, respectively. In summary, in a chloride ion erosion environment, the EC composite repair material exhibits optimal performance in terms of the mass change rate, compressive strength, flexural strength, and chloride ion concentration when the defoamer dosage is 1%. Above 1%, the enhancement effect becomes less significant. Therefore, it is determined that the optimal defoamer dosage in a chloride ion erosion environment is 1%.

A comprehensive analysis of the above experiments revealed that the EC composite repair material achieved optimal performance in an erosive environment when the waterborne epoxy resin accounted for 50% of the mass of ultrafine cement, while waterborne polyurethane and defoamer constituted 5% and 1% of the waterborne epoxy resin’s mass, respectively. Specifically, the optimal proportion of EC composite repair material in the erosive environment was as follows: superfine cement/water-based epoxy resin/water-based epoxy curing agent/water-based polyurethane/defoamer/water = 100:50:50:2.5:0.5:30.

4. EC Composite Repair Material Performance

4.1. Flowability of EC Composite Repair Material

Flowability is one of the critical properties of crack repair materials. Only repair materials with excellent flowability can penetrate deep into cracks, fully fill the defects, and achieve the optimal repair effect. After testing, it is found that the flowability of EC composite repair material with the optimal ratio is 181 mm, meeting the grouting requirements for repair materials. Therefore, EC composite repair material can be used to repair tunnel lining cracks.

4.2. Setting Time of EC Composite Repair Material

The setting time of repair materials is one of the important indicators that reflects their performance. The setting time of EC composite repair material is measured through a setting time measurement test and is presented in

Table 11. Setting time of EC composite restorative materials.

Setting Time	Starting Time	End Time	Time/min
Initial setting time	16:20:35	17:32:42	72
Final setting time	16:20:35	17:55:36	95

.

The initial setting time of EC composite repair material is 72 min, which provides ample time for mixing and grouting during construction, conducive to on-site construction operations. The final setting time is 95 min, enabling EC composite repair material to bond quickly in cracks and achieve rapid crack repair.

4.3. Compressive Strength of EC Composite Repair Material

The compressive strength curves of the EC composite repair material and CM repair material (used as the control group) are shown in Figure 5.

According to Figure 5, in a chloride ion environment, the 7 d compressive strength of EC composite repair material is 26.44 MPa, and the 28 d compressive strength is 32.16 MPa. The 7 d compressive strength reaches 82.21% of the 28 d compressive strength. It can be seen that the strength of the EC composite repair material grows faster in the early stage, and it is 2.07, 1.85, and 1.733 times higher than the compressive strength of CM repair material specimens in the same period. Furthermore, compared with CM repair material, EC composite repair material contains waterborne epoxy resin organic components, which can react with cement hydration products to form a dense three-dimensional mesh structure. EC composite repair material has the advantages of high pre-strength, small shrinkage and deformation, strong adhesion, good water resistance, strong resistance to chlorine ion erosion, and high ultimate strain.

The 7 d compressive strength of the EC composite repair material can reach 26.44 MPa, while its 28 d compressive strength can reach 32.16 MPa. According to Code for Design of Concrete Structures (GB50010-2010 [41]), the standard axial compressive strength values for C30 and C40 concrete are 20.1 MPa and 26.8 MPa, respectively. The 28 d compressive strength of this repair material significantly exceeds these standards. Therefore, EC composite repair material is fully capable of replacing a portion of concrete as the secondary lining of tunnels, and it can also satisfy the compressive strength requirements for cracks in tunnel lining structures.

4.4. Flexural Strength of EC Composite Repair Material

The specimens, when maintained in the chloride ion solution for the corresponding age, are taken out, and the flexural strengths of the EC composite repair material and CM repair materials are measured through flexural testing. The results are shown in Figure 6.

As can be seen from Figure 6, the 7 d, 14 d, and 28 d flexural strengths of EC composite repair material are 7.62 MPa, 8.92 MPa, and 10.15 MPa, respectively, which are 3.37 times, 2.50 times, and 2.44 times higher than the flexural strengths of CM repair material specimens of the same age. The EC composite repair material exhibits high early flexural strength and strong resistance to bending deformation, making it suitable for meeting the requirements of tunnel lining crack repair in erosive environments.

4.5. Bond Strength of EC Composite Repair Material

Bond strength is an important index for evaluating the repair effect of crack repair materials, and the quality of the repair materials primarily depends on the bonding force between the old and new interfaces. The measured bond strengths of EC composite repair material and CM repair material in a chloride ion erosion environment are shown in Figure 7.

From Figure 7, the bond strengths of the EC composite repair material at 1 d, 3 d, 7 d, and 28 d are 0.79 MPa, 1.16 MPa, 1.88 MPa, and 2.62 MPa, respectively. For the CM repair material, the bond strengths at 1 d, 3 d, 7 d, and 28 d are 0.3 MPa, 0.96 MPa, 1.22 MPa, and 2.5 MPa, respectively; thus, they are all lower than those of the EC composite repair material. According to JCT2381-2016 [38] Repair Mortar, the interface bending tensile strength (i.e., bond strength) for ordinary flexible repair mortar after 28 days is ≥1.5 MPa, while that for ordinary rigid repair mortar is ≥2 MPa. This repair material meets the specification requirements, possesses excellent bonding properties, and is capable of effectively bonding tunnel lining cracks, preventing secondary cracking, and enhancing the service life of the lining.

5. SEM Test Analysis of EC Composite Repair Material

The EC composite repair material and CM repair material after 28 d of maintenance in a chloride ion solution are observed with an electron scanning microscope at magnifications of 8000× and 30,000×, respectively. The results of the scanned images are shown in Figure 8.

As can be seen from Figure 8, the SEM diagrams of EC composite repair material and CM repair material are quite different. It can be observed from Figure 8a,c that the hydration products of CM repair materials exhibit a significant presence of needle-like ettringite crystals and flocculated C-S-H gels, accompanied by some individual particles. The internal structure is relatively loose, with numerous pores and cracks, indicating a lack of tight material connections. Conversely, as depicted in Figure 8b,d, the hydration products of EC composite repair materials demonstrate a denser distribution of substances, such as C-S-H gel and flocculent ettringite. A membrane-like material attached to the cement hydration products can also be seen. This membrane-like material is a polymer film produced by the reaction between the waterborne epoxy resin emulsion and waterborne epoxy curing agent. The polymer membranes fill the pores between the hydration products, allowing the hydration products to interconnect closely through the polymer membrane, thus reducing the internal porosity. It is this polymer film that gives the EC composite repair material a dense internal structure, excellent mechanical properties, and bond strength. It impedes chloride ion infiltration and improves anti-erosion performance. By comparing the SEM diagrams, it can be concluded that EC composite repair material is a suitable material for repairing cracks in tunnel linings in erosive environments.

6. Conclusions

(1)

When the composition ratio of the EC composite repair material is set at ultrafine cement/waterborne epoxy resin/waterborne epoxy curing agent/waterborne polyurethane/defoamer/water = 100:50:50:2.5:0.5:30, the material achieves optimal performance in terms of the mass change rate, compressive strength, flexural strength, and resistance to chloride ion erosion in a chloride ion environment.

(2)

The EC composite repair material with an optimal composition ratio exhibits a fluidity of 181 mm, an initial setting time of 72 min, and a final setting time of 95 min, which meets the relevant specification requirements and effectively repairs tunnel lining cracks.

(3)

In a chloride ion erosion environment, the EC composite repair material with an optimal composition ratio achieves a 7-day compressive strength of 26.44 MPa and a 28-day compressive strength of 32.16 MPa, which are 1.85 times and 1.733 times higher than that of the CM repair material, respectively. Its 7 d flexural strength is 7.62 MPa, and its 28 d flexural strength is 10.15 MPa, representing values that are 2.50 times and 2.44 times higher than flexural strength of the CM repair material. Additionally, its 1 d bond strength reaches 1.86 MPa, 7 d bond strength reaches 2.82 MPa, and 28 d bond strength reaches 3.02 MPa, all exceeding the CM repair material. These values satisfy both specifications and the strength requirements for tunnel lining crack repair materials.

(4)

SEM showed that the internal structure of CM material is relatively loose, with many pores and cracks, resulting in poor material connectivity. In contrast, the EC composite repair material exhibited a denser distribution of hydration products, such as C-S-H gel and plate-like ettringite. The polymeric film formed by the reaction between the waterborne epoxy resin emulsion and the curing agent filled the pores between the hydration products, giving the EC composite repair material a dense internal structure, excellent mechanical properties, strong bond strength, and outstanding resistance to erosion, making it suitable for repairing tunnel lining cracks in erosive environments.