Adhesion Strength Analysis of Synthetic Polymer Rubberized Gel on Diversely Wetted Concrete Surfaces

Abstract

In this study, the relationship between viscosity of synthetic polymer rubberized gel (SPRG) used for waterproofing and adhesive strength on varying degrees of wetted surfaces is examined. First, SPRG specimens of different ratios of viscosity modifiers were prepared (3% to 10%, interval of 1% per specimen type). These specimens were installed onto substrate surfaces of varying degrees of humidity ratio (0%, 10%, 20%, 30%, 50%, 70% and 100%). Upon following the standard installation procedure of the SPRG specimens, standard pull-off tests were conducted. Results of the test showed that a consistent relationship exists between the viscosity of synthetic polymer rubberized gel (SPRG) and its adhesive strength on different degrees of wetted surfaces. Linear regression analysis and subsequent calculation of the respective coefficient of determination leads to the result that viscosity is a relevant factor for achieving stable adhesion when it comes to the usage of SPRG waterproofing. The findings of this study could have significant implications for the development and application of adhesive gel waterproofing materials in various industries.

1. Introduction

One conventional waterproofing method for concrete surfaces is the use of synthetic polymer rubberized gels (SPRG), which can adhere strongly to the surface and create a protective barrier against water penetration. SPRGs are waterproofing material that are composed of synthetic rubber and process oil as their main ingredients, and they have a gel-like state in solid form with adhesive properties. SPRGs are mainly used in non-exposed parts of construction and civil engineering sites, and are most commonly used for external waterproofing of underground structures.

Viscosity is a measure of a fluid’s resistance to flow, while adhesion is the ability of a substance to stick to a surface. In this regard, it naturally follows that highly viscous fluids or materials are more likely to adhere to a surface than less viscous fluids [1]. However, the effectiveness of adhesion is influenced by the degree of wetness of the surface, as moisture can interfere with the adhesive strength and, in turn, overall waterproofing performance [2]. Existing studies indicate that when a surface is wet, it can affect the surface energy and surface tension of the surface, which can in turn affect the ability of a material or fluid to adhere to it, such as when a hydrophilic surface will have a lower contact angle with water when it is wet, which can increase the adhesive forces between the surface and the fluid [3]. However, in the case of an SPRG installation area (substrate surface), where moisture and condensation occur frequently, if adhesion performance to the construction surface is not secured, moisture can be generated through the interface with the construction surface, and, further, waterproofing performance deterioration and leakage can occur through the lifting problem of the waterproofing layer. Therefore, for waterproofing materials applied to underground structures construction sites, securing adhesion performance in humid conditions can be considered important for long-term waterproof durability. In contrast, viscosity is a property of the fluid or material itself, hence the relationship with adhesion effectiveness to the wetness of the surface is not mutually correlative in every case [4]. In the case of SPRG, there has not yet been an officially published source that investigates this parameter and outlines a quantitative relation between SPRG viscosity and adhesion performance on a wet surface.

The property of SPRG that is most sought after in the field of leakage repair and waterproofing is its viscous property, which allows the material to respond to substrate movement (crack displacement), and therefore sufficient fluidity should be secured [5]. Furthermore, unlike asphalt mastics, SPRGs have been able to exhibit construction and adhesive performance in moderate environments. In contrast, while it is generally accepted that higher viscosity will help waterproofing and adhesion performance, if the viscosity is too high then the SPRG workability and response to substrate movement will decrease [6]. As of this moment, very few studies have been concerned with any experimental results that discuss a balanced trade-off between viscosity and workability/performance [7]. With regards to SPRG materials, on the topic of adhesion/cohesion, there have been recent studies on the effectiveness and significance of cohesive strength and developing a quantifiable evaluation method [8]. Preeda et al. also provide a study on the effect of moisture on the bonding of surface of aggregates [9]. Pertrie summarizes with clarity the methods by which moisture affects adhesives, sealants and coatings [10]. Francke et al., in recent studies, provide an experimental study on the mechanics of adhesion failure of waterproofing coatings on terrace tiles [11]. Jin et al. present studies on the adhesion performance of waterproof coatings on frozen substrate surfaces of concrete bridge decks [12]. Youkou et al. provide an extensive study on the three dimensional material point method based on single-root complex that can be referenced for material analysis [13]. Yeon et al. also discuss the impact of wet surface adhesion when under the effect of substrate displacement of waterproofing materials.

The presence of studies on the theme of adhesion performance and moisture indicates that waterproofing construction can often proceed without properly securing a dry environment, and the employed waterproofing materials are not able to secure proper adhesion. However, there is still a significant lack of understanding, and high-level academic investigations being undertaken on this subject with regards to waterproofing materials. SPRGs are a relatively new type of waterproofing material that have entered the market; in the case of China and Korea, SPRGs have been used without the proper analysis on their limitation with regards to wet surface adhesion. For these reasons, it is important to investigate the adhesion strength and waterproofing performance of SPRGs on varying degrees of wetted concrete surfaces. This study aims to analyze the adhesion strength and waterproofing properties of a specific type of synthetic polymer rubberized gel on concrete surfaces with different levels of wetness. The following sections will discuss the methodology, whereby SPRGs of different viscosities (types D1 to D8, with the viscosity modifier incrementally increasing by 1%) will be installed on concrete substrates. The concrete substrates will be prepared such that during SPRG installation the substrates will have varying levels of humidity ratio (0%, 10%, 20%, 30%, 50%, 70%, and 100%), repeated 3 times for each conditioned substrate type. The SPRG specimens will undergo standard adhesive strength testing, whereby the measured adhesive strengths will be compared between the different conditions. The results will then be plotted on linear regression graphs throughout the humidity conditions and a coefficient of determination will be used to assert the reliability and consistency of the data and an optimal viscosity will be provided. The results of this study can provide insight into the suitability of synthetic polymer rubberized gels for waterproofing applications on wetted concrete surfaces, and help guide the selection and use of these materials in civil engineering projects.

2. Materials and Methods

2.1. Material Composition and Properties

Experimental studies on the development of asphalt based grout injection sealant made of waste oil and waste rubber confirmed that increased viscosity leads to an increase in adhesive strength, and in turn an increase in hydrostatic pressure resistance [14]. On this premise, a viscosity modifier has been used to increase the viscosity of SPRG during the manufacturing of recycling type adhesive components, and the adhesive gel is mixed by adjusting the content to enhance the waterproofing performance.

Based on the different compositions (represented hereafter by D1–D8 per different viscosity modifier ratio content) of the specimen, a preliminary set of testing was conducted: (1) viscosity measurement; and (2) a Land and Housing (LH) specification based testing of wet surface adhesion, in compliance with International Standard Organization (ISO) Technical Specification (TS) 16774 Test methods for repair materials for water-leakage cracks in underground concrete structures—Part 4: Test method for adhesion on wet concrete surface [15]. The ISO TS 16774 test was conducted as a means to check ahead of time whether or not a pull-off test is possible by confirming surface adhesion onto the substrate surface.

For this study, one product type of SPRG was selected and prepared with different viscosity-adjusting agent concentrations to investigate the adhesion performance on wet substrate surface. The concentration of viscosity-adjusting agents was formulated by incrementing by 1% per specimen type (D1 at 3% viscosity modifier content–D8 at 10% viscosity modifier content). The composition of each test specimen was prepared by including other components corresponding to the optimal concentration ratio of the recycled material of 30%, confirmed in the previous study, into each component’s ratio in a total ratio of 100%. For each D type, three specimen samples were prepared, and the experimental results were derived based on the average result of the three specimens. The concentration of the viscosity-adjusting agents and other components is presented in

Table 1. SPRG Specimen Component and Modifier Ratio.

Components	Mixture Ratio for Each SPRG Specimen Type (Unit: %)
	D1	D2	D3	D4	D5	D6	D7	D8
Waste Oil + Waste Rubber	30	30	30	30	30	30	30	30
Asphalt	26.8	26.4	26.0	25.6	25.2	24.8	24.4	24.0
Viscosity Modifier	3	4	5	6	7	8	9	10
Asphalt Modifier	6.0	5.9	5.9	5.8	5.7	5.6	5.5	5.4
Filler	25.5	25.1	24.6	24.3	23.9	23.5	23.2	22.8
Adhesive Coating Agent	8.7	8.6	8.5	8.3	8.2	8.1	7.9	7.8
Total	100	100	100	100	100	100	100	100

.

2.2. Material Preparation and Viscosity Testing

To measure the viscosity of the resource-cycling adhesive gel with different viscosity-adjusting agent concentrations, the apparent viscosity of the test specimens according to the Brookfield method in Korean Standard (KS) M ISO 2555:2002 Plastics-Liquid, paste and semi-paste, viscosity determination using a rotational viscometer was tested [16]. The test specimens were placed in a container of approximately 500 mL without bubbles and then kept in a constant temperature chamber until reaching the test temperature of (20 ± 1) °C. After that, the spindle of the viscometer was inserted into the test specimen to measure the viscosity. The test conditions and status of the viscosity measurement test are presented in

Table 2. Viscosity Measurement Apparatus Specification.

Item	Spindle Type	Rotation Speed	Referenced Standard
Viscosity Measurement Apparatus	T-F	10 rpm/min	KS M ISO 2555:2002

and Figure 1.

The viscosity measurement test results for different viscosity control agent concentrations are shown in

Table 3. Viscosity measurement results of the SPRG specimens (D1–D8).

Specimen and Viscosity Modifier Ratio		Viscosity Result (mPa·s)
Specimen Label	Viscosity Modifier Ratio (%)
D1	3	2,760,800
D2	4	3,125,100
D3	5	3,894,800
D4	6	3,931,400
D5	7	4,581,200
D6	8	5,085,700
D7	9	5,520,500
D8	10	6,409,500

and Figure 2. The results of the viscosity measurement test showed an increase in viscosity as the ratio of viscosity modifier increased, and it was confirmed that all samples satisfied the LH specification of a minimum viscosity of 2,000,000 mPa·s for the adhesive gel.

2.3. Wet Surface Adhesion Test Based on ISO TS 16774

SPRG specimens were subsequently tested using the wet surface adhesion test in compliance to the test specifications outlined in ISO TS 16774 Part 4. The wet surface adhesion test on the LH technical data sheet specifies that tested samples must satisfy the quality standard stipulated in the relevant standard; this requirement is for the material to not lose adhesion within 60 s after being installed onto a mortar based substrate submerged in water. The results of the wetting adhesion performance test, including the adhesion time and patterns, according to the ratio of viscosity modifier are presented in

Table 4. ISO TS 16774 Test results for SPRG types (D1–D8).

	Wet Surface Adhesion Testing
Results
	D1	D2	D3	D4
	D5	D6	D7	D8

below. The results of this preliminary test indicate that SPRGs samples of D1 to D8 are capable of being tested by a pull-off method.

3. Results and Discussion

In the case of Korean standards, moisture adhesion performance is limited to evaluation regimes, as outlined in ISO TS 16774 Part 4, and further detailed analysis is not mandatory. The nature of the evaluation method in ISO TS 16774 Part 4 is qualitative, in that the requirement is only that the tested material should not lose adhesion within 60 s. This type of result is not sufficient to provide a concise evaluation and investigation on the relationship between SPRG viscosity and wet surface adhesion. Therefore, to clearly analyze the response characteristics of SPRG to varying degrees of humid environment, quantitative evaluation is required. When adhesive gel is applied to vertical walls of structures, such as exterior walls, problems such as detachment, adhesion failure, or waterproof sheets attached to the adhesive gel can occur, especially in humid environments, where the probability of a decrease in adhesion performance causing related problems is high. Therefore, follow-up pull-off testing was conducted to quantitatively evaluate and assess the changes to the adhesion strength of SPRGs on varying degrees of humid substrate surface.

3.1. Selection of Moisture Content Conditions Based on a Test Specimen

Prior to the testing evaluation of the waterproofing performance of resource-recycling adhesive gel based on the response to a humid environment, the moisture environment was set by adjusting the functionality of the test surface, where the adhesive gel was applied, to create a humid background surface. The moisture condition of the test surface was referenced to the state of the background specified in Section 3.1.6 of the Construction Standard Specification (KCS) 41 40 01 for waterproofing construction [16], and the surface humidity condition was set to below 30% for methods applicable in a humid state. Thus, humidity conditions were set to 0% for dry conditions, 10%, 20%, and 30% for functional moisture conditions, and 50%, 70%, and 100% for simulating very humid conditions as a means to test the adhesive performance limitation of the SPRG specimens. The SPRG was immediately applied to the humid conditioned substrate surface, and the test specimens were prepared under conditions identical to those of actual construction in a humid environment (23 ± 1 °C, 65–85% RH). The moisture condition and composition status of the test specimens used in the testing evaluation are shown in Figure 3 below.

3.2. Pull-Off Evaluation Method

To confirm the adhesive performance of an SPRG according to the substrate surface humidity ratio, the specimens were tested using the adhesive performance test method for the construction of waterproofing materials, as specified in KS F 3211:2015, with modifications [17]. The test substrate was prepared using a metal mold with inside dimensions of approximately 210 mm × 70 mm and a thickness of approximately 20 mm, adjusted according to the method specified in KS L ISO 679. The substrate was cured for 24 h at a temperature of (20 ± 3) °C and a humidity of 80% or more, and then removed from the mold and allowed to age for six days at (20 ± 2) °C in water, followed by a further seven days of conditioning in the standard state. The bottom surface of the substrate was sufficiently ground with a 150-grit abrasive paper according to KS L 6003, and then the surface humidity ratio was measured. After applying the SPRG to the substrate, three dollies were placed on each of SPRG test specimen. Any adhesive protruding from the side during the attachment process was removed, and the four sides of the attachment were cut using a grinder cutter until reaching the substrate. The adhesive performance test was conducted by fixing the test specimen and dolly to a tensile testing machine and measuring the maximum load by applying tension at a vertical speed of 2 mm/min to the sample surface. The calculation was performed according to Equation (1). The test specimens and test status are shown in Figure 4.

$$T_f=\frac{T_n}{A}$$

(1)

where;

• $T_f=\mathrm{Adhesive} \mathrm{Strength} \left(\mathrm{N}/{\mathrm{mm}}^2\right)$
• $T_n=\mathrm{Maximum} \mathrm{Load} \left(\mathrm{N}\right)$
• A = Specimen cross-section area (${\mathrm{mm}}^2$)

3.3. Adhesion Strength Measurement Test Results

The adhesion performance test results (averaged between the results of the three dollies per specimen type) according to the humidity ratio of the substrate surface are shown in

Table 5. Pull-off test results for SPRG specimens according to different humidity ratios.

Evaluation Result of Adhesion Performance according to Substrate Surface Humidity (N/mm2)
Humidity Condition (%)	Adhesion Strength per Specimen Type (N/mm2)
	D1	D2	D3	D4	D5	D6	D7	D8
0	0.0132	0.0156	0.0185	0.0197	0.0201	0.0213	0.0238	0.0255
10	0.0123	0.0141	0.0175	0.0182	0.0189	0.0206	0.0222	0.0236
20	0.0107	0.0131	0.0156	0.0173	0.0179	0.017	0.0198	0.0214
30	0.0108	0.0128	0.0156	0.0163	0.0163	0.0171	0.0201	0.0215
50	0.0082	0.0108	0.0118	0.0136	0.0124	0.0128	0.0143	0.0163
70	0.0098	0.0113	0.0126	0.0128	0.0153	0.0143	0.0167	0.0183
100	0.0075	0.0092	0.0109	0.0109	0.0118	0.0122	0.0129	0.014

and Figure 5.

As a result of the adhesion performance test according to the moisture content of the substrate, it was, as expected, confirmed that the adhesive strength decreased as the moisture content of the substrate surface increased for all specimens (D1–D8). Furthermore, adhesion strength increased relative to the increase in viscosity as well. The results indicated that proportional correlation was confirmed in which the wet surface adhesion performance increased as the content of the viscosity modifier increased.

3.4. Analysis on the Correlation of SPRG Viscosity and Adhesion Strength on Wet Surface

To analyze the correlation of SPRG viscosity and changes to the adhesion strength on varying humidity ratios of substrate surface, linear regression analysis based on the specimens of different viscosity modifier content ratio (D1–D8) and the respective adhesive strength throughout different humidity ratios was conducted.

Based on the linear regression analysis, the coefficient of determination (denoted as R²) was derived to provide the basis of correlation. The correlation coefficient is typically calculated as the square of the correlation coefficient (r) between two variables, measuring the strength and direction of the linear relationship between the two variables, while the coefficient of determination indicates the proportion of variance in the dependent variable that is explained by the independent variable(s) [18,19]. The coefficient of determination was calculated using the following Equation (2):

$$R^2=1-\frac{S{S}_{res}}{S{S}_{tot}}$$

(2)

where

R² = Coefficient of determination

$S{S}_{tot}$= Total sum of squares

$S{S}_{res}$= Residual sum of squares.

Based on the linear regression, an appropriate viscosity range corresponding to the applicable range of surface humidity ratio condition is proposed at the end.

3.5. Correlation Analysis and Consideration of Adhesion Performance according to Humidity Ratio of Substrate Surface

Regression analysis was conducted based on Table 5, individually for the specimen types, which evaluated the adhesion performance of the adhesive gel waterproofing material according to the substrate surface humidity ratio and viscosity, to analyze the correlation between adhesion performance and substrate surface humidity ratio. Regression analysis was carried out using the SPSS regression analysis program, and the analysis results are considered relevant when the determination coefficient (R²) is 0.8 or higher, indicating a high explanatory power. Figure 6 below lists the regression analysis results of specimens D1 to D8:

The regression analysis results of the adhesion performance, according to the substrate surface humidity ratio with a viscosity modifier content of 3%(D1) to 10%(D8) shown in Figure 6, were used to derive the linear regression equation and the coefficient of determination. The results are listed in

Table 6. Coefficient of Determination of Linear Regression Relation of Viscosity (Specimen Type) based on the Increase of Substrate Surface Relative Humidity Ratio (%).

Specimen Type	Linear Regression Equation	Coefficient of Determination (R2)
D1	y = −5.6 × 10−3x + 0.0126	0.9362
D2	y = −5.9 × 10−3x + 0.0148	0.9531
D3	y = −7.9 × 10−3x + 0.0178	0.9631
D4	y = −8.6 × 10−3x + 0.0190	0.9895
D5	y = −8.7 × 10−3x + 0.0196	0.9732
D6	y = −9.5 × 10−3x + 0.0203	0.8964
D7	y = −1.1 × 10−2x + 0.0230	0.9705
D8	y = −1.1 × 10−2x + 0.0246	0.9746

Note: y: Adhesive Strength (N/mm²). x: Substrate Surface Humidity (%).

below.

The coefficient of determination explaining the correlation between the substrate surface humidity ratio and the adhesion performance of SPRGs with varying viscosity were all above the required value of 0.8, ranging from the lowest 0.8964 (D6) to the highest of 0.9895 (D4). The results have shown that the regression equation has high reliability due to high ranges of coefficient of determination, and it can be concluded that there is a correlation between the adhesion performance changes of the adhesive gel waterproofing material according to the substrate surface humidity ratio. Based on these factors, a reliable methodology for recommending SPRG viscosity modifier content (limited to the SPRG material type tested in this study) is provided in the following section.

3.6. Recommended SPRG Viscosity Modifier Content for Respective Surface Humidity Ratio Ranges

Based on the adhesive strength results and the subsequent coefficient of determination, it was confirmed that the adhesive strength of the SPRG on a wet surface can be affected by the viscosity of the waterproofing agent, and that the adhesion strength increases as the viscosity increases. On the basis of the high coefficient of determination and the reliability of the relation between viscosity and wet surface adhesion performance limited to the SPRG materials, the adhesion strength difference each specimen type between the 0% humidity ratio condition (completely dry) and 100% humidity ratio condition (completely wet) were derived.

Given the scope of this study, the results would indicate that the greater difference of the range signifies high variance of performance (indicating a low level of consistency). Although D6 and D8 display a significantly high level of viscosity, meaning the difficulty of workability at site, it is not impossible to work with. In contrast, D1–D3 have the lowest viscosity, indicating low manufacturing costs as well as labor due to higher workability, at the risk of loss of adhesion performance. It is expected at all times that waterproofing installation proceed in accordance to manufacturer specifications, whereby construction conditions should maintain an as dry as possible condition on the concrete or substrate surface, but as this is not always the case, builders should always be aware of limitations of the adhesive performance of SPRGs on various wet surface conditions. As the question remains to be whether the trade-off on the workability and performance is worth the benefits, Figure 7 was derived to provide an illustration on the adhesion strength difference derivation results as a means to recommend an optimal viscosity modifier content for the SPRGs for various surface humidity ratio conditions.

In the case of the SPRG, if the upper limit viscosity range for workability during construction exceeds 5,000,000 mPa·s, poor workability at the construction site has been documented to be an issue [14]. In this regard, despite the high viscosity variants of the specimens (D6–D8, marked as red in Figure 7) having notably higher ranges of adhesive strength, it is not recommended for an SPRG with this range of viscosity to be used. Furthermore, the adhesion strength differences at the 0% humidity ratio condition and 100% humidity ratio condition were also higher as the viscosity increased, indicating that the reduction in adhesive strength is also greater as the viscosity increases. On the other hand, specimens that had low viscosity, while having had low overall adhesive strength, also had more consistent adhesive strength performance (indicated by the lower adhesion strength differences at the 0% humidity ratio condition and 100% humidity ratio condition). Furthermore, lower viscosity naturally indicates easier workability, thus this study can recommend (based on the limitations of this study scope) SPRG of D3 type level of viscosity that has moderately high adhesive strength performance and low adhesive strength difference. However, this conclusion is arbitrary, and it is not yet determined whether the optimal viscosity is offered by D1/D2 or D4/D5. To determine a more concise conclusion, further investigation is necessary. This parameter and the exact relationship should be further investigated in the future based on the data of different types of SPRG products.

4. Conclusions

This study aimed to determine the optimal viscosity level for SPRG materials for wet surface application on substrate surface. It is important to be aware that, with the development of new waterproofing materials with inherently higher adhesive performance than most other types of existing waterproofing materials, they come with the downsides that (either or both) (1) their workability performance is low and (2) they have higher costs. With these circumstances, if the use of materials like SPRG is strongly advised by the designers when building new structures, attempts on construction costs reductions will usually follow, one method of which is by reducing the allotted construction time. In these circumstances, cases can occur where waiting for the recommended dry level of concrete before waterproofing application is a luxury builders are not willing to afford, as a common misunderstanding is that the adhesion strength of the installed waterproofing material will still be adequate enough. The problem is that there is not yet clearly experimented data results on an SPRG-like material’s adhesive strength performance on wetted concrete surface and, as the data of this study shows, the adhesive strength is quite varying. Therefore, it is very important to alert contractors to these types of information through the publication of this paper.

Specifically, this paper sought to analyze the relationship between viscosity and different rates of change in adhesive performance in varying humidity ratios, which had not been studied in previous research, and to quantitatively determine the viscosity level at which stable adhesion is possible for different functional ratios of the backing surface. The key findings of this study are as follows:

(1)

A regression analysis was conducted to examine the correlation between viscosity-adjusting agent concentration and changes in viscosity, adhesive performance for different backing surface humidity ratios, and adhesive performance for different viscosities. Based on this regression analysis, we determined the optimal viscosity range that can adapt to the humidity environment for each humidity ratio of the backing surface. Additionally, viscosity standards were provided that meet the KS quality criteria for commercializing resource-circulating adhesive waterproofing agents: a minimum viscosity of 2,500,000 mPa·s and a maximum viscosity of 5,000,000 mPa·s, considering on-site workability.

(2)

Based on the regression equations derived from the adhesive strength evaluation, the adhesion strength difference of each specimen type between the 0% humidity ratio condition (completely dry) and 100% humidity ratio condition (completely wet) were derived. A conjecture was provided that these findings can help in developing a stable mixing design for manufacturing SPRG by deriving an optimal trade-off of viscosity modifier ratio and workability, enabling more consistent quality control during their application on site.

This study is significant in that it provides insight into how to improve the adhesion stability of existing adhesive waterproofing materials in humid environments. However, this study is limited to the waterproof engineering aspects of one product type of SPRG. To build a stable database, further research and analysis on the performance evaluation that include more types of SPRGs is necessary. This could help improve the quality stability of waterproofing agents under continuous weathering conditions, making it essential to consider as future research directions.