Evaluation of Viscoelastic Adhesion Strength and Stability of Composite Waterproofing Sheet Using Non-Hardening Viscoelastic Synthetic Polymer-Based Rubber Gel

Abstract

Recently, a non-hardening type synthetic polymerized rubber gel (hereinafter referred to as NHV-SPRG) composite waterproofing sheet has been used on construction site as a new waterproofing technology. In this study, a test method is proposed in which a composite waterproof sheet is attached to an area of 150 mm in length and 100 mm in width on the mortar-based substrate specimen, and subsequently peeled off at 180 ° vertically to measure the “peel load (N)” at 10 points of 10 mm intervals (P1~P10, from 30 mm point to 120 mm point). Value obtained by dividing the average value of the measured peeling load by a width of 100 mm was defined as “viscoelastic adhesion strength (N/mm)”. The viscoelastic adhesive strength evaluated for the NHV-SPRG composite waterproof sheet of 4 types (VG-1, VG-2, VG-3, VG-4) was an average of 1.99 N/mm. To examine the effectiveness of the viscoelastic adhesive strength, the adhesion stability was evaluated based on the peel load’s coefficient of variation, grade of the linear regression line of the peel load, and peel load percent relative range in peel load for each section of the composite waterproofing layer. As a result, the VG-2 type was measured to have the highest viscoelastic adhesive strength, but the VG-1 type was confirmed to have the highest adhesive stability. Considering these results, it is judged that even a product with high adhesion can have varying stability in terms of performance that can affect construction precision. The test method and performance standard value for the viscoelastic adhesion strength proposed as an evaluation index through this study are expected to be used as a quality control standard to secure the on-site adhesion stability of the NHV-SPRG composite waterproof sheet.

1. Introduction

Modern, advanced construction structures require high-performance waterproofing technology to prevent water leakage for the purpose of maintaining long-term safety and continuous performance. Therefore, research for the development of high-performance waterproofing materials and construction methods that can meet future smart construction structures is continuously being conducted in the waterproofing industry [1]. Recently, in construction sites, composite waterproofing sheets reinforced by laminating a coating and sheet material to replace the existing single ply waterproofing layer is attracting attention as a relatively new waterproofing technology [2]. The manufacturing method of such a composite waterproofing sheet is largely divided into two processs; one is a method of making a waterproof layer by combining a coating-type material such as curable urethane or non-curable synthetic rubber gel with a sheet-type material such as an improved asphalt sheet or synthetic polymer-based sheet at a factory or site. The other is a method to manufacture the waterproofing layer by impregnating and combining synthetic rubber gels and asphalt compounds on both sides with a core material [3]. According to the research and field application verification results, these composite waterproofing layers are 3–4 times larger than the existing single waterproofing layer in terms of structural behavior response, temperature stability, water pressure resistance, and durability [4]. In particular, among these composite waterproofing sheets, in which a non-hardening viscoelastic synthetic polymer-based rubber gel (hereinafter referred to as NHV-SPRG) composite waterproofing sheet) and asphalt compound, etc., is impregnated on both sides with a non-woven fiber reinforcement is preferred as new waterproofing technology [5]. Due to these advantages, NHV-SPRG are actively used not only in Korea, but also in Southeast Asian countries such as Vietnam and Singapore. In particular, in China, quality control of composite waterproofing sheets is managed based on their own standard such as JC/T 2428-2017 “Non-curable rubber modified asphalt coating for waterproofing” as a new waterproofing material [6].

An essential performance evaluation item for the field application of composite waterproofing sheets is an adhesion test that confirms satisfactory adhesion between the waterproofing layer and the concrete substrate surface. The installation method of composite waterproofing materials involves rigid or semi-rigid attachment methods in which the composite waterproofing layer is attached to the surface of a concrete structure with a synthetic resin-based adhesive, asphalt-based adhesive, or polymer cement-based adhesive [6]. In general, performance evaluation methods and quality standards for hard or semi-rigid attachments are commonly used in many countries such as ASTM, JIS, and KS. However, it is difficult to find an adhesion test method or quality standard for a non-rigid or soft waterproof layer, which are commonly associated with that of NHV-SPRG. Therefore, it is difficult to evaluate and quantitatively assess the adhesion strength of the different NHV-SPRG composite waterproofing products existing in the market, and the underlying concrete surface with the general rigid adhesion method. Due to the adhesion mechanism of the existing hard adhesive waterproof layer and the non-hardening soft adhesive layer interface being different, a new evaluation index, method, and criteria should be defined differently. Therefore, a new evaluation method and performance standard specifically designed for NHV-SPRG type of viscoelastic waterproofing materials are required. Existing NHV-SPRG type composite waterproofing sheets have been positively evaluated as being more effective than a single waterproofing layer in terms of structural behavior response, temperature stability, water pressure resistance, and durability, and a clear evaluation method and evaluation criteria are still yet to be established for the adhesion performance of the waterproofing layer to concrete. As such, disputes over quality and safety have arisen among developers, contractors, and supervisors. Sarnowski et al. discuss the impact on the damages to viscoelastic properties of polymer modified bitumen for bridge pavements due to high processing temperature, resulting in the loss of durability of the waterproofing layer [7]. Goikoetxeaundia et al. provide a research on the effect of polymer concentration of the loss tangent of viscoelastic bitumen polymer waterproofing materials, and based on the resulting elastic modulus model evaluate the adhesion capacity of the waterproofing material on roof surface [8]. Dong Soo Ahn et al. researched the effect of screw mixing duration and temperature during the installation preparation of viscoelastic asphalt waterproofing materials and their changing physical properties [9]. F. Saulnier et al. present an analysis for the dissipative process during adhesion failure of viscoelastic polymer materials and classify and evaluate different types of adhesion energy [10]. Kumhar et al. analyze the love wave vibration in a pre-stressed fluid-saturated viscoelastic material in a half-space conditioned environment and evaluate the properties of the material based on affecting factors on gravity, stress, porosity, and viscosity, etc. [11].

In regards to the above circumstances, this study proposes a “viscoelastic adhesion performance” and “evaluation regime” that cater to the unique qualities of an NHV-SPRG material. The scope of this study is limited to examining viscoelastic adhesion performance based on adhesion strength testing redesigned based on the existing KS F 2400, and the derived viscoelastic adhesion strength (quality standard) and effectiveness will be examined, and statistical analysis is performed to determine the correlation between the adhesion strength and subsequent adhesion stability analysis on the composite waterproofing sheet of NHV-SPRG will be carried out.

2. Necessity of Viscoelastic Adhesive Performance Evaluation Method for NHV-SPRG Composite Waterproofing Sheet

2.1. Characteristics of NHV-SPRG Composite Waterproofing Sheet

2.1.1. Composition of NHV-SPRG Composite Waterproofing Sheet

The basic composition and structure of NHV-SPRG composite waterproofing sheet is shown in Figure 1. The NHV-SPRG composite waterproofing sheet is comprised of asphalt compound, synthetic rubber, uses a non-woven fabric as the main core material, and is impregnated with non-hardening viscoelastic coating materials on both the upper and lower sides of a non-woven fabric. The non-woven fabric affects tensile performance, tear performance, elongation rate, and sheet stiffness as a composite waterproof sheet material. After impregnation, a protective layer is laminated on the upper surface of the PE or HDPE film, and the lower surface is composed of a sheet-type layer with a release paper. In addition, a release paper is placed at the bottom surface of the non-curing viscoelastic coating layer for convenience purposes during transportation and installation after the completion of manufacturing.

2.1.2. Types of Viscoelastic Adhesion (Limited to Composite Waterproofing Sheet Properties)

Non-hardening viscoelastic gel, the main material serving as a waterproofing sheet, adds non-hardening materials such as synthetic rubber, oil, fluidizing agent, etc. to asphalt compound, which is the basic constituent material, such that viscosity and elasticity are simultaneously expressed and has a mechanism of adhesion according to the cohesion that occurs inside [8,9] as shown in Figure 2.

The coating layer has high flexibility and non-hardening (non-curing) viscoelastic properties due to the asphalt mixed synthetic rubber gel compound used [10]. The range of deformation specified in ISO TS 16774 “Test methods for repair materials for water-leakage cracks in underground concrete structures-Part 6 Test method for response to the substrate movement” [11] is allowed up to 0~2.5 mm, and the relevant test evaluation confirmed from the study results that there is a behavioral response ability [12,13]. On the other hand, since the rheological property and viscoelasticity are affected by temperature change according to the non-hardening characteristics, it is a material that requires management in the range of solid content and temperature of use [14]. Therefore, in this study, it is necessary to set up a test method that can discriminate the response load such as fatigue behavior and temperature by reflecting these analysis data.

2.2. Viscoelastic Adhesive Performance Index and Test Method of NHV-SPRG Composite Waterproofing Sheet

2.2.1. Adhesive Performance Evaluation Method for NHV-SPRG Composite Waterproofing Sheet

The NHV-SPRG composite waterproofing sheet is generally installed on non-exposed waterproofing method for aboveground slabs and underground positive-side walls of concrete structures. Performance evaluation of the attachment stability to the concrete substrate surface has to consider response to complex construction environmental conditions such as water pressure, earth pressure, groundwater flow, structure behavior, wetness, etc. International adhesion strength test methods for waterproofing materials similar to the NHV-SPRG composite waterproofing sheet were investigated for reference purposes. In China, JC/T 2428-2017 [15], and in Korea, KS F 4934 Self-adhesive rubberized asphalt waterproofing sheet [16] were investigated. Also, for NHV-SPRG materials, ISO TR 16475 [17], ISO TS 16774 and KS F 4935 [18] stipulate the maintenance, usage and test methods as leak repair materials. In China, quality is managed according to JC/T 2428-2017 Non-curable rubber modified asphalt coating for waterproofing standards. Among these test methods, the peel-off resistance test governs the adhesive strength evaluation of waterproofing sheets similar in product and construction form to NHV-SPRG composite waterproofing sheet, were considered.

2.2.2. Peel-Off Adhesion Strength Test (KS F 4934)

As has been illustrated in Section 2.1.2, due to the varying properties, material mixture ratio and viscosity of the NHV-SPRG component in the composite waterproofing sheet, there are variables that will determine whether cohesive failure or adhesive failure occurs first during adhesion strength testing. Whether based on the thickness and toughness of the gel, area of unadhered sections, amount and angle of pressure during installation. It is very difficult to quantify these factors and consider them during evaluation, but an evaluation regime is nevertheless necessary. Therefore, rather than directly comparing the differing degrees of adhesion strength, an overall statistical analysis of adhesion state of NHV-SPRG composite waterproofing sheet is proposed. To propose this method, first an applicable test method (peel-off method) has to be considered.

In this regard, a peel-off resistance test method outlined in KSF 4934 is first referred to. An illustration of the peel-off resistance test method as an adhesive strength test method applicable to KS F 4934 is shown in Figure 3. This test method is a test to evaluate the stability of the joint (or overlap section) between the sheet material and the sheet material by measuring the adhesion strength. The advantage and compatibility of this method is that it allows an observation of the state of adhesion throughout the entire selected area of the specimen, which if applied to NHV-SPRG, a statistical analysis of the varying adhesion strength throughout the area can be conducted. The test method is performed by applying tensile force on the opposite sides of the specimen where the sheet material is adhered and affixed such that the cross-sections are at 180° in both directions as shown in Figure 3.

For this test method, two identical waterproofing sheets with a width of 50 mm and a length of 110 mm are overlapped over 50 mm in length, and reinforced three times with a manual compression roller device, and then the tensile force is measured by attaching an anchor that cover up to 10 mm unbonded end at 180° with a tensile tester. The specimen is mounted on a UTM, where the anchors installed to the UTM apply pulling tension force on the specimen at a speed of 200 mm/min. The peeling loads at 5 points (P1, P2, P3, P4, P5) in the peeled section (length) are used to calculate the adhesive strength using the following Equation (1).

$$T_{\mathrm{B}}=\frac{P_{\mathrm{B}}}{W}$$

(1)

where;

• $T_{\mathrm{B}}$: Peel off resistance (N/mm)
• $P_{\mathrm{B}}$: Maximum Load (N)
• W: Width of specimen (50 mm)

Based on this method of applying a 180° tensile method of peeling resistance, a performance test can be cited as a method for evaluating the resistance (viscosity) of the viscoelastic gel material against the force of the NHV-SPRG composite waterproof sheet flowing down from the vertical wall in the direction of gravity.

2.2.3. Peel-Off Adhesion Strength Test (KS F 2400)

KS F 2400 [19] outlines a different version of the peel-off test method to evaluate waterproofing sheet adhesive strength as shown in Figure 4. The test method procedure is as follows: (1) a cellulose fiber reinforced cement board is prepared as base substrate with a length of 200 mm and a width of 70 mm along with a self-adhesive waterproofing sheet (specimen) sample with a length of 180 mm and a width of 60 mm; (2) primer is applied on the fiber reinforced cement board and dry it under standard conditions. 90 mm of the prepared waterproofing sheet is installed to the base substrate, and the remaining 90 mm is covered with release film to prevent adhesion to the remaining base substrate surface; (3) sheet section adhered to the cellulose fiber reinforced cement board is pressed 3 times with a manual roller compactor, and set to rest for 2 h in standard room temperature condition; (4) after installing the specimen (sheet + board) to the UTM, an anchor is fixed to the end of the sheet material that is not attached to the base substrate to the UTM jig. Tensile load(N) is subsequently measured while the anchor applies a pulling load at 90°, and value Pi(N) is recorded at 5 separate intervals (P1, P2, P3, P4, P5) in the 40 mm section after excluding the initial peeling length of 20 mm section as shown in Figure 5. The peeling loads at 5 points (P1, P2, P3, P4, P5) in the peeled section (length) are used to calculate the adhesive strength.

In this study, when the NHV-SPRG composite waterproofing sheet is installed on a concrete wall or floor, the peel-off method outlined in KS F 2400, and the peeling angle conditions outlined in the method KS F 4934 on substrate surface can be cited as the most appropriate method to evaluate the resistance (viscosity) of the NHV-SPRG materials based on the peeling action and adhesion failure from the substrate surface.

2.3. Evaluation Index, Method, and Criteria for Adhesion Performance of NHV-SPRG Composite Waterproofing Sheet

Compatibility, repeatability, and objective calculation should be reflected as the most important basics in proposing new evaluation indicators (scales, items), evaluation methods, and evaluation criteria for the proposed evaluation regime for NHV-SPRG composite waterproofing sheets. In other words, similar waterproofing materials related to each other should be applicable in common, and there should be consistency in reproduction. In addition, it should be possible to continuously verify the quality by reflecting the unique properties of the waterproofing material. Therefore, this study proposes a test method that is suitable for the intrinsic properties of the non-hardening NHV-SPRG composite waterproofing sheet and the mechanism of the waterproofing layer.

2.3.1. Evaluation Method

The NHV-SPRG composite waterproofing sheet should be implemented in a way that can simultaneously measure viscosity and elastic loads in evaluating the viscoelastic adhesion strength of non-hardening viscoelastic gel, which is the main material. Therefore, in this study, the test method used in the existing standard is applied, while improving the test method to reflect the field situation, and it is presented such that the performance evaluation results through this improved method can be implemented as a standard of the Viscoelastic Adhesion Strength and Stability Test Method.

2.3.2. Evaluation Criteria and Index

It has been made clear that the evaluation index (scale, item) for the Viscoelastic Adhesion Strength and Stability Test Method for NHV-SPRG composite waterproofing sheet cannot be the same as the index found in the conventional evaluation index of existing hardening type waterproofing sheets. These index only assess whether the measured adhesion strength meets the required minimum standard criteria. Viscoelastic adhesion performance of the NHV-SPRG composite waterproofing sheet depends on the viscosity and elastic strength of the non-hardened asphalt mixed synthetic polymer gel used in the NHV-SPRG composite waterproofing sheet, but as there are different failure modes (cohesive and adhesive), simple assessment of adhesion strength is insufficient. Index on overall adhesion stability of NHV-SPRG composite waterproofing sheet need to be newly devised.

3. Viscoelastic Adhesive Strength and Stability Testing Method

3.1. Test Materials

3.1.1. Types of NHV-SPRG

The main components and characteristics of 4 types material used for this study are as follows in

Table 1. The composition and properties of NHV-SPRG composite waterproofing sheets of 4 types.

Type	Composition	Property
VG-1	Acrylamide, persulfate (one or two of sodium, ammonium, potassium), asphalt, other additives, etc.	- 85–90% solidity - High-viscosity + low-viscosity non-hardening type fluid gel - Diaphragm toothed structure (diaphragm support structure)
VG-2	Asphalt, inorganic filler for viscosity control, processor oil, strength modifier, heat-resistance modifier, adhesion modifier, flow prevention additive, waste tire, etc.	- 95–99% solids - High Viscosity Uncured Mastic Asphalt - Polar covalent structure of lipophilic group and hydrophilic group
VG-3	Asphalt, bentonite, oil, rubber, water-soluble polymer resin, other additives etc.	- 90–95% solids - High tack, non-hardening bentonite rubberized asphalt - Diaphragm toothed structure (diaphragm support structure)
VG-4	Asphalt, inorganic filler for viscosity control, processor oil, asphalt modifier, strength modifier, ammonium, potassium, asphalt, other additives, etc.	- 85–90% solidity - High-viscosity + low-viscosity non-hardening type fluid gel - Diaphragm toothed structure (diaphragm support structure)

. The test materials are the gel-type materials with flexibility by fusing and bonding inorganic components such as synthetic rubber, polymer resin, and bentonite with asphalt as the main component. Each NHV-SPRG type used in this experiment originates from manufacturers in Korea that are partaking in this experimental project to assist in the development of the proposed test method, and are subsequently labeled as VG-1, VG-2, VG-3, and VG-4 using the abbreviation of viscoelastic gel.

3.1.2. Mortar-Based Substrate and Test Specimen

(1)

Substrate preparation (mortar base)

In this case, tension is first applied by the cohesive load between the molecules inside the viscoelastic layer. To reflect this viscoelastic adhesion mechanism, the size of the blank specimen and specimen (sheet) is adjusted and prepared based on the conditions outlined in the following

Table 2. The size of the mortar specimen and adhesion area for the viscoelastic strength test.

Parts	Viscoelastic Material Test Method Part
Substrate base size	320 × 120 mm
Waterproofing sheet specimen size	480 × 100 mm
Specimen adhesion surface area	150 × 100 mm

and Figure 6 to increase the attachment area within the range that could be mounted on the tensile test device such that sufficient cohesive load can be exerted. This method is currently under development, and thus the size conditions are subject to variation for future versions of this evaluation method.

Depending on the condition of the surface of the base specimen, the adhesion pattern with the waterproof layer may come out without coherent consistency. In order to determine a clear result value while taking this into consideration, it is necessary to prepare a base specimen that reflects the field properties. In particular, in order to reflect the viscoelastic properties unlike the general adhesion aspect, a mortar-based substrate specimen based on a standard formulation is required. In compliance with the KS code, the mortar-based specimen was manufactured by processing a dedicated STS mold, as is shown in Figure 6 by complying the size of the background specimen set above in Table 2. Substrate specimen mold is made of an STS (stainless steel) plate. Based on the standard mortar mixing ratio (C:S:W = 1:2.45:0.5), the base mortar specimen was cast with a thickness of 320 mm × length 120 mm × thickness 6 mm, and after 14 days of settling (curing period). Furthermore, the 4 types of NHV-SPRG composite waterproofing sheet are applied to meet the specifications as shown in

Table 3. The 4 types NHV-SPRG composite waterproofing sheets applied on substrate specimens.

State	Specimen Types
	VG-1	VG-2	VG-3	VG-4
Specimen Image

and the laboratory environment was set to a general standard temperature (20 ± 2) °C and humidity (60 ± 10)%.

3.1.3. Test Specimens and Apparatus

Viscoelastic adhesive strength measuring device automatically records the force and displacement such that during a 180° pulling test, NHV-SPRG specimen can be peeled off, as it is in the attached state where the crosshead movement speed is constant and the maximum cohesive force being exhibited can be applied to measure the adhesive strength between the base mortar and waterproofing material. An adjustable thermostat was used to set the room temperature to 60 °C, the tensile speed is adjusted to 200 min/min. The concept and process of the testing is illustrated in Figure 7.

3.2. Test Method for Evaluation of Viscoelastic Adhesive Strength

Viscoelastic Adhesion Strength and Stability Test Method for the 4 types of NHV-SPRG composite waterproofing sheet were conducted by installing the specimen on to UTM apparatus as is shown in Figure 7a. As the following

Table 4. The NHV-SPRG composite waterproofing sheet specimen and peel off test summarized.

State	Specimen Types
	VG-1	VG-2	VG-3	VG-4
(a) Sheet Specimens
(b) Setting for Testing
(c) After Testing

a–c, the NHV-SPRG composite waterproofing sheets were peeled from substrate at 180° with a tensile speed of 200 min/min.

3.3. Evaluation Method of Viscoelastic Adhesive Performance

3.3.1. Viscoelastic Adhesive Strength

The viscoelastic adhesive strength of each specimen was measured from the peel load value from 1 mm to 150 mm, except initial values from 1 mm to 29 mm and latter values from 121 mm to 150 mm, and peel load value at the respective Pt intervals (P1, P2, P3, P4, P5, P6, P7, P8, P9, P10) from 30 mm to 120 mm were measured (as shown in Figure 8). The average value (N) of value of 10 points of the three specimens is derived. Using this derived average value (N), the viscoelastic adhesive strength (N/mm) is calculated.

Figure 8. 180° peel-off test method and data graph example; (a) testing process illustrated, (b) data procurement example illustrated.

Figure 8. 180° peel-off test method and data graph example; (a) testing process illustrated, (b) data procurement example illustrated.

3.3.2. Adhesion Stability

The attachment stability is reviewed using the peel load values for each section of the 4 types of NHV-SPRG composite waterproofing sheets derived in Section 3.3.1. Load and adhesion strength calculation methods were conducted in compliance to the standard format outlined in KS F 4934 “Self-adhesive waterproof sheet.”

In general, when NHV-SPRG is peeled off from the interface of the substrate, stress is simultaneously generated in the interfacial adhesion between dissimilar materials and the cohesion interface, that the rate at which the two types of separation (adhesive or cohesive) can differ significantly based on the material composition, which in turn can have different effects on overall adhesion stability. For example, Figure 1 shows that when the sample waterproof sheet is peeled off from the surface of the mortar specimen, it is confirmed that the adhesion load is read more closely at the interface of different materials. On the other hand, as shown in Figure 1, some materials display a different effect by showcasing when tension is applied to the cross-sectional area due to the viscoelastic properties, before stress is applied to the adhesion between the coating layer and the substrate. For this purpose, in this study, to analyze the relationship between viscoelastic adhesion strength and stability difference based on the peel load’s coefficient of variation, grade of the linear regression line of the peel load, and peel load percent relative range.

(1)

Coefficient of Variation

For this parameter, adhesion strength stability is determined by calculating the coefficient of variation based on the standard deviation of the 10 tensile load intervals (P1 to P10). Standard deviation refers to the value of how scattered one data value is from the mean, and the value obtained by subtracting the mean of the data from each variate of data is called the deviation from the mean of each variate. In this case, the larger the absolute value of the deviation means that the variance is far from the mean and the smaller the absolute value of the deviation is, the closer the variance is to the mean. In addition, the coefficient of variation is a technique used to compare data with different units of measurement, and generally refers to a value obtained by dividing the standard deviation by the mean. In the case of the non-hardened asphalt composite waterproofing sheet reviewed in this study, it is difficult to secure consistent adhesive strength over the entire area due to its viscoelastic properties. As confirmed in the above viscoelastic adhesive strength test result, the peeling load value decreased and increased, and increased and decreased for each section from P1 to P10; that is, when the non-hardening type viscoelastic waterproofing material adheres to the base concrete, it is judged that the adhesive load is lowered due to the variables (high and low compression load, voids, latency, protrusions, etc.) acting on the base concrete. The coefficient of variation calculated by reflecting these aspects will be able to judge the adhesion stabilization according to the size of the value. Based on the example graphs provided in Figure 9, it can be interpreted that there is adhesion stabilization when the value of the coefficient of variation is calculated by Equation (2):

$$P_{cv}=\frac{\sqrt{\frac{{{\displaystyle \sum }}^{\hspace{1em}}{\left|x-\mu \right|}^2}{N}}}{\mu }$$

(2)

where;

• $P_{cv} :\mathrm{Coefficient} \mathrm{of} \mathrm{variation} \mathrm{of} \mathrm{peel} \mathrm{load} \mathrm{standard} \mathrm{deviation}$
• $x:\mathrm{peel} \mathrm{load} \mathrm{sample} \left(\mathrm{N}\right)$
• $\mu :\mathrm{Average} \mathrm{peel} \mathrm{load} \left(\mathrm{N}\right)$
• $N :\mathrm{Number} \mathrm{of} \mathrm{data} \left({\mathrm{number} \mathrm{of} \mathrm{P}}_t, 10\right)$

Figure 9. A stability analysis comparison example of peel load data standard deviation; (a) a case with high standard deviation, (b) a case with low standard deviation.

Figure 9. A stability analysis comparison example of peel load data standard deviation; (a) a case with high standard deviation, (b) a case with low standard deviation.

(2)

Grade of the Linear regression Line

For this parameter, adhesion strength stability is determined by calculating the angle grade of the regression curve. In general, a regression line refers to a curve showing the correlation between two variables based on the average value of another variable for a certain value of one variable. When this technique is applied to the study, it can be predicted that the change in adhesive load occurs according to the grade of the slope of the regression slope as shown in Figure 10; that is, it can be interpreted that the higher the grade of the regression curve, the more unstable the adhesion state, and the lower the grade, the more stable the adhesion state. Therefore, in this study, the grade of the regression line of the peel load values of P1 to P10 for each of the four derived types is calculated using Equation (3) to determine adhesion stability.

P_RG = (100%) y/x

(3)

where;

• P_RG: Grade of the linear regression line formed from the peel load over peel length data (%)
• y: Peel load data (N)
• x: Peel length data (mm)

(3)

Percent relative range

For this parameter, adhesion strength stability is determined by calculating the ratio of the average coefficient compared to the average value for the maximum and minimum values of the derived peeling load values. In other words, in this study, as shown in Figure 11, the wider the gap between the maximum peel load value and the minimum peel load value, the more unstable the adhesion state, and the narrower the interval, the more stable the adhesion state. Percent relative range is calculated using Equation (4) below.

$$(\frac{\left({\mathrm{P}}_{\max }+{\mathrm{P}}_{\min }\right)}{N}/\left({\mathrm{P}}_{\max }-{\mathrm{P}}_{\min }\right))\times 100\%$$

(4)

where;

• ${\mathrm{P}}_{\max }$: Maximum peel load (N)
• ${\mathrm{P}}_{\min }$: Minimum peel load (N)
• $N :\mathrm{Number} \mathrm{of} \mathrm{data} \left({\mathrm{number} \mathrm{of} \mathrm{P}}_t, 10\right)$

Figure 11. A stability analysis comparison example of percent relative range of peel load; (a) case with high percent relative range, (b) case with low percent relative range.

Figure 11. A stability analysis comparison example of percent relative range of peel load; (a) case with high percent relative range, (b) case with low percent relative range.

4. Results and Discussion

4.1. Viscoelastic Adhesion Strength

(1)

VG-1 Specimen

The measurement result of the peeling load (N) of the VG-1 specimen is shown in

Table 5. VG-1 viscoelastic adhesive strength test results.

	Peel Length P Interval (mm)										Average Peel off Load and Adhesion Strength
P(t)	30	40	50	60	70	80	90	100	110	120	Average Peel off Load (N), Ps		Visco-Elastic Adhesion Strength (N/mm)
	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
①	190.5	198.0	205.0	217.5	225.0	202.0	210.0	206.5	215.0	185.0	205.5	204.3	2.04
②	184.0	201.5	196.0	213.0	208.0	211.5	199.0	209.0	220.5	189.0	203.2
③	185.5	198.0	198.8	215.0	221.0	217.3	201.0	205.0	216.0	185.3	204.3

, by 10 points (30, 40, 50, 60, 70, 80, 90, 100, 110, 120 mm points) for 3 samples. The peeling load (N) and their average load of 204.3 N were derived. The viscoelastic adhesive strength obtained by dividing the average peeling load value by the attachment width of 10 mm was calculated as 2.04 N/mm.

(2)

VG-2 Specimen

The measurement results of the peeling load (N) of the VG-2 specimen are shown in

Table 6. VG-2 Viscoelastic adhesive strength test results.

#	Peel Length P Interval (mm)										Average Peel off Load and Adhesion Strength
P(t)	30	40	50	60	70	80	90	100	110	120	Average Peel off Load (N), Ps		Visco-Elastic Adhesion Strength (N/mm)
	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
①	293.5	314.0	313.0	314.5	315.0	306.5	270.0	243.0	235.0	251.0	285.6	263.6	2.64
②	265.0	292.0	294.5	290.5	285.5	280.5	277.0	271.5	274.0	280.0	281.1
③	79.5	129.5	172.5	211.5	247.5	271.0	277.5	285.0	283.0	284.5	224.2

, by 10 points (30, 40, 50, 60, 70, 80, 90, 100, 110, 120 mm points) for 3 samples. The peeling load (N) and their average load of 204.3 N were derived. The viscoelastic adhesive strength obtained by dividing the average peeling load value by the attachment width of 10 mm was calculated as 2.64 N/mm.

(3)

VG-3 Specimen

The measurement results of the peeling load (N) of the VG-3 specimen are shown in

Table 7. VG-3 viscoelastic adhesive strength test results.

#	Peel Length P Interval (mm)										Average Peel off Load and Adhesion Strength
P(t)	30	40	50	60	70	80	90	100	110	120	Average Peel off Load (N), Ps		Visco-Elastic Adhesion Strength (N/mm)
	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
①	266.0	284.0	271.0	267.5	262.0	259.9	262.0	244.5	110.0	239.0	246.6	234.1	2.34
②	272.5	226.0	208.0	202.0	209.5	219.0	220.5	220.0	214.5	209.5	220.2
③	251.5	247.0	243.5	243.0	242.0	235.0	226.0	230.0	222.5	216.5	235.7

, by 10 points (30, 40, 50, 60, 70, 80, 90, 100, 110, 120 mm points) for 3 samples. The peeling load (N) and their average load of 204.3 N were derived. The viscoelastic adhesive strength obtained by dividing the average peeling load value by the attachment width of 10 mm was calculated as 2.34 N/mm.

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VG-4 Specimen

The measurement results of the peeling load (N) of the VG-4 specimen are shown in

Table 8. VG-4 viscoelastic adhesive strength test results.

#	Peel Length P Interval (mm)										Average Peel off Load and Adhesion Strength
P(t)	30	40	50	60	70	80	90	100	110	120	Average Peel off Load (N), Ps		Visco-Elastic Adhesion Strength (N/mm)
	P1	P2	P3	P4	P5	P6	P7	P8	P9	P10
①	89.0	91.0	93.5	100.0	101.5	95.5	92.0	90.5	89.5	88.5	93.1	95.4	0.95
②	86.0	82.0	81.5	96.0	90.5	82.0	83.5	82.0	113.0	115.2	91.2
③	89.0	72.5	71.0	82.0	93.5	117.5	126.5	134.0	110.0	122.0	101.8

, by 10 points (30, 40, 50, 60, 70, 80, 90, 100, 110, 120 mm points) for 3 samples. The peeling load (N) and their average load of 204.3 N were derived. The viscoelastic adhesive strength obtained by dividing the average peeling load value by the attachment width of 10 mm was calculated as 0.95 N/mm.

(5)

Consideration of viscoelastic adhesive strength of NHV-SPRG

The measurement result of viscoelastic adhesive strength showed highest values in the order of VG-2 > VG-3 > VG-1 > VG-4 from a quantitative point of view. Through this experiment, the viscoelastic adhesive strength of the four materials showed adhesive strength of VG-4 was lower than that of the other three materials, but the average strength was 1.99 N/mm. Therefore, in this experiment, this average value can be proposed as a quality standard value. Furthermore, it was confirmed that the test method proposed in this study can measure the viscoelastic adhesion strength of the NHV-SPRG composite waterproofing sheet in the field.

However, as a result of confirming the increase or decrease of the peel load value for each section from P1 to P10, all four types showed different types of peel load values. Taken together, in the case of VG-1 Type, the 30~60 mm section shows a decreasing trend, but increases in the 70~110 mm section and decreases again at the 120 mm section, showing a decreasing → increasing → decreasing trend. In the two samples, VG-2 type was increased in the early stage and decreased in the latter half, showing an increase → decrease trend. Conversely, the other one showed a high decrease in the early stage and a high increase in the latter part, showing a high decrease → increase trend. VG-3 Type generally increased in the early stage and decreased in the latter half, showing an increase → decrease trend. The VG-4 type generally decreased and increased in some sections, but the increased peel load value was significantly lower than the above three types, but generally decreased → increased.

As described above, although the 4 types show different ranges of the viscoelastic adhesion strength, the trend in the peel load value variation at peel-off section interval shows an increase or decrease, confirming a relation to an overall adhesion stability with regards to the adhesion area of the whole specimen. The verification method is to subdivide the increase/decrease pattern of the peel load value for each P1~P10 section derived from the viscoelastic adhesion strength, and calculate the coefficient of variation, grade of the linear regression line, and percent relative range. Adhesion stability was evaluated according to the methodology outlined in the following Section 4.2 using basic statistical analysis techniques.

4.2. Adhesion Stability Evaluation

4.2.1. Coefficient of Variation

The average value of the coefficient of variation was confirmed to be 6% for VG-1, 15.9% for VG-2, 11.3% for VG-3, and 13.9% for VG-4. In summary, based on the calculated coefficient of variation from the standard deviation of peel load, adhesion stabilization was shown in the order of VG-1 > VG-3 > VG-4 > VG-2.

Table 9. The coefficient of variation of peel load data.

Specimen		Average Peel Load (N)	Standard Deviation (N)	Coefficient of Variation (%)
VG-1	①	205.5	12.25	6
	②	203.2	11.37	6
	③	204.3	13.28	6
	Avg.	204.3	12.30	6
VG-2	①	285.6	32.61	11
	②	281.1	9.58	3
	③	224.2	73.87	33
	Avg.	263.6	38.68	15.9
VG-3	①	246.6	49.64	20
	②	220.2	19.75	9
	③	235.7	11.55	5
	Avg.	234.1	26.98	11.3
VG-4	①	93.1	4.56	5
	②	91.2	12.95	14
	③	101.8	23.09	23
	Avg.	95.4	13.54	13.9

shows the summarized result of the calculated results.

4.2.2. Linear Regression Line Grade

The linear regression line was derived for each of the peel load throughout the peel load changes of the Pt (P1~P10). For each VG type, the linear regression equation was calculated and shown in the respective Figure 12a–d, and the slope of the each of the equations were used to calculate the grade. Based on Equation (2) in Section 3.3.2, it is determined that the method for deriving the slope and grade is similar (a relation of Y and X value), but the direction of the slope is not considered for the grade. Without considering the trend of decrease and/or increase of adhesion strength, and by considering only the grade (which does not consider the vector of the angle) of the linear regression line, a simpler and clearer comparison of adhesion strength stability is possible.

The average value of the derived slope coefficient (angle) was confirmed to be 1.19% for VG-1, 4.29% for VG-2, −5.55% for VG-3, and 2.85% for VG-4. In summary, based on the calculated slope and in turn the grade of the linear regression line, adhesion stabilization was shown in the order of VG-1 > VG-4 > VG-2 > VG-3.

Table 10. The regression line grade calculation results.

Specimen		Slope	Regression Line Grade (%)
VG-1	①	1.24	1.24
	②	0.19	0.19
	③	2.15	2.15
	Avg.	1.19	1.19
VG-2	①	22.4	22.45
	②	−0.92	0.92
	③	−8.65	8.65
	Avg.	4.29	4.29
VG-3	①	−3.71	3.71
	②	−3.16	3.17
	③	−9.76	9.77
	Avg.	−5.55	5.55
VG-4	①	6.25	6.25
	②	2.64	2.64
	③	−0.36	0.36
	Avg.	2.85	2.85

shows the summarized result of the calculations.

4.2.3. Percent Relative Range

The ratio of the average coefficient to the derived maximum/minimum value was confirmed to be 18.7% for VG-1, 43.4% for VG-2, 39.1% for VG-3, and 37.6% for VG-4. In summary, based on the ratio of average coefficients, adhesion stabilization was shown in the order of VG-1 > VG-4 > VG-3 > VG-2.

Table 11. The percent relative range calculation results.

Specimen		Max. Peel Load (N)	Min. Peel Load (N)	Average Peel Load (N)	Average Percent Relative Range (%)
VG-1	①	185	225	205.45	19.5%
	②	184	220.5	203.15	18.0%
	③	182	223	203.28	18.5%
	Avg.	183.67	222.83	203.96	18.7%
VG-2	①	235	315	285.55	28.0%
	②	265	294.5	281.05	10.5%
	③	79.5	285	224.15	91.7%
	Avg.	193.17	298.17	263.58	43.4%
VG-3	①	110.01	284	246.591	70.6%
	②	202	272.5	220.15	32.0%
	③	216.5	251.5	235.7	14.8%
	Avg.	176.17	269.33	234.15	39.1%
VG-4	①	88.5	101.5	93.1	14.0%
	②	81.5	115.2	91.171	37.0%
	③	71	134	101.8	61.9%
	Avg.	80.33	116.90	95.36	37.6%

shows the summarized result of the calculations.

4.2.4. Considerations

Using the peel-off load values for each section of the NHV-SPRG composite waterproofing sheet of the 4 types derived above, the coefficient of variation of the standard deviation, the linear regression line grade, and the percent relative ratio based on the maximum and minimum load were calculated and the adhesion stability was determined (as shown in Figure 13).

Based on the results of evaluation of adhesion stability for each type derived above, a correlation of adhesion on the base substrate surface for each specimen type can be derived as shown in Figure 14. It can be interpreted as a stable adhesion state. In addition, the waterproof materials of VG-4, VG-2, and VG-3 types can be interpreted as having similar adhesive stability as all three types were found to have almost the same range.

Through these results, it can be determined that NHV-SPRG composite waterproofing sheets with high adhesive strength do not necessarily have adhesive stability as has been demonstrated in this experimental study. It is thought that the stability of materials may change depending on the quality and mixing ratio of the raw materials used, the precision of the equipment used, and the temperature conditions during construction. Furthermore, the size of the specimen, condition of the concrete substrate base and speed of the testing must continue to be reconsidered for validation purposes. The purpose of the article has been to state that NHV-SPRG composite waterproofing, mainly due to the viscoelastic nature of the non-hardening component, cannot be tested for their adhesive strength using the conventional methods outlined the example test methods of KS F 4934 and KS F 2400 among others. It is the limitation of the presented model in this study to state that a new adhesive strength evaluation method, such as the one demonstrated in this study in some format of this Viscoelastic Adhesion Strength and Stability Test Method, should be developed and practiced in order to comprehensively evaluate material with distinct viscoelastic properties such as NHV-SPRG composite waterproofing sheets

5. Conclusions

Recently, as the NHV-SPRG composite waterproofing sheet has been used internationally as a waterproofing material for concrete structures, the need to prepare standards for quality control of adhesion strength has been emphasized. A non-hardening viscoelastic adhesive concept used in the NHV-SPRG composite waterproofing sheet have shown to respond with high resistance against behavior of microcracks and joints occurring in the underlying concrete by using the property that elastic deformation. In this regard, there is a limit to evaluating the adhesion performance of non-hardening flexible waterproofing materials using the existing general adhesion performance evaluation methods. While the method still requires validation, this study has shown that it is necessary to present a new evaluation system (evaluation index, evaluation method, evaluation standard) specifically for NHV-SPRG type waterproofing materials, and that it is possible to evaluate them based on the parameters of stability rather than minimum required adhesive strength. Therefore, through this study, a test method, quality control index, and quality standard for evaluating the viscoelastic adhesion strength of non-hardening viscoelastic gel were proposed, and the effectiveness of the NHV-SPRG composite waterproofing sheet attachment stability according to the quality standard was confirmed as follows;

(1)

In order to reflect the material mechanism of non-hardening viscoelastic properties, as a new test method, 180° vertical tension and peel-off test were determined. Based on this, the viscoelastic adhesive strength and stability of the NHV-SPRG composite waterproofing sheets of 4 types were evaluated with respect to the peeling load (N) values for each section. As a result, the viscoelastic adhesive strength was measured as VG-1 2.04 N/mm, VG-2 2.64 N/mm, VG-3 2.34 N/mm, and VG-4 0.95 N/mm. It appeared in the order of VG-2 > VG-3 > VG-1 > VG-4.

(2)

Through this study, the viscoelastic adhesive strength of the four materials showed that the value of VG-4 was lower than that of the other three materials, but the average value was 1.99 N/mm. Therefore, this average value can be proposed as a quality standard value. Also, it was confirmed that the test method proposed in this study can measure the viscoelastic adhesion strength of the NHV-SPRG composite sheet in the field.

(3)

In the evaluation of the adhesion stability between the NHV-SPRG composite waterproofing sheet and mortar substrate, although the 4 types show different aspects of the viscoelastic adhesion performance, the change in the peel load value for each peeling section shows a change in increase or decrease, the coefficient of variation using the peel load value (limited to 10 points), the grade of linear regression line, and percent relative range to the maximum/minimum value were compared. It was confirmed that VG-1 was the most stable adhesion state, VG-4, VG-2, and VG-3 were confirmed to have similar adhesive stability.

(4)

Summarizing these results, the quantified viscoelastic adhesive strength of the VG-2 type was the highest, but in the adhesive stability evaluation analyzed by the peel load (N) value for each section, it was confirmed that the VG-1 type waterproofing material was high. Through these results, it was found that NHV-SPRG composite waterproofing sheets with good adhesive strength do not have good adhesive stability. Therefore, as a future study, it is necessary to consider the mixing ratio and quality of raw materials, precision of equipment use, and temperature conditions during construction in order to maintain continuous adhesive stability.