Evaluating the Effect of Curing Time and Resting Time on Moisture Damage Resistance of Asphalt Mixtures Using the Pull-Off Tensile Strength (POTS) Test

Abstract

This article explores the effectiveness of the Pull-off Tensile Strength (POTS) test as a tool for evaluating the moisture damage resistance of asphalt mixtures. Currently, indirect tensile strength (ITS) and the Hamburg Wheel Tracking (HWT) test are used, but they have limitations such as expensive equipment and being heavy. The POTS test is a low-cost and portable alternative. This study investigated the effect of curing time and resting time on the POTS of asphalt-aggregate systems by subjecting samples to different curing times and resting times before testing their tensile strength under dry and wet conditions. The results show that the tensile strength decreases with increasing curing time or exposure to water, indicating that the debonding process between asphalt and aggregates occurs more rapidly with aging. The tensile strength ratio (TSR) of the POTS test for all three asphalt binder types increases with resting time and curing time, with the highest values observed at a curing time of 2 h and a resting time of 5–15 min. Additionally, this study found a strong linear relationship between the tensile strength ratios of ITS and POTS tests, regardless of curing time and resting time. Overall, the POTS test is a promising alternative for evaluating moisture damage resistance in asphalt mixtures.

1. Introduction

Moisture susceptibility testing is a critical aspect of asphalt mixture design and evaluation, as it can significantly impact pavement performance, safety, and maintenance costs. Moisture damage can occur due to the penetration of water into the asphalt mixture, causing the loss of strength and cohesion in the binder and leading to cracking, rutting, and other types of distress.

The development of tests to evaluate the moisture susceptibility of asphalt mixtures dates back to the 1930s [1]. Over the years, numerous tests have been implemented to identify the vulnerability of asphalt mixtures to moisture damage and to simulate the loss of strength that occurs in pavement structures [2,3]. Despite the continuous improvements made in moisture susceptibility tests, there is still a need for a reliable and practical laboratory method that can accurately simulate moisture damage in the field, according to Diab and You (2013) [4].

The laboratory testing procedure for moisture susceptibility of asphalt mixtures is a challenging process. It requires the ability to fully simulate field conditions, such as environmental conditions, traffic, and construction practices. According to Diab and You (2013), efforts were made to develop a test procedure that could accurately determine the susceptibility of an asphalt pavement to moisture damage. However, none of the moisture susceptibility tests were widely accepted due to a lack of repeatability, difficulty in the process, expensive equipment, or a lack of quantitative results [4].

Moisture susceptibility tests typically have a “conditioning” and “evaluation” phase [2]. In the conditioning phase, the asphalt mixture sample is subjected to conditions that simulate the deterioration in the field, including environmental conditions, traffic loads, climatic conditions, air void levels, and others. In the evaluation phase, the sample is then assessed through visual evaluation (qualitative) and physical testing (quantitative). The visual evaluation determines the percentage of retained asphalt coating after the conditioning process, while physical testing evaluates the strength or modulus of the sample. The ratio between the result from the conditioned sample and the result from the unconditioned sample is then computed. If the ratio is less than the standardized value, the sample is considered moisture susceptible [5]. Moisture susceptibility tests can be divided into two categories: tests on loose mixtures (qualitative tests) and tests on compacted mixtures (quantitative tests) [6]. Some of the national standard tests currently used by public agencies include AASHTO T 165/ASTM D 1075, AASHTO T 283/ASTM D 4867, ASTM D 3625, ASTM D 4867, and AASHTO T 324 [7,8,9,10,11].

The indirect tensile strength (ITS) test and the Hamburg Wheel-Track (HWT) test are the most commonly used methods to test the moisture damage resistance of asphalt mixtures [6]. These tests are utilized to determine the susceptibility of asphalt mixtures to moisture damage and to simulate the loss of strength that may occur in pavement structures due to environmental exposure, traffic loads, and other factors [6]. Ongoing developments and refinements have led to these tests becoming more reliable and practical in providing accurate results over the years. However, it is important to note that despite the continuous efforts to improve moisture susceptibility tests, it is still challenging to fully replicate field conditions in a laboratory setting, which could impact the accuracy of the results obtained from these tests. The tests are not effective in identifying adhesive and cohesive failures in asphalt binder and mastic. These tests also have drawbacks such as being expensive and heavy. To overcome these challenges, researchers have turned to the Binder Bond Strength (BBS) test, which is more efficient, requires less equipment and space, and is easier to transport [12,13]. BBS has become a popular choice for studying the resistance of asphalt mixtures to moisture damage, with bond strength being a crucial factor in evaluating the ability of the binder to withstand moisture. The adhesion between asphalt and aggregate is measured in terms of adhesive bond energy [14]. In the past, most researchers focused on the loss of adhesion bond or separation of the bitumen and aggregate surface, ignoring the loss of cohesion, which is more prevalent [15]. The best definition of moisture damage, according to Caro et al. (2008), is provided by Kiggundu and Roberts (1988), who described it as “the gradual functional deterioration of a pavement mixture caused by the loss of adhesive bond between the bitumen and the aggregate surface and/or the loss of cohesive resistance within the bitumen, primarily due to water” [16,17]. Therefore, a test performed directly on the asphalt-aggregate system can effectively evaluate the impact of water on both cohesive and adhesive failure types, resulting in a better understanding of the moisture sensitivity of asphalt mixtures [15,18,19].

Canestrari et al. proposed a method to evaluate the bonding properties of asphalt and aggregate using the Pneumatic Adhesion Tensile Testing Instrument (PATTI), which is repeatable, dependable, and practical [18]. However, there is no standard method to assess moisture damage in the asphalt-aggregate connection. Moraes et al. (2011) conducted research using the Binder Bond Strength (BBS) test to examine the influence of moisture on the adhesion of asphalt and aggregates [12]. They found that the adhesion between asphalt and aggregate is influenced by the type of binder improvement and the pretreatment time under moisture. Chaturabong and Bahia (2016) used the BBS test to investigate the impact of moisture on the cohesiveness of asphalt mastic and discovered that the cohesiveness of mastic is a crucial factor in understanding moisture damage, while adhesive failures are secondary in well-coated and manufactured mixtures [13]. The composition of mastic and its sensitivity to water must be controlled to ensure that pavements have an acceptable level of resistance to moisture.

Moreover, in addition, the use of the Gibbs surface free energy (SFE) method has been recently described in the NCHRP RRD 316 report for assessing moisture resistance and determining adhesion between aggregates and asphalt [20]. Many studies utilizing the SFE method have demonstrated that aging leads to a reduction in energy ratios for asphalt-aggregate combinations, which indicates a decrease in moisture resistance [20,21,22,23,24,25,26]. Xiao and Huang (2022) concluded that the aging process caused a decrease in the dry adhesion energy and an increase in the cohesion energy of the asphalt, leading to reduced wettability of the asphalt over aggregates. As a result, the coating quality of mixtures made with pre-aged asphalt was significantly lower. Also, if the specimen has undergone such aging, the asphalt may not be completely displaced from the aggregate during the test, even if moisture damage has occurred [27].

Recently, there has been a study working on the development of the Pull-off Tensile Strength (POTS) test, which employs the same underlying concept as the Binder Bond Strength (BBS) test. According to Ratchabut and Chaturabong (2020), the temperature for wet conditioning should be set to the maximum PG grade because this method is considered a direct tensile strength method and higher temperatures are necessary [28]. The initial findings indicate a strong correlation between the ITS and POTS tests. However, the curing time and resting time need to be considered as when subjected to moisture conditioning, the debonding process between asphalt and aggregates accelerates as the degree of aging increases [27].

Therefore, it is essential to develop a reliable and practical laboratory method for simulating moisture damage in the field to assess the susceptibility of asphalt mixtures to this type of failure. However, developing a reliable and practical laboratory method for simulating moisture damage in the field has been challenging. The existing tests have limitations in identifying adhesive and cohesive failures in asphalt binder and mastic, which can affect the accuracy of moisture susceptibility testing. Moreover, there is a need to consider the effects of different environmental conditions and construction practices on moisture susceptibility, which can vary depending on the location and climate.

This study focused on evaluating the effect of curing time and resting time on moisture damage resistance of asphalt mixtures using the Pull-off Tensile Strength (POTS) test. The authors believe that POTS is a useful tool for testing due to its advantages such as low-cost equipment and low weight [28]. However, the authors acknowledge that there are still parameters that need to be considered for developing a more promising test. The study aimed to investigate these effects and provided insights into the relationship between curing time, resting time, and moisture damage resistance of asphalt mixtures.

2. Materials and Methods

2.1. Materials

All the bitumen used in the experiment was sourced from Tipco Asphalt Co, Ltd. The laboratory design consisted of determining the impact of curing time and resting time on the moisture damage resistance of the asphalt mixture. The study used 3 different asphalt binders: AC60-70, AC60-70 + carbon black, and Polymer Modified Asphalt (PMA) using styrene-butadiene-styrene (SBS). The Performance Grades (PG) of all binders were evaluated (

Table 1. PG of asphalt binders.

Type of Asphalt Binder	Performance Grade
AC60-70	PG58
AC60-70 + Carbon black	PG76
PMA	PG76

). A commonly used mix gradation was selected and controlled for air voids at 4% as presented in Figure 1. Limestone from Chonburi was selected as aggregate and filler. The factors in the study are shown in

Table 2. Summary of factors for asphalt mixture samples.

Factor	Descriptions
Aggregate type	Limestone
Filler types	Limestone
Mixture types	Dense
Binder types	AC60-70, AC6070 + carbon black, and PMA
Replication	3

. The samples were evaluated using both indirect tensile strength and pull-off tensile strength tests.

2.2. Asphalt Mixture Preparation

The materials were prepared according to ASTM D6926. The aggregates were heated at 100 °C for 24 h to eliminate moisture. The mixture samples were heated for two hours at a constant temperature of 150 °C to decrease their viscosity. Specimens were compacted with a Marshall hammer (75 blows each) to 101.6 mm diameter and 65–75 mm height. After curing for 24 h, volumetric measurements were taken. The Marshall stability test was performed following AASHTO T283 after conditioning, and then ITS was evaluated.

2.3. Experimental Methods

2.3.1. Indirect Tensile Strength Test

To assess the resistance of the asphalt mixture to moisture damage, the ITS test was performed as per AASHTO T283. The test involved measuring the static indirect tensile strength of a specimen by applying a loading rate of 51 mm/min. The split tensile strength was calculated by dividing the peak load by geometric factors, as shown in Equation (1):

$$S_t=\frac{2000P}{\pi tD}$$

(1)

where S_t = indirect tensile strength (kPa), P = maximum load (N), t = specimen height immediately before test (mm), and D = specimen diameter (mm).

Two testing conditions were performed: dry and wet. Samples were left at room temperature for 24 h before being tested in the dry condition. In the wet condition, samples were stored in a temperature-controlled water bath set at 60 ± 1 °C for 24 h. The Superpave system was tested to assess its susceptibility to moisture using the ITS testing device. The samples were evaluated under both conditions and measured for tensile strength. The tensile strength ratio (TSR) was calculated to determine the best mixes for moisture resistance, as shown in Equation (2).

$$\mathrm{T}\mathrm{S}\mathrm{R}=\frac{\mathrm{I}\mathrm{T}\mathrm{S}\left(\mathrm{w}\mathrm{e}\mathrm{t}\right)}{\mathrm{I}\mathrm{T}\mathrm{S}\left(\mathrm{d}\mathrm{r}\mathrm{y}\right)}\times 100$$

(2)

2.3.2. Pull-Off Tensile Strength (POTS) Test

In the 1970s, the POTS test was developed in England as a means of evaluating the strength of in situ concrete in response to issues arising from the use of high-aluminum cement [29]. Additionally, this test can be employed to verify the quality of adherence of the concrete repair material [30]. The POTS test has been standardized in both the UK and the USA (ASTM C 1583) and is recognized as a means of estimating compressive strength in the field [31]. This approach was derived from the standard specified in ASTM C 1583, which is used for measuring strength in concrete. This test could be used to verify the quality of adherence of the concrete repair material and is suitable for both field and laboratory use. The POTS test indicates the direct traction force (tensile strength) of the material in an asphalt-mixed surface layer of a steel disc. The tensile strength could be calculated using the measured tensile force that causes breakage. Figure 2 shows the POTS device and all its settings.

The required apparatus includes a core drill, a core barrel with diamond-impregnated bits, a steel disk, and a tensile loading device with a load-indicating system. The materials required include epoxy adhesive material for bonding the steel disk to the test specimen. Before testing, the asphalt mixture samples had to be prepared in the correct order. Two sets of asphalt mixture samples were prepared, one for the dry condition and one for the wet condition, and both were maintained at ambient temperature for 24 h. The test procedure involves drilling a 5 cm diameter and 4 cm thick circular cut using the coring equipment perpendicular to the surface and attaching the steel disk to the top of the test specimen using the epoxy adhesive as shown in Figure 2b. The tensile loading device is attached to the steel disk using the coupling device, and the tensile load is applied to the test specimen so that the force is parallel to the axis of the cylindrical test specimen. The samples were subjected to two different curing conditions. The first condition involved curing for 24 h at the maximum temperature of the performance grade (PG) in dry conditions. The second condition involved curing in a water bath at the maximum temperature of the PG for a specific time in wet conditions. According to Ratchabut and Chaturabong (2020), the wet conditioning temperature should be set to the maximum PG as this method is considered the direct tensile strength method and requires a higher temperature. If a lower temperature is used, the steel disk would not detach from the asphalt mixture. Therefore, the wet conditioning temperature was set to the maximum PG, which was 58 °C for AC60-70, 76 °C for AC60-70 + carbon black, and 76 °C for PMA. The study took into account both curing time and resting time as factors. Three different curing times (2, 4, and 6 h) were applied based on previous studies on wet conditioning curing time. Resting time was also incorporated into the experimental design based on preliminary results, as different resting times produced different results. To determine the resting time, trial tests were carried out to find the minimum resting time that would not damage adhesion before testing. The resting times were finalized as 5, 10, and 15 min. After the sample was prepared, the tensile load was applied at a constant rate to ensure that the tensile stress increased at a rate of 35 ± 15 kPa/s [5 ± 2 psi/s]. The bond strength in the asphalt mixtures material was reported upon failure in the bond. POTS testing was conducted to determine the tensile load, and Equation (3) was used to calculate the tensile strength.

$$\mathrm{Tensile}\mathrm{Strength}(\mathrm{psi}[\mathrm{MPa}])=\frac{\mathrm{T}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{l}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}\left(\mathrm{l}\mathrm{b}\mathrm{f}\left[\mathrm{N}\right]\right)}{\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{o}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{k}\left(\mathrm{i}{\mathrm{n}}^2\left[{\mathrm{m}\mathrm{m}}^2\right]\right)}$$

(3)

3. Results

3.1. Effect of Curing Time and Resting Time on Pull-off Tensile Strength

The samples were kept in a water bath set to the appropriate PG temperature for 2, 4, and 6 h. The experiment included trials with no rest time where the samples were immediately tested after removal from the bath. Therefore, the resting time was introduced by keeping the samples in 25 °C tap water for 5, 10, and 15 min before testing. The results of the asphalt mixture testing are presented in Figure 3, which illustrates the observed failures.

Table 3. Pull-off tensile strength (POTS) of dry and wet conditions.

Curing Time	Resting Time (Minutes)			Tensile Strength (MPa)
		AC60-70	CV (%)	AC60-70 + Carbon Black	CV (%)	PMA	CV (%)
Dry		0.92	1.7	0.66	0.8	1.05	0.3
Wet-2 h	5	0.32	1.6	0.21	1.9	0.42	1.7
	10	0.54	2.5	0.30	1.8	0.71	1.6
	15	0.65	1.5	0.40	1.7	0.83	5.6
Wet-4 h	5	0.26	2.3	0.15	3.6	0.34	1.5
	10	0.41	5.9	0.26	4.2	0.55	0.5
	15	0.54	6.3	0.35	5.5	0.70	0.6
Wet-6 h	5	0.23	1.2	0.14	2.3	0.33	1.0
	10	0.37	1.9	0.21	2.3	0.48	4.4
	15	0.43	1.3	0.26	2.4	0.60	5.4

shows the effects of curing time and resting time on the pull-off tensile strength (POTS) of the asphalt-aggregate systems, which were tested using the POTS tester.

Table 3 shows the relationship between curing time, resting time, and the tensile strength of asphalt mixtures under different conditions. There were three different concrete mixtures represented: AC60-70, AC60-70 mixed with carbon black, and PMA. The asphalt mixture samples were tested under two conditions: dry and wet (after being exposed to water for 2 h, 4 h, or 6 h). The resting time referred to the amount of time the sample was allowed to rest before testing. The table shows that, in general, the tensile strength decreased as the curing time or exposure to water increased. The addition of carbon black to the AC60-70 mixture and the PMA mixture seemed to result in a reduction in compressive strength compared to the pure AC60-70 mixture. This suggests that the bond between the asphalt and aggregate interface became weaker as the curing time in water increased, leading to a lower pull-off strength value.

Table 4. TSR values of the POTS test.

Curing Time	Resting Time (Minutes)	TSR
		AC60-70	CV (%)	AC60-70 + Carbon Black	CV (%)	PMA	CV (%)
Wet-2 h	5	0.35	4.2	0.32	2.5	0.40	3.2
	10	0.59	3.5	0.45	7.2	0.68	5.6
	15	0.70	4.5	0.61	8.7	0.79	8.5
Wet-4 h	5	0.28	3.2	0.23	2.5	0.32	5.5
	10	0.45	2.9	0.39	0.8	0.53	0.5
	15	0.59	6.1	0.53	3.5	0.67	3.2
Wet-6 h	5	0.25	7.2	0.22	3.2	0.31	2.4
	10	0.40	1.3	0.32	4.5	0.46	1.5
	15	0.47	2.3	0.39	5.4	0.57	1.1

displays the TSR values of the three types of asphalt: AC60-70, AC60-70 with carbon black, and PMA under different curing times (Wet-2 h, Wet-4 h, and Wet-6 h) and resting times (5, 10, and 15 min).

For each curing time, the table shows the TSR values for the three types of asphalt at different resting times. The TSR for all three asphalt binder types (PMA, AC60-70, AC60-70 + carbon black) increased with resting time and curing time, with the highest values observed at a curing time of 2 h and a resting time of 5–15 min. The highest TSR for PMA was 0.40–0.79 (curing time 2 h), 0.32–0.67 (curing time 4 h), and 0.31–0.57 (curing time 6 h). The highest TSR for AC60-70 was 0.35–0.70 (curing time 2 h), 0.28–0.59 (curing time 4 h), and 0.25–0.47 (curing time 6 h). The highest TSR for AC60-70 + carbon black was 0.32–0.61 (curing time 2 h), 0.23–0.53 (curing time 4 h), and 0.22–0.39 (curing time 6 h).

Overall, the TSR values for AC60-70 with carbon black were lower than the TSR values for AC60-70 and PMA at each curing time and resting time. This suggests that the addition of carbon black to AC60-70 asphalt had a positive effect on its TSR properties. The TSR values for all three types of asphalt decreased as the curing time increased, indicating that longer curing times result in stronger asphalt.

3.2. Sensitivity Analysis

The sensitivity analysis was to examine how the tensile strength ratio of the pull-off tensile strength method changes as the resting time of the mixture samples changes. This analysis aimed to determine the impact of resting time on the TSR values. The results of this analysis could help identify the optimal resting time for POTS testing and guide the development of more effective and efficient testing procedures.

The tensile strength ratio changed over time because of the effect of resting time.

Table 5. The sensitivity analysis of the POTS-TSR with different resting times.

Asphalt Type	TSR
	Resting Time (Minutes)
	5	%Diff (5–10)	10	%Diff (10–15)	15
2 h curing time
AC60/70	0.35	42%	0.59	16%	0.70
AC60-70 + Carbon black	0.32	29%	0.45	26%	0.61
PMA	0.40	41%	0.68	14%	0.79
4 h curing time
AC60/70	0.28	37%	0.45	24%	0.59
AC60-70 + Carbon black	0.23	41%	0.39	25%	0.53
PMA	0.32	38%	0.53	21%	0.67
6 h curing time
AC60/70	0.25	38%	0.40	13%	0.47
AC60-70 + Carbon black	0.22	32%	0.32	18%	0.39
PMA	0.31	32%	0.46	20%	0.57

compares the properties of three types of asphalt: AC60/70, AC60-70 with carbon black, and PMA at different curing times (2 h, 4 h, and 6 h). The table shows the measured TSR (tensile stress ratio) and %Diff (percent difference) at different resting times (5, 10, and 15 min).

At a the 2 h curing time, the TSR values for AC60/70, AC60-70 with carbon black, and PMA were 0.35, 0.32, and 0.40, respectively. The %Diff values indicate that there was a 42% difference between the TSR at 5 min and 10 min for AC60/70, while there was a 29% difference for AC60-70 with carbon black and 41% difference for PMA. The TSR and %Diff values for the other resting times are also shown in the table.

At the 4 h and 6 h curing times, the TSR and %Diff values changed for all three types of asphalt, but the overall trend showed that the TSR values for AC60-70 with carbon black were lower than the TSR values for AC60/70 and PMA. The %Diff values also showed that the difference between the TSR at different resting times decreased as the curing time increased. The lowest TSR values and the smallest %Diff values were observed at the 6 h curing time. The conclusion was that longer rest time leads to less sensitivity in TSR.

3.3. Correlation between TSR-POTS and TSR-ITS

The results of the indirect tensile strength and pull-off tensile strength tests were compared to understand the relationship between the two methods in determining the moisture damage resistance of the asphalt mixture. The tensile strength ratio (TSR) from both tests was used as an indicator of moisture damage. The correlation between the TSRs from the two tests provides evidence for the potential of POTS as an alternative method for measuring moisture damage in asphalt mixture. The comparison of the TSRs from the two methods helps to understand the accuracy and reliability of the POTS test in quantifying the moisture damage resistance of the asphalt mixture.

Table 6. Tensile strengths by the ITS test.

Condition	Tensile Strength (MPa)
	AC60-70	CV (%)	AC60-70 + Carbon Black	CV (%)	PMA	CV (%)
Dry	1041.34	4.3	727.02	10.0	1060.18	4.5
Wet	968.73	5.7	645.10	9.1	992.83	3.1

shows the values of the ITS test on asphalt mixtures using different asphalt binders in dry and wet conditions. The results indicate that PMA had the highest value in dry conditions (1060.18 MPa) while AC60-70 with carbon black had the lowest value (727.02 MPa). In wet conditions, PMA also had the highest value (992.83 MPa) while AC60-70 with carbon black again had the lowest value (645.10 MPa). The values between the dry and wet conditions had a small difference, with wet conditions showing slightly lower values. The coefficient of variation (CV) was also provided for each value and ranged from 2% to 5%. The results indicate that the TSR values for all types of asphalt binder were within the typical specification threshold and showed high resistance to moisture damage. The highest TSR value was 0.97 for PMA and the lowest TSR value was 0.89 for AC60-70 with carbon black (Figure 4). The findings indicate that the minimum tensile strength ratios, as specified by the typical threshold of 0.7, were surpassed by all the TSR values.

The tensile strength ratio (TSR) was an important factor in evaluating the strength and durability of asphalt mixtures. The relationship between the TSR from POTS (Performance-Oriented Testing System) and ITS (Indirect Tensile Strength) could be used to assess the feasibility of using POTS as a substitute method for estimating moisture damage. Curing time and resting time had a significant impact on TSR values, and it was important to consider these factors when comparing the TSR from POTS and ITS.

The results of the comparison between the TSR values of ITS and POTS tests (as shown in Figure 5) suggest that there was a strong linear relationship between the two tests, even though there was no equality between them. The results also showed that the slope relationship between ITS and POTS is significant for all three curing times (2, 4, and 6 h), with an R² range of 0.68–0.92, indicating a moderate to strong correlation between the tensile strength ratios obtained from the ITS and POTS tests in this study. The 2 h curing trend tended to be closer to the equivalent line compared to 4 and 6 h curing. This indicates that TSR values from 2 h curing were easier to shift to the equivalent line compared to longer curing times. Additionally, the results showed that all samples in the ITS test were above the equality line, which was likely due to the difference in the concept of strength obtained from ITS and POTS tests. ITS used compressive strength, which tended to yield lower values of strength compared to the tensile strength used in the POTS test. The TSR values from the ITS test showed positive and rational relationships with those from the POTS test, suggesting that both tests could provide useful information about the strength and durability of asphalt mixtures. It is important to note that the correlation coefficients can be influenced by various factors, such as the sample size, test conditions, and material properties.

4. Discussion

The research aimed to assess the resistance of moisture damage on asphalt-aggregate systems by evaluating their moisture susceptibility. Both Indirect Tensile Strength and Pull-Off Tensile Strength methods were used in the experiment. The samples were subjected to water bath conditioning for varying durations of 2, 4, and 6 h, and then tested for their pull-off tensile strength under both dry and wet conditions, with different resting times of 5, 10, and 15 min.

The results showed that the tensile strength decreases as the curing time or exposure to water increases. This is corresponding to the statement from previous studies that the debonding process between asphalt and aggregates (i.e., the separation of the asphalt from the surface of the aggregate) occurs more rapidly as the level of aging increases, resulting in moisture susceptibility [27]. The TSR of the POTS test for all three asphalt binder types increased with resting time and curing time, with the highest values observed at a curing time of 2 h and a resting time of 5–15 min. This is logical since, as the resting time increases, more moisture in the asphalt mixture evaporates, resulting in increased stiffness.

The results suggest that there was a strong linear relationship between the tensile strength ratios of the ITS and POTS tests, regardless of the curing time and resting time. The R² values, which indicate the degree of correlation between the variables, were high for most of the samples, indicating a strong correlation.

These findings were consistent with a prior study on the development of POTS testing, which also showed a strong relationship between the variables. The comparison between TSR values of ITS and POTS tests showed a strong linear relationship between the two tests. The current study demonstrated a similarly strong relationship between variables as a prior study on the development of POTS testing, as shown in Figure 6 [28]. The findings were comparable to the stripping point observed in the Hamburg Wheel Tracking (HWT) test for samples with the same gradation but different filler types and contents, which exhibited a similar trend to the TSR values in the POTS test [32]. The current study extended this finding by showing that the relationship holds for a range of curing and resting times. However, all samples in the ITS test were above the equality line, which may be due to the difference in the concept of strength obtained from the ITS and POTS tests. ITS uses compressive strength, which tended to yield lower values of strength compared to the tensile strength used in the POTS test. The results also showed that the resting time has an impact on the relationship between the tests. The 2 h curing time with a 5 min resting time had the highest correlation coefficient (y = 0.5649x + 0.7166, R² = 0.7259), while the 6 h curing time with a 5 min resting time had the lowest correlation coefficient (y = 0.4833x + 0.7924, R² = 0.6833). This suggests that a shorter resting time may lead to a stronger correlation between the tests.

Overall, the TSR values from the ITS test showed positive and rational relationships with those from the POTS test, suggesting that both tests could provide useful information about the strength and durability of asphalt mixtures. The choice of test may depend on the specific application and conditions of the pavement.

5. Conclusions

This study investigated the impact of curing time and resting time on the TSR values of the Pull-off Tensile Strength test. It was found that the tensile strength of asphalt mixtures decreased with longer curing time in water. PMA binder had the highest TSR values compared to AC60-70 and AC60-70 + carbon black, with the highest TSR values observed at 2 h of curing time and 5–15 min of resting time. Longer resting time led to less sensitivity in TSR and the addition of carbon black to the AC60-70 mixture resulted in a reduction in tensile strength. Both ITS and POTS tests can provide useful information on the strength and durability of asphalt mixtures, and further studies are needed to verify the optimum resting time and to test different resting times for contrast performance.