Evaluation of the Impact of Oxidation and Moisture Conditioning on Indirect Tensile Asphalt Cracking Test Parameters and the Tensile Strength Ratio

Abstract

This study evaluated the effect of laboratory oxidation and testing temperature on asphalt mixtures’ moisture damage. The purpose of this study was to present a modified American Association of State Highway and Transportation Officials (AASHTO) T283 testing procedure that is more sensitive to moisture damage in asphalt mixtures. This study assessed the original test conditioning (OTC), short-term oxidative conditioning (STOC), and long-term oxidative conditioning (LTOC). Three performance grades (PGs)—PG 64-22, PG 76-22, and PG 52-34—and three test temperatures—the ambient laboratory temperature (25°C), asphalt intermediate temperature, and climate intermediate temperature (CIT)—were selected. To better understand the oxidation and moisture conditioning effects, AASHTO T283’s Tensile Strength Ratio (TSR%) was utilized. Furthermore, additional analysis of the load–displacement curve parameters was carried out using the Indirect Tensile Asphalt Cracking Test, where an interaction diagram that included the mixture toughness (G_f), ductile–brittle behavior ratio (l₇₅/m₇₅), and their interactions with CT_index was created. Statistical analysis was conducted using analysis of variance and Tukey’s post-hoc analysis to determine an appropriate oxidation and testing temperature where moisture damage is most sensitive. Results showed that higher oxidation and CIT had the greatest impact on TSR%. Moisture conditioning increased the CT_index and oxidation reduced the CT_index. The increase in CT_index was attributable to an increasing l₇₅/|m₇₅| and decreasing G_f. The CT_index decreased because of reducing l₇₅/|m₇₅| and increasing G_f. The statistical analyses showed that asphalt mixtures are more susceptible to moisture damage at STOC and LTOC than OTC. Overall, the findings highlight the need for an alternative oxidation conditioning and testing at the CIT to more accurately detect moisture damage in asphalt mixtures.

Keywords

moisture damage/stripping asphalt mixture evaluation and performance aging pavement structural testing and evaluation asphalt characterization

Environmental factors (oxidative aging) usually dominate at the early age of asphalt pavement and moisture damage typically comes into play after few years of pavement construction ( 1 ). During the asphalt pavement service life, moisture damage occurs in two ways: reducing the cohesion in the asphalt binder (cracks within the binder) and compromising the adhesion between the asphalt binder and aggregate (also known as stripping). Reduced cohesion and adhesion bonding can significantly expedite the deterioration of asphalt pavement ( 2 ). As a result, highway agencies and the pavement industry established criteria for designing moisture damage resistant asphalt mixtures.

Moisture damage of asphalt mixtures is usually evaluated using two types of tests: qualitative tests performed on loose mixes and quantitative tests performed on compacted specimens. The most common tests on loose samples are as follows: (1) the static immersion test (American Association of State Highway and Transportation Officials [AASHTO] T182) ( 3 , 4 ); and (2) the boiling water test (ASTM D3625) ( 5 , 6 ). Qualitative tests are carried out on asphalt-coated aggregate mixes submerged in water. When compared to quantitative tests, these experiments are unable to reproduce the pore pressure, traffic conditions, or mix design parameters required to understand the moisture susceptibility of asphalt mixes. In addition, the results of these tests are primarily qualitative, and their analysis is subjective because it is based on the evaluator’s judgment and experience.

Quantitative moisture-induced damage tests can be carried out on lab-compacted specimens, field cores, or slabs. Test procedures basically carry out different types of moisture conditioning to asphalt specimens, followed by a comparison of indirect tensile strength (ITS) between the unconditioned (UC) and conditioned asphalt specimens. The modified Lottman test (AASHTO T283) is the most widely used procedure for assessing moisture damage in asphalt mixtures. Other tests, such as the immersion-compression test (ASTM D1075/AASHTO T165) ( 7 , 8 ) and Tunnicliff–Root test (ASTM D4867) ( 9 , 10 ), were also developed. The immersion-compression test was withdrawn from the ASTM database in 2019 ( 11 ), while the Tunnicliff–Root test is similar to the widely used AASHTO T283 ( 3 , 12 ). Other quantitative tests, such as the Hamburg wheel tracking test (AASHTO T324) ( 13 , 14 ), saturated aging tensile stiffness (SATS) ( 15 , 16 ), environmental conditioning system (ECS) (AASHTO TP34) ( 17 , 18 ), and moisture-induced sensitivity test (MIST) ( 19 , 20 ), are also available.

Despite the widespread use of AASHTO T283, several potential improvements can still be made to address some of its shortcomings. Researchers and highway agencies alike have pointed out various limitations of the AASHTO T283 test ( 21 – 25 ). Some of these limitations include the following:

it does not always predict moisture sensitivity as observed in the field (22, 23, 26 –28);

it shows disagreement in results obtained from 100-mm Marshall and 150-mm Superpave gyratory samples ( 23 );

it does not offer a clear justification for poor or good performance ( 24 ); and

it allows for only a narrow range of water-saturation levels and one single freeze–thaw cycle.

With regards to lack of relatability of AASHTO T283 results to the field, researchers observed that mixes might satisfy the laboratory tensile strength ratio (TSR%) criterion (e.g., min. 80%); however, that was not reflected in field performance (22, 23, 26 –28). Solaimanian et al. ( 22 ) reported that laboratory TSR% values for good-performing field asphalt specimens were unexpectedly low, whereas those for poor-performing mixtures were unexpectedly high. There is also no consensus among studies on the agreement between AASHTO T283 results obtained from Marshall- and Superpave gyratory-compacted samples. For example, a survey conducted by the AASHTO materials reference laboratory (AMRL) reported that using 100 mm (4 in.) Marshall specimens for the AASHTO T283 test had better agreement with the field performance ( 29 ). In contrast, Epps et al. ( 23 ) reported that using 150 mm Superpave gyratory compactor (SGC) specimens when conducting the AASHTO T283 test showed better correlation with field performance than using 100 mm Marshall-compacted specimens. With respect to the subjectivity of testing and not offering a clear justification for poor or good performance, the evaluation criterion for AASHTO T283 relies only on a pass/fail and allows a narrow saturation degree of 70%–80% and one cycle of freeze–thaw.

In addition to the above limitations, the oxidative conditioning procedure (to simulate pavement aging) in the current AASHTO T283 test may not necessarily be representative of field climatic conditions. A severely aged asphalt binder (or mixture) can accelerate the occurrence of moisture damage in flexible pavements ( 1 , 30 , 31 ). After being built for many years, asphalt pavements suffer moisture damage, where aging has already taken place. In other words, aging is a major factor when characterizing the moisture damage of asphalt mixtures. This was observed by Bazuhair et al. ( 1 ), where they monitored multiple performance characteristics of asphalt pavement over the course of eight years in the field. The early performance of the asphalt pavement was seen to be dominated by the stiffening effects of oxidation, with the tensile strength increasing throughout the first four to five years and Hamburg rutting decreasing, whereas in future years, as the effects of moisture became more dominant, the tensile strength started to decrease and Hamburg rutting began to increase.

Epps et al. ( 23 ) also evaluated the effect of different oxidative conditioning variations of loose and compacted mix methods. Asphalt mix oxidization was identified as a significant factor influencing both UC and moisture-conditioned (MC) asphalt mixtures. Previous studies have reported that the sensitivity of moisture-induced damage is significantly affected by asphalt binder and mixture oxidative conditioning ( 32 – 34 ). Liang ( 35 ) evaluated the effect of oxidation on the susceptibility of asphalt mixtures to moisture-induced damage and found that oxidative conditioning increased their susceptibility.

In summary, oxidative conditioning and moisture damage of asphalt mixtures are frequent causes of pavement distresses. AASHTO T283 is the most commonly used test for evaluating moisture-induced damage of asphalt mixtures. However, previous studies have highlighted several limitations of the AASHTO T283 test where conflicting observations were seen in the literature. Few studies have considered an appropriate oxidative conditioning levels for asphalt mixtures where moisture damage has the most severe effects (highest sensitivity to moisture damage). Therefore, additional research is needed to fill these gaps in the current practice and to establish consensus among researchers and state agencies with respect to moisture damage of asphalt mixtures.

Research Goal and Scope

The goal of this research study was to develop a modified laboratory test procedure based on AASHTO T283 that better captures the moisture sensitivity of asphalt mixtures. Specifically, the objectives of this study were to assess the impacts of the asphalt oxidation conditioning and testing temperature on the severity of moisture-induced damage and recommend reasonable oxidative conditioning to be applied during AASHTO T283 testing. Asphalt binder types used in two different climatic regions were selected for this study to encompass a broader range of climatic regions. In addition, a detailed analysis of the AASHTO T283 load–displacement curve using Indirect Tensile Asphalt Cracking Test (IDEAL-CT) parameters was performed to characterize the mechanical response at a deeper level than only using the ITS.

Materials and Experimental Plan

Materials

Three different asphalt binder grades were chosen in this study. These included the following: performance grade (PG) 52-34, which is commonly used in cold regions, such as Alaska and Canada ( 36 ); PG 64-22 and styrene-butadiene-styrene (SBS)-modified PG 76-22, which are commonly used in warmer regions, such as New Jersey ( 37 ). Asphalt mixtures meeting the Federal Aviation Administration (FAA) P-401 specifications for low-weight aircraft (60,000 pounds or less) were prepared as part of this study ( 38 ). Mixes were designed at 50 gyrations (N_des) and a design air void content (AVC%) of 3.5% ± 0.5% with a minimum voids in mineral aggregate (VMA) value of 15%. The mixes were prepared using granite aggregates with a nominal maximum aggregate size (NMAS) of 12.5 mm, and the dense-graded gradation used in this study is presented in Figure 1. The same aggregate blend was used with the three binder types to design three mixes (the results are shown Table 1). As seen from Table 1, the mixture prepared using PG 52-34 obtained an optimum binder content (OBC%) of 5.7%, which was slightly higher than the 5.5% obtained for mixtures prepared using PG 64-22 and PG 76-22.

Figure 1.

Gradation curve for Federal Aviation Administration P-401 asphalt mixtures.

Table 1.

Mix Design Results

Binder type	OBC (%)	AVC (%)	VMA (%)
PG 64-22	5.5%	4.0%	16.4%
PG 76-22	5.5%	3.7%	16.4%
PG 52-34	5.7%	3.6%	16.6%

Note: OBC = optimum binder content by total mixing weight; AVC = air void content; VMA = voids in mineral aggregate; PG = performance grade.

Experimental Plan

The experimental plan was designed to evaluate the impact of asphalt mixture oxidative aging and testing temperature on asphalt mixture performance. In general, the experimental plan covered three oxidative conditioning levels: (1) original test conditioning (OTC); (2) short-term oxidative conditioning (STOC); and (3) long-term oxidative conditioning (LTOC). Three testing temperatures were also selected: (1) the ambient laboratory temperature (ALT) (25°C); (2) the asphalt intermediate temperature (AIT); and (3) the climate intermediate temperature (CIT). The AIT was selected to consider the intermediate temperature of the asphalt binder, as suggested in Note 5 of the IDEAL-CT specification (ASTM D8225-19). The equation for AIT is expressed in Equation 1:

AIT = \frac{PG HI + PG LT}{2} + 4

(1)

where AIT is the AIT (°C), PG HI is the binder high PG temperature (°C), and PG LT is the binder low PG temperature (°C).

The AASHTO T283 testing temperatures used in this study are shown in Table 2. It is noted that the specimens prepared with the PG 64-22 mixture shared both the ALT and AIT of 25°C, making it a neutral case. The specimens prepared with PG 76-22 were tested at 25°C and an increased AIT of 31°C. Finally, the specimens prepared with PG 52-34 were evaluated at 25°C as well as a decreased AIT of 13°C. Based on the Long-term Pavement Performance (LTPPBind) database, PG 52-34 is usually used in cold regions (i.e., Alaska and Canada) where the CIT is 13°C, whereas 25°C is the actual CIT for pavements constructed using PG 76-22 and PG 64-22.

Table 2.

Temperatures Used when Conducting American Association of State Highway and Transportation Officials T283 Testing

Binder grade	Ambient laboratory temperature (°C)	Asphalt intermediate temperature (°C)	Climate intermediate temperature (°C)
PG 64-22	25	25	25
PG 76-22	25	31	25
PG 52-34	25	13	13

Note: PG = performance grade.

The analysis of parameters obtained from the load–displacement curves included determining TSR% by dividing the ITS of MC specimens by that of UC specimens. Further, the UC and MC comparison was carried out for the parameters used in the IDEAL-CT and cracking tolerance index (CT_index) calculation (i.e., G_f, l₇₅, and |m₇₅|), according to ASTM D8225-19. Figure 2 presents an overall flow chart of the experimental plan utilized in this study; specific details with respect to specimen preparation, conditioning, and testing are provided in the subsequent sections.

Figure 2.

Laboratory experimental plan.

Specimen Preparation

Specimen preparation followed AASHTO T283 specifications. Asphalt binders were heated in an oven and occasionally stirred to maintain homogeneity. The heated asphalt binder was then mixed with preheated aggregates to obtain loose-asphalt mixtures. At this point, loose-mix oxidative conditioning procedures were performed, as applicable, following the oxidation procedures described in the next section. Loose-mix samples were then allowed to cool for 2.0 ± 0.5 h at room temperature. After cooling down, loose-mix samples were placed in a 60°C ± 3°C oven for 16 ± 1 h (i.e., the AASHTO T283 mix oxidation process). Once loose-mix oxidative conditioning and AASHTO T283 aging were completed, mixes were then heated to the P-401 compaction temperature of 157°C (315°F) for 2.0 h ± 10 min and compacted using a SGC targeting 7.0% ± 0.5% AVC%.

Loose-Mix Oxidative Conditioning Procedures

Previous studies have demonstrated that loose-mix oxidation yields more uniform conditioning than oxidation of compacted specimens ( 39 – 43 ). Therefore, loose-mix conditioning was selected for use in this paper as follows.

OTC: In this level, no additional loose-mix oxidation of asphalt mixtures was conducted. Sample preparation for OTC consisted of cooling the loose mix after mixing to room temperature, followed by AASHTO T283 oxidation of 16 h, then heating and compacting the loose mix at the compaction temperature. This serves as the control (i.e., standard practice based on typical T283 testing).

STOC: STOC consisted of loose-mix oxidative conditioning for 2.0 h at the mix compaction temperature (i.e., 157°C or 315°F). Note that STOC was performed during the mix design phase of this work to establish the mix designs.

LTOC: LTOC consisted of loose-mix oxidation at 85°C for 120 h (5 days). This practice followed procedures in AASHTO R30 for long-term conditioning; however, it is important to note that R30 specifies conditioning of compacted specimens rather than loose mix. This departure from typical R30 practices was an intentional step taken.

Moisture Conditioning

As in normal AASHTO T283 testing, only a subset of all specimens was set aside for moisture conditioning, including a freeze–thaw cycle, while the other subset was tested without moisture conditioning. Moisture conditioning first required vacuum saturating specimens to 70%–80% saturation. These were then wrapped in plastic film and placed in a plastic bag containing 10 mL ± 0.5 mL of water. Specimens were subjected to a freeze–thaw cycle in a freezer at −18°C ± 3°C for 16–24 h before being removed from the plastic bag and conditioned (thawed) in a 60°C water bath for 24 ± 1 h.

IDEAL-CT

In 2017, the IDEAL-CT was developed by Zhou et al. ( 44 ). The test method adopted by ASTM D8225-19 describes the determination of the cracking tolerance index, CT_index, at 25°C, and other parameters from the same load–displacement curve used in AASHTO T283 (Figure 3). Parameters from T283 and D8225 are investigated in this paper for evaluating moisture damage. Multiple research studies have adopted the IDEAL-CT to evaluate the post-peak performance of asphalt mixtures (36, 45 –48). Five different parameters were considered in the IDEAL-CT calculations, as shown in Equation 2:

C T_{index} = \frac{t}{62} \times \frac{l_{75}}{D} \times \frac{G_{f}}{| m_{75} |}

(2)

where CT_index is the cracking tolerance index; G_f is the failure energy (J/m²); |m₇₅| is the absolute slope of load and displacement from 85% to 65% post-peak load (N/m); l₇₅ is displacement corresponding to 75% of the peak load at the post-peak stage (mm); t is the specimen thickness (mm); and D is the specimen diameter (mm).

Figure 3.

IDEAL-CT parameters: (a) load-displacement curve parameters and (b) IDEAL-CT interaction chart parameters.

With regards to moisture damage evaluation, IDEAL-CT parameters do not consider the peak load (P₁₀₀) accounted for in TSR% calculations to be the deciding factor for moisture damage performance in asphalt mixtures. Instead, the IDEAL-CT detects other post-peak parameters obtained from the load–displacement curve, such as G_f, which reflects the energy required to break the specimen (considering the area under the curve and the work of fracture [W_f]), where a higher G_f is desirable for better performance. Similarly, |m₇₅| provides an evaluation of the post-peak rate of failure in an asphalt specimen (i.e., crack propagation rate). As seen in Figure 3a, a lower |m₇₅| is desirable for better resistance to crack propagation. Lastly, l₇₅ gives an indication of the cracking deformation tolerance at 75% peak load (P₇₅) after the peak, where a higher value is desired. These parameters used in the CT_index calculation can also be assessed individually to evaluate the effect of the oxidative conditioning and testing temperature on moisture damage. The CT_index values are also sensitive to oxidation, where previous studies reported a decrease in CT_index as asphalt mixtures were exposed to oxidative conditioning ( 36 , 49 , 50 ).

To highlight the impacts of oxidation on moisture damage using IDEAL-CT parameters, the findings were assessed further by plotting average G_f against the average l₇₅ over |m₇₅| ratio (l₇₅/|m₇₅|) on an interaction diagram developed by the National Center for Asphalt Technology (NCAT) ( 51 ). The interaction diagram also contains a sequence of CT_index contour curves that connect the input test parameters of G_f and l₇₅/|m₇₅| to the final CT_index value. Here, G_f represents the toughness of asphalt mixtures, while l₇₅/|m₇₅| describes their relative ductile–brittle behavior. According to Figure 3b and Equation 2, a greater CT_index will result from an increase in G_f and l₇₅/|m₇₅|. Consequently, asphalt mixtures with greater CT_index values will be closer to the upper right corner of the interaction diagram. It is possible for data points on a contour curve to have the same CT_index value but different G_f and l₇₅/|m₇₅| values. A specific mixture combination can produce a high G_f and a low l₇₅/|m₇₅|, while another mixture combination can produce a low G_f and a high l₇₅/|m₇₅|, with both yielding the same CT_index. The interaction diagram provides a comprehensive vantage point for interpreting IDEAL-CT results, as opposed to relying solely on the CT_index parameter.

According to Figure 3b, it is expected that the influence of oxidative conditioning would result in an increase in G_f and a reduction in l₇₅/|m₇₅|, resulting in an expected decrease in CT_index. In contrast, moisture conditioning is expected to weaken the asphalt mixes, therefore reducing G_f (weakening the toughness of the mix) and increasing l₇₅/|m₇₅| (pseudo-enhancing the ductility of the mix). Although it depends on the degree of G_f reduction, the increased l₇₅/|m₇₅| may actually yield a higher CT_index, which is contrary to the anticipated effect of moisture conditioning on asphalt mixtures. Therefore, it is determined that l₇₅/|m₇₅| and CT_index may not be suitable parameters for evaluating moisture-induced damage, and G_f may be better suited for moisture damage evaluations.

Laboratory Performance Results and Discussion

Indirect Tensile Strength and Tensile Strength Ratio (TSR%)

Impact of Binder Type on ITS and TSR%

Figure 4, a–c, presents the 25°C (ALT) ITS and TSR% for mixtures prepared using PG 64-22, PG 76-22, and PG 52-34, respectively. As can be seen from Figure 4, the tensile strength of asphalt mixtures increased with a higher asphalt grade. Asphalt mixtures prepared using PG 76-22 showed the highest ITS, followed by PG 64-22 then PG 52-34. This is expected, since PG 76-22 is SBS polymer modified and stiffer than the other binder types. As seen in Figure 4a, oxidative conditioning increased ITS, but this was more prominent in UC mixtures (2–2.4 times that of OTC) than MC (1.5–1.6 times that of OTC) mixtures. As a result, moisture sensitivity increased meaningfully, with TSR% decreasing from above 100% to 85% (STOC) to 71% (LTOC). Figure 4b displays a similar trend for the PG 76-22 mixture with the key difference being that the overall ITS values are higher than for PG 64-22; otherwise, TSR% trends with oxidation are similar (99% to 85% [STOC] to 68% [LTOC]). Similar trends were reported in previous studies ( 52 – 55 ) where ITS increased and TSR% decreased as the oxidation level increased.

Figure 4.

Indirect tensile strength (ITS) and tensile strength ratio (TSR%) results for asphalt mixtures tested at 25°C: (a) mixes with PG 64-22, (b) mixes with PG 76-22 and (c) mixes with PG 52-34.

Figure 4c is different from Figure 4, a and b . The effect of oxidative conditioning showed a slight ITS increase from 0.3 MPa at OTC to approximately 0.6 Mpa at STOC and LTOC levels. This suggests that mixtures prepared using the softer PG 52-34 binder and tested at ALT (25°C) were minimally affected by oxidation as measured by ITS and TSR%. Interestingly, while TSR% decreased from 97% to 89% when STOC was performed, it increased to above 100% after LTOC, which is not logical. The ITS values for PG 52-34 mixes were consistently lower than those of other asphalt mixes and oxidation levels, ranging from 0.52 to 0.62 Mpa for STOC and LTOC, respectively. These values were the lowest across all asphalt mixes and oxidation levels, even lower than the values for the PG 64-22 mixes at OTC (i.e., 0.65 Mpa).

Figure 4c indicates that the oxidation process did not appear to have a significant effect on the ITS values for the PG 52-34 mixes, unlike the PG 64-22 and PG 76-22 mixes, as shown in Figure 4, a and b , respectively. However, it is noted that the lower ITS values observed for the PG 52-34 mixes may be attributed to the softer nature of the binder, rather than the oxidation process alone. Moreover, the testing of moisture damage for mixes with softer binders, such as PG 52-34 at the original intermediate testing temperature of 25°C, may not be representative of the actual intermediate temperature of the asphalt binder, which is typically lower (i.e., 13°C in this case). This difference in temperature can affect the performance of the asphalt mix, especially for softer binders. Therefore, while the results suggest that oxidation did not have a substantial impact on the ITS values for the PG 52-34 mixes, it is important to consider other factors, such as the stiffness of the binder and testing temperatures, when evaluating the moisture damage of asphalt mixtures under different oxidation levels.

Impact of Testing Temperature on ITS and TSR%

Figure 5, a and b , presents the ITS and TSR% for mixtures prepared using PG 76-22 and PG 52-34 and tested at the AIT, respectively. As seen from Figure 5a, increasing the test temperature from 25°C to 31°C caused a reduction in the ITS of PG 76-22 mixes, as would be expected.

Figure 5.

Indirect tensile strength (ITS) and tensile strength ratio (TSR%) results at the asphalt intermediate temperature: (a) mixes with PG 76-22 at 31°C and (b) mixes with PG 52-34 at 13°C.

The findings show that with the increasing oxidative conditioning level, the moisture sensitivity remains the same (within ≈4%). The TSR % was 64% ± 2%, indicating that increasing the testing temperature above the ALT of 25°C to the AIT negatively affected the sensitivity of moisture damage to the effect of oxidative conditioning. Interestingly, it is worth noting that the OTC TSR% dropped noticeably from 25°C to 31°C, going from 99% to 66% (Figure 4b compared to Figure 5a). Effectively, the results suggest that changing from the ALT to the AIT has a meaningful impact on TSR% with no additional oxidation, but testing at the AIT has no sensitivity to oxidation compared to testing at the ALT.

As seen from Figure 5b, reducing the test temperature from 25°C to 13°C led to increasing ITS values, as expected. The ITS value also progressively increased from 0.60 to 1.48 Mpa from OTC to LTOC. Unlike that of 25°C testing, the TSR% showed greater and more rational sensitivity to oxidative conditioning when tested at the AIT. The TSR% values reduced from 113% (OTC) to a relatively similar TSR of 86% at the STOC level and 85% at the LTOC level, where a higher effect of moisture conditioning can be observed. In the case of PG 52-34, the ALT was a less appropriate testing temperature, and testing at the AIT greatly alleviated the inconclusive results from ALT testing.

It is important to note that the impact of the binder type and testing temperature was limited to the aggregate type and gradation used in this study, as well as the moisture conditioning protocols (i.e., 70%–80% saturation and one single freeze–thaw cycle) specified in AASHTO T283. However, other factors, including temperature, saturation levels, freeze–thaw cycles, moisture diffusion, and other potential moisture-related weather conditions, would provide a more accurate correlation with field performance. According to previous studies ( 23 , 56 , 57 ), increasing the saturation level and the number of freeze–thaw cycles would ultimately lead to a reduction in the ITS and TSR%.

IDEAL-CT Interaction Charts

Impact of Binder Type on the IDEAL-CT

Figure 6 presents IDEAL-CT interaction diagram results at the ALT. Overall, moisture conditioning increased CT_index for all oxidative conditioning levels. Higher CT_index implies that the mixture fatigue cracking performance has improved, although this is not a rational interpretation. In Figure 6, moisture conditioning, which was expected to weaken asphalt strength, increased l₇₅/|m₇₅| and CT_index, indicating that moisture conditioning had the contrary anticipated effect of moisture conditioning on asphalt mixtures: higher CT_index and l₇₅/|m₇₅|.

Figure 6.

Indirect Tensile Asphalt Cracking Test interaction diagram (25°C) results: (a) mixes with PG 64-22, (b) mixes with PG 76-22 and (c) mixes with PG 52-34.

As seen from Figure 6a, the UC results for PG 64-22 mixtures showed a reduction in the CT_index after STOC and LTOC because of an increase in G_f (toughness behavior) and a reduction in l₇₅/|m₇₅| (more brittle behavior). Moisture conditioning increased the CT_index because of increasing l₇₅/|m₇₅| (more ductile behavior) and reducing G_f (low toughness). The highest impact of moisture conditioning was seen at the LTOC level, where the highest drop in G_f was seen.

As can be seen in Figure 6b, PG 76-22 mixtures after oxidative conditioning had high G_f (between 11,000 and 13,000 J/m²) and low l₇₅/|m₇₅| (between 0.2 and 2.2 mm²/kN). The largest decrease in G_f because of moisture conditioning was seen after LTOC. Mixtures prepared using the softer PG 52-34 binder (Figure 6c) had low G_f values (between 2900 and 6500 J/m²) and high l₇₅/|m₇₅| values (between 1.8 and 6.2 mm²/kN).

It can be observed that moisture damage behaviors are more evident when G_f is higher. Moisture conditioning for PG 64-22 and PG 76-22 mixes at the LTOC level had the greatest drop in G_f. However, PG 52-34 mixes had increased G_f at OTC and LTOC oxidation levels (following the above 100% TSR% for these conditions) and equal G_f at the STOC level.

Impact of Testing Temperature on the IDEAL-CT

Figure 7 presents IDEAL-CT interaction diagrams when testing samples at the AIT. Figure 7a shows that specimens prepared using PG 76-22 and tested at 31°C presented lower CT_index sensitivity because of oxidation and moisture conditioning. Moisture conditioning reduced G_f and increased l₇₅/|m₇₅|, which is the expected behavior. However, the impacts of oxidation were less evident when tested at the AIT rather than at the ALT.

Figure 7.

Indirect Tensile Asphalt Cracking Test interaction diagram asphalt intermediate temperature results: (a) mixes with PG 76-22 at 31°C and (b) mixes with PG 52-34 at 13°C.

Mixes with PG 52-34 showed that decreasing the testing temperature from 25°C to 13°C (Figure 7b) led to an increase in G_f and reduction in l₇₅/|m₇₅|. In contrast to testing at 25°C, higher sensitivity to oxidation and moisture conditioning, reflected in G_f and l₇₅/|m₇₅|, resulted in greater impacts to the CT_index (CT_index values range from as low as 60 to as high as 310). Moisture conditioning only affected PG 52-34 mixes at the STOC oxidation level. However, G_f was the only parameter that demonstrated susceptibility to moisture damage. Although moisture conditioning for LTOC oxidation and PG 52-34 mixes obtained slightly higher G_f values, higher oxidation had a greater impact when testing at 13°C than at 25°C. As 13°C is the CIT for PG 52-34 and 25°C is the CIT for PG 76-22, the results suggest that, compared to the ALT and AIT, the CIT is the testing temperature at which asphalt mixtures exhibited the highest moisture susceptibility.

Statistical Analysis

A multi-factor analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) post-hoc analysis were conducted to detect the statistical significance of the binder type, oxidative conditioning, and testing temperature to the sensitivity of moisture conditioning. The analysis was performed in three stages: the first analysis was with the goal of evaluating the statistical significance among three oxidative conditioning levels (i.e., OTC, STOC, and LTOC) with ITS as the response variable; the second analysis was performed to compare the differences between the three different binder types (ITS was used as the response variable) to evaluate the effect of moisture conditioning on asphalt performance. An ANOVA and Tukey’s HSD analysis were performed on both the ALT of 25°C and the AIT. The analysis was conducted at the 95% confidence level (or p-value ≤ 0.05 for a significant impact). Table 3 presents the results for ANOVA and Tukey’s HSD post-hoc analysis.

Table 3.

Analysis of Variance Results for Indirect Tensile Strength

Analysis of variance (ANOVA)
Factor	F-value	p-value
Binder Type	114.032	<.001*
Oxidation Level	216.103	<.001*
Moisture Conditioning	74.523	<.001*
Testing Temperature	1.046	0.309
Binder Type*Oxidation Level	6.283	<.001*
Binder Type*Moisture Conditioning	18.622	<.001*
Binder Type*Testing Temperature	196.219	<.001*
Oxidation Level*Moisture Conditioning	24.549	<.001*
Testing Temperature*Oxidation Level	0.144	0.866
Testing Temperature*Moisture Conditioning	1.919	0.169
Tukey’s HSD post-hoc
Impact of oxidative conditioning
Oxidative conditioning (I)	Oxidative conditioning (J)	Mean difference (I−J)	p-value
OTC	STOC	−.3967*	<.001*
OTC	LTOC	−.5296*	<.001*
STOC	LTOC	−.1329*	<.001*
Impact of binder type
Binder type (I)	Binder type (J)	Mean difference (I−J)	p-value
PG 64-22	PG 52-34	.3289*	<.001*
PG 64-22	PG 76-22	−0.0332	0.425
PG 52-34	PG 76-22	−.3621*	<.001*

Note: * = statistically significant at a 95% confidence level; HSD = honestly significant difference; OTC = original test conditioning; STOC = short-term oxidative conditioning; LTOC = long-term oxidative conditioning; PG = performance grade.

As seen in Table 3, the ANOVA results had statistical significance with a p-value of <.001 at different binder types, oxidation levels, and moisture conditioning, whereas the testing temperature’s p-value was 0.309, indicating no statistical significance; however, this comparison included all binder types (ITS range between 0.3 and 2.04 MPa), whereas comparing the testing temperature for the binder types individually (i.e., Binder Type*Testing Temperature) shows a significance of testing temperature with a p-value of <.001. This indicates that when PG 76-22 and PG 52-34 were tested at different temperatures, the values were statistically significant. Tukey’s HSD analysis showed that all oxidation levels (OTC, STOC, and LTOC) show significantly different ITS results from each other. This indicates that ITS is significantly affected by the loose-mix conditioning at all oxidation levels. Tukey’s HSD post-hoc results confirm the data analysis results where all combinations were significantly affected at different oxidation levels. The impact of binder types showed that mixes prepared using PG 64-22 were statistically significant from mixes prepared using PG 52-34. This indicates that only PG 52-34 was statistically significant at both testing temperatures.

Summary of Findings

This study evaluated the effect of asphalt oxidation and testing temperature on asphalt mixture moisture damage. Asphalt mixtures were evaluated using two New Jersey asphalt binder grades (PG 64-22 and PG 76-22) and one Alaska and Canada asphalt binder grade (PG 52-34). Testing was conducted at the ALT (25°C), AIT, and CIT. In addition, three oxidation levels (OTC, STOC, and LTOC) were evaluated to determine an appropriate level of oxidation and testing temperature at which damage is best discerned. Additional analysis was conducted using the AASHTO T283 load–displacement curve and IDEAL-CT parameters. As opposed to relying on the CT_index parameter (final IDEAL-CT parameter), an interaction diagram was established to better study and comprehend the effects of oxidation and moisture conditioning on the load–displacement parameters. The diagram included plotting G_f against l₇₅/|m₇₅|. The diagram also contained a sequence of CT_index contour curves that connected the input test parameters to the final CT_index value. Finally, statistical analysis was conducted to evaluate the effect of oxidation on the moisture susceptibility of asphalt mixtures. Based on the results of this study, the following findings were drawn.

Tensile Strength Ratio

Based on the AASHTO T283 test results at the ALT, moisture damaged asphalt mixtures more as the level of oxidation increased. The greatest impact of oxidation on TSR% was seen at the LTOC level. This was seen for mixes prepared using PG 64-22 and PG 76-22 and tested at 25°C, where the TSR% values were 71% and 68%, respectively. However, testing PG 52-34 mixes at 25°C yielded increased TSR% after the LTOC level where TSR% was 116%. The greatest impact on PG 52-34 mixes at 25°C was seen at the STOC level.

Based on the AASHTO T283 test results at the AIT, the impact of oxidation was better discerned when evaluating TSR% for mixes prepared using PG 52-34 at the AIT (i.e., 13°C), where the lowest TSR% was seen at the STOC (86%) and LTOC (85%). PG 76-22 mixes when tested at 25°C showed the maximum impact of oxidation on TSR%. However, both the intermediate temperatures were the CIT for the binder types. These findings suggest that the CIT is the appropriate testing temperature, which showed higher moisture damage.

An ANOVA and Tukey’s post-hoc statistical analysis also support the conclusion that moisture conditioning had the most significant impact at the CIT, all at the STOC and LTOC levels, where a statistical significance (i.e., p-value < 0.05) was seen for PG 64-22, PG 76-22 at 25°C, and PG 52-34 at 13°C.

IDEAL-CT Parameters

Based on the IDEAL-CT results, moisture conditioning increased the CT_index and oxidation reduced the CT_index. The increase in CT_index because of moisture conditioning was mainly attributable to an increasing l₇₅/|m₇₅|. The decrease in CT_index because of oxidation was primarily caused by a reduction in l₇₅/|m₇₅| and increase in G_f.

Moisture conditioning had an impact only on G_f, and this effect was observed at LTOC for mixes with PG 64-22 and PG 76-22, and at STOC for mixes with PG 52-34.

Conclusions

Based on the findings from this study, the following conclusions were drawn.

Effect of Oxidation on TSR%: The study found that long- and short-term oxidation had a greater impact on moisture damage compared to the original test procedure when characterizing the moisture damage of asphalt mixtures.

Effect of Testing Temperature on TSR%: Testing at the climatic intermediate temperature applicable for each binder is recommended instead of fixed testing at 25°C, as the TSR% results were lowest when mixes were tested at the climatic intermediate temperature.

Load–Displacement Curves Parameters: The study found that G_f, obtained from the IDEAL-CT method, has good potential for characterizing the moisture damage of asphalt mixtures.

Recommendations

Based on the conclusions from this study, the following recommendations were drawn.

Effect of Oxidation on TSR%: It is recommended to revise AASHTO T283 to consider alternative oxidation conditioning for characterizing moisture damage in asphalt mixtures when conducting TSR% testing.

Effect of Testing Temperature on TSR%: It is recommended that the testing for TSR% should be conducted at the climatic intermediate temperature applicable for each binder, instead of at a fixed temperature of 25°C.

Load–Displacement Curves Parameters: It is recommended that the IDEAL-CT parameters should be considered when evaluating moisture damage in asphalt mixtures, and parameters such as G_f should be included in the evaluation of asphalt mixtures.

Study Limitations and Future Work

The findings of this study are restricted to the aggregate type, gradation, and moisture conditioning protocols used. It is recommended that future research considers assessing the laboratory performance of various asphalt mixtures produced by different plants, particularly those that incorporate recycled asphalt pavement (RAP). In addition, the impact of loose-mix oxidation on asphalt performance should be studied for various climatic regions, considering factors such as testing temperatures, saturation levels, and freeze–thaw cycles. A comprehensive microstructure evaluation for moisture damage and an evaluation of the mechanical properties of aggregate and asphalt binders is also advocated.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: A. Alfalah, D. Offenbacker; data collection: A. Alfalah, D. Offenbacker, A. Ali, Y. Mehta; analysis and interpretation of results: A. Alfalah, D. Offenbacker, A. Ali, Y. Mehta; draft manuscript preparation: A. Alfalah, D. Offenbacker, A. Ali, Y. Mehta, M. Elshaer, B. Cox. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The tests described and the resulting data presented here, unless otherwise noted, are based on work supported by the US Army ERDC under PE W81EWF01291720, Project T26 “Innovative Construction Materials for Cold Regions.”

ORCID iDs

Daniel Offenbacker

Yusuf Mehta

Ayman Ali

Ben C. Cox

The use of trade, product, or firm names in this document is for descriptive purposes only and does not imply endorsement by the U.S. Government. Permission was granted by the Director, Cold Regions Research and Engineering Laboratory to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents

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