1. Materials

In this study, normal (unmodified) WMA concrete quality was investigated for application to the rural road pavement. Two maximum sizes (13 and 19 ㎜) of granite-based aggregates were used for dense-graded surface course mixtures. The screenings and limestone powder were used as the fine aggregate and mineral filler, respectively, for asphalt concrete mixture. Three binders were prepared using two WMA additives and a base asphalt (AP) which shows the performance grade (PG) of 64-22 (Asphalt Institute, 2002).

The two WMA additives (Fig. 1) include Evotherm (EV), the emulsion type additive, and K-Pearl (KP), the wax and oil-based pellet-type additive were used for producing WMA mixtures. EV was melted into the heated asphalt at 160^oC and blended with a spatula for 3-5 minutes before use. However, KP was added to the mixer just before mixing with asphalt and aggregates in the mixing bowl. Designation, description and content by weight of each additive are shown in Table 1.

Fig. 1

Photographs showing (a) Evotherm (EV) and (b) K-Pearl (KP)

Since the rural road pavement is deteriorated due to environmental damage, in addition to traffic loading, the moisture resistance after freezing-and-thawing (F-T) was evaluated by tensile strength ratio (TSR). Since moisture resistance can be enhanced by using hydrated lime (Kim, 2015; Kim et al., 2019; Lesueur, 2010), a hydrated lime (HL) was added (by 1 wt. % of total mix) to a WMA mixture for moisture resistance test. Total 6 mixtures (2 aggregates × 1 gradations × 3 binders) were produced for the surface course.

2. Binder Test

The kinematic viscosity (KS F 2392) of the asphalt binder sample was measured in two different temperatures, 135^oC and 115^oC, using a rotational viscometer. The PG grade of each binder was measured using the dynamic shear rheometer (DSR) and bending beam rheometer (BBR). The binder stiffness (G^*/sinδ) was measured from 64^oC, which is high-temperature PG grade of base asphalt, up to the temperature where binder fails to pass the limit. Using BBR, beam stiffness and m-value were measured to estimate the low temperature PG of binder (Asphalt Institute, 2002).

3. Mix-Design

All WMA mixtures were short-term aged for 2 hours in a 130^oC oven, while HMA mixture was short-term aged for 1 hour in a 160^oC oven. For mix-design, the specimen was compacted by 75 gyrations using a Superpave gyratory compactor. The optimum asphalt content (OAC) of HMA was determined using the specification limits of four properties; the air-void ratio, void in mineral aggregate (VMA), voids filled with asphalt (VFA) and the deformation strength (S_D). The OAC was selected at the air-void ratio of 4%, as long as VFA and VMA satisfied specification limits by the Korean Guide (MLIT, 2017). Once the OAC of HMA was determined, the same OAC was used for the WMA mix with a minor adjustment by checking physical properties.

4. Deformation Strength (SD)

The S_Dis the strength property representing resistance against deformation at high-temperature (60^oC) under a static-mode loading applied on top center of a briquette specimen of asphalt concrete (Doh et al., 2007; Kim et al., 2004(a); Kim et al., 2011). The specimen is soaked in 60^oC water for 30 minutes and then placed in the specimen holder for measuring S_D, as shown in Fig. 2(a). Static loading is applied at the speed of 30 ㎜/min through the loading head (40 ㎜ diameter with a 10 ㎜ radius (r) of round cut at the bottom edge) until failure, as shown in Fig. 2(b) (Baek et al., 2009; Park et al., 2008). From the load (P) and deformation (v) curve of the test, designated as “Kim Test,” the peak load, P and the vertical deformation, v at P were read and used in Eq. (1) for S_D calculation (Kim et al., 2004(a)).

Fig. 2

(a) Kim Test setting and (b) P-v curve acquired from a Kim Test

where S_D is the strength against deformation (MPa), P is the maximum load (N), and v is the vertical deformation (㎜) at maximum load.

Since the S_D shows a good correlation with rut characteristics of dense grade asphalt concrete, it was adopted as a standard criterion in the Korean asphalt mix-design guide (Kim et al., 2006; Kim et al., 2011), in which the specific criteria of strength level are given. The criteria for S_D in Korean guide (MLIT, 2017) are S_D ≥ 3.20 MPa and S_D ≥ 4.25 MPa for the 2^nd class road and 1^st class highway pavements, respectively. Detailed Kim Test procedures are given elsewhere (Doh et al., 2007; Kim et al., 2004(a); 2004(b); 2011; MLIT, 2017).

5. Wheel Tracking Test

It is a widely held hypothesis that the wheel tracking (WT), used in much research, gives a reasonable prediction of rutting in the field (Kim et al., 2011, 2018). The rut depth data collected from wheel paths under the simulated conditions are assumed to indicate the rut characteristics of the asphalt concrete in the field (Brown and Gibb, 1996). The data used in this study were from a wheel tracker that consisted of a steel wheel (200 ㎜ in diameter and 50 ㎜ in width) rolling on top of a slab specimen on a base plate moving with a 200 ㎜ stroke back and forth. A WT test was performed for 7,200 passes at 60^oC at a speed of 40 cycle/min for 90 minutes, with a pressure of 689.4 kPa (100 psi). The final rut depth data at 3,600 cycles were used for analysis.

6. Indirect Tensile Strength Test and Tensile Strength Ratio

The tensile strength (S_T) of asphalt concrete was measured by indirect tension test (KS F 2382) at 25^oC on 100 ㎜ diameter, 63 ㎜ thickness specimen. A static loading is applied at the speed of 50 ㎜/min through loading strips on top and bottom of specimen (Fig. 3). The S_T was calculated using Eq. (2).

Fig. 3

Indirect tensile strength test setting

where S_T is the indirect tensile strength (kPa), P is the peak load (N), D is the specimen diameter (㎜), and t is the specimen thickness (㎜).

In this study, to examine durability of asphalt concrete for rural road pavement, the tensile strength ratio (TSR) was evaluated after freezing-and-thawing (F-T) conditioning. Since the lower strength asphalt concrete shows, in general, lower durability during in service in the pavement, a WMA mixture, which showed the lower strength values ( and ) on average, was only selected to evaluate TSR. The specimens (diameter of 100 ㎜, heigh of 63 ㎜) were prepared with air void of 7±1% for a F-T cycle, for which a freezing at -18^oC for 16 hours and then submerging at 60^oC water for 24 hours for wet-conditioning by AASHTO T 283 (2014). The specimen, picked out from 60^oC water, was submerged into the 25^oC water bath for two hours before S_T testing. The dry specimen was kept in an environmental chamber controlled at 25^oC for 48 hours. Three specimens were used for each conditioning and TSR was calculated by Eq. (3) using the average value for each condition.

where TSR (%) is the tensile strength ratio, S_Twet is the indirect tensile strength (kPa) of wet-conditioned specimen, and S_Tdryis the indirect tensile strength (kPa) of dry specimen.

1. Evaluation of Asphalt Binder

Table 2 shows binder test results. WMA binders, which showed PG 64-22, showed lower kinematic viscosity at 135and 115^oC than AP (base asphalt). The compaction temperatures of HMA and WMA are considered approximately 135 and 115^oC, respectively. Those temperatures are the mixture temperatures, which were cooled down, when compacted by roller after wide spreading by the paver. Therefore, kinematic viscosity of two binders were tested at 135 and 115^oC.

Comparing kinematic viscosity of HMA and WMA binders, as shown in Fig. 4, the WMA binder using EV were 27.4% and 13.6% lower than HMA binders at 115^oC and 135^oC, respectively. The WMA binder using KP was 23.0% lower than HMA binder at 115^oC, but similar to each other at 135^oC. The test result indicated that the WMA binders used in this study will be more effective for compaction at lower temperature, i.e., at 115^oC, than the HMA binder.

Fig. 4

Comparison of kinematic viscosity of HMA and WMA binders

All binders of HMA and WMA show similar failure temperature from 66 to 69^oC. Therefore, WMA binders were not observed to be inferior to the HMA binders, even though WMA additives, which might cause softening of the binder at high service temperature, were contained. The low temperature grade was all measured to be -22^oC, even though there were some differences in failure temperature to pass the maximum stiffness and minimum m-value.

2. Evaluation of Asphalt Concrete

Table 3 shows physical properties measured from the specimens prepared using the optimum asphalt content (OAC) and Table 4 shows three properties of HMA and WMA concretes. The values of S_D ≥ 3.20 MPa, WT rut depth ≤ 5 ㎜ and S_T ≥600 kPa are, in general, considered an acceptable level of engineering properties of asphalt concrete pavement for the 2^nd class road pavement (ScDOT, 2007). The 2^nd class road includes most of the two-lane roadways with daily traffic volume less than 1,000 ESAL (equivalent single axle loading). Therefore, WT rut depth of 5 ㎜ and S_T of 600 kPa were considered as guidelines of WMA concrete for rural roads.

All the HMA and WMA concretes satisfied the limits of S_D and S_T, although one HMA and one WMA mixes were slightly over the WT limit. However, those values are still very close to the 5 ㎜, which can be considered acceptable for rural roads pavement. Figs. 5~7 illustrated individual values of each test for more detailed analyses.

In Fig. 5, S_D values of 19 ㎜ showed higher, on average, than those of 13 ㎜. In case of 13 ㎜, WMA concretes showed higher S_D, but for the case of 19 ㎜, WMA conversely showed lower S_D values. However, still all values satisfied the 3.2 MPa limit, indicating that WMA concretes have high enough strength for 2^nd class roadways and rural roads. In general, the asphalt concrete made of the larger aggregate shows stronger resistance against rutting. But according to this result, the use of asphalt concrete with 13 ㎜ aggregate was found to be strong enough for rural road pavement.

Fig. 5

Comparison of deformation strength (S_D) of HMA and WMA concretes

The short-term aging temperatures of WMA and HMA were 130^oC and 160^oC, respectively. It is possible to state that the WMA additive performed properly at WMA temperatures (130^oC) for mixing. Therefore, the WMA concrete appeared to be compacted properly at 30^oC lower than the HMA temperature. This will be a good indication that the WMA will be workable properly at a cooled-down temperature for a rural working environment.

Fig. 6 shows wheel tracking test results. In general, as expected from S_D results, WT rut depths of 13 ㎜ were higher than those of 19 ㎜, indicating 19 ㎜ being better in rut resistance. However, it was difficult to see a pattern of difference between WMA and HMA concretes. The limit of 5 ㎜ rut depth is the guideline for the 2^nd class roadway pavement (ScDOT, 2007), therefore, a somewhat higher limit, such as 6 ㎜ or less, might be allowable for rural road pavement. According to this test result, even though some mixes showed a little higher than 5 ㎜ rut depth, the rut resistance of WMA concrete with 13 ㎜ aggregate seems to be strong enough for rural road pavement.

Fig. 6

Comparison of wheel tracking (WT) rut depth of HMA and WMA concretes

The tensile strength (S_T) was measured at 25^oC and illustrated in Fig. 7. According to S_T, WMA concretes revealed similar or somewhat higher strengths than HMA concretes. Therefore, in case of tensile strength, an ambient temperature property, basically no difference was observed between HMA and WMA concretes prepared using the same PG grade binders. Therefore, the normal (unmodified) WMA concretes seemed to be strong enough for rural road pavement because the ambient (25^oC) and high (60^oC) temperature performance were good enough when they were compared with those of HMA concretes.

Fig. 7

Comparison of tensile strength (S_T) of HMA and WMA concretes

Table 5 shows test results of tensile strength ratio (TSR) of rural road asphalt concretes. Since a HMA and a WMA were below 75%, which is the limit value of TSR by AASHTO (2014), those mixes were not satisfactory without hydrated lime (HL). However, by adding HL by 1 wt % of total mix, the TSR levels for both WMA mixes were improved to be superior levels, i.e., 97% or higher. If an asphalt concrete shows high TSR level, the pavement serviceability life will be extended with great durability under freezing weather condition. Therefore, it would be better to used HL in WMA mixtures for rural road pavement.

3. Correlation Analysis

The S_D is essential property used as one of the mix-design criteria of asphalt concrete in Korea mix-design guide (MLIT, 2017). Therefore, many studies evaluated the correlation of other properties with S_D for HMA concretes (Lee et al., 2004; Kim et al., 2004(a); 2004(b)). This study examined correlation of WT rut depth and S_T with S_D using WMA concrete data only (Figs. 8 and 9).

Fig. 8

Relation of WT rut depth and S_D of WMA concrete

Fig. 9

Relation of S_T and S_D of WMA concrete

The WT rut depth was reduced by S_D increase with R² ≒ 0.80, indicating that the asphalt concrete showing higher S_D would be stronger against rutting. The S_T was increased by S_D increase with R² ≒ 0.70, indicating that the asphalt concrete with higher S_D would be stronger against tensile stress. Since S_T is used as an index of cracking resistance for asphalt pavement, the asphalt concrete with higher S_D will be considered tougher against cracking.

According to these correlation analyses, the S_D, the most fundamental property measured during the mix-design stage, can be used as a criterion of WMA concrete for rural road pavement. The regression curve between WT rut depth and S_D in Fig. 8 showed that the WT rut depth of 5.3 ㎜ at S_D of 3.2 MPa was shown to decrease with S_D increase. Using the regression model in Fig. 8, the WT rut depth of 5.3 ㎜ can be estimated by S_D = 3.2 MPa. By getting rid of the decimal point from 5.3 ㎜ for practical use, therefore, the S_D ≥ 3.2 MPa was considered as a reasonable level for satisfying 5 ㎜ of the rut depth guideline.

The relation of S_T with S_D showed that the regression line meets approximately 700 kPa at the S_D of 3.2 MPa. Since 700 kPa is higher than the previously mentioned S_T of 600 kPa guideline, the S_D of 3.2 MPa will be strong enough for satisfying the S_T performance.

Therefore, S_D ≥ 3.2 MPa is suggested as a criterion of asphalt concrete for rural road pavement because the S_D level was observed to satisfy both rutting resistance (measured by WT) and cracking resistance (measured by S_T) guidelines. Although this level of S_D is the same as the 2^nd class road pavement requirement, it is recommended for rural road pavement. If a lower level of S_D is applied, considering low level traffic volume, the pavement will be susceptible to environmental distresses and traffic loading, because there is no official maintenance program provided for the rural road in most cases.