A Modified Method for Determining Impact Locking Point of Asphalt Mixtures Based on Hammer’s Dynamic Response

For the Superpave Gyratory Compactor (SGC), the concept of locking point was developed to identify the compatibility of asphalt mixes and to alleviate potential aggregate crushing in the mould, which is defined as the gyrations related to specimens’ height change. Inspired by this concept, for Marshall impact compaction, previous studies redefined the impact locking point as the blow number at the acceleration pulse of each blow reaching a stable stage. Unlike the gyratory locking point, the impact locking point is based on the change in acceleration, which might be related to the change in the specimen stiffness. However, determining the impact locking point by comparing the two successive blow’s acceleration pulse is time-consuming and inconvenient based on previous studies. To improve this, a modified method is proposed based on the change in the peak acceleration during impact compaction and applied on test results from three typical Marshall compactors, and a comparison to locking points of gyratory compaction is also made to verify the validity. The results indicate that the modified method is applicable for the American Marshall compactor. For Chinese Marshall compactors, further study should focus on the accelerometer’s alternate installation method to avoid noise effects.

Keywords:Marshall compactor; Accelerometer; Dynamic response; Superpave gyratory compactor

The quality of asphalt mixture is strongly related to the air void content. Previous studies showed that a 1% increase in air voids (over the base air-void level of 7%) would result in a 10% loss in asphalt pavement life [1]. Therefore, the air voids of conventional mixtures at the end of construction should be below 8% to avoid an inappropriate permeability. The final in-situ air voids should be approximately 4% after opening to traffic to reduce rutting [2]. In the laboratory, the target air void plays a crucial role in determining the asphalt content in the asphalt mixture design. The compaction methods evolve from static compression to hand tamping, impact, kneading, gyration, vibration and simulated rolling [3]. At present, the commonly used methods include Marshall, Hveem and gyratory compaction methods [4]. As one type of gyratory compactions, the Superpave gyratory compaction (SGC) is the most widely acceptable mixture design method in the United States [5], with several advantages such as decreasing variability, reducing aggregate breakdown, and accommodating a larger aggregate size [6]. Based upon the densification curve obtained from the SGC, numerous volumetric parameters could be obtained to describe the compatibility of various asphalt mixtures, such as the percentage of the maximum theoretical specific gravity (Gmm) at the initial number of gyrations (Nini) [7], the compaction energy index (CEI) [8], the compaction slope and locking point indicating the resistance to permanent deformation [9]. The locking point is defined as the moment during compaction after which the coarse aggregate’s structure interlock and further compaction effort may lead to aggregate degradation without further densification. Initially, the locking point concept was originated to alleviate the potential aggregate breakdown in SGC specimens due to over-compaction [10]. Vavrik et al. [9] defined the locking point as the first of three gyrations with an identical height after two gyrations of identical height [9]. Anderson and Bahia modified the definition of the locking point as the first gyration in the first appearance of three gyrations at the same height proceeded by two sets of two gyrations at the same height [11]. Besides, several agencies introduced various definitions of locking points including Alabama Department of Transportation (DOT) and Georgia DOT [12,13]. Currently, the typical definitions of the locking point could be classified as follows: 1) the first occurrence of two consecutive gyrations with the same specimen height (Locking Point 2-1), 2) the second occurrence of two consecutive gyrations with the same specimen height (Locking Point 2-2), and 3) the third instance of two consecutive gyrations with the same specimen height (Locking Point 2-3) [14].

Unlike the United States, the Marshall impact compactor is still widely used in the rest of the world, including China. According to the molds’ size, there are mainly two types of Marshall compactors with the same compactive effort in unit volume [15], including the standard Marshall compactor with a mold of 101.6-mm in diameter and the largescale Marshall compactor with a mold of 152.4-mm in diameter. Both the Marshall compactors employ the same volumetric parameters as the SGC, , such as air void, VMA, density, etc. However, unlike the SGC which is capable of recording the accurate specimen height data, the Marshall compactor couldn’t offer any data related to volumetric parameters during compaction. In addition, volumetric indicators cannot directly reflect the strength or stiffness change of asphalt mixtures despite using gyratory compaction or impact compaction. Previous studies showed that dynamic response during impact compaction had a strong correlation with the stiffness of filled materials. In 1976, Clegg devised a handheld peak acceleration meter for both field and laboratory as a strength indicator for base-course materials [16]. Polaczyk et al. [18] employed an accelerometer mounted on an American Marshall compactor’s hammer to record the dynamic response data during the impact compaction process. Based on the hammer acceleration pulse during the compaction, they proposed the concept of impact locking point by comparing the pattern of acceleration pulse for each blow (i.e., the maximum acceleration and duration) [17,18]. When the acceleration pulse reaches a stable pattern, the below number is identified to describe the compatibility of asphalt mixtures. However, this evaluation method requires to compare the acceleration pulse for each blow through visual comparison, which results in issues such as inevitable structure noise, inaccuracies in the comparison, and an occupancy of time. To improve the application of impact locking point, the following challenges or works should be solved or performed: first, a more convenient and accurate method should be developed to determine the impact locking point; second, various typical Marshall Compactors with different structure noise should be tested to check the stability of this method; third, diverse types of asphalt binder should also be taken into account to check the applicability of this method. In addition, the installing location of accelerometer should be investigated to better avoid the noise effect.

The objective of this study is to develop a more convenient and practical method for determining the impact locking point by constructing a complete densification curve during compaction. For this purpose, fourteen types of asphalt mixtures, including base mixtures, surface dense mixtures, and stone mastic asphalt mixtures (SMA), and three Marshall compactors, including American standard Marshall compactor, Chinese standard and large-scale Marshall compactor were included in this study. Accelerometers were installed on the compactors to monitor the hammer’s dynamic response. Also, locking points of gyratory compaction for the same mixtures were obtained to verify the validity of the proposed new method for various Marshall compactors in America and China.

Materials

Ten types of Tennessee asphalt mixtures were employed in this study, as shown in Table 1 & 2. The mixtures were divided into 3 groups: 1) base (mix A, B, BM2, and C), 2) surface (mix D, E, and TLD), and 3) stone mastic asphalt mixtures (SMA9.5, SMA12.5 and SMA19.0) based on Tennessee specifications. The limestone aggregates originated from East Tennessee, involving #4 stone, #5 stone, #56 stone, #7 stone, and #10 screenings. The asphalt PG 64-22 was employed for all mixtures. The asphalt mixtures were mixed at the temperature of 154.4 °C and compacted at 143.3 °C to eliminate the temperature effect. Table 2 lists the detailed requirements for ten types of asphalt mixes design.

Table 1:Gradation design (Tennessee).

Table 2:Requirements for Asphalt Mixes Design (Tennessee).

To validate whether the modified method is applicable for asphalt mixtures compacted with Chinese Marshall Compactors, four common types of asphalt mixtures (AC-25C, AC-20C AC-13C, and SMA-13 as shown in Table 3 & 4) in Shanghai, China, were also studied. Heavy-traffic asphalt and limestone aggregate were used in AC-25, AC-20C, and AC-13C mixtures, and one modified asphalt and one type of basalt aggregate were used in the SMA-13 mixture. The mixing temperature was 180 °C and the compacting temperature was 170 °C for all mixtures.

Table 3:Gradation design (Shanghai).

Note: “*” means the grade of aggregates is 5-15mm.

Table 4:Requirements for Asphalt Mixes Design (Shanghai).

To verify the applicability of the proposed method in this study, three types of Marshall compactors were employed, including American standard Marshall Compactor as shown in Figure 1, Chinese standard and large-scale Marshall compactor as shown in Figure 2. The mix design was conducted by using a standard Marshall Compactor with a 101.6-mm mold both in Tennessee and Shanghai. For a large-scale Marshall compactor with a mold of 152.4mm in diameter, a larger specimen is beneficial to the moving and interlocking of coarse aggregates and improve density [15,19]. In the premise of equivalent compaction effort per unit volume, for a specimen of 152.4mm in diameter, the design number of impact compaction is 1.5 times that of a 101.6-mm specimen. For instance, the SMA mixture is compacted by 50 blows on each side in 101.6- mm mold, whereas 75 blows on each side are needed in a 152.4- mm mold.

Figure 1:American Marshall Compactor (Φ101.6-mm).

Figure 2:Chinese Marshall Compactors (Φ101.6-mm and Φ152.4-mm)

As shown in Figure 3, although the standard Marshall Compactor in America and China have the same weight and falling height, the Marshall hammer’s structure and shape of them are different. Compared to the American Marshall hammer which is cylinder-shaped, the Chinese Marshall hammer is flat in front and behind and cylindrical on the left and right sides. In addition, for the Chinese Marshall compactor, the mold is fixed and the pressure head rotates during compaction, whereas the mold rotates an angle of 90 degrees after each blow and the pressure head is fixed for the American Marshall Compactor.

Figure 3:Structural Differences between Marshall Compactors.

For each Marshall Compactor, the dynamic response of the system was monitored by placing an accelerometer on the impact hammer, as shown in Figure 1 & 2. The accelerometer measured the rebound of the hammer during each blow. A data acquisition system was employed to gather acceleration data and transmit it to the software. This system consisted of four modules, involving a sensor, an adaptor, the data acquisition equipment, and the management software (LabVIEW), as shown in Figure 4. In this study, an accelerometer with a capacity of 49050m/s2 (5,000 g) and an error range of ±1% (i.e., ±50g) was mounted on the Marshall hammer both in USA and China, and the sampling frequency was set to 10000Hz.

Figure 4:Data acquisition system.

The Superpave gyratory compactor with a rate of 30 gyrations per minute, compacting stress of 600kpa, a gyration angle of 1.25° was employed to obtain the gyratory locking point for all the mixtures. The specimens were compacted in 150-mm molds and 100-mm molds and recorded the height change during gyratory compaction.

Figure 5 shows a typical impact pulse for each blow recorded by accelerometers, which is the 1st, 50th, 100th, 108th, 125th and 150th blows of Mix D compacted by the US standard Marshall compactor. In each figure the scattered points are the raw acceleration data collected by the data acquisition system, and the fitted curve is smoothed by five points moving average for removing noise and other undesirable quantities. During impact compaction, as shown in Acceleration-Time curves, the positive peak value of acceleration starts at 370.1145g with an impact duration of 0.003 seconds for the first blow, then constantly evolves until reaching 516.8130g at the 108th blow as the impact duration decreases to 0.0025 seconds. After the 108th blow, the positive peak acceleration fluctuates at a stable range between 488.3875g to 516.8130g, and the impact duration remains unchanged at 0.0025 seconds. According to the method of previous studies, the locking point for mix D could be determined at blow 108. However, this method has to compare the Acceleration-Time Curves at each blow to determine the impact locking point, which is time-consuming and inconvenient.

Figure 5:Acceleration-Time Curves of Mix D.

Figure 6a illustrates the whole acceleration data from blow 109 to 110. The interval of two continuous impacts is approximately one second. In general, most of the recorded data are during the separation of hammer and specimen and not helpful for mixture evaluation. Figure 6b presents an impact pulse section for the duration of 3.725 seconds to 3.785 seconds. Due to the resonance aroused by the impact between the hammer and steel pad (the hammer does not directly impact on the specimen surface), the acceleration data are both affect by the specimen strength and structure noise. The stable noise fluctuates in the error range of -14.87g to 17.1g, which is acceptable in measuring error range (±50g ). In Figure 6c the vertical axis represents the peak acceleration for each blow and the horizontal axis is the number of impact compaction. The peak accelerations are attained from five points moving average curve based on the original acceleration data. Further, a trend line is constructed by using three points moving average curve. It could be observed that the strength of specimen increased gradually from the beginning to the 75th blow. Due to the change of specimen surface, the acceleration value drops sharply but swiftly resumes the previous value and finally reaches a stable value at the end of compaction. It can be identified easily that the final stable platform exists from blow 109 to blow 150. Therefore, the 109 blow could be easily identified as the locking point for this specimen.

Figure 6:Process of determining impact locking point.

Furthermore, as presented in Table 5, the maximum (or minimum) difference is the maximum (or minimum) acceleration minus the average acceleration during relatively stable compaction. The final stable compaction stage’s error range is from -11.79g to 7.48g, narrowing the stable section’s error range (-14.87g to 17.1g) between two successive blows and is also acceptable in the measuring error range (±50g ). Hence, the impact locking point is determined at blow 109 in a rational range.

Table 5:Error range analysis.

Asphalt mixtures of base course in tennessee

By using the proposed method, Figure 7 illustrates the constructed densification curves for the mix A, mix B, mix BM2, and mix C as the base mixtures in Tennessee. In general, with the growth of the blow number, the average peak accelerations for each blow initially increase sharply before the 30th blow, then reach a relatively stable but still increasing state, indicating that the stiffness increases slightly. Utilizing the proposed method to determine the impact locking point, mix B, mix BM2 and mix C are interlocked at blow 143, blow 139 and blow 128, respectively, by reach a stable value at the end of compaction. However, it isn’t easy to obtain the exact locking point for mix A due to remaining scattered points at the end of compaction.

Figure 7:Peak Acceleration Curves of Asphalt Mixtures of Base Course

Asphalt mixtures of surface course in tennessee

Similarly, for mix D and mix E as surface mixtures in Tennessee, it can be seen from Figure 8 that the tendency of average peak acceleration curves presents a stable flat after the initial drastic rising stage, demonstrating that the locking points could be determined at the 109th blow for mix D and 102nd blow for mix E accordingly. The peak acceleration for mix TLD is rather scattered. Although the curves’ tendency follows an increasing of value, no locking point could be identified, which may be due to the issues in specimen preparation.

Figure 8:Peak acceleration curves of asphalt mixtures of surface course.

Stone mastic asphalt mixtures in tennessee

SMA mixtures have a framework-dense structure in coarse aggregates. As shown in Figure 9, their densification curves are different from that of other base and surface curves. In general, the SMA densification curves grow steadily and constantly from the initial period before reaching the final stable platform. The impact locking point could be verified at blow 106 and blow 102 for SMA12.5 and SMA19.0 mixture, respectively. However, for SMA9.5, the interlocking state is hard to identify, although the peak value fluctuates up and down in the same range.

Figure 9:Peak acceleration curves of asphalt mixtures of SMA mixtures.

The results of the error analysis of peak accelerations after the locking point are listed in Table 6. The difference between peak acceleration and the average value of acceleration during the interlocking stage determines the stable fluctuation range. Compared to the measuring error range in Table 6, the maximum and minimum differences are entirely tolerant for determining the locking points.

Table 6:Error range analysis.

To further verify the applicability of impact locking points using the proposed method, A comparison of impact locking points to gyratory locking points by using 4-inch mold and 6-inch mold was conducted in Table 7. In this study, the gyratory compaction is defined as the first gyration in the first occurrence of three gyrations at the same height proceeded by two sets of two gyrations at the same height [11]. The method to determine the impact locking point in previous study based on comparing each impact pulse was also included in Table 7 [17,18]. As shown in Table 7, the impact locking points in seven of ten mixes determined by the modified method are highly close to those determined by the previous method. In both evaluation methods, mix A cannot reach the locking point. The locking point for mix TLD and SMA9.5 mixture could not be identified by the modified method, whereas the previous method could identify it for these mixes. Also, it is observed that the larger the nominal maximum particle size (NMPS), the more compaction effort was needed in the formation of aggregates skeleton.

Table 7:Correlation of locking points.

As expected, for SMA (as dense framework structure) and other mixes (as suspended dense framework structure) compacted by gyratory compactor with 4” mold and 6” mold, the locking points present a strong linear relationship as shown in Figure 10a). For mixes excluding SMA, the impact locking points (in 4” mold) determined by the modified method had a strong linear relationship with the gyratory locking points (in 4” mold) in Figure 10b. There are only two data points for SMA mixes, so no correlation analysis could be performed.

Figure 10:Comparison of locking points among various compactors.

Figure 11 shows the densification curves for four typical types of Chinese asphalt mixes compacted by the Chinese Marshall Compactors. The same mixes were compacted by both the Chinese standard and large-scale Marshall Compactor.

Figure 11:Peak acceleration curves with chinese Marshall compactors.

Compared to the peak acceleration curves in Tennessee, Shanghai’s mix results show a similar increasing trend; however, the final stable platform is missing. Comparing to the Marshall Compactor curves with a 101.6-mm mold, the large-scale Marshall Compactor with a 152.4-mm mold could offer a more consistent and less scattered curve. However, no locking point could be speculated in all the curves.

Based on the above results, the utilization of modified method for determining impact locking points of asphalt mixtures is practical and efficient. For Tennessee’s mixes, the impact locking points are affected by the mixes’ nominal maximum particle size (NMPS). The smaller NMPS are related to a less compaction effort to achieving the locking point due to the high asphalt content with particular lubrication effect. This phenomenon could be identified through the locking point decided by the modified method. However, there are still three exceptions, including mix A, mix TLD and SMA 9.5 mixture. For mix A, the NMPS is greater than 25.4mm, and the specimen is compacted in a 4-inch mold, which may not accommodate the big aggregate well. As the number of impact compaction increases, aggregates are more prone to be broken not to interlock each other so that the stiffness of specimen may vary during the final stage of compaction. The locking points also could not be observed for mix TLD and SMA 9.5 mixture as surface mixes. The reason might be that the impact compaction effort is inappropriate with the mix aggregate gradation. These types of surface mixes are prone to be compacted easily. Therefore, during the final stage, the impact compaction may over-compact the mixes and disturb the mix structure. Nonetheless, the relationship between the volumetric parameters and stiffness parameters should be further studied.

For the Chinese mixes, the peak acceleration curves of specimens compacted in a 152.4-mm mold present less variance for each blow than those in a 101.6-mm mold, which may indicate less noise effect during compaction for the large Marshall Compactor. The specimen in a 152.4-mm mold contains more materials and has a larger surface area, which helps to eliminate the variance. However, no locking points could be identified in all curves. By considering the difference between the Chinese Marshall Compactor and American Marshall Compactor, the reason could be due to the various distance of the falling hammer to the base plate or the difference of hammer shape, which may result in the signal noise of the accelerometer during impact compaction. Hence, the mounting position of accelerometer should be investigated to improve the sensitivity for Chinese Marshall Compactors in future studies.

In the further study, for the Chinese Marshall compactor, several factors should be taken into account for determining the impact locking point as follows: 1) the installation method of accelerometer to avoid noise effect; 2) diverse types of asphalt binder; 3) influence of aggregate gradation; 4) compaction temperature and asphalt content; 5) correlation to the mechanistic performance.

In this study, three types of Marshall Compactors were employed to compact asphalt mixes in suspended dense framework structure and dense framework structure. The results of dynamic response during compaction were recorded and analyzed. A novel method was developed to filter the noise and data not in the compact duration when determining impact locking points of asphalt mixes. To verify the applicability of this evaluation method, the impact locking points were compared to the locking points of gyratory compaction and that from the original method. The main conclusions are drawn as follows:
A. The modified method is effective to determine the impact locking point based on the stiffness change of specimens.
B. The impact locking points demonstrate a strong linear correlation with the gyratory locking points for the mixes excluding SMA.
C. The dense framework structures are easier to be compacted than the suspended dense framework structures, which was identified by different locking points among those asphalt mixes in Tennessee.
D. Mix A, mix TLD, and SMA9.5 cannot identify an obvious stable acceleration peak during the final stage, which indicates that the nominal maximum particle size plays a crucial role in obtaining a stable stiffness when reaching its highest density.
E. The data of Chinese mixes during the compaction process present a less stable tendency. Although the data from the large-scale Marshall Compactor presents are more stable than the standard Marshall Compactor, the locking point is still hard to identify. Future studies should be performed to improve the sensitivity for Chinese Marshall Compactors.

The authors are grateful for the support by Shanghai Road and Bridge Group Co. LTD. to this study. This paper’s content only reflects the authors’ views, who are responsible for the facts and the accuracy of the data presented herein. This research was funded by the Shanghai Housing and Urban-rural Construction Management Commission (2023-002-051).

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