Investigation on the use of non-contact digital ski sensor in the South Korea expressway network

TEXT: Augusto Cannone Falchetto, Aalto University, Espoo, Finland and Ki Hoon Moon, Korea Expressway Corporation, Dongtan, South Korea

FIELD AND LABORATORY INVESTIGATION ON THE USE OF NON-CONTACT DIGITAL SKI SENSOR IN THE SOUTH KOREA EXPRESSWAY NETWORK

In this study, the effect of using Non-Contact Digital Ski (NCDS) on the smoothness of asphalt pavements and its impact on the low temperature behavior of the paved mixture is evaluated. For this purpose, the International Roughness Index (IRI) was measured on testing sites paved with and without NCDS. Field asphalt mixture cores were then tested with the Bending Beam Rheometer (BBR) to determine the evolution of thermal stress. It was found that the NCDS paving method results in considerably lower IRI coupled with a moderate reduction in thermal stress, suggesting a potential benefit of this technology also on the low temperature behavior of the mixture.

Augusto Cannone Falchetto

is a faculty in the Department of Civil Engineering. During his international career, he worked in different countries such as Germany, Japan and U.S. His research focuses on infrastructure materials and pavement engineering and combines experimental research, advanced analysis, and modeling. ROADS that are characterized by a smooth and even surface are beneficial from both economical and environmental viewpoints resulting in less fuel consumption, reduced vehicle maintenance, and potentially limited greenhouse gas emissions (Bae et al. 2018).

The International Roughness Index (IRI, m/km) is commonly used to indirectly evaluate the smoothness of a pavement surface (Ziari et al. 2016). Higher IRI indicates poor pavement smoothness with a threshold limit of 1.6m/km in South Korea (MOLIT 2017).

Commonly, the Long Ski (LS) system, is used in combination with String-Line (SL) for controlling asphalt pavement smoothness in South Korea expressway construction. However, this method presents limitations resulting in lower IRI. This has led to the development of the Non-Contact Digital Ski (NCDS) system. Different from the previous solutions, the

Long Ski (LS) String Line (SL) (Physical)

Physical contact

ASPHALT PAVEMENT SURFACE

Asphalt paving direction

Fig. 1. LS+SL (left) and NCDS systems (right).

Fig. 2. Non-Contact Digital Ski (NCDS) system on a South Korean expressway - Ho-Nam.

Non Contact Digital Ski (NCDS) Non Contact Sonic sensor Measurement (Non-physical)

ASPHALT PAVEMENT SURFACE

Asphalt paving direction

1 2

Figure 11: Comparison of the thermal stress results (Site A and B)

Site Average results of IRI (m/km) NCDS A (260m) 1.04 B (540m) 1.03 C (170m) 0.81 D (130m) 0.96 E (170m) 0.68 LS+SL Difference [NCDS ]-[ LS+SL] 1.68 -0.64

1.58 -0.55

1.23 -0.42

1.50 -0.54

1.48 -0.80

An example of computed thermal stress curves for site A and corresponding statis-tical analysis (p-value) is shown in Figure 3.

Table 1. Measured IRI(m/km) using different smoothing solutions.

NCDS does not require physical contact between the smoothing equipment and the sub-layer. The design thickness of the paved layer is automatically calculated during the paving process through a non-contact distance measuring algorithm relying on multi-sonic sensors data (Bae et al. 2018).

A schematic of LS+SL and NCDS systems is presented in Figure 1.

OBJECTIVE AND RESEARCH APPROACH In this study, the impact of the use of NCDS in improving the asphalt pavement smoothness is investigated and compared to the combined LS+SL method conventionally adopted in the construction of expressway composite pavement in South Korea. As a pavement smoothness evaluation parameter, IRI (m/ km) was selected (Gillespie et al. 1980).

For this purpose, IRI results obtained on five testing sites when using LS+SL and NCDS systems were compared. In addition, Bending Beam Rheometer (BBR) mixture creep tests (Marasteanu et al. 2009) were performed to evaluate the low temperature cracking resistance of asphalt mixture samples cored from field sites that were paved with NCDS and LS+SL methods.

ASPHALT PAVEMENT SMOOTHING TECHNOLOGIES

In the SL+LS system, steel sticks are placed, every 5~10m, on the surface along the path where the asphalt material is paved. This method is widely adopted in the pavement industry due to its simplicity.

However, the need for a track bottom reference surface and the limited flexibility on curved sections has led to the development of newer solutions such as the Non-Contact Digital Ski (NCDS) system. This relies on 3-4 groups of multi-sonic sensors (cartridges) attached to a long single beam (5 to 13m in length).

During construction, a large beam with multi-sensor cartridges is attached to the side of the asphalt paver. Each thickness measurement provides 15 to 20 single data per side (depending on the number of sensor arrays - 3 or 4).

As the asphalt paver proceeds, the distance with the surface is measured, and the paving layer thickness is computed. This information is simultaneously provided to the paver allowing the instantaneous determination of the quantity of asphalt material that is discharged up to the specific/design pavement level. An application of the NCDS system on the Korean expressway is shown in Figure 2.

FIELD AND LABORATORY TESTING Five different testing sites (A to E) of recent constructions were selected from the Korean Expressway network to compare the pavement surface smoothing performance of NCDS and LS+SL systems. The sites present different lengths and consist of an asphalt layer on the top of existing Jointed Concrete Pavements (JCP) at the end of their service life, finally resulting in a composite pavement. IRI was measured with professional equipment and the data analyzed with the pavement analysis program PROVAL (2019).

In the present study, IRI results were generated every 20cm, and hence, approximately more than 5,000 data were used for IRI analysis for each pavement site. Along with pavement smoothness evaluation, sample coring was performed for preliminary

mechanical performance evaluation of the paved mixture as a function for the smoothing system used. Simple creep tests were conducted on asphalt mixture using the BBR (Marasteanu et al. 2009).

Based on the experimental measurements, thermal stress was computed with a one-step mathematical solution using the Laplace transformation approach (Cannone Falchetto et al. 2020).

DATA ANALYSIS The IRI results are summarized in Table 1. Approximately 34~54% higher IRI values were derived when performing the paving work with the LS+SL device compared to the NCDS system. Moreover, when the NCDS system was used, relatively lower IRI values (0.68~1.04) were observed suggesting remarkable pavement smoothness improvements. This suggests that the NCDS approach can provide remarkably better pavement evenness and smoothness even when balancing the benefits with the time required for the initial calibration process.

An example of computed thermal stress curves for site A and corresponding statistical analysis (p-value) is shown in Figure 3.

The plot shows that relatively lower thermal stress values were found when NCDS technology was used in place of the conventional SL+LS smoothing technology. This is true for all five test sites. In addition, the statistical analysis suggests a possibly favorable material performance associated with the use of the NCDS device. CONCLUSIONS In the present study, the possibility of using a newer asphalt pavement surface smoothing technology known as Non-Contact Digital Ski (NCDS) was evaluated and compared with the conventional Long Ski - String-Line (LS+SL) adopted for the construction of the main road network in South Korea.

After a series of IRI measurements, evaluation, and comparison for the two smoothing systems in different construction sites, it was found that remarkably lower and better IRI values could be obtained when the NCDS approach was selected.

Moreover, relatively lower thermal stress values were observed from the field mixture when NCDS was applied, suggesting potentially superior performance against low temperature cracking with potentially additional benefits, such as enhanced driving experience associated with a hypothetically reduced thermal cracking.

ACKNOWLEDGMENTS The authors would like to gratefully acknowledge Korea Expressway Corporation Pavement Research Division (KECPRD) for access to the testing sites and the experimental support.

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