Abstract
Skid resistance of pavements plays a crucial role in ensuring the safety of road users, particularly under wet weather conditions. The standardized method for evaluating the skid resistance of aggregates, AASHTO PP103, requires using a three-wheel polishing device. That test procedure requires significant time for specimen preparation, testing, and monitoring of aggregate stockpiles. This study aims to propose alternative test methods to reduce the time required for the evaluation of skid resistance of aggregates. Two approaches were examined as alternatives to the AASHTO PP103 method. The first approach involved using a micro-deval abrasion device to polish the aggregates before evaluating the skid resistance of the aggregates using a dynamic friction tester. In the second approach, a multiple linear regression equation was proposed that uses conventional mechanical performance test results as independent variables to predict skid resistance. Twenty-one aggregate samples were collected from various sources in Texas for this study. The study results indicate that both approaches can be used as surrogate methods for AASHTO PP103 for rapid screening of the skid resistance of aggregates, which could help manage aggregate quality control and assurance.
Keywords
Skid resistance, one of the crucial long-term functional performance parameters of pavements, has been extensively studied. Numerous studies reveal a linear relationship between the skid resistance of surface mixtures and the occurrence of accidents at a particular location (
Skid resistance (a.k.a. frictional resistance and coefficient of friction) refers to the ratio of frictional force developed between the tire and the surface to the normal weight on the tire in the direction of vehicle movement (
Several departments of transportation (DOTs), such as the Maryland DOT, have adopted the DFT to assess the frictional resistance of aggregates as part of their aggregate stockpile quality management (
Some DOTs conduct alternative aggregate tests to assess the mechanical characteristics of aggregates (
The polishing resistance of aggregates is highly dependent on the minerals present in them. Generally, aggregates containing high-hardness minerals exhibit better frictional performance, while those with low-hardness minerals perform poorly (
In previous studies, the aggregate surfaces were subjected to abrasion using the MDAL test (
The first objective of this study is to utilize the MDAL test to polish aggregates before determining their frictional performance using the DFT. The correlation between the results from this approach and those from the AASHTO PP 103 methods is examined to demonstrate the potential use of this approach as an alternative test for AASHTO PP 103.
The second objective of this study is to develop a model for predicting the frictional performance of aggregates using conventional aggregate test results as input variables. Since some of these aggregate parameters have shown a correlation with the frictional performance of aggregates, this approach seems feasible. Many DOTs regularly perform tests on aggregates as part of their quality control and assurance programs. Assuming the regression equations perform well in predicting the frictional resistance of aggregates, they can be used for rapidly monitoring and estimating aggregate frictional resistance. As a result, these methods can be potentially used as surrogate test methods for AASHTO PP103 for the evaluation of aggregate frictional resistance.
Methods
Experimental Plan
Table 1 shows the 21 different aggregate sources that were considered in this study. The collected aggregates were evaluated for their mechanical properties, namely, the MDAL, LAAL, SM, and AIR tests. The test results of the mechanical properties are also summarized in Table 1. The collected aggregates were examined for alternative friction test methods in two phases. The first phase involved evaluating the polishing resistance of aggregates according to the AASHTO PP 103 method. The second phase focused on assessing the frictional resistance of aggregates after subjecting them to the micro-deval abrasion test.
List of Aggregates used in this Study
Acceptable limits were extracted from Texas Department of Transportation specifications.
Aggregate Ring Preparation Method for Polishing Using the TWPD
The AASHTO PP 103 method was utilized to prepare aggregate rings and determine their frictional performance. This method involves polishing the aggregate rings using the TWPD and conducting friction tests using the DFT. Aggregates passing a 12.5 mm (1/2 in.) sieve and retained on a 9.5 mm (3/8 in.) sieve were acquired and washed to remove any fine particles adhered to the aggregate surface for the ring preparation. The washed and fines-free aggregates were dried in an oven at 110°C until their weight remained constant, indicating the absence of moisture. Hereafter, the dried aggregates are referred to as standard aggregates. After cooling at room temperature, the aggregates were meticulously placed in a mold to create aggregate rings, as shown in Figure 1, a–c. The aggregate particles were tightly arranged and seated on a flat surface. A binding agent of quick-setting cement was applied to the sample and allowed to cure at room temperature for 5 h to hold the aggregates in place (Figure 1d). Figure 1e shows a TWPD aggregate ring specimen after curing. Figure 2 depicts the aggregate ring in the TWPD during the polishing process. The prepared aggregate rings were polished using the TWPD for up to 100k cycles to evaluate their frictional resistance.

Aggregate ring preparation process: (

Aggregate ring under polishing using an in-house fabricated three-wheel polishing device.
Micro-Deval Abrasion Loss Test
The standard aggregates were tested using the micro-deval abrasion test, following the ASTM D6928 procedure (
Aggregate Ring Preparation After Micro-Deval Aggregates
The aggregates retained on the 1.18 mm (No. 16) sieve after the micro-deval abrasion test were used to prepare aggregate rings for assessing the frictional resistance. The preparation of aggregate rings involved placing the abraded aggregates on Bondo filler and curing them for 5 h. Figure 3 displays the micro-deval abrasion test apparatus and showcases the standard aggregates before and after the micro-deval abrasion test. Figure 4 illustrates the process of preparing aggregate rings after abrading the aggregates using the micro-deval device.

(

Aggregate ring preparation process with aggregates after the micro-deval process: (
Dynamic Friction Test on Aggregate Rings
The frictional resistance of both the TWPD aggregate rings and micro-deval aggregate rings was tested using the DFT following ASTM E1911 (

Various components of the dynamic friction test (DFT) device.
Conventional Tests for the Evaluation of the Mechanical Performance of Aggregates
In addition to the MDAL test, other conventional mechanical performance tests were conducted in this study, namely the LAAL, SM, and AIR tests. For the LAAL test, the standard aggregates were added to the drum used for this testing method. Later, the drum was subjected to 500 revolutions, and the LAAL value was calculated following the ASTM C131/C131M procedure. For the SM test, the standard aggregates were tested using the solution prepared using the salt of anhydrous and crystalline hydrate as per ASTM C88/C88M. The AIR test was performed using a concentrated hydrochloric acid (HCl) solution following the test procedure ASTM D3042.
Results and Discussion
Figure 6 depicts the friction values of the 21 aggregates after 100k TWPD polishing cycles on replicate rings. The average of triplicate trials was reported as the friction value for each test. The error bars for each value represent ±1 standard deviation of the three measured values.

Friction values of 21 aggregates after 100k polishing cycles.
Table 2 presents the distribution of the friction values. The selected aggregates exhibited friction values ranging from 0.10 to 0.50, indicating a wide range of aggregate friction values. The materials selected are biased toward low-friction materials to be representative of the stockpiles available within the region. This study focused solely on the friction values corresponding to 100k TWPD, cycles as numerous studies (
Number of Aggregate Samples within an Interval of 0.10
Figure 7 illustrates the friction values with the DFT of the 21 aggregates subjected to micro-deval abrasion before aggregate rings were prepared. Again, the average friction values of the three DFT trials are reported in the graph. As reflected in the error bars, the results are not as repeatable as those shown in Figure 6. One possible reason for the higher variability in the results could be the method of preparation. To reduce the overall test duration, the aggregates were directly placed in the Bondo paste, which might have led to a more uneven surface. Inherently, uneven surfaces pose slightly higher forces on the DFT device rubber pads when compared with an even surface. Further studies are required to elucidate the impact of rough surfaces on the DFT measurement and equipment.

Friction values of micro-deval aggregate rings.
Figure 8 illustrates the correlation between the friction values from the TWPD and micro-deval abrasion methods for the 21 aggregates. The friction values from the two abrasion methods are reasonably well correlated, as judged by an

Correlation between the three-wheel polishing device (TWPD) method and the micro-deval method.
Table 3 summarizes the Pearson correlation (
Pearson Correlation between Dynamic Friction Value (DFV) and Independent Variables
Out of the 21 aggregates, 15 were randomly selected to develop a multiple linear regression (MLR) equation (training), while the remaining six were set aside to assess the veracity of the formulated equation (testing). Before proposing the MLR equation for predicting the aggregate friction value (
where
Different Scenarios and their Prediction Capabilities

Performance of the multiple linear regression (MLR) equation: (

Predicted friction values for 21 aggregates by Equation 5.
Conclusions
This study aimed to explore the feasibility of using rapid alternative methods as surrogates for the AASHTO PP103 method in assessing the frictional resistance of aggregates. Two alternative approaches were examined. The first approach involved abrading the aggregates first using a micro-deval device, followed by evaluating the frictional resistance of the abraded aggregates using the DFT. The second approach utilized conventional aggregate test results as independent variables to predict the frictional resistance of aggregates.
The AASHTO PP103 method requires approximately 30 h from sample preparation to polishing up to 100k cycles using a TWPD, to testing with the DFT. Alternative 1, which utilizes micro-deval abrasion, requires only 8 h for abrasion and friction evaluation. Alternative 2 utilizes conventional aggregate test results (MDAL, LAAL, and AIR).
Both alternative methods exhibited reasonable correlations, with
Footnotes
Acknowledgements
The authors would like to express their appreciation to the undergraduate research assistants Hector Garcia, Daren Perez, and Adrian Gonzalez of the Center for Transportation Infrastructure Systems (CTIS) for their dedicated efforts in conducting the aggregate testing. Gratitude is also extended to the staff of the CTIS.
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: G. Sandeep Reddy, I. Abdallah, S. Nazarian, R. Izzo; data collection: G. Sandeep Reddy; analysis and interpretation of results: G. Sandeep Reddy; draft manuscript preparation: G. Sandeep Reddy, M. Montoya, I. Abdallah, S. Nazarian, R. Izzo. 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 author(s) received financial support from the Texas Department of Transportation.
