Chapter 3 Alabama Department of Transportation Evaluation of Open-Graded Friction Course
3.2 Mix Design and Performance Testing
Prior to the construction of the three test sections at the NCAT Test Track, five OGFC mixtures were designed in 2012 based on a 12.5-mm OGFC mix design previously approved by ALDOT.
The approved OGFC mix was designed based on ALDOT’s OGFC mix design procedure and consisted of 91 percent #78 granite aggregate, 8 percent M10 granite aggregate, 1 percent baghouse fines, 0.3 percent cellulose fiber, and 6 percent PG 76-22 asphalt modified with polymer. The approved mix design did not include an antistrip agent, but the tensile strength
ratio determined in accordance with AASHTO T283 without a freeze-thaw cycle was 0.87, which is higher than the commonly used tensile strength ratio criterion of 0.80 in Alabama. A
description of each OGFC mix design follows.
• The first OGFC mixture was a 12.5-mm mix with cellulose fiber deigned similar to the state approved mix design except that this mix design had a lower air void content than a typical OGFC mixture approved by the state.
• The second OGFC mixture, which was later selected for Section E9A, was designed with a 9.5-mm NMAS gradation instead of a 12.5-mm NMAS gradation. This mix design consisted of 44 percent #78 granite aggregate, 50 percent #89 granite aggregate, 6 percent M10 granite, 0.3 percent cellulose fiber, and 6 percent PG 76-22 asphalt modified with polymer.
• The third OGFC mixture, which was later selected for Section E9B, was designed with a 12.5-mm NMAS gradation similar to the one utilized in the state approved OGFC mix design. It had 91 percent #7 granite aggregate, 8 percent M10 granite aggregate, 1 percent baghouse fines, and 6 percent PG 76-22 asphalt modified with polymer.
However, this mix design had 0.5 percent synthetic fiber instead of the cellulose fiber used in the state approved mix design to prevent draindown of the thick asphalt binder film from aggregate particles.
• The fourth OGFC mixture was designed similar to the third OGFC mix, except that this mix had both 0.3 percent synthetic fiber and 0.3 percent cellulose fiber.
• The same 12.5-mm NMAS gradation was used in the fifth OGFC mix design, which was later selected for Section E10. However, the mix design for Section E10 had 6.5 percent asphalt modified with GTR, which consisted of 5.8 percent base asphalt modified by adding 12 percent GTR (by weight of asphalt binder). The GTR was a minus No. 30 mesh size, and the base asphalt was a PG 67-22. No fiber was added to the mix in order to determine whether GTR alone could prevent drain-down and provide resistance to raveling.
Table 1 summarizes the design gradations for the 9.5-mm and 12.5-mm OGFC mixtures. All the binders were pre-blended with an antistrip agent at a dosage of 0.5 percent by weight of the base binder. During the mix design, samples were prepared in the NCAT laboratory and compacted to 50 gyrations to measure air void content, Cantabro stone loss, and splitting tensile strengths.
Table 1 Design Aggregate Gradations for OGFC Mixtures
Sieve Size
(mm) Sieve Size (in)
Percent Passing
9.5-mm OGFC (E9A) 12.5-mm OGFC (E9B and E10)
19 ắ 100.0 100.0
12.5 ẵ 98.5 95.7
9.5 3/8 87.2 56.1
4.75 #4 32.4 15.7
2.36 #8 9.8 9.5
1.18 #16 5.7 7.1
0.6 #30 4.2 5.7
0.3 #50 3.1 4.5
0.15 #100 2.1 3.4
0.075 0.075 1.4 2.6
Figure 2 shows the sample air voids and Cantabro loss results for the five OGFC mixtures evaluated during mix design. These two mixture properties were found to be important for the performance of OGFC mixtures in the field. An OGFC mixture with higher air voids is expected to drain water off the pavement surface quicker, and the minimum air void content for OGFC mixtures was estimated to be approximately 15 percent in previous research cycles of the NCAT Test Track. In addition, an OGFC mixture with lower Cantabro stone loss results is anticipated to have better resistance to raveling, and the maximum Cantabro loss for OGFC mixtures
compacted to 50 gyrations is 15 percent. A further discussion of the results shown in Figure 2 follows.
Figure 2 Comparison of Sample Air Voids and Cantabro Loss
• Except for the 9.5-mm mix, the other mixes shown in Figure 2 used the same 12.5-mm design gradation. Also, except for the GTR mix, the other mixes had a polymer modified PG 76-22 binder and cellulose and/or synthetic fiber. The three mixtures selected for
evaluation at the NCAT Test Track included the 9.5-mm mix with cellulose fiber (Section E9A), 12.5-mm mix with synthetic fiber (Section E9B), and 12.5-mm mix with GTR modified binder (Section E10). The air voids determined for the three mixtures selected were close to or above the minimum air void content of 15 percent, and the Cantabro loss results were all lower than the maximum Cantabro loss threshold of 15 percent.
• The 12.5-mm mix with cellulose fiber (the first mix in Figure 2) was similar to the ALDOT- approved mix design with known field performance from previous pavements on
Interstate 85, which are a few miles away from the NCAT Test Track. Compared to the three OGFC mixes selected for the experiment, this mix had lower air voids and a similar Cantabro stone loss. The 12.5-mm mix design with both cellulose and synthetic fibers (the fourth mix in Figure 2) was not selected because it had air voids and Cantabro loss results similar to the mix that had only the synthetic fiber. Thus, adding cellulose fiber into an OGFC mixture with synthetic fiber did not provide added benefits.
Figure 3 compares splitting tensile strength test results obtained during mix design for four OGFC mixes. While the ALDOT procedure for OGFC mix design requires the tensile strength ratio to be determined in accordance with AASHTO T 283 without a freeze-thaw cycle, the splitting tensile strength test (results shown in Figure 3) was conducted with a freeze-thaw cycle as the conditioned splitting tensile strength determined with a freeze-thaw cycle was found to be a good indicator of OGFC mixture performance in the field. In addition, a minimum splitting tensile strength of 50 psi was proposed for a performance-based mix design in a previous study (2). This threshold is shown as the dash line in Figure 3.
Figure 3 Comparison of Tensile Strength Test Results for OGFC Mixtures In Figure 3, the 9.5-mm mixture with cellulose fiber provided the highest tensile strengths.
Among the 12.5-mm OGFC mixes, the GTR mix without fiber had the highest tensile strengths
the 12.5-mm mix with cellulose fiber failed this requirement significantly. In addition, the three selected OGFC mixtures passed the conditioned splitting tensile strength threshold of 50 psi while the 12.5-mm mix with cellulose fiber failed this proposed requirement. The moisture conditioning and freeze-thaw cycle in AASHTO T 283 had a significant effect on the conditioned splitting tensile strength of this mix.