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10 C H A P T E R 3 Experimental Design Experimental Plan In this implementation effort, the project team worked with three state highway agencies (GDOT, SCDOT and ALDOT) in three PFC construction projects. For each construction project, the activities that were conducted to implement the performance-based PFC mix design procedure are illustrated in Figure 3- 1. Figure 3-1. Activities Conducted to Implement Performance-Based PFC Mix Design Procedure for Each Construction Project. As shown in Figure 3-1, the implementation effort in each construction project started with the state highway agency sharing the PFC mix design that was conducted based on its PFC mix design procedure. NCHRP 20-44/Task 18 Mix Design Verification Obtaining agencyâs PFC mix design Sampling raw materials Batching/compacting test specimens Conducting performance tests Comparing to proposed thresholds Final Mix Design A dj us tin g m ix pr op or tio ni ng Pass Fail Construction Sampling plant mix Compacting test specimens Conducting performance tests Comparing properties of lab and plant mixtures Final Report
11 The project team then sampled raw materials that would be used in the construction project to verify the agencyâs PFC mix design based on the performance-based mix design procedure proposed at the conclusion of NCHRP Project 01-55. Raw materials were then prepared, and performance test specimens were compacted and tested. Performance test results were compared with the thresholds proposed in the performance-based mix design procedure to determine if the PFC mix design would meet the expected performance or if the mix design would be adjusted for performance testing until it met all the proposed thresholds. The three PFC mix designs were used in three construction projects by GDOT, SCDOT and ALDOT. During construction, samples of plant-produced mixtures were taken for performance testing. Performance test results of the plant-produced mixtures were then compared with those of the corresponding laboratory- prepared mixtures tested during the mix design verification to determine their differences. The following sections include information about the materials and PFC mix designs provided by the state highway agencies, followed with performance tests conducted to verify the agencyâs mix designs and their respective thresholds proposed in the NCHRP Project 01-55 performance-based mix design procedure. Materials and Agencyâs PFC Mix Designs The PFC mix design used in the GDOT project was made of granite aggregate, including #7 stone (65 percent), #89 stone (29 percent), W10 aggregate (5 percent) and 1 percent hydrated lime. The mix also had 0.3 percent cellulose fibers based on total weight of aggregate. The mixing temperature was 315oF. Table 3-1 summarizes the design gradation and optimum asphalt binder content for the mixture that met the respective GDOTâs specification limits. Table 3-1. GDOT Mix Design. Sieve (Std) Sieve (Metric) %Passing Limits 3/4 inch 19 mm 100 100 1/2 inch 12.5 mm 97 85-100 3/8 inch 9.5 mm 71 55-75 No. 4 4.75 mm 17 15-25 No. 8 2.36 mm 6 5-15 No. 200 0.075 mm 3.0 2-5 Optimum AC PG 76-22 6.2% 6.0-7.25% Table 3-2 summarizes the design gradation, optimum binder content, and other properties of the PFC mix design used in the SCDOT project. The design gradation was made of granite aggregate with 64 percent #7 aggregate, 35 percent #89 aggregate, and 1 percent hydrated lime. The design gradation and optimum binder content met the specification limits shown in Table 3-2. The PFC mix design also included 0.5 percent Evotherm M-1 as the SCDOT specifications allow contractors to produce PFC mixtures below 270oF with a chemical warm mix additive without cellulose fibers to prevent binder draindown. Other mixture properties shown in Table 3-2 include the Cantabro loss and binder draindown, and both met the specification limits.
12 Table 3-2. SCDOT Mix Design. Sieve (Std) Sieve (Metric) %Passing Limits 3/4 inch 19 mm 100 100 1/2 inch 12.5 mm 99 92-100 3/8 inch 9.5 mm 69 55-75 No. 4 4.75 mm 19 15-30 No. 8 2.36 mm 8 5-15 No. 200 0.075 mm 1.6 0-4 Optimum AC PG 76-22 6.0% 5.5-7.0% WMA Evotherm M-1 0.5% Cantabro Loss 12.3% < 15% Draindown 0.3% < 0.5% A summary of the PFC mix design used in the ALDOT project is provided in Table 3-3. The design gradation included slag aggregates (8 percent 1/2â and 11 percent 3/8â), sandstone materials (50 percent #7 and 30 percent #8 stone) and 1 percent baghouse fines. The mix also had 0.3 percent cellulose fibers to prevent binder draindown. The design gradation and optimum binder content met all the limits set in the ALDOT specifications. Table 3-3. ALDOT Mix Design. Sieve (Std) Sieve (Metric) %Passing Limits 3/4 inch 19 mm 100 100 1/2 inch 12.5 mm 97 85â100 3/8 inch 9.5 mm 71 55â65 No. 4 4.75 mm 17 10â25 No. 8 2.36 mm 6 5â10 No. 200 0.075 mm 3.0 2â4 Optimum AC PG 76â22 6.2% Min 6% Test Methods and Proposed Design Thresholds Prior to developing an experimental plan for NCHRP Project 20-44(18), the project team discussed with several SHAs the possibility of implementing the NCHRP Project 01-55 performance-based mix design procedure for PFC. Based on these discussions, the consensus was that raveling was still the leading form of distress affecting PFC performance, and that PFC typically did not fail because of cracking or rutting even though they could be concerns in the future. Thus, SHAs would like to implement a mix design procedure with performance tests that specifically address raveling as the first implementation step before considering other performance tests to address rutting and cracking. For this reason, optional rutting and
13 cracking tests, namely HWT and I-FIT, proposed in the draft performance-based mix design procedure were not included in the experimental plan for NCHRP Project 20-44(18). Binder Content and Aggregate Gradation of Plant-Produced Mixture The asphalt content of the sampled plant-produced PFC mixes was confirmed using the ignition oven. Testing was performed in accordance with AASHTO T 308, Determining the Asphalt Binder Content of Asphalt Mixtures by the Ignition Method. The test temperature was the standard 1,000°F. A minimum of 2 replicates were tested for each plant-produced mixture and the asphalt content was provided by the printed ticket. After ignition, a washed sieve analysis was performed on each sample in accordance with AASHTO T 30, Mechanical Analysis of Extracted Aggregate, to determine the gradation. The gradations and asphalt contents were then compared to the original job mix formula (JMF) for each mixture. Specimen Air Voids The volumetric properties were calculated based on data collected at the NCAT laboratory. One of the most important volumetric properties for PFC mixtures is air voids at the design (laboratory) compaction level (Ndes) as this property was found to correlate to the mix permeability in NCHRP Project 01-55 (Watson et al., 2018). To calculate air voids, both the bulk (Gmb) and theoretical maximum (Gmm) specific gravities of PFC mixture were determined. While Gmm was determined in accordance with AASHTO T 209, Gmb was determined following two methods: (1) a dimensional method in which the height and diameter of each compacted specimen was measured to calculate the volume and then Gmb, and (2) an automatic vacuum sealing (Corelok) method described in ASTM D6752, Bulk Specific Gravity and Density of Compacted Bituminous Mixtures Using Automatic Vacuum Sealing Method. For both methods of measuring Gmb, the PFC specimens were compacted in a Superpave Gyratory Compactor with Ndes of 50 gyrations, which is specified in both ASTM D7064, Standard Practice for Open-Graded Friction Course (OGFC) Mix Design, and AASHTO PP 77, Standard Practice for Materials Selection and Mixture Design of Permeable Friction Courses (PFCs). It should be noted that all specimens tested in this implementation effort were compacted to Ndes of 50 gyrations, but the amount of loose mix in the mold was adjusted to target certain specimen heights required for each performance test conducted in this study. The specimen air voids were then determined based on the Gmm and Gmb values of the mix. Two air void contents are reported for each PFC mixture, including one using the Corelok method and the other using the dimensional method. Draindown Draindown occurs when asphalt binder drains from the aggregate particles in the PFC mixture and settles at the bottom of the silo, transport vehicles, and/or construction equipment. Draindown is due to several factors, the greatest of these being the absence of a stabilizing agent to hold the thick binder film in place. The draindown testing was conducted according to AASHTO T 305, Draindown Characteristics in Uncompacted Asphalt Mixtures, using a 2.36 mm mesh sieve for the draindown baskets. According to AASHTO T 305, the lower test temperature should be equivalent to the production temperature and the higher test temperature should be 15°C (27°F) above the production temperature, to account for the anticipated fluctuation in production temperature. The samples were conditioned in the basket over a pie plate for 1 hour. The maximum recommended amount of draindown allowed is 0.3 percent. The draindown is recorded as the percent of material that is on the pie plate after the 1-hour conditioning.
14 Cantabro Test The Cantabro test is used to determine the durability of an asphalt mixture. It is primarily used for evaluating PFC mixes but has more recently been used to evaluate other asphalt mixture types. The test is conducted in accordance with AASHTO TP 108, Standard Method of Test for Determining the Abrasion Loss of Asphalt Mixture Specimens. Test specimens are individually placed in the Los Angeles Abrasion machine, without the steel charges, and tested for 300 revolutions at a rate of 30 to 33 revolutions per minute. The loose material is then discarded and the final specimen weight is recorded to calculate the percent weight loss (or Cantabro loss) for each specimen. Tensile Strength Ratio The tensile strength ratio (TSR) test was conducted in accordance with AASHTO T 283, Resistance of Compacted Bituminous Mixture to Moisture Induced Damage, with slight modifications to accommodate PFC mixes. The modifications are recommended in the ASTM D7064 test procedure. The specimens were compacted to the design gyration level and height instead of the target height in the procedure of 95 mm. While this differs from the specification, the height of the specimens is included in the final calculations so this change is accounted for in the final results. The weight of the design specimens was altered slightly for these specimens to target a height of 110 to 115 mm in order to ensure that the specimens fit inside the breaking head. The specimens were saturated at 26-in Hg (660.4-mm Hg) below atmospheric pressure for 10 minutes and then the saturated specimens were frozen in plastic concrete cylinder molds. The specimens were kept submerged under water while freezing to keep the interior voids filled with water. The rest of the test procedure was followed according to the specification. The specimens were tested for indirect tensile (IDT) strength on a Marshall Stability press at a rate of 2 inches per minute. The IDT strength of the mixes was determined from the peak load recorded on the device and the specimen dimensions. The conditioned and unconditioned IDT strengths and their ratio (TSR) are reported for each mixture.
