Experimental Procedure And Results A Engineering Performance of HMA with PCSs

1. Soil Classification

Six contaminated soils provided by NJDEPE from sites identified as containing soils with less than 1% total petroleum hydrocarbons (TPHs) were selected for testing and for characterization. Three soils were contaminated with heating oil, and the other three were contaminated with gasoline. The degree of contamination for oil-contaminated samples was determined by the Soxhlet oil and grease extraction method (USPHS standard method for the analysis of water and wastewater). Table 1 gives the soil classification data and lists the type and amount of contaminants in each soil.

2. Stability or Strength of HMA with PCSs

All the asphalt concrete samples were designed for New Jersey surface coarse mix (NJ 1-3). A control mix was designed and tested for comparison. The mix designs to include soils were prepared for each of the six soils based on the sieve analysis data. (Please refer to Meegoda et al. [8] for grain size analysis data for soils and aggregates and the NJ 1-3 specifications.) The optimum percentages that may be used in the NJ 1-3 mix based on aggregate blending for the above six soils were as follows: Soil 1, 35%; soil 2, 10%; soil 3, 20%; soil 4, 15%; soil 5, 10%; and soil 6, 15% [8], These percentages are much higher than the current practice of 5% PCSs. Once the maximum amount of PCS that may be added to HMA was determined, the suitability of such an addition was evaluated. The Marshall stability test was performed using ASTM D1559. Based on the New Jersey Department of Transportation (NJDOT) requirements for Marshall strength, bulk density, air voids, voids in mineral aggregates (VMA), and flow, the optimum asphalt content for each mix was selected. The optimum asphalt contents based on Marshall test results for the control mix and for the six contaminated soils are shown in Table 2 [8]. Table 2 also shows the dry density, Marshall stability, air voids, VMA, and flow values at the optimum asphalt contents for the control as well as for HMA made with each soil type. The New Jersey specifications for high traffic volume 1-3 mix are also shown in Table 2.

If an asphalt concrete meets all state specifications and if it is a workable mix, then it is accepted as a paving material. Except for the control mix and HMA mixed with PCS No. 4, as shown in Table 2, all the HMA mixes with PCS were acceptable as paving materials. The control mix and the HMA mix with soil No. 4 had low flow values. A higher flow value can be selected for all three mixes from Marshall test results with higher asphalt contents, but it will result in lower stability values (lower than 8006 N). If a lower Marshall strength value

Table 2 Optimum Properties of Asphalt Concrete with PCSs for NJ 1-3 Mix

Asphalt concrete Allowable for

Table 2 Optimum Properties of Asphalt Concrete with PCSs for NJ 1-3 Mix

Asphalt concrete Allowable for

property

NJ 1-3 Mix

Control

Soil 1

Soil 2

Soil 3

Soil 4

Soil 5

Soil 6

Strength (N)

>8006

8006

8228

8450

10229

8450

8317

10452

Flow (0.25 mm)

>6.0

4.0

11.0

8.0

7.5

3.5

6.5

7.7

Air voids (%)

2.0-8.0

7.0

7.5

3.0

5.7

8.0

4.0

3.4

VMA (%)

>13.0

18.0

17.8

14.0

16.8

18.0

14.7

14.2

Density (kN/m3)

N/A

24.3

24.8

24.5

24.1

23.4

24.6

24.5

Optimum asphalt

4-8

5.0

4.5

4.5

5.0

4.5

4.5

(say 667IN) is acceptable, then the control mix and the HMA made with soil No. 4 are acceptable as paving materials. Based on the test results shown in Table 2, it can be stated that the HMA with PCSs produced better asphalt concrete than the control. This may be due to the better blend obtained by adding natural soils.

3. Durability of HMA with PCS

ASTM D4867 describes the test procedure for determining the effect of moisture on asphalt concrete mixtures, a factor that is very important for the durability of hot mix asphalt concrete. It has a section on freeze-thaw conditioning of a mixture. However, the freeze-thaw and wet-dry tests require only one cycle each of freeze-thaw or wet-dry. There was no rationale for the selection of one cycle of freeze-thaw or wet-dry to evaluate the moisture damage when, in the real world, asphalt concrete pavements are subjected to several freeze-thaw and wet-dry cycles under service conditions before they are removed for resurfacing. Therefore, an experimental program was designed to evaluate the moisture damage of the control mix and one mix prepared with PCS No. 3. In this experiment HMA specimens were subjected to several cycles of freeze-thaw and wet-dry. For this experiment, 18 specimens of control mix and 18 specimens of HMA mixed with PCS No. 3 were used. The experimental test results of an extended freeze-thaw test are plotted in Figures 1 and 2. Figures 3 and 4 show similar data for wet-dry tests.

In this extended durability test, the control mix and HMA with PCS No. 3 were tested for one, three, seven, and 14 cycles, with each cycle taking approximately 48 hr. Upon completing 14 freeze-thaw and wet-dry cycles, data were collected and graphically displayed. It can be concluded from these test results that as the temperature drops to freezing temperatures for the freeze-thaw specimen, the asphalt concrete contracts and becomes brittle, creating tiny cracks on the surface of the sample, which provide entry points for water. Water inside the specimen causes moisture damage due to stripping and volume expansions during subsequent freeze cycles. With the increase in the number of freeze-thaw cycles, the cracks get larger, letting more water into the specimen and eventually leading to the failure of the specimen. The data from cyclic freeze-thaw tests indicate that the percentage swell increased rapidly during the first cycle and then gradually reached a maximum before the specimen failed. It is believed that the specimen reached its maximum percentage swell when it was totally saturated, and stripping occurred before complete failure. The tensile strength ratio also decreased rapidly during the first cycle and also began to level off to zero strength after 14 cycles. The behavior seems to be similar for the control mix and the mix containing PCS No. 3, suggesting comparable durability

NUMBER OF CYCLES

Figure 1 Extended freeze-thaw test for control mix and HMA with PCS No. 3—TSR values. (From Meegoda et al. [9],

NUMBER OF CYCLES

Figure 1 Extended freeze-thaw test for control mix and HMA with PCS No. 3—TSR values. (From Meegoda et al. [9],

Figure 2 Extended freeze-thaw test for control mix and HMA with PCS No. 3—swell values. (From Meegoda et al. [9].)

values for these two mixes. It also seems that there is a correlation between the percentage swell and the tensile strength ratio. The data show that most of the strength is lost during the first cycle, and hence there was no need to test beyond one freeze-thaw cycle as suggested by ASTM D4867.

The cyclic wet-dry test also indicated that the first cycle was the point where attention must be focused. For the wet-dry test, the tensile strength ratio declined during the first '

6-i

5.5-

5

4.5-

4

0?

3.5-

1

-J

3-

III

§ C/i

2.5-

2

1.5-

1-

0.5-

CONTROL

6 8 10 NUMBER OF CYCLES

6 8 10 NUMBER OF CYCLES

Figure 3

goda et al.

Extended wet-dry test for control mix and HMA with PCS No. 3—TSR values. (From Mee-[9].)

Figure 3

goda et al.

Extended wet-dry test for control mix and HMA with PCS No. 3—TSR values. (From Mee-[9].)

4 6 8 10 12 14 NUMBER OF CYCLES

Figure 4 Extended wet-dry test for control mix and HMA with PCS No. 3—swell values. (From Meegoda et al. [9].)

cycle and increased thereafter. This is believed to be due to oxidation of asphalt during the drying cycle. Therefore, it was concluded that the first wet-dry cycle should yield the critical conditions.

It can be concluded from these tests that freeze-thaw and wet-dry tests with one cycle indicate whether a specimen is durable, and a strength loss of more than 20% indicates that the specimen will not withstand harsh weathering conditions.

Table 3 Durability of HMA with PCSs

HMA mix

Wet-dry test

Freeze-thaw test

Control

91.7

82.3

HMA with soil 1

98.0

89.0

HMA with soil 2

89.3

100.0

HMA with soil 3

87.2

93.9

HMA with soil 4

83.8

87.0

HMA with soil 5

93.4

98.4

HMA with soil 6

100

100

Table 3 shows the tensile strength ratio (TSR) values of wet-dry and freeze-thaw tests performed based on ASTM D4867 for the control mix and for the HMA made with six soils. TSR values for HMA with PCSs were not significantly different from that of the control, indicating that HMA with PCSs produced durable asphalt concrete. Some of the wet-dry and freeze-thaw test results show that the HMA with PCS are better than the HMA produced with virgin aggregates.

4. Permeability of HMA Prepared with PCSs

Hydraulic conductivity tests were performed on laboratory-compacted HMA specimens. The specimens were placed on top of a 100 mm diameter flexible-wall permeameter. A cell pressure of 50 psi (344kPa), a back-pressure of 30 psi (206 kPa), and a desired pressure difference (mainly 1 psi or 7 kPa) were applied to the specimen. Twenty-four hours after the in-flow became equal to the outflow and when the hydraulic conductivity did not show further reduction, the permeability test was stopped. The permeability test was conducted concurrently on three specimens of HMA made with the same PCS. A bladder accumulator was connected between the permeameter and the pressure panel to separate the permeant from the distilled water used in the pressure panel. This procedure eliminated the contamination of the pressure panel.

At the end of the permeability test, the specimen height was measured. Figure 5 shows typical permeability test results where the variation of hydraulic conductivity with time is shown. The average saturated hydraulic conductivities of the control mix and HMA mixes with PCSs are shown in Table 4.

Table 4 shows the saturated hydraulic conductivity data for HMA with and without PCSs. Only one mix (with PC No. 2) showed a lower saturated hydraulic conductivity value than the control mix. However, the saturated hydraulic conductivity values of all the HMA mixes with PCSs were less than 2.0 x 10-6, a characteristic value for low permeable clay-type soils and hence can be considered acceptable. Table 4 also shows the other parameters of asphalt matrix that contribute to the saturated hydraulic conductivity. It appears that the combination of air voids, asphalt content, and dl0 size (aggregate size corresponding to the 10% finer fraction in the gradation curve) control the saturated hydraulic conductivity. With a higher percentage of air voids, one would expect a higher hydraulic conductivity as a larger fraction of asphalt concrete matrix will be porous, allowing fluid to flow through those voids. In geotechnical engineering, it is an accepted fact that dl0 size controls the hydraulic conductivity of soils, with soils that have higher di0 sizes having higher hydraulic conductivity values. However, since all the HMA mixes tested in this research were NJ 1-3 mixes, there should not be drastic differences in di0 size as shown in Table 4; hence its influence here is minimal.

The asphalt content will also play a major role when it is higher than optimum, as the excess asphalt cement, after coating all the particles, will block the interconnecting voids,

150 Time (hrs)

Figure 5 A typical hydraulic conductivity test result. (From Meegoda et al. [9].)

150 Time (hrs)

Figure 5 A typical hydraulic conductivity test result. (From Meegoda et al. [9].)

Table 4 Hydraulic Conductivities and Other Related Parameters of HMA with PCSs

Saturated hydraulic

dio size in the

HMA mix

conductivity (cm/sec)

mix (mm)

Air voids (%)

Asphalt content (%)

Control

2.3E-07

0.18

4.7

4.75

HMA with soil 1

3.3E-07

0.21

5.6

4.50

HMA with soil 2

1.6E-07

0.13

6.9

4.50

HMA with soil 3

1.6E-06

0.12

7.5

5.00

HMA with soil 4

1.0E-06

0.27

6.9

4.25

HMA with soil 5

8.3E-07

0.14

7.3

4.50

HMA with soil 6

4.6E-07

0.15

6.3

4.50

causing a reduction in hydraulic conductivity. However, since all the mixes were tested around the optimum asphalt content values for the corresponding mixes, its contribution to the test results are minimal too. Therefore, the hydraulic conductivity values should increase with the percentage of air voids, and there appears to be a direct correlation between percentage of air voids with the hydraulic conductivity except for mix having PCS No. 3. We suspected measurement errors for this mix [9]. Therefore, it can be concluded that the addition of PCSs may not change the saturated hydraulic conductivity of the asphalt concrete and the change is due only to the difference in air voids, dm size, and asphalt content associated with the mix design [9],

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