Transportation Research Record 1034

Effect of Mixing Temperature and Stockpile

Moisture on Asphalt Mixtures

THOMAS W. KENNEDY and GERALD A. HUBER

ABSTRACT

Studies have indicated that mixtures produced in drum plants have workability and short-term performances equal to those of mixtures produced in conventional plants and that long-term performance will be equivalent. This work, however, has led to questions about the effect of lower mixing temperatures and higher aggregate moisture contents. In addition, there is a need for information on the effects of stockpile and asphalt-mixture moisture contents for conventional batch and drum plants for use in developing cost-effective specifications. To determine these effects, the Center for Transportation Research at the Univer­sity of Texas at Austin and the Texas State Department of Highways and Public Transportation initiated a series of field experiments to evaluate the engi­neering properties of asphalt mixtures produced with a range of stockpile mois­ture contents, a range of mixing temperatures, and both drum and conventional batch plants. The first phase of the study, discussed here, involved a variety of aggregate types and different plants in Texas. Mixing temperatures ranged from 175°F to 325°F and stockpile moisture contents varied from dry to satu­rated. The engineering properties evaluated were Hveem stability, tensile strength, resilient modulus, and moisture susceptibility characteristics. Es­sentially, for the studies conducted, there were no significant effects pro­duced by stockpile moisture and mixing temperatures for either batch or drum plants. Both plants were able to remove most of the moisture although this probably increased fuel costs and lowered possible production rates. Density was the major factor that affected the engineering properties and was indi­rectly related to mixing temperature.

35

I n the past decade the use of drum mix asphalt p lants has increased signific a ntly and t he t r end i s expec t e d t o continue i new plant s ales are pr edomi ­nantl y o f this type because of t heir simplicity , lower initial cost, and lower operating cost.

and conventional batch plants with a variety of ag­gregate type s.

Several studies have indicated that the mixture produced in drum mix plants h as workability and s hoI't- t e r m performance qualities eq ual to t hose of a mi xtur e p r oduced i n conventional plants . Ther e is also an i ndication t hat long-te rm pe r f o rmance of both will be equivalent (1). However, these studies evaluated material produced only under existing specifications and no effort was made to establish the performance qualities of mixtures produced out­side of existing specifications.

The work done to assess and compare the proper­ties of drum-mixed and conventionally mixed asphalt mixtures led to the question of whether the drum mix plant c ould produce acceptable mix tures at lower t empera tur es and h igher moisture conten ts than are normal with a batch plant . In addition , t here is a need for information about the effects of stockpile or mixture moisture contents for both conventional plants and drum plants . If the specifications could be changed without detrimental effects on the mi x­ture properties, significant savings could be re­alized.

To determine these effects the Center for Trans­portation Research (CTR) and the Texas State Depart­ment of Highways and Public Transportation (Texas SDHPT) through their cooperative research program initiated a series of experiments performed on as­phalt plants under regul ar field cond it ions. These experiments were conducted on both dr um mix pla nts

This paper is a report on the first phase of the study and contains a summary of the experimental program, problems encountered, results, and con­clusions.

EXPERIMENTAL PROGRAM

Study Desig n

The study involved an experiment in which asphalt mixtures containing different aggregates were pro­duced at various combinations of mixing temperatures and stoc kpile moisture cont ents (Table 1). This basic e xper iment was repea ted at seve r al different asphalt plants in Texas.

TABLE 1 Moisture-Temperature Experiment Deaign

Asphalt Mixture Cold Feed Aggregate Mixing Moisture Content Temperature (oF) Dry Wet Saturated

175 x x x 225 x x x 250 x x x 275 x x x 325 x x x

36

Experimental Mixing Temperatures and Stockpile Moisture Contents

The mixing temperature was varied from 325°F to 175°F, which is well below current operating temper­atures, and the stockpile moisture contents were varied from virtually dry to nearly saturated, The three levels of moisture content used were referred to as dry, wet, and saturated, Dry stockpiles were ootained by drying the aggregate before mixing. Wet stockpiles were defined as the natural stockpile moisture, and saturated aggregates were obtained by applying water to small stockpiles of individual aggregates, Mixing temperature measured at the dis­charge of the mixture was varied by changing the burner flame control. The temperature of the asphalt cement before mixing was held constant throughout the experiments at an estimated 275°F to 300°F,

Asphalt Plants used for Experimentation

The three asphalt plant sites used in the study were located at Austin (drum and batch plants), ·Mathis (batch plant), and Alice (drum plant). Mathis and Alice are near Corpus Chris tie. Two different con­tractor-producers were involved.

Materials

Three asphalt mix designs were used (Table 2). The major components were hard limestone for the Austin aggregate and sandstone for both the Mathis and Alice aggregates, The asphalt cement used for all projects was an AC-10. The Alice and Mathis aggre­gates were susceptible to moisture damage and as part of the regular contract work a l percent hy­drated lime slurry was sprayed on the cold feed belt.

TABLE 2 Location and Description of Aggregate Mixtures

Asphalt Plant

Location

Austin

Mathis

Alice

Type

Drum/Batch

Batch

Drum

Aggregate Type•

Crushed sandstone Crushed limestone Limestone screenings Field sand Crushed sandstone Crushed limestone Sandstone screenings Field sand Crushed sandstone Sandstone screenings Field sand

11 All mlxtwe designs were 1/2-in. maximum size aggregate.

Sampling Program

Samples of both asphalt mixture and raw aggregate were gathered during each experimental run. The as­phalt mixture was taken from a truck after the mate­rial had been mixed at the desired temperature and test specimens were prepared in the field laboratory.

Raw aggregate samples were gathered to determine aggregate moisture contents at key points in plant processing.

Specimen Preparation and Conditioning

Specimens, 2 in. high by 4 in. in diameter briquets, were molded using a Texas gyratory shear compactor.

Transportation Research Record 1034

Three compaction procedures were used and are re­ferred to as standard, modified-standard, and mod­ified:

• Standard compaction. Specimens were prepared using the standard procedures of the Texas SDHPT in which the mixture was compacted at 250°F (2) •

• Modified-standard compaction. Specimens were prepared using the same method except that the com­paction temperature was the same as the plant mixing temperature.

• Modified compaction. Specimens were prepared at the plant mixing temperature to a target density of 7 percent air voids.

Two conditioning methods were applied to each of the specimens:

• Dry conditioned, Specimens were stored at room temperature for several days.

• Wet conditioned. Specimens were vacuum satu­rated under 26-in. mercury vacuum, put through a freeze-thaw cycle, then tested at room temperature .

Test Methods

The three basic test methods used were the Hveem stability test, the static and repeated-load in­direct tensile tests, and the Texas boiling test.

Hveem Stability Test

Hveem stabilities were deter:mined using the Rveem stabilometer according to Texas SDHPT standard pro­cedures <ll . Compacted asphalt mixture specimens 2 in. high by 4 in. in diameter are loaded at a con­stant strain to a maximum vertical load of 5,000 lo. The horizontal force generated by the vertical load is measured as a pressure on the stabilometer wall and is used to calculate the Hveem stability.

Static Indirect Tensile Test

The indirect tensile test <1> was conducted to esti­mate the tensile strength of the asphalt mixtures. A cylindrical specimen was loaded with a compressive load distributed through steel loading strips curved to fit the specimen. The specimen ultimately failed by splitting along the vertical diameter. The ten­sile strength was calculated from the applied load at failure and the specimen dimensions. Tests were conducted at 75°F,

Repeated-Load Indirect Tensile Test

To determine the resilient modulus of elasticity, the repeated-load indirect tensile test was used in which approximately 20 percent of the static failure load was applied repeatedly to the specimen C.! and ASTM D 4123-82) at a testing temperature of 75°F. The load-vertical deformation and the load-hori­zontal deformation relationships were recorded and the resilient modulus was calculated.

Texas Boiling Test

The Texas boiling test is a rapid method of evalu­ating the moisture susceptibility or stripping po­tential of aggregate-asphalt mixtures <.2.t. In this test a visual observation is made of the extent of stripping of the asphalt from aggregate sur.'aces

Kennedy and Huber

after the mixture has been subjected to the action of boiling water for a specified time. The amount of stripping is determined by a visual rat i ng, ex­pressed in terms o f the percentage of asphalt re­tained on a scale of 0 to 100 percent.

Other Tests

Other tests used on asphalt mixtures were conducted according to standard methods of the Texas State Department of Highways and Public Transportation 11> a nd ASTM D 2041-78 and included asphalt extraction and recovery (Abscon process) , asphalt penetration and viscosity of the extracted asphalt, sieve analy­sis of the asphalt mixture, Rice theoretical maximum specific gravity (ASTM 2041) , bulk specific gravity of compacted specimens , and moisture content deter­minations of the coated and uncoated aggregate ( 250°F).

EXPERIMENTAL RESULTS

In the -experiment design, only mixing temperature and stookpile moisture content were to be varied i however , other mixture parameters were found to change during field production of the e xperimental mixtures , which complicated the evaluation. An i n i ­tial attempt was made t o a nalyze the data using var­ious statistical methods. These methods, however, did not yield useful results and a decision was made to abandon the use of regression and analysis of variance with consideration of covariates as analyt­ical tools. Instead, the uncontrolled variables were examined individually to determine if any was likely to cause a significant effect on the experiment re­sults.

A complete listing and analyses of the test re­sults are given elsewhere (.§_).

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Uncontrolled Variables

The uncontrolled variables in the experiment were moisture in the asphalt mixture, voids in the min­eral aggregate, air voids, asphalt content, and as­phalt penetration.

Stockpile Mpisture Content

Although the stockpile moisture contents were con­trolled qualitatively as- dry, wet, and saturated, the actual moisture contents were dependent on ag­gregate type, atmospheric conditions, length of time the experiment was conducted, and technique used to introduce moisture (Figures l and 2) •

Within each aggregate type the moisture contents for each of the three stockpile moisture conditions were relatively uniform except for the Austin aggre­gate. This aggregate was used at the stockpile mois­ture content and the variation of this content is attributable to the 9-month period during which the experiment was conducted.

Moisture in the Asphalt Mixture

Most asphalt mixtures had moisture contents of less than about 0.5 percent, especially for the rela­tively nonporous Austin aggregate. Nevertheless, for high stockpile moisture contents and low mixing tem­peratures, moisture contents did exceed 0.5 percent and approached l or 2 percent (Figure l) • Thus the moisture content of the asphalt mixture was influ­enced by the stockpile moisture content, the mixing temperature, and the aggregate porosity.

As expected, moisture content increased with in­creased stockpile moisture, decreased as mixing tem­perature increased, and was higher for the Mathis and Alice aggregates that were more porous than the

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FIGURE I Relationship between mixing temperatures and moisture content of the asphalt mixtures.

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Transportation Research Record 1034

Stockpile Moisture

225 250 275 325

Mixing Temperature,''F

Drum Plant

Austin Aggregate Mathis Aggregate Alice Aggregate

F1GURE 2 Degree of saturation of aggregate.

Austin aggregate. During the experimental runs the stockpile moisture content and the mixing temper­ature were controlled and no attempt was made to produce a given asphalt mixture moisture.

Specimen Density

Density was estimated in terms of voids in the min­eral aggregate (VMA) and air voids because the spe­cific gravities of the various aggregates differed and the asphalt contents of the mixtures also dif­fered. Variation in VMA and air void content is given elsewhere (6). The variation in both VMA and air voids was relatively small for any given compac­tion procedure and thus it is believed that density variations probably did not have a significant ef­fect on the laboratory test results.

Asphalt Content

An attempt was made to maintain the asphalt content at the design percentage for each experimental run. However, the extracted asphalt contents were found to vary considerably.

The daily plant testing done by Texas SDHPT on nonexper imental mixtures indicated that the asphalt contents varied ±0.l percent from the design. Thus there is no evidence to indicate that the plant cal­ibration was not accurately supplying the proper as­phalt volume. Unfortunately, state personnel did not test any of the experimental mixtures and a direct comparison of extraction results cannot be made.

It is believed that an experimental error of un­determinable origin has affected the indicated as­phalt contents of the experimental mixtures but that the actual contents were approximately equal to the design value. Therefore asphalt content was treated as a constant throughout the analysis of experiment results.

Extracted Asphalt Penetration

The penetration of the extracted asphalt was mea­sured to determine the amount of hardening that had occurred under the experimental conditions. Results indicate that the amount of asphalt hardening was not significantly affected within the range of mix­ing temperatures studied1 however, the higher mixing temperatures did cause a penetration reduction of 5 to 10.

Hardening of the asphalt was found to be affected by the stockpile moisture content. High stockpile moisture contents decreased the amount of hardening that the asphalt experienced during mixing. The dif­ference in penetration over the range of moisture contents is approximately 10. This behavior may have been caused by the presence of increased humidity in the pugmill or the drum mixer with increasing stock­pile moisture, which in turn may have retarded the evaporation of the lighter fractions of the asphalt and also reduced oxidation.

Summary of Test Results

Tests were conducted on each of the experimental mixtures to determine the Hveem stability, tensile strength, resilient modulus of elasticity, tensile strength ratio, and asphalt retained after the boil­ing test. The results are the average result of du­plicate or triplicate specimens for each experiment condition.

Effect of Mixing Temperature

The effect of mixing temperature on each of the mea­sured engineering properties is shown in figures and is discussed for each of the engineering properties. It should be noted that it was difficult to achieve uniform coating of the aggregates with mixing tem­peratures below about 200°F.

Kennedy and Huber

Hveem Stability

As shown in Figure 3, mixing temperature produced a slight increase in Hveem stability for the standard and modified-standard specimens: however, the in­crease was of no practical significance and the ef­fects were inconsistent. The modified compaction specimens did not exhibit any significant change in stability with different mixing temperatures.

Compaction procedure was observed to produce a major · effect on Hveem stability. The modified com­paction specimens that had significantly lower den­sities and higher air voids also had significantly lower stabilities.

Tensile Strength

The tensile strengths of the standard specimens show a tendency to increase with increased mixing temper­ature, but the degree and consistency of this trend is variable (Figure 4).

For Austin aggregate, the standard compaction specimens and, to some degree, the modified compac­tion specimens exhibited an increase in tensile strength with increasing mixing temperatures. This trend is also apparent for the modified-standard compaction specimens of the Mathis aggregate. No relationship with mixing temperature existed for the other mixture conditions.

The compaction procedure used to prepare the specimens had the greatest effect on the tensile strength values. The modified, modified-standard, and standard specimens have tensile strengths that lie within distinct separate bands.

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Resilient Modulus of Elasticity

A discernible relationship between mix:i.ng tempera­ture and resilient modulus of elasticity does not appear t0 exist in the observed experimental results (Figure 5). The measured values for the Alice aggre­gate suggest that the resilient modulus of elastic­ity is fairly uniform regardless of mixing tempera­ture i however, Austin and Mathis aggregates do not exhibit the .same uniformity. The observed values vary over a wide range and the variation did not appear to be related to the mixing temperature.

Moisture-Damage Resistance

Two evaluation techniques used to determine the moisture-damage resistance of the experimental mix­tures were the indirect tensile strength ratio and the Texas boiling test.

Tensile Strength Ratio

To determine the tensile strength ratio two sets of specimens were tested. One set was subjected to a moisture saturation, freeze-thaw condition and the other was not. The tensile strength ratio was ex­pressed as the ratio of the wet-conditioned tensile strength to the dry-conditioned tensile strength. The acceptance level of the tensile strength ratio was 0. 70, which often is used to identify a non­moisture-susceptible aggregate mixture.

The tensile strength ratio was found to be sig­nificantly affected by the applied compaction effort

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Mixing Temperature, °F Mixing Temperature, °F Mixing Temperature, °F

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FIGURE 3 Relationship between mixing temperature and Hveem stability.

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Mixing Temperature, °F

Austin Aggregate

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Mixing Temperature, °F

Mathis Aggregate FIGURE 4 Relationship between mixing temperature and tensile strength.

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Mixing Temperature, °F

Mathis Aggregate FIGURE 5 Relationship between mixing temperature and resilient modulus of elasticity.

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Mixing Temperature, °F

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Kennedy and Huber

(Figure 6). The modified compaction specimens had a lower density and were more susceptible to water entry during vacuum saturation. As a result these specimens lost a larger proportion of the dry ten­sile strength than did the standard specimens that were more resistant to water entry.

The effect of mixing temperature on the measured tensile strength ratios of the modified compaction specimens was not consistent for all the aggregates tested. Austin aggregate results suggest that in­creased mixing temperature caused an increase in the tensile strength ratioi however, for the other ag­gregates, with the exception of the wet stockpile moisture specimens of Alice aggregate, no consistent effect was observed.

Boiling Test

The resistance to moisture damage as measured by the percentage of retained asphalt after the boiling test increased with increased mixing temperature (Figure 7). This trend is relatively consistent throughout the three aggregates and shows substan­tial changes in retained asphalt across the range of temperatures tested.

Mathis and Alice aggregates were known to be moisture susceptible aggregates, and as part of the nonexper !mental work both were treated with a hy­drated lime slurry before mixing to improve their resistance to moisture damage. The Austin aggregate, a non-moisture-susceptible aggregate, was not treated. The relationship between mixing temperature and retained asphalt is similar for all three aggre­gates. Both treated and nontreated aggregates show substantial losses in retained asphalt at lower mix­ing temperatures. At higher mixing temperatures the

41

effect of the lime slurry treatment is evidenced by the acceptable percentage of retained asphalt.

The percentage of asphalt retained after the boiling test is known to increase for asphalts that have lower penetrations. The penetration of the ex­tracted asphalt discussed earlier did decrease some­what with increasing mixing temperature1 however, the change was not large enough to account for the significant changes that were observed in the per­centage of retained asphalt.

Effect of Stockpile Moisture

The effect of stockpile moisture on each of the en­gineering properties is shown in the accompanying figures. The Austin aggregate was not considered be­cause only one stockpile moisture condition was studied.

Hveem Stability

The experiment results do not indicate that stock­pile moisture had a major effect on Hveem stability (Figure 8). The saturated stockpile conditions caused a decrease but the change was neither consis­tent nor large. The major influence on the Hveem stability results was density. The values for each of the compaction methods generally lie in distinct groups.

Tensile Strength

The measured values of tensile strength were rela­tively uniform and generally did not vary with

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Mixing Temperature, °F

Mathis Aggregate FIGURE 6 Relationship between mixing temperatme and tensile strength ratio.

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Alice Aggregate

42

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Mixing Temperature, °F Mixing Temperature, °F Mixing Temperature, °F

Austin Aggregate Mathis Aggregate Alice Aggregate FIGURE 7 Relationship between mixing temperature and asphalt retained after boiling test.

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Alice Aggregate FIGURE 8 Relationship between stockpile moisture condition and Hveem Hlability.

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Kennedy and Huber

stockpile moisture condition (Figure 9). The most significant variable that affected tensile strength was density, as shown by the separate groupings of the test results.

Resilient Modulus of Elasticity

The results indicated that the resilient modulus of elasticity was not affected by the stockpile mois­ture condition (Figure 10). The values for the Alice aggregate showed no change for varying stockpile moisture contents and the results for the Mathis aggregate, though varied, did not show a trend,

Moisture-Damage Resistance

Moisture-damage resistance was measured in terms of the tensile strength ratio and the percentage of retained asphalt as measured by the Texas boiling test.

Tensile Strength Ratio

A general trend existed between the stockpile mois­ture condition and the measured tensile strength ratio (Figure 11). The ratio, or the resistance to moisture damage, tended to increase with increasing stockpile moisture contents although the amount of the increase was generally not large.

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43

Texas Boilfng Test

A definite relationship between the asphalt retained after the Texas boiling test and the stockpile moi s ­ture content was evident (Figure 12). The retained asphalt significantly increased as the stockpile moisture content increased, indicating a higher re­sistance to moisture damage. The greatest increase in resistance occurred at the lower mixing tempera­tures. Gene_rally the increase of retained asphalt became less as the mixing temperature increased.

Repeatability of Experiment

A detailed study of the repeatability of the experi­ment was not conducted as part of the project . Dur­ing the Mathis experiment, however, the portion with saturated stockpile moisture and low mixing tempera­ture was repeated and the resulting measured proper­ties were compared . Most of the measured properties were s i milar except possibly the resilient modulus of elasticity and the Hveem stability, both of which exhibited variations.

These variations cannot be explained by the as­phalt contents or the gradation, both of which were the same. Nevertheless, the results suggest that despite some variation of measured results the re­peatability was acceptable for runs performed within a relatively short time span.

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Alice Aggregate FIGURE 9 Relationship between stockpile moisture condition and tensile strength.

44

1450

"iii .Ill: 1300

~ .... ·c; ;:

1150 IA IV ii:i .... 0 IA 1000 ::I "3 'a 0

::::& 850 .... c ~ 'iii 700 G> a:

550

Batch Plant

~

I .. >(

0

/·······o a ,...:::·.· ......... :\~·~/···············• .... . .

(j

Dry Wet Sat.

Stockpile Moisture Condition

Mathis Aggregate

"iii .II:

~ .... ·c; ,.-; IV ii:i .... 0 IA ::I "3 'a 0

::::& .... c ~ ·; G> a:

1450 Drum Plant

1300 Legend

0 175°F Mixing temp . (200°F !or Mathis)

1150 ~ 225°F Mixing temp.

() 250°F Mixing temp.

a 275°F Mixing temp .

e 325°F Mixing temp. 1000 •••••Standard compaction specimens

........ ()

850 ~ ............ : ....... o

;;;:~·;:;?:,:: -::: . ... .." 700 a,... ~ ................... a

550

400'-----l~~~~~'--~~~~.L.-­Dry Wet Sat.

Stockpile Moisture Condition

Alice Aggregate FIGURE 10 Relationship between stockpile moisture condition and resilient modulus of elasticity.

0 ;:

1.20

1.05

0.90

=. 0.75

.c .... CJI c ! 0.60 .... rn .! 'iii 0.45 c G> I-

0.30

0.15

Batch Plant

_..o .. 0:.: -·~:'.' ... ..Q ;. .. /"'-­ca...~·····" ·····~"!>

o··· ...... ....,'.···· ........... ~ '!' .. ... - _ ......... ....

-~ ............. -...... · . ..

-""7 vr .. . (.•" .... ...

Legend 0 175°F Mixing temp.

(200'F for Mathis)

~ 225°F Mixing temp.

() 2SO"F Mixing temp.

a 275"F Mixing temp.

e 325°F Mixing temp.

Common . Acceptance Level

••••••· Standard compaction specimens

" - • Modified-standard compaction specimens

- Modified compaction specimens 0.00 ...___, _____ ...._ ____ __. __

Dry Wet Sat.

Stockpile Moisture Condition

Mathis Aggregate

0 ;:

1.20

1.05

0.90

=. 0.75

.c .... CJI c G> 0.60 .. .... rn .! ·; 0.45 c G> I-

0.30

Drum Plant

......... ~········ .......... ,.

,.,,:::·:::::·::.='~,,,::··:::::::::: . SI:: ........ , •. :: ... , .•

O~ ........ ·: ...... ···~···• ....... .. .. ......... ·

0.15~

o.ool I Dry Wet Sat.

Stockpile Moisture Condition

Alice Aggregate FIGURE 11 Relationship between stockpile moisture condition and tensile strength ratio.

Kennedy and Huber

120

105

~ 90 m :E ·s m 75 ... G> .. .... < 'O 60 G> c iii .. G> 45 a: .. iii .c ~ 30 <

15

Ory

Lime Treated Mixture Batch Plant

Wet Sat.

Stockpile Moisture Condition

Mathis Aggregate

45

120

Lime Treated Mixture

Drum Plant

105

~ 90 m • 325°F :E ·s

75 m ... G> .. .... < 'O 60 G> c 'iii .. G> 45 a:

:!:! ca .c ~ 30 <

15

0 Dry Wet Sat.

Stockpile Moisture Condition

Alice Aggregate FIGURE 12 Relationship between stockpile moisture condition and asphalt retained after the boiling test.

CONCLUSIONS AND RECOMMENDATIONS

The primary objective of this study was to determine the effects of mixing temperature and stockpile moisture on selected engineering properties of as­phalt mixtures produced in conventional batch as­phalt plants and drum mix plants. On the basis of the findings of this study several conclusions were reached and some proposed recommendations can be made.

General Conclusions

1. Several uncontrolled variables encountered during the experiments (moisture content of asphalt mixture, voids in the mineral aggregate, air voids, asphalt content, and asphalt penetration) caused some var iabili·ty in the results; however, they did not mask the effects of the controlled variables completely.

2. Although some variation in tensile strength, tensile strength ratio, and boiling test results was observed, no signifiCant difference was identified between mixtures produced in the batch plant and the drum plant for all conditions of stockpile moisture and mixing temperature.

3. Both types of asphalt plants were able to remove most or all of the moisture from the stock­pile aggregate although removal resulted in higher fuel costs and lower production. The higher fuel costs were measured and documented for a drum mix plant and it is believed that the effect would be similar for a batch plant.

4. Density was the major factor that affected the properties of the experimental mixtures.

5. Asphalt mixtures with moisture contents above 1.5 percent were difficult to produce and then only at very low mixing temperatures.

Effects of Mixing Temperature

1. Mixing temperature had a slight effect on Hveem stabilityi however, the effects were small and inconsistent.

2. The effect of mixing temperature on tensile strength was found to be dependent on aggregate type. Tensile strength increased with increasing mixing temperature for the hard limestone aggregate (Austin) in both the drum and the batch plant. The standstone aggregate (Mathis and Alice) did not show a similar effect.

3. Resilient modulus of elasticity did not ap­pear related to mixing temperature. Significant vari­ations were measured for different experimental conditions; however, these variations were quite random.

4. There was an indication that resistance to moisture damage, as measured by the boiling test, improved with increased m1x1ng temperature. A slightly improved tensile strength ratio with in­creased mixing temperature was also observed.

5. Increased mixing temperature did not signifi­cantly increase the amount of asphalt hardening that occurred during mixing for the temperature range of the experiment.

6. Asphalt mixtures with mixing temperatures

46

below 200 ° F could not be produced with a uniform coating of asphalt.

Effects of Stockpile Moisture Content

1. Stockpile moisture content did not affect Hveem stabilities.

2. Stockpile moisture content did not affect tensile strengths.

3. Stockpile moisture content did not affect the resilient modulus of elasticity.

4. A slight indication of increased moisture­damage resistance, as measured by the tensile strength ratio, was observed with increased stock­pile moisture contents.

5. Reduced asphalt hardening during mixing was observed for increased stockpile moisture contents.

Recommendations

1. The experimental study did not identify any clear relationships between mixing temperature, stockpile moisture content, and the properties of asphalt mixtures produced in either batch or drum mix plants for the aggregates studied. Further test­ing of other aggregates should be conducted to de­termine if the observations of this study are gen­erally valid or if the results are dependent on aggregate type. As a minimum, one or two aggregates that are porous and retain moisture should be studied.

2. The results of the experimental study indi­cate that the measured engineering properties were primarily affected by the density of the mixture. Although the compaction procedure used on the road­ways was not monitored and documented as part of the experiment, the location of the Mathis and Alice ex­perimental mixtures is known and a coring program should be undertaken to determine the in-place den­sities. The results will determine the densities and engineering properties that were achieved without alteration of the regular compaction procedures.

ACKNOWLEDGMENT

The research presented in this paper was conducted as part of the Cooperative Highway Research Program sponsored by the Texas State Department of Highways

Transportation Research Record 1034

and Public Transportation and the Federal Highway Administration.

REFERENCES

1. H.L. Von Quintus, T.W. Kennedy, and J, Epps. Op­erational and Performance Characteristics of Drum Mixer Plants. Report FHWA-RD-82. Brent Rauhut Engineering, Austin, Tex., Aug. 1982.

2. Manual of Testing Procedures, Bituminous Sec­tion, 200-F Series. Texas State Department of Highways and Public Transportation, Austin, 1978.

3. J. N. Anagnos and T. W. Kennedy. Practical Method of Conducting the Indirect Tensile Test. Re­search Report 98-10. Center for Transportation Research, Bureau of Engineering Research, The University of Texas at Austin, Aug. 1972.

4. T.W. Kennedy and J .N. Anagnos. Procedures for the Static and Repeated-Load Indirect Tensile Tests. Research Report 183-14. Center for Trans­portation Research, Bureau of Engineering Re­search, The University of Texas at Austin, Aug. 1983.

5. T.W. Kennedy, F.L. Roberts, and J.N. Anagnos. Texas Boiling Test for Evaluating Moisture Sus­ceptibility of Asphalt Mixtures. Research Report 2 53-5. Center for Transportation Research, Bu­reau of Engineering Research, The University of Texas at Austin, May 1983.

6. T.W. Kennedy and G.A. Huber. The Effects of Mix­ing Temperature and Stockpile Moisture on As­phalt Mixtures. Research Report 358-1. Center for Transportation Research, Bureau of Engineer­ing Research, The University of Texas at Austin, Aug. 1984.

The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This paper does not constitute a standard, specification, or regulation.

Publication of this paper sponsored by Committee on Characteristics of Bituminous Paving Mixtures to Meet Structural Requirements.