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The effects of damage evaluation techniques on the prediction of environmental damage in asphalt mixtures
ARTICLE IN PRESS
Building and Environment 42 (2007) 288–296 www.elsevier.com/locate/buildenv
The effects of damage evaluation techniques on the prediction of environmental damage in asphalt mixtures Saad Abo-Qudais Civil Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan Received 9 May 2005; received in revised form 7 June 2005; accepted 2 August 2005
Abstract The main objective of this study was to investigate the effects of evaluation methods on the prediction of environmental damage (stripping) of hot mix asphalt (HMA). To achieve this objective, four different environmental damage evaluation techniques were used to evaluate asphalt mixtures prepared using different mix parameters. The environmental damage evaluation techniques include either percent reduction in indirect tensile strength or Marshall stability or percent increase in static creep deformation due to environmental damage. In addition, theTexas boiling test which is based on visual evaluation of the percent of coated aggregate was used for environmental damage evaluation. The findings of this study indicated that the estimated environmental damage is significantly affected by the method of evaluation. Retained indirect tensile strength and retained stability were found to be less sensitive to environmental damage. Moreover, the static creep test was the only method used that was able to monitor the effect of used asphalt and aggregate gradation on the environmental damage of HMA, while the effect of antistripping additives on reduction stripping was easily monitored by the Texas boiling and static creep test. Finally, it was found that different environmental damage evaluation techniques revealed different results of using calcium stearate hydroxide. r 2005 Published by Elsevier Ltd. Keywords: Stripping; Environmental damage; Aggregate gradation; Hot mix asphalt
1. Introduction Stripping is the phenomenon in which loss of adhesion between asphalt cement and aggregate surface occurs in bituminous mixtures. This typically begins at the bottom of the hot mix asphalt (HMA) layer and progresses upward. Stripping usually causes distresses in pavements, which lead to poor pavement performance and to an increase in the maintenance cost [1]. Environmental damage is one of the most difficult distresses to identify because it can take numerous forms and it is influenced by different variables. Some of these variables are related to the materials forming the HMA, i.e., aggregate, asphalt, cement, and antistripping agent. Others are related to the weather conditions, compaction, air voids, testing method, handling, and storage of additives. Moreover, stripping is a rate process based on the viscosity/temperature dependency of bituminous binders. Also, stripping occurrence is a function of the surface tension between aggregate and bitumen. Several studies have been conducted in both laboratory and field in an attempt to reduce the environmental damage potential in HMA. Most of these studies have considered only one method of evaluation and many of them involved the use of antistripping agents to prevent or minimize environmental damage.
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Abo-Qudais and Al-Shweily [2] investigated the effect of environmental damage on creep behavior on HMA. In this study, the effect of antistripping additives, aggregate gradation, and type of asphalt on the HMA environmental damage and creep behavior before and after environmental damage were evaluated. The findings of this study indicated that conditioning of HMA specimens has a significant effect on the increase of creep deformation. Also, aggregate gradation, asphalt type, and the type of antistripping additive have a considerable effect on creep deformation. This is especially true for conditioned specimens. For both unconditioned and conditioned mixes prepared using either middle or upper limits of ASTM specification for dense graded aggregate, the creep deformation of mixes prepared using 80/100 asphalt were less than those for mixes prepared using 60/70 asphalt. In contrast, conditioned specimens prepared using an open graded aggregate gradation (12.5% air voids) indicated that the creep deformation of mixes prepared using 80/100 were less than those using 60/70 asphalt. Antistripping additives showed significant effects on reducing environmental damage and creep deformation. Mixes using calcium stearate hydroxide additive showed less environmental damage and creep deformation than those using limestone dust additive. In another study, Abo-Qudais and Almulqi [3] evaluated the effectiveness of different types of chemical compounds in reducing environmental damage in HMA using the Texas boiling test. The result of the study indicated that, among evaluated compounds, calcium stearate hydroxide has a significant effect in reducing environmental damage for asphalt mixes prepared using limestone and basalt aggregate. 2. Research objectives In order to achieve the aim of this study and to evaluate the parameters that affect the environmental damage and creep behavior of HMA, the main objectives of this study were to study the effect of environmental damage evaluation method on the estimated extent of stripping. 3. Research approach 3.1. Materials used The materials used in this study were different types of aggregate, asphalt, and antistripping additives. These materials are described in the following sections. 3.1.1. Aggregate The aggregate used in this study was 100 crushed limestone obtained from the quarries of Al-Huson in the northern part of Jordan. Although limestone aggregate has low stripping potential, it was used in this study to better evaluate the factors affecting the HMA stripping and to assess the effectiveness of different evaluation techniques at low stripping levels. Any method has the capability to detect low levels of stripping, it is expected to have the capability to detect high levels of stripping. Table 1 summarizes the physical properties of the aggregate used. The three aggregate gradations evaluated in this study were:
Gradation A: upper limit of ASTM specifications for dense aggregate gradation. Gradation B: middle limits of ASTM specifications for dense aggregate gradation. Gradation C: middle limits of ASTM specification for open aggregate gradation. Fig. 1 shows the aggregate size distribution of the three gradations used in this study.
3.1.2. Asphalt Two types of asphalt cement with different penetrations were used in this study:
Asphalt 1: 60/70 penetration grade, and Asphalt 2: 80/100 penetration grade.
The asphalts were obtained from Zarqa petroleum refinery. Table 2 summarizes the physical properties of the asphalt used. 3.1.3. Antistripping additives In this study, two types of antistripping additives were used: limestone dust and calcium stearate hydroxide. The limestone dust was waste material, a residue of sawing rocks to produce building masonry. It is available in large quantities in Jordan as a by-product waste material. The used dust was passed through sieve number 100.
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290 Table 1 Properties of the aggregate used Aggregate size
ASTM test designation
Bulk specific gravity (dry)
Apparent specific gravity
Absorption (%)
Coarse aggregate Fine aggregate Mineral filler
C127 C128 C128
2.552 2.485 —
2.625 2.590 2.573
3.1 4.6 —
100 90 Percent Passing
80
Upper limit of ASTM for dense aggregate gradation (A) Mid limits of ASTM for dense aggregate gradation (B) Mid limits of ASTM for open aggregate gradation
(C)
70 60 50 40 30 20 10 0 0.01
0.1
1 Seive opening (mm)
10
100
Fig. 1. Aggregate size distribution of different aggregate gradations used in preparing the hot mix asphalt specimen.
Table 2 Properties of the asphalt used Test
Ductility at 25 1C (cm) Penetration at 25 1C, 100 g (0.1 mm) Softening point (1C) Flash point (1C) Fire point (1C) Specific gravity at 25 1C
ASTM test
D D D D D D
113 5 36 92 92 70
Results AC 60/70
AC 80/100
110 64 50 319 325 1.01
118 92 45.5 312 318 1.01
The calcium stearate hydroxide [Ca(C17H35 COO){OH}] is prepared as described by Abo-Qudais and Almulqi [3]. This compound is prepared by neutralization of stearic acid (C17H35COOH) with an equimolar quantity of calcium hydroxide {Ca (OH)2}. One hundred grams of stearic acid was melted in a suspension containing 26.0 g of calcium hydroxide in 50.0 g of water at 80 1C. The mixture was stirred vigorously until a syrupy material was obtained and then it was poured into a plastic dish and left for several days at room temperature for drying. One advantage of this additive is its low cost relative to other chemical antistripping additives. The Texas boiling test (ASTM D 3625) was used to estimate the optimum amounts of additives in HMA. The additives were added to the mix at seven different levels (3%, 5%, 7%, 10%, 20%, 25%, and 100% by weight of asphalt cement). Twenty and fifteen percent were found to be the optimum amounts of limestone dust and calcium stearate hydroxide, respectively [2]. 3.2. Mix design methodology (optimum asphalt content) To determine the optimum asphalt content by weight of total mix, for each aggregate gradation, Marshall mix design procedures (ASTM D 1559) were followed. Three specimens of each asphalt content (3.5%, 4.0%, 4.5%, and 5.0% for mixes prepared using gradation C, and 4.5%, 5.0%, 5.5%, 6.0%, and 6.5% for mixes prepared using gradations A and B)
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were prepared. A total of 42 specimens were tested for stability, flow, air voids, unit weight, and voids in mineral aggregate (VMA). The optimum asphalt content was determined based on these parameters. The optimum asphalt content was calculated as the average of asphalt content that meets maximum stability, maximum unit weight, and 4.0% air voids. The resulting optimum asphalt content was checked whether it achieved the specification limits of the five parameters (stability, flow, air voids, unit weight, and VMA). The resulting optimum asphalt contents were 5.6%, 5.3%, and 4.2% for mixes prepared using gradation A, B, and C, respectively. 3.3. HMA specimen fabrication Compacted Marshall specimens used in the Marshall stability, indirect tensile strength, and creep tests were prepared according to the Asphalt Institute Manual (MS-2) and ASTM D 1559. The specimens were compacted by Marshall compactor using 50 blow per each side of the specimen. The average air voids of the prepared specimens were 4.8, 4.1, and 12.8 for mixes prepared using gradation A, B, and C, respectively. The loose specimens used in the Texas boiling test were prepared by mixing 100 gm of the aggregate with the optimum asphalt content. Antistripping agent calcium stearate hydroxide was melted into the asphalt, while the limestone dust was added to the aggregate before mixing with asphalt, since adding limestone dust to asphalt will raise the asphalt viscosity. This makes it too hard to mix the asphalt with the aggregate. 3.4. Moisture conditioning The Texas boiling test (ASTM D 3625) was used to evaluate the stripping potential of loose (uncompacted) HMA specimens. The loose HMA was placed in boiling water for 10 min and every 3 min the mix was stirred for 10 s using a glass rod. Then the mix was removed from the water and placed on a white surface to determine, visually, the percent of areas coated by asphalt. For compacted HMA, moisture conditioning was used to evaluate the effects of water saturation and accelerated water freezing–thawing cycle on compacted bituminous mixtures. The HMA specimen conditioning was performed according to AASHTO T283 by immersing the compacted specimens in water and applying vacuum to achieve saturation levels between 55% and 80%. Then the specimens were exposed to a freezing temperature of –1873 1C for 16 h and thawing at 60 1C for 24 h. The effect of conditioning on the environmental damage of compacted HMA was evaluated by the static creep test, indirect tensile strength, and Marshall stability. 3.5. Methods of environmental damage evaluation 3.5.1. Static creep test The static creep test was conducted, at 30 1C, by applying a static stress of 100 kPa for 1 h followed by unloading for 15 min. The universal testing machine (UTM) was used for this purpose. The tests were performed according to the following procedures: after capping the two ends of the specimen, it was placed in the loading machine under a conditioning stress of 10 kPa for 600 s. Then the conditioning stress was removed and a stress of 100 KPa was applied for 3600 s. Three specimens were tested for each combination of type of asphalt, type of aggregate gradations, type of additives, and mode of conditioning. The original height of the specimens was measured before capping, while the axial deformation was measured during the creep test using the linear vertical displacement transducers (LVDTs). Accumulated microstrain was calculated as the ratio between the measured deformation to the original specimen height according to the following equation: ¼ Dh=h0 , where e is the accumulated microstrain occurred in the specimen after 1 h of loading at 30 1C, h0 the original specimen height (the original distance between specimen loading surfaces), Dh the axial deformation (change in distance between specimen loading surfaces). The environmental damage effect on creep behavior was evaluated based on the percent increase in creep due to conditioning. It is calculated as the ratio between the difference in creep value between unconditioned and conditioned specimens to that of unconditioned specimens, according to the following equation: % Increase in creep ¼
creep of conditioned specimens creep of unconditioned specimen 100%. creep of unconditioned specimen
3.5.2. Marshall stability The Marshall stability test (ASTM D 1559) also was used to evaluate the effect of conditioning. The values of percent reduction of Marshall stability were compared to the percent increase in HMA creep values. The reduction in Marshall
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stability is calculated according to the following equation: % Reduction in stability ¼
stability of unconditioned specimens stability of conditioned specimens 100%. stability of unconditioned specimens
3.5.3. Indirect tensile strength The indirect tensile strength test (ASTM D 4123) was performed by loading of the cylindrical Marshall specimen (102 mm in diameter and about 63 mm in height) with a compressive load acting in parallel to and along the vertical diametrical plane distributed on the specimens through 12.7-mm-wide steel bars to obtain a uniform loading on the specimens. The specimens were placed in a water bath at 25 1C for 30 to 40 min, prior to loading, then they were dried and tested. The indirect tensile strength was calculated according to the following equation: TS ¼ 2P=pdt, where TS is the indirect tensile strength (Pa), P the applied load (N), d the diameter of specimen (m), and t the thickness of specimen (m). The indirect tensile strength was used to evaluate water damage (stripping) by comparing the reduction in tensile strength due to specimen conditioning. The reduction in tensile strength due to conditioning is calculated according to the following equation: % Reduction in strength ¼
strength of unconditioned specimens strength of conditioned specimens 100%. strength of unconditioned specimens
The percent reduction in HMA indirect tensile strength is compared to the percent increase in creep strain and the percent reduction in Marshall stability of specimens prepared using the same mix parameters. 3.5.4. Texas boiling test The Texas boiling test also was used to evaluate environmental damage. The boiling test used in this study was similar to that in ASTM D 3625, but the aggregate gradation used in this study was different from those specified in ASTM D 3625. As mentioned earlier, three different aggregate gradation (A, B, and C) were under evaluation in this study. The percent reduction in the aggregate surface area coated by asphalt due to conditioning in the boiling water was used as an indication of environmental damage occurrence. The reduction in HMA aggregate-coated areas is calculated according to the following equation: % Reduction in areas coated by asphalt ¼
% coated area in unconditioned specimens % coated area in conditioned specimens 100%. coated area in unconditioned specimens
4. Results and discussion The environmental damage in the prepared HMA specimens was evaluated using the static creep test, Marshall stability, indirect tensile strength, and Texas boiling tests. The effects of the method of evaluation on the environmental damage in mixes prepared using different aggregate gradation, type of asphalt, and type of antistripping additives are summarized in Figs. 2–8 and discussed in the following sections. Each data point presented in this study represent the average of three specimens prepared using the same mix parameters and tested under the same conditions. Statistical analysis of collected data indicated that the variability in results (based on the calculated standard deviation) among specimens prepared from the same mix parameters and tested under the same conditions is low and within the expected variability of these parameters. Also, statistical analysis, using t-test at 95% confidence level, indicated that there is a significant difference between specimens prepared from different mix parameters or tested under different conditions. The percent increase in creep due to conditioning, obtained by data collected by Abo-Qudais and Al-Shweily [2], was compared to percent reductions in Marshall stability and indirect tensile strength due to the same conditioning procedures. Results of the comparisons summarized in Fig. 4 indicate that in general, there is an inverse relationship between percent increase in creep and percent reduction in indirect tensile strength and Marshall stability. However, this relation was weak, since a small reduction in stability sometimes causes a relatively small increase in creep. A similar reduction in stability was accompanied with a high increase in creep in other cases. For mixes prepared using C aggregate gradation, the Marshall stability and indirect tensile strength tests indicated that calcium stearate hydroxide has a negative effect, while the static creep test indicated that the same additive has a positive effect regardless of the aggregate gradation used in preparing the HMA. For mixes prepared using C aggregate gradation
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Fig. 2. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following upper limit of ASTM specifications for dense aggregate gradation.
Fig. 3. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following mid limits of ASTM specifications for dense aggregate gradation.
Fig. 4. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following mid limits of ASTM specifications for open graded aggregate gradation.
and 80/100 asphalt, the reduction in indirect tensile strength due to conditioning increased from 22.5% to 55.5% as calcium stearate hydroxide was added to the mix, while for the same mix the increase in creep due to conditioning reduced from 100.0% to 56.7% as calcium stearate hydroxide was added. The Texas boiling test showed similar trends to those of the creep test results; however, the difference in percent coated aggregate was small. 4.1. Sensitivity of evaluation methods to the type of asphalt used The effect of different types of asphalt used in preparing HMA on environmental damage by different evaluation methods, summarized in Figs. 2–4, indicated that the indirect tensile strength, the Marshall stability, and the Texas boiling
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Fig. 5. Effect of aggregate gradation and type of asphalt used on percent reduction in HMA indirect tensile strength.
Fig. 6. Effect of aggregate gradation and type of asphalt used on percent reduction in HMA Marshall stability.
Fig. 7. Effect of conditioning on percent increase in HMA creep strain.
Fig. 8. Effect of aggregate gradation and type of asphalt used on percent reduction in aggregate surface area coated by asphalt.
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tests were not able to monitor the effect of different types of asphalt on stripping susceptibility, while the creep test indicated that using 60/70 asphalt in the mix will produce HMA that has better resistance to environmental damage compared to those using 80/100 asphalt. This effect of the asphalt type was more significant in mixes prepared using aggregate gradations B and C. The percent increase in creep for mixes prepared using gradation B and without any type of additives was 25.2% and 55.6% for asphalt 60/70 and 80/100, respectively. In general, the percent increase in creep due to conditioning varies from 7.5% to 41.6% for mixes that used 60/70 asphalt while it varies from 13.8% to 100% for mixes that used 80/100 asphalt. This can be explained by the fact that the amount of absorbed asphalt in mixes prepared using 60/ 70 asphalt, as reported in earlier study by the authors [2], was higher than those for mixes prepared using 80/100 asphalt, even though the viscosities of the two asphalt during mixing was almost the same, since mixing temperature of HMA prepared using 60/70 asphalt was higher. The higher amount of absorbed asphalt, for mixes prepared using 60/70 asphalt, improved the mechanical bond between asphalt and aggregate, and hence improve the environmental damage resistance leading to less increase in creep deformation. Another reason behind better stripping resistance of HMA prepared using 60/ 70 asphalt is the higher softening point of 60/70 asphalt as shown in Table 2. Higher softening point enable the asphalt to keep better adhesion with the aggregate as the mix is exposed to high temperature during conditioning. 4.2. Sensitivity of evaluation method to type of antistripping additive The indirect tensile strength and Marshall stability tests indicated that, for HMA prepared using gradations A and B, limestone dust antistripping additive improved the stripping resistance significantly. While for mixes prepared using gradation C, the same antistripping additive caused either negative or insignificant impact on environmental damage. The above-mentioned results agree with those obtained by Maupin [4] who indicates that high air voids in HMA in the field make the stripping more severe, regardless of what type of antistripping additive was used in the mix. The same tests (indirect tensile strength and Marshall stability) indicated that calcium stearate hydroxide antistripping additive has an inconsistent effect on environmental damage resistance. Mixes prepared using different aggregate gradations and asphalt types indicated different effect of calcium sterate hydroxide additive on environmental damage. When the creep test was used to evaluate the environmental damage resistance, the results indicated that both limestone dust and calcium stearate hydroxide improve the environmental damage resistance significantly regardless of the type of aggregate gradation or the type of asphalt used in preparing the mix. Moreover, the effect of using calcium stearate hydroxide was more significant than limestone dust. The paired samples t-test between mixes prepared using calcium stearate hydroxide and those prepared without using additives indicated that the difference is significant at 95% confidence interval with t-values of 5.04 and 11.87 for mixes used asphalt 60/70 and 80/100, respectively. For mixes prepared using gradation A and 60/70 asphalt, the percent increase in creep strain due to conditioning was 7.5, 11.8, and 25.2 for mixes using calcium stearate hydroxide, limestone dust, and without any type of additive, respectively. The Texas boiling test indicated that the two types of antistripping additives used improve the environmental damage resistance. Also, using calcium stearate hydroxide improves the environmental damage resistance better than using limestone dust; however, the difference in the percent of reduction of coated areas was small as shown in Figs. 2–4. 4.3. Sensitivity of evaluation method to aggregate gradation used Indirect tensile strength and Marshall stability results distinguished the effect of using open aggregate (C) gradation on the occurrence of stripping. HMA prepared using C aggregate gradation showed greater reduction in both indirect tensile strength and Marshall stability due to conditioning; however, the same two tests were not able to distinguish in a consistent manner between mixes prepared using A gradation and those prepared using B gradation. On the other hand, the creep test distinguished clearly the effect of aggregate gradation on the environmental damage resistance of HMA. For mixes prepared using 60/70 asphalt, those using gradation C showed the highest increase in creep due to conditioning, followed by those using gradation A. Mixes using gradation B showed the lowest increase in creep due to conditioning, which means it has the highest stripping resistance. For mixes prepared using 80/100 asphalt, again those that used gradation C showed the highest increase in creep due to conditioning, followed by those using B gradation. Mixes prepared using gradation A showed the lowest increase in creep due to conditioning. The Texas boiling test was not able to assess the effect of aggregate gradation on the stripping of HMA. 5. Conclusions Four environmental damage evaluation techniques were used to evaluate the effect of different mix parameters on HMA environmental damage. Based on the above results and discussion, the following conclusion can be drawn: 1. the predicted environmental damage is significantly affected by the methods of evaluation used,
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2. indirect tensile strength, Marshall stability, and Texas boiling tests were not able to monitor the effect of the type of asphalt used in the HMA on the stripping occurrence, while the static creep test was capable of monitoring this effect, and 3. the static creep test is the only test that was able to differentiate the effect of aggregate gradations on stripping resistant.
6. Recommendation for further studies Based on the finding of this study, it is recommended to: 1. compare the resulted HMA environmental damage using other evaluation techniques such as the resilient modulus to those based on evaluation techniques considered in this study, and 2. study the effect of using other types of asphalt, antistripping additive, aggregate, and aggregate gradation on the results of different evaluation techniques.
References [1] Lottman RP. Laboratory test method for predicting moisture induced damage to asphalt concrete. Transportation research record no. 843. Washington, DC: Transportation Research Board; 1982. p. 88–95. [2] Abo-Qudais SA, Al-Shweily H. Effect of antistripping additives on environmental damage of bituminous mixtures. Building and Environment Journal 2005, accepted for publication. [3] Abo-Qudais SA, Almulqi M. New chemical antistripping additives for bituminous mixtures. Journal of ASTM International 2004, accepted for publication. [4] Maupin Jr GW. Evaluation of stripping in Virginia’s pavements. Transportation research record report no 1681, 1999. p. 37–42.
Building and Environment 42 (2007) 288–296 www.elsevier.com/locate/buildenv
The effects of damage evaluation techniques on the prediction of environmental damage in asphalt mixtures Saad Abo-Qudais Civil Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan Received 9 May 2005; received in revised form 7 June 2005; accepted 2 August 2005
Abstract The main objective of this study was to investigate the effects of evaluation methods on the prediction of environmental damage (stripping) of hot mix asphalt (HMA). To achieve this objective, four different environmental damage evaluation techniques were used to evaluate asphalt mixtures prepared using different mix parameters. The environmental damage evaluation techniques include either percent reduction in indirect tensile strength or Marshall stability or percent increase in static creep deformation due to environmental damage. In addition, theTexas boiling test which is based on visual evaluation of the percent of coated aggregate was used for environmental damage evaluation. The findings of this study indicated that the estimated environmental damage is significantly affected by the method of evaluation. Retained indirect tensile strength and retained stability were found to be less sensitive to environmental damage. Moreover, the static creep test was the only method used that was able to monitor the effect of used asphalt and aggregate gradation on the environmental damage of HMA, while the effect of antistripping additives on reduction stripping was easily monitored by the Texas boiling and static creep test. Finally, it was found that different environmental damage evaluation techniques revealed different results of using calcium stearate hydroxide. r 2005 Published by Elsevier Ltd. Keywords: Stripping; Environmental damage; Aggregate gradation; Hot mix asphalt
1. Introduction Stripping is the phenomenon in which loss of adhesion between asphalt cement and aggregate surface occurs in bituminous mixtures. This typically begins at the bottom of the hot mix asphalt (HMA) layer and progresses upward. Stripping usually causes distresses in pavements, which lead to poor pavement performance and to an increase in the maintenance cost [1]. Environmental damage is one of the most difficult distresses to identify because it can take numerous forms and it is influenced by different variables. Some of these variables are related to the materials forming the HMA, i.e., aggregate, asphalt, cement, and antistripping agent. Others are related to the weather conditions, compaction, air voids, testing method, handling, and storage of additives. Moreover, stripping is a rate process based on the viscosity/temperature dependency of bituminous binders. Also, stripping occurrence is a function of the surface tension between aggregate and bitumen. Several studies have been conducted in both laboratory and field in an attempt to reduce the environmental damage potential in HMA. Most of these studies have considered only one method of evaluation and many of them involved the use of antistripping agents to prevent or minimize environmental damage.
Tel.: +962 272 01 000; fax: +962 270 95 123.
E-mail address: [email protected]. 0360-1323/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.buildenv.2005.08.005
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Abo-Qudais and Al-Shweily [2] investigated the effect of environmental damage on creep behavior on HMA. In this study, the effect of antistripping additives, aggregate gradation, and type of asphalt on the HMA environmental damage and creep behavior before and after environmental damage were evaluated. The findings of this study indicated that conditioning of HMA specimens has a significant effect on the increase of creep deformation. Also, aggregate gradation, asphalt type, and the type of antistripping additive have a considerable effect on creep deformation. This is especially true for conditioned specimens. For both unconditioned and conditioned mixes prepared using either middle or upper limits of ASTM specification for dense graded aggregate, the creep deformation of mixes prepared using 80/100 asphalt were less than those for mixes prepared using 60/70 asphalt. In contrast, conditioned specimens prepared using an open graded aggregate gradation (12.5% air voids) indicated that the creep deformation of mixes prepared using 80/100 were less than those using 60/70 asphalt. Antistripping additives showed significant effects on reducing environmental damage and creep deformation. Mixes using calcium stearate hydroxide additive showed less environmental damage and creep deformation than those using limestone dust additive. In another study, Abo-Qudais and Almulqi [3] evaluated the effectiveness of different types of chemical compounds in reducing environmental damage in HMA using the Texas boiling test. The result of the study indicated that, among evaluated compounds, calcium stearate hydroxide has a significant effect in reducing environmental damage for asphalt mixes prepared using limestone and basalt aggregate. 2. Research objectives In order to achieve the aim of this study and to evaluate the parameters that affect the environmental damage and creep behavior of HMA, the main objectives of this study were to study the effect of environmental damage evaluation method on the estimated extent of stripping. 3. Research approach 3.1. Materials used The materials used in this study were different types of aggregate, asphalt, and antistripping additives. These materials are described in the following sections. 3.1.1. Aggregate The aggregate used in this study was 100 crushed limestone obtained from the quarries of Al-Huson in the northern part of Jordan. Although limestone aggregate has low stripping potential, it was used in this study to better evaluate the factors affecting the HMA stripping and to assess the effectiveness of different evaluation techniques at low stripping levels. Any method has the capability to detect low levels of stripping, it is expected to have the capability to detect high levels of stripping. Table 1 summarizes the physical properties of the aggregate used. The three aggregate gradations evaluated in this study were:
Gradation A: upper limit of ASTM specifications for dense aggregate gradation. Gradation B: middle limits of ASTM specifications for dense aggregate gradation. Gradation C: middle limits of ASTM specification for open aggregate gradation. Fig. 1 shows the aggregate size distribution of the three gradations used in this study.
3.1.2. Asphalt Two types of asphalt cement with different penetrations were used in this study:
Asphalt 1: 60/70 penetration grade, and Asphalt 2: 80/100 penetration grade.
The asphalts were obtained from Zarqa petroleum refinery. Table 2 summarizes the physical properties of the asphalt used. 3.1.3. Antistripping additives In this study, two types of antistripping additives were used: limestone dust and calcium stearate hydroxide. The limestone dust was waste material, a residue of sawing rocks to produce building masonry. It is available in large quantities in Jordan as a by-product waste material. The used dust was passed through sieve number 100.
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290 Table 1 Properties of the aggregate used Aggregate size
ASTM test designation
Bulk specific gravity (dry)
Apparent specific gravity
Absorption (%)
Coarse aggregate Fine aggregate Mineral filler
C127 C128 C128
2.552 2.485 —
2.625 2.590 2.573
3.1 4.6 —
100 90 Percent Passing
80
Upper limit of ASTM for dense aggregate gradation (A) Mid limits of ASTM for dense aggregate gradation (B) Mid limits of ASTM for open aggregate gradation
(C)
70 60 50 40 30 20 10 0 0.01
0.1
1 Seive opening (mm)
10
100
Fig. 1. Aggregate size distribution of different aggregate gradations used in preparing the hot mix asphalt specimen.
Table 2 Properties of the asphalt used Test
Ductility at 25 1C (cm) Penetration at 25 1C, 100 g (0.1 mm) Softening point (1C) Flash point (1C) Fire point (1C) Specific gravity at 25 1C
ASTM test
D D D D D D
113 5 36 92 92 70
Results AC 60/70
AC 80/100
110 64 50 319 325 1.01
118 92 45.5 312 318 1.01
The calcium stearate hydroxide [Ca(C17H35 COO){OH}] is prepared as described by Abo-Qudais and Almulqi [3]. This compound is prepared by neutralization of stearic acid (C17H35COOH) with an equimolar quantity of calcium hydroxide {Ca (OH)2}. One hundred grams of stearic acid was melted in a suspension containing 26.0 g of calcium hydroxide in 50.0 g of water at 80 1C. The mixture was stirred vigorously until a syrupy material was obtained and then it was poured into a plastic dish and left for several days at room temperature for drying. One advantage of this additive is its low cost relative to other chemical antistripping additives. The Texas boiling test (ASTM D 3625) was used to estimate the optimum amounts of additives in HMA. The additives were added to the mix at seven different levels (3%, 5%, 7%, 10%, 20%, 25%, and 100% by weight of asphalt cement). Twenty and fifteen percent were found to be the optimum amounts of limestone dust and calcium stearate hydroxide, respectively [2]. 3.2. Mix design methodology (optimum asphalt content) To determine the optimum asphalt content by weight of total mix, for each aggregate gradation, Marshall mix design procedures (ASTM D 1559) were followed. Three specimens of each asphalt content (3.5%, 4.0%, 4.5%, and 5.0% for mixes prepared using gradation C, and 4.5%, 5.0%, 5.5%, 6.0%, and 6.5% for mixes prepared using gradations A and B)
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were prepared. A total of 42 specimens were tested for stability, flow, air voids, unit weight, and voids in mineral aggregate (VMA). The optimum asphalt content was determined based on these parameters. The optimum asphalt content was calculated as the average of asphalt content that meets maximum stability, maximum unit weight, and 4.0% air voids. The resulting optimum asphalt content was checked whether it achieved the specification limits of the five parameters (stability, flow, air voids, unit weight, and VMA). The resulting optimum asphalt contents were 5.6%, 5.3%, and 4.2% for mixes prepared using gradation A, B, and C, respectively. 3.3. HMA specimen fabrication Compacted Marshall specimens used in the Marshall stability, indirect tensile strength, and creep tests were prepared according to the Asphalt Institute Manual (MS-2) and ASTM D 1559. The specimens were compacted by Marshall compactor using 50 blow per each side of the specimen. The average air voids of the prepared specimens were 4.8, 4.1, and 12.8 for mixes prepared using gradation A, B, and C, respectively. The loose specimens used in the Texas boiling test were prepared by mixing 100 gm of the aggregate with the optimum asphalt content. Antistripping agent calcium stearate hydroxide was melted into the asphalt, while the limestone dust was added to the aggregate before mixing with asphalt, since adding limestone dust to asphalt will raise the asphalt viscosity. This makes it too hard to mix the asphalt with the aggregate. 3.4. Moisture conditioning The Texas boiling test (ASTM D 3625) was used to evaluate the stripping potential of loose (uncompacted) HMA specimens. The loose HMA was placed in boiling water for 10 min and every 3 min the mix was stirred for 10 s using a glass rod. Then the mix was removed from the water and placed on a white surface to determine, visually, the percent of areas coated by asphalt. For compacted HMA, moisture conditioning was used to evaluate the effects of water saturation and accelerated water freezing–thawing cycle on compacted bituminous mixtures. The HMA specimen conditioning was performed according to AASHTO T283 by immersing the compacted specimens in water and applying vacuum to achieve saturation levels between 55% and 80%. Then the specimens were exposed to a freezing temperature of –1873 1C for 16 h and thawing at 60 1C for 24 h. The effect of conditioning on the environmental damage of compacted HMA was evaluated by the static creep test, indirect tensile strength, and Marshall stability. 3.5. Methods of environmental damage evaluation 3.5.1. Static creep test The static creep test was conducted, at 30 1C, by applying a static stress of 100 kPa for 1 h followed by unloading for 15 min. The universal testing machine (UTM) was used for this purpose. The tests were performed according to the following procedures: after capping the two ends of the specimen, it was placed in the loading machine under a conditioning stress of 10 kPa for 600 s. Then the conditioning stress was removed and a stress of 100 KPa was applied for 3600 s. Three specimens were tested for each combination of type of asphalt, type of aggregate gradations, type of additives, and mode of conditioning. The original height of the specimens was measured before capping, while the axial deformation was measured during the creep test using the linear vertical displacement transducers (LVDTs). Accumulated microstrain was calculated as the ratio between the measured deformation to the original specimen height according to the following equation: ¼ Dh=h0 , where e is the accumulated microstrain occurred in the specimen after 1 h of loading at 30 1C, h0 the original specimen height (the original distance between specimen loading surfaces), Dh the axial deformation (change in distance between specimen loading surfaces). The environmental damage effect on creep behavior was evaluated based on the percent increase in creep due to conditioning. It is calculated as the ratio between the difference in creep value between unconditioned and conditioned specimens to that of unconditioned specimens, according to the following equation: % Increase in creep ¼
creep of conditioned specimens creep of unconditioned specimen 100%. creep of unconditioned specimen
3.5.2. Marshall stability The Marshall stability test (ASTM D 1559) also was used to evaluate the effect of conditioning. The values of percent reduction of Marshall stability were compared to the percent increase in HMA creep values. The reduction in Marshall
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stability is calculated according to the following equation: % Reduction in stability ¼
stability of unconditioned specimens stability of conditioned specimens 100%. stability of unconditioned specimens
3.5.3. Indirect tensile strength The indirect tensile strength test (ASTM D 4123) was performed by loading of the cylindrical Marshall specimen (102 mm in diameter and about 63 mm in height) with a compressive load acting in parallel to and along the vertical diametrical plane distributed on the specimens through 12.7-mm-wide steel bars to obtain a uniform loading on the specimens. The specimens were placed in a water bath at 25 1C for 30 to 40 min, prior to loading, then they were dried and tested. The indirect tensile strength was calculated according to the following equation: TS ¼ 2P=pdt, where TS is the indirect tensile strength (Pa), P the applied load (N), d the diameter of specimen (m), and t the thickness of specimen (m). The indirect tensile strength was used to evaluate water damage (stripping) by comparing the reduction in tensile strength due to specimen conditioning. The reduction in tensile strength due to conditioning is calculated according to the following equation: % Reduction in strength ¼
strength of unconditioned specimens strength of conditioned specimens 100%. strength of unconditioned specimens
The percent reduction in HMA indirect tensile strength is compared to the percent increase in creep strain and the percent reduction in Marshall stability of specimens prepared using the same mix parameters. 3.5.4. Texas boiling test The Texas boiling test also was used to evaluate environmental damage. The boiling test used in this study was similar to that in ASTM D 3625, but the aggregate gradation used in this study was different from those specified in ASTM D 3625. As mentioned earlier, three different aggregate gradation (A, B, and C) were under evaluation in this study. The percent reduction in the aggregate surface area coated by asphalt due to conditioning in the boiling water was used as an indication of environmental damage occurrence. The reduction in HMA aggregate-coated areas is calculated according to the following equation: % Reduction in areas coated by asphalt ¼
% coated area in unconditioned specimens % coated area in conditioned specimens 100%. coated area in unconditioned specimens
4. Results and discussion The environmental damage in the prepared HMA specimens was evaluated using the static creep test, Marshall stability, indirect tensile strength, and Texas boiling tests. The effects of the method of evaluation on the environmental damage in mixes prepared using different aggregate gradation, type of asphalt, and type of antistripping additives are summarized in Figs. 2–8 and discussed in the following sections. Each data point presented in this study represent the average of three specimens prepared using the same mix parameters and tested under the same conditions. Statistical analysis of collected data indicated that the variability in results (based on the calculated standard deviation) among specimens prepared from the same mix parameters and tested under the same conditions is low and within the expected variability of these parameters. Also, statistical analysis, using t-test at 95% confidence level, indicated that there is a significant difference between specimens prepared from different mix parameters or tested under different conditions. The percent increase in creep due to conditioning, obtained by data collected by Abo-Qudais and Al-Shweily [2], was compared to percent reductions in Marshall stability and indirect tensile strength due to the same conditioning procedures. Results of the comparisons summarized in Fig. 4 indicate that in general, there is an inverse relationship between percent increase in creep and percent reduction in indirect tensile strength and Marshall stability. However, this relation was weak, since a small reduction in stability sometimes causes a relatively small increase in creep. A similar reduction in stability was accompanied with a high increase in creep in other cases. For mixes prepared using C aggregate gradation, the Marshall stability and indirect tensile strength tests indicated that calcium stearate hydroxide has a negative effect, while the static creep test indicated that the same additive has a positive effect regardless of the aggregate gradation used in preparing the HMA. For mixes prepared using C aggregate gradation
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Fig. 2. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following upper limit of ASTM specifications for dense aggregate gradation.
Fig. 3. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following mid limits of ASTM specifications for dense aggregate gradation.
Fig. 4. Effect of testing method on estimated environmental damage in HMA prepared using aggregate following mid limits of ASTM specifications for open graded aggregate gradation.
and 80/100 asphalt, the reduction in indirect tensile strength due to conditioning increased from 22.5% to 55.5% as calcium stearate hydroxide was added to the mix, while for the same mix the increase in creep due to conditioning reduced from 100.0% to 56.7% as calcium stearate hydroxide was added. The Texas boiling test showed similar trends to those of the creep test results; however, the difference in percent coated aggregate was small. 4.1. Sensitivity of evaluation methods to the type of asphalt used The effect of different types of asphalt used in preparing HMA on environmental damage by different evaluation methods, summarized in Figs. 2–4, indicated that the indirect tensile strength, the Marshall stability, and the Texas boiling
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Fig. 5. Effect of aggregate gradation and type of asphalt used on percent reduction in HMA indirect tensile strength.
Fig. 6. Effect of aggregate gradation and type of asphalt used on percent reduction in HMA Marshall stability.
Fig. 7. Effect of conditioning on percent increase in HMA creep strain.
Fig. 8. Effect of aggregate gradation and type of asphalt used on percent reduction in aggregate surface area coated by asphalt.
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tests were not able to monitor the effect of different types of asphalt on stripping susceptibility, while the creep test indicated that using 60/70 asphalt in the mix will produce HMA that has better resistance to environmental damage compared to those using 80/100 asphalt. This effect of the asphalt type was more significant in mixes prepared using aggregate gradations B and C. The percent increase in creep for mixes prepared using gradation B and without any type of additives was 25.2% and 55.6% for asphalt 60/70 and 80/100, respectively. In general, the percent increase in creep due to conditioning varies from 7.5% to 41.6% for mixes that used 60/70 asphalt while it varies from 13.8% to 100% for mixes that used 80/100 asphalt. This can be explained by the fact that the amount of absorbed asphalt in mixes prepared using 60/ 70 asphalt, as reported in earlier study by the authors [2], was higher than those for mixes prepared using 80/100 asphalt, even though the viscosities of the two asphalt during mixing was almost the same, since mixing temperature of HMA prepared using 60/70 asphalt was higher. The higher amount of absorbed asphalt, for mixes prepared using 60/70 asphalt, improved the mechanical bond between asphalt and aggregate, and hence improve the environmental damage resistance leading to less increase in creep deformation. Another reason behind better stripping resistance of HMA prepared using 60/ 70 asphalt is the higher softening point of 60/70 asphalt as shown in Table 2. Higher softening point enable the asphalt to keep better adhesion with the aggregate as the mix is exposed to high temperature during conditioning. 4.2. Sensitivity of evaluation method to type of antistripping additive The indirect tensile strength and Marshall stability tests indicated that, for HMA prepared using gradations A and B, limestone dust antistripping additive improved the stripping resistance significantly. While for mixes prepared using gradation C, the same antistripping additive caused either negative or insignificant impact on environmental damage. The above-mentioned results agree with those obtained by Maupin [4] who indicates that high air voids in HMA in the field make the stripping more severe, regardless of what type of antistripping additive was used in the mix. The same tests (indirect tensile strength and Marshall stability) indicated that calcium stearate hydroxide antistripping additive has an inconsistent effect on environmental damage resistance. Mixes prepared using different aggregate gradations and asphalt types indicated different effect of calcium sterate hydroxide additive on environmental damage. When the creep test was used to evaluate the environmental damage resistance, the results indicated that both limestone dust and calcium stearate hydroxide improve the environmental damage resistance significantly regardless of the type of aggregate gradation or the type of asphalt used in preparing the mix. Moreover, the effect of using calcium stearate hydroxide was more significant than limestone dust. The paired samples t-test between mixes prepared using calcium stearate hydroxide and those prepared without using additives indicated that the difference is significant at 95% confidence interval with t-values of 5.04 and 11.87 for mixes used asphalt 60/70 and 80/100, respectively. For mixes prepared using gradation A and 60/70 asphalt, the percent increase in creep strain due to conditioning was 7.5, 11.8, and 25.2 for mixes using calcium stearate hydroxide, limestone dust, and without any type of additive, respectively. The Texas boiling test indicated that the two types of antistripping additives used improve the environmental damage resistance. Also, using calcium stearate hydroxide improves the environmental damage resistance better than using limestone dust; however, the difference in the percent of reduction of coated areas was small as shown in Figs. 2–4. 4.3. Sensitivity of evaluation method to aggregate gradation used Indirect tensile strength and Marshall stability results distinguished the effect of using open aggregate (C) gradation on the occurrence of stripping. HMA prepared using C aggregate gradation showed greater reduction in both indirect tensile strength and Marshall stability due to conditioning; however, the same two tests were not able to distinguish in a consistent manner between mixes prepared using A gradation and those prepared using B gradation. On the other hand, the creep test distinguished clearly the effect of aggregate gradation on the environmental damage resistance of HMA. For mixes prepared using 60/70 asphalt, those using gradation C showed the highest increase in creep due to conditioning, followed by those using gradation A. Mixes using gradation B showed the lowest increase in creep due to conditioning, which means it has the highest stripping resistance. For mixes prepared using 80/100 asphalt, again those that used gradation C showed the highest increase in creep due to conditioning, followed by those using B gradation. Mixes prepared using gradation A showed the lowest increase in creep due to conditioning. The Texas boiling test was not able to assess the effect of aggregate gradation on the stripping of HMA. 5. Conclusions Four environmental damage evaluation techniques were used to evaluate the effect of different mix parameters on HMA environmental damage. Based on the above results and discussion, the following conclusion can be drawn: 1. the predicted environmental damage is significantly affected by the methods of evaluation used,
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2. indirect tensile strength, Marshall stability, and Texas boiling tests were not able to monitor the effect of the type of asphalt used in the HMA on the stripping occurrence, while the static creep test was capable of monitoring this effect, and 3. the static creep test is the only test that was able to differentiate the effect of aggregate gradations on stripping resistant.
6. Recommendation for further studies Based on the finding of this study, it is recommended to: 1. compare the resulted HMA environmental damage using other evaluation techniques such as the resilient modulus to those based on evaluation techniques considered in this study, and 2. study the effect of using other types of asphalt, antistripping additive, aggregate, and aggregate gradation on the results of different evaluation techniques.
References [1] Lottman RP. Laboratory test method for predicting moisture induced damage to asphalt concrete. Transportation research record no. 843. Washington, DC: Transportation Research Board; 1982. p. 88–95. [2] Abo-Qudais SA, Al-Shweily H. Effect of antistripping additives on environmental damage of bituminous mixtures. Building and Environment Journal 2005, accepted for publication. [3] Abo-Qudais SA, Almulqi M. New chemical antistripping additives for bituminous mixtures. Journal of ASTM International 2004, accepted for publication. [4] Maupin Jr GW. Evaluation of stripping in Virginia’s pavements. Transportation research record report no 1681, 1999. p. 37–42.