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Effect of antistripping additives on environmental damage of bituminous mixtures
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Building and Environment 42 (2007) 2929–2938 www.elsevier.com/locate/buildenv
Effect of antistripping additives on environmental damage of bituminous mixtures Saad Abo-Qudais, Haider Al-Shweily Civil Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan Received 15 February 2005; accepted 6 May 2005
Abstract The main objective of this study was to investigate the effect of environmental damage (stripping) on the hot mix asphalt (HMA) creep behavior. To achieve this objective, 24 different HMA combinations using different mix parameters were evaluated for their resistance to static creep deformations. The mix parameters include: two types of asphalt cement, three types of aggregate gradation, and two types of antistripping additives (limestone dust and calcium stearate hydroxide). Marshall specimens were prepared at optimum asphalt contents and tested for static creep. Part of the prepared specimens were exposed to freezing–thawing conditioning according to AASHTO T283. The findings of this study indicated that conditioning of HMA specimens has a significant effect on the increase of creep deformation. This is especially true for open graded aggregate gradation mixes. For unconditioned mixes and conditioned mixes, prepared using mid and upper limits of ASTM specification for dense graded aggregate, the creep deformation of mixes prepared using 80/100 asphalt was less than that for mixes prepared using 60/70 asphalt. The opposite trend was noticed for conditioned specimens prepared using open graded aggregate gradation. Antistripping additives showed significant effect on reducing stripping and creep behavior. Mixes which used calcium stearate hydroxide additive showed less stripping and creep deformation than those which contained limestone dust additive. r 2005 Published by Elsevier Ltd. Keywords: Stripping; Environment; Asphalt; Mixture; Creep
1. Introduction Road pavements represent a huge investment. Most surfaced roads are made of flexible pavement. One of the most important problems in flexible pavement is stripping, which is the loss of adhesion between asphalt coating and aggregate in the bituminous mixture caused by the existence of moisture. Stripping might cause different types of distresses such as: creep, raveling, rutting, shoving, and cracking. Hot mix asphalt (HMA) environmental damage (stripping) is a material compatibility problem. If the materials are compatible, it will form strong and Corresponding author. Tel.: +962 27201000; fax: +962 270 95123.
E-mail address: [email protected] (S. Abo-Qudais). 0360-1323/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.buildenv.2005.05.017
long-term bonds, so that the HMA resists stripping. However, if the mix has poor bonding, its resistance to stripping will be less. In this case, the bonds should be enhanced or the material combinations should be changed to achieve satisfactory performance. Stripping 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 agents. Other variables are related to the weather conditioning, 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 surface tension between aggregate and bitumen [1].
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Antistripping additives are categorized into the following groups: cationic surfactants, iron naphthenate, hydrated lime, oregano saline, Portland cement, and other products. The main objective of adding these additives was to convert a hydrophilic (water loving) aggregate surface to a hydrophobic (water hating) aggregate [1]. Tunnicliff and Root [2] presented findings of a survey regarding the use of antistripping agents in bituminous mixtures. The survey indicated that there were over 100 antistripping agents available, and there were a very large number of testing procedures used by United States of America agencies to evaluate the effectiveness of the antistripping additives. Maupin [3] used Lottman’s test to evaluate the effect of different types of asphalt cements and antistripping agents on the stripping susceptibility of HMA. The results indicated that the Lottman’s test measured no differences in stripping susceptibility of HMA with different types of asphalts. However, significant differences in stripping susceptibility were detected when different additives were used. In another study, Maupin [4] performed field investigations of the effectiveness of antistripping additives in Virginia. The results of the study indicated that significant visual stripping was detected at many sites. Also, chemical additives performed no better than hydrated lime. Creep is an important factor in flexible pavement design. This is especially true for pavements exposed to heavy traffic and high tire pressure. In pavement, it is most likely that permanent deformation occurs in the upper layers rather than in the subgrade. The creep test is usually used to estimate the rutting potential of asphalt mixtures by applying a static load to a HMA specimen and measuring the resulting permanent deformation with time [1]. An experimental program was conducted by AboQudais [5] to characterize the viscoelastic deformation using the static creep test. The study evaluated the effect of aggregate gradation and temperature on the permanent deformation of HMA. The results indicated that the effect of temperature variation on creep deformation is more significant at low temperature and long loading time than those at high temperatures and short loading time.
2. Research objectives In order to achieve the aim of this study and to evaluate the parameters that affect stripping and creep behavior of HMA, the objectives of this study were to evaluate: 1. The effect of stripping and antistripping additives on HMA creep behavior. 2. The possibility of using different locally available and low cost materials as antistripping agents to reduce the environmental damage of HMA pavements.
3. Research approach 3.1. Materials used The materials used in this study were different types of aggregate, asphalt, and antistripping additives described as follows: 3.1.1. Aggregate The aggregate used in this study was 100 percent crushed limestone obtained from the quarries of Al-Huson in the northern part of Jordan. Table 1 summarizes the physical properties of the aggregates used. Three aggregate gradations were evaluated in this study:
Gradation A: upper limit of ASTM specifications for dense aggregate gradation. Gradation B: mid limit of ASTM specifications for dense aggregate gradation. Gradation C: mid limit of ASTM specifications for open aggregate gradation.
Fig. 1 shows the aggregate size distribution of the three gradations used. 3.1.2. Asphalt Two types of asphalt cement with different penetrations were used in this study:
Asphalt 1: 60/70 penetration grade, this asphalt has performance grading (PG) of 70–10.
Table 1 Properties of aggregate used Aggregate
Coarse aggregate Fine aggregate Mineral filler
ASTM test designation
C127 C128 C128
Dry bulk specific gravity
Apparent specific gravity
Absorption (%)
Limestone
Basalt
Limestone
Basalt
Limestone
Basalt
2.424 2.485 2.552
2.531 2.632 2.692
2.573 2.590 2.625
2.621 2.680 2.710
3.1 4.6 5.1
3.1 3.6 5.0
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100 Upper limit of ASTM for dense aggregate gradation (A)
90
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80
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70 60 50 40 30 20 10 0 0.01
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1
10
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Table 2 Properties of 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 designation
Result AC 60/70
AC 80/100
D 113 D5
110 64
118 92
D D D D
50 319 325 1.01
45.5 312 318 1.01
36 92 92 70
Asphalt 2: 80/100 penetration grade, this asphalt has performance grading (PG) of 58–22.
The asphalt was obtained from Zarqa local 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 obtained from industrial waste. It results from sawing rocks to produce building masonry, so it is available in large quantities in Jordan as waste material. The dust used was passed through sieve number 100. The calcium stearate hydroxide [Ca(C17H35COO){OH}] was prepared as described by Abo-Qudais and Almulqi [6]. This
compound was 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, then it was poured in a plastic dish and left for several days at room temperature for drying. One advantage of this additive is its low cost. The Texas boiling test (ASTM 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. 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 D1559) were followed. Three specimens of asphalt contents 3.5%, 4.0%, 4.5%, and 5.0% for mixes prepared using gradation C, and 5%, 5.5%, 6%, and 6.5% for mixes prepared using gradations A and B were prepared. A total of 36 specimens were tested for stability, flow, air voids, unit weight, and voids in mineral aggregate. The optimum asphalt content was determined based on these parameters.
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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 voids in mineral aggregate (VMA)). The resulting optimum asphalt content were 5.6%, 5.3%, and 4.2% for gradation A, B, and C, respectively. 3.3. HMA specimens fabrication The specimens were prepared according to the Asphalt Institute Manual (MS-2) and ASTM D1559. Antistripping calcium stearate hydroxide was dissolved in the asphalt since it was easy to form a homogenous solution of asphalt and calcium stearate hydroxide, while the limestone dust was added to the aggregate before mixing with asphalt. The reason for that was limestone dust increases the viscosity of asphalt when added directly to the asphalt. This makes it too hard to mix the asphalt with the aggregate. 3.4. Moisture conditioning Moisture conditioning that simulates the environmental damage in the field was used to evaluate the effects of water saturation and accelerated water conditioning with a freezing–thawing cycle of compacted bituminous mixtures in the laboratory. The HMA specimen’s conditioning was performed according to AASHTO T166 by immersing the specimens in water and exposing them to a vacuum to achieve saturation levels between 55% and 80%. Then the specimens were exposed to freezing at a temperature of –1873 1C for 16 h and thawing at 60 1C for 24 h. The effect of stripping on HMA creep was evaluated by using the static creep test. 3.5. Static creep test This test is considered to be very important in obtaining data for estimating potential deformation of the vehicle wheel paths and ranking bituminous mixtures on the basis of their resistance to permanent deformation. The static creep test was conducted by applying a static stress of 100 kPa for 1 h at 30 1C 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 sides of the speci-
% Increase in creep ¼
Fig. 2. Static creep test setup.
men, it was placed in the loading machine (Fig. 2) 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, after which the load is removed and the deformation recovery was monitored for 900 s. Three specimens were tested for each type of aggregate, 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 and the original specimen height according to the following equation: ¼ Dh=ho , where e is the accumulated microstrain that occurred in the specimen during certain loading time at certain temperature, ho the original specimen height (the original distance between specimen loading surfaces), Dh the axial deformation (change in distance between specimen loading surfaces). Creep test results for different mixes and conditioning are summarized in Figs. 3–5, while Fig. 6 summarizes the accumulated microstrain after 60 min of loading. The stripping effect on creep behavior was evaluated based on the percent of increase in creep due to conditioning. It was calculated as the ratio between the difference in creep value between unconditioned and conditioned specimens to that of unconditioned specimens.
Creep of conditioned specimens Creep of unconditioned specimen 100% Creep of unconditioned specimen
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Fig. 3. Effect of antistripping additives on creep behaviour. (a) 80/100 asphalt and upper limit of dense aggregate gradation, (b) 80/100 asphalt and midlimit of dense aggregate gradation, (c) 80/100 asphalt and midlimit of open aggregate gradation, (d) 60/70 asphalt and upper limit of dense aggregate gradation, (e) 60/70 asphalt and midlimit of dense aggregate gradation, (f) 60/70 asphalt and midlimit of open aggregate gradation.
Fig. 7 summarizes the results of the percent of increase in creep values due to conditioning.
Marshall stability and indirect tensile strength are discussed in the following sections. 4.1. Effect of antistripping additives
4. Results and discussion The prepared specimens were evaluated by performing the static creep test using the UTM. The effects of conditioning and aggregate gradation, type of asphalt, and type of antistripping additives on the creep behavior of HMA are summarized in Table 3 and discussed in the following sections. Also, the effect of stripping on
The purpose of adding antistripping additives was to improve bonding between asphalt and aggregate (adhesion force). The effect of antistripping additives on HMA creep behavior was evaluated using static creep. Two types of additives, limestone dust or calcium stearate hydroxide, were evaluated in this study for their effect on stripping and creep behavior.
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Fig. 4. Effect of aggregate gradations of creep behavior of hot mix asphalt. (a) 80/100 asphalt and unconditioned, (b) 60/70 asphalt and unconditioned, (c) 80/100 asphalt and conditioned, (d) 60/70 asphalt and conditioned.
4.1.1. Effect of antistripping additives on conditioned specimens Fig. 3 shows the effect of limestone dust on the creep behavior of HMA prepared by using different mix parameters. Based on this figure, it can be seen that limestone dust has a significant effect on reducing creep deformation. This was true for the three types of aggregate gradations and two types of asphalt used in preparing the HMA. This effect might be explained by the fact that limestone dust contains calcium which increases interaction between aggregate surface and asphalt binder, so it improves the bond between asphalt and aggregate. The average creep microstrain after 60 min of loading was 11,367 for mixes prepared using midlimits of ASTM specification for aggregate gradation. The initial microstrain was equal to 6230 while the timedependent microstrain was 5137. This means that the rate of increase in creep deformation was 1.44 microstrain/s. Fig. 3 also indicated that calcium stearate hydroxide had a more significant effect in reducing creep deformation of HMA. This effect of calcium stearate hydroxide may be explained by the fact that the calcium stearate hydroxide is a surface-active material which causes an increase in the bond between hydrocarbons in asphalt and aggregates. The average creep microstrain after 60 min of loading was 9991 for mixes prepared using midlimits of ASTM specification for aggregate gradation. The initial microstrain was equal to 6358 while the time-dependent microstrain was 4633. This
means that the rate of increase in creep deformation was 1.3 microstrain/s, which is less than that in mixes using limestone dust. This is due to less stripping talking place in mixes using calcium stearate hydroxide additive compared to mixes using limestone dust. 4.1.2. Effect of antistripping additives on unconditioned HMA specimens Unconditioned HMA specimens were tested to evaluate the effect of antistripping additives on unconditioned specimens without exposing them to freezing–thawing. The effect of limestone dust on HMA is shown in Fig. 3, indicating that there is no significant effect of limestone dust on creep behavior of HMA. Similar effects were noticed regardless of the type of asphalt and of the type of aggregate gradation used in preparing the mix. However, the same figure indicates that the addition of calcium stearate hydroxide to HMA reduces creep deformation. This trend was similar to that observed in other mixes prepared using different aggregate gradation and types of asphalt. This effect of calcium stearate hydroxide might be caused by the fact that this type of additive enhanced the bond between the aggregate and asphalt, leading to less creep. 4.2. Effect of aggregate gradation Aggregate gradation was found to have a significant influence on the creep behavior of HMA. Aggregate
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gradation influences air voids (AV) and VMA, and so it affects the creep behavior. The effect of aggregate gradation on creep behavior for different mixes are shown in Fig. 4. This figure shows the relationship between accumulated microstrain and the time for different HMA prepared using different aggregate gradations. The mixes prepared using gradation C had the highest creep deformation, followed by the mixes prepared using gradation B. While the mixes prepared using gradation A show the lowest creep deformation, a similar trend was noticed for the two types of asphalt and the type of additives used in preparing HMA. The AV in mixes prepared using gradations A, B, and C mixes were 4.6, 5.0, and 12.3, respectively. The average creep microstrain after 60 min of loading were 11,367, 9984, and 9044 for mixes prepared using asphalt 60/70 and gradation C, gradation B, and gradation A, respectively. The elastic microstrain for gradation A was equal to 5120 while the viscous microstrain is 3924. This means that the rate of increase in creep deformation is 1.10 microstrain/s. For gradation B, the elastic microstrain was equal to 6802 while the viscous microstrain was 3182. This means that the strain rate was 0.88 microstrain/s. For mixes using gradation C, the elastic microstrain was equal to 6900 while the viscous microstrain was 4467 causing a microstrain increase rate of 1.24 microstrain/s. For the unloading portion of the creep test (deformation recovery), gradation C mixes had the lowest deformation recovery after unloading compared to gradations A and B. The average deformation recovery microstrain after 15 min of unloading was 457, 579, and –707 for mixes prepared using gradation C, A, and B, respectively. The above-mentioned results can be explained by the fact that the dense aggregate gradation having maximum density provides increased stability through the increase in interparticle contact and reduces VMA. This
Fig. 6. Creep accumulated microstrain after 60 min of loading.
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Fig. 7. Percent increase in creep due to conditioning. Table 3 Ranking of different mixes according to their resistance to creep Rank
According to creep microstrain at 60 min
According to percentage increase in creep due to conditioning
1 2
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3
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4 5
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6
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7
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17 18 19 20 21 22 23 24
leads to more resistance to the applied load. Meanwhile, the open aggregate gradation has less aggregate interaction due to a large amount of voids that reduces
Middle limit of dense gradation, asphalt 60/70, no additive, conditioned Open gradation, asphalt 60/70, calcium hydroxide additive Middle limit of dense gradation, asphalt 80/100, chemical additive Upper limit of dense gradation, asphalt 60/70, no additive, conditioned Upper limit of dense gradation, asphalt 80/100, no additive, conditioned Open gradation, asphalt 60/70, lime dust additive Open gradation, asphalt 60/70,no additive, conditioned Middle limit of dense gradation, asphalt 80/100, no additive, conditioned Open gradation, asphalt 80/100, lime dust additive
interaction between aggregate in addition to having less amounts of fine aggregate which reduces the adhesion between coarse aggregate grains.
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Fig. 4 was constructed to show the effect of aggregate gradation on creep behavior of HMA conditioned specimens. HMA prepared using gradation C had the highest creep deformation, followed by the mix prepared using gradation B, while the mix prepared using gradation A showed the lowest creep deformation. The average creep microstrain after 60 min of loading was 18,402, 12,343, and 9594 for mixes prepared using gradation C, gradation B, and gradation A, respectively. The initial microstrain for gradation A was equal to 5203 while the time-dependent microstrain was 4391. This means that the rate of increase in creep deformation was 1.22 microstrain/s. For gradation B, the initial microstrain was equal to 6080 while the time-dependent microstrain was 6263, indicating that the rate of increase in creep deformation was 1.77 microstrain/s. For gradation C, the instantaneous microstrain was equal to 11,500 while the time-dependent microstrain was 6902. This means that the rate of increase in creep deformation was equal to 1.92 microstrain/s. For the unloading portion of the creep test (deformation recovery), the gradation C HMA had the lowest deformation recovery compared with the other gradations. The average recovery microstrain after 15 min of unloading were 503, 481, and 249 for mixes prepared using gradations A, B, and C, respectively. These results can be explained by the fact that there are more AV in mixes using gradation C compared to that using gradations A and B. The large AV content is created by using a larger percentage of coarse aggregate, this facilitates the entering of water into the mix during conditioning leading to more stripping in mixes prepared using gradation C. Particle size is also important when considering the surface energy concept, because the surface areas and surface energy effects of very fine sizes are larger in relation to particle mass. 4.3. Effect of asphalt type The effect of asphalt used in preparing specimens is shown in Fig. 5. These figures indicated that unconditioned HMA prepared using asphalt 1 demonstrated higher creep values than those of mixes prepared using asphalt 2. These results agree with those obtained by Abo-Qudais [5], and Salter and Shkaarshi [7]. For conditioned specimens, no specific trend for the effect of asphalt type was noticed, as shown in Fig. 6. Mixes prepared using these upper limit of the ASTM dense aggregate gradation showed trends similar to those of unconditioned specimens. Also, specimens prepared using midlimits of the ASTM dense aggregate gradation showed trends similar to those of unconditioned specimens; however differences in creep deformation between specimens prepared using asphalt 1 and those using asphalt 2, was less significant. Specimens prepared using open graded aggregate
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gradation show a trend opposite to those of unconditioned specimens. Mixes prepared using 60/70 asphalt showed better resistance to increases in creep due to stripping than mixes prepared using 80/100 asphalt penetration. This is especially true for mixes prepared using open graded aggregate gradation, in which the percent of increase in creep deformations of mixes using 80/100 asphalt was almost 2.5 times of those using 60/70 asphalt. The above-mentioned results can be explained by the fact that the asphalt of high viscosity resists displacement by water much better than those of low viscosity. However, low viscosity is desirable during mixing operations because it has more wetting power than the ones having high viscosity, so it will coat the aggregate better, leading to more resistance to stripping. For control specimens, the average creep microstrain after 60 min of loading was 9984 and 7926 for mixes prepared using asphalt 1 and asphalt 2, respectively. For asphalt 1, the instantaneous microstrain was equal to 6140 while the time-dependent microstrain was 3844. This means that the increase rate in creep deformation was 1.10 microstrain/s. For asphalt 2, the initial microstrain was equal to 4925 while the time-dependent microstrain was 3006, which means that the rate of increase in creep deformation was 0.84 microstrain/s. For the unloading portion of the creep test (deformation recovery), the HMA prepared using asphalt 1 showed a higher deformation recovery than that of mixes prepared using asphalt 2. The average microstrain recovery after 15 min of unloading were 681 and –577 for mixes prepared using asphalt 1 and asphalt 2, respectively. These results indicate that the creep deformation and recovery decreased as penetration of asphalt increased. 4.4. Effect of conditioning The resulting percent of increase in creep due to conditioning, presented in Fig. 7, indicated that the effect of conditioning (stripping) on creep deformation was more detrimental for mixes prepared using open graded aggregate gradation. Among the evaluated mixes, the one prepared using calcium hydroxide additive showed the highest resistance to increase in creep due to stripping, while the one without the additive showed the least resistance.
5. Conclusions This study aimed to evaluate the effect of stripping on creep behavior. Based on the study results, the following conclusions can be drawn:
Mixes prepared using calcium hydroxide mixes demonstrate better resistance to stripping than those prepared using limestone dust additive.
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Unconditioned and conditioned mixes prepared using midlimits and upper limits aggregate gradation using 60/70 asphalt with low asphalt penetration showed higher creep deformation than those using 80/100 asphalt. However, the opposite trend was noticed for conditioned mixes prepared using open graded aggregate gradation.
References [1] Robert RL, Kandhal PS, Brown ER, Lee D, Kennedy TW. Hot Mix Asphalt Materials, Mixtures, Design, and Construction, 1st ed. Lanham, Maryland: NAPA Education Foundation; 1999. 490p. [2] Tunnicliff DG, Nebraska O, Root RE. Use of antistripping additives in asphalt concrete mixtures. Report no 274, National Cooperative Highway Research Program (NCHRP), 1984.
[3] Maupin Jr GW. Implementation of stripping test for asphaltic concrete. Transportation research record report no 712, Transportation Research Board, Washington, DC, 1979. p. 8–12. [4] Maupin Jr GW. Follow-up field investigation of the effectiveness of anti-stripping additives in Virginia. Project report no 9398-010-940, Virginia Transportation Research Council, 1997. 22p. [5] Abo-Qudais SA. Time-temperature and time-aggregate gradation superposition in asphalt mixes. Road materials and pavement design. France: Lavoisier Publisher; 2004, accepted for publication. [6] Abo-Qudais SA, Almulqi M. New chemical antistripping additives for bituminous mixtures. Journal of ASTM International, PA, USA: ASTM International; 2005. submitted for publication. [7] Salter KJ, Shkaarshi MTO. Effect of ambient temperature and thermal cycling on the creep of bituminous paving materials. Transportation research record no 1228, Transportation Research Board, National Research Council, Washington, DC, 1990. p. 106–111.
Building and Environment 42 (2007) 2929–2938 www.elsevier.com/locate/buildenv
Effect of antistripping additives on environmental damage of bituminous mixtures Saad Abo-Qudais, Haider Al-Shweily Civil Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan Received 15 February 2005; accepted 6 May 2005
Abstract The main objective of this study was to investigate the effect of environmental damage (stripping) on the hot mix asphalt (HMA) creep behavior. To achieve this objective, 24 different HMA combinations using different mix parameters were evaluated for their resistance to static creep deformations. The mix parameters include: two types of asphalt cement, three types of aggregate gradation, and two types of antistripping additives (limestone dust and calcium stearate hydroxide). Marshall specimens were prepared at optimum asphalt contents and tested for static creep. Part of the prepared specimens were exposed to freezing–thawing conditioning according to AASHTO T283. The findings of this study indicated that conditioning of HMA specimens has a significant effect on the increase of creep deformation. This is especially true for open graded aggregate gradation mixes. For unconditioned mixes and conditioned mixes, prepared using mid and upper limits of ASTM specification for dense graded aggregate, the creep deformation of mixes prepared using 80/100 asphalt was less than that for mixes prepared using 60/70 asphalt. The opposite trend was noticed for conditioned specimens prepared using open graded aggregate gradation. Antistripping additives showed significant effect on reducing stripping and creep behavior. Mixes which used calcium stearate hydroxide additive showed less stripping and creep deformation than those which contained limestone dust additive. r 2005 Published by Elsevier Ltd. Keywords: Stripping; Environment; Asphalt; Mixture; Creep
1. Introduction Road pavements represent a huge investment. Most surfaced roads are made of flexible pavement. One of the most important problems in flexible pavement is stripping, which is the loss of adhesion between asphalt coating and aggregate in the bituminous mixture caused by the existence of moisture. Stripping might cause different types of distresses such as: creep, raveling, rutting, shoving, and cracking. Hot mix asphalt (HMA) environmental damage (stripping) is a material compatibility problem. If the materials are compatible, it will form strong and Corresponding author. Tel.: +962 27201000; fax: +962 270 95123.
E-mail address: [email protected] (S. Abo-Qudais). 0360-1323/$ - see front matter r 2005 Published by Elsevier Ltd. doi:10.1016/j.buildenv.2005.05.017
long-term bonds, so that the HMA resists stripping. However, if the mix has poor bonding, its resistance to stripping will be less. In this case, the bonds should be enhanced or the material combinations should be changed to achieve satisfactory performance. Stripping 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 agents. Other variables are related to the weather conditioning, 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 surface tension between aggregate and bitumen [1].
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Antistripping additives are categorized into the following groups: cationic surfactants, iron naphthenate, hydrated lime, oregano saline, Portland cement, and other products. The main objective of adding these additives was to convert a hydrophilic (water loving) aggregate surface to a hydrophobic (water hating) aggregate [1]. Tunnicliff and Root [2] presented findings of a survey regarding the use of antistripping agents in bituminous mixtures. The survey indicated that there were over 100 antistripping agents available, and there were a very large number of testing procedures used by United States of America agencies to evaluate the effectiveness of the antistripping additives. Maupin [3] used Lottman’s test to evaluate the effect of different types of asphalt cements and antistripping agents on the stripping susceptibility of HMA. The results indicated that the Lottman’s test measured no differences in stripping susceptibility of HMA with different types of asphalts. However, significant differences in stripping susceptibility were detected when different additives were used. In another study, Maupin [4] performed field investigations of the effectiveness of antistripping additives in Virginia. The results of the study indicated that significant visual stripping was detected at many sites. Also, chemical additives performed no better than hydrated lime. Creep is an important factor in flexible pavement design. This is especially true for pavements exposed to heavy traffic and high tire pressure. In pavement, it is most likely that permanent deformation occurs in the upper layers rather than in the subgrade. The creep test is usually used to estimate the rutting potential of asphalt mixtures by applying a static load to a HMA specimen and measuring the resulting permanent deformation with time [1]. An experimental program was conducted by AboQudais [5] to characterize the viscoelastic deformation using the static creep test. The study evaluated the effect of aggregate gradation and temperature on the permanent deformation of HMA. The results indicated that the effect of temperature variation on creep deformation is more significant at low temperature and long loading time than those at high temperatures and short loading time.
2. Research objectives In order to achieve the aim of this study and to evaluate the parameters that affect stripping and creep behavior of HMA, the objectives of this study were to evaluate: 1. The effect of stripping and antistripping additives on HMA creep behavior. 2. The possibility of using different locally available and low cost materials as antistripping agents to reduce the environmental damage of HMA pavements.
3. Research approach 3.1. Materials used The materials used in this study were different types of aggregate, asphalt, and antistripping additives described as follows: 3.1.1. Aggregate The aggregate used in this study was 100 percent crushed limestone obtained from the quarries of Al-Huson in the northern part of Jordan. Table 1 summarizes the physical properties of the aggregates used. Three aggregate gradations were evaluated in this study:
Gradation A: upper limit of ASTM specifications for dense aggregate gradation. Gradation B: mid limit of ASTM specifications for dense aggregate gradation. Gradation C: mid limit of ASTM specifications for open aggregate gradation.
Fig. 1 shows the aggregate size distribution of the three gradations used. 3.1.2. Asphalt Two types of asphalt cement with different penetrations were used in this study:
Asphalt 1: 60/70 penetration grade, this asphalt has performance grading (PG) of 70–10.
Table 1 Properties of aggregate used Aggregate
Coarse aggregate Fine aggregate Mineral filler
ASTM test designation
C127 C128 C128
Dry bulk specific gravity
Apparent specific gravity
Absorption (%)
Limestone
Basalt
Limestone
Basalt
Limestone
Basalt
2.424 2.485 2.552
2.531 2.632 2.692
2.573 2.590 2.625
2.621 2.680 2.710
3.1 4.6 5.1
3.1 3.6 5.0
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100 Upper limit of ASTM for dense aggregate gradation (A)
90
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80
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1
10
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Table 2 Properties of 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 designation
Result AC 60/70
AC 80/100
D 113 D5
110 64
118 92
D D D D
50 319 325 1.01
45.5 312 318 1.01
36 92 92 70
Asphalt 2: 80/100 penetration grade, this asphalt has performance grading (PG) of 58–22.
The asphalt was obtained from Zarqa local 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 obtained from industrial waste. It results from sawing rocks to produce building masonry, so it is available in large quantities in Jordan as waste material. The dust used was passed through sieve number 100. The calcium stearate hydroxide [Ca(C17H35COO){OH}] was prepared as described by Abo-Qudais and Almulqi [6]. This
compound was 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, then it was poured in a plastic dish and left for several days at room temperature for drying. One advantage of this additive is its low cost. The Texas boiling test (ASTM 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. 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 D1559) were followed. Three specimens of asphalt contents 3.5%, 4.0%, 4.5%, and 5.0% for mixes prepared using gradation C, and 5%, 5.5%, 6%, and 6.5% for mixes prepared using gradations A and B were prepared. A total of 36 specimens were tested for stability, flow, air voids, unit weight, and voids in mineral aggregate. The optimum asphalt content was determined based on these parameters.
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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 voids in mineral aggregate (VMA)). The resulting optimum asphalt content were 5.6%, 5.3%, and 4.2% for gradation A, B, and C, respectively. 3.3. HMA specimens fabrication The specimens were prepared according to the Asphalt Institute Manual (MS-2) and ASTM D1559. Antistripping calcium stearate hydroxide was dissolved in the asphalt since it was easy to form a homogenous solution of asphalt and calcium stearate hydroxide, while the limestone dust was added to the aggregate before mixing with asphalt. The reason for that was limestone dust increases the viscosity of asphalt when added directly to the asphalt. This makes it too hard to mix the asphalt with the aggregate. 3.4. Moisture conditioning Moisture conditioning that simulates the environmental damage in the field was used to evaluate the effects of water saturation and accelerated water conditioning with a freezing–thawing cycle of compacted bituminous mixtures in the laboratory. The HMA specimen’s conditioning was performed according to AASHTO T166 by immersing the specimens in water and exposing them to a vacuum to achieve saturation levels between 55% and 80%. Then the specimens were exposed to freezing at a temperature of –1873 1C for 16 h and thawing at 60 1C for 24 h. The effect of stripping on HMA creep was evaluated by using the static creep test. 3.5. Static creep test This test is considered to be very important in obtaining data for estimating potential deformation of the vehicle wheel paths and ranking bituminous mixtures on the basis of their resistance to permanent deformation. The static creep test was conducted by applying a static stress of 100 kPa for 1 h at 30 1C 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 sides of the speci-
% Increase in creep ¼
Fig. 2. Static creep test setup.
men, it was placed in the loading machine (Fig. 2) 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, after which the load is removed and the deformation recovery was monitored for 900 s. Three specimens were tested for each type of aggregate, 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 and the original specimen height according to the following equation: ¼ Dh=ho , where e is the accumulated microstrain that occurred in the specimen during certain loading time at certain temperature, ho the original specimen height (the original distance between specimen loading surfaces), Dh the axial deformation (change in distance between specimen loading surfaces). Creep test results for different mixes and conditioning are summarized in Figs. 3–5, while Fig. 6 summarizes the accumulated microstrain after 60 min of loading. The stripping effect on creep behavior was evaluated based on the percent of increase in creep due to conditioning. It was calculated as the ratio between the difference in creep value between unconditioned and conditioned specimens to that of unconditioned specimens.
Creep of conditioned specimens Creep of unconditioned specimen 100% Creep of unconditioned specimen
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Fig. 3. Effect of antistripping additives on creep behaviour. (a) 80/100 asphalt and upper limit of dense aggregate gradation, (b) 80/100 asphalt and midlimit of dense aggregate gradation, (c) 80/100 asphalt and midlimit of open aggregate gradation, (d) 60/70 asphalt and upper limit of dense aggregate gradation, (e) 60/70 asphalt and midlimit of dense aggregate gradation, (f) 60/70 asphalt and midlimit of open aggregate gradation.
Fig. 7 summarizes the results of the percent of increase in creep values due to conditioning.
Marshall stability and indirect tensile strength are discussed in the following sections. 4.1. Effect of antistripping additives
4. Results and discussion The prepared specimens were evaluated by performing the static creep test using the UTM. The effects of conditioning and aggregate gradation, type of asphalt, and type of antistripping additives on the creep behavior of HMA are summarized in Table 3 and discussed in the following sections. Also, the effect of stripping on
The purpose of adding antistripping additives was to improve bonding between asphalt and aggregate (adhesion force). The effect of antistripping additives on HMA creep behavior was evaluated using static creep. Two types of additives, limestone dust or calcium stearate hydroxide, were evaluated in this study for their effect on stripping and creep behavior.
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Fig. 4. Effect of aggregate gradations of creep behavior of hot mix asphalt. (a) 80/100 asphalt and unconditioned, (b) 60/70 asphalt and unconditioned, (c) 80/100 asphalt and conditioned, (d) 60/70 asphalt and conditioned.
4.1.1. Effect of antistripping additives on conditioned specimens Fig. 3 shows the effect of limestone dust on the creep behavior of HMA prepared by using different mix parameters. Based on this figure, it can be seen that limestone dust has a significant effect on reducing creep deformation. This was true for the three types of aggregate gradations and two types of asphalt used in preparing the HMA. This effect might be explained by the fact that limestone dust contains calcium which increases interaction between aggregate surface and asphalt binder, so it improves the bond between asphalt and aggregate. The average creep microstrain after 60 min of loading was 11,367 for mixes prepared using midlimits of ASTM specification for aggregate gradation. The initial microstrain was equal to 6230 while the timedependent microstrain was 5137. This means that the rate of increase in creep deformation was 1.44 microstrain/s. Fig. 3 also indicated that calcium stearate hydroxide had a more significant effect in reducing creep deformation of HMA. This effect of calcium stearate hydroxide may be explained by the fact that the calcium stearate hydroxide is a surface-active material which causes an increase in the bond between hydrocarbons in asphalt and aggregates. The average creep microstrain after 60 min of loading was 9991 for mixes prepared using midlimits of ASTM specification for aggregate gradation. The initial microstrain was equal to 6358 while the time-dependent microstrain was 4633. This
means that the rate of increase in creep deformation was 1.3 microstrain/s, which is less than that in mixes using limestone dust. This is due to less stripping talking place in mixes using calcium stearate hydroxide additive compared to mixes using limestone dust. 4.1.2. Effect of antistripping additives on unconditioned HMA specimens Unconditioned HMA specimens were tested to evaluate the effect of antistripping additives on unconditioned specimens without exposing them to freezing–thawing. The effect of limestone dust on HMA is shown in Fig. 3, indicating that there is no significant effect of limestone dust on creep behavior of HMA. Similar effects were noticed regardless of the type of asphalt and of the type of aggregate gradation used in preparing the mix. However, the same figure indicates that the addition of calcium stearate hydroxide to HMA reduces creep deformation. This trend was similar to that observed in other mixes prepared using different aggregate gradation and types of asphalt. This effect of calcium stearate hydroxide might be caused by the fact that this type of additive enhanced the bond between the aggregate and asphalt, leading to less creep. 4.2. Effect of aggregate gradation Aggregate gradation was found to have a significant influence on the creep behavior of HMA. Aggregate
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gradation influences air voids (AV) and VMA, and so it affects the creep behavior. The effect of aggregate gradation on creep behavior for different mixes are shown in Fig. 4. This figure shows the relationship between accumulated microstrain and the time for different HMA prepared using different aggregate gradations. The mixes prepared using gradation C had the highest creep deformation, followed by the mixes prepared using gradation B. While the mixes prepared using gradation A show the lowest creep deformation, a similar trend was noticed for the two types of asphalt and the type of additives used in preparing HMA. The AV in mixes prepared using gradations A, B, and C mixes were 4.6, 5.0, and 12.3, respectively. The average creep microstrain after 60 min of loading were 11,367, 9984, and 9044 for mixes prepared using asphalt 60/70 and gradation C, gradation B, and gradation A, respectively. The elastic microstrain for gradation A was equal to 5120 while the viscous microstrain is 3924. This means that the rate of increase in creep deformation is 1.10 microstrain/s. For gradation B, the elastic microstrain was equal to 6802 while the viscous microstrain was 3182. This means that the strain rate was 0.88 microstrain/s. For mixes using gradation C, the elastic microstrain was equal to 6900 while the viscous microstrain was 4467 causing a microstrain increase rate of 1.24 microstrain/s. For the unloading portion of the creep test (deformation recovery), gradation C mixes had the lowest deformation recovery after unloading compared to gradations A and B. The average deformation recovery microstrain after 15 min of unloading was 457, 579, and –707 for mixes prepared using gradation C, A, and B, respectively. The above-mentioned results can be explained by the fact that the dense aggregate gradation having maximum density provides increased stability through the increase in interparticle contact and reduces VMA. This
Fig. 6. Creep accumulated microstrain after 60 min of loading.
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Fig. 7. Percent increase in creep due to conditioning. Table 3 Ranking of different mixes according to their resistance to creep Rank
According to creep microstrain at 60 min
According to percentage increase in creep due to conditioning
1 2
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7
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leads to more resistance to the applied load. Meanwhile, the open aggregate gradation has less aggregate interaction due to a large amount of voids that reduces
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interaction between aggregate in addition to having less amounts of fine aggregate which reduces the adhesion between coarse aggregate grains.
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Fig. 4 was constructed to show the effect of aggregate gradation on creep behavior of HMA conditioned specimens. HMA prepared using gradation C had the highest creep deformation, followed by the mix prepared using gradation B, while the mix prepared using gradation A showed the lowest creep deformation. The average creep microstrain after 60 min of loading was 18,402, 12,343, and 9594 for mixes prepared using gradation C, gradation B, and gradation A, respectively. The initial microstrain for gradation A was equal to 5203 while the time-dependent microstrain was 4391. This means that the rate of increase in creep deformation was 1.22 microstrain/s. For gradation B, the initial microstrain was equal to 6080 while the time-dependent microstrain was 6263, indicating that the rate of increase in creep deformation was 1.77 microstrain/s. For gradation C, the instantaneous microstrain was equal to 11,500 while the time-dependent microstrain was 6902. This means that the rate of increase in creep deformation was equal to 1.92 microstrain/s. For the unloading portion of the creep test (deformation recovery), the gradation C HMA had the lowest deformation recovery compared with the other gradations. The average recovery microstrain after 15 min of unloading were 503, 481, and 249 for mixes prepared using gradations A, B, and C, respectively. These results can be explained by the fact that there are more AV in mixes using gradation C compared to that using gradations A and B. The large AV content is created by using a larger percentage of coarse aggregate, this facilitates the entering of water into the mix during conditioning leading to more stripping in mixes prepared using gradation C. Particle size is also important when considering the surface energy concept, because the surface areas and surface energy effects of very fine sizes are larger in relation to particle mass. 4.3. Effect of asphalt type The effect of asphalt used in preparing specimens is shown in Fig. 5. These figures indicated that unconditioned HMA prepared using asphalt 1 demonstrated higher creep values than those of mixes prepared using asphalt 2. These results agree with those obtained by Abo-Qudais [5], and Salter and Shkaarshi [7]. For conditioned specimens, no specific trend for the effect of asphalt type was noticed, as shown in Fig. 6. Mixes prepared using these upper limit of the ASTM dense aggregate gradation showed trends similar to those of unconditioned specimens. Also, specimens prepared using midlimits of the ASTM dense aggregate gradation showed trends similar to those of unconditioned specimens; however differences in creep deformation between specimens prepared using asphalt 1 and those using asphalt 2, was less significant. Specimens prepared using open graded aggregate
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gradation show a trend opposite to those of unconditioned specimens. Mixes prepared using 60/70 asphalt showed better resistance to increases in creep due to stripping than mixes prepared using 80/100 asphalt penetration. This is especially true for mixes prepared using open graded aggregate gradation, in which the percent of increase in creep deformations of mixes using 80/100 asphalt was almost 2.5 times of those using 60/70 asphalt. The above-mentioned results can be explained by the fact that the asphalt of high viscosity resists displacement by water much better than those of low viscosity. However, low viscosity is desirable during mixing operations because it has more wetting power than the ones having high viscosity, so it will coat the aggregate better, leading to more resistance to stripping. For control specimens, the average creep microstrain after 60 min of loading was 9984 and 7926 for mixes prepared using asphalt 1 and asphalt 2, respectively. For asphalt 1, the instantaneous microstrain was equal to 6140 while the time-dependent microstrain was 3844. This means that the increase rate in creep deformation was 1.10 microstrain/s. For asphalt 2, the initial microstrain was equal to 4925 while the time-dependent microstrain was 3006, which means that the rate of increase in creep deformation was 0.84 microstrain/s. For the unloading portion of the creep test (deformation recovery), the HMA prepared using asphalt 1 showed a higher deformation recovery than that of mixes prepared using asphalt 2. The average microstrain recovery after 15 min of unloading were 681 and –577 for mixes prepared using asphalt 1 and asphalt 2, respectively. These results indicate that the creep deformation and recovery decreased as penetration of asphalt increased. 4.4. Effect of conditioning The resulting percent of increase in creep due to conditioning, presented in Fig. 7, indicated that the effect of conditioning (stripping) on creep deformation was more detrimental for mixes prepared using open graded aggregate gradation. Among the evaluated mixes, the one prepared using calcium hydroxide additive showed the highest resistance to increase in creep due to stripping, while the one without the additive showed the least resistance.
5. Conclusions This study aimed to evaluate the effect of stripping on creep behavior. Based on the study results, the following conclusions can be drawn:
Mixes prepared using calcium hydroxide mixes demonstrate better resistance to stripping than those prepared using limestone dust additive.
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Unconditioned and conditioned mixes prepared using midlimits and upper limits aggregate gradation using 60/70 asphalt with low asphalt penetration showed higher creep deformation than those using 80/100 asphalt. However, the opposite trend was noticed for conditioned mixes prepared using open graded aggregate gradation.
References [1] Robert RL, Kandhal PS, Brown ER, Lee D, Kennedy TW. Hot Mix Asphalt Materials, Mixtures, Design, and Construction, 1st ed. Lanham, Maryland: NAPA Education Foundation; 1999. 490p. [2] Tunnicliff DG, Nebraska O, Root RE. Use of antistripping additives in asphalt concrete mixtures. Report no 274, National Cooperative Highway Research Program (NCHRP), 1984.
[3] Maupin Jr GW. Implementation of stripping test for asphaltic concrete. Transportation research record report no 712, Transportation Research Board, Washington, DC, 1979. p. 8–12. [4] Maupin Jr GW. Follow-up field investigation of the effectiveness of anti-stripping additives in Virginia. Project report no 9398-010-940, Virginia Transportation Research Council, 1997. 22p. [5] Abo-Qudais SA. Time-temperature and time-aggregate gradation superposition in asphalt mixes. Road materials and pavement design. France: Lavoisier Publisher; 2004, accepted for publication. [6] Abo-Qudais SA, Almulqi M. New chemical antistripping additives for bituminous mixtures. Journal of ASTM International, PA, USA: ASTM International; 2005. submitted for publication. [7] Salter KJ, Shkaarshi MTO. Effect of ambient temperature and thermal cycling on the creep of bituminous paving materials. Transportation research record no 1228, Transportation Research Board, National Research Council, Washington, DC, 1990. p. 106–111.