Effect of Hydrated Lime on Moisture Susceptibility of Asphalt Mixtures

Moisture induced damage can cause a progressive deterioration in the performance of asphalt pavement by the loss of adhesion between asphalt binder and aggregate surface and/or loss of cohesion within the binder in the presence of water. The objective of this paper is to improve the asphalt mixtures resistance to moisture by using hydrated lime as an anti-stripping additive. For this purpose, two types of asphalt binder were utilized; asphalt grades (40-50) and (60-70) with one type of aggregate of 19.0 mm aggregate nominal maximum size, and limestone dust as a mineral filler. Marshall method was adopted to find the optimum asphalt content. Essentially, two parameters were determined to evaluate the moisture susceptibility, namely: The Index of Retained Strength and the Tensile Strength Ratio. The hydrated lime was added by 1.0, 1.5, and 2.0 percentages (by weight of aggregate) using the saturated surface dry method. It was concluded that using hydrated lime will improve the moisture damage resistance. This was adopted as the value of tensile strength ratio increased by 24.50 % and 29.16% for AC (40-50) and AC (60-70) respectively, furthermore, the index of retained strength also increased by 14.28 % and 17.50 % for both asphalt grades. The optimum hydrated lime content founded to be 1.5 %.


INTRODUCTION
For most societies, if not all, highways networks play a vital role in the economic and social development.In Iraq, due to the rapid social changes within the last years, the number of vehicles has an enormous growth rate, furthermore, highways pavement suffers from dramatically more loads due to the absence of transportation alternatives for goods and passengers in these days.Thus, more studies are required to overcome this progressive demands for constructing and rehabilitation roads considering the consumption of billions of dinars every year in this field.In general, most of highways in Iraq are constructed using asphalt concrete pavement.The fact is that highways do not last for the designed period because of many factors that contribute to the asphalt pavements performance, one of these factors is the moisture damage which is considered as one of the most important factors that reduce the asphalt pavement structural capacity causing different distresses to develop in pavement and reduce its life cycle, thus, highway engineers make more effort towards the improvement of the asphalt pavement resistance to moisture.Different additives were tested to perform as anti-stripping agents, some of these materials made a significant improvement in asphalt pavement moisture resistance as stated by Perez and Pasandin, 2017.

BACKGROUND
Moisture sensitivity in asphalt pavement is a global issue affecting the performance and cycle life of asphalt pavement mixtures and can basically be defined as the loose of adhesive bonding between asphalt binder and aggregate surface and/or loose of cohesion within the binder or aggregate.Asphalt pavement sensitivity towards moisture is a complex phenomenon contributed by many factors including but not limited to physical and chemical properties for the binder and aggregate as well as the design procedure and construction quality.Moisture damage in asphalt pavement can lead to many distresses within asphalt structure such as bleeding, cracking, rutting, raveling and localized failures as reported by Hicks, et al., 2003.To demonstrated this distress, Kiggunda and Roberts, 1988, referred to six mechanisms behind moisture damage in asphalt pavement mixtures which are: detachment, displacement, spontaneous emulsification, pore pressure, hydraulic scour, and pH instability and the effects of environment or climate on asphalt-aggregate materials system that may have a significant effect on moisture damage.Several theories tried to explain the adhesion bond between asphalt binder and aggregates including: mechanical theory, chemical reaction theory, surface energy theory, and molecular orientation theory while no one of these theories provides full explanation of the mechanism by which adhesion works as declared by Hicks, 1991.To improve asphalt pavement resistance to moisture damage, many steps must be considered, among these, is the addition of anti-stripping agents.Anti-stripping agents are basically classified into two groups; those added to the asphalt binder which are usually chemical liquids called "Liquid Anti-Stripping Agents" and those added directly to the aggregate like: lime, fly ash, Portland cement, flue dust, polymers, cement Klein, and many others as illustrated by Epps, et al., 2003.The primary goal of adding anti-stripping agents is to reduce asphalt mixtures moisture sensitivity by improving the bond between the asphalt binder and the aggregate.Another important consideration when using anti-stripping additives is its ability to maintain good HMA properties, i.e, the additive must not negatively affect other desirable properties of the HMA in addition to its ability to improve HMA resistance to moisture as reported by Sebaaly, 2007.Lime has been widely used both as filler and additive to resist moisture in asphalt mixtures.
Three major forms of lime are used with asphalt mixtures according to Hunter and Ksaibati, 2002; Hydrated Lime (Ca(OH)2), Quick lime (CaO), and Dolomitic Lime (CaO.MgO).In this paper, Hydrated Lime (Ca(OH)2) was used as anti-stripping, thus the term (lime) refers to .0Hydrated Lime in this paper.The National Lime Association, NLA, 2003, suggested that the lime may be added by a content ranged from 1.0 to 2.0 % by dry weight of total aggregate in different ways; dry lime to dry aggregate, dry lime to wet aggregate, lime slurry to dry aggregate, or mixed with asphalt.The mechanism by which lime acts as anti-stripping agent is not fully understood, Selim, 1997, assumed a chemical interaction involved between the calcium in the lime with the silica in the aggregate and found that lime has proven to work effectively with different types of aggregate.Lime is considered to be very chemically active in nature which makes it effective to reduce moisture damage in asphalt pavements.In the same direction, the mechanical characteristics of lime were also approved by Huang, et al., 2010, when they studied the effect of the lime particle fineness when lime was used as anti-stripping additive with asphalt mixtures, they pulverized a commercially available lime into smaller particle by using abrasion machine with different revolution speeds, then a concentration of 1.0 % of the pulverized lime was added to the asphalt mix and tested for moisture damage.The results showed an increase of TSR's values.

MATERIAL CHARACTERIZATION
Asphalt cement, aggregate, and filler used in this work have been characterized using routine type of tests and the results were compared with the State Corporation for Roads and Bridges Specifications, SCRB, 2003.

Asphalt Cement
Two types of asphalt cement were used in this work.Asphalt cement grade (40-50) which was obtained from Al-Durrah Refinery and asphalt cement grade (60-70) which was obtained from Al-Sheaba Refinery.Physical properties of both asphalt cement types are listed in Table 1 and  Table 2.All tests results meet the SCRB R/9, 2003 specification.

Coarse Aggregate
The coarse aggregate (crushed) was brought from the hot mix plant of Amant Baghdad.The source of the aggregate was from Al-Nibaee quarry.According to SCRB R/9, 2003 specifications, the sizes of coarse aggregate ranged between 3/4 in.(19 mm) and No.4 sieve (4.75mm).A routine physical tests were conducted to investigate the aggregate properties which are listed in Table 3.

Fine Aggregate
The fine aggregate (river and crushed sand) is the part of asphalt mixtures that passes sieve No.4 (4.75mm) and retained on sieve No.200 (0.075mm) according to SCRB R/9, 2003.It was brought from the same hot mix plant of Amant Baghdad and consists of hard, tough, grains, free of injurious amount of clay, loam or other deleterious substances.The physical properties of fine aggregate are listed in Table 3.

Mineral Filler
Filler is a material passing sieve No. 200 (0.075 mm).It is thoroughly dry and free from lumps or aggregations of fine particles.It was decided to use limestone dust as filler in preparing the asphalt mixture due to its availability and relatively lower cost.It was brought from lime factory at Karbala province.The physical properties are shown in Table 4.

Additive
To improve the ability of asphalt mixtures to resist the harmful effect of moisture presence, the hydrated lime was used as anti-stripping additive.Lime was added at 1.0, 1.5, and 2.0 % by weight of aggregate using SSD method (i.e.adding dry lime to wet aggregate).Water was added with 5.0 % , by weight of aggregate, to the aggregate without mineral filler, then lime was added and properly mixed with the aggregate and then left to dry in an oven for 2 hours at 150 C.After drying, the mineral filler was added to the combination and mixed with desired asphalt content.Chemical properties of hydrated lime are listed in Table 5.
Table 5.Chemical properties of hydrated lime

EXPERIMENTAL WORK
The experimental work phase was started by Marshall test to find the optimum asphalt content for asphalt cement grade (40-50) and asphalt cement grade (60-70).In order to evaluate the potential of asphalt mixture to resist the damage effect of moisture, two parameters were determined, namely: Tensile Strength Ratio (T.S.R) and Index of Retained Strength (I.R.S).For the purpose of determining these parameters, the indirect tensile strength and compressive tests were employed.

Marshall Method
This test measures the resistance of asphalt mixtures specimens to plastic flow when loaded on the lateral surface by means of Marshall apparatus according to ASTM (D-6927).This method includes the preparation of cylindrical specimens which were 101.6 mm in diameter and 63.5 mm in height.The tested specimens were evaluated for Marshall stability, flow value, percent of air voids (AV) and percent of voids in mineral aggregate (VMA).The optimum asphalt content for AC (40-50) was 5.0 % and 4.8 for AC (60-70), test results are listed in Table 6.

Indirect Tensile Strength Test
The moisture susceptibility of the bituminous concrete mixtures was evaluated according to ASTM (D-4867).The result of this test yields the indirect tensile strength (I.T.S) value and the tensile strength ratio (T.S.R).For his test, a set of asphaltic specimens were prepared for each mix according to Marshall procedure and compacted to 7±1 % air voids using trial mixtures as shown in Fig. 1.This high targeted air voids content is not meant to mimic the actual field conditioning process but to accelerate the moisture damage in a manner that can be measured under laboratory conditions.The set consists of six specimens and was divided into two subsets, one set (control) was tested at 25°C and the other set (soaked) was conditioned at 60 C water for 24 hours and then tested at 25°C.The test involved loading the specimens with a compressive load at a rate of 50.8 mm/min which acting parallel to and along the vertical diametric plane through 12.5 mm wide steel strips that was curved at the interface with the specimens.These specimens failed by splitting along the vertical diameter.The indirect tensile strength (I.T.S) value is calculated according to Eq. ( 1), while the value of tensile strength ratio (T.S.R) is determined by Eq. (2).

Index of Retained Strength Test
This test covers measurement of compressive strength loss resulting from the action of water on compacted asphaltic mixtures.This procedure is fully covered by ASTM (D-1075).Six cylindrical specimens with a dimensions of (101.6 mm *101.6 mm) were prepared according to ASTM (D-1074).The mixtures were compressed at top and bottom under an initial pressure of 1MPa (150 psi) to set the mixture against the sides of the mold, after that, the required pressure of 20 MPa (3000 psi) was applied for a two minutes and the specimen was left to cool at room temperature for 24 hours as demonstrated by Ismael and Al-Harjan, 2018.The specimens were extruded from the molds and the bulk specific gravity was obtained following the procedure described in test method ASTM (D-2726).The set of six specimens was sorted into two groups of three specimens so that the average bulk specific gravity was approximately the same for both groups.One set was tested in dry condition by storing in air bath for 4 hours at 25±1˚C before applying an axial load at a rate of 5.08 mm/min and the failure load was recorded as S1.The second group was immersed in water path for 24 hours at 60˚C and was transferred to another water path at 25 ˚C for 2 hours to bring the specimens to the test temperature before applying the same load rate and the failing load was recorded as S2.
The index of retained strength is calculated using the following formula:

Marshall Test
The addition of hydrated lime as an additive to the asphalt mixture caused an increase in Marshall stability by (2.6, 8.69, and 4.34) % for asphalt grade 40-50 and by (7.14, 13.26. and 9.18) % for asphalt grade 60-70.As noticed, the maximum increment in stability occurred at 1.5 % of lime content.This increase was accompanied by a reduction in flow values by (12.10, 15.15, and 21.20) % for asphalt grade 40-50 and by (3.57, 10.71, and 14.28) % for asphalt grade 60-70.The air voids in asphalt mixtures decreased with the addition of lime, whereas, it decreased by (2.5, 5.0 and 8.75) % for asphalt grade 40-50 and by (0.0, 2.5, and 5.0) % for asphalt grade 60-70.The values of bulk density for asphalt mixtures containing hydrated lime were slightly increased with the increase of lime content.The justification of these behaviors can be attributed to the fact that addition of hydrated lime increased the fine materials in the asphalt mixtures, subsequently, the air voids decreased despite the fact that these materials absorbed more asphalt.The reduction in air void values causes the bulk density to increase resulting in an obvious growing increment in Marshall stability.
The test results are gathered in Tables 7 and 8 and graphically presented by Figs 2 and 3.

Tensile Strength Ratio
The addition of designated amount of hydrated lime to the asphalt mixtures increased both of dry and wet tensile strength values.Consequently, the values of T.S.R for both types of asphalt are elevated, whereas, the maximum increment of increase occurred at 1.5 % of lime content.As for using asphalt grade (40-50), herein, the maximum T.S.R value was increased by 24.50 % over the control mixture.In the same way, for mixtures fabricated by utilizing asphalt grade (60-70), the maximum T.S.R value was higher by 29.16 % over the control mixture.This improvement in moisture damage resistance might be scientifically interpreted by the effect of chemical action due to the presence of Ca ++ that is available in the lime material which increased the bond between the aggregate surface texture and the film of asphalt cement.All the necessary data concerning this test are listed in Tables 9 and 10 and demonstrated in Figs. 4 and 5.

Index of Retained Strength
Following the same trend of improvement as for tensile strength ratio, incorporating the hydrated lime in mixtures preparation enhance the values of dry and wet compressive strength.Hence, the magnitude of Index of Retained strength was also raised.The best results occurred at a percent of 1.5.The maximum increment of increase in I.R.S value was 14.28 % over the control mix for AC (40-50) while for AC (60-70) this value became 17.5 %.The justification of this behavior of improvement is similar to the previous interpretation.The output of this test are summarized in Tables 11 and 12 and illustrated visually by Figs. 6 and 7.

Figure 1 .
Figure 1.Relationship between No. of blows and air voids.

Table 4 .
Physical properties of mineral filler.

Table 6 .
Marshall test results for wearing course

Table 11 .
Index of retained strength test results for asphalt grade (40-50).