DOKUZ EYLÜL UNIVERSITY
GRADUATE SCHOOL OF NATURAL AND APPLIED
SCIENCES
DETERMINATION OF MOISTURE
SUSCEPTIBILITY CHARACTERISTICS OF
POLYMER MODIFIED HOT-MIXED ASPHALT
by
İ
smail Çağrı GÖRKEM
August, 2008
DETERMINATION OF MOISTURE
SUSCEPTIBILITY CHARACTERISTICS OF
POLYMER MODIFIED HOT-MIXED ASPHALT
A Thesis Submitted to the
Graduate School of Natural and Applied Sciences of Dokuz Eylül University In Partial Fulfillment of the Requirements for the Degree of Master of Science
in
Civil Engineering, Transportation Engineering Program
by
İ
smail Çağrı GÖRKEM
August, 2008
ii
M.Sc THESIS EXAMINATION RESULT FORM
We have read the thesis entitled “DETERMINATION OF MOISTURE
SUSCEPTIBILITY CHARACTERISTICS OF POLYMER MODIFIED HOT-MIXED ASPHALT” completed by İSMAİL ÇAĞRI GÖRKEM under
supervision of ASSISTANT PROFESSOR DR. BURAK ŞENGÖZ and we certify that in our opinion it is fully adequate, in scope and in quality, as a thesis for the degree of Master of Science.
Assist.Prof.Dr. Burak ŞENGÖZ
Supervisor
Assist.Prof.Dr. Serhan TANYEL Assoc.Prof.Dr. Gökdeniz NEŞER
(Jury Member) (Jury Member)
Prof.Dr. Cahit HELVACI Director
iii
ACKNOWLEDGEMENTS
Firstly I would like to express my sincere appreciation to my advisor, Assist.Prof.Dr Burak ŞENGÖZ, for his valuable guidance, continuous support and encouragement throughout this study. I also express my special thanks to Assist.Prof.Dr. Serhan TANYEL and Dr. Ali TOPAL for their helpful recommendations and supports.
I would like to thank to Assist.Prof.Dr. Cumhur AYDIN for his guidance, continuous support, and significant recommendations about my life.
Lastly, I would like to express my deepest thanks and best wishes to my family, my father Mithat GÖRKEM who is always a model engineer for me, my mother Meral GÖRKEM, and my sister Pelin GÖRKEM for their guidance, and endless supports. I would like to dedicate this study to my family.
iv
DETERMINATION OF MOISTURE SUSCEPTIBILITY
CHARACTERISTICS OF POLYMER MODIFIED HOT-MIXED ASPHALT ABSTRACT
By the increase in traffic volume, tire pressure, axle load and changes in environmental conditions cause serious premature failures in pavements such as rutting, fatigue and low temperature cracking as well as moisture induced damage. One of the major causes of premature pavement failure is also the moisture induced damage of asphalt concrete layer. Many variables affect the amount of water damage in asphalt concrete layer such as the type of aggregate, bitumen, mixture design and construction, level of traffic, environment and the additive properties that are introduced to the bitumen, aggregate or bitumen-aggregate mixture.
This study is the aimed to determine the effect of additives such as elastomeric (SBS) and plastomeric (EVA) polymer modified bitumen (PMB) on the stripping potential and moisture susceptibility characteristics of hot mix asphalt (HMA) containing different types of aggregate (basalt-limestone aggregate mixture and limestone aggregate). The stripping properties and moisture susceptibility characteristics of the samples have been evaluated by means of captured images and Nicholson Stripping Test (ASTM D 1664) as well as Modified Lottman Test (AASHTO T 283) respectively.
The results indicated that both elastomeric (SBS) and plastomeric (EVA) polymer modification increased the resistance of asphalt mixtures to the detrimental effects of water. Moreover, it was found out that samples prepared with EVA PMB exhibits more moisture susceptibility compared to samples prepared with SBS PMB.
Keywords: Polymer modified bitumen; Stripping; Water damage; Moisture
v
POLİMER MODİFİYE BİTÜMLERLE ELDE EDİLEN SICAK KARIŞIMLARIN SUYA KARŞI DUYARLILIKLARININ İNCELENMESİ
ÖZ
Trafik hacmindeki ve dingil yüklerindeki artışlar ve üretim hataları, yollardan beklenen performansı ve hizmet ömrünü düşürmekte, tekerlek izi oluşumu, yorulma ve düşük sıcaklık çatlakları gibi bozulmalara sebep olmaktadır. Kaplamanın öngörülen ömür ve konfor düzeyinin sağlanması, büyük ölçüde karışımlarda kullanılan bitümlü bağlayıcının özelliklerine bağlıdır. Kaplamalarda meydana gelen en önemli deformasyon türlerinden biri de suya bağlı bozulmalardır. Bitümlü kaplamalarda suya bağlı bozulmaların miktarını etkileyen kullanılan agreganın çeşidi, bitümlü bağlayıcını özelikleri, karışım tasarımı ve yapım özellileri, trafik, çevresel etkenler, bitümlü bağlayıcıya eklenen katkıların özellikleri gibi birçok etmen bulunmaktadır.
Bu çalışmanın amacı, elastomerik (SBS) ve plastomerik (EVA) polimer modifiye bitümlerle (PMB) hazırlanan ve farklı agrega türleri (bazalt-kalker agrega karışımı ve yalnız kalker agregası) içeren sıcak karışım asfaltların soyulma potansiyeli ve neme karşı hassasiyet özelliklerinin saptanmasıdır. Hazırlanan örneklerin bu özellikleri, Nicholson Soyulma Deneyi (ASTM D 1664), Modified Lottman Test (AASHTO T 283) deneylerinden bulgular ve Leica S 8 AP0 Stereo Mikroskobu kullanılarak elde edilen fotoğraflar yardımıyla değerlendirilmiştir.
Deneysel sonuçlara göre, elastomerik ve plastomerik polimer modifikasyonların her ikisinin de suya bağlı bozulmaların asfalt karışımlar üzerindeki etkisini düşürdükleri ve kaplamanın neme karşı direncini arttırdıkları gözlemlenmiştir. Ayrıca yapılan çalışmalardan elde edilenlere göre, EVA PMB ile hazırlanan karışımların SBS PMB ile hazırlanan örneklere göre neme karşı hassasiyet üzerine etkisinin daha az olduğu saptanmıştır.
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CONTENTS
Page
THESIS EXAMINATION RESULT FORM………...ii
ACKNOWLEDGEMENTS………...iii
ABSTRACT………iv
ÖZ………v
CHAPTER ONE – INTRODUCTION……….……1
CHAPTER TWO – DEFORMATIONS………...….……...4
2.1 Permanent Deformations………...……….4
2.2 Cracking……….………7
2.2.1 Fatigue cracking……….7
2.2.2 Low Temperature Cracking………...9
2.3 Moisture Damage……….10
CHAPTER THREE – WATER-INDUCED DAMAGE………...11
3.1 Factors Which Influence Moisture Damage………17
3.1.1 Type of Aggregate………...18
3.1.1.1 Surface Energy Theories………..18
3.1.1.2 Chemical Bonding………21
3.1.1.3 Mechanical Interlock………23
3.1.2 Type of Bitumen………..24
3.1.3 Mixture Design and Construction………25
3.1.4 Environment……….27
3.1.5 Traffic………...27
3.1.6 Anti-Stripping Additive Properties………..28
vii
3.2.1 Static Immersion Test………..29
3.2.2 Boiling Water Test………...30
3.2.3 Immersion – Compression Test………...31
3.2.4 Conditioning with Stability Test………..32
3.2.5 Texas Freeze – Thaw Pedestal Test……….32
3.2.6 Hamburg Wheel Tracking Test………33
3.2.7 Environmental Conditioning System………...34
3.2.8 Standard Method of Test for Resistance of Compacted Hot – Mixed Asphalt (HMA) to Moisture – Induced Damage (Modified Lottman Test – AASHTO T 283)………35
CHAPTER FOUR – MODIFICATIONS………...37
4.1 Definition and Aim of Modification………37
4.2 Types and Applications of Polymers………...40
4.2.1 Elastomers………42
4.2.2 Plastomers………53
4.3 The Effects of Polymers on Moisture – Induced Damage………...54
4.3.1 Polymeric Aggregate Treatment Mechanism………...55
CHAPTER FIVE – EXPERIMENTAL………..57
5.1 Applied Test Conducted on PMBs and Test Apparatus………...57
5.1.1 Bitumen Penetration Test……….57
5.1.2 Softening Point Test……….58
5.1.3 Thin Film Oven Test………59
5.1.4 Storage Stability Test………...59
5.1.5 Ductility Test………60
5.2 Materials………...61
5.2.1 Bitumen………61
viii
5.2.3 SBS Kraton D 1101………..64
5.2.4 Evatene 2805 (EVA)………65
5.3 Preparation of SBS and EVA PMBs………66
5.4 Test Methods………67
5.4.1 Nicholson Stripping Test ……….69
5.4.2 Modified Lottman Test (AASHTO T 283)………..69
5.4.2.1 Apparatus………..69
5.4.2.2 Sample Preparation………...70
5.4.2.3 Moisture Conditioning………..71
5.4.2.4 Test Procedure………..71
CHAPTER SIX – RESULTS AND DISCUSSIONS………..76
6.1 Nicholson Stripping Test Results……….76
6.2 Modified Lottman Test Results………79
CHAPTER SEVEN – CONCLUSION AND RECOMMENDATIONS………..82
1
CHAPTER ONE INTRODUCTION
Environmental factors such as temperature, air, and water can have a profound effect on the durability of asphalt concrete mixtures. In mild climatic conditions where good-quality aggregates and asphalt cement are available, the major contribution to the deterioration may be traffic loading, and the resultant distress manifests as fatigue cracking, rutting (permanent deformation), and raveling (Terrel & Al-Swailmi, 1994). But when a severe climate is in question, these stresses increase with poor materials, under inadequate control, with traffic as well as with water which are key elements in the degradation of asphalt concrete pavements. Moisture-induced damage within hot mix asphalt (HMA) pavements is a world wide issue that decreases the lifespan of the highways. Moisture damage is caused by distress mechanisms induced by the presence or infiltration of moisture and manifests itself in a phenomenon referred to as stripping, where the asphalt binder is “stripped” from the aggregate.
To alleviate or to control the deformations which have been caused by high traffic loads and some environmental effects such as water damage, various researches have been performed leading to utilization of anti-stripping additives such as traditional liquid additives, hydrated lime and quick lime (Hunter, 2001).
With the introduction of polymers, the utilization of polymer modified bitumen other than the utilization of anti-stripping additives in hot mix asphalt to reduce the moisture susceptibility characteristics of the mixture has gained wider attention by the scientists in the last few years.
Polymer is a derived word meaning of many parts. Polymers can be thought of as long chemical strands that are made up of many smaller chemicals (monomers) that are joint together end-on-end. The physical and chemical properties of a polymer will depend on the nature of the individual molecular units, the number of them in each polymer chain and their combination with other molecular types.
The main reasons to modify bitumens with polymers can be summarized as;
– to obtain softer blend at high temperatures and reduce cracking, – to reach stiffer blends at high temperatures and reduce rutting, – to reduce viscosity at layout temperatures,
– to increase the stability and the strength of the mixtures, – to improve the abrasion resistance of blends,
– to improve fatigue resistance of blends, – to improve oxidation and aging resistance, – to reduce structural thickness of pavements, – to reduce life costs of pavements.
Two basic types of polymers are used in modified bitumen of road applications: i)elastomers, ii) plastomers.
An elastomer is a polymer that has a flexible 'rubber' backbone and large sidechains in its structure. Styrene butadiene styrene (SBS) is an example of this type. Thermoplastic elastomers derive their strength and elasticity from a physical cross-linking of the molecules into a three dimensional network (British Petrol, 1997).
A plastomer is a polymer that will deform in a plastic or viscous manner at melt temperatures and becomes hard and stiff at low temperatures, i.e. the structure is reversibly broken down with the application of heat. Whereas elastomers can improve the resistance to rutting as well as low temperature and fatigue cracking, plastomers will generally only improve the resistance to rutting (British Petrol, 1997.). EVA polymers are easily blended into asphalt by simple low shear mixing.
This study is the aimed to determine the effect of elastomeric (SBS) and plastomeric (EVA) polymer modified bitumen (PMB) on the stripping potential and moisture susceptibility characteristics of hot mix asphalt (HMA) containing different types of aggregate (basalt-limestone aggregate mixture and limestone aggregate).
The stripping properties and moisture susceptibility characteristics of the samples have been evaluated by means of captured images and Nicholson Stripping Test (ASTM D 1664) as well as Modified Lottman Test (AASHTO T 283) respectively.
4
CHAPTER TWO DEFORMATIONS
A road no matter how well- designed and constructed may fail some time during its service life because of some unforeseeable environmental factors. The only way of elongating its service life and keeping its riding qualities at the desired level is satisfactory maintenance and strengthening. The choice of maintenance or strengthening depends upon the degree of failure.
Various deformations or failures occur in bituminous pavements. Some of these are;
– inherent properties of hot mix asphalt mixtures, – improper choice of design methods and materials, – local traffic factors and climatic conditions,
– inadequate quality control or lack of proper control in construction.
The types of deformations in HMA pavements have been investigated and have been classified by researchers. According to the Strategic Highway Research Program (SHRP) A-417 (1994), deformations can be categorized as;
– permanent deformations (rutting)
– cracking (fatigue cracking and low temperature cracking) – moisture damage
2.1 Permanent Deformations
One of the most serious types of structural distresses of asphalt concrete is the permanent deformation, which could be referred to as rutting. Permanent deformation occurs in the pavement layers or subgrade as a result of consolidation or movement of the materials due to traffic loads (Huang, 1993.). Permanent deformation presents an accumulation of small amounts of unrecoverable deformation that occur each time a load is applied (Topal, 2001).
Rutting has two principal causes:
– causes related with sub layers,
– causes related with bituminous layers.
The rutting related with below the asphalt layer is caused by too much repeated being applied to the subgrade, subbase or base. Figure 2.1 represents the rutting from weak sub layers.
Figure 2.1 Rutting from weak subgrade
Although stiffer paving materials will partially reduce this type of rutting, it is normally considered a structural problem rather than a materials problem. Essentially, there is not enough pavement strength or thickness to reduce the applied stress to a tolerable level. It may also be caused by a pavement layer that has been unexpectedly weakened by the intrusion layers rather than in the asphalt layers (Asphalt Institute, 1996.)
The type of rutting of most concern to asphalt mix designers is deformation in the asphalt layers. This rutting results from an asphalt mixture without enough shear strength to resist repeated heavy loads. Figure 2.2 represents rutting from weak mixture. Original profile Asphalt Layer Subgrade Deformation
Figure 2.2 Rutting from weak mixture.
A weak mixture will accumulate small, but permanent, deformations with each truck pass, eventually forming a rut characterized by a downward and lateral movement of the mixture. The rutting may occur in the asphalt surface course, or the rutting that shows on the surface may be caused by a weak underlying asphalt course.
Rutting of a weak mixture typically occurs during the summer under higher pavement temperatures. While this might suggest that rutting is solely a bitumen problem, it is more correct to address rutting by considering the combined resistance of the mineral aggregate and bitumen (Asphalt Institute, 1996.).
As a term of “depth of rut” is a criterion used in many countries. When the depth of rut reaches a certain level, the pavement is considered to have failed. Depending upon the depth, either maintenance or strengthening decision is taken. The criterion for depth of rut used in different countries varies between 1.0 and 2.5 cm.
In the UK, for example, the failure condition is defined as 2.5 cm (1 inch) of permanent deformation in the wheel- tracks measured from the original level or 1.3 cm (0.7 inch) measured under a 2 m (6 feet) straight-edge as shown in Figure 2.3 (Uluçaylı M, 1976).
Weak Asphalt Layer
Original Profile
Figure 2.3 Failure condition based on rut depth (British Method)
The oxidation of the bitumen effects the rutting in the bituminous pavements. Due to become more brittle by oxidation, the cracks can occur. Even though crack formation, resistance against deformation may increase.
2.2 Cracking
Cracks are the deformations which occur on the surface of pavement with different form, width and depth by the effects of traffic, environment and climatic conditions. Cracks can be defined in two main groups:
– Fatigue cracking
– Low temperature cracking
2.2.1 Fatigue Cracking
Fatigue cracking is considered a major structural distress of pavements and is a load-associated distress mechanism. Fatigue cracking is a chain of interconnected cracks caused by failure of asphalt surface or stabilized base under cyclic traffic loading (Huang, 1993). “Bottom-up” cracking begins at the bottom of the asphalt surface where the tensile stress or strain is highest under the wheel load. The cracking then propagates upwards toward the surface where longitudinal cracks
0.7 inç(13 mm) P er m ane nt D ef or m at ion
Slow Lane Fast Lane
appear. Figure 2.4 represents critical strains and both bottom-up and longitudinal cracks.
Figure 2.4 Critical strain and fatigue cracking on pavements
Longitudinal cracks run parallel to the pavement's centerline and are indicative of the beginning of fatigue cracking. Due to repetitive loading the cracks connect and develop a pattern that resembles the skin pattern on an alligator and is termed “alligator cracking”. In the case of thick pavements, the cracks may propagate at the surface and migrate downwards which is referred to as “top-down” cracking. Excessive or severe alligator cracking can lead to potholes. Potholes occur when there is a hole left after interconnected cracks create a small piece of pavement that is broken from the pavement surface. Potholes may also be formed during freeze-thaw cycling or localized disintegration within the bituminous pavement layer (Huang, 1993).
Fatigue cracking occurs due to a loss of structural support. Moisture has an effect on the structure of the pavement in two possible locations: at the subgrade or base layers and within the compacted bituminous layer. The subgrade or base layers can lose support due to poor drainage and during the thawing process. Stripping may
occur as a result of high tensile stresses in the bottom of the bituminous layer. The stripped area will not provide any support so the effective compacted bituminous layer thickness is decreased. Further, fatigue cracking allows moisture infiltration, which can lead to further damage and the onset of other distress mechanisms (Copeland, 2007).
2.2.2 Low Temperature Cracking
Low temperature cracking is caused by adverse environmental conditions rather than by applied traffic loads. It is characterized by intermittent transverse cracks that occur at a surprisingly consistent spacing.
Low temperature cracks from when an asphalt pavement layer shrinks in cold weather. As the pavement shrinks, tensile stresses build within the layer. At some point along the pavement, the tensile stress exceeds the tensile strength and the asphalt layer cracks. Low temperature cracks occur primarily from a single cycle of low temperature, but can develop from repeated low temperature cycles.
The asphalt binder plays a key in low temperature cracking. In general, hard asphalt binders are more prone to low temperature cracking than soft asphalt binders. Asphalt binders that are excessively aged, because they are unduly prone to oxidation and/or contained in a mixture constructed with too many air voids, are more prone to low temperature cracking.
Thus, to overcome low temperature cracking engineers must use a soft binder that is not overly prone to aging, and control the in-place air void content and pavement density so that the binder does not become excessively oxidized (Asphalt Institute, 1996.).
2.3 Moisture Damage
Asphalt pavement failures are typically classified as stability (load) or durability related failures. Moisture damage is signified by loss of strength or durability in an asphalt pavement due to the effects of moisture and may be measured by the asphalt mixture’s loss of mechanical properties (Little, 2003.). The integrity of an asphalt concrete pavement depends on the bond between aggregate and asphalt cement. Moisture in the form of liquid or vapor can degrade this bond and lead to the first stage of failure which is deterioration of the asphalt-aggregate bond or “stripping” followed by the second stage which is premature failure of the pavement structure. Kiggundu et al. define stripping (moisture-induced damage) as: “The progressive
functional deterioration of a pavement mixture by loss of the adhesive bond between the asphalt cement and the aggregate surface and/or loss of the cohesive resistance within the asphalt cement principally from the action of water.” (Kiggundu & Roberts, 1988.).
11
CHAPTER THREE WATER-INDUCED DAMAGE
Moisture damage in asphalt concrete pavements is a primary cause of distresses in the asphalt pavement layers. The existence of water in asphalt pavement is often one of the major factors affecting the durability of HMA. The water induced damage in HMA layers may be associated with three mechanisms:
– loss of adhesion – loss of cohesion
– breaking of the aggregates due to severe environmental conditions
In the first mechanism, the water gets between the asphalt and aggregate and strips the asphalt film away, leaving aggregate without asphalt film coverage, as illustrated in Figure 3.1 and Figure 3.2. This is because the aggregates have a greater kinship for water than asphalt binder.
Figure 3.2 Displacement and detachment of asphalt binder in the presence of moisture
The second mechanism includes the interaction of water with the asphalt cement that reduces the cohesion within the asphalt cement.
Moisture damage is a complex process that is influenced by material factors, their combinations, construction, and external effects such as environment and loading (Solaimanian et al., 2003.). These factors influence physical properties of an asphalt mixture such as air void content, mechanical strength, and stiffness. When moisture is introduced and transported through the mixture and individual materials, deterioration may occur in the form of detachment, displacement, spontaneous emulsification, pore pressure or hydraulic scour (Kiggundu & Roberts, 1988, Terrel & Al-Swailmi, 1994.). As a result, major pavement failure modes may occur such as cracking and permanent deformation.
Water may enter a pavement layer from the top (road surface), bottom, and sides. Run-off water primarily can enter the road surface via surface cracks. Water can enter from the side and bottom from a high water table in the cut areas or from seepage. According to Kandhal, the most common water movement is upward from under the pavement by capillary action. This is due to poor subbases or subgrades that lack proper characteristics such as sufficient permeability that can lead to improper drainage (Kandhal, 1994.). Thus, the subsurface is saturated with moisture that can migrate upwards to the asphalt-aggregate mixture.
Once water is present, there are three ways water may influence an asphalt mixture:
(i) a flow field, (ii) static water, and
(iii) water present in aggregates (Kringos, 2007.).
If a flow field is present, water may wash away the mastic in a process termed “advective transport”, weaken the binder, and eventually attack the bond between asphalt and aggregate (Kringos & Scarpas, 2005.). Static moisture may weaken the binder and attack the bond between asphalt and aggregate. Wet aggregates become an issue if the aggregates are not thoroughly dried during mixture production. The moisture within the wet aggregate may weaken the aggregate or move towards the asphalt-aggregate interface and weaken the bond between asphalt and aggregate. The two primary modes of failure are softening of the binder which results in cohesive failure and loss of bond strength between asphalt binder and aggregate referred to as adhesive failure (Copeland, 2007).
Claisse (2005) describes the primary transport processes through concrete which are used to develop the three primary moisture transport processes through compacted asphalt mixtures: pressure-driven flow, diffusion, and thermal migration. Diffusion occurs when particles of two or more substances intermingle as the molecules move from regions of higher to lower concentration. In other words, ions will migrate between solutions until they both achieve the same concentration. Thus, diffusion is driven by concentration gradients. Moisture diffusion can also occur in a gas when the concentration of water vapor is higher in one region than another. This allows movement of water through unsaturated compacted bituminous mixtures. Moisture typically reduces the stiffness of the binder and mastic through diffusion which may lead to cohesive failure.
In a solid, water moves from hot or warm regions to cold regions and the rate at which water moves is determined by the solid’s permeability. Similarly, in a saturated mixture, ions will also move from hot towards a cold area. An ion that is moving rapidly in hot water has a greater probability of migrating through the asphalt mixture. This is an important consideration considering the highly dependent nature of asphalt mixture properties on temperature (Claisse 2005.).
There are also internal asphalt mixture processes that affect the transport processes:
– adsorption,
– capillary suction, and – osmosis.
Adsorption is used to describe any process that binds an ion (temporarily or permanently) to the asphalt mixture and prevents the ion from moving. Adsorption may be a result of a chemical process or physical surface effects.
Capillary suction occurs when water is drawn into the fine voids in compacted mixtures with wet surfaces. Capillary suction is due to surface tensions and mixtures with finer pore structures experience higher capillary suction pressures. In dense graded mixtures this may be compensated by the limitation of flow due to impermeability. Water may move in both vertical directions, up and down, due to gravity or capillary suction.
Osmosis depends on a semi-permeable membrane in which water may pass but material dissolved in the water cannot pass through easily. This causes a flow from the weak solution to the stronger solution. Water will pass through asphalt by osmosis and can eventually reach the aggregate surface causing stripping of the asphalt from the aggregate (Copeland, 2007).
Stripping typically begins at the bottom of the compacted bituminous layer where tensile stresses are greatest due to cyclic traffic loading. The stripping then progresses upward to the surface. The surface layer can be replaced; however stripping in the load bearing layer does not provide support so the effective compacted bituminous layer thickness is decreased. This may lead to pavement cracking and surface rutting and to loss of serviceability (Lottman et al., 1974.). According to Kandhal et al., there are four “essential ingredients” that encourage stripping: presence of water, high air void content, high temperature, and high stress (Kandhal & Rickards, 2002.).
Two primary mechanisms are associated with moisture damage in asphalt pavements: loss of cohesion and loss of adhesion (Terrel & Al-Swailmi, 1994.). Cohesion refers to the interaction between the asphalt mastic and water; moisture may weaken the asphalt binder, which can lead to severe loss of durability and strength. Adhesion as a failure mechanism relates to the degradation of the bond between the aggregate and the asphalt. Although degradation of the aggregate or weak aggregates may damage an asphalt mixture moisture-related failure due to aggregate strength loss is rare (Stuart, 1990.).
Cohesion is defined as the intermolecular force that holds molecules in a solid or liquid together. At the macro level of a compacted bituminous mixture, cohesive forces constitute the integrity of the material. At the micro level, considering asphalt film surrounding aggregate, cohesion may be defined as deformation under load that occurs at a distance from the aggregate substrate and beyond the influence of mechanical interlock and molecular orientation (Terrel & Al-Swailmi, 1990.). Cohesive forces develop in the mastic and are influenced by the viscosity of the asphalt binder. The viscosity of asphalt binder is dependent on temperature and cohesive forces developed in the asphalt mixture are inversely proportional to temperature.
Loss of cohesion due to moisture typically occurs in the asphalt mastic. Water can affect cohesion in various ways such as deterioration of the mastic due to saturation and void swelling. Water may behave like a solvent in asphalt and result in reduced strength and increased permanent deformation. Asphalts that retain the most amount of water have been shown to accumulate damage at a more rapid pace (Cheng et al., 2002.). In the extreme case, the presence of water (saturation) can result in bituminous emulsion: a suspension of minute globules of bituminous materials in water (ASTM, 1997.). A greater tendency is the occurrence of an inverted emulsion where water becomes suspended within the asphalt binder in spheres (Miknis et al., 2005.).
Adhesion is the molecular force of attraction in the area of contact between unlike bodies that acts to hold them together. Loss of adhesion may be used to refer to the amount of energy required to break the bond between asphalt and aggregate (Kanitpong & Bahai, 2005.).
There are some factors that affect the adhesion between aggregate and bitumen. These are;
– Surface tension (i.e. surface free energy) of the asphalt and the aggregate – Chemical composition of the asphalt and aggregate
– Viscosity of the asphalt
– Surface texture of the aggregate – Porosity of the aggregate – Cleanliness of the aggregate
– Moisture content and temperature of aggregate during mixing with asphalt cement (Terrel & Al-Swailmi, 1990.).
3.1 Factors Which Influence Moisture Damage
Moisture damage, as mentioned above, is a complex mechanism that influences the properties of bituminous pavements. When moisture is introduced and transported through the mixture and individual materials, deterioration may occur. However, the level of deterioration can be influenced by some other factors which are represented in Table 3.1.
Table 3.1 Factors which influence moisture damage
1-Aggregate ● Aggregate Composition -degree of acidity or pH -surface chemistry -types of minerals -source of aggregate ● Physical Characteristics -angularity -surface roughness -surface area -gradation -porosity -permeability
● Dust and Clay Coating ● Moisture Content
● Resistance to Degradation
2-Bitumen
● Chemical Composition ● Grade or Hardness
● Crude Source and Refining Processı
3-Mixture Design and Construction
● Air Void Level and Compaction ● Permeability and Drainage ● Film Thickness 4- Environmental ● Temperature ● Freeze-Thaw Cycles ● Moisture Vapour ● Pavement Age 5- Traffic
3.1.1 Type of Aggregate
Studies that have been used to evaluate the effects of aggregates on the degree of damage are generally separated into three concepts (Stuart, 1990.):
– Surface energy theories,
– The degree of chemical bounding, – The degree of mechanical interlock.
Adhesion, stripping and even other forms of moisture damage are thought to be related to a combination of all three concepts. Although the procedures and theories under all three concepts evaluated asphalt-aggregate-water interactions, most studies have been concerned with evaluating the effects of aggregates rather than the effects of bitumen. It is generally believed that the type of aggregate has a greater effect on moisture susceptibility.
Surface energy theories deal mainly with how materials reduce their surface free energies to obtain more thermodynamically stable conditions. Chemical bonding studies try to relate adhesion to the chemistry of materials and the chemical reactions that occur. In both concepts, it is hypothesized, but not confirmed, that molecules in the asphalt interphase and at the interface can oriented themselves to improve adhesion. The interphase region of the bitumen is that part of the bituminous layer between the bulk bitumen and the interfacial region where the aggregate and asphalt contact and adsorption occurs. Both concepts evaluate the same bonding phenomena but in different ways. The degree of mechanical interlock deal principally with the physical properties of the aggregate (Majidzahed & Brovold, 1968.).
3.1.1.1 Surface Energy Theories
Surface energy theories states that molecules in the interior of a liquid of solid are closely packed and are in equilibrium with themselves, while surface molecules have unbalanced forces.
Surface free energy (force x length) is the difference between the energy in the surface molecules and in the interior molecules. It is equal to the surface tension (force/length) times the surface area of the material. Surface free energy is the energy stored in the surface. Materials tried to minimize the amount of surface free energy. For two liquids having similar viscosities, the liquid with the lower surface tension will spread more readily on a solid than the liquid with the higher surface tension because it has a lower amount of surface free energy per unit area. In general, surface tension and surface free energy will decrease with an increase in temperature (Stuart, 1990.).
Values of the interfacial tensions of bitumen, water and some materials are shown in Table 3.2.
Table 3.2 Interfacial tensions and work separations
Types of Materials Interfacial Tensions (Dyn/cm) Rocks or Minerals and Bitumen
Limestone and Bitumen 21–26
Slag and Bitumen 22–30
Quartz and Bitumen 30
Rocks or Minerals and Water
Limestone and Water 58–64
Slag and Water 83
Asphalt and Water 0–16
Sand and Tar 40.5
As shown in Table 3.2, surface tensions, accordingly surface free energies, between limestone and bitumen is lower than the others; this represents that the bonding between limestone and bitumen is much stronger.
Studies on bitumen indicate that water, which is highly polar, can strip asphalt from most aggregates because polar liquids are better able to reduce the surface energies of aggregates than non-polar or partially polar liquid such as bitumens (Stuart, 1990.).
As shown in Table 3.2, the works of separation between aggregates and water are greater than between aggregates and bitumen. Thus, water has a greater selective wetting power and should produce stripping. However, whether stripping will occur should be determined using the combination of bitumen and water, as the bitumen and water have their own interfacial tension.
According to interfacial tension approach, the vectors in Figure 3.3 can be formulated as;
γSB - γSW = γWB * cosθ
where;
γSB = Interfacial tension between aggregate and bitumen
γSW = Interfacial tension between aggregate and water
γWB = Interfacial tension between bitumen and water
θ = Angle of bitumen contact
Figure 3.3 Contact angle in aggregate-bitumen-water phase
Contact angles and interfacial tensions are also dependent on;
– the test temperature,
– aggregate characteristics which effect roughness, such as the use of weathered versus polished surfaces and the degree of absorption,
– possibly the size of the liquid drop, – possibly the atmospheric pressure.
In general, there is an inverse ratio between contact angle and adhesion. If contact angle decreases, adhesion increases.
The majority of researchers who have studied the causes of stripping hypothesize that interfacial energy relationships are primarily responsible for adhesion and stripping mechanisms. However, the literature indicates that surface energy theories of adhesion and concepts based on minimum surface free energy and contact angle have not adequately described the adhesive properties of bitumen-aggregate-water systems.
3.1.1.2 Chemical Bonding
Concepts and research on chemical bonds between bitumen and aggregates indicate that these two materials may form chemical bonds, such as water-insoluble covalent bonds, which effect adhesive strength (Majidzahed & Brovold, 1968., Thelen, 1958.). Most studies on chemical bonding have been very simple such as those that indicate how bitumen and aggregates should bond according to their degree of acidity or pH. The pH of a material indicates its hydrogen-ion activity. Values less than 7 represent increasing hydrogen-ion concentration and increasing acidity, while values greater than 7 indicate decreasing hydrogen-ion concentration and increasing alkalinity.
Chemical bonding concepts based on measuring pH state that more bonds will be formed between an acidic material and basic material than between two materials that are either both acidic or both basic, and the degree of bonding will be greatest between a strongly acidic material and strongly basic material. Even though bitumens are amphoteric, or are capable of functioning as a base or an acid, they have generally been considered slightly acidic in most chemical bonding studies.
Thus, it is hypothesized that the basic aggregates should provide good adhesive properties while acidic aggregates should bond poorly (Gzemski, 1948.).
According to Mack, aggregates can be studied in two sections (Mack, 1964.);
– Hydrophilic
– Hydrophobic (lipophilic)
Acidic types of aggregates are considered hydrophilic and should strip. Bitumen has a lower pH than water and thus the water should tend to displace most of the bitumen chemical groups and be adsorbed itself.
Basic aggregates are lipophilic and should not strip. In this case, the lower pH of the bitumen compared to water is desirable. Some reports define basic aggregates as hydrophobic. However, very few aggregates are known to repel water. Either definition opposes surface energy concepts where most aggregates are considered hydrophilic. An additional compaction is that through hydroxylation; partially stripped aggregates in contact with water can be become more hydrophilic over time (Mack, 1964.).
As mentioned above, most of the aggregates have both basic and acidic characteristics. When chemical composition is considered, degree of acidity is related with silica contents directly. Silica content is the one of the most significant factor that affects the adhesion between aggregate and bitumen. The aggregates with high silica content have acidic characteristics. Table 3.3 represents the chemical analysis of some rocks.
Table 3.3 Typical chemical analysis of rocks in percentages.
Types of Aggregate Silica Content (%)
Granite 68.3 Quartzite 74.2 Sandstone 76.1 Limestone 3.8 Dolomitic-Limestone 16.2 Basalt 51.7 3.1.1.3 Mechanical Interlock
Increased aggregate angularity and surface roughness increases the mechanical interlock, which may help to resist the effects of moisture damage. However, complete wetting and the uniform film thickness may be more difficult to obtain with aggregates having high angularities. Asphalt films at sharp edges may be very thin and more susceptible to breaking. Increased angularity and surface roughness also increased the surface area, or contact area, between the aggregate and the bitumen. This may also increase the mechanical grip and bitumen demand, but any beneficial effects are confounded with the change and variability in film thickness (Stuart, 1990.).
The angularity of the aggregate and the contact area between the bitumen and the aggregate can be increased by crushing the aggregate, but changes in surface energy factors must also be considered. Crushing may increase the number of unbalanced forces, which may increase or decrease the susceptibility to moisture damage depending on the type of aggregate and bitumen. Verbal reports indicated that crushing generally increases the susceptibility to stripping, while weathering generally has the opposite effect (Thelen, 1958.).
The mechanical interlock and contact area between the bitumen and the aggregate can also be varied by manipulating the aggregate gradation. In the literature, the information and the studies about this subject is so limited. Some adjustments in the percentages of the different aggregates may lead less susceptible to moisture damage (Stuart, 1990.).
A higher aggregate porosity generally increases the contact area and bitumen absorption, and thus possibly the mechanical grip. Furthermore, the effect of ling-term absorption of bitumen or specific asphalt chemical functional groups on the susceptibility to moisture damage is unknown. The permeability of the aggregate provides the diffusion of the water from one point to another and this is unwanted factor.
Dust and clay coatings must also be considered because they inhibit an intimate contact between the bitumen and aggregate and provide channel s for penetrating water. It has also been hypothesized that finely divided mineral matter may cause stripping by emulsifying small amounts of bitumen when water is present, but this appears to be an insignificant factor if it occurs (Mathews, 1958.).
An increase in moisture content of the aggregate may also decrease adhesion if the water is not thoroughly dried from the aggregate surfaces or pores. Even if the aggregates are thoroughly dried by the mixing plant, they still may have several molecular layers of adsorbed water, which will decrease the number of unbalanced forces on the surface of the aggregate. The effects of this strongly adsorbed water on adhesion and moisture damage are unknown (Thelen, 1958.).
3.1.2 Type of Bitumen
The stiffness of bitumen can have an effect on moisture damage. The viscosity of the heated bitumen must be sufficiently low during mixing to allow complete coating and absorption. Mixing time is equally important. After coating, stiffer bitumens are generally harder to peel from an aggregate at ambient temperatures or take longer to
peel and thus have more resistance to moisture damage. However, it is unknown if the increased resistance to moisture damage with increased stiffness is really due only to the stiffness or cohesiveness of the binder (Kennedy et al., 1984.).
Studies concerned with the effects of asphalt chemical composition on moisture damage have been limited. Most studies have been concerned mainly with effects of the type of aggregate. Plancher et al. have been studied chemical groups of bitumens and the replacing characteristics of these groups with water (Plancher et al, 1977.). Their studies indicate that asphalt chemical functional groups most easily displaced by water are carboxylic acids, anhydrides, and sulfoxides. However, the mentioned researchers could not manage to develop any modeling about the effects of carboxylic acid and sulfoxide groups on moisture induced damages.
The crude source of the asphalt and the refining process are important because the chemical composition of bitumens within a grade may be very different (Stuart, 1990.).
3.1.3 Mixture Design and Construction
The air void level and the permeability of mixture, which are influenced by the degree of compaction, bitumen content and aggregate gradation, are important because they control the level of water saturation and drainage. In general, the percentage of air voids which can be filled with water increases with an increase in permeable air void level. One exception is with open-graded mixtures where air void levels of 15 to 25 percent allow water to drain (Abson & Burton C, 1966).
Figure 3.4 represents the effect of air void contents on the resistance of mixtures. This ratio can be obtained by the division of indirect tensile stress of conditioned samples to indirect tensile stress of dry (unconditioned) samples.
Figure 3.4 Relationship between air voids and retained mix strength after moisture conditioning according to pessimum voids theory
According to Terrel and Swailmi, the changes of percentages of air voids can be investigated in to three sections (Terrel & Swailmi, 1987).
First one is the mixtures that contain low air voids. This type of mixtures is impermeable due to low air void content. Low air voids in mixtures can be ob tained by increasing the amount of bitumen and adjustment of gradation.
Second one is the mixtures that contain pessimum air void. As a term of pessimum is the opposite of the term of optimum. This type of mixtures decreases the service life and performance of pavements as well as highly sensitive to effects of moisture damage.
Third one is open-graded mixture and enables to drainage of water. 0 10 0 5 10 15 20 R et ai ne d M ix S tr engt h (% ) Air Void % Impermeable Pessimum Air
Void %
Free Drainage
3.1.4 Environment
There are several environmental factors which can affect the degree of moisture damage besides the amount of rainfall and water in pavement.
Pressures and water movements due to freezing and thawing can rapture asphalt films and thus may promote stripping (Way, 1974). Cracks caused by low temperatures or fatigue stresses may promote stripping because they allow the entrance of water.
Temperature can also have an effect. Field experience has indicates that cool rainfalls and rapid drops in temperature while a pavement mixture is being placed or cured can have harmful effects on adhesion. Also, pavements placed in cool seasons may be more difficult to compact, and thus have higher air void levels and permeabilities than pavements placed in warmer weather. This may increase the susceptibility to moisture damage. During the live of the pavement, high temperatures may promote healing in dry weather, although in wet weather, the decrease in viscosity associated with the high temperature may decrease the resistance to moisture damage (Hallberg, 1950).
Aging increases the stiffness of bitumen and thus may decrease the susceptibility to moisture damage. Aging also changes the chemistry of the bitumen and surface energies. The effects of these changes on moisture damage are unknown and are confounded with the increase in hardness and the fact that moisture damage is also time dependent (Stuart, 1990.).
3.1.5 Traffic
Stresses from traffic and the effects of water interact to cause pavement failure (Mack, 1964.). Aggregate edges may be very susceptible to breaking because at these edges, the stress may be high while the film thickness may be low. Mechanical vibrations and pore pressures also may force water into bitumen-aggregate interfaces
(Lottman & Johnson, 1970.). The extent that these two factors have on damage is unknown. Pore pressure is often hypothesized to be major influence on the rate of damage. Traffic also wears and can scour the bitumen coating from aggregates on the surface of the pavement and can create cracks. However, decreased pavement air voids and permeability due to traffic may reduce the susceptibility to moisture damage in some cases.
3.1.6 Anti-Stripping Additive Properties
The use of anti-stripping additives in mixtures can significantly affect the degree of moisture damage. Anti-stripping additives that have been used in practice or tested in the laboratory include:
– Traditional liquid additives – Metal ion surfactants
– Hydrated lime and quicklime – Silane coupling agents – Silicone
Polymer type anti-stripping additives will be discussed in chapter four as the headline called “modifications”.
By the light of mentioned above, the factors that affect the moisture damage can be summarized as follows (Stuart, 1990.):
– The aggregate drying temperature is increased because higher temperatures liberate more water or water vapor.
– The angularity, roughness and absorption of the aggregate increase because tese increase the mechanical interlock.
– The asphalt film thickness increases.
– The mixture air void level and permeability decrease. – Drainage is improved.
– The resistance of aggregate to thermal, freeze-thaw, chemical or other disintegration mechanisms increases.
– Dust coating are removed because dust inhibits an intimate contact between bitumen and aggregate and provide channels for penetrating water.
– Using of anti-stripping additives such as polymers.
3.2 Moisture Damage Tests
Numerous tests are available to determine the moisture susceptibility of a mixture. Some of these test methods are tried to define below.
3.2.1 Static Immersion Test (AASHTO T182/ASTM1664)
The 3/8 in (9.5 mm) to # 4 sieve (4.75 mm) fraction of a coarse aggregate is coated with 5.5 percent bitumen by weight of the aggregate and immersed in distilled water at 25 °C (77 °C). After 16 to 18 hours, the degree of coating is estimated to be either above or below 95 percent.
3.2.2 Boiling Water Test (ASTM D3625)
Boiling water tests are similar to static immersion tests except that the loose sample is either placed in boiling water or water at ambient temperature which is then brought to boil. The amount of coating is estimated after one minute of boiling. Coating degree between aggregates and bitumen can be determined visually by the ratio of coated area to all whole area of aggregates. Figure 3.6 represents boiling water test.
Figure 3.6 Boiling water test
This test is rarely used because of high temperature. High temperature influences the additive characteristics that are used against moisture damage.
3.2.3 Immersion-Compression Test
In this test, cylinders, which are 10.2 cm(4 in) in diameter by 10.2 cm (4 in) in height, are compacted by the double plunger method and a standard level of compaction of 30.7 MPa (3000 psi) for 2 minutes. Specimens to be conditioned are soaked in distilled water at 48.9 °C (120 F°) for 4 days or at 60 °C (140 °F) for 1 day. The specimens are then tested at 25 °C (77 °F) in compression using a rate of 1.27 mm/min (0.05 in/min). A group of unconditioned specimens are also tested at the same time and temperature. Specimens are grouped so that both the dry and wet are approximately equal bulk specific gravities and air void levels. As with most tests which evaluate stripping using a chemical measurement, moisture damage is based on the retained ratio. A retained ratio equal to or above 70 percent is usually required for acceptability.
Figure 3.7 Specimen and apparatus for the immersion-compression test
3.2.4 Conditioning with Stability Test (AASHTO T245)
Conditioning with stability test is applied with same procedure of immersion-compression test. However, there is a difference from AASHTO T 165. While in immersion-compression test, 10.2 cm (4 in) height cylinders are used, in conditioning with stability test, this cylinder is 6.4 cm (2.5 in). In addition, Marshall Test apparatus are used in this test.
3.2.5 Texas Freeze-Thaw Pedestal Test
This test involves subjecting a cylindrical specimen to a series of freeze-thaw cycles until cracking is visually observed. A specimen is first frozen at -17.8 °C (0°F) and placed in an oven with 48.9 °C (120 °F). This freeze-thaw cycle duration is nearly 24 hours. If there is no cracking at the end of this cycle, then the procedure is repeated. If the specimen survives 20-25 cycles with no observed cracking, then it is deemed moisture damage resistant. An obvious disadvantage of this test is that it is extremely time consuming. There is also only a fair correlation between lab and field results (Figure 3.8) (Taylor B A, Comparison of the Evaluator of Rutting and
Stripping of Asphalt with the Rotary Asphalt Wheel Tester, M.Sc. Thesis, Arkansas Tech University, 2002).
Figure 3.8 Fixed base Texas Freeze-Thaw Pedestal Test apparatus.
3.2.6 Hamburg Wheel Tracking Test
Test methods have been developed that combine moisture with cyclic traffic loading such as the Hamburg Wheel Tracking Device (HWTD) shown in Figure 3.9. The HWTD includes cyclic loading conditions and saturation of compacted asphalt mixtures. The HWTD is used to predict permanent deformation potential and moisture damage of HMA. The samples used for the test are typically 260 mm (10.25 in.) wide, 320 mm (12.5 in.) long and 38 mm (1.5 in.) to 100 mm (4 in.) thick. The sample is compacted to 7.0 ± 2.0 percent air voids or some other designated air void content. The samples are submerged in water at 50 °C (122 °F), but the temperature can be specified within a range from 25 °C to 70 °C (77 °F to 158 °F). A steel wheel 47 mm (1.85 in.) wide is rolled across the surface of each submerged sample at a load of 705 N (158 lbs). The wheel passes over each sample fifty times per minute at a maximum velocity of 34 cm/sec (1.1 ft/sec) in the center of the sample. Each sample is loaded for 20,000 passes or until the average linear variable displacement transducer (LVDT) displacement is 40.90 mm (1.6 in.). The test takes approximately six and a half hours (Aschenbreher et al., 1995).
The rut depth (i.e. deformation) is plotted versus the number of passes and the results usually show a curve with two distinct steady-state portions, see Figure 3.10. The first portion denotes the creep (i.e. rutting) slope and the second portion begins when there is a sudden increase in the rate of deformation.
Figure 3.10 Hamburg Wheel-Tracking Device Test Results 3.2.7 Environmental Conditioning System (AASHTO TP 34)
This method was developed under SHRP Project A-003A. This method uses samples that are 102 ± 2 mm in diameter and height and compacted to 7.5 ± 0.5 percent air voids (Figure 3.11). They are then placed in a latex membrane and sealed with silicone. The specimen is placed in an ECS loading frame where the air permeability and the dry unconditioned resilient modulus (MR) are measured. The
specimen is then vacuum-saturated to determine the water permeability. The next step is dependent on the geographic climatic region where the HMA will be used. In warm climatic regions three cycles of a six hour hot cycle is used, and for the cold regions a cold cycle is added. The specimens are subjected to cyclic loading during the test which is intended to simulate traffic. The specimens are subjected to cyclic loading during the test which is intended to simulate traffic. If the ratio is less than 0.70, then the sample fails the test and is considered moisture susceptible. An indirect tensile strength can be measured at the completion of the all cyclic loading
tests. The sample may also be broken apart to provide a visual determination of the severity of the stripping. Advantages of this method are that it is nondestructive and a direct comparison of MRs can be seen on the same specimen. A couple of disadvantages of this method are that it is not as severe as the Modified Lottman Test and this method is more expensive than comparable methods (Taylor, 2002).
Figure 3.11 Load frame inside environmental cabinet
3.2.8 Standard Method of Test for Resistance of Compacted Hot-Mixed Asphalt (HMA) to Moisture-Induced Damage (Modified Lottman Test) (AASHTO T 283)
The aim of the modified Lottman Test is to evaluate susceptibility characteristics of the mixture to water damage. This test is performed by compacting specimens to an air void level of 7% ± 1.0. Three specimens are selected as dry (unconditioned) and tested without moisture conditioning; and three more are selected to be conditioned by saturating with water (55%–80% saturation level) followed by a freeze cycle (-18 °C for 16 h) and subsequently having a warm-water soaking cycle (60 °C water bath for 24 h). The specimens are tested for indirect tensile strength
(ITS) by loading the specimens at a constant rate (50 mm/min vertical deformation at 25 °C) and the force required to break the specimen is measured.
CHAPTER FOUR MODIFICATIONS
This chapter consists of the aim of modification, the materials that in use modification process and polymers.
4.1 Definition and Aim of Modification
Modification can be defined as mixing of the additives to the bitumen for improving the binder ductility and the aging characteristics of a binder, leading to a more durable pavement (Glover, 2006).
Bitumens have two important properties which provide the using as binder, being thermoplastic and visco-elastic.
Visco-elastic means that when a force is applied, the structures making-up the bitumen will distort as well as flow, and this feature of bitumen can observed in hot-mix asphalt (HMA). Viscous flow is irrecoverable movement. Distortion is coverable movement and describes elastic behavior. The relative amount of viscous and elastic response that bitumen exhibits to an applied force depends on its chemical make-up and its temperature. More viscous materials at lower temperatures will respond more elastically to a short-time applied force. The material will deform rather than flow. Less viscous materials at higher temperatures, and under the action of a force applied for some time, will flow. On removal of the applied force, the material will exhibit little recovery of shape.
Thermoplastic means that the viscosity of bitumens reduces on heating and increases on cooling. The process is reversible (Hunter, 1994). Most of the pavement deformations are related with these properties of bitumen and mixture.
For many years, researchers and development chemists have experimented with modified bitumens, mainly for industrial uses, adding asbestos, special fillers,
mineral fibres and rubbers. In the last thirty years many researchers have looked at a wide spectrum of modifying materials for bitumens used in road construction. Table 4.1 details the majority of bitumen modifiers and additives that have been examined.
Table 4.2 contains a summary of some of the modifiers used with bitumens and asphalts in service situations and (√) indicates where they enhance the performance of the asphalt.
Table 4.2 benefits of different types of modifier
Improvements made by adding polymers to bitumens icluding (Choquet & Ista, 1992.):
– Increasing the viscosity of the binder in service, – Reducing the thermal susceptibility of the binder,
– Widening the range of plasticity (difference between ring and ball softening temperature and Fraass’ breaking temperature),
– Increasing the cohesion of the bitumen,
– Increasing the resistance to the permanent deformations, – Improving the resistance to the fatigue at low temperatures,
– Improving the binder-aggregate adhesion (higher viscosity of the binder), – Slowing down the aging process (thicker film of binder around the aggregate)
For the modifier to be effective and for its use to be both practicable and economic, it must;
– be readily available,
– resist degradation at asphalt mixing temperatures, – blend with bitumen,
– improve resistance to flow at high road temperatures without making the bitumen too viscous at mixing and laying temperatures or too stiff or brittle at low road temperatures,
– be economical.
The modifier, when blended with bitumen, should:
– maintain its premium properties during storage, application and in service – be capable of being processed by conventional equipment
– be physically and chemically stable during storage, application and in service – achieve a coating or spraying viscosity at normal application temperatures.
4.2 Types and Applications of Polymers
The properties of bitumen and mixtures can be improved by various bitumen modifications such as sulfur, organic polymers and rubbers, the advantages of which were indicated by researchers. Most of the using bitumen modification is polymer addition (Whiteoak & Read, 2003.). The classification of polymers are given in Table 4.3.
Table 4.3 Classification of Polymers
Notation Name Type
SBS Styrene-Butadiene-Styrene Elastomer
SBR Styrene-Butadiene Rubber Elastomer
EPDM Terpolymer Of Ethylene Propylene And Dipolyene Elastomer Plastomer
NR Natural Rubber Elastomer
PE Polyethylene Plastomer
PP Polypropylene Plastomer
APP Atactic Polypropylene Plastomer
IPP Isotactic Polypropylene Plastomer
PVA Polyvinyl Acetate Plastomer
EVA Ethylene Vinyl Acetate Plastomer
EMA Ethylene Methyl Acrylate Plastomer
PIB Polyisobutylene Elastomer
PVC Polyvinyl Chloride Plastomer
PS Polystyrene Plastomer
SIS Styrene-Isoprene-Styrene Elastomer
Polymer is a derived word meaning "of many parts". Polymers can be thought of as long chemical strands that are made up of many smaller chemicals (monomers) that are joined together end-on-end. Polymers can therefore be made up of different numbers of the monomers and therefore they can have different 'chain lengths'. Only certain chain lengths may be suitable for a particular polymer type when used in bitumen. For example, the polymer 'polystyrene' is made up of many styrene molecules linked together one after the other. A copolymer has two different sorts of repeating molecular units. Block copolymers have these repeating molecular units in a regularly occurring block pattern.
The physical and chemical properties of a polymer will depend on the nature of the individual molecular units, the number of them in each polymer chain and their combination with other molecular types. Consequently, the different polymers
behave in different ways and generally the different PMBs have to be tried out in bitumen applications before they can be considered suitable (British Petrol, 1997.). The most commonly used polymers are given in Table 4.4.
Table 4.4 Most commonly used polymers for the modification of bitumen Type of Polymer Widely Known Name Areas of Usage
SBS Copolymer Thermoplastic Rubber (Elastomer)
Performance of lower temperature Performance of higher temperature Surface treatments and isolations Hot mixtures
Filling cracks
EVA® Copolymer
EBA Copolymer Thermoplastics (Plastomer)
Performance of higher temperature Surface treatments and isolations Holding of aggregate
Hot mixtures Rutting resistance Filling cracks Polyethylene Thermoplastic Rubber Rutting resistance SBR
Performance of lower temperature Filling cracks
Holding of aggregate Rutting resistance Natural Rubber (Latex) Rubber Holding of aggregate Filling cracks Epoxy Resins Thermo set
Rutting resistance Hot mixtures
Higher skid resistance
Two basic types of polymer are used in modifying bitumen for road applications:
– Elastomers (thermoplastic) – Plastomers
4.2.1 Elastomers
Of the four main groups of thermoplastic elastomers – polyurethane, polyether– polyester copolymers, olefinic copolymers and styrenic block copolymers – it is the styrenic block copolymers that have proved to present the greatest potential when blended with bitumen (Bull & Vonk, 1984.).
Styrenic block copolymers, also termed thermoplastic rubbers or thermoplastic elastomers, may be produced by a sequential operation of successive polymerisation of styrene–butadiene–styrene (SBS) or styrene–isoprene–styrene (SIS). Alternatively, a di-block precursor can be produced by successive polymerisation of styrene and mid-block monomer, followed by a reaction with a coupling agent. Thus, not only linear copolymers but also multi-armed copolymers (known as star-shaped, radial or branched copolymers) can be produced; these are often referred to as radial or branched copolymers, as shown in Figure 4.1.
Figure 4.1 A linear and branched thermoplastic elastomer
Thermoplastic elastomers derive their strength and elasticity from a physical cross-linking of the molecules into a three-dimensional network. This is achieved by the agglomeration of the polystyrene end-blocks into separate domains, as shown schematically in Figure 4.2, providing the physical cross-links for a three-dimensional polybutadiene or polyisoprene rubbery matrix. It is the polystyrene end-blocks that impart strength to the polymer and the mid-block that gives the material its exceptional elasticity (Vonk & Van Gooswilligen, 1989.). At temperatures above the glass transition point of polystyrene (100 °C) the polystyrene softens as the
domains weaken and will even dissociate under stress, thus allowing easy processing. Upon cooling, the domains re-associate and the strength and elasticity is restored, i.e. the material is thermoplastic.
The quality of the polymer dispersion achieved is influenced by a number of factors (Collins et al., 1991):
– the constitution of the bitumen
– the type and concentration of the polymer – the shear rate applied by the mixer.
When the polymer is added to the hot bitumen, the bitumen immediately starts to penetrate the polymer particles causing the styrene domains of the polymer to become solvated and swollen. Once this has occurred, the level of shear exerted on the swollen particles is critical if a satisfactory dispersion is to be achieved within a realistic blending time. Thus, medium or, preferably, high shear mixers are required to adequately disperse thermoplastic elastomers into the bitumen.
