The development of modern highways has always depended upon the material available to build them
The development of modern highways has always depended upon the material available to build them. Early attempts to seal a pavement with tar or pitch led to the use of bitumen and to the high performance materials currently in use worldwide. This process of development continues today for improvement of the existing materials to perform better by modification through addition of additives and admixtures.
The primary function of the pavement is to give the users a smooth, comfortable and safe ride at economical cost. One of the main drawbacks of bituminous pavement materials is that they combine `elastic’ and `plastic’ behavior, that is, when they deflect under load, a small part of the deflection becomes permanent. After individual loads these permanent deformations are practically invisible, but repeated loading of this effect can lead to rutting. The phenomenon is particularly prevalent in warm climates where the growing weights or tire pressures of the transport vehicles are present.
The behavior of flexible pavement is very complex due to the inter-relation between factors influencing its performance. Some of the major observed asphalt pavement problems can be listed as (Thompson et. al. 1983):
- Thermal and fatigue cracking
- Hardening of binder
- Moisture susceptibility and stripping
In order to deal with these problems, use of different types of additives in asphalt concrete mix is in use in various countries. For example, different types of “Filler Material” available are one such type of additive, which is known to affect the properties of the Asphaltic Concrete (AC) mixes.
Therefore any refinement of knowledge for use of such additives in AC mixes and their potential benefits, will find a good place in today’s world where lot of concern exist for these in widespread problems of major importance.
1.2 PROBLEM DEFINITION
Bangladesh is climatically a tropical country with combination of all type of weathers and an average 2000 mm of precipitation annually. To provide communication between the towns, huge investment has been placed in constructing quality roads that covered great distances, under extreme climatic and topographical conditions. Road pavement of the national, regional and major feeder roads is mainly of AC surfacing and designed for a design life of 15-20 years before any major maintenance and rehabilitation is needed. However, during the past few years, these roads with AC layers have been experiencing early distress and deterioration leading to failures ultimately.
Apart from heavy axle loads, high tire pressure and climatic conditions, use of locally available low quality aggregate in road construction is one of the major contributors to failures. The construction industry faces a difficult situation in finding good performance aggregate. On highways and urban roads many damaged spots can be seen after the seasonal rains, especially in eastern half of the country where aggregate are weaker comparatively and sensitive to water. Since the transportation of good quality aggregate from nearby country is uneconomical, in the eastern half of the country. Therefore, certain modifications in AC mixes produced by using the local aggregate of Bolaganj source is needed to ensure a durable mix by incorporating additives.
There are a number of factors which may affect the performance of an AC layer. The major factors known to affect the material characteristics and behavior under traffic loading are; asphalt type and content, temperature variation, aggregate type and gradation, air voids, mix density, filler type and wheel loading or stress level. Among these the asphalt content and quality of the AC mix has direct effects on the stability and durability.
Considerable research and development has been done to achieve a mix which can satisfactorily resist the major distresses and water sensitively problems in pavements. One of the major steps towards this is achieved by incorporating additives in AC mixes to improve its temperature and water susceptibilities, especially for extreme and tropical climate regions. Use of additives to significantly improve the properties of the AC mixes such as temperature and water susceptibilities, strength and durability had been reported by researches in countries like USA, India and Saudi Arabia (Ronald et. al. 1989). Such promising results could present a cure for different types of distress and deterioration in the pavement in Bangladesh.
Among various types of additives and modifiers “Filler Material” is one, which is considered to improve the AC mix properties without affecting much on the overall economy AC pavements. There are different kinds of filler material available and is in use in different regions depending upon the improvement needed and the relevant functions they provide. Although different kind of filler used in AC mixes may result in performance improvements or better economy. However, each one has it’s own limitation. For example:
a) Hydrated lime is widely used as an anti-stripping agent in AC mix, but it slightly increases the bitumen requirement in the mix thereby affecting economy. However, the achieved benefits in the improvement of properties and durability are considerable and feasible.
b) Asbestos fiber is reported to be an excellent filler material in AC mixes, but due to health hazard the use is discouraged References
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demand Resources 1983).
Therefore, correct selection and use of a particular type and amount of filler additive among various available and new emerging products becomes important to ensure a properly designed AC mix as per the local existing environmental and loading conditions.
The study is designed to investigate the engineering properties of AC mixes modified with hydrated lime as part of the filler with regards to effectiveness and look for the improvements obtained as compared to conventional mix without lime.
1.3 OBJECTIVES OF THE STUDY
Bangladesh has a very limited choice of suitable locally available stone sources for producing aggregate for road pavement. The imported stone is consumed mainly in the Western part of country due to cost effectiveness. In the Eastern part of the country the local available stone of Bolaganj (Sylhet) is mainly used in aggregate production for the road pavement. Bolaganj stone have been reported to have an affinity towards water due to its hydrophilic character and AC mixes produced by using aggregate of these stone is susceptible to moisture and less durable thereby. In consideration of high annual rainfall and long rainy season, a necessity of modification of AC mixes is a need to control the moisture susceptibility aspects, which leads to stripping and raveling in the AC surfacing.
Therefore this study is taken up with an objective to:
i) To review the available literature on the moisture susceptibility problems in the AC mixes, the types of the additives and modifiers and the effects of these on AC mixes in general and hydrated lime as modifier of AC mixes in particular
ii) Conduct laboratory tests on the specimens of AC mix without lime and modified AC mixes in varying percentage of hydrated lime to determine the engineering properties with respect to water susceptibility aspects.
iii) Compare the properties of AC mix without lime and modified AC mixes with lime to evaluate the effects on moisture susceptibility behaviour and come up with the conclusions.
1.4 SCOPE AND APPROACH OF STUDY
The findings of the study shall be applied in using the modified AC mixes with lime produced by using aggregate of crushed Bolaganj stone boulder with confidence in actual works to check and control the problems arising from water.
The scope of the study involves the following tasks:
1. Material characterization i.e., testing and evaluation of aggregate of Bolaganj origin and bitumen of Eastern Refinery (Bangladesh) in the laboratory for their quality and conformance with the specifications.
2. Mix design using Marshall Method to arrive at the optimum mix for the AC wearing course gradation of RHD specification (2001).
3) Study characteristics of the modified AC mixes with hydrated lime in different percentages and carry out comparative analysis with the control optimum AC mix without lime.
To achieve the study objectives, a systematic approach consisting of three main interconnected phases have been framed:
The first phase consists of material collection and characterization to evaluate their quality with respect to conformance with applicable specifications. The second phase involves conducting mix design by Marshall Method and laboratory tests on control optimum AC mix without lime and modified AC mixes with lime in different percentages related to moisture susceptibility. The third phase involves data analysis, interpretation of results, conclusions and recommendation.
1.5 THESIS LAYOUT
Chapter 1 presents the an overview of the problems being experienced with respect to moisture susceptibility in context of Bangladesh in AC mixes, the objectives, scope and approach of the study to arrive at conclusions.
Chapter 2 is on a review if the literature on the subject matter with regards to modification of AC mixes by various additives in general and by modification using hydrated lime in particular. The chapter also covers the findings of researchers in the past to take account of those in the current study
Chapter 3 presents the details related to characterization tests on constituent materials, mix design of AC mix for optimum mix and testing of Optimum AC mix without lime and modified AC mix with lime in varying doses for moisture susceptibility aspects.
Chapter 4 presents test results, their analysis and evaluation adopting statistical approach and discussions for final conclusions on effects of modifications of AC mixes by lime.
Chapter 5 presents the final findings and conclusions of the study. Recommendations for further researches and studies are also included in this chapter.
The concept of modifying asphalt mixtures is certainly not new, but has become much more prominent during the past few years due to frequent occurrence of problems in respect of early distresses and deteriorations in Asphalic Concrete (AC) mixes. To address theses problem for remedy one of the techniques being practiced in modern to modify the AC mixes by incorporation of various additives and admixtures. The factors that have some influence on an increased interest in modifying AC mixes include at least the following:
- Traffic factors have increased including heavier loads, higher volume, and higher tire pressures.
- To accommodate the shift from larger projects such as the Interstate System to smaller projects such as maintenance of the existing road network.
- Higher costs have created a tendency to construct thinner pavements, thus reducing the service lives of pavements.
- Environmental and economic pressure to dispose of certain industrial waste materials (i.e., tires, glass, ash etc) has prompted the idea of concerting them to additives in AC mixes.
A family of products and processes are aimed at a variety of pavement application. The highway engineer knows that the complexity of pavement distress requires a choice of repair or rehabilitation options (one or two methods may not suffice). The appropriate modification of AC mixes has broadened the choices available to the engineer.
Engineers who are familiar with the field performance of bituminous pavements generally agree on three potential modes of distress (Ronald et. al. 1989):
i) Repeated load (fatigue cracking)
ii) Non-load (thermal cracking)
i) Raveling (Loss of adhesion)
ii) Stripping (moisture damage)
Although most bituminous pavements perform satisfactorily, problems still do occur. Consequently, there is an increased interest in making changes that include several possibilities:
- Improved pavement design (structural, damage, materials, etc.)
- Revision of specifications for paving materials and pavements.
- Improvement in the quality control of construction
- Improvement of binders systems through modifications by additives.
All of the above will contribute in improving the performance of the pavement system. However, the improvement of binder system through modification by additives has gained a primary interest (Terrel et. al. 1986)
2.2 ADDITIVE AND MODIFIERS
An additive to the AC mix is a material which would normally be added during mix production, to improve the properties and/or performance. A bitumen extender is an additive which replaces a part of the mineral filler that would normally be used in the AC mix, and may additionally result in performance improvements or better economy (Terrel et. al 1986).
The justification or reasons for using an additive or extender would include the following:
- Solve or alleviate a pavement problem
- Realize some benefits such as:
iv) Application and Performance
Vehicle weights, traffic volume, and tire pressures are steadily increasing and demanding more and more from the pavement structures. Engineers face with serious problems with the quality of paving material. Often aggregates are transported from long distances at high cost because local aggregate supplies of high quality have been depleted. As a result, additives to AC mixes have been widely accepted by the paving industry for the present time. The concept of additives is logical, and results from laboratory testing look positive. Even though field test results using many additives are incomplete, many of those responsible for pavement quality are willing to use because the results appear to be favorable (Joe, W.B., 1991).
2.2.1 Types of Additives and Modifiers
The generic classification has led to the following types of additives (Ronald et. al 1989):
- Crumbed Rubber
- Anti-stripping agents
Each of the additives noted above provides benefits and improvements to the AC mix, either actual or perceived. The impetus to use one or more of these modifiers is generally based upon several factors. For example, a user agency may have a particular pavement problem and is in need of a solution. They in turn seek out additives or modifiers that provide some hope. Another approach has been to seek new markets for materials that are already available and have traditionally been used in other applications (Terrel et. al. 1986).
2.2.2 Filler as Additive and Modifier
Any fine powder added to bituminous mixture in the course of manufacture, and which has been processed to such a degree of fineness that not less than 85 percent by weight passes a 0.075 mm sieve is called “Filler” (Khanna, P.N., 1992).
Examples of Filler are:
1. Mineral Fillers:
i) Crusher fines/dust
iii) Portland Cement
iv) Fly ash
v) Granite dust
2. Fiber material filler:
i) Natural Fibers:
a) Asbestos (Hazardous)
b) Rockwool (Non-Hazardous)
ii) Man-made Fibers:
c) Steel Fibers
i) Carbon Black
iii) China Clay and Fuller’s earth
Mineral Fillers: They are generally considered to be fine inert mineral materials a high proportion (at least 65 percent by ASTM and AASHTO specifications) of which will pass the 0.075 mm sieve”
The description is improved by adding a statement to the effect that filler is important because of the surface area involved, and that properties of a pavement which may be improved by the use of filler include strength, plasticity, amount of voids, resistance to water action, and resistance to weathering. In short, if filler is to be adequately described it is necessary to turn to the literature to try to determine what others have learned about it, or to attempt independent analysis in the laboratory and field.
Fiber material filler: Fiber provides some sort of reinforcement in the AC mixes. They also provide a finely divided material in the mix with a high surface area that permits the application of thicker than normal films of asphalt cement on the aggregate (Ronald et. al., 1989).
Natural, Synthetic and Steel Fibers have all been used in AC mixes. The usual approach is to incorporate very fine, short Fibers into the AC mixture, depending upon their form, chemistry, and intended function.
According to the research, fiber linkage is a mechanism that may explain the resistance to rutting which asbestos imparts in bituminous paving mixtures. Selective adsorption on the short chrysotile asbestos fiber could bond or link together the heavy viscous bitumen fraction. Pavement stability against rutting would then depend on the strength of the heavy fraction, the amount present in the paving mix, and the proportion adsorbed by the asbestos fiber (Thomos et. al., 1962).
Apart from asbestos, non-hazardous Rockwool fibers have been used in Great Britain and France, where fibers were added to the bituminous mixes during mixing, it was reported that it improved resistance to reflective cracking, deflection and there were no construction constraints (Terrel et. al., 1986).
2.2.3 Influence of Mineral Filler on AC mixes
Extensive research, most of it from the early part of the century, has been done on the properties of mineral filler and its influence on asphalt concrete mixtures.
Richardson, C, (1941) was one of the first investigators to report on the effects of mineral fillers. He postulated that the function of the filler is more than mere void filling, inferring that some sort of physico-chemical interaction occurs when fine mineral dust is added to AC mixes.
Traxler, R. N., (1937) considered size and size distribution as fundamental filler properties that affect the void content. More recent work by Traxler confirms his earlier findings (Anderson el. at., 1982).
Mitchell et. al., (1939) also attempted to find a single parameter that would adequately predict the ability of mineral filler to stiffen the bitumen in the AC mix. The data were obtained for mineral filler asphalt mixtures with relatively small concentrations of solids. The results indicated that the bulk settled volume of filler in benzene is a good predictor of the performance of the mineral filler.
A very extensive series of experiments on mineral fillers and mineral filler binder system has been reported by Rigden, (1954). In particular, he studied the relationship between filler properties and the viscosity of mineral filler binder mixtures. At filler bitumen ratios similar to those found in typical asphalt concrete mixtures, the fillers stiffened the asphalt by as much as three orders of magnitude. His data also indicate that fillers affect the temperature and water susceptibility of the AC mixes, however, the stiffening effect did not correlate with any of the fundamental properties of the fillers.
The theology of mineral filler asphalt systems has been studied by Winniford, R. S., (1961) using the sliding plate micro-viscometer. Winniford suggested that the role of the filler is more than volume filling, and postulated additional stiffening mechanisms including.
1) A gelatin of the asphalt by the mineral surface, which increases the non Newtonian flow characteristics and lowers temperature susceptibility.
2) Formation of thick viscous coatings which increase the effective solids concentrations, and
3) Surface shielding by adsorbed asphaltenes. It was also shown that the stiffening effect of the mineral fillers was more pronounced with smaller sized materials.
Warden et. al., (1959) presented data on filler asphalt mixes in conjunction with filed observations. This study was motivated by field failures that were attributed to filler type. An easily measured parameter was sought that would predict the performance of the filler in the field. The tests performed on the fillers were empirical tests in use in the late 1950’s. A reexamination of the early work by Traxler again demonstrated that no single parameter was sufficient to predict the behavior of different mineral fillers. The softening point of the filler bitumen mixtures was found to be critical with respect to filler type.
Puzinauskas, V. P., (1968) reporting on The Asphalt Institute study of mineral filler concluded that the mineral filler plays a dual role in bituminous mixes. Stated that “they are part of the mineral aggregate and they fill the interstices and provide contact points between larger aggregate particles, when mixed with asphalt mineral fillers form a high consistency binder or matrix which cements larger aggregate particles together”.
Craus ET. al., (1978) dealt with the effect of the physicochemical properties of filler on mix performance. In particular, they examined the geometric characteristics (shape, angularity, and surface texture), adsorption intensity at the filler asphalt interface, and the selective adsorption of the filler asphalt system. They concluded that the physicochemical interaction between filler and bitumen increased with the adsorption intensity, geometric irregularities, and selected adsorption of the fillers. The authors concluded that a single test on mineral filler cannot be expected to predict the behavior of the filler in the bituminous mixes.
2.2.4 Theory of Filler
Two fundamental theories, based on the results of studies, observations, and experience, have emerged regarding the functions of fillers in bituminous mixes:
Filler theory: The filler theory postulates that “the filler serves to fill voids in the mineral aggregates and thereby create a denser mix”. This theory presumes that each particle of the filler is individually coated with bitumen and that such coated particles, either discrete or attached to an aggregate particle; serve to fill the voids in the aggregate. By virtue of such filling of voids, mixes of higher stability and density can be attained (Ladis H. Csanyi, 1962).
Mastic Theory: The Mastic theory proposes that he filler and bitumen combine to form mastic, which fill the voids and also bind aggregate particles together into a dense mass. When filler is added to bitumen, part of it will have a mechanical function where physical contact is not established and then filler and bitumen work together in the form of what can be called a binder. This finest portion of filler will be suspended in the asphalt, changing the properties of binder films. It will act as filler within the bitumen itself, since it will replace a certain amount of asphalt in the mixture. Mastic of this type is harder, stiffer, tougher, and possesses lower temperature and water susceptibility than the original bitumen (Hamad I. Al Abdul Wahab, 1981).
2.2.5 Filler Attributes
The desired practical and functional quality attributes in a filler material should include the following (Warden et. al., 1959):
The filler in the bituminous mix must be non critical. Variations in the filler content which may be expected under normal plant operation must not cause undesirable fluctuations in the physical properties of the pavement. The yardstick or means of judging is the sensitivity of all the following quality attributes as a function of filler–bitumen ratio.
The quantity of filler desired for functional reasons must not unfavorably affect the mixing, placing and compaction of the bituminous mixture. In other words at the desired concentration to meet design criteria the mortar softening point or consistency must not be so high that the mix is unworkable.
Added mineral filler should be economical (availability and cost) and should be readily transported, stored, proportioned and mixed with customary equipment. Yardsticks for storing and proportioning are that the filler be non-hygroscopic and do not form lumps or cake or bridge in the bins.
A completed pavement surfacing must be stable and durable over a wide range of temperature and over an extended period of time. This means that from the functional viewpoint the type and quantity of filler in the bituminous mixture must be such that the optimum void is maintained within the desired limits, both initially and after ultimate compaction, and that there is sufficient resistance to deformation by traffic at the highest service temperature. Concurrently the filler must not decrease the resistance to water or the bond of the bitumen or mortar to the aggregate and must not decrease durability through loss of flexibility by inducing cracking of the pavement.
2.2.6 Role of Filler in AC mixes
In general the functions of filler can be listed as follows (Khanna., P. N., 1992):
1. To increase the viscosity of the binder and hence increase density and stability of the mixture.
2. To enable a thicker film of binder to be held by the mixes.
3. To improve the resistance of the binder to weathering.
4. To increase the effective volume of the binder
5. To reduce the apparent temperature and water susceptibility of the mixture (for dense surfacing-filler/binder mixtures have lower temperature and water susceptibility than straight binders of the same viscosity).
6. To reduce the brittleness of a mix in cold weather, where the quantity of the filler can be considerably increased.
7. To obtain a close texture on the surface after compaction
Researchers have related the void properties of the filler to the Marshall Mix properties. For example, Hudson et. al., (1962) have related the activity coefficient to Marshall Stability. The activity coefficient is defined as the bulk volume of the filler to the solid volume of the filler. The bulk volume of the filler was determined from the settled volume of the filler in kerosene. For a given mixture, it was found that the activity coefficient is related to Marshall Stability. It was concluded, however, that the stability is a function of both filler type and concentration.
Craus et al., (1978) concluded that the physicochemical interaction between filler and bitumen increased with the adsorption intensity, geometric irregularities, and selected adsorption of the fillers. These effects strengthen the filler bitumen bonds producing a mixture with a higher strength.
Summarizing the key points from the state of the art review on mineral fillers it can be concluded that:
1. Mineral fillers stiffen asphalt, and the degree of stiffening varies significantly between different fillers.
2. For a given filler source the finer the filler the greater the stiffening effect.
3. Although performance varies for different fillers, there are no exact tests that can adequately predict their performance.
4. Different fillers may react differently with different bitumen
2.3 LIME AS FILLER AND MODIFIER TO AC MIX
Hydrated Lime, Calcium Hydroxide Ca (OH)2, commonly used in soil stabilization have also traditionally been used in AC mixes as a filler to improve the properties. Lime perhaps has special binding qualities in addition to the role of filler. It has been used for the purpose of providing stiffening or reinforcement to the bitumen as well as ‘Filling in’ the voids in the aggregate matrix (ES-2, Asphalt Institute).
“Hydrated lime” is a dry powder obtained by hydrating quicklime with enough water to satisfy its chemical affinity, forming a hydroxide due to its chemically combined water (Robert S. Boynton, 1980). It has a surface area of 17-24 m2/gram.
2.3.2 Effect of Lime in AC Mixes
Hydrated lime has gained considerable recognition as a useful additive for improving the performance of bituminous pavements. It is added to some low grade aggregate to render them suitable in bituminous mixes for use in highway construction. Sometimes, it is difficult to coat certain aggregate with bitumen because of their siliceous or acidic surfaces. Hydrated lime, which is highly alkaline, starts a chemical reaction that changes the character of the aggregate surfaces and neutralizes any acidic properties present in the aggregate. Adding hydrated lime often improves the coat-ability and bonding properties of bitumen to these aggregate (Robert S. Boynton, 1980).
Thomas W Kennedy et. al., (1984) in their study on “Techniques for reducing moisture damage in AC” reported that hydrated lime has been found to be a very effective additive. Indirect Tensile test results indicated that hydrated lime was effective in reducing stripping and moisture damage effects.
Plancher, et. al., (1977) in their research suggested that hydrated lime absorbs carboxylic acids in the bituminous mixes which increase the water resistance and asphalt aggregate bonds.
Welch et. al., (1977) studied effect of hydrated lime on bitumen and aggregate mixtures and found that hydrated lime changes the mechanical properties of the mixtures. It has been shown by investigators that the addition of a small quantity of basic oxides such as calcium hydroxide, calcium oxide, and Portland cement helps to maintain adhesion in the presence of water, and retard oxidative hardening (Ishai et. al., 1977).
The report on “Lime Treatment of Asphalt Mixes to reduce age hardening and improve flow properties” by a distinguished scientist, Peterson J. Claine indicated that lime treatment on AC mixes reduced asphalt age hardening, increased the high temperature stiffness of un-aged asphalts, reduced the stiffness in aged asphalts at higher temperatures and increased the asphalt tensile elongation at low temperatures. These effects benefit asphalt pavements by increasing asphalt durability, reducing rutting, shoving and other forms of permanent pavement deformation, improving fatigue resistance in aged pavements, and improving pavement resistance to low temperature transverse cracking. These benefits are in addition to the well documented effect of lime in increasing the resistance of pavements to moisture damage (Peterson, J Claine., et. el., 1987).
2.3.3 Method and stage of lime addition
The batch of mineral aggregate shall be dried to 300o F, the required quantity of additive shall be added to the aggregate, and the entire mass shall be thoroughly mixed until a uniform distribution of additive has been achieved. Care shall be taken to minimize loss of lime to the atmosphere in the form of dust. It is unified that the addition of hydrated lime to AC mixes does increase stability and reduce the hardening rate of bitumen present in the mixes.
Hydrated lime is usually added to aggregate at the pug-mill. It may serve as filler in the aggregate material. Researches have established that the addition of hydrated lime can increase bitumen content in the AC mixes over the normal bitumen content without risks of raveling or bleeding in the completed pavement. This produces a firmer, denser and more durable surface and considered effective in improving the water resistance of asphalt concrete (Peterson, J Claine., et. al., 1987).
2.4 MIX DESIGN AND EVALUATION METHODS
The major properties of AC paving mixtures are stability, durability, flexibility and skid resistance (in case of wearing surface). The mix design methods are the process and procedures to establish the aggregate particle size distribution and to determine the corresponding design asphalt content that would let the AC mix to perform satisfactorily, particularly with respect to stability and durability aspects.
Stability is defined by many engineers is the “resistance to deformation” with an implied emphasis towards resistance to flow or rutting, including resistance to tensile, compressive, and shear stresses that causes failure in a pavement surface. Durability has been defined as the resistance to the effects of weather and its combination with other forces. Durability is enhanced with high content of bitumen, however, resistance to flow or deformation is impaired with high bitumen content. As a consequence, the amount of bitumen to be used in a bituminous paving mix must be in a balance to optimize durability but yet maintain adequate stability (Jimenez, R. A., 1986).
There are many mix design methods used throughout the world such as Marshall Mix design method, Hubbard field mix design method, Hemet Mix design method, Asphalt Institute’s Triaxial method of mix design etc. Out of these Marshall Mix design method is used in this study and discussed in detail.
2.4.1 Mix Design by Marshall Method
The Marshall procedure as applied to design and control of bituminous mixtures used was evolved during the period from World War II to late 1950’s by U. S Army Corps of Engineers. Motivation for its development came from the need for a mix design procedure to proportion aggregate and bitumen border to sustain increasing wheel load and tire pressure of military aircrafts during World War II. In order to achieve these needs, Corps began an investigation to select a test apparatus that was simple and easily portable and could be used in the field for control purposes. The second phase of this study was to determine the method of compacting laboratory specimen in order to achieve the density as that obtained in field. The third phase of this investigation was the establishment of satisfactory design criteria and control procedure (Ziauddin, A. Khan, 1988).
The Corps of Engineers selected a testing machine and a method of bituminous mix design conceived by Bruce Marshall of Mississippi State Highway Department. The Marshall Test procedure has been standardized by the American Society for Testing and Materials by ASTM designation D-1559 “Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus”. The procedure and design criteria, is adopted by RHD with some modifications to suit the environmental conditions in Bangladesh and shown in Table 2.1.
The use of these criteria must be limited to hot bituminous paving mixes using penetration grades of bitumen and containing aggregate size of 1 inch or less. The corps of Engineers found that, in order to have the proper balance between durability and stability, the air voids in the total mix should be limited to between 3 and 5 percent and the voids in the aggregate mass filled with bitumen 75 and 85 percent. The RHD standards requires the air voids to be between 3 and 5 percent for AC wearing course mixes and between 3 and 7 percent for base coarse mixes.
Since its development in 1940’s the Marshall method has increasingly been accepted by highway agencies throughout the world to design and control the bituminous paving mixtures. A review of literature indicates that Marshall Stability is a measure of tensile strength. Smith V. R, (1984) wrote in his research paper that the Marshall Stability values are affected primarily “by the tensile strength or cohesion properties of a mixture”. Benson, (1952) found a linear relationship between Marshall Stability and cohesion-meter value. It would seem to be apparent that the Marshall test does give a measure of tensile strength and that the success in preventing shear deformation (rutting) failure come form the control of aggregate texture and gradation, bitumen content, and compaction.
During the past few years, other supplementary tests such as indirect tensile test, resilient modulus and creep test etc. have been used to evaluate the engineering properties of AC mixes.
Table 2.1: Mix Design Criteria for AC Wearing Course (RHD Project Specification – 2006) for heavy traffic > 1 msa.
|Marshall Mix Criteria||Min. Max.|
|Compaction (No. of blows on each face)||75|
|Stability, kN||8.2 –|
|Flow, mm||2.5 4.5|
|Percent Air Voids||3 5|
|Percent voids in mineral aggregate||15 20|
|Marshall Stability Flow Ratio||2.5|
|Percent Voids filled||75 85|
|Percent Bitumen Content||5.5 6.5|
|Percent Loss of Stability on immersion||25|
2.4.2 Indirect /Diametral / Split Tensile Strength
The indirect tensile test is one type of tensile strength test used for stabilized materials. The indirect tensile test can be used to characterize bituminous mixes in terms of (Kennedy, T.W., 1977):
a) resilient elastic properties,
b) properties related to thermal cracking,
c) properties related to fatigue cracking, and
d) properties related to permanent deformation
The test is non-repetitive Indirect Tensile Strength and conducted by loading a cylindrical specimen with a single compressive load which acts parallel to and along vertical diametrical plane. This loading configuration develops a relatively uniform tensile stress perpendicular to the direction of applied Load and along the vertical diametrical plane, which ultimately causes specimen to fail by splitting along vertical diameter. The development of stresses within cylindrical specimen subjected to load is reported by (Kennedy et. al., 1977).
The test has been standardized as AASHTO T 283 under the title “Resistance of Compacted Asphalt Mixtures to Moisture Induced Damage”. Diametral Tensile Strength is also called Split Tensile Strength is a form of Indirect Tensile strength for measuring the change in diametral tensile strength resulting form the effect of saturation and accelerated water conditioning of compacted mix to simulate with field condition. The tests are conducted on the unconditioned test specimens and on conditioned test specimens after immersion in water at controlled temperature to find the Tensile Strength Ratio (TSR) parameter. The result is used for prediction of long term stripping susceptibility from water on the bituminous mix and evaluating the performance of Anti-Stripping additives.
The equation employed in calculating the tensile strength is:
in FPS units ?T = 2P max/?hD …….. (2.1)
in SI units ?T = 2000P max/?hD …….(2.2)
Where: (?T) is Indirect/splittensile strength; (Pmax) is the load at failure, (D) diameter and (h) the thickness of test specimen.
Tensile Strength ratio (TSR) = (?T2 / ?T1) ………. (2.3)
Where: (?T1)is average tensile strength of unconditioned subset of test specimen and (?T2) is the average tensile strength of the conditioned subset of test specimens
2.4.3 Resilient Modulas
The elastic modulus of asphalt treated material can be determined by means of the diametral resilient modulus (MR) device. This test is basically a repetitive load test using the stress distribution principles of the indirect tensile test. Like the non-repetitive indirect tensile test, the main advantage of this test procedure is the simplicity of the test equipment as well as the ability to test asphalt specimens similar in size to those used for the widely known Marshall and Hemet tests.
A repetitive (pulsating load) of 0.1 second duration and 0.9 second dwell time is applied diametrically to the test specimen. The dynamic load, in turn, results in dynamic deformations across the horizontal diametrical plane. These deformations are recorded by transducers mounted on each side of the horizontal specimen axis. Knowledge of the dynamic load and deformation allows the MR value to be calculated. Thus, for an applied dynamic load of ‘P’ for the duration‘t’ to produce a resulting horizontal dynamic deformation (?h), the modulus or MR value is (Yoder, E. J., et. al., 1975):
MR =P (? + 0.2374) / (t ?h) ……… (2.4)
A commonly used value of Poisson’s ratio (m) for AC mix materials is 0.35
2.4.4 Fatigue and Permanent Deformation
“Fatigue” is the phenomena of repetitive load induced cracking due to a repeated stress or strain level below the ultimate strength of the material. Fatigue tests may be conducted by several test methods and various specimens. Repeated load indirect tensile (split tensile) test have also been used. The research work had been carried out at Ohio State University also which is based upon fracture mechanic principles applied to a more mechanistic solution of the fatigue problem (Yoder, E. J., et. al., 1975).
A common method for evaluating the fatigue characteristics of the AC mix is by repeated flexural testing. In this testing the specimen is applied with a repeated load having sine wave, with a certain adjusted loading and unloading (rest) time. Because of the effect of varying stiffness upon AC fatigue tests, a temperature control system is used around the flexural load device. The range in stress level is selected so as to yield a range in fatigue life between 100 to 1,000,000 repetitions.
Fatigue testing may be conducted under two types of controlled loading. They are either (i) Controlled stress or (ii) Controlled strain. In the controlled stress mode a constant load is continuously applied to the specimen. Because of progressive damage to the specimen, a decrease in stiffness results in. This, in turn, causes an increase of the actual flexural strain with load applications. For the controlled strain (deflection) approach, the load is continuously changed to yield a constant beam deflection. This results in a stress that continuously decreases with load applications, However, since controlled stress conditions give more conservative estimate of the fatigue life (Nf)and is easy to apply, this test may be safely employed. For controlled stress testing, conducted in the laboratory, the effect of stiffness may be accounted for by plotting the fatigue results in a critical tensile strain applied (?) versus (Nf) relationship on logarithmic scale. This results in a relationship for fatigue tests of the form (Tayebali et. al., 2004):
Nf = 4.9016 x 10-2 x(e)0.03029 VFB x (?)-3.8034 x (S0)-0.98505 …….. (2.5)
S0 = 8.560 x (G0)0.9130 .…….. (2.6)
Where (S0) is the Dynamic flexural stiffness and (G0) is dynamic shear stiffness
“Permanent Deformation” is a longitudinal depression that forms in the wheel track due to consolidation in one or more of the pavement layers due to repeated traffic load applications. The depressions or ruts are of concern for at least two reasons:
i) if the surface is impervious, the ruts trap water and at depths of 0.2 inch, hydroplaning (particularly for passenger cars) is a definite threat.
ii) As the ruts progress in depth, steering becomes increasingly difficult, leading to added safety concern.
For pavements in hot tropical climates and subjected to large number of heavy vehicles and/or vehicles operating at high tire pressures, rutting can be a controlling factor. The relationship at a particular number of load repetitions can be stated as:
?p = f( ?ij )= (5.9055 x 10-3)x Yt ……….. (2.7)
Where (Yt) is the total vertical deformation, mm
Shell researchers have developed a pavement design system in which rutting potential of asphalt concrete is characterized by a simple “Creep Test”. This has lead to the establishment of an empirical link between rheological properties of bitumen and visco-plastic behaviour of asphalt concrete. The test has been designed for the following purposes (Kamyar Mahboob, 1990):
i) To measure compressive stiffness or compliance properties of mixture
ii) To establish plastic flow potential of AC mixes under various stress states in terms of visco-plastic strains.
Based on the research of Van der Poel, (1954) the Creep deformation of cylindrical specimen under a uni-axial, static compressive load is measured as a function of time. In this test, a constant stress (?0) is applied to the specimen and the resulting time dependent strain (?t) is measured. For permanent deformation characterization the relevant quality is the stiffness modulus of mix (Smix) defined as (Ziauddin A Khan, 1988):
Smix = (?0 / ?t) ………….. (2.8)
Where: (?0) is Applied Stress, (?t) is measured strain at time (t) and equell to ?h / h0, ?h is Change in height of specimen and h0 is the Original height of the specimen.
Based on the studies, the researchers had recommend a minimum creep modulus of 80 Mpa (120,000 psi) at 40 oC and stress of 200 kpa (30 psi) for conditions of heavy, slow moving traffic.
Stripping and damages from water susceptibility is a serious problem faced by the Road project implementing agencies. Two types of testing procedure have been developed to predict the moisture susceptibility of AC mixes: strength and subjective. In strength test, the TSR data have been widely accepted. The Marshall Test equipment can be used by replacing the testing head with an accessory suitable to test the Indirect Tensile Strength in accordance with procedure AASHTO T 283 without major additional expenditures.
MATERIAL CHARACTERIZATION, MIX DESIGN AND MOISTURE SUSCEPTIBILITY TESTS
Material characterization consists of evaluation of engineering properties of component materials i.e., bitumen and aggregate, mix design include determination of design Asphalt content for layer gradation by Marshall procedure and moisture susceptibility tests covers Marshall stability and Indirect tensile strength determinations. The sequence of testing is shown in Figure 3.1
Figure 3.1: Flow Diagram of Material Testing and Mix Design
3.2 MATERIAL SELECTION AND TESTING
Bolagonj Boulder stone is the only source of stone, locally available in Eastern Part of the Bangladesh. Hence the aggregate of this stone source is chosen for the present study. The aggregate fractions for the study have been colleted from stone crushing plant Dhaka. Crushed stone fine aggregate containing fines which is a by-product from stone crushing also collected from the same plant for use as fine aggregate and filler in the experimental work of this study.
The aggregate were subjected to testing as per ASTM standard test methods to evaluate the properties which are of significance for AC mix aggregate. The tests include Los Angles abrasion test, Water absorption test, Sand Equivalent, plasticity, and specific gravity test for coarse and fine aggregates. The test results together with project specifications limit of RHD are summarized in Table 3.1. These results are in agreement with RHD project specifications (2006) for AC wearing course.
Table 3.1: Test Result of Aggregate
|TEST||Wearing course||RHD Project Specifications|