IntroductionThe bituminous binder functions as a waterproof, thermoplastic adhesive. In other words, it acts as the glue that holds the mineral aggregates together to act as a structural layer. In its most common form, bituminous binder is simply the residue from petroleum refining. To achieve the necessary properties for paving purposes, binder must be produced from a carefully chosen crude oil blend, and processed to an appropriate grade. Increasing axle loads, climatic variations and traffic growth has posed challenge to paving industry to look into the demands made on the bitumen pavement construction. In this regard, as early as in 1980’s modification to base bitumen was done by addition of certain additives like polymer’s, natural rubber, crumb rubber, plastic’s etc., to enhance the mechanical behavior of bitumen by physical modification. Also chemical modification’s was attempted in the form of polyethylene, poly-phosphoric acid, etc. The use of modified bitumen can serve a number of purposes. It can target a specific improvement in the bitumen, such as permanent deformation (rutting) or low temperature cracking. Benefits that may be derived from binder modification include:
- Improved consistency
- Reduced temperature susceptibility
- Improved stiffness and cohesion
- Improved flexibility, resilience and toughness
- Improved binder aggregate adhesion
- Improved resistance to in-service aging
Study Objective and ScopeThe objective of this study can be listed as follow:
characterization of the modified and conventional binders through an elaborative laboratory investigations
In this investigation, four types of modified binders and a conventional binder were studied. Modified binders like styrene-butadiene-styrene polymer modified binder (PMB) of two grades viz., PMB-40 and PMB-70, Crumb Rubber Modified Binder (CRMB) of two grades viz., CRMB-55 and CRMB-60, natural rubber modified binder (NRMB), waste plastic modified binder (WPMB) and conventional binder (60/70 grade) were studied. From these investigations, the properties affecting the in-service performance in comparison with the specifications developed by other countries was assessed.
BackgroundThere is a marked difference between the maximum and minimum temperatures in the country, so a flexible pavement should be capable of resisting to the extreme temperature variations and to prevent pavement distress. In this regard, a binder modification is an effective tool to reduce the temperature susceptibility and improve the strength. Hence an ideal modifier should have enhanced cohesion and very low temperature susceptibility throughout the ranges of the temperature to which it will be subjected in service. Its resistance to permanent deformation and fatigue characteristics should be high. It should have at least the same adhesion qualities as conventional binders and material should be such that the whole composition of the mix should be homogenous.
Modified Binders Used In Road ApplicationsTable 1 shows a generic classification system that was used to define and classify modifiers, as well as other additives in bituminous mixes (IRC: SP: 53-2002).
Rubber CrumbsRubber used for these products is derived from pneumatic tyres that have been processed by mechanical means and should be substantially free from ground fabric, steel and other contaminants, including moisture. When introduced to the hot binder the rubber swells through absorption of the aromatic fractions of the binder. As a result of the high blending temperature some of the rubber dissolves in the binder and some is de-vulcanized (14).
Properties and Field Performance of Modified BindersIn order to relate binder properties to pavement performance, it is necessary to know the fundamental relationships between binder properties and mix properties. To minimize the deterioration of a flexible pavement due to influence of traffic and climate, the bituminous layers should be stiff enough at elevated service temperature to avoid permanent deformation (rutting), show good load-associated fatigue resistance, possess good stripping resistance (low water susceptibility), and have good flexibility at low temperature (resistance to low temperature cracking. All of these performance-related properties of the mix are influenced to some extent by binder properties (5, 6, 10, 12 & 15).
AgingAging is induced by chemical and/or physical changes and is usually accompanied by hardening of the binder. In road applications, binder is exposed to aging at three different stages: (i) storage, (ii) mixing, transport and laying, as well as (iii) during service life. Aging is a very complex process in neat binders and the complexity increases when modifiers are added. The aging properties of neat binders are normally characterized by measuring rheological properties such as viscosity and softening point before and after artificial aging in the laboratory. This procedure is not sufficient in the case of modified binder since thermolytic degradation of the modifier may occur during aging and the fragments formed may contribute to a lowering of the consistency. Therefore, when assessing the aging properties of modified binder, further characteristics, such as elastic recovery and chemical composition have to be evaluated (1 & 16). Indications of improved aging properties by admixture of polymers to the binder have been reported in recent publications (20 & 22).
Temperature SusceptibilityIn cold climates, cracking in pavements may be an extensive problem. Low temperature cracking is caused by thermally induced tensile stresses when these exceed the tensile strength of the pavement material. The main factor influencing the degree of cracking at low temperature is found in the binder properties. Several papers have indicated that the addition of polymers to binders may increase resistance to low temperature cracking (13 & 17 to 20). However, validation of laboratory methods by field performance tests is necessary before a more definite opinion on the matter can be given.
Specifications Based on Performance(3)
ExperimentsThe main laboratory experiment programme envisages the quantitative analysis in assessment of rheological and empirical properties of both neat and modified binder. This chapter presents properties of neat and modified binders. Experiments are conducted under unaged and TFOT (thin film oven test) aged conditions.
MaterialsThe materials selected for the present investigations are:
- 60/70: Conventional neat binder
- PMB-40: SBS modified binder
- PMB-70: SBS modified binder
- CRMB-55: Crumb rubber modified binder
- CRMB-60: Crumb rubber modified binder
- NRMB-70: Natural rubber modified binder
- WPMB-40: Waste plastic modified binder
ResultsThe test results are shown in the Tables 3 to 9.
Discussions and Conclusions
There are three critical working ranges for bituminous binders: a range of high temperature and long loading times during which the binders may flow, entailing a risk of rutting of the mixes, a range of low temperature and long loading times during which the mixes are liable to crack under the effects of thermal stress and a range of low temperature and short loading times during which the binder is brittle and may give rise to mechanical cracking. The tests used currently for binder specifications yield little information on the behavior of binders in these critical ranges. Fortunately, advances in rheometers have made it possible to perform dynamic tests at a wide range of temperatures and frequencies, from which various rheological parameters (e.g., complex modulus and phase angle) in different conditions can be obtained. However, recent research (1, 2, 10 & 15)has indicated that dynamic parameters are useful for predicting performance-related properties.
Aging occurs during the production of the bituminous mix and during its service life as pavement layer. The circumstances during different aging stages vary considerably. Standardized aging test methods simulate the aging that occurs during the production of the pavement. To simulate long term aging in service, the PAV test has been developed in SHRP. The test is performed after RTFOT or TFOT aging.
The chemistry of binder is very complex and is even more complex after the admixture of modifier. No specifications including require– ments on the chemical composition of binder or modified binder have been found in the literature. It is doubtful whether this type of requirement should be included at all in specifications, at least not for plain binder. When characterizing the aging properties of modified binders, some chemical tests could be suitable for specification purposes.
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