Needed Characterization of Modified Binders in India

The thermo mechanical properties of bituminous binder have a major effect on its subsequent in-service performance. The rheology of conventional binders is relatively simple and behavior can be predicted through the use of traditional tests such as penetration, softening point and viscosity testing at various temperatures. On the other hand the rheology of modified binders is highly complex and, although the results from traditional tests may indicate a significant improvement in properties, the in-service performance of these binders is not easily categorized. This paper presents detailed investigations on the physical properties of modified binders in comparison with conventional binder using current Indian specifications. Firstly, it gives a brief description on modification of bitumen its advantages and limitations. It also looks into the present specifications that are followed in India and the need for improved specifications for performance based binder characterization, which may provide fundamental explanations with respect to in-service performance.

Introduction

The 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

However, there needs to be a way to evaluate whether the performance of the modified bitumen is cost effective.

Study Objective and Scope

The 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.

Background

There 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 Applications

Table 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).

Polymer

A polymer is a very large molecule comprising hundreds or thousands of atoms formed by the successive linking of one or two, or occasionally more, types of small molecule into chain or network structures (4). To achieve the goal of improving binder properties, a selected polymer should create a secondary network or new balance system within binders by molecular interactions or by reacting chemically with the binder. The formation of a functional modified binder system is based on the fine dispersion of polymer in binder for which the chemical composition of binders is important. The degree of modification depends on the polymer property, polymer content and nature of the binder.

Rubber Crumbs

Rubber 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 Binders

In 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).

Aging

Aging 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 Susceptibility

In 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⁾

In most countries, current binder specifications are viscosity or penetration graded and typically based on measurements of viscosity, penetration, ductility and softening point. These measurements are not sufficient to describe properly the linear viscoelastic and failure properties of binder that are needed to relate binder properties to mixture properties. These specifications and test methods are not performance related, because they, lack adequate low-temperature measurements, do not include fundamental binder properties which may be related to fundamental mixture properties or to pavement performance, are not appropriate for measuring consistency at the upper service temperatures, and do not consider long-term in-service aging. In most cases, specification used or proposed for modified binders are derivatives of the specifications of neat binders and are supplemented with tests such as tensile strength and elastic recovery. This is also the reason for developing new specifications within the Strategic Highway Research Program (SHRP), seen in Table 2. In SHRP, new powerful tools for the evaluation of bituminous binders have been developed. The SHRP binder specifications are said to be performance related. In this specification, new testing instruments, such as the bending beam rheometer and the direct tension tester, are employed. The instruments are used to measure more fundamental properties such as the inverse of loss compliance, storage modulus, stiffness and strain at failure. The parameters determined have been proposed collectively as being related to the rutting, fatigue and thermal cracking behavior of binders (2). Short term aging is simulated using the rolling thin film oven test (RTFOT) and long term aging using the pressure aging vessel (PAV). It is important to emphasize that proposed SHRP specifications are intended for both neat and modified binders, and allow selection of a binder based on the climate in which it is expected to perform.

Experiments

The 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.

Materials

The 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

All samples were subjected to a number of characterization evaluations according to IS: 73-2006, IS: 15462-2004 and IRC: SP: 53-2002 in Asphalt Laboratory, IIT Madras by the methods outlined in the above specifications.

Results

The test results are shown in the Tables 3 to 9.

It is seen from the results both conventional and modified binders to satisfy the specification requirements. Only in case of WPMB, it failed to satisfy the elastic recovery criteria on both unaged and TFOT aged conditions. Elastic behavior indicates that the binder recovers most or all of its initial shape when the load that caused deformation is removed. The elastic recovery of a binder is commonly used to measure the fatigue resistance of a binder or its ability to absorb large stresses without necessarily cracking or deforming. From this, it may be observed that use of waste plastics as modifier forms a rigid phase or network, imparting no elastic recovery properties to the base binder but induces a high stiffness. Loss in weight is also higher than the specified limits in case of WPMB, which may attribute that use of waste (recycled/ non-virgin) plastic as a modifier to the base binder gradually changes over time due to heat, oxidation, ultra violet radiation and loss of volatile components. Viscosity measurements of unaged PMB’s were higher than the specified which may due to higher concentration of polymer itself. Both elastic recovery and viscosity of NRMB was lesser than the CRMB’s. This may be due to use of stiffer base binder in case of CRMB’s.

Discussions and Conclusions

The aim of this paper is to provide a summary of information found in the current literature regarding test methods, specifications and performance of modified binders. The main purpose of material testing is to characterize the material in question, in such a way that the characteristics measured can be used to predict behavior in practice. For tests on binders to be valid, the tests must be sensitive to properties of the bituminous pavements, such as resistance to rutting, load associated fatigue and low temperature cracking. In general, when assessing the quality of modified binders, traditional methods developed for testing neat inder, are used. These specifications are in general based on empirical test methods, such as penetration, softening point, ductility and viscosity measurements, the performance relations of which are not always obvious. There are several reasons why empirical methods, nevertheless, are used. Above all, these test methods have been used for a very long time, and present knowledge on binders is for the most part based on results obtained using these methods. To describe the properties of modified binders in an overall situation, the modified binder specifications should be supple– mented by other test parameters, such as dynamic mechanical analysis and compatibility.

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.

Conclusion

In conclusion, using the traditionally tests used to characterize the binder, it is very difficult to analyze and predict its in-service performance due to the complexity of the various modified binders as a function of base binder and the type as well as the content of the modifier. Hopefully, the dynamic mechanical analysis and rheological studies, which are intended for both neat and modified binders, may be more suitable in predicting binder performance on roads compared with conventional tests. The pressure aging vessel (PAV) simulates the age hardening of bitumen during the first 5-10 years of pavement service life. The pressure aging vessel conditioning allows further testing by the dynamic shear rheometer and bending beam rheometer to evaluate the binder’s performance following aging to evaluate whether the performance of the modified bitumen is cost effective in its service period.

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