When is enough good enough?



A 2015 study by the Western Regional Super Pave Center at the Department of Civil and Environmental Engineering of the University of Nevada, Reno, proved that Gilsonite-modified binders significantly extend the useful life of asphalt pavement. This is good news for agencies looking to reduce the long-term costs of asphalt pavement maintenance.

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Findings from comprehensive testing demonstrated that Gilsonite-modified binders showed significant improvement in:

  • Tensile strength– The addition of Gilsonite significantly increased both the unconditioned and moisture-conditioned tensile strength.
  • Compressive strength– Gilsonite-modified binders showed significant improvement in compressive strength.
  • Rutting strength– Gilsonite-modified binders significantly increased projected pavement life. In all but one case, adding Gilsonite yielded a ten-fold increase.
  • Fatigue resistance– The projected pavement life is 5 to 5 times longer with the Gilsonite-modified binders.

Gilsonite reduces material and saves money

Gilsonite increases the viscosity of asphalt so roadways resist deformation and fatigue, even under temperature extremes – unlike other modifications or high modulus asphalt based on RAP.

Industry reports of Gilsonite-modified asphalt lasting more than twice as long as unmodified mixtures is common. In addition, the high modulus provided by Gilsonite allows the base and binder courses to be up to 20% thinner while still providing the same level of performance, enabling the use of less material. Gilsonite can also act as a lower-cost, performance-enhancing extender in a mixture including SBS polymers.

By significantly extending the useful life of asphalt pavement and allowing a reduction in the amount of paving material required, Gilsonite has proven to offer substantial cost effectiveness.

 Gilsonite is superior for recycling asphalt

Only Gilsonite-modified binder bonds with and stabilizes recycled binder, making recycled asphalt as strong and durable as new. By contrast, polymer-modified binders form large molecules that cross-link into a matrix that recycled binder cannot bond with, leaving pavement vulnerable to breakdown.

No other additive can match all the benefits of Gilsonite:

  • High strength and fatigue resistance
  • Reduced temperature susceptibility
  • Superior deformation resistance
  • Increased resistance to water stripping
  • Reduced pavement thickness
  • Safe and environmentally responsible

 Gilsonite is proven to perform

Gilsonite unique properties make a road that is less susceptible to high-temperature and deformation performance issues. Gilsonite can be added directly at the hot mix plant without additional equipment.

Gilsonite makes roads stronger

To last longer Compared to a typical road that might last 10 years, Gilsonite produces a stronger, better-bonded asphalt that can last 25 years.

 Use Gilsonite as a cost-effective component of modified polymer

Modified polymer additives can increase binder performance under certain extreme conditions. However, SBS polymers cost more than twice as many as Gilsonite. A more cost-effective solution is to combine Gilsonite and SBS polymers to achieve the desired grade.

 Asphalt with Gilsonite resists rutting

Gilsonite increases the viscosity of asphalt so roadways resist deformation and fatigue, even under temperature extremes – unlike other modifications or high modulus asphalts based on RAP.

Gilsonite outperforms other bitumen modifiers

  • Unique source with high purity deposits
  • Extracted with little contamination
  • Ideal softening point for bitumen modification
  • Natural antioxidation properties resist age hardening and degradation

 Building stronger roads for more than 100 years

Gilsonite is the trademarked name for entities, a natural asphalt discovered in the Uinta Basin of northeastern Utah. Since the 1860s, We Gilsonite Company has been producing Gilsonite to strengthen asphalt roads. Gilsonite is readily available in Meltable bags.

Gilsonite is versatile, cost-effective and adds a new dimension of strength to asphalt.

Gilsonite is a naturally occurring hydrocarbon resin that adds tremendous strength, durability, and longevity to asphalt roads. It is a cost-effective additive with unmatched performance.

Evaluation of Gilsonite-modified asphalt mixtures

Cost-effective additive improves strength, reduces rutting and cracking
Gilsonite has been used as an additive to enhance the performance of asphalt pavements for more than 100years. While field experience had demonstrated the advantages by using Gilsonite, the unique properties of Gilsonite-modified asphalt had not been well-documented due to the lack of a comprehensive evaluation.

However, a 2015 study by the Western Regional Super Pave Center at the Department of Civil and Environmental Engineering of the University of Nevada, Reno, thoroughly analysed various asphalt pavements using the latest advancements in materials testing, pavement modelling, and life-cycle analysis.

Key points of the study

The evaluation was conducted under rigorous standards:
> An independent supplier provided control samples and Gilsonite-modified versions of two performance grades (PG) of Asphalt: PG64-28, indicating performance engineered for a pavement temperature range of 64°C to –28°C; and PG76-16, designed for a pavement temperature range of 76°C to –16°C.
> All asphalt binders met the applicable state highway agencies’ specifications based on the Super pave PG System for Asphalt Binders.
> The optimum binder content (OBC) of each asphalt mixture was identified by following the Super pave Volumetric Mix Design Method.
> Laboratory equipment included the industry recognized Asphalt Mixture Performance Tester (AMPT).
> The engineering properties of the asphalt mixtures were measured at their OBCs in terms of the dynamic modulus E* master curve.
> The following performance characteristics of the asphalt mixtures were evaluated:

  • Resistance to rutting in terms of the Flow Number
  • Resistance to thermal cracking in terms of fracture temperature and fracture stress
  • Resistance to fatigue cracking in flexural bending

Quantifying performance

Gilsonite has long been used to increase the performance of asphalt binders. Now, the respected study has documented its advantages.

Super pave PG characteristics used in testing

The Super Pave Performance Grading (PG) system uses the rheological properties of the asphalt binder to identify its performance under the prevailing environmental conditions at the location of the project. The characteristics of the binders used in the testing are summarized in the table below.

A close examination of the data shows that the addition of Gilsonite to the PG 76-16 mixture resulted in an asphalt binder that is less susceptible to short-term and long-term aging.

The industry standard

The most commonly used asphalt binder specifications in the U.S. are the Super pave Performance Grading (PG) system.

The same amount of binder, significant increases in strength

The Super pave system determines optimum binder content (OBC) based on an air voids level of 4% while meeting the remaining specified volumetric properties. Tests showed that the OBC was comparable to control samples and Gilsonite modified samples. The PG64-28 mixtures required the addition of hydrated lime in order to meet moisture sensitivity specifications. Gilsonite modified binders showed significant improvement in both tensile strength (TS) and unconfined compressive strength (UCS).

Summary of optimum binder contents and lime contents, unconditioned and moisture-conditioned TS and UCS properties of the various mixtures

Dynamic modulus (E*) represents strength and stability

The American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical Design uses the dynamic modulus (E*) master curve as the engineering property of the asphalt concrete layer to evaluate the structural response of asphalt pavement under various combinations of traffic loads, speed, and environmental conditions.

A higher E* property indicates a stronger, more stable mix and leads to lower stresses generated in the asphalt pavement under given loading and environmental conditions. The master curves data in the figure below indicate that the Gilsonite mixtures exhibit significantly higher E* properties compared to the control mixtures over the entire range of loading frequency.

Capturing a range of variables

The dynamic modulus curve represents various combinations of loading frequency and temperature.

The data in the charts below compare the E* property of the mixtures at 10Hz loading frequency representing a truck traveling at 60 mph (108 kph). The 104° F (40° C) and 70° F (21° C) temperatures were selected since they represent the critical temperature of rutting and fatigue, respectively.

Gilsonite helps reduce rutting

Gilsonite increases the viscosity of asphalt so roadways resist deformation and fatigue, even under temperature extremes.

Resistance to rutting

The flow number property was evaluated to assess the rutting resistance of mixtures and to determine the rutting model coefficients.

The figure below compares the rutting models of mixtures. The PG64- 28 Gilsonite-modified mixture exhibited a lower rutting model at 104°F (40°C) than the control mixture, indicating that the Gilsonite mix would offer higher resistance to rutting. The PG76-16 Gilsonite-modified mixture exhibited a slightly higher rutting model at 104°F (40°C) than the control mixture.

As shown below, adding Gilsonite to the PG76-16 mixture increased the flow number at 60°C (140°F) by 66% (754 vs. 454). The addition of Gilsonite to the PG64-28 mixture resisted the tertiary flow at 48°C (118°F). This indicates that Gilsonite-modified mixtures will offer significantly more resistance to rutting at the elevated pavement temperature.

Flow numbers of the PG76-16 and PG64-28 mixtures

Relevant sample preparation

Since rutting is an early pavement life failure, the mixtures for the flow number test were only short-term aged.

Resistance to thermal cracking

To determine an asphalt mixture’s ability to withstand low temperatures without cracking, tests measured both the temperature at which fracturing occurred as well as the stress required to initiate fractures.
In the PG64-28 samples, both the control and the Gilsonite modified binders exceeded cold temperature specifications and thus were not expected to crack. The Gilsonite-modified PG76-16 sample showed cracking at 2°C warmer than specifications. However, the calculated crack initiation energy was 20% higher than the control, so it would not be expected to crack under normal conditions.

Thermal cracking properties of the evaluated mixtures

Resistance to fatigue cracking

The resistance of the various mixtures to fatigue cracking was evaluated using the flexural beam test wherein a beam specimen was subjected to a constant bending moment over the centre portion of the specimen. Initial flexural stiffness was measured at the 50th load cycle. Fatigue failure was defined as the number of cycles corresponding to a 50% reduction in the initial stiffness.
The figures below compare the fatigue models of mixtures. The cycles-to-failure tests showed comparable results for the PG76-16 control and Gilsonite modified samples. In the PG64-28 mixtures, the control sample exhibited slightly higher fatigue relationships than the Gilsonite mixture. It should be noted, however, that the calculated values do not account for the effect of the much higher E* in the Gilsonite-modified binder.

Fatigue cracking models for the PG76-16 and PG64-28 mixtures at 70°F

Testing at the right time

Thermal cracking and fatigue cracking are later pavement life’s failures, generally occurring after five years. Test mixtures were aged accordingly.

Mechanistic analysis assesses the effect of actual conditions

In addition to sophisticated laboratory tests, the university’s evaluation included mechanistic analysis. A lab test designed to compare the fatigue cracking models of two asphalt mixtures only assesses the relative behaviour of the mixtures without any indication on their relative impacts on pavement life.

The main advantage of the mechanistic analysis is its ability to combine the engineering property of the asphalt mixture (E*) with its rutting and fatigue cracking characteristics to determine the true impact of traffic loads on pavement life. The analysis evaluated thin pavements – <4” asphalt cement (AC) layer and 8” crushed aggregate base (CAB) layer – and thick pavements – 6” AC layer and 12” CAB layer. Dynamic conditions included the heavy load imposed by a legally loaded 18-wheeler traveling at 60 mph (96 kph) without any braking, and traveling at 10 mph (16 kph) with braking.

The mechanistic analysis of rutting resistance demonstrated that the Gilsonite-modified mixtures had ten times the projected pavement life in every instance except one. For fatigue resistance, the Gilsonite-modified mixtures had a projected pavement life of 1.5 to almost 5 times that of the control mixture.

Summary of the comparative mechanistic analyses of the various mixtures

In theory and in practice

Mechanistic analysis completes the picture begin with lab testing.

Vertical and horizontal forces affecting pavement life

When an 18-wheeler is in the free rolling condition, the vertical loads are distributed evenly among the various axles. Under braking, there is a significant re- distribution of the vertical loads among the axles and the development of significant horizontal loads at the tire/pavement interface. These horizontal loads significantly increase the shear and vertical stresses within the AC layer and represent the main cause of accelerated failures in rutting and shoving at intersections and on off-ramps.

Distributions of axle loads of 18-wheeler at free rolling and braking conditions

Mechanistic analysis findings:

  • The PG64-28 control mixture cannot be adequately designed to withstand the braking action of the 18- wheeler for both the thin and thick pavement structures.
  • The PG76-16 and PG64-28 Gilsonite-modified mixtures significantly improved the rutting life of the thin and thick pavements under the no-braking and braking conditions.
  • The PG76-16Gilsonite mixture significantly improved the fatigue life of the thin pavement under the no-braking and braking conditions.

Bearing the load

The mechanistic analysis shows which mixtures significantly improved road life.

Specific conclusions and recommendations

In summary, the evaluation program concluded that the addition of Gilsonite to the PG76-16 neat and PG64-28 polymer-modified asphalt binders resulted in unique, measurable characteristics that offer excellent alternatives in the following situations:

  • The PG76-16mixture provides extremely high resistance to rutting and shoving with excellent long-term aging characteristics for use in the wearing course of asphalt pavements.
  • Both the PG76-16and PG64-28 mixtures provide extremely high resistance to fatigue cracking beneficial for use as the binder/base course of Perpetual asphalt pavements.
  • The PG64-28mixture can withstand the braking action at traffic lights on urban streets and off-ramps.

Cost effectiveness

By imparting properties that increase resistance to rutting, shoving and fatigue cracking, the addition of Gilsonite can significantly extend the useful life of asphalt pavement. Industry reports of Gilsonite modified asphalt lasting more than twice as long as unmodified mixtures are common.

In addition, the high modulus provided by Gilsonite allows the base and binder courses to be up to 20% thinner while still providing the same level of performance, enabling the use of less material.

Gilsonite can also act as a lower-cost, performance enhancing extender in a mixture including SBS polymers.

By significantly extending the useful life of asphalt pavement and allowing a reduction in the amount of paving material required, Gilsonite has proven to offer substantial cost effectiveness.

Gilsonite in road asphalt

Gilsonite (Natural bitumen) are natural occurring hydrocarbon substances characterized by a high softening point (above 110° C) in the class known as asphaltite. They are mined much like other minerals and sold essentially in their native state. They are fully compatible with asphalt and have long been known as asphalt hardeners and reinforcing agents. Gilsonite is currently sold all over the world as a road bitumen modifier in the form of a dry bulk solid granular powder.

The mixture of Gilsonite (Natural bitumen) into road asphalt relates to a base stock composition comprising a blend of natural bitumen and a rubber latex residue. The base stock may be used alone as a paving material or emulsified to form a natural bitumen emulsion having a variety of uses. The mixture of natural bitumen enforces asphalt against rotting and the thickness of the asphalt with becoming thin.

The present invention is directed to an asphalt cement composition formed from petroleum asphalt, natural bitumen, reactive oil, and an elastomer. The elastomer has a gel content of up to about 95 percent by weight.

Gilsonite (Natural bitumen) modified road asphalt have been particularly successful in highly stressed traffic areas. Natural bitumen, as the majority constituent, has been combined with virgin polymers such as styrene – butadiene – styrene (SBS) and Ethyl Vinyl Acetate (EVA). Gilsonite modified asphalt binders generally do not increase asphalt binder content requirement in pavement mixtures.
Performance grading of asphalt binders and pavement mixtures became a reality with the conclusion of the “FHWA” 50 million dollars, Strategic Highway Research Program, “SHRP”, in March of 1993. “SHRP” developed new asphalt binder specifications and test criteria based on the engineering properties related to pavement performance.

Nowadays the importance of asphalt, because of its use in road networks, airports, highways et cetera, is obvious to everyone all over the world.

Asphalt with an adequate quality is important in various aspects:
Economics: A considerable portion of Municipalities’ budgets, their Civic Engineering and Roads and Transport sections, are spent on manufacturing, applying and maintaining asphalt. If asphalt can be manufactured to have a two to three times longer effective lifetime the costs related to asphalt maintenance would be reduced commensurately. It is also obvious it would be more economical if cheaper substances could be used as modifiers or the bitumen for asphalt.
Environmental: The bitumen that exists in asphalt, due to containing heavy aromatic substances that can be released into the environment in small amounts after rainfall, pollute the environment, specifically underground waters. This pollution can be reduced by replacing a small percentage of the bitumen with green substances such as Gilsonite (Natural bitumen) or using complexing materials.
Driving: It is obvious that good asphalt affects driving. Nowadays, good quality asphalt with low deformation susceptibility, low-temperature susceptibility, high resistance to water stripping and high friction rate will help driving safety at high speeds and in different weather conditions (cold, warm, freezing).
Protecting and Saving Mineral and Natural Resources: Stone materials are used, alongside bitumen and other added substances, in manufacturing asphalt. By adding to the effective lifetime of asphalt the use of those materials would also be decreased.

To increase the rigidity and lifetime of asphalt, and to achieve the goals mentioned above, in advanced countries, different mixtures such as PPA, EVA, and SBS are used as modifiers and fillers. These materials are both expensive and toxic. And so producing a modified with lower manufacturing cost, lower toxicity, and equal or better quality is among the goals of asphalt scientists.
In Iran, for the last decade, asphalt modifiers have not been used due to their high prices. This has led to a dramatic drop in asphalt quality and effective lifetime, so much as Iranian asphalt has a lifetime equal to one-third of American and European asphalt.
In this regard, this cooperative, using micronized Gilsonite powder with a mesh size of 200 and solubility of more than 85% in carbon disulphide, has made efforts to modify asphalt to have higher rigidity and friction, and lower penetration rate, manufacturing cost, and toxicity.
We have used Gilsonite, as a powerful modifier, as both feeder and volumizer. Gilsonite is a by-product of bitumen that can replace up to 15% refinery bitumen.

Goal: The produced powder is directly used in warm road bitumen, reducing the bitumen by 10-20. The micronized Gilsonite powder is used as a modifier in hot asphalt to add to its functionality, rigidity, stability and lifetime. Also, Gilsonite decreases penetration rate and increases viscosity, softening point, thermal resistance, resistance to deformation and resistance to water stripping. Also, this powder, due to having nitrogen-containing heterocyclic materials, adds friction between the road and the cars’ wheels, that, in turn, will increase the quality of driving in the rain and while braking.
Lab results show that asphalt rigidity with 10% Gilsonite is 1357 kilograms while in the absence of any Gilsonite it is 1097 kilograms. Asphalt modified with Gilsonite has more rigidity, resistance and effective lifetime than regular asphalt. In addition, it is cheaper and more environmentally-friendly and contains more bitumen.

Gilsonite mixture in road asphalt

It has been discovered that asphalt cement can be toughened with natural bitumen and thinned with a reactive oil. Reactive oil is an oil that contains a high content of unsaturated fatty acids. By using a reactive oil, the oil would cure or react after application of the asphalt cement to the highway, thereby allowing for a lower viscosity application by curing later to prevent rutting.

Gilsonite preferably one having a melting or softening point near about 300° F. So that it is more easily softened and blended with the petroleum asphalt at such temperatures.

An asphalt composition consisting essentially of;

  • 100parts by weight of a petroleum asphalt;
  • From about 1to about 10 parts by weight of a natural bitumen;
  • From about 1to about 10 parts by weight of a thinning reactive oil comprising at least about a 60
  • Percent unsaturated fatty acid content having from about 14to 24 carbon atoms, and
  • From about 1to about 10 parts by weight of an elastomer.

The advantage of using Gilsonite in road asphalt

  • Powdered Gilsonite is added to asphalt pavingmixes to improve pavement performance. In the case of doing this project, the lifetime of asphalt in Iran will be doubled and tripled.
  • With this method, the price of asphalt will be decreased by around 10%.
  • No chemical polymer modifiers would be needed anymore to modify bitumen.
  • 60% lower prices of all manufactured Polymer bitumen in comparison to its chemical and synthetic counterparts.
  • There would no longer be the need to import chemical and synthetic polymer
  • Increased thermal stability of bitumen (refinery bitumen will melt at temperatures above 70degrees Celsius and crack at 0 degrees Celsius while this kind of polymer bitumen has a thermal tolerance range of –30 to +110 degrees Celsius).
  • Decrease in bitumen’s penetration rate and increase in resistance to water stripping
    Improves load-bearing capacity and reduces highway rutting and shoving. Increase in its Marshall load frame capacity.
  • Improves ductility and resistance to low-temperature cracking. Increase in bitumen’s softening point temperature.
  • Improves the workability of the mixture during paving.
  • Increase in bitumen’s Brookfield viscosity
  • Increases the durability of the pavement;
  • The decrease in the resulting bitumen’s toxicity in comparison to refinery bitumen and the synthetic.
  • Polymer kind due to the decrease in unsaturated materials.
  • The possibility of producing entirely based on domestic resources and technology.
  • The asphalt modified with polymer bitumen has a higher PG and can be mixed, unlike other modifiers, with a low roller force with other asphalt mixtures.
  • The resulting polymer bitumen are highly water repellent and tolerant against polar and acidic and alkaline dissolvent.
  • Stronger and increases asphalt’s adhesion to aggregates.
  • Natural bitumen function by making the pavements harder.

Typically, 5% to 15% Gilsonite is added to the mixture’s binder. It is generally regarded that Gilsonite reduces pavements’ low-temperature properties, making them susceptible to thermal cracking. Natural bitumen (Gilsonite) melted into hot asphalt will reduce penetration and increase the viscosity of the asphalt binder. Gilsonite may also be mixed with aggregate prior to combining with the asphalt binder.
The features of this product, such as softening point, moisture, grain grading (mesh 200), ash percentage, ash texture, solubility in carbon disulphide, the percentage of aromatic carbons et cetera are controlled and optimized. The company has presented this product for sale after conducting numerous experiments and production in the cities of Kermanshah and Mehran in Iran and Erzurum in Turkey and under the supervision of these cities’ municipalities. And currently, this product (modifier for bitumen, asphalt and bitumen by-products) is released for sale with consistently great quality and appropriate prices.

Evaluation of Gilsonite and Styrene-Butadiene-Styrene Composite Usage in Bitumen Modification of the Mechanical Properties of Hot Mix Asphalts


This study is an investigation of the rheological properties of bituminous binders and the mechanical properties of hot mix asphalts (HMAs) when a combination of styrene-butadiene-styrene (SBS) and Gilsonite (G) is added to the bitumen. In this work, HMAs were prepared with two binders containing a single additive (5% SBS and 18% G) and three binders containing two different additives (4% SBS+6% G), (3% SBS+10% G) and (2% SBS+13% G), which have the same Super pave performance levels. Tests carried out on the mixes demonstrated that the most effective additives relative to Marshall stability, retained Marshall stability, and indirect tensile strength values were 18% G and 3% SBS+10% G; the most effective additives relative to stiffness and fatigue life were 18% G, 2% SBS+13% G, and 3% SBS+10% G; and the most effective additives against moisture-induced damage were 5% SBS, 2% SBS+13% G, and 3% SBS+10% G. A consideration of all of the study’s findings indicates that the utilization of a combination of SBS and G provides more benefits than utilizations of SBS or G alone.

 Gilsonite is superior for recycling asphalt

Only Gilsonite-modified binder bonds with and stabilizes recycled binder, making recycled asphalt as strong and durable as new. By contrast, polymer-modified binders form large molecules that cross-link into a matrix that recycled binder cannot bond with, leaving pavement vulnerable to breakdown.

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