Overview

Research has proven that the degree of bonding between pavement layers can significantly affect the overall performance of the pavement structure or overlay. Zydex Industries specifically addresses this unique problem of permanent bonding between asphalt layers with its asphalt additive – NanoTac.

In a study undertaken by the National Center for Asphalt Technology (NCAT) titled: “Effects of NanoTac Additive on Bond Strength and Moisture Resistance of Tack Coats” by Adam Joel Taylor, P.E. (2011), laboratory analysis measured the effects of NanoTac in a tack coat application. The results provide an interesting view into the performance of NanoTac’s organosilane chemistry and its unique bonding characteristics.

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NanoTac is an asphalt pavement additive derived from organosilane technology. NanoTac is designed to convert the surface of the treated material (untreated aggregate, soil, or HMA) from a water-loving (hydrophilic) surface to a water-repelling (hydrophobic) surface. The NanoTac additive is added to a diluted tack coat material by blending a small quantity of the additive with the water used to dilute the emulsion. This is designed to improve the water-resistance and bonding strength of the tack coat interface. The NanoTac additive is also designed to lower the surface tension of a cationic emulsion with which it is blended, reducing droplet size and improving spray coverage. NanoTac is also intended to wet, penetrate, and set quickly to reduce the problem of ‘tire pickup,’ a common problem with construction traffic driving over newly placed tack coats, removing the tack, rendering it ineffective.

Effects of Poor Bond Strength

Poor bonding between pavement layers has been known to decrease the structural bearing capacity of a pavement inducing pavement distresses and failures. Problems commonly associated with debonding are premature slippage cracking, top‐down cracking, and fatigue cracking. Past research shows debonding can reduce the pavement’s fatigue life by more than 50% prompting the need for extensive repairs such as full‐depth patches or complete reconstruction.

The best way to prevent debonding from occurring is through good construction practices. A quality tack coat material that provides sufficient bond strength should be used and uniformly applied to the pavement surface. Common issues with tack coat application include non‐uniform spray (often due to clogged nozzles on the spray truck) and tire pickup (tires from construction traffic removing the tack). Quality construction practices are critical to achieving a quality tack coat interface in the field.

The NanoTac Study

The study was performed to evaluate the effect of the NanoTac additive on tack coat bond strength, spreadability, dry time, tire pickup, and moisture susceptibility.

The study also evaluated the properties of a diluted cationic slow setting (CSS) emulsion containing the NanoTac additive versus the properties of a diluted emulsion with no additive.

Testing Procedures

The interlayer bond strength of each core was determined using ALDOT‐430 Standard Test Method for Determining the Bond Strength Between Layers of an Asphalt Pavement. The control emulsion application had a 30% AC residual while the NanoTac modified application had only a 10% AC residual.

Summary

Based on the results of this study, the following conclusions can be made:

The NanoTac modified emulsion (10% AC residual) with a lower residual AC content provided equivalent bond strength to that of a control emulsion (30% AC residual) on an unmilled (new HMA) surface.

All optimum bond strength values for both the control and NanoTac emulsions were greater than 150 psi (1.03MPa). Construction standards recommend a minimum value of 100 psi (0.69MPa) to have adequate bond strength in the field.

For a copy of this study, please contact your regional representative.

Chemical Soil Stabilisation is the process of blending and mixing chemical additives to improve the engineering properties of the soil. The efficiency of the stabilisation depends on the reaction between the chemical/stabilizer used and the soil particle composition.

Soil stabilisation using chemical additives is adopted not only to arrest its shrinkage & swelling potential, but also to improve its strength, workability & durability, thus improving their overall performance. The increased performance implies reduced the maintenance costs, significantly improving the overall lifecycle costs of the pavement. It also offers other advantages such as reduced dusting requirements, controlled volume changes to soil and better workability.

The performance of the treated soil depends on the ability of the additives to react with the soil, which places a great emphasis on choosing the stabilizer best suited for a particular type of soil, taking into consideration, not only the improvement of its engineering properties but also considering its economic and environmental impact.

It has been a common practice to use calcium based stabilizers such as cement and lime for stabilisation, due to its notable effect of reduction in plasticity, swelling & shrinkage as well as the increased bearing strength. However, there are certain limitations to using these additives. Stabilisation of soils containing sulphate minerals when treated with lime, causes adverse chemical reactions & formation of an expansive crystalline mineral, causing heaving and other pavement distresses. Exposure of lime treated soils to wetting and drying cycles results in the loss of cohesion between soil and lime particles. Lime & cement applied in the powder form is known to burn unprotected workmen and also causes harmful environmental effects. Moreover, cement manufacturing emits large amounts of CO 2 . Fibers have been considered an effective alternative to help prevent cracks; however, its effectiveness in restricted to certain type of soils.

The use of bitumen emulsion for soil stabilisation is also a notable practice as it imparts water resistance to the soil particles, increases the load bearing capacity & stability of the soil. However, it may not be a cost effective solution as the soil particles require a high dosage of bitumen emulsion in order to provide the same/better level of bonding. Foamed bitumen, i.e. mixture of air, water and bitumen, is also being used for soil stabilisation. It has been proven that by applying foamed bitumen, produces a semi-rigid layer of stabilised soil.

New and emerging technologies such as enzymes and silanes are being preferred over traditional stabilisation methods, to solve the problems of swelling and shrinkage as well as to improve the load bearing capacity and compaction densities of poor soils, with no harmful effects on the environment. Major advantage of using chemical additives for soil stabilisation is that it enables the use of in- situ/locally available soils and aggregates. This decreases the effort and time required for hauling good quality materials from borrow areas, allowing for an efficient use of natural resources. This results in minimized cost of hauling, thereby reducing the cost of construction.

Warm Mix Asphalt (WMA) is a variant of Asphalt that allows the production and placement of asphalt mixes at lower temperatures, which translates to energy and cost savings. There are a wide variety of additives based on different technologies that allow the production of Warm Mix Asphalt mixes. These technologies can be classified as:

1.Foaming
2.Organic Additives
3.Chemical Additives or Surfactants

Foaming signifies the introduction of water into the asphalt mix, which, due to the high temperature of the mix, gets converted into steam, and generates bubbles that causes foaming of the bitumen binder. This expansion reduces the viscosity of the binder which facilitates aggregate coating and compaction at lower temperatures. However, the involvement of water in the foaming process makes these mixes more susceptible to moisture damage, resulting pavement distresses such as rutting, during its service life.

Additionally, the use of organic additives such as waxes, is also an adopted practice for the production of Warm Mix Asphalt. However, it has been found that waxes affect the fatigue and low temperature performance of these mixes. The melting point of the waxes in the range of 85 – 1150C, allows it to be completely soluble in the binder. It lowers the viscosity of the mixes, thereby facilitating free movement and coating of the aggregates. However, during the cooling phase, waxes have a tendency to crystallize, which increases the stiffness of the mix, and causes the problems of workability and density achievement.

Moisture damage results in poor adhesion between bitumen and aggregates, resulting in loss of cohesion, which results in the loss of stability. Chemical additives or surfactants are relatively newer technology for war mix asphalt that works at the microscopic interface level of the aggregates and bitumen. They facilitate easier movement of mixes, leading to better workability and requires relatively lower energy levels for mixing and compaction. Some of these warm mix asphalt additives also improves the coating efficiency in the mixes, thereby preventing adhesion failures.

Zydex offers next generation warm mix asphalt technology thatreduces the surface tension of the mixes to enable faster and better coating. It enables temperature reduction during mix production and compaction. It provides better workability to enable cold climate paving and also facilitates consistent & easy compaction, even at low ambient temperatures.

These technologies have enabled the use of Warm Mix, providing both environmental benefits as well as better working conditions, thus allowing us to take a step towards a greener & sustainable future.

Perpetual Pavements are pavements designed and constructed to last > 50 years, with maintenance restricted to the wearing course alone. In order for the pavement to perform through its design life, it is imperative to provide a strong, flexible bottom layer that resists cracking under heavy traffic loads and cyclic weather variations.

Soil bases are critical layers for any pavement, as it transfers the vehicular load to the ground level, allowing smoother rides. They form the foundation for the long term performance of the pavement. However, factors such as unavailability of good materials, variability in the available soils and greater affinity to water challenge the conventional construction methods as they lead to failures causing undulations and cracks. This presents a compelling case to find solutions that can improve the engineering properties of the in-situ soil. Various methods such as the use of geogrids and geotextiles, grouting, increasing the thickness of the subgrade etc. have been adopted over the years, to construct a strong base. One such method is soil stabilisation.

Soil stabilisation is the process of blending and mixing different materials, to improve the engineering properties of the soil. It not only helps to increase the strength of the soil, but also allows to arrest its swelling potential, allowing for better stress transfer thus preventing premature failures. Stabilizers such as cement, lime and bitumen emulsions have been used traditionally for soil stabilisation. However, each method comes with its own disadvantages, be it the environmental or health hazards, or cost effectiveness. Also, with depleting resources, the focus on sustainable construction practices has been ever increasing. New and improved methods are being preferred as they are cost-effective and allows construction of high performance, durable roads with reduced material consumption. This has paved the way for new and emerging chemical technologies that allows to improve the properties of the soil without any harmful effects, along with a keen eye on easy-to-use solutions that have proven its efficacy on field on field and widely accepted.

Zydex offers Terrasil (organosilane) and Zycobond (acrylic co-polymer), which are easy to apply additives for soil stabilisation, that significantly reduces water permeability, imparts higher strength and enhanced flexibility through nano-bonding. These properties make the soil bases non-deforming and dimensionally stable. As this change in the engineering property is permanent in nature, it enables the construction of all-weather water resistant, durable roads. This significantly improved strength and higher modulus with flexibility of soil bases can be utilized for reducing the overall thickness of the pavement, thus delivering a sustainable and perpetual road at the same cost.

Perpetual Pavements are pavements designed and constructed to last > 50 years, with maintenance restricted to the wearing course alone. Perpetual pavements have high structural strength and offer long term resistance to two most devastating distresses i.e. structural rutting and fatigue cracking, keeping the compressive and tensile strains below the failure limits.

Traditionally, highways are designed for a 20 year design life, whereas perpetual pavements are expected to perform for 50 years or more. These pavements are designed with a strong, flexible bottom layer that resists cracking under heavy traffic loads and cyclic weather variations. Several practices such as soil stabilisation using materials like cement, lime, bitumen emulsion and installation of geogrids & geotextiles are adopted to ensure a strong and dimensionally stable base. These practices help improve the engineering properties of the bottom layers. New and emerging technologies such as organosilanes & polymers, offers further improvement in terms of moisture resistance, higher strength and flexible nano bonding. This increases the durability (lifespan) of the pavement layers significantly. Use of emulsion additives also help in effective bonding of the different layers for uniform load/stress transfer.

The bituminous layers of the pavements offer flexibility and resistance to deformations. However, factors such as material variability and operational inefficiencies causes serious problems which lead to premature failures. For example, improper coating of the bitumen on aggregate surface results in water ingress and delamination of the layers, thereby reducing its ability to withstand heavy loads. Therefore, bitumen additives are used at times, to improve the properties of the bituminous mixes. Bitumen additives such as anti stripping agents improve the cohesion between aggregate & bitumen and ensures a tightly bonded structure, thereby facilitating better stress transfer under heavy loads. This eventually translates to longer pavement life. Innovative practices such as the use of Warm Mix Asphalt provides additional environmental benefits by facilitating reduced odor & emissions, lesser fuel consumption and paving friendliness.

The benefit of restricting distress formation to the top layer, is to ensure that when distresses exceed tolerance limits, the distressed top layer can be removed and resurfaced with an asphalt overlay. Cost effective solutions such as microsurfacing or slurry seal can be adopted to ensure a smooth ride quality, thereby reducing the maintenance costs and time.

Perpetual pavements, therefore, offer a durable, long-lasting road, with cost-effective maintenance that restricts traffic disruption thereby saving time for road users and the maintenance crew. Reduced maintenance also conserve the use of the scarce natural resources such as aggregates and bitumen, to provide significant environmental benefits as well as cost savings.

The future lies in high-strength pavements that are faster to construct using limited natural resources, and when constructed for perpetuity, will never need to be reconstructed.