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1. INTRODUCTION 1.1 Why Interlayer is required? 

Pavements that exhibit cracking (fatigue or thermal cracking), are commonly rehabilitated by overlaying with a new surfacing layer

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Rehabilitation of pavements is one of the major activities that the engage time and resources of many highway agencies around the world.

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Rehabilitation of road pavements is essential, as bad roads have ed for many accidents and loss of lives.

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Overlaying the old pavement with new surfacing material is one of the commonly used methods for rehabilitating deteriorated pavements.

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For both flexible and composite pavements, a common technique used by many agencies for preventive maintenance and/or rehabilitation was simply to construct a thin Hot Mix Asphalt (HMA) overlay, normally between 1 and 2 inches (25 and 50 mm) thick.

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Repairing cracked roads by overlaying has been found to be a short term solution. This is the case because the deficiencies in the old road are rapidly reflected at the surface as a result of the combined effects of thermally induced stresses and traffic loading. This phenomenon referred to as reflective cracking is one of the more serious problems associated with the use of thin overlays.

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Reflective cracking is first initiated at the bottom of the overlay material and grows until it appears at the surface. Once a reflective crack reaches the surface, it creates a path for the flow of surface water into the lower layers of the pavement.

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Methods for controlling reflective cracking and extending the life of overlays consider the importance and effectiveness of overlay thickness and proper asphalt mixture specification.

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In the past Asphalt mixes have been improved and even modified by adding a variety of materials and number of potential solutions has also been evaluated including unbound granular base “cushion courses” and wire mesh reinforcement. All have found to be either marginally effective or extremely costly.

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So an innovative measure is required for tackling this problem.

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1.2 Innovative Maintenance Strategy: 

The crack resistance of the overlay can also be enhanced via interlayer systems.

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This involves the use of interlayer materials between the existing pavement and the overlay.

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These materials are required to have lower stiffness than both the overlay and the existing pavement.

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Therefore, they are referred to as Stress Absorbing Membrane Interlayers (SAMIs).

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They are termed SAMIs because they relieve stress and reduce strain concentration in the crack region, therefore prolonging the life of the overlay.

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Stress Absorbing Membrane Interlayers (SAMIs) designed to dissipate energy by deforming horizontally or vertically, therefore allowing the movement (vertical/horizontal) of the underlying pavement layers without causing large tensile stresses in the asphalt overlay.

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The application of stress-relieving systems at the interface between the overlay and the old pavement surface reduces the shear stiffness of the interface.

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Asphalt additives do not stop reflective cracking, but do tend to slow down the development of cracks and convert a large crack in the old pavement into a multiple small cracks in the overlay.

1.3 Crack Reducing Interlayer (CRL)/ Stress absorbing membrane interlayer (SAMI) Hot mix asphalt (HMA) overlays are the traditional method for protecting deteriorating pavements. They strengthen the pavement structure, reduce roughness and restore skid resistance. However, existing cracks in the pavement begin to appear in the new surface within one or two years. It is common to see all cracks reflect through the new overlay in three to five years. According to observations, cracks reflect at a rate of one inch per year. To address the problem, pavement designers typically increase the overlay thicknesses. When the cracks enable water infiltration, freeze-thaw cycles and repeated loadings cause the asphalt surface to deteriorate at the crack and eventually ravel resulting in roughness and potholing. The most serious problems are poor ride and deterioration of the base structure caused by moisture permeating through the cracked pavement. While no product will stop reflective cracking, it can be retarded with the use of interlayers.

Causes of cracking: Cracks in the highway emanate from either i. The surface, where the surface course is fatigued by the traffic. ii. The sub-base, from movement of the underlying layers 2

Benefits of Crack Reducing Interlayers: i.Seal and waterproof existing pavement surface. ii.Retard reflective cracking. iii.Disrupt pathway for water from surface into the pavement structure. iv.Allow thinner lifts of HMA overlays. v.Cost-effective protection for prolonging the service lives of HMA overlays Interlayers can be used on any asphalt or concrete pavement to be overlaid with HMA, including streets, roads, highways and airfields. Any structure or base problems and severe cracks or potholes should be addressed before interlayer construction. Any quality paving project requires quality materials and quality construction

Fig: cross section of a paved road with a paving fabric interlayer

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2. IRC 37: 2012 

In case of pavements with cementitious base, a crack relief layer is provided between the bituminous layer and the cementitious base which delays considerably the reflection crack in the bituminous course.

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This may consist of crushed aggregates of thickness 100mm of WMM conforming to IRC/MORTH Specifications or Stress Absorbing Membrane Interlayer (SAMI) of elastomeric modified binder at the rate of about 2 litre/m2 covered with light application of 10 mm aggregates to prevent picking up of the binder by construction traffic (AUSTROADS).

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A SAMI layer using elastomeric modified bitumen provided over the cementitious layer delays the cracks propagating into the bituminous layer.

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A crack relief layer of wet mix macadam of thickness 100 mm sandwiched between the bituminous layer and treated layer is much more effective in arresting the propagation of cracks from the cementitious base to the bituminous layer.

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Above figure shows a four layer pavement consisting of bituminous surfacing, cemented base, cemented subbase and the subgrade. For traffic > 30 msa, VG 40 bitumen is used. DBM has air void of 3 per cent after rolling (Bitumen content is 0.5 per cent to 0.6 per cent higher than the optimum).

Note: 

Cracking of cemented base is taken as the life of pavement. Minimum thickness of bituminous layer for major highways is recommended as 100 mm as per the AASHTO 93 guidelines. Stress on the underside of the bituminous layer over un-cracked cemented layer is compressive.

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Cemented layers normally develop transverse and longitudinal cracks due to shrinkage and thermal stresses during hydration and during the service life. Hence a layer of Stress Absorbing Membrane Interlayer (SAMI) of elastomeric modified binder (AUSTROADS 2004) is to be provided over the cemented base to resist reflection cracking.

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Geotextile seal and many other commercially available synthetic products available commercially have the promise to retard crack propagation in the bituminous layer.

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SAMI is not very effective if the crack opening is more than 3 mm.

2.1 Successful trial: 

Cemented bases with SAMI using modified bitumen topped with DBM and BC performed well in trial sections Krishnagiri-Topurghat Road Project (Tamil Nadu).

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It was designed for 150 msa traffic. Periodical inspections indicated fine transverse cracks at about 25 m spacing after three years and riding quality was maintained within the acceptable limits.

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Light spray of diluted bitumen emulsion across the fine cracks before the monsoon was done as a preventing maintenance to preclude any entry of water during the monsoon. FWD deflection studies also indicated low deflections.

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2.2 Comparison of thickness of layer with SAMI and without SAMI layer:

Without any Interlayer

With Crack Relief Interlayer of Aggregate

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With SAMI Interface

 From the above three figures, for a particular traffic 50 msa and CBR 3% i. Subbase thickness reduces by around 130mm when CT subbase with any interface is introduced. ii. Base thickness is reduced by 150mm if crack relief layer is introduced and reduced by 90mm if SAMI is laid. iii. For DBM, when crack relief layer and SAMI is present its thickness reduced by 85mm. iv. But one can observe that the thickness of SDBC increased by 10mm when interlayer is present. v. Crack Relief Layer of Aggregate of 100 mm is introduced.

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3. AUSTROADS Elastomers provide flexibility and elasticity leading to improved resistance to cracking and permanent deformation. They are used in sprayed seals and asphalt to make the binder more flexible and resilient, as well as in crack sealing materials and some emulsion applications. SBS (styrene-butadiene-styrene) at high concentrations is particularly useful for crack control in strain alleviating membrane (SAM) and strain alleviating membrane interlayer (SAMI) applications and improved resistance to permanent deformation in asphalt. SBS is one of the most common polymer types used in modified binders. Conventional bitumen (AS2008) is an effective binder for most road surfacing applications. However, the performance of bitumen may be enhanced by the addition of selected polymers or the use of multi-grade bitumen. Strain-Alleviating Membranes (SAMs) and Strain Alleviating Membrane Interlayers (SAMIs) are used for reducing the emergence of reflective cracking. SAMI is kind of sprayed seal. As per AUSTOROADS Stress Absorbing Membrane Interlayer is called as Strain Alleviating Membrane Interlayer

3.1 Sprayed Seal: Sprayed seal types are classified and/or selected based on the size, or sizes of aggregate, type of binder and the number of applications of binder and aggregate. Sprayed seals may also be reinforced with the incorporation of geotextile fabrics or glass fibres.

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3.1.1 Strain Alleviating Membrane (SAM): SAM is a sprayed seal wearing surface treatment used for minimizing the risk of reflection cracking on existing cracked surfaces, or where the potential for cracking exists. Typical applications are:  where the ingress of water may damage the underlying pavement  As a pavement preservation or holding treatment to maintain pavement integrity and defer more costly rehabilitation. In single/single seals, a minimum application rate of 1.5 L/m2 is recommended for effectiveness as a SAM. In double/double seals, the same polymer modified binder can be used in both applications or a lower modification or conventional bitumen can be used in the second application.

3.1.2 Strain Alleviating Membrane Interlayer (SAMI): A SAMI is a sprayed seal membrane consisting of a polymer modified binders (PMB) covered with an aggregate (typically 10 mm nominal size) which is applied prior to an asphalt overlay. It is designed to relieve tensile strains developed in underlying layers, and so lessen the likelihood of cracking in the overlay. SAMIs are generally used:  on cracked pavements, or where future cracking is anticipated  To waterproof pavements and bridge decks prior to asphalt overlay. 1. For effectiveness as a SAMI, a minimum modified binder application rate of 1.8 L/m2 is recommended. 2. Where a SAMI is to be overlaid with asphalt with little or no public trafficking, the normal conditions for spraying of polymer modified binders (minimum pavement temperature, cutting practice, etc.) do not apply. SAMI applications should use little or no cutter oil (2% maximum) to avoid the risk of diffusion of cutter oils into the asphalt binder and consequent softening or bleeding of the asphalt. 3. Where the application of the overlay is delayed for more than a few days, it should be designed as a SAM to avoid flushing/bleeding problems, particularly on roads with higher/heavier traffic. Restrictions on the use of cutter oil, however, still apply. For SAM and SAMI applications, assessment of pavement condition includes investigation of the type of cracking to be treated and its associated activity (rate of movement).

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3.2 Performance Conditions: Prior to selection of a sprayed seal treatment, an assessment must be made of service conditions including traffic, pavement condition and temperature regime.

3.2.1 Traffic Loading and Site Severity: The below tables refers to the type of selection of binder in SAMI applications and for incorporating that several conditions are taken:

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3.2.2 Assessment of Crack Activity and Crack Severity: As discussed earlier, for SAM and SAMI applications, assessment of pavement condition includes investigation of the type of cracking to be treated and its associated activity (rate of movement). Rate of Movement: Slow rate of movement or environmental cracking is induced by changes in moisture content and/or diurnal or seasonal changes. Rapid rate of movement or traffic induced cracking is induced by the action of repetitive traffic loading. Where large, seasonally active environmental cracks are present, and where crack movement is greater than 0.5 mm, a modified binder alone is unlikely to provide a long-term solution, and a geotextile reinforced seal using an unmodified or lightly modified bitumen binder may be considered as an alternative.

Crack severity: High cracking severity is defined by conditions where some crack widths are greater than 2 mm and/or the incidence of surface defects (patches, etc.) is frequent. Low cracking severity is applicable where almost all crack widths are ≤ 2 mm and the incidence of surface defects is isolated.

Note:  Prior to using SAMs, a program of crack sealing of existing cracks wider than 2 mm is advisable.  Where large cracks are present and where crack movement is > 0.5 mm, a PMB alone may not provide a long-term solution and a geotextile seal may be a more effective alternative. 11

3.3 Binder Selection: The binder is selected as per the above performance conditions.

NOTE: In SAMI applications, where the seal is to be covered with asphalt within a short period (e.g. several days), it is undesirable to add any cutter oil. However, if used, the maximum added should not exceed 2 parts of cutter oil per 100 parts of PMB.

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4. AASHTO It is used with flexible layer rehabilitation as a means to retard reflective cracks, prevent water intrusion, and (in the case of SAMI-R (rubberized)) enhance pavement structural strength. A SAMI may be placed between layers of new flexible pavement, such as on a leveling course, or on the surface of an existing flexible pavement. Synthetic fabrics and stress absorbing interlayer ie SAMI is effective in controlling the reflection of low and medium severity alligator cracking. They may be useful in controlling reflection of temperature cracks, particularly when used in combination with stress filling.

4.1 Types of SAMI: a. Rubberized (SAMI-R). SAMI-R is a rubberized chip seal. b. Geosynthetic Pavement Interlayer (GPI), consists of asphalt-imbued geotextile a. SAMI-R Since SAMI-R is a rubberized chip seal that can act as a moisture barrier, it should be used with caution in hot environments where it could prevent underlying moisture from evaporating.

b. Geosynthetic Pavement Interlayer (GPI) It consists of asphalt-imbued geotextile. It should not be placed directly on coarse surfaces such as a chip seal, OGFC, areas of numerous rough patches, or on a pavement that has been cold planed. Coarse surfaces may penetrate the fabric and the paving asphalt binder used to saturate the fabric may collect in the voids or valleys leaving areas of the fabric dry. For the GPI to be effective in these areas, use a layer of HMA prior to the placement of the GPI. GPI is ineffective in the following applications: I.

When placed under rubberized hot mix asphalt (RHMA). This is due to the high placement temperature of the RHMA-G mix, which is close to the melting temperature of the GPI.

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For providing added structural strength when placed in combination with new flexible pavement.

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III.

In the reduction of thermal cracking of the new flexible pavement overlay.

4.2 Requirements for thickness of SAMI layer as per California Department of Transportation. The SAMI-R is placed under non-rubberized hot mix asphalt that is engineered for reflective crack retardation; the equivalence of a SAMI-R depends upon the type of base material under the existing pavement. 

When the base is a treated material, a SAMI-R placed under HMA (Hot Mix Asphalt) or OGFC( Open Graded Friction Course) is considered to be equivalent to 0.10 foot of HMA.

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When the base is an untreated material SAMI-R is equivalent to 0.15 foot of HMA.

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A Geosynthetic Pavement Interlay (GPI) placed under HMA that is engineered for reflective crack retardation provides the equivalent of 0.10 foot of HMA.

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