A New BS for Spray Applied Stress Absorbing Membrane Interlayers (SAMIs)

Stress absorbing membrane interlayers (SAMIs) are introduced into an asphalt pavement for a number of reasons. They are primarily used to reduce the incidence of reflective cracking when overlaying a cement bound or jointed concrete  pavement.  They also can have a reinforcing function when they incorporate a geogrid.

This new British Standard, BS 4342:2023  “Spray applied bitumen system for stress absorbing membrane interlayers. Materials, installation, and testing – Specification”, is limited to SAMI providing reflective crack control. It provides no information on the performance of the specified systems in delaying reflective cracking. It is targeted at providing a consistent high-quality installation of the relevant system.        

There are a range of different materials and techniques available for these SAMIs, including asphalt, geo-synthetics, and bituminous binders, which can include bitumen emulsion, paving grade and modified bitumen. Whilst PD 6691 provides guidance on the requirements for asphalt which may be used for SAMIs and ISO NP TR 18228-10 “Design Using Geosynthetics: Part 10 – Asphalt pavements” gives guidance on design of interlayers with geosynthetics, including geogrids,  currently only BS 434-2 has a paragraph (albeit very short) on the installation of bituminous sprays. Therefore, there was a need for a standard to manage the quality of sprayed bituminous interlayers from process of production to installation.

A new British Standard BS 9434:2023 “Spray applied bitumen system for stress absorbing membrane interlayers– Materials, installation, and testing – Specification” has recently been published.  The new standard contains 10 pages, presenting the installation requirements for two kinds of SAMI. A SAMI of bitumen emulsion spray containing chopped reinforcing fibres, and a spray applied SAMI with three kinds of protecting layer.  The protecting layer includes options for the use of paving fabric protection, aggregate protection, and slurry surfacing protection. Conformity of type approval installation trial to the British Standard is addressed by the plant and method statement in accordance with the installer’s quality plan and ongoing surveillance of the quality of installations by the contractor himself under ISO 9001. There is a recommendation that this is supported by National Highway Sector Scheme 13, as used for surface dressing and other surface treatments.