Spray seals
GENERAL
Bitumen is an effective binder for most road surfacing applications. However, the properties of road grade bitumens may be enhanced by the addition of selected polymer to:
* improve aggregate retention provided adhesion is established
* minimise or delay reflection cracking
* minimise bleeding
* reduce water penetration
* improve shear resistance in high traffic situations
* reduce temperature dependence of properties
* reduce oxidation in high temperature regimes
The use of polymers in sprayed seals may also be beneficial in situations for:
* the improvement of aggregate retention during the early curing period of an emulsion seal
* the improvement in resistance to lateral shear in situations of high stress due to turning and braking manoeuvres
PMBs have been used as sprayed seals for both SAM (strain alleviating membrane) and SAMI (strain alleviating membrane interlayer) applications.
Strain Alleviating Membrane (SAM)
A SAM consists of a scrap rubber bitumen or polymer modified binder with aggregate (typically 10 to 14 mm nominal size) in single or multiple applications. Larger aggregate used with the thicker binder films is found to be more effective in inhibiting crack propagation. A SAM is used for:
* reducing reflection cracking
* waterproofing pavements and bridgedecks
* improving aggregate retention in high traffic stress areas
SAMs are effective on pavements exhibiting reflection cracking where crack widths are less than 2 mm and where deflection behaviour of the pavement has not deteriorated to the stage where pumping of fines to the pavement surface is a problem.
Strain Alleviating Membrane Interlayer (SAMI)
A SAMI is an interlayer consisting of a scrap rubber bitumen or polymer modified bitumen binder with aggregate (typically 10 to 14 mm nominal size) which is applied prior to an asphalt overlay. It is designed to dissipate tensile strains developed in underlying layers and so lessen the likelihood of fatigue failure in the overlay. Interlayer properties (i.e. low modulus) may become a factor in analysis of pavement stress-strain behaviour in pavement design and should be considered. SAMIs are generally used:
* on cracked pavements
* to waterproof pavements and bridgedecks
Scrap Rubber
When scrap rubber is used, the grading and morphology of the rubber particles, together with the digestion conditions (time and temperature) m bitumen, will influence the properties of the final blend.
PRECAUTIONS
The use of PMBs should not be regarded as a substitute for good design and practice. Their use imposes additional constraints to those applicable to seals with conventional binders. These constraints include:
– the avoidance of placement at low temperatures (pavement & ambient) which may lead to lack of adhesion and loss of aggregate
– ensuring the compatibility of the binder with the proposed aggregate precoat and the adhesion agent has been confirmed by testing with the proposed aggregate
– the avoidance of moisture at the binder stone interface or on the surface to be sealed
– the avoidance of dust on the aggregate the provision of precise aggregate spreading operations the exclusion of fluxing agents from the binder
– extreme care in the selection and use of cutting agents
– strict adherence to recommended binder temperatures (typically 180*C – 200°C for hot sprayed sealing applications and 80’C 90°C for emulsion applications).
EXPECTED PERFORMANCE
To date, monitoring of SAM and SAMI treatments indicates:
* the performance of treatments is dependent on the degree and rate of crack movement and on crack width
* pretreatment of cracked pavements is essential
* the treatments are effective in minimising the reflection of fine to hairline cracks (< lmm) from the underlying surface
* with medium to coarse cracking (up to 5mm), the treatment merely delays reflection cracking, however the resultant reflection cracking should be less severe than the cracks in the original underlying surface
* the treatment cannot be expected to prevent the reflection of wide active cracks from the underlying surface
* concrete bridgedecks may require priming prior to the application of a PMB seal to ensure proper bonding. The presence of curing compounds on concrete bridge decks can cause problems and compatibility with PMB should be investigated
* PMB treatments are most effective in dealing with cracking caused by surfacing deterioration and binder hardening, and are less effective for treating severe structural or environmental cracking
* these treatments can arrest degradation of distressed pavements by preventing further ingress of water
* selected PMBs will reduce the effects of shear on winding roads and high stress areas
* these treatments are considered to be methods of strategic control of surfacing distress and not necessarily the elimination of the distress
* SAMI treatments can prevent water ingress despite some reflective cracking
DESIGN
It should be noted that seal design technology needs to be modified when SAM and SAMI techniques are used. The binder application rates will be generally higher when PMB sprayed seals are used. Aggregate application rates also need to be increased to accommodate the differences in aggregate packing as the ALD is not always attainable with PMBs.
