PMBs may be used in all types of asphalts (Eg. open graded, dense graded and gap graded). Asphalt may incorporate PMBs to meet specific requirements and to improve service performance.
It must be remembered that the binder constitutes only one component of the asphalt mix and therefore the performance and properties of any PMB asphalt will also be affected by the aggregate skeletal structure, aggregate quality, adhesion properties, compaction, etc.
Furthermore, the benefits of using PMBs in asphalts in new construction depend on the pavement design methodology, layer thicknesses and the location of the PMB asphalt layer in the pavement structure.
Conventional mix design and testing techniques are applicable to PMB asphalts. However, to realise the increased benefits from polymer modification of the binder, structural characterisation of the PMB asphalt is recommended.
Rutting in asphalt (dense or open graded) occurs due to the plastic flow of the material. Plastic flow is irreversible and may be due to high applied stresses caused by vehicles or sustained elevated temperatures on a hot day. This permanent deformation is irreversible and therefore results in a displaced or rutted asphalt surfacing.
Cracking in a pavement surface can be due to shrinkage or fatigue. Shrinkage cracks undergo thermal movements over a range of strains varying from zero to very high. Some of the higher thermal movements are too great for a SAM or SAMI treatment. Fatigue cracking of a pavement occurs due to the cumulative effect of vehicular movements over the pavement within a finite time. These are best treated with a SAM or SAME application.
Tests are being developed to better measure directly the properties of binders required to achieve these benefits. These are given in the Specifications and Test Methods for Polymer Modified Binders
PMB asphalt mix can be designed according to standard mix design procedures. A PMB with properties most suited to the desired performance requirements is chosen. The resultant mix design should be adjusted to accommodate any volumetric requirement of the selected modifier where this effectively contributes to the volume of the solids (Eg, scrap rubber). The structural characteristics of the mix should then be checked and further optimised if necessary by using mechanical testing methods.
Experience to date indicates that binder contents higher than those normally used in conventional mixes (with a consequent reduction in voids) may be acceptable for PMB asphalts and may enhance fatigue performance.
The design of an asphalt mix must take into account the following factors:
A. The identified purpose of the mix. ie. resistance to rutting, fatigue resistance, high strength, low noise, high skid resistance in conjunction with,
B. The environment in which it is being used ie. the service
temperatures at the site and traffic levels and loadings.
Before considering the use of a modified binder in a demanding application, it should be recognised that significant performance gains can be obtained by prudent choice of aggregate grading and aggregate shape and texture, binder class etc.
MANUFACTURING AND PLANT MIXING
Due to the higher viscosity of PMBs, the mixing conditions should be adjusted according to the manufacturer’s recommendation. These recommendations may entail increased mixing temperature and duration, and specific measures to minimise heat loss during transit.
PLACEMENT AND COMPACTION
Placement and compaction of PMB asphalt may vary from conventional mix, requiring an elevated compaction temperature according to the manufacturer’s recommendation and a modified compaction technique to compensate for possible decreased workability. Additional rolling equipment may be required to ensure immediate compaction. Care should be taken to avoid “pick-up”. Compaction techniques may include the extended use of steel wheel rollers and delaying access to rubber tyred rollers. The delay may be minimised by preheating the rubber tyres. Due to specific temperature requirements and reduced workability, joints require particular attention. Transverse joints should be minimised by careful scheduling of delivery.
Open graded mixes, particularly with SBS modified binders, require even greater attention to temperature and timing.