Segragation in Hot Mix Asphalt...

A proper gradation of aggregate is essential for performance of a mix.  Sometimes during plant mixing or placement of Hot Mix Asphalt (HMA), smaller aggregates are separated from larger aggregates leaving a fragmented mix.  For example segregation occurs when a patch crew throws shovels of HMA in a hole rather than placing the HMA.  Segregation reduces the performance of the mix by changing the stability of the product and compromising the air void content.

Somewhat related to segregation and evaluated with the same methodology is stripping in HMA.  Stripping occurs when the asphalt covering the aggregate no longer adheres to the aggregate.  This may happen with moisture and heat or be related to the aggregate.  Observing the patterns from geophysical images, measuring them, and interpreting the results can often identify suspect stripping.  GPR can often provide detailed screening, which can be followed up by other means, including geophysical NDT methods and destructive sampling.

Techniques Recommended

Surface NDT Methods:

• Ground penetrating radar (GPR).
• Spectral Analysis of Surface Waves (SASW) / MASW.
• Impact Echo.
• Acoustic-ultrasonic.
• Infrared Thermography.

A brief note on Infrared Thermography:  It can successfully be applied to this application, particularly if variations in paving processes during initial construction are being identified.  Variations in temperature within the asphalt as it is being unloaded from dump trucks, routed beneath the paving equipment and rolled, or "shoveled-in" HMA are areas that appear as thermal variations of significance in the infrared image.  IR can be used very effectively for both QC and initial baseline condition assessment (QA of sorts).  While other methods are available or under development, the ones listed provide cost-effective determination of pavement thickness.