Handle9: I’d be interested in hearing from @benoire on this as someone who has actual experience with designing roading projects.

Howdy folks!

Ironically I have just tasked my engineering team with looking at concrete roads again to see if there is value in developing some rules around the use of concrete on certain types of road/usage e.g. bus/freight lanes, bus interchanges etc.

So a breif basis of road pavement/surface design for Auckland - The stuff that transport operates on consists of 3 distinct manmade layers (sub-base, basecourse and surfacing) on a natural ground formation with a minimum defined strength (CBR). 

• Sub-base is generally a GAP type product (65mm) and this is to bring the strength (CBR) of the ground up to around 20-30%
• Basecourse:

   

  • cementiously bound AP40/M4 spec aggregate (increased CBR and various other engineering advantages at the cost of increased construction cost and time)
  • unbound (laid and rolled AP40/M4 spec aggregate but not hydraulically locked together so only useful on lower volume roads with low heavy vehicles)
  • Asphalt - generallly laid as structural AC and this is the best hardest wearing product as long as subgrade is good - also the most expensive.
• Surfacing:

   

  • Chipseal - cold emulsion or hot applied bitumen seal coat with roadchip sprayed on top - essneitally provides a waterproof membrane and skid resistance from the chip - generally used on unbound road pavements.
  • Asphaltic - either as part of an Structual AC system or a thin surface course with product defined based on ESA and other stuff.

Chipseal is 1/4 of the cost of Asphalt but lasts about 1/2 the time so it works out, on an average road <10000 vehciles per day and 5% HCV, as the cheapest option long term with full asphalt being reserved for roads >10K vpd or larger HCV %ages.

So to the concrete conversation - in the standard pavement makeup concrete would replace the base course and surfacing layers and has a lower requirement for CBR from the formation.  Concrete has the advantage of spanning softspots in the ground, is hard wearing, long life and highly resistant to heat damage (environment and vehicle), potholes, rutting etc. if intact and built correctly (expansion/conctraction jointing and controlled crack jointing at defined points).  Its major distadvantages have been construction time, reinforcemen needs, noise generation and once cracked the failures propegate fast across the surface leading to massive failure and time/cost to fix.

With the use of polymer fibre reinforcement in concrete the polymer is dosed at the batching plant to the right amount and can achieve the same strength increases as a 20mm high tensile steel reinforcement could, but can be cut through with a diamond saw without issue.  This negates a major problem with concrete with respect to additional work, utility placement etc. 

However all of this doesn't negate the major problem with concrete which is time to construction (gotta acheive that 7 day min strength for use), ride quality or the noise which is considerable as concrete has no sound dampening properties - UK got around this by building composite pavements, concrete roadbase and asphalt wearing course.

Noise and ride quality cannot be overcome really without using an asborbancy layer like the UK do with the composite pavement but we're looking in to dry mixed concrete which only needs ~72 hours to cure for use and can be laid like a leanmix concrete so no slumping or issues on steeper grades.

So why would I, as the head of engineerinng and design for Auckland Transport, consider concrete?  Well we're struggling with the HPMV trucks which drag their rear axles leading to lots of shear in an asphalt surface and therefore failure, the electric buses are just heavy and pounding the network on the frequent network routes and the increased expecation of hot weather days is going to cause havoc causing bitument to soften.  Concrete pavements can offer advantages in these areas and so if we target the correct lane / corridor then it may work as fibre reinforced concrete is not far of the cost of asphalt and with the massively longer design life will lead to whole of life savings that are quite considerable.

concrete is not a fit for all roads pavement and for all situtions but its something we need to consider, espeically as we're about to publish some work on low carbon concrete which effecively puts the emission to the floor and makes it highly viable against asphalt with RAP (recycled asphalt) from a sustainability perspective.. 

some random rumblings on a saturday am :-)