I was at a pavement meeting this week for a road tender project. These meetings never used to happen, as the choice was blacktop of what thickness (except DBFOs) and the market place used to sort that out, usually after an award. As the rules are new in HD 26/06 and the now much clearer IAN 73/06, 2009 update, there is a part of the meeting that is educative. It's not that its complicated to understand as you still end up with thicknesses of layers, it's the choice that complicates. In this choice matrix are grey areas, the largest is the defining of the subbgrade CBR with any certainty, this particularly with soils that are not all weather types. The usual case. The other grey area that could be tempting particularly with traffic levels > 80 MSA, is modifying site won materials to make a CBGM subbase. These grey areas is where the ground investigation can help or hinder, usually the latter. Ground investigations rightly concentrate on structure foundations, I would like to see as much effort put into subgrade conditions and classification of fill, and I don't mean the usual hundreds of plastic and liquid limits, I mean MCVs, shear strengths, OMCs, recompacted CBRs etc. Geotechnical engineers who determine the laboratory testing on road projects take note. Ground investigation has plenty of issues not the least of them being the clauses and caveats  that allow no responsibility to be subject on those that design and have the Ground investigation work carried out. So back to choices, some are made for you in the clients documents, traffic, PSV etc, and in Scotland  there is no point in an HBM D as we are not allowed to go that thin, and of course  departures. Underlying these choices is engineering but the driving force is cost. So it falls on the poor estimator to price up the choices, here's a scenario given you choose restricted designs, for traffic > 80 MSA. Needs a CBGM subbase for a class 3 foundation, thickness varies with subgrade CBR, then to cost are; mix in place or plant mix, plant on site, or a nearby one off site? Plant on site has the benefit of using unregistered tippers running on red diesel. HBM (leanmix) as a base, or blacktop? Which HBM and which type of blacktop. Then compare your costs with each other ( this is predicated on receiving sensible costs) Is HBM A, B or C with 180mm of blacktop more expensive than blacktop over  CBGM foundation class 3. Then there's the program, CBGM and HBM have a no traffic rule for usually 7 days, fine with a project with plenty of lanes, what about ties ins? What if the client insists on keeping a certain number of lanes open at peak traffic flows, do you build temporary diversions as you have to use a CBGM subbase and leave it for seven days? the once available alternative  being a full blacktop tie in pavement laid on a night shift ready for the AM 4 x 4 school run. Has land been made available for temporary diversions? Then if you ever bottom that out a director type will ask about performance designed foundations!!  How did he find out about them ?  The risk with performance design  is proof  of what you have built  is not just thickness but proving performance, and while the deflectograph is still the rough filter for pavement approval for Transport Scotland, then.................... Drive you round in circles, choices!!

6 Responses to “Pavement choices”

  1. May 19, 2009 at 8:41 pm

    Let’s make life interesting, go for the performance design.

  2. Ian June 12, 2009 at 6:29 pm

    Well DMRB, typing that has just made me smile, good acronym for a pseudonym. Voting for a performance design gives me a profile of you, consultant type, product of a uni, revels in facts and figures, particularly if there’s a published paper in it. Push the design boundaries when there’s no risk for you. Bet you wear a tie every working day and classic value for money brogue shoes, in that your children will get wear from them?
    DMRB the most unlikely lad

  3. Ian June 12, 2009 at 6:41 pm

    What did my blog say about director types? why not performance designs? Well my director called me today as i was in HQ, well regional office as Inverness is now HQ. An aside, but in his carpeted premises I was instructed to deliver a talk on pavement design and the new European specs on concrete. Fine, but not to our graduates, as you can tell them anything, and it doesn’t matter anyway for if you don’t use the info imparted it’s lost in two weeks, same for this audience but its agents to project managers so they have present knowledge which means better delivery and factual or f**ked.
    Directors!

  4. November 8, 2010 at 2:10 pm

    Ian, I am struggling with UK spec to here (Tashkent).

    HBM

    I read the advice as if over 6% cement (our spec) you can lay at 3 deg C. You do not need to consider freezing later and you can traffic once the PTR says so, or because it is crushed rock and has mechanical interlock. Cracking will occur thus you use crack inducers. You seem to see it different. Sorry we are stuck with a short time to complete, I am out of it with materials now, and the materials engineer we have is shit. Meanwhile the Russian Spec wants 14 DAYS. Need to get EOT and shorten this period or we are in shit.

    • Ian November 8, 2010 at 2:35 pm

      Well John Anthony, someone was asking me where you were the other day, now I know, but have forgotten who asked me.
      kinda understood were you’re coming from, if you want to reduce time for trafficking the UK spec allows this immediately if a PTR is used. If you want to protect from frost then usually a strength of 2/3 N/mm2 will suffice. you could make cylinders or cubes and cure them on site this will give you a frost protection guide and if and when you get 8 N/mm2 I would say you could traffic the layer. this could be dangerous as it might take more than 14 days to get to 8N/mm2 or MPa as the EU makes us use. Any CBGM/HBM over 10MPa at 7 days lab cured needs pre-cracking at 3m intervals. Can’t remember the temperature laying rules for laying but you can find them on. http://www.standardsforhighways series 800 of vol 1 of the MCHW

  5. November 8, 2010 at 4:13 pm

    Thanks Ian, I owe you one

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