Materials Man

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!!

Running up to Christmas, just had company Christmas lunch, do you notice when the loud music starts the older guys head for the door? I am part of that congregation.

Anyway I am old enough to remember digging sand replacement densities in lean mix and wondering at the mysteries of comparing them to a theoretical density. Then there used to be a minimum strength, then a minimum and a maximum, then if my memory is correct, a minimum again. Whether it was the blacktop lobby or a diminishing amount of long greenfield road jobs, the good old leanmix was no longer in favour. perhaps also by then enough roads had been built and the resultant transverse cracks were apparent. Again by memory (this site is for conversation, not for me to reel off details checked and verified, thats for you lot!) so memory, was not lean mix a not allowed roadbase on Scottish roads?

I am getting round to HD 26/06 and IAN 73/06, isn’t the IAN wonderful reading? not, but as I see it trunk roads, 80MSA stuff, brings in lean mix, OK it’s HBM now and you have to design it on 365 day results, add PFA, and test beams, cylinders, cubes and any other testable shape, for creep, compression, bending strength, indirect tensile strength and tensile strength, whow. why is it engineers (them with degrees) want to test brittle materials for everything? is it because they can? Flexible materials get of lightly, or did, another post!

where was I? Christmas lunch top up happening! Oh yes back to the past lean mix road bases are in, and the only fight back the blacktop boys have is EME2, how lucky is that that a flexible material is stiff? Trust the French, and why? Well the trunk roads in France are essentially DBFOs (design build finance and operate) so the contractors control the research. Yes, the most Social (with development) part of Europe allows the market place to sort out the roads.

Ask yourself how many “developments” in UK blacktop have come from Germany and France?

Sounds like a bit of a rant, but Talisker makes you type, see what I saying, where’s the leadership is road design? lean mix good, lean mix bad, new lean mix necessary for trunk roads but test it for everything and have design parameters for year old tested specimens.

explain that to an estimating manager!

David?