Materials Man

I did deliver my pavement design talk to my senior colleagues on two separate occasions this week. Fortunately my stumbling performance was politely received, my highland hosts have an imbued politeness, which contrasts with my part weggie assertiveness. (Pure dead aggression, bye the way, know what I mean?)

To the talk, my spreadsheet brings together everything I put in text, and highlights, “where’s the money”. I have set it at 80 MSA  as that would appear to be the “design” standard adopted by Transport Scotland for most trunk roads. Perhaps so, if you have the money, order the best you can.

As I mentioned before class 4 designs are far too expensive, this cost comes from the foundation, the class 4 pavement is the cheapest as it is the thinnest. So I’m not sure where the highways agency thinks a class 4 foundation will be used, perhaps I need to run a few more designs with different foundation CBRs.  All blacktop surfaced foundations are more expensive than hydraulically bound materials at 80 MSA. this should worry the blacktop boys as even the very good EME2 material does not feature as a base. I also believe I can reduce my HBM prices as I have induced cracks in  HBM foundations when I now believe I was being conservative.

Other features that emerged were discussions on what needs cracked, it seems clear that mixes with 10 N/mm2 at 7 days, laid widths more than 4.75m and all HBM bases with a blacktop surface, need induced cracking. the other item was laying tolerances, it is clear that series 700 is not up to speed with the new requirements. No negative tolerances are a particular problem and need careful thought and discussion at tender time and when subsequently placing a sub-contract. Do both pre-tender?

A adjunct to pavement design was raised by me, Transport Scotland have road compliance testing at years 3, 4 and 5, this includes the deflectograph. If this shows potential defects, then investigation follows. If your pavement suffers from debonding, it probably will show as a potential defect. It is vital therefore, to “prove” your design was built to the DMRB, to record your bond coat applications, with all the attendant spraybar and product certificates. This will be necessary till a bond test is introduced. Also have the lab that cores the pavement from the upper layers to record and photograph the whole core before any splitting is performed.

I did a talk to our graduates recently on pavement design with a construction director, I did the technical bit he did the money / risks bit. Directors don’t get to be directors in a LTD company without merit. For those of you mentally compiling a list of  directors without merit,  just ask yourself if they were good for the company, ie made money? So there are certain learning opportunities watching a director who has had training and experience that I will never have. It was a complete focus on “where’s the money?” ” and how can you lose it?” A money risk workshop.

Although I am never oblivious to money, it  can be a blinkered vision when you  concentrate on the technical bit, consultant pavement designers take heed!! for what I am about to reveal may sound counter intuitive.

So when instructed to do a similar talk to senior staff I decided to take it all of it on board myself, and what I have completed (but not yet delivered) was an introduction to the design principals followed by a lengthy exercise on IAN 73 and HD 26.

Now the exercise I speak of is a spreadsheet where teams will fill in the thicknesses from the design charts in IAN 73 and HD 26 for a MSA of 80, for various CBR formations,  and discussion will follow. I have Scottish market prices on the spreadsheet. This reveals certain outputs:

performance designs are more economical than restricted.

HBM bases are more economical than blacktop bases.

The cost of the whole foundation plus pavement has various drivers, but the foundation costs predominate, despite them being around a fifth of the whole pavement + foundation cost.

This means  the choice of pavement , ie cheapest with risk analysis applied, should mean that HBM  and performance design will predominate and class 3 and class 4 foundations are far too expensive.

This has singular outcomes particularly if oil/bitumen prices soar, which does affect HBMs cost, but not to the same extent as blacktop.

I did ask HA if a economical appraisal had been conducted and the answer  was no, but I have read that IAN 73 and HA 26/06 have been subject to an economical appraisal.

Well if my spreadsheet is correct then there are two (50% of the available performance designs) that will never be used !!

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!!