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.

One Response to “Pavement Design 2”

  1. Ian November 9, 2009 at 7:26 pm

    I did a spreadsheet exercise today on a 40 MSA pavement using a CBR of 10 and a CBR of 15, and I did the same for 80 MSA. For the 40 MSA there was one class 3 that was cheaper than all the restricted designs, and all the HBM base designs were cheaper than blacktop. With the 80 MSA there was no class 3’s or 4’s cheaper than a restricted design, and all the HBM base designs for class 2 single and class 2 with capping being cheaper. Again both bases highlighting as “commercial” were HBMs, the 40 MSA a 500 MPa one and the 80 MSA a 200 MPa, both singles, ie no capping. Well with a subgrade CBR of 10 and 15 you should not need capping unless you have serious wet weather and almighty site traffic.
    It is obviously down to prices on my spreadsheet which base will be chosen, but the fact remains Class 3 foundations figure sometimes and class 4 foundations are not commercially valid. That’s my humble opinion anyway !!

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