to prime or not to prime...
to prime or not to prime...
(OP)
Just wondering what the current concensus is on applying a bituminous prime coat to the aggregate base course on roadway prior to placing asphalt concrete (assuming major urban arterial road)





RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
It is similar to other "standard" practices that we see which have become just habit and for no good reason.
A glaring example is codes requiring gravel as backfill for underdrains.
If we wanted, I am sure a long list can be made up of these useless requirements.
RE: to prime or not to prime...
RE: to prime or not to prime...
Here is why I question the use of a prime coat and am seeking advice: Does the application of a prime coat reduce the infiltration of rain into the base-mix asphalt?
I've been involved on several projects where there was "failure" of the "approved" subgrade during construction. The geotechnical testing firm and civil engineers were claimed for inadequate subgrade approval and inadequate civil design. The latter claim that the civil design should have included subdrains. What actually happened was the contractor prepared the subgrade, placed the dense-graded stone base and applied only the base mix asphalt, using this as a working surface during construction through the winter. Well, falling weather readily infiltrated the base mix asphalt, saturated the stone base and the construction traffic rendered the subgrade to mud, thus failing the subgrade, stone base and base-mix asphalt. One sample of the base mix asphalt that I collected displayed a permeability of 0.01 cm/sec. Needless to say, base mix asphalt does not provide much of a hydraulic barrier.
So, if a prime coat would substantially improve (i.e., reduce) the permeability of an exposed base mix asphalt, maybe it's a good idea?
Comments?
f-d
¡papá gordo ain’t no madre flaca!
RE: to prime or not to prime...
My concern with not specifying what the Industry Institute and the DOT’s recommend is that if you are called upon to defend your work under a failure condition, you are solely accountable.
RE: to prime or not to prime...
The original point was to use a prime between the aspahlt and aggregate base. Yes, it has always been done (generally) and, while offering no "strength" value, it does have attributes that will help a pavement behave better over the long term and especially the short term like fattdad's example indicates. Chip steel pavements - the aggregate of the single surface course of aggregate on the base course offers no structural strength - but if you don't use it on your aggregate base course, you'll have problems.
One of the first jobs I was on was an asphalt pavement breaking up under traffic - it was only 25 mm thick and was supposed to be rural road with little in the way of heavy trucks. New development changed all that. But what I found was that there was no adhesion of the asphalt layer with the dusty base course - and hence the sliding/shoving of the asphalt near stop signs.
My thoughts . . .
RE: to prime or not to prime...
RE: to prime or not to prime...
Above, a member alluded to the fact that heavy construction traffic was "design traffic." I would argue that the heavy construction traffic shouldn't have much of an effect on the roadway. The trucks and equipment should easily be able to use the roadway at all interim conditions, with no detrimental effects. As I understand it, the limiting factor in pavement life is the repeated (long term) traffic loadings, not the relatively short heavy construction traffic.
As for an answer to the original question, I don't have an answer, but have often wondered the same. My "method" is to use the tack and prime coats for traffic loading situations such as roadways, and to leave it out for parking lots, driveways, etc. Although I have no basis other than that that's what the DOT requires for their roadways.
RE: to prime or not to prime...
e.g. Acceptance on Friday, weather event expected over the weekend, base course scheduled for Monday = Prime.
Tack coat is best applied when overlaying or if the previous layer has cooled. (RoT: cooled for more than 24 hours)
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RE: to prime or not to prime...
RE: to prime or not to prime...
RE: to prime or not to prime...
(A) I’d offer that this part of the Contractor’s “means and methodology”. On an early career interstate project, the paving contractor used a belt placer. Haul trucks were unloaded alongside of the paver, not in front of it. They were never on the subgrade.
I dug out an old AASHTO Flexible Pav’t Design charts provided for Daily Equivalent Axle Loads, DEALs, and ran some quick AASHTO pavement numbers. Assuming the design shows a need for the finished pavement’s composite S/N of 3.0:
Rock = 0.1 / inch, 6” equals a S/N = .6
Bit = 0.4 / inch, 6” yields a S/N = 2.4
How many contractor’s DEALs are running on your subgrade? Running 1000 tons of bituminous in tandems equals 67 DEALs. Assuming a soil support value of 2 to 3, that requires a S/N of around 4. I don’t see how this could be met with 6” of rock providing a S/N of 0.6.
My experience is that the Contractor will attempt to deflect their overloading of the pavement as a defect in the Design. This is simply not the case in the above example. The Contractor needs the correct equipment on hand, even if it means renting something.
RE: to prime or not to prime...
What does your computations show if the subgrade (natural) is a soft silty clay with water table at or very near the surface? How many loaded trucks can you run on that without pumping, rutting, etc? Until you get the pavement structure thick enough to take loads of the heavy construction trucks (as they do drive on the building road in my experience) their loads and the problems with precipitation cause problems. I've seen a lot of road bed chewed up in poor weather by heavy loaded trucks - and you have to go back and rehab before you can place succeeding layers.
The major point you made, though, is, that if you can use buggies and load into hoppers off the road, this is the better way (asphalt); but I'm not sure how this would be done for spreading select subgrade/subbase/base or slipform concrete pavement. Thanks for the interest.
RE: to prime or not to prime...
In my humble opinion, if final cover (course layer PCC or HMAC) is going to be inmediately placed over the base, there is not a real reason to use the prime coat, I'll directly apply the tack coat and then pave, however, in the real world this situation usually does not happen, even in the states (I assume that those who refer to DOT's are US based.)
Perhaps the most common practice arround the globe, is to place the base and some days, some weeks or even several weeks later place the course layer. Meanwhile final layer is placed the base is used for multiple purposes, as socking deck, as parking lot, as course surface. In a hihgway dumptruks, ready mix concrete trucks, HMAC dump trucks and any other heavy piece of equipment you can said will road over the base. In comercial or industrial facilities is always the same history. So,I think that prime coat is intended not to protect the base against moiture entering or to provide and additional strenght or bond, my opinion is that prime coat it is intended to protect base against moisture lost and maintain surface aggregates bonded. Water entering should not be a problem since one of the multiple functions of the base is precisely provide an adequate sub-drainage in order to keep the pavement structure as dry as possible, if base accumulate water, there is not a problem related with traffic estimation, it is probably a problem with designing considerations, in highway construction, base usually can freelly drain the entering water through both sides of the roadway, however, in situations like parking lots or urban roadways this free drainage does not exist in that case when designing a higher value of lost support should be used and consequently a thicker pavement structure will result.
Guys nevertheless, this prime coat is not the panacea, it is just a temporary and quite limited protection and if base is under heavy loads or bad weather, perhaps the layer should be repaired before final layer will be placed, maintaining the layer in moisture and compaction will be necessary even if base remains unprotected for a few days with no traffic, so, i think that the question should be:
To place the base now or to rapair it later?