to prime or not to prime... 3

[cvg]

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)

 

[civilperson]

Unneccessary unless you are planning staged construction. If the entire structural section is to be built prior to traffic, then a prime coat is superfluous. It could help if you are only building a fraction of the final depth and using the fraction for traffic.

 

[cvg]

what if part of the road is paved with one lift of base asphalt and traffic is routed on that section while the rest of the street is being done? would that warrant using a prime coat?

 

[civilperson]

Staging construction traffic needs a tack coat between lifts of asphalt, (sweeping and tack), but is not a reason for the prime coat.

 

[eea]

I had this same question about 15 years ago. Why do I need to bond the top of the aggregate to the bottom of the hot mix asphalt (HMA)? The answers were mixed, but a concern is of the base aggregate's affinity for asphalt (AC). Non-primed base could possibly pull some of the AC out of the HMA. I've seen most DOTs prime aggregate base course as standard practice.

 

[cvg]

eea, thanks for the reply. I agree that our State and County DOT's do prime. However, a city that I am working for doesn't appear to have a standard requirement for prime coat and even though I have seen it done and recommended it, I could not give a good reason to do it.

 

[Dozerman56]

Here in NC, the land of extremely moisture sensitive soils, DOT is really the only entity that specifies prime coat, and then usually only if you're going to put the stone down and leave it unpaved for an extended period.

 

[oldestguy]

When it comes to designing pavements for thickness, based upon soils and traffic, as well as climate, etc. I know of no method that gives any benefit for the prime coat. And for good reason- no benefit.

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.

 

[eea]

cvg, I should have also offered that the Asphalt's Institute's Principles of Construction of HMA Pavements list priming of the aggregate as part of their standard practice. I used to wonder how effective was tack coat until I took core samples and found I couldn't separate the lifts without using a hammer (glue, strong stuff). As far as a cost standpoint, what are you talking, $10k per mile at $2 per gallon at 0.375 gal/SY for a two lane pavement? Pretty cheap insurance if things go awry.

 

[fattdad]

I don't recommend using a prime coat between base-mix asphalt and surface mix asphalt - maybe I should. I fully agree with oldesguy - in my method of pavement design (AASHTO or VDOT), there is no structural value for the prime coat and as a result, it doesn't factor into a design strength.

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!

 

[eea]

fattdad, under what conditions would a partially completed road be capable of bearing design traffic, let alone possibly greater construction traffic loads? I’ve seen a number of local projects hammer down on the contractor trying to use failure excuse as anything but “his own doing” from premature / excessive loading. I just witnessed 3 miles of interior roadways at a mall being paved with a concrete pumper truck to keep concrete trucks from over-loading the aggregate subbase.

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.

 

[BigH]

eea - construction traffic is almost always the "worst" case scenario. How can you build a highway (I'm not talking a little sideroad) without trucks traveling on it? Trucks have to travel on subgrade to spread subbase. They have to travel on subbase to travel on base course. They have to travel on the base course to place the first layer of asphalt mix - or concrete if rigid pavement. If they were worried about the few concrete trucks breaking down the base course, then I doubt the road was designed for anything but cars - and hence the concern. The probelem that fattdad was alluding to was that the road was used an excessivly long period of time in wet weather. He is correct in that a coarse base-mix will not be impermeable and that the granular base will become wet. The big problem there was likely that the base course did not drain well - water can come from top down as well as bottom up.
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 . . .

 

[armyengr]

Is there a concensus on the use of Prime coats in private projects. It appears from the previous replies that most DOTs are moving towards requiring them but how about private parking and "driveways?"

 

[jartgo]

I think several of the posts above are using the terms "prime coat" and "tack coat" interchangeably. My definitions are as follows: Prime Coat, applied to the ABC before the first course of asphalt concrete. Tack Coat, applied between courses of asphalt concrete.

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.

 

[Rjeffery]

Perhaps the best use of prime coat is to protect the subbase that has been compacted and accepted but the base course of asphalt will not be placed immediately.

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)

 

[BigH]

jthompson - It was not alluded that the construction traffic was the "design" traffic but that the subgrade receives heavier loads during construction than it does during the pavement's lifetime. Look at the structural numbers. Of course, the long term behaviour of a finished pavement depends on the ADT, traffic loading, degradation of strength (increase in strain) with time, etc. But, you get a bit of wet weather on a subgrade and are trying to lay the subbase - it ruts. Once finished with stronger layers on top, it is a different story. Note, too, that no reference was made to problems once the base course asphalt was placed. The behaviour of concern was construction behaviour. Agree with your observation that prime and tack appear to be used rather loosely at times.

 

[eea]

Just to help with the confusion, our State DOT uses tack and prime under the same pay item - prime and spec's different application rates for each.

 

[jartgo]

BigH, I see now that I was confusing two separate posts above.

 

[eea]

BigH, you questioned "How can you build a highway (I'm not talking a little sideroad) without trucks traveling on it? Trucks have to travel on subgrade to spread subbase. They have to travel on subbase to travel on base course. They have to travel on the base course to place the first layer of asphalt mix - or concrete if rigid pavement. If they were worried about the few concrete trucks breaking down the base course, then I doubt the road was designed for anything but cars - and hence the concern."

(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.

 

[BigH]

eea - Agree that newer equipment allows situations where there are buggies and the heavier trucks are loaded on the side of the road - but, my friend, you are in America on big jobs and it may, over the years have become the norm for some places. I doubt that such is the case in most developing countries and in smaller state and county highways where there is little money to build construction roads alongside the main road/highway. Your computations and observations on how to construct, in my view, do not fully take into account the variety of construction conditions that are detrimental for building on the road - weather, limited ROW, etc. For instance, I am building a road through a mountainous area - we cut the uphill slope at 1H:2V for some 30 m in height - the immediate downhill slope (pavement plus 1 m shoulder) goes down the hill for several hundred metres. How can I build a construction service road in this situation without huge financial costs? Such would make the new road unfeasible. Countries like Laos, Cambodia, etc. do not have the money for such niceties.
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.