Asphalt Prime Coat

For you construction people, any stone that is put in as a base course requires a asphalt prime coat to avoid any water infiltration and to keep the internal moisture in the stone base. SO, my arguement is that if the contractor plans on laying asphalt as soon as the base is tight bladed, what purpose does the prime coat serve?

My boss says that you get some bonding and that it prevents future problems such as wicking of the asphalt out of the asphaltic concrete. It could also cause the mat to slide if the top has excessive stone dust. My point is that the prime is added after the tight blading and if is strictly for keeping water in/out, why not just lay directly on the stone but keep it damp to avoid dusting. It saves the contractor money & does not produce an inferior product in my opinion, but I do not have vast experience with asphaltic concrete.

Does anyone know of long term problems? I know that this is not considered a bonding agent between the base and binder, but is there something I'm overlooking? Thanks for any input.

The only other reason I see in having a prime coat is to consolidate or stabilze the fines. I often delete this item during construction mainly because of weather and prime coat it is not a common practice in my region(seems like the designers always throw it in and it is deleted in the field*). I never seem to have enough dry days to guarantee proper curing.

*only when it is used in a flexible pavement design

akwooly said:

The only other reason I see in having a prime coat is to consolidate or stabilze the fines. I often delete this item during construction mainly because of weather and prime coat it is not a common practice in my region(seems like the designers always throw it in and it is deleted in the field*). I never seem to have enough dry days to guarantee proper curing.
*only when it is used in a flexible pavement design

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We removed it from all our construction jobs in NJ. I deal with Airport pavements and after many years of debate the FAA does not require it anymore. Plus that stuff is messy.

We placed the binder course today without the prime coat. One thing I noticed that is usually a 2" binder "rolls" out about an inch to an inch and a half. This time it rolled out about 3 inches and even more in some places. Anyone else ever observed this?

are talking about "tack coat"? It shouldnt have much of an impact on the asphalt thickness.

I know its getting a bad rap from the environmental folks, there is an approved alternate in some states.

Road Guy said:

I know its getting a bad rap from the environmental folks, there is an approved alternate in some states.

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You won't get any out of me!

JR

sehad said:

We placed the binder course today without the prime coat. One thing I noticed that is usually a 2" binder "rolls" out about an inch to an inch and a half. This time it rolled out about 3 inches and even more in some places. Anyone else ever observed this?

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I'm guessing the contractor was paid by the ton rather than by area covered.

I've found prime to be a waste of money that often causes problems due to sticking to the tires of the delivery trucks, etc. We have the contractor wet down the area before paving on new base.

I work in the asphalt paving industry. I agree with Cement on both his points.

Specs that call for prime coat are usually outdated. It is rarely used anymore in that application, especially if it a cutback prime coat as opposed to a emulsified prime coat. With the cutbacks you are spraying kerosene or gasoline onto the subbase and waiting for it to evaporate. It not "environmentally friendly"

Also, the prime coat would have almost no bearing on the material "squishing-out". The contractor was most likely paid by the ton, so he's happy to cover an additional area.

Andrew

I am currently in the middle of a five mile base and pave project on a rural county road. The contractor wanted to waive the prime coat, stating additional curing time would be needed, delaying the project even more. They were granted the waiver (I disagreed with the waiver). When the binder layer was being placed, the breakdown roller kept sliding the mix. So, I requested the contractor to lightly wet the base infront of the spreader. The contractor didn't "lightly" wet the base, in some places it was soaked. A week later, we have base failure. I believe this all could have been avoided, if the relatively inexpensive prime coat had been placed.

AJK said:

I work in the asphalt paving industry. I agree with Cement on both his points.
Specs that call for prime coat are usually outdated. It is rarely used anymore in that application, especially if it a cutback prime coat as opposed to a emulsified prime coat. With the cutbacks you are spraying kerosene or gasoline onto the subbase and waiting for it to evaporate. It not "environmentally friendly"

Also, the prime coat would have almost no bearing on the material "squishing-out". The contractor was most likely paid by the ton, so he's happy to cover an additional area.

Andrew

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We know that rollout is going to happen so on a 12' lane we usually lay about 11' 10" to account. Yes the contractor is paid by the ton, however we run a yield and he is responsible for anything over that yeild. So we regulate him to where he is not just putting out hotmix and us paying for it. We use emulsified asphalt as a prime coat. Our spec says that it's only to prevent water infiltration, but our District Construction Engineer disagrees and has a list of things he "thinks based off of his experience" that the emulsion does. Don't you love when a senior engineer has no concrete reason but plays the experience card when you disagree and he can't back up his statements?

rolltyde said:

I am currently in the middle of a five mile base and pave project on a rural county road. The contractor wanted to waive the prime coat, stating additional curing time would be needed, delaying the project even more. They were granted the waiver (I disagreed with the waiver). When the binder layer was being placed, the breakdown roller kept sliding the mix. So, I requested the contractor to lightly wet the base infront of the spreader. The contractor didn't "lightly" wet the base, in some places it was soaked. A week later, we have base failure. I believe this all could have been avoided, if the relatively inexpensive prime coat had been placed.

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What was used to construct the base? I've seen base failures when the contractor used a 610 stone and didn't have weep holes cut to allow for water to pass through the shoulders, if the shoulders were soil. However, I would say that the base failures were not due to him not having a prime coat. I would even say that they were not related to the water being added. A LOT of water has to be added to make a good base fail if it is properly drained.

I've had a contractor hit me with the delay the prime causes as well. Truth is, if the project was bid then that should be in his bid. In most cases, the prime will cure out overnight so if it is placed in the evening causing no delay at all. I'm not a big fan of prime coat, but to me that's a crappy arguement to not use it. Plus, cutting the prime out of a 5 mile project was a pretty significant savings to him. He saved the material, labor to put it out, trucking to get it there, and a tack truck. This adds up over 5 miles.

My apologies, I forgot to mention the locations of the wet base. The project road is an existing gravel road with an ADT of 110. So, when the contractor was filling the water truck from local fire hydrants, he did not have to get of traffic. He was able to fill up while in the roadway (with traffic control of course). With a water hose with a bad leak and spillage over a couple days of constant filling got the base pretty wet. I could be wrong. Maybe this didn't cause the base failure, but the fatigue is only showing up in these locations. It supprised the heck out of me, because the densities on the base (taken every 500 ft) was at least 100% compaction.

As far as the waiver on the prime, my argument was: He bid it, He should have to do it.

rolltyde said:

My apologies, I forgot to mention the locations of the wet base. The project road is an existing gravel road with an ADT of 110. So, when the contractor was filling the water truck from local fire hydrants, he did not have to get of traffic. He was able to fill up while in the roadway (with traffic control of course). With a water hose with a bad leak and spillage over a couple days of constant filling got the base pretty wet. I could be wrong. Maybe this didn't cause the base failure, but the fatigue is only showing up in these locations. It supprised the heck out of me, because the densities on the base (taken every 500 ft) was at least 100% compaction.
As far as the waiver on the prime, my argument was: He bid it, He should have to do it.

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Now that makes more sense. However, I've seen 95% - 100% desnified bases fail due to having too much moisture plenty of times. Just because it is at optimum density does not mean that the optimum moisture is acceptable. I would say that was your problem. And if it was gravel, too much moisture will make it move pretty easily. I'm betting there was a good bit of fines in there as well. Also, if you were getting over 100% compaction very regular, I would check your Proctor again, just my opinion.

I'm with you on the last statement about the bid, to a certain extent. From my experience, I would give any future contractor a choice. Put out the prime, OR give me a rebate so that you won't have to. It puts the ball in their court for a decision instead of you having to give a yes or no. Also, at some point in a job there is going to be a place where the contractor can stick it to you if he wishes. If you've worked with him, some will work with you back to save you money. But this depends on the contractor. I would tell some contractors they had to put it out no questions asked, but that's because they're assholes anyways so it makes me feel better.

We use primes here in Texas in construction still and the only prime coats I have experienced problems with are the "environmentally sensitive" primes. This is probably due to the fact that we don't know nearly as much about them as we do the standard prime coats. I know that there are some primes that are out there that can hurt the environment but contractors used to use this as fertilizer and a stabilizer on embankment. I have old inspectors/engineers tell me all the time that if you ever saw it, the grass was ALWAYS greenest where they used prime as the stablizer and you can go to some of the roads and still tell that something different was done in those areas...but who knows.