Asphalt Density Testing by Nuclear Methods

[DirtDelight]

Hello All,

Am presently working on a trench paving job. Our firm had been directed to test the AC 3/4 " state mix after the material had been placed and cooled. Sometimes 3 or 4 days later. The total section of AC is 8" per plan. The contractor is placing in 3 lifts. An issue about why the AC has not been tested during placement has arose.We were then directed to test during placement. The issue is that the first mat has been approx 220 degrees,then the contractor starts placing the second mat.My concern is that the finish rolling has been performed while material is to hot or not finished rolled at all before placement of the second lift.Breakdown rolling has been completed with the material between 315-225 degrees. Does anyone have an opinion on this.... The city agency, who has concerns, is receiving information from another lab that asphalt can be tested at any temperature and any size lifts. They are passing this along to the oversight agency who is questioning. Is there a guidline for the proper temperatures to test the AC ???
Any help would be really appreciated.

Peace............

 

[GeoPaveTraffic]

In my expereince, testing is generally started after one finish rolling pass. Temperature at this pass varries with the mix. Testing after the mix has cooled to the point that rolling can no longer be accompished makes no sense. If the density is low, it can't be improved, so why test?

One problem I do see is placing additional lifts when the underlying lift is that hot. The underlying lift should be under 150 degrees, lower is better.

 

[Ron]

Make sure your instrument is calibrated at the temperatures you will encounter. Take cores for correlation to nuke. Some nuclear gages are capable of high temperature use while others are not.

Your initial temperatures look a bit high....is this consistent with your local practice?

Wear good boots!

 

[darthsoilsguy2]

Careful with the QA verses QC line. If you work for the owner then you are QA. The contractor is responsible for providing QC as part of their job. Our SOP is to monitor truck temps, observe rolling and nuke test, and return after all is done for coring. Enough of that though.

I use to think the heat didn't affect the thin-lift nuke gauge. but last week our most senior tech told me the numbers do change as it cools... i don't know how much, but i'll find out if i have the opportunity.

All that said, asphalt compaction will probably not be the cause of future problems for an 8" deep trench application. Failure is more likely to be at the sidewalls from temperature stresses and water intrusion/freezing, or from settlement of the underlying soils if not compacted well.

 

[dirtsqueezer]

Numbers sound reasonable to me. Your breakdown roller will be used pretty much after the mix hits the pavement, followed by your mid-sized and then your finish roller.
It is my experience that there is a sweet spot in the temperature readings where one achieves compaction, as the mix cools and gets sticky; I was told about 175-225F. You just don't get compaction at higher temperatures because the asphalt's not sticky enough. Lower temps and of course it just freezes up. This probably varies with the mix. The thing is, unless you have a lazer thermometer, you don't know exactly what the temperature is anyway. This is why you need someone on site, there to test as they roll so they know when they've reached compaction and so they don't over-roll it.
Once you've reached compaction on your lower lift, I don't see why you can't place the next lift for compaction. The thing is, subsequent lifts will take longer to compact, or cool, as you've got insulation from that lower heated lift. But, like I said, if you have someone there on site, they can tell you when that compaction's been reached. Otherwise the possibility exists that they put the same effort as the first lift, and leave it too warm to compact, and there go your numbers.
It sounds like someone may have made an error in judgement. Aside from those first lifts, you can still measure the top and get your numbers there, using either cores or the easier route of the batch plant supplied rice. Either way, sounds like someone's going to have hell to pay. Glad it's not you!

 

[mb27]

In my experience, i've never seen an asphalt company lay layer upon layer of asphalt in the same day - usually a base layer or two on separate days, followed by a surface layer at a later day. I assume it is alright, in theory, to place another layer of asphalt on another layer so long as the mix is consistent and it's not blazing hot. I would at least let it cool to the point where running a roller over it makes little to no movement prior to placing another lift.

and as for this: "asphalt can be tested at any temperature and any size lifts", I don't know what kind of nukes they're using, but I'm going to call bunk on that one. However, it could be they're being purposely vague - asphalt CAN be tested at any temperature and any size lifts, but it's definitely not the smartest thing to do. I always wait to begin testing at around 210 degrees fahrenheit...maybe if i'm bored i'll start at about 225. Any higher and you'll not only be wasting your time, but you run the risk of frying your nuke.

 

[gtrayler]

mb27, many times on highways we will setup a second paving crew to lay a second lift. The second crew will wait until first mat is around 180 - 150 before starting second mat. Many times we will have two crews with 2 to 3 miles betweeen them.

Many full depth repair jobs requier 6 to 8 inches of full depth mix placed and traffic opened up by the end of the day. During summer it is a push the get every lift cool enough.