Bird Baths/Low Spots in AC Paving 3

[roscoe]

Does anyone have a good method of quality control of "bird baths" or low spots in Asphalt Concrete Paving construction. I am supervising the construction of some roads, and have found some bird baths after the paving was already complete. I would like a method of either preventing it from happening, or quality control during paving and subsequent corrective measures. The spec calls for using water to flood the newly paved areas. However, this is not very practical in the field.

 

[jimbo]

I spent a lot of my Civil career in AC work, and have the opinion that inadequate crown is the primary cause of premature pavement failure. (2% is sadly inadequate).
Leave the remedy of the problem now to the contractor.

 

[Qshake]

JImbo has good advice...leave it for the contractor to correct this time.

However, just as a AC contractor will have a tack truck ready to go, he should also have a water truck ready to be used at the inspectors discretion especially if is written that way in the specs - which most state specs require contractors to aid the inspection team.

IMHO, the problem may be many-fold yet it seems likely to be a common problem associated with

1. Existing holes/swales in pavement that are not properly wedged prior to mainline paving

2. Too much horsing around with the paving machine controls

Good inspection will deal with both problems and should avoid costly removal work by the contractor later. Thus he should be glad that its pointed out before hand.

 

[Ron]

Birdbaths and surface undulation are checked during construction by using a straightedge. If the paving contractor knows he has to meet a straightedge criteria, his performance will likely improve.

Flooding a pavement to check surface quality is absurd.

A common QA/QC criterion for surface profile is to have no more than a 1/4-inch variation in 10 feet. This is checked with a 10-foot straightedge in longitudinal, transverse, and diagonal directions. Milling and overlaying is the proper course of repair. "Skin" patches for birdbaths are not acceptable. Cut out and replace or mill and overlay.

QShake's comments are good as well. His experience is similar to mine.

 

[Qshake]

One more thing to consider that Ron brought to mind about that straight edge...
For concrete paving a "profilograph" is used to determine the acceptable level of surfacing. It may be that this is also used in ACC pavement as well. Having a straight edge following the paver, luteman and roller is also a good idea.

 

[roscoe]

Thanks for your comments gents! Very helpful.

 

[CSainte]

I have often found bird baths on the plans before costruction. This is particularly true at street intersections where both streets are on a grade and you have curb and gutter. The designer will have spot flowline elevations plainly showing you will have a bird bath on one of the curb returns, frequently in a wheel chair ramp. I don't know how they can calculate all this stuff and not see this.

Another problem location is when you have an overside drain at the bottom of a long sag parabolic vertical curve. These are difficult to find with a hand level. We usually put the dike in first, bring in the water truck, and pay extra work to saw out the dike at the low point. A little extra expense, but guaranteed to find the right place.

Charlie Sainte Claire
Caltrans

 

[bearit]

Minor Birdbaths have occurred in our park path. This occurred immediately after paving over an existing dense, stable gravel path. The path is approx 8 feet wide and was designed to be crowned for drainage. What is the consequence with allowing skin patches?

 

[ACtrafficengr]

If you had a stable gravel path, it is likely to make a poor base for a paved path. Unpaved gravel works best with a fines content too high for a pavement base.

------------------------------------------
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."

Wm. Phelps Eno, ca. 1928

 

[Ron]

Skin patches do not perform well under traffic loadings. They tend to ravel and disintegrate starting at the edges where there is an attempt to "feather" them.

If you path only receives foot traffic or very light vehicles, then you might consider a skin patch using a very fine asphalt mix.

 

[justbuildit]

These are very good responses. I find them to be by experienced people. In my experience skin layovers quickly disenigrate over time with vehicular loading. We always make a clean sawcut, gut out the area, and repave. This could be a deep AC section if the area is small. Similar to repairing a pothole.

 

[saruman26]

In my experience, patches work well as long as the minimum asphalt depth of the pavement patch is 1" (assuming 9-12.5 mm aggregate), and 2" for 19.5mm aggregate. Make sure the contractor mills and preps the surface with a tack coat prior to making the patch.

 

[InspectorGuru]

Good subgrade/ base is the esential to keep depressions at bay. This means good QA testing of subgrade for compaction and moisture, weather in a cut or fill area of the proposed mainline. If the area is in a cut area, proof-rolling with a loaded triaxle or other heavy equipment, will display the failing subgrade areas. Any areas would then need to be rectified and pass inspection again. It sounds like a lot, but in the long run, it is worth it: the contractor can't argue the point of fault, you will end up with a good product that will not be scarred by patches and/or milling, and you will have a happy client. Water over the finished product??? In my twenty years in the industry, I have never heard that one. Use straight-edge or a string-line or a profilograph-- something that can have REAL technical documentation. Food for thought: If after full-depth paving you see depressions in the surface, the subgrade HAS GOT to be bad.

 

[woodleigh]

Assuming you can't pave up against PCC curb/gutter pan, design an AC flowline grade at no flatter than 0.5%. If you are putting in even lifts of AC, and you have a decent contractor, you shouldn't get any birdbaths. If you design flatter than 0.5% and expect ac to be paved without birdbaths you aren't being realistic.

If you have to pave using several lifts of varying thickness and don't have any type of automatic controls, after each lift take shots on on what you just paved, then write down the fills on the pavement, then do your best to pave to those fills. As AC from behind the screed will compact down 20 to 25% after rolling,try to build up grades so that the final lift (pass) is a consistant thickness (so that the final lift compacts down the same amount). Then use a 12' straightedge with a carpenters level. When the mix is still hot and after the initial breakdown compaction, slide the straightedge/level up and down the flowline, find any high spots pound them out with the vibratory roller (before the ac cools).

 

[ACtrafficengr]

"If you design flatter than 0.5% and expect ac to be paved without birdbaths you aren't being realistic."​

Or you're rehabbing a road and can't adjust the grade much (e.g., a city street)


------------------------------------------
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."

Wm. Phelps Eno, ca. 1928

 

[woodleigh]

"Or you're rehabbing a road and can't adjust the grade much (e.g., a city street)"

and you don't care about a few bird baths.

 

[Hoagie]

Perform an "as-built" of your base prior to paving

check the asbuilt for any slopes at or less than 2.0%

If you have a problem with the base, then correct prior to placing AC.