The base asphalt pavement alligator cracked, displaced and possibly heaved over two miles of subdivision road. This occurred though the base pavement was installed at standards greater than required and at different seasons. Is it likely from the asphalt, gravel, and soil quality, the installation, or excessive homebuilding loader and concrete truck traffic, and what is common for repair and final paving options at this point, beef it up, or tear it out and add geotextile base?
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base asphalt pavement cracked
- Thread starter 2321Selma
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Describe the pavement: is it rutted? Potholes? Subgrade pumping observed? How have the pavement shoulders fared?
What was the pavement design? Have you verified the dimensions of the pavement structure? The quality of the materials? What is the subgrade? What were the pavement design assumptions?
Have you had any unusual weather prior to and during the period in question?
We have had "failure" of asphaltic pavements in San Antonio because they were "too strong." (3-4" HMAC, 4-6" "black base", 8" cement treated base. All for residential streets. Sheesh. Reinforced concrete would have been cheaper.) These occurred on residential streets in older areas of town, not on major thoroughfares.
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Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
What was the pavement design? Have you verified the dimensions of the pavement structure? The quality of the materials? What is the subgrade? What were the pavement design assumptions?
Have you had any unusual weather prior to and during the period in question?
We have had "failure" of asphaltic pavements in San Antonio because they were "too strong." (3-4" HMAC, 4-6" "black base", 8" cement treated base. All for residential streets. Sheesh. Reinforced concrete would have been cheaper.) These occurred on residential streets in older areas of town, not on major thoroughfares.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
KRSServices
Civil/Environmental
Selma,
- The "aligator" cracking you described, where are they in relation to the "lanes" travelled by the heavier vehicular traffic?
- How old is the asphalt?
- Was the finished grade proof rolled before placement of the asphalt?
- You referenced different seasons for construction. Does that pertain to the entire road structure or only the asphalt?
Without the benefit of looking at the design and the base specifications, wherein you stated the actual paving standards exceeded the standards that were required. Suspecting a clay or clay/silt subgrade, it is quite likely that either the subgrade was insuficiently compacted (or is wet) or the base gravels are not adequate or their gradation is not to spec. As a designer, I subscribe to my general rule that the base and subgrade should be able to support the heavy loading (cement trucks) without flexure or deflection whatsoever. Asphalt should be considered as a surfacing and not be an integral structural member, even though many practices integrate it into their stuctural design.
I have run across this countless times and there are some very cost effective and durable repairs that can be done, but they will be dependant on the subgrade soils, the loading design asnd the typical cross section of the road (ie. urban or rural). In other words, I would need a little more information first.
KRS Services
- The "aligator" cracking you described, where are they in relation to the "lanes" travelled by the heavier vehicular traffic?
- How old is the asphalt?
- Was the finished grade proof rolled before placement of the asphalt?
- You referenced different seasons for construction. Does that pertain to the entire road structure or only the asphalt?
Without the benefit of looking at the design and the base specifications, wherein you stated the actual paving standards exceeded the standards that were required. Suspecting a clay or clay/silt subgrade, it is quite likely that either the subgrade was insuficiently compacted (or is wet) or the base gravels are not adequate or their gradation is not to spec. As a designer, I subscribe to my general rule that the base and subgrade should be able to support the heavy loading (cement trucks) without flexure or deflection whatsoever. Asphalt should be considered as a surfacing and not be an integral structural member, even though many practices integrate it into their stuctural design.
I have run across this countless times and there are some very cost effective and durable repairs that can be done, but they will be dependant on the subgrade soils, the loading design asnd the typical cross section of the road (ie. urban or rural). In other words, I would need a little more information first.
KRS Services
Rick -
As a designer, I subscribe to my general rule that the base and subgrade should be able to support the heavy loading (cement trucks) without flexure or deflection whatsoever.
I would have said, "with minimal flexure." After all, there's no such thing as an infinitely stiff pavement, particularly if it's a flexible pavement structure...
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Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
As a designer, I subscribe to my general rule that the base and subgrade should be able to support the heavy loading (cement trucks) without flexure or deflection whatsoever.
I would have said, "with minimal flexure." After all, there's no such thing as an infinitely stiff pavement, particularly if it's a flexible pavement structure...
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
KRSServices
Civil/Environmental
Actually, I really meant NO observable flexure when proof rolling the subgrade or the base. If flexure is predominate prior to the asphalt being placed, the ACP will undoubably fail, never mind in five to ten years. On any of my jobs, if flexure was noted on the subgrade it was ripped and recompacted, particularly if it was a clay or clay/silt material. On critical structures, the base was proofed as well, but rarly had flexure if the subgrade was ok.
KRS Services
KRS Services
Something that may have happened is the granular base becoming saturated and the construction traffic getting on to the thinner pavement structure too soon. While I agree in general with Focht3 that the base, etc. should support constrution traffic, there is a point at which no such traffic should go on for obvious reasons such as oversaturation, etc. This could, then, be overloading in a temporary adverse condition.
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2321Selma
Sounds ominously like base failure to me. There are a number of possibilities as to the cause of the problem; poor subgrade, deficient structure, etc. I have seen similar problems in residential developments. The bulk of the heavy loadings (outside bus routes) commonly occur in the first few years during house construction. Exacerbating the problem is that, in some jurisdictions, a staged pavement is required on newly constructed roads. So the full structure is not yet present during the most severe loading conditions.
Unless the ACP mix and/or compaction/lay-down properties are grossly substandard, I would be less inclined to focus on this component of the road structure. I agree with others that proof rolling is a valuable tool during construction. It is cautioned, however, that proof rolling be accompanied with soil testing to ensure that adequate moisture is present. (I have seen excessively dry, crusty subgrade proof-rolled with great results only to have it turn to crap when subject to moisture increase).
At this point, I would consider deflection testing the road using Benkelman Beam, Dynaflect,etc... From this, an overlay can be designed and applied if required. Total failure areas will require removal. The best method of reparing these areas will depend upon the conditions exposed.
Sounds ominously like base failure to me. There are a number of possibilities as to the cause of the problem; poor subgrade, deficient structure, etc. I have seen similar problems in residential developments. The bulk of the heavy loadings (outside bus routes) commonly occur in the first few years during house construction. Exacerbating the problem is that, in some jurisdictions, a staged pavement is required on newly constructed roads. So the full structure is not yet present during the most severe loading conditions.
Unless the ACP mix and/or compaction/lay-down properties are grossly substandard, I would be less inclined to focus on this component of the road structure. I agree with others that proof rolling is a valuable tool during construction. It is cautioned, however, that proof rolling be accompanied with soil testing to ensure that adequate moisture is present. (I have seen excessively dry, crusty subgrade proof-rolled with great results only to have it turn to crap when subject to moisture increase).
At this point, I would consider deflection testing the road using Benkelman Beam, Dynaflect,etc... From this, an overlay can be designed and applied if required. Total failure areas will require removal. The best method of reparing these areas will depend upon the conditions exposed.
are there any records indicating the as-built thickness and compaction? Was there QA testing done by the City? What do those show? Have there been any forensic testing - coring to verify base and pavement thickness? You may need to investigate this before deciding on a repair method.
[blue]SirAl[/blue] is right: I re-read the original thread, and it sounds like:
[ul][li] subgrade failure,[/li]
[li] bad base material, and/or[/li]
[li] excessive loadings.[/li][/ul]
I'd look at the dimensions of the various layers first - to be sure that the section isn't "short." Then I'd check the subgrade and base material properties. If that doesn't indicate a problem, then it has to be excessive loading.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
[ul][li] subgrade failure,[/li]
[li] bad base material, and/or[/li]
[li] excessive loadings.[/li][/ul]
I'd look at the dimensions of the various layers first - to be sure that the section isn't "short." Then I'd check the subgrade and base material properties. If that doesn't indicate a problem, then it has to be excessive loading.
Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.
dirtdoctor
Materials
I think all of the responses in terms of what caused your failure are correct. Now to the fixes, for varying conditions:
1) If this is a curb and gutter section of roadway, then you can't elevate grades (i.e. overlay). Identify distressed areas - if total percentage of full depth repair (removing all asphalt to allow for a minimum 6" patch) is greater than 20% of total area, then in-place cold process recycling (CPR) would be the cheapest most cost-effective repair option. Should cap off CPR with at least 220 psy (2"
of asphalt surface. If total patching area is less than 20%, the do isolated FDR followed by surface overlay.
2) IF not a curb and gutter section and grades can be altered, once majority (85%) of construction traffic is complete, level the roadway with 200 psy of open-graded popcorn mix (max agg size 1/2", approx. 5% AC). This will halt vertical crack propagation. Then, install a 165 to 220 psy asphalt surface mix.
1) If this is a curb and gutter section of roadway, then you can't elevate grades (i.e. overlay). Identify distressed areas - if total percentage of full depth repair (removing all asphalt to allow for a minimum 6" patch) is greater than 20% of total area, then in-place cold process recycling (CPR) would be the cheapest most cost-effective repair option. Should cap off CPR with at least 220 psy (2"
of asphalt surface. If total patching area is less than 20%, the do isolated FDR followed by surface overlay.2) IF not a curb and gutter section and grades can be altered, once majority (85%) of construction traffic is complete, level the roadway with 200 psy of open-graded popcorn mix (max agg size 1/2", approx. 5% AC). This will halt vertical crack propagation. Then, install a 165 to 220 psy asphalt surface mix.
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