Hello Arsadie,
As with anything else, the problem has to be defined before it can be solved. What is the mode of failure that has been observed? Reflection cracking (cracks, joints in the underlying concrete "reflecting" to the surface), rutting, raveling (fretting), delamination/slippage? It is difficult to suggest remedies without knowing the actual problem, because those that I listed have different causes. Others have discussed problems at the surface of the asphalt layer-I'll go deeper into the pavement to what is the most likely culprit-the underlying concrete pavement.
Typically, the biggest problem with asphalt overlays of concrete pavement is reflection cracking, and deterioration of those cracks under traffic. These cracks are caused by movements in the concrete pavement resulting from poor load transfer at the joints and cracks. The remedy for this depends a lot on project constraints-particularly surface drainage and the ability to deal with significant changes to the roadway profile. For jointed-plain concrete, cracking and seating can stabilize the movement of the slabs and minimize cracking through the overlay. Breaking and seating is a similar approach used for concrete pavements reinforced with deformed bars. These techniques reduce the size of the slabs and firmly seat them onto the underlying subbase, thus reducing movement. Another technique, rubblization, completely eliminates slab action by breaking or fracturing the concrete into small pieces in place.
If there is evidence of pumping of fines from the subbase, subsurface drainage has to be improved to enable good long-term performance.
When using any fractured slab technique, you should consider placing at least three lifts of HMA-a minimum of 15 cm total-more depending on the traffic loading/pavement structural design requirements for the project. The reason for the number of lifts is to achieve the desired ride quality, since a fractured surface will not have a smooth profile.
Another approach to consider is an open-graded asphalt crack relief layer placed directly on the concrete pavement. These have been shown to be effective in significantly delaying reflection cracking, and in providing an outlet for moisture in the pavement. The problem with this is that the resulting roadway profile will ultimately be raised by at least 20 cm, which might not be practical for situations other than rural highways.
Last but not least, the asphalt mix design/materials criteria and construction practices must be appropriate for the conditions as others have noted in their comments. Refer to Asphalt Institute MS-2 for mix design details whether using Superpave or Marshall mix design procedures. Polymer (SBS) - modified asphalt binders are proven to improve resistance to rutting and cracking. It's difficult to provide a specific recommendation for a binder grade/specification without knowing more details about the project, but the key features of the specification should be multiple stress, creep recovery (MSCR) requirements for compliance (Jnr) and recovery (%R), and the upper limit for DSR measured at 25°C after RTFO conditioning.
I hope this is helpful.
Gary L. Fitts, P.E.
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Gary Fitts P.E., M.ASCE
Manager, Market Development
Garden Ridge TX
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Original Message:
Sent: 12-08-2020 10:52 AM
From: Arsadie Monte
Subject: Thesis for Highway/Materials Engineering
Hi, I am looking for a topic in preparation to my thesis in Master of Science in Civil Engineering. I am interested on the subject of Highway or Materials Engineering. Mostly the concrete pavement (280mm thk) highway in my region is being constructed with an asphalt overlay (100mm thk). The lifespan of the asphalt overlay (mixed with some aggregates) is 5 years. Isn't it a bit expensive to remove it after 5 years and replace it with a new one? Not to mention the labor cost. Anyway, can you suggest or recommend something that I can place at the top of the asphalt overlay or add something like mixture with the asphalt to prolong the lifespan of the asphalt overlay? Any help will be much appreciated.
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Arsadie Monte R.Eng, M.ASCE
Civil Engineer
DPWH - Lanao Del Norte
Iligan City
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