transition ramp between overlayed PQC and flexible runway

[tubsdua]

We are executing a runway work for overlaying the turning pads of an operational runway. The overlay is unbonded type (white topping) i.e 75 mm DBM over existing rigid pavement and followed by 375 mm of 44 Kg/sq cm PQC.

Now the issue raised here is that after relaying the turning pad will be 450 mm higher than the flexible portion of runway and a ramp is to be provided with a gradient of 1:100 (as per ICAO). The ramp is to be laid up to 45m into the flexible portion to cater for 450mm of level difference.

Does anyone has any experience in a situation like this and how was the ramp provided? What is the post laying feedback and any lessons learnt or corrections reqd?

A few methodologies are attached as pdf files. The PQC is being layed with a fixed form paver. Points to be looked into:-

(a) The options with sloping PQC on flat ground is questionable (option 1 and
3).Maybe possible on sloping ground (option 2)?
(b) Laying of bituminous layers on PQC will have high possibility of
reflective cracking. Any comments/ideas on these aspect?
(c) Is excavating the existing flexible pavement recommendable (option 2 and 3)?

Suggestions and comments are welcome.

Best.

 

[tubsdua]

The option 3 was missed out.

 

[Ron]

Keep in mind this is an international forum, so please explain your acronyms.

 

[tubsdua]

Yes sure,

PQC - Pavement quality concrete (44 Kg/sqcm for us)
DBM - Dense bituminous macadam

 

[Ron]

I do not like the idea of "sandwiching" a relatively thick layer of asphalt in between a highly loaded concrete section and a concrete section below. Deformation in the asphalt will increase the cracking potential for the overlay.

I assume the DBM is being used for crack mitigation from the lower concrete layer. If so, this can be accomplished with a much thinner section or even a geotextile.

I also assume you are using the whitetopping in areas having high abrasion and potential fuel spills...otherwise, why not use a bituminous surface course. Keep in mind that if the goal of the whitetopping is to withstand fuel spills, all cracks in the whitetopping must be meticulously maintained or petroleum products will get into the DBM, causing all sorts of problems.

As for the thickness transition, you show a sharp transition below the surface. That's not good. That will result in a crack at the surface along the interface. Keep the thickness of the topping consistent and make the transition below that level.

Simplify the design.

 

[tubsdua]

Hello Ron,

I agree with you on the aspects you have brought out. As a matter of fact I have recommended option 3 i.e having a Concrete ramp altogether with milling of existing flexible portion. Which means the transition is at the below level and the topping of concrete is constant.

Secondly you are also right that Dense asphaltic macadam (DBM) is for crack mitigation between the two layers of concrete. The design methodology is 'unbonded overlay'. For avoiding deformation of asphaltic layer, a compaction of 95% has been specified.

For the benefit of people who may refer to this post I am attaching a few references based on which this methodology and the decision of transition ramp has been taken.