Asphalt Wearing Course above Concrete Deck?? 2

[Contech2011]

Dear Friends,

We have Project specifying asphalt wearing course 50mm thick average laid over concrete deck (350mm thick). Between the two there lies a 4mm thick torch applied waterproofing membrane. Now, when you apply the membrane to the concrete and lay hot-mix asphalt wearing course 50mm average thick, the follwoing problems are encountered,

1. Due to air-packets between the concrete and the membrane the asphalt gets cracked from below since the air trying to expel the asphalt outwards. This is due to improper application of the membrane.

2. The structure is a marine jetty, where 50 tonnes capacity crane manouver to load and unload stuff. Due to such heavy static loads of cranes and forklifts, the wearing course is getting scaled-off or peeled-off below the tyres of such equipments.

3. The entire area of arphalt is deteriorated within 4 months of laying due to above reasons.

We are in serious trouble and tried check the marshall properties of the laid asphalt. The results match with the specs, stability and loss of stability is ok. This problem is only at the areas above the concrete deck and not where we have laid the same asphalt over aggregate base course. We reckon following may be the reasons for failure,

a. Separation layer of membrane causing debonding between the asphalt and concrete. Air packets is the main reason.
b. Lesser thickness of asphalt for such loading.

Can somebody help us please.

 

[Ron]

You have several problems. The bond between the asphalt and concrete or a base course should be capable of shear transfer in order to withstand the relatively high lateral loads at the bottom of the asphalt layer, induced by the tire loads. You have a shear plane break in there with the membrane...won't work unless the membrane is fully bonded to the concrete AND bonded to the asphalt AND transmits shear directly without much deformation. That won't happen with a torch applied system, or for that matter, with any thick membrane system.

Blisters in the membrane will do exactly as you have described. They are air pockets that get heated upon application of the HMA, thus the HMA is compacted onto an air layer that later shrinks back down, thus causing a crack in the HMA around the blister....debonding is inevitable.

I am assuming the membrane is for waterproofing protection of the concrete below, since I doubt there's anything below the concrete that matters with regard to waterproofing. You don't really need it. The section needs to be designed as a pavement, not a roof. It won't work as either the way it was specified and installed.

Remove the asphalt. Scabble and prime the concrete. Place new asphalt. Keep the reflective cracks that will come from the concrete sealed by maintenance.

 

[Drumchaser]

It has been a standard practice (in my area) to apply a two course surface treatment on exposed bridge slabs before any HMA is placed to avoid just what you are experiencing.

A surface treatment is hot AC sprayed down and aggregate placed on top ~ this procedure is also referred to as a chip seal.

Plane off HMA and goop back down to concrete, apply the TCST, replace the ACP.

 

[Ron]

Agree with Drumchaser...a chip seal should work fine.

 

[ACtrafficengr]

Membranes like this are often used on asphalt overlaid bridge decks to protect the reinforcement from water and deicing salts. Can I assume that since it is a marine jetty, that measures have already been taken to protect the steel against chlorides?

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