Scarifying Concrete for drainage

[VAStrEngr]

Considering options for placing a timber deck on top of a concrete slab on a single lane, one span bridge. I am concerned that water will collect between the timber and concrete and rot the wood decking too quickly. I am considering scarifying the top of the deck as a way to drain the water from beneath the decking but either need to slope the entire deck or vary the depth of the scarfs to permit drainage to one side.

Sloping the entire deck isn't desireable but I am not sure if it is possible to scarify concrete at a variable depth from one side of the bridge to the other.

Does anyone have experience actually doing this in the field to konw if this is possible?

 

[ACtrafficengr]

I'm curious why. Is this a permanent wearing course, or a temporary thing, say, to protect the deck while running tracked vehicles over it?

The deck should already have a 2% slope towards one or both sides. If so, it's probably easier to groove the bottoms of the timbers. Any carpenter with a router would be able to do that.

Most decks are grooved or tined to allow water to displace outward or lengthwise out from underneath car tires. NY has switched to longitudinal grooving, since it supposedly works as well and is quieter, but you may be able to find a contractor in your area that can still do transverse grooving.

Grooving a 1/4" or so shouldn't make much difference, but you should probably check the record plans to make sure you'll have enough cover over your rebar after grooving.


"...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]

If the wood is in contact with the concrete, whether there are drainage kerfs or not, you'll get deterioration in an exposed application. Even PT lumber will not withstand wet contact with concrete.

One of the problems I've seen with timber decks, particularly thicker sections, is that rot occurs through the center of the member, appearing to leave the exterior intact. That, of course, was not on applications like yours, but does show that timber can deteriorate in different ways.

For your application, you might consider an asphalt coating on the bottom of the timber materials, if you have no other choice and cannot otherwise isolate the timber from the concrete. Keep the kerfs with the kerf bottom sloped to drain.

 

[VAStrEngr]

AC, the bridge isn't so much a traffic bridge as it is a pedestrian bridge that will occasionally see a traffic/construction load. The reason that a concrete deck is going down as "Phase I" is that the bridge will serve as a means of access for construction activities occuring beyond it and then will be transformed in "Phase II" to a park feature which is where the timber deck comes along (to match other features at the park). In phase I, the idea is that they can beat up the concrete deck (with plenty of cover above the reinforcing) and then we will add the timbers on later.

In any case, I think scarifying the deck with a slight slope in addition to using an ashpalt coating on the timber is a decent approach.

I also wonder if using a drainage panel, such as is done with exterior plaza or green roofs might help keep moisture away from the decking. Any thoughts on this approach?

 

[port125]

Have you considered some of that wood colored plastic deck lumber?
It doesn't rot alot and it you get the recycled kind someone may be able to clame it for points towards some enviromental do-good award.

 

[a2mfk]

If its pedestrian traffic only and you can support the decking as often as you wish (since there is concrete deck underneath), have you considered a composite product like Hardi decking? Though a much larger up front cost, for something like you are describing it may be a worthwhile investment.

The concern would be the type and amount of occasional traffic from vehicles, this type of decking is only 1-1/4" or something similar off the top of my head...

 

[HeavyCivil]

You can adjust the depth of man-operated scarifiers. I've only had to do this in the field as a laborer one time, and it is miserable, slow work. My point is that you'll burn up many man-hours and, while you'll be able to get some pitch, it's unlikely it will come out as good as you're probably picturing it.

Not knowing what the geometry is, couldn't you just build a timber deck from the beginning? Are you worried about tracked vehicles beating it up? You could build your deck, then ledger-lok 3" plank to it for the duration of construction, then remove the plank. Think wetland mats for heavy equipment access.

 

[MiketheEngineer]

Could you set the deck up off the concrete - say on concrete or steel curbs??

 

[VAStrEngr]

Man, I love this site...thanks for all of the help.

I have considered a composite or FRP decking, perhaps even a two-layer deck where the lower layer is the FRP and the upper layer is a heavy timber (timber was a specification from the owner).

Any vehicles on the bridge after the initial construction phase would be an occasional maitenance vehicle or emergency vehicle. For that reason, I am not totally concerned with them damaging the decking with traffic.

VTEIT, that is a great point, thank you for your tip.