Sacrificial Wood Concrete Forms - Good or Bad Idea?

[8thStreet]

I have a project where the owners believes that sacrificial wood forms will be more cost effective than forming fill.

The project is a new walkway / deck on the outside of a long commercial building. The deck is 10 feet out from the building - about 4 feet tall - and about 200 feet long. Like a very long loading dock.

I have been suggesting a wood and steel deck system, but the contractor thinks concrete would be cheaper and easier, but they dont want to bring in tons of fill because of complications with the existing business. They propose building sacrificial forms out of wood to form the concrete walkway. The wood forms would be left in place under all of the concrete. Obviously - I would design the concrete to clear span.

Are there any issues with leaving wood formwork inside of the concrete structure at an exterior location? What are some of the reasons this is an atypical type of forming?

Thanks

 

[SlideRuleEra]

What are some of the reasons this is an atypical type of forming?

In the southern USA, termites... long after the contractor is gone.

http://www.SlideRuleEra.net

 

[msquared48]

In the Pacific Northwest, black mold, which is very toxic, as well as termites.

Mike McCann, PE, SE (WA)

 

[DTGT2002]

What is supporting the perimeter? A wall? Can you leave one of the short ends open to strip the material?


Its just crawlspace formwork at that point - but could have confined space concerns.

 

[DTGT2002]

Going back to your questions, wood is typically not abandoned in place for forms due to cost of lost material, risk of leaving a combustible material in a space, risk of termites and risks associated with the breakdown of said material in the work.


If you are building a supporting wall at the leading edge of the deck, attaching an angle to that wall and spanning metal deck to the wall might be reasonable, potentially with a intermediate cmu wall or temporary wood center support line.

Another option for abandonment in the work would be Styrofoam or geofoam to fill the space but not require the heavy trucking. But still require a lot of material shipping and handling.

 

[CANPRO]

If not disrupting existing business is of concern, maybe precast panels is an option. Timber or maybe steel frames 10'x4' at some regular spacing and then drop the concrete panels on top.

 

[8thStreet]

Thanks for all of the suggestions. There will be a concrete or block wall at the front edge. I have suggested the full span metal deck from front wall back to the building - but I was concerned about deck rusting out over the long term, but I guess that is similar to rotting wood form work.

I think i am going to provide the detail with a big note that says all form work to be removed via the short end. I still dont think this is the most cost effective method, but at least they can price it and find out.

 

[BAretired]

If the space is totally enclosed by concrete or masonry, would fire or termites be a problem?


BA

 

[SlideRuleEra]

BA - Yes, see "Concrete & Termites", masonry is even easier for them.

http://www.SlideRuleEra.net

 

[BridgeSmith]

So the formwork rots or gets eaten by termites eventually. Does it matter once the concrete cures and is self-supporting?

 

[StrucDesignPE]

I have suggested the full span metal deck from front wall back to the building - but I was concerned about deck rusting out over the long term, but I guess that is similar to rotting wood form work.

I wouldn't worry about the deck rusting, just design the slab to be self-supporting, even if you have to increase the thickness the depth of the form decking. We have done this with crosswalk pedestrian bridges due to concerns of building maintenance not keeping up with the paint on the underside of the decking. Eventually, chemicals used on the roads will cause the deck to corrode and become useless. At least as it rusts away it won't develop mold or attract termites.

 

[CANPRO]

Termites might not be an issue for the self-supporting concrete, but all of the adjacent businesses/tenants might be too fond of having a well fed colony of termites as neighbors.

 

[BAretired]

Good points, SlideRuleEra and CANPRO. I had not thought about termites being a problem by just being in the neighborhood, but it's much more sensible not to attract the little blighters by providing a never ending supply of food.

BA

 

[azcats]

This seems like an ideal situation for geofoam type material as mentioned above. Lightweight, installed quickly, your forming problem goes away, and you're not left with a perpetual void.

 

[BAretired]

A perpetual void is bad because?

I can offer one suggestion. If there are any openings in that long length of wall, wasps and other stinging critters could build nests inside the void.

BA

 

[BUGGAR]

If you drive across the Dumbarton Bridge in CA, keep your feet up because the piers were built using stay-in-place wood interior forms. This was built early '80s.

 

[azcats]

I have no issue with perpetual voids provided they're 'sealed.' My understanding was that this case would be open to earth below and that just doesn't seem like a great idea. But I really have no experience either way. If my thoughts are incorrect I'm always happy to be educated.

 

[bones206]

This thread is well timed, as my colleague just proposed stay-in-place forms for the exact same application, but for a project that hasn't yet begun construction.

The note requiring the contractor to remove the forms seems like a bad idea to me. Definitely a confined space that could be dangerous and become expensive with personnel training, air quality monitoring, etc. I wouldn't want to be sucking in fumes from form-release agents inside a tunnel like that.

I vote for geofoam as the next best option to traditional soil fill. It can handle pretty heavy loads and allows you to design a thinner, less reinforced slab, which could offset the cost of the foam.

 

[BAretired]

If frost heave is possible, I would prefer a void instead of any type of fill material.

BA