huangyhg

leave rebar exposed for future construction?
i have to plan for a future clarifier to be built adjacent to the pump station and clarifier i am currently designing. that is, eventually, it will be two clarifiers with a pump station between, but only the one clarifier will be built in this contract. what's the best way to detail the foundation so that it can be expanded later? could i leave rebar exposed--coated and protected--ready to receive the future basin mat? is there a "soft" concrete that could be demo'd later? or should i just let the next guy worry about it?
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i have done this very thing - leave the rebar exposed but protected from the elements. do you need a water stop between the two adjacent structures? need to think about that too....greenstreak has some good guidance.
threaded bar couplers such as lenton formsavers might be a good solution.
you could use screww in rebar splices fixed to the formwork so that nothing projects past the face of the slab. the new rebar would just need to be supplied threaded to fit.
this would be much easier than chipping away lean mix concrete.
they do a similar thing with ferrules in precast panels.
on electric generating stations we would leave exposed rebar that was coated and wrapped with asphaltic compounds. technically, this worked well... the problem was no matter how carefully the plans were detailed to show what had been done the next engineer of record would not accept the existing rebar as satisfactory for use.
the best compromise solution that we (the owner) found was to leave clearly defined area where the existing concrete could be drilled (in the future) and appropriate size rebar dowels grouted, epoxied, etc. into place.
we're dealing with a similar situation. our client wishes to build 3 sbr tanks, 2 now and one at a later date. we considered dowel bar splicers. problem is, it may be years before the 3rd unit is built, and we were afraid the slicers would corrode. we've decided to build the double unit and have the third tank constructed separately, a few feet away with no attachment.
i have successfully used the threaded couplers. i have also used concrete poured around rebar. joints were detailed so that the removal of the concrete was somewhat easier. of the two methods, i would use the threaded couplers again if it met the project requirements.
i've used galvanized bent rebar, both exposed and enclosed in a plywood 'box'. i've also used a weak mix concrete that is removed at the time the addition has been added. it's a matter of approaching the existing as though the addition is proceeded with. i've often encountered buildings that an addition was intended and have found some parts have been missed. i've also done many buildings with a future addition that the addition was never built.
dik
if the joint is going to be submerged after the future construction, you'll need to embed waterstop and protect it. all of the engineers that routinely do this work have a typical detail for this situation. you might want to look at other projects and check what they're doing.
as far as your original question, i agree with sperlingpe. the threaded couplers are the way to go.
the real problem with this detail is to make sure the contractor puts it in at all. it's hard to get them to put in quality work to help out the guy on the next project.
it would be good to hear from engineers who have been on the other end of this scenario. has anyone attached a concrete tank to an existing tank that was fitted with mechanical couplers? how old was the existing tank? how did it go?
we're on both ends. we're currently expanding a plant where one phase is about 3 years old and the other is 12 years old. the couplers are in pretty good shape. their placement isn't as precise as maybe we would hope for but it's usually good enough.
the waterstop is the tougher issue. it needs to be protected, but not so protected that we damage it getting off the protection. i've seen several ways to do this all of which work preetty good. using class c (2500 psi) concrete, galvanized sheet metal, redwood all are ok, but it's never as good as new.

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