huangyhg

value engineering session
i just had a value engineering session on one of my jobs and they came back with a few comments. see below: if anyone has some suggestions on how to respond, i would appreciate it.
1. eliminate upturned keys in concrete starter walls.
2. reduce wait time between adjacent concrete placements from 10 days to 3 days.
3. use steel bar joists and metal decking in lieu of pre-cast concrete planks for roof construction.
4. do not use hydrophylic expanding rubber waterstops.
5. use fly ash in concrete.
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1. would lead to poor waterproofing.
2. can construction sequencing be changed to lessen the impact (i.e. do two sparate stages at once or start the upper shoring at an earlier stage).
3. give them an alternate typical design so that they can price it up.
4. not an option.
5. dont know the specifics so cannot comment.
1) i'm not a believer in keys anywhere. but you'll still need the starter walls.
2) we use three days sometimes, but the contractor needs to be pretty good.
3) it will save money now, but will cost money for future remodels.
4) nope, no, nada, nein, no way.
5) less than 20% is no problem. anything more might increase cure times (see 2 above) or cause other issues.
generally agree with jed, but on item (2) it really depends on the type of joint you are talking about. three days should be plenty for any strength issues. if the issue is shrinkage, there is a paradox: the earlier you place the second element, the less cracking due to restraint parallel to the element, but the greater propensity for the joint to open perpendicular to the joint.
i think it depends greatly on the intended use of the structure. ilike up turned keys as wter is not as likely to be trapped at the bottom of the key. should the water freeze, the concrete can be damaged. however, i agree with jed that in may situations, you do not need keys. don't know the particulars of the situation, but if someone wants to substitue, let them propose the budget alternative cost. be sure he ads fees for redesign. i think the use of hydrophylic water proofing again depends on the application. simple rubber waterstops are often used. how important is it to ensure no water gets through the joint? fly ash is a common substitue. the concrete supplier may have priced the concrete planning to use some fly ash in liue of cement. it has pluses and minuses. for one it will finish well, but is more difficult than just cement. it also depends on what type of fly ash. slag is another common substitute. i would talk to the concrete suplier's quality control engineer about the mix.
1. eliminate upstand key and use bentonite strip if waterproofing is required.
2. three days is likely ok. if not a strength (flexure and particularly shear).
3. depends on the use and cost, but steel bar joists and deck is likely less costly, but, you may end up with with a lesser roof.
4. hydrophillic rubber waterstops are ok; depends on how critical water penetration is; what is their alternative?
5. fly ash is good and depends on the type of concrete. not uncommon in these areas to use 25% fly ash with type 10 (gu) to provide equivalent of type 50 (hs) concrete.
dik
1. nevered used projecting keys, always recessed keys.
2. most concrete mixes will get strength in 3 days now, but tests should confirm that. also recommend some way of alternating pours to maximize time between adjacent pours as strength is not the single factor in placing new concrete adjacent to existing. just start playing with the extended reinforcing and see what happens.
3. agree with dik that overall durability and use should be factor what roof selection is.
4. water stop depends on application.
5. fly ash is not bad overall especially when dealing with mass pours. however, this issues goes back to item 2 wherein concrete strength is affected.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
added note... with high amounts of fly ash, strength gain may be slower...
dik
also... hc slab can reduce the thickness of the roof construction by a foot or so... also building height... this can offset added costs.
dik

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