allowable slump for concrete with superplasticizer 4

[chekre]

hi guys,
what is the acceptable range for concrete slump (during a slump test) for a ready mixed concrete with superplasticizer ?
the poured element is a reinfoced concrete element. Is 21 cm ok ?
THank you.

 

[MrHershey]

Disagree that slump is an engineering decision that we'd be passing off to the contractor. Historically it's been specified primarily to control water content and workability. Controlling water content is an admirable goal, but most engineers now specify a maximum water to cement ratio for durability reasons anyways (and are even required to show exposure classes for concrete if using ACI 318-08 or newer). For the purposes of limited the water content, doing both is at best redundant and at worst conflicting. For workability, I would argue this is not an engineering decision but means and methods. Obviously the concrete needs to be workable enough that it can get into voids and be distributed well without being so workable that it damages the formwork. But if the final end product is a well-consolidated pour with the minimum strength and maximum water content that I need for my design, I'm not sure why the slump should matter to me, the engineer.

I would still advocate that slump be tested for each batch so we have some idea of consistency between batches of the same mix design as noted above. But that's more of a comparative test. I want each slump to come out to about the same number because that indicates that the batches are relatively consistent and I can expect my final results to be somewhat consistent. I don't necessarily care what that number is, so long as it's consistent.

Call it lazy engineering, I guess. I'd call it giving the contractor the flexibility to build as he/she sees fit as long as the final product provides what we need.

 

[chekre]

since u guys seem to have enough experience on the site, lets say the slab thickness required is 40cm and the cover specified to be 30mm. What is the tolerance which u adopt ? do u accept 50 mm while the design show minus 2cm ?

 

[jayrod12]

Yes. If they're giving me 20mm more cover than asked for in a 400mm deep slab then I have zero complaints.

 

[chekre]

Ok but is there any rule regarding this subject ? Too much cover will decrease ur effective depth and may necessitates additional bars. Any limitations ?

 

[jayrod12]

Use your engineering judgement. If it feels like it is too much then check it, if not then don't worry about it. In your example it is off by 5% of the depth, which I would personally likely accept.

You would also want to check the crack control parameter Z as increasing the depth has an impact on that as well.

if they were off 20mm in a 200mm deep slab then it's more cause for concern.

 

[TXStructural]

It is generally accepted that slump is a bad indicator of anything useful, but lacking anything better it can provide an indication of consistency of a mix from batch to batch.
Doing slump post-modifier is really useless for checking concrete quality. Flow tests are valuable as a way make sure concrete will consolidate and aggregates will stay in suspension.