super-plasticizer for high-rise concrete structure
 

super-plasticizer for high-rise concrete structure
i am a structural estimator and have priced many higrise concrete structures from 20 stories up to 50+ stories and just run into a debate of super-p use.
after spent time digging out these old project data, i found out that every project is different in specifying super-p, even the same structural engineer would specify no super-p on this job but specified super-p on shearwall & columns with more than 10,000psi on similar project a few months later.
do you know if there is specific aci requirements about the use of super-p on highrise concrete?
thanks

in general, aci isn't big on requiring admixtures. they recommend air entrainment for freeze thaw resistance, but very little else.
super p is a constructability aid or a crutch for contractors, depending on who you ask. it can be touchy to use. it doesn't affect the strength or other structural properties of the concrete and that's what aci is interested in.
high strength concrete with super plasticizer has to be done by an experienced contractor in this field. eventhough, super plasticizers can drastically reduce the amount of water needed for a given slump, but at the same time can create multitude of other problems if not used properly. loss of workability within very short duration is one of many examples. it will have implications, with set time, generation of heat of hydration and associated microcracks in the concrete.
i have been using super p concrete for years, all the time, and have experienced no detrimental effects at all. the contractors like it becuase it's so easy to pump and work with, and i like it because now i'm not always fighting them adding water to improve workability. we typically have a mix with both super p and a set retarder. expense is the only problem i know of.
thank you all, so far the recommendations i got from structural engineers about the use of super-p are
1. highrise with 10,000psi or higher strength of concrete in shearwall and/or col (with pearock, set retarder, etc.)
2. if developers have no objection, use pearock and super-p everywhere, which makes concrete pumping easy and smoot, but cost more money
any thought?
i have nevr used 10,000psi concrete. 5,000 is about my limit, and most of my work has been decks(elev & at grade) and walls. super p is alright, but is very fussy. there is low mid & high range which refer to how long it lasts. once it is over the workability of the concrete changes dramatically. personally for 4,000 psi mixes and down, i perfer to add water., however, over 5, the water may significantly effect strength. matter of fact 10,000 will be a very fussy mix and i would imagine only a couple of plants in the area will produce it. i would talk to their technical department to get their input. otherwise, if they don't like it you will hear about it at submittal time when everyone is under the gun to move the project and there is no time to refine the design.
10,000psi is about 70n/mm2. i think it will be difficult to achieve without a super plasticiser.
you will need a low water cement ratio, to do this you decrease the water and increase the cement. but at increased levels of cement the heat of hydration will reach an unacceptable level.
to counteract this you reduce the water cement ratio not by adding additional cement but by using a superplastiser to enable the water content to be reduced.
the other point is about using "pearock". if i assume that this typically refers to a 10mm crushed aggregate then i doubt the effectiveness unless really necessary due to congested rebar. 20mm aggregate has a lower surface area per weight than 10mm aggregate. it then follows that if you can use the 20mm aggregate you need less cement "paste" to coat it, this has benefits for the water cement ratio.
the other issue with high strength concretes - say in the range 70 - 100 n/mm2 - is heat generation and cracking.
you should also give some attention to constructibility of the wall structures considering drop height at the time of placement. if your super-p mix gets above an 8" slump, the potential for segregation is increased. i agree super-p is appropriate for pumped 10ksi concrete. i believe from an estimating standpoint, the super-p cost would be very much offset by the lost work due to clogged pipes and timed-out trucks if you weren't using it. super-p "evaporates" quick so things can still get sticky fast.
whh47 -
you were questioning why a structural engineer would specify super p or scc on some jobs and not on others. -
here is one possible reason.- it may depend on the contracting method and the type of structure.
i have many associates that will gladly spec more sophisticated concrete materials if it is a project with known contractors, subcontractors and supliers going in. this provies a level of confidence and reliability and more of a "team" approach.
on other jobs, with the "old boy scout" method of design, bid, low bidder and heavier/more intense site controls and testing, they may often go with materials that are more suited toward a more varied "cast of characters". frequently, these types of publically funded projects can get high visability if there is a hint of a real or perceived problem.
dick
dick, thank you, you're right it can be because of contracting method.
one job, completed by our firm six years ago, as i tracked it out, was on design-built basis, had super-p and pearock mix on entire tower 54 stories. i found out that the owner, a/e and concrete contractor agreed to do that.