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.

 

[jayrod12]

We've stopped specifying slump as it's a useless test (I'm likely to get torn apart for that comment) .

If the slump is too high they can just let it sit in the truck longer. If too low then they may add water to make it pass.

All I specify is that the contractor must specify his desired slump ahead of time to ensure workability and as long as the concrete meets the w/cm ratio and the other specs I've asked for then I don't care.

 

[steellion]

I think we typically spec 8-11 inches, so 21 cm would be in that range. Slump for super-plasticized concrete doesn't mean much though, since the goal is a more liquefied mix.

To me, measuring slump for any concrete mix serves as a sanity check more than anything. If you have 20 trucks with the same slump range, and the 21st truck is way off, could be indicative of a problem with that concrete before it is placed.

 

[Ron]

Yes...slump is to measure two forms of consistency. The first is the consistency (apparent viscosity/flowability) for workability, to allow proper placement and consolidation of the concrete. The second is consistency for batch-to-batch comparison as steellion noted. Both have importance if you want to control the properties of the concrete you specify.

In my occasionally humble opinion, waiting to determine the performance of concrete is not a good way to assess its potential quality. This must be done at the time of batching/placing/finishing.

The mix design for superplasticized concrete should be cognizant of both the pre-dosing slump and the post-dosing slump; whether the dosing takes place at the plant (most commonly done) or at the site (less common these days). Only the slump of the pre-dosing condition has any relevance to the historically predictable properties of concrete, with the obvious exception of workability.

 

[jayrod12]

Our specifications note that the contractor can choose the slump he desires for workability. This slump must be shown on both the submitted mix design and delivery ticket.

I don't particularly care what that slump is or how they get the slump to what they want (plasticized or not). As I said previously, it needs to provide the performance I'm asking for. I'm not a mix designer, nor am I a concrete placer. I leave things like that up to the professionals.

For what it's worth. I've seen plasticized concrete that had immeasurable slump because it was basically soup. But all the specs came back as designed and the concrete still looks fantastic.

 

[Ron]

jayrod12....how can you know what the performance will be when you take no control over the input properties?

I leave things like that up to the professionals.

That's the problem....they are likely not professionals in the subject either. The contractor probably knows less about the properties of concrete than you. The ready mix supplier should know more than all on the team; however, they only supply what they are asked. You are letting a contractor with little or no training in the properties of construction materials make an engineering decision for you. The chances of that biting you in the a$$ might actually be small; however, when it does happen it can happen big.

The contractor will make a decision based solely on cost and expedience. You would be making a decision based on an engineering need. There's a big difference.

Sorry, but I disagree with you on this one.

 

[chekre]

Thank u for ur different opinions.
another question: during the inspection, i saw electrical cables passing inside the concrete slab. personally, i dont have an objection since the cable diameter is small (less than 5cm) And spacing between the cables is ok (>1 or 2 meters). It becomes fichy when the cables converge to a single point. So the spacing will decrease and u will have congestion of many cables in one particular area which will reduce considerably the concrete section strength.
obviously, a bad coordination between the designs engineers (electrical and strucural).
What do u do in such case ? Is there any limitation regarding the spacing or location of electrical cables ?

 

[Ron]

chekre...conduit congestion and other obstructions within the concrete are common problems. These must be checked by the structural engineer for impact on the concrete section. In some cases, it only increases the cracking potential in the area; however, it can be detrimental to the structural integrity of the section.

 

[chekre]

Yeah right ! So it is an engineering judgment ! Miscoordination between the different trades ia the main cause. There must be some rules like allowable spacing between the conduits. and if this spaxcing is not respected, then we should get back to the design enginner to check the concrete section.

 

[Ron]

Conduit, like rebar, should be no closer together than about twice the nominal coarse aggregate size. This allows proper consolidation of the concrete around the conduit or rebar.

 

[KootK]

Engineering judgment is what needs to go into the limits that you're looking to have respected. Most firms develop in house guidelines something like this:

1) Min spacing at least 2x aggregate.
2) Min spacing at least 3x largest of adjacent conduits.
3) Conduits must exist within middle 1/3 of slab depth.
4) No more than two layers of crossing conduit anywhere.
5) Extra rebar where conduit is excessively dense.
6) Conduit tied in groups where density is excessive.

And still, where all of the conduit meets up in an electrical room, you're probably going to have to visit site and apply some additional judgement. I've taken to trying to build shaft walls around electrical rooms so that the structural demand on the slabs inside is minimal.

I'm with Jayrod on the slump business. Maybe it's a Canadian thing. I ask for stamped mix designs and a stamped quality control plan. Those stamps invariably belong to professionals more qualified than I when it comes to mix design and quality control. And, while I won't spec a slump value, that does not preclude slump measurement from being used as a monitored parameter of the QC program.

The closest that I ever came to legal problems regarding concrete specification was on a super flat industrial SOG. I was too heavy handed in the specification of the concrete and had to have the finishers school me on what really needed to be done to achieve our goals. Complex projects can't be realized unless the white collars place some trust/faith in our blue collared brethren.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

And by trust/faith, I also mean liability. Liability appropriately allocated to the party best suited to handle it in this instance in my opinion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[chekre]

thank u KootK.
Regarding the conduits, if we are dealing with 1 layer of conduit pipes, its location within the slab is not a big deal since it wont reduce the section strength. Things become scary when a big amount of conduits are there. I think the best solution in order not to jeopardize the structure is an architectural one and is to provide a shaft.

 

[jayrod12]

Thanks KootK, you put that much more eloquently than I could have.

 

[KootK]

The performance specification thing makes for an interesting discussion. A few years back I was involved in some significant infrastructure work where the local municipality required everything to be "High Performance Concrete". The big fish concrete supplier who got the job refused to provide me with their mix designs because they considered them trade secrets. I kicked up a fuss and the municipality instructed me to back down.

In the end, my concrete specification basically got whittled down to "awesome concrete, I want frickin' awesome concrete". And all objective indicators point to my having got it. Once I adapted to it, the arrangement was kind of liberating. I really do just want frickin' awesome concrete.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I thought that the before/after photos below might interest you chekre. I didn't come across conduit grouping requirements until fairly recently. As the photos shown below illustrate, they can be surprisingly effective. According the field rep that took the photos, the time lag between photos is six hours.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CANPRO]

If my memory serves me correctly (from my concrete testing days), I think the slump test is only valid up to a certain value...at which point I think you need to a different test which measures the spread of the concrete. I don't have the testing spec here, but I think the limit was less than 8".

 

[chekre]

thank u KootK. The above photos are helpful. I think placing the conduits within the middle third is an intelligent manoeuvre since stress will be minimum. However, i think in such congested areas the designer shall be consulted specially if such arrangement is near the column (risk of punching failure).

 

[KootK]

I don't think that I'd allow any horizontal conduit within the critical punching shear perimeter unless there were oodles of reserve capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dcarr82775]

ACI318 has rules for conduit placement similar to KootK's list. As for embedded conduits, the electrical engineer is useless, they always tell me size/spacing/layout of conduits is not in their scope, it is the contractor who determines that. So you just go out and after having a small breakdown curled up in the fetal position rocking slowly back and forth when looking at the slab in the area of the electrical room you stand back up and start figuring out how to get it good enough so you can sleep.

We have had success requiring the plumbers and electricians to submit sleeve layout drawings so we can get the bigger opening handled before we get on site, but the conduits are always a problem. They need to be there, so sometimes cleaning them up as in KootK's pic is the best you can do (still a terrible jumbled mess bottom center but better than it started).

I am just barely old enough to get my curmudgeon on, so I still spec slumps but since I am a modern man I will allow deviations at the contractor's request if in my judgement the overall mix still looks good. I see no point in having a stamped mix design or QA program by the supplier.