Segregation - fresh concrete 1

[ATSE]

ACI 301 and other spec guides talk about segregation, but the discussion seems very subjective.
What hard tools does the inspector have to accept or reject ready-mix concrete based on segregation.
My question comes from concern about a project where fresh concrete will be transported in a 1-yard bucket about 25 minutes up a rough-terrain road using a forward-reach forklift ("GradAll")?

 

[concretemasonry]

You better to be there to see how the concrete is handled and placed if you are worried about segregation. - Otherwise you have no observation on whether there is segregation of the placed concrete. A good mix without excessive water could work well if care was taken on placement.

Segregation of the concrete is applied to when the inspector sees the concrete and not on suspected problems that might be encountered later.

If it is a bid document, cover all the bases.

That is why the code is cannot address all possibilities.

Dick

 

[Ron]

How old will the concrete be at final placement? I'd be concerned about initial set and the slump at placement. Will there be any means of remixing before placement?

I would also expect that your issue might be more of lack of consolidation, rather than segregation.

 

[ATSE]

The ready-mix truck takes 20 minutes to get to the bottom of the mountain, where it meets the GradAll. The GradAll takes another 25 minutes, so 45 minutes in all. A line pump (like a Putz TK-50) will be used to place the concrete (40 ft max). As you've probably seen, pumps provide some mixing in the hopper; not sure if this is adequate to consider "re-mixed" (seems like it is).
Also, a retardant such as Pozzolith or Daratard will be used, and the concrete will be vibrated.
I've seen segregated concrete before, but don't know if simple re-mixing will "fix" this, and need adequate backing before I reject a mix.

 

[Ron]

Provided you have a reasonable slump at the pump, the "remixing" you get through the hopper and line should be OK....just don't let them wet the concrete down in the hopper.

 

[dik]

Often lack of consolidation fron lack of proper vibrating is the cause of 'segregation'. For true segregation, I've found improper placement due to reinforcing steel in wall being the culprit more often than not.

Dik

 

[bb29510]

unless you have a high fluid mix, segragation is really diffucult to do. A normal 4 inch slump is just not going happen on its own. now people here will say is does, but slump has more effect on segragation than vibration does

 

[Ron]

brownbagg...you got part of it right. Slump has something to do with segregation, but it is only one aspect. The mix design (proportion of coarse to fine aggregate), course aggregate size, configuration of the formwork and rebar (as dik noted), placement technique (chute, trimie, pump), and yes, vibration.

Concrete with a 4-inch slump can segregate, as can concrete with a 2-inch slump and an 8-inch slump. Just because you haven't seen it happen, doesn't mean that it can't happen.

 

[ATSE]

Gentlemen,
Thanks for the discussion.
One last question: Is there a recognized standard for calling fresh concrete "segregated?" That is, if you're physically at the site during concreting, what criteria would you use before you could say with confidence - "I will not accept this concrete since it is no longer homogeneous..." Besides risking your life at the hands of an angry mob, the engineer has to have some criteria.

I have seen localized segregation from over-vibrated concrete (usually at the top of a wall), but wouldn't know how to measure that, or put numbers to it.

 

[dik]

When birds can fly through it, you have some really bad honeycombing...

Dik

 

[Ron]

..or when you core a hole in a mat foundation and the wash water runs out the side of the mat 15 feet away and over a foot below the surface.

 

[jmims912]

You know...I'm thinking its a good and fair question and it makes me realize that for me it is a judgment call... not as random or subjective as that may sound, but, in fact based on a fair amount of experience. Yet, not something that I can easily relay to another. I'd say build experience as much as possible by viewing placements as much as possible...and better still, find a way to place samples of concrete(perhaps from the excess from the last truck)and purposely mistreat them...you can't beat hands on experience and "learning from mistakes"...

 

[conceng]

You could reference ASTM C 94, Appendix A1 which has to do with mixer uniformity. You would sample from near 15% of the load and 85% of the load and wash them each through a #4 sieve. They shouldn't vary more than 6%. This intended from the mixer, so you still may have to argue with your contractor as to how this applies to placement equipment.

Greg

 

[mugzy1]

The mix design should be built robust enough to handle the environment you will be placing it in. Rheology control is the key to your success here.

http://www.todaysconcretetechnology.com/forum/

 

[darthsoilsguy2]

Site-batch! Site-batch! Site-batch!

by my count, at best you can order a 2 yrd truck. Segregation shouldn't be a problem, but all concern for which can be cured with a well-graded mix.

Here are some other issues to consider.

If they get two trips per truck, how will the old concrete in the pump line affect overall quality... maybe they will dose it with some water to keep it fluid. Maybe the truck will dose between pours because he has been sitting there spinning for a long time... Will they try to work on wet days and possibly damage this access way. Will weather, truck arrival, or equipment failures affect the delivery so that the contractor will feel obliged to place old concrete because its better than having a monster cold joint.

Also, it has been my experience that truck drivers just destroy concrete when they add water to a small load.