Self-compacting concrete 2

[KimWT]

Hi,

When you use self-compacting concrete, don't you need to be concerned about aggregate segregation problem
unless it is extreme case like using vibrator severely.
A salesman from admixture company explained about SCC use in precast concrete industry.
He did not mention about aggregate segregation at all, but air pocket or bug hole problem.

For example, when I pour SCC to produce 60ft long precast double tees of 28" tall stems and 4" thick flange,
what sequence of concrete placement should I follow:
1)do I pour stems and flange together? Make concrete mix truck pass once. Or
2)do I pour one stem first and then pour other stem and flange later. What time interval is good? Or
3)Can I pour SCC with mix truck stationed at one location and let it flow to everywhere? Or
4)Should I pour a part of stem and wait until internally entrapped bubbles can escape and pour remained portion later?
(Because 28" is too deep for the internal air to move to the top.)

Otherwise, it depends on which specific chemical is used.
I saw an European guideline say "NO GENERAL RULE."
So, aren't there generally accepted rules on concrete flow (max) distance or maximum hydraulic head?

Thanks for your time!

 

[Teguci]

When you use self-compacting concrete, don't you need to be concerned about aggregate segregation

Yes, when they perform the pancake test you should see the aggregate flow all the way to the outside of the ring. This performance should be confirmed in the mix design submittal prior to the onsite pancake test. Any separation at the edge of the test is grounds for rejection - do not use.

what sequence of concrete placement should I follow

Theoretically, you can pick a corner of the flange and put it all in there. I'd suggest limiting transportation of the concrete to about 6 ft though. A flow distance of 10 m has been observed. Although its SCC, I think a little vibration at the congested ends of tee stems is still a standard practice.

 

[KimWT]

Thank you very much for your input!
BTW, how can you limit the concrete flow (up to 6ft)?
In this case, 60ft long precast DT, isn't SCC flowing continuously to the bulkhead?
Do you want to install temporary blocks to limit concrete flowing?
I don't this is practical solution.

 

[Teguci]

First, I've never put a pour sequence together for precast, I just have a little experience with SCC, so take my advice on this with a grain of salt. What I mean is to limit the required transportation to 6 ft. If it goes further than 6 ft (it will) that's OK. Just make sure they deposit subsequent pours along the tee at 12 ft spacings. I'd think the critical locations are the 4 ends of the tee stems. Pour down the length of one stem, then the other and then fill the flange. Vibrate the forms at the ends of the tee stems and you should be good.

If you want reassurance, watch this video. They are pouring the full tee stems from the middle and the concrete is just flowing around the dapped tee reinforcement.

https://www.youtube.com/watch?v=Ykin8xWSrf4

 

[TehMightyEngineer]

We did a large pour of precast arches recently using SCC. Was pulling my hair out trying to get minor bug holes and lift lines out of the pieces. They were 8 feet deep. Some pieces had them, some did not, almost at random. However, we had a long fall for the concrete into the form and we always had bugholes at the placement points. We suspected that the concrete was trapping air against the form and not releasing it. We were also pouring slabs and what was essentially counterfort blocks for a headwall. Those pieces turned out perfect every time.

Tried using 2 placement points and we had twice as much bugholes. It was minor enough that it was easy to patch but was just so frustrating that we couldn't solve it the correct way. Faster pours worked better than slow pours, though. We did try very light vibrating of the form at the pour points to no effect (no bad effects either). Occasionally we had some soupy mixes but never once had poorly consolidated aggregate. Tried rodding the ends of the arch where we got the minor lift lines to no noticeable effect (though it didn't hurt).

Overall I'd say limit yourself to a single pour point down the middle of the piece and pour the entire piece in one pour if possible, as fast as possible. Don't do two pours for the stem and flanges. We were placing the concrete using 1 cu. yd. buckets and thus could only do it in multiple pours. With the quick set times, if we were just a tiny bit off in mix consistency or has a slight delay you could see a faint lift line (though it was always well bonded, just unsightly).

Experiment though, each form will react to SCC different from what we found. We didn't have a single piece rejected even with all our experimentation so it's not going to be a big problem to try a few different methods when you start out. If you want, do not feel that you can't lightly vibrate but I'd check by hand to make sure the aggregate isn't settling; plus vibrating really beat out the #1 benefit of SCC which is ease and speed of placement.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KimWT]

Isn't the compressive strength test for SCC different from normal concrete test?
Rodding (consolidating) is limited in SCC.

 

[msquared48]

I thought the term was "self-consolidating", not "self-compacting".

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

Self-consolidating and self-compacting are used interchangeably from my understanding. Self-consolidating is what I call it, personally.

SCC will cure differently and I suppose all things equal SCC will have different strengths than a similar mix of normal concrete. However, there's no reason you can't have a mix design that gets you the properties you're looking for from SCC.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com