Had an interesting conversation with the guy I plan to have do the concrete placement for my, hopefully soon, ICF house project. He has never poured a complete ICF house before but is a very well experienced commercial and residential concrete contractor.

I gave him a copy of the PCA article by John Gajda on concrete consolidation, and a copy of Mark Ross's comments in this recent thread, http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/4/postid/22844/view/topic/Default.aspx, concerning "gravelling." He readily understood Mark's concerns. In pours he does, particularly into foundation forms, he said you have to avoid pouring even on the form ties, let alone rebar, because it causes the concrete to segregate. And nothing beats a pencil vibrator for consolidation. Even there you have to be careful as some vibrators vibrate so little as to be almost useless.

Now for the interesting part. He said the ACI specifications call for the free fall of concrete from the end of the hose to be no more than 4 feet. In commercial placements, if the foundation is like 8' or more high, a tube extension has to be added to the pump hose and dropped down into the foundation to within 4' of the landing surface, be it the footer or the previous lift. Greater drops, like on the order of 8' or more, can even cause regular foundation forms to blow out. In concrete block wall where certain cells are filled with concrete and rebar, he knows of cases where the concrete blocks at the bottom of a long drop have blown out. And styrofoam ain't near as strong as wood or concrete! The ties do add a tremendous amount of strength to the overall system, though. In a regular foundation 6" wide the rebar will be all tied together in a pattern that is only 2 rebars thick and will be offset from the center a little bit. This provides clearance for a 3" tube to drop down as low as needed.

But this isn't the case with ICF. In a 6" ICF block the usual instructions are to lay the horizontal rebars in alternate slots in the webs so that when you put the vertical bar in you can drop it in between the alternating runs of the horiz rebar. This captures the vertical bar without having to tie it all together. Also, the rebar is placed pretty much dead center in the block space. What this means then is there is only about 2" free space to try to drop a concrete hose and extension. I don't know that you can pump concrete through a 2" hose, as a practical matter.

He strongly urged I lay up the wall only about 4' to 5', not the full 9', and pour that first. In a full 8' or 9' wall you can get by by starting the pour in one area and get a pile built up, and then pour onto that pile and let the concrete flow down to the bottom. That way you keep the free fall to 4' or less. This will work OK if the wall is continuous. But, he says, you have doors and windows in the usual house wall, which means you have to start a new pile at each one. In my case there's 24 doors and windows and 12 corners and tees. He said if I lay up the wall to 5' and pour that, there will virtually no problem with the "gravelling" issue nor consolidation. It does mean the vertical bars will be in segments and have to be lapped. He argues that the total time for the concrete placement will be virtually equal if done in one pour, or two. There would be an additional mobilization charge for the pumper, an additional bit of waste to prime the pumper a second time, and an additional cold joint.

Gene's argument is that in the end the final product will be better and there will be much less risk of bad concrete placement, therefore it's worth the little bit of extra cost and time to do it this way.

Your thoughts? I wonder if this method had been followed on some of the bad placements that have been mentioned in this forum, if they wouldn't have been good instead of bad.

Although I will probably be contradicted I do not know of any ICF builder that would pour in this manner! Most would build the wall pour the 4' lifts and go around how ever many times is needed, consolidating properly as they go.

That is the reality of pouring a wall and is no different than when they pour concrete for sky scrapers take a gander some time at the distance that they drop concrete.

I was also at a seminar during world of concrete in Las Vegas a couple of years back when the speaker was asked that exact question about concrete segregation I can not remember the name of the speaker but his answer was that with the new concrete designs specifically wall mix that you could drop it 100' with out segregation, and that it in reality is being droped the entire length of the boom from its highest point any how, with that said I would talk to the concrete supplier in your area and get there opinion on this issue.

No one could afford to build any thing with concrete under the conditions that you have mentioned above.

Respectfully Cattail Bill

Here's some stuff I cut and pasted from ACI's website regarding the freefall of concrete.
________________________________________________________________________________________

Q. Is there a maximum acceptable free-fall distance for concrete placement?

A. The American Concrete Institute’s two main standards governing building construction, ACI 318-05 and 301-05, don’t directly address the issue of free fall of concrete. Other ACI documents currently provide the following information on the free fall of concrete:

Section 5.4.1 of ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” cautions:

“Arrange equipment so that the concrete has an unrestricted vertical drop to the point of placement or into the container receiving it. The stream of concrete should not be separated by falling freely over rods, spacers, reinforcement, or other embedded materials. If forms are sufficiently open and clear so that the concrete is not disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks or chutes is favorable. Concrete should be deposited at or near its final position because it tends to segregate when it has to be flowed laterally into place.”

Section 3.5.6 of ACI 336.1-01, “Specification for the Construction of Drilled Piers,” cautions:
“Guide placement of free-fall concrete so as not to hit the reinforcement, hole sides, or anchor bolt assemblies. Vibration of concrete free falling more than 20 ft is not required.”

In the Specification’s Notes to Owner’s Representative, ACI 336.1-01 has two other pieces of information:
1. If some pier diameters are less than 30 in. (750 mm), review the Specification and modify. If pier diameters are less than 30 in. (750 mm), certain elements of the Specification can be inappropriate, such as the permitted use of free-fall concrete and any requirements for physical downhole inspection. The risk of free-fall concrete scraping the sides of the shaft while falling increases dramatically as the shaft diameter decreases below 30 in. (750 mm), and physical bottom inspection of pier diameters less than 30 in. (750 mm) is impractical.
2. Specify if free-fall concrete is not permitted or if the free-fall height is limited. Recent research on free-fall concrete has confirmed that free fall does not cause segregation, at least for fall heights up to 60 ft (18 m) and pier diameters as small as 3 ft (1 m) with 10 in. (750 mm) diameter cages. Even accidentally hitting the reinforcing bar cage does not appear to result in measurable segregation (ADSC-FHWA report on “The Effects of Free-Fall Concrete in Drilled Shafts,” [1994]); however, hitting the reinforcing bar cage may displace the cage and should be avoided. Thus, free-fall limits may be desirable in small diameter shafts deeper than 60 ft (18 m).

ACI 336.3R-93, “Design and Construction of Drilled Piers,” states:
“It is also permissible to allow free fall of concrete as long as it can be directed vertically on the centerline of the shaft, and it does not hit the sides of the shaft or the reinforcement cage.”

In the June 2001 issue of ACI’s magazine, Concrete International, an article by Suprenant cites several references summarizing the effects of free fall. He concludes that free fall of concrete directly over reinforcing bar or at high slumps doesn’t cause segregation or reduce compressive strength, but restricting free-fall height does decrease concrete production rates and increase owner costs.

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From my experience dropping concrete up to 12' high in 6" forms, it's pretty easy to direct the flow of the concrete straight down between the webs (and these are webs on 6" centers) and make it miss most of the rebar.

Properly using an internal vibrator will reconsolidate any materials that do get segregated.

DO NOT try to place the concrete by piling it up and letting it run horizontally down off the pile. Thats probably the worst thing you can do.

Bruce Cain, P.E.
ICF Building Systems
Mt. Juliet, TN

Bruce,

Great read. Your last statement is the most diff to understand. I have invisioned keeping a wave going without the impact of a hard 'splash'. Sounds like I had it 180 deg off!  I have 12' wall we are about to pour. Some areas are steped an additional 40", so quite some drop. We have planned on filling the lowest areas first and getting it all to level. Now as far as vibration; can I realistly get a vibrator down nearly 16'? Or do you think a re-bar rattler might take care to the first lift? I'll bet you would not advize re-bar rattling as you go to upper lifts due to the setting of the lowests lifts. Point being as the first lift starts to set, don't re-consolidate because it will have the opposite effect?

Patrick T.

You can get a vibrator into walls 12' deep.  Do the first lift 4' or 5' and as you go around for the second lift do not re consolidate the bottom lift.  If you need to put a piece of tape around the vibrator so you know when to stop.  As Mark stated before there are more things to interfere with consolidation than just the re-bar.  You can "punch vibrate" if needed.  Just push a 7/8"or 1" vibrator through the foam with it turned on.  It will drill a nice neat hole in the foam and consolidate the concrete near the punch point. 
It is costly to have a pump truck "come another day".  We have done some gable walls all the way to the top, all the same day. 

Bruce
Thank You for taking the time to post what I had read in the past. I was hoping someone would post at least a link, but you went one better!!

I agree with Bill, and probably a lot of installers....set up your wall and pour the complete wall. 4' wall pours, and then continue to erect blocks and pour another 4' is to me, a foolish approach!

I do think "graveling" may happen, but I don't believe it is a prevalent result of wall pours in ICFs; I think more people have problems understanding what their particular mix and slump is doing, and not recognizing how to change their technique!

Another thought. I use Viscosity Modifiers in my mix designs, these actually bind the coarse and fine aggregates while increasing the flowability of the concrete. By binding the concrete it is able to fall longer distances. I have not had any segregation issues with my walls since adding viscosity modifiers. Educating yourself on some of the admixtures that are currently available, I feel, is a great move to ensure solid walls. And of course always always vibrate your concrete with internal vibration.

ICF Contractor

Patrick,
I probably should have said "run laterally" instead of "run horizontally" in my post.

If you let the wall fill up by piling the concrete in one place and letting it run laterally to where it needs to be, then the webs will act like a screen and hold the aggregate back and let the liquid through. Now THAT will give you segregation that you can't put back together with a vibrator! Just keep the hose moving back and forth to fill up the wall somewhat evenly raising the level of the concrete up in lifts of about 4' at a time. Remember too that the speed that you pour and the height of the lift is temperature (and slump) dependent. You can pour faster when it's warm than when it's cold.

That "hard splash" you invisioned is actually good as it helps consolidate the concrete in the lower levels. Thats why you don,t need to vibrate concrete that drops more than 20'.

Tom,
I'd like to add one clarification to your statement about not reconsolidating the bottom lift when you vibrate the lift above. That is correct except you should let the vibrator penetrate the bottom lift a few inches to help "stitct" the lifts together.

ICF Contractor,
I didn't want to leave you out so let me rephrase one of your statements, if I may. "And of course ALWAYS vibrate your concrete with internal vibration."

Thanks

Bruce Cain, P.E.
ICF Building Systems
Mt. Juliet, TN

Looks like the responses have ended. Thanks for all your input. It actually is what I pretty much expected, but I thought I'd see what got stirred up!

I'll pass this along to my concrete man. He did say, "It's your house and I'll do do what you want me to do, but I'm just telling you from my experience in the field...." He mentioned he's seen the demonstrations at the concrete shows about plastizers and such and watching concrete slide past plexiglass windows. All looks really great at the show, but it doesn't ever seem to quite pan out the same in the field. He has many years of experience placing concrete, never has a call back, and doesn't cut corners. My brother said one day, "I can look at a curb from across the street and tell you whether Gene placed it or not."

I had the feeling that his comments weren't in sync with the way you guys, who do it all the time, do it. I didn't want to dismiss his concerns out of hand so I will share your comments with him, and hopefully give him greater confidence in doing a full 8' to 9' pour. I will in any case be doing my walls in two pours, the first being from footing to just above floor level, and then the rest later. This is because the ground has a 4' slope and I will have stepped footings with a crawl space. I'll pour the foundation wall portion and then build the floor. That way we'll have the floor to work from for the remainder of the wall. The tallest part of the first pour will be about 5'.

Again, thanks for the input.

Dmaceld:
I too would disagree with your concrete sub though respect the amount of time he seems to have already investigated in the ICF method.

We have had to pour six inch slump in walls 19 ft. high, to the top, in less than two and a half hours. So I am not sure the better ICF forms are in fact weaker than wood forms... Generally in residential work, we end up pouring a max. 13 ft., as labor demands escalate above this threshold.

Consistent methods for rebar placement, not just mix designs, are also key to pouring and consolidating effiectively.
Kevin

I assume (and that usually is a bad thing) that the reason most pros don't like to pour in two stages, i.e a 5' pour and a 6' pour for a 11'  wall is

the extra time and pump truck costs. 

I am a first timer and owner builder that has extra time and am willing to eat the extra pump truck cost to make sure that the job goes correctly.

Are there any quality or structural reasons to do the pour in one shot, or is it a time and money question?  My walls are 6" core Nudura and 10.5'

tall.  If I did it in two pour's is the finished product not as good? 
 

Irishwhip,
I see no reason to break up a 11' wall pour into two pours. If you brace properly, pour in lifts using the right mix and consolidate as you go on a 6" core wall. Have a couple extra hands to just make everything easier. I've seen plenty of Nudura projects go fine pouring mainly 8" cores to around 12 ' many times without problems. I'm a rep with BuildBlock and knoe if you use a good block properly, you'll end up with great results.

Good Luck
Dave

Thanks for the reply Dave. I was actually wanting to use Buildblock but you did not 10" core available. Our house is going to be built above grade on 10" columns supporting a 10" by 16" deep elevated beam. The 6" walls will be placed on the elevated beam.  But I have to pour the first course of 6" block so we can attache the floor system, then place and pour the last 10' feet of wall. I was just curious that since I had to basically make two pours anyway, if I was to place maybe three courses of blocks instead of just one if there was any advantages or disadvantages to that.  It seems that it would be less risky to make a 4' and a 6' pour than a 1.5' and a 9' pour.

Several things to add if ya-all dont mind.

First, pouring above 4-5 feet in a lift, does trap air in the lower sections and often, even with agressive vibration, you cannot remove it all, resulting in air pockets, usually found about 4-5 feet below the top of the pour down to the footing, so then the concrete, which is by definition a solid homogenous mass, is not truely concrete.

Second, Stiffer mixes with larger course aggregate means less segregation on impact, however a much higher opportunity for voids if improper consolodation is completed. What is almost ideal with a stiff mix, is two fold when it comes to consolodation. One is that it passes less pressure to the forms and lowers the overal form pressure dramatically, and second has a much larger active area with internal concrete vibrators.

Third, Additives can be very dangerous and cause more problems than they are worth. They tend to cause unecessary packing of the course aggregate, especially in pumps with reducing fittings, whereas good old concrete, does the job very well, easy to place, less segregation, stronger walls, etc. etc. etc. The list goes on. Proper placement of such mixes works, and works well.

Fourth Drop distances vary by design, so there really is no real effective height as such that shoulb be stipulated. Please note very carefully that the "codes" are there to ensure that reinforced concrete has all the qualities, without the need for a PHD to work with it. They are guidlines, based on a variety of assumptions, not neccessarily accurate. The PHD, can make the call, from an educated standpoint, hence a good ICF Pro, can also make the call from an educated standpoint.

Lastly, What gets a lot of installers, is a change in location of work, in which suddently the concrete places and behaves differently. This is very, very common, and has been the downfall of many a good ICF Pro.

I would reccomend to anyone one key item. Check your work, check every pour during and after placement, look closely to find any issues, and work to correct them. We all make mistakes, but ignorance is never an excuse for a bad job. Our livelyhoods depend on it.

Mark Ross

we have done vertical pouring of 16 feet using a patented light wieght concerte. with no Air trapment, and no blow out either.

Checking your own work is sometimes the hardest thing you do on the job.  Many times all is well, but sometimes you will discover that all is not as precieved.  It's always better that you find your own mistakes if any, and fix them. 

I understand that since I am a first, and the "hurricane god "willing last time home builder that there is an art to all of this. It seems from the outside looking in that it is a combination of engineering (science) and real world experience (art). I see many different professionals achieving results with slightly different methods.
My question is if all things being equal is there a quality issue with pouring a 10' wall in two stages, say 4' and 6'?

Only if it is done improperly most of us who have poured for a long time can tell what is going on with our pour as well as accuratley tell how much concrete we need for the job if we have extra some where there is a void, this is not to say that even the seasoned pro will not get a small void but those are few and far between and so small that they will not effect the structural integrity of the building

Thank you Bill