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(Structural)
(OP)
14 Jan 05 19:07My firm has a general note disallowing pea stone pump mixes preferring larger aggregates and yet no one has been able to give me a good reason why. Most says it's intuitive that it's relatively difficult to achieve even a 4000psi mix with pea stone or that it has worse than average shrinkage characterstics.
Since I'm regularly asked by contractors if I will allow it I have taken the side that it is not just because it must not be good if they want to use it. I assume they like it because they can pump it with a smaller grout type.
Any input on it's poor characteristics it would be appreciated.
2
Ron(Structural)
14 Jan 05 20:03The size of the aggregate does not generally dictate the ultimate strength. 4000 psi concrete can easily be achieved with "pea rock"; however....
There are several reasons you should continue to require larger aggregate. The smaller aggregate creates more surface area, that must be covered by cement paste. To do this, the supplier must add cement. When he adds cement, he must also add water. All of this creates workability issues and significantly greater shrinkage. In short, use the largest practicable aggregate size for the application as this will provide you with a more viable and workable concrete mix with good durability.
(Structural)
16 Jan 05 23:48....yeah Ron -
just to add a bit - the shrinkage Ron mentions causes lots of messy things called cracks....larger aggregates tend to scare those cracks away.
(Mechanical)
30 Jan 05 02:34My guess is your firm has been burned on several jobs that didn't pump properly. Most likely, the cement content was shorted.
I don't see any reason to shy away from pea rock.
As a concrete pumper, specifying a "pumpable pea-gravel mix" , or a "pumpable mix" whenever a pump is used or anticipated is the mark of an engineer with field experience. There are tailgate loads and pumpable loads. Pumpable mixes cost more and herein lies the devil in the details.
The task in the drawing room is a mix design that accomplishes the goal and then there is an eye to economy. Often, the fact that the mud will be pumped is left out of the equation! Hence the beginning of a bloodbath that leaves many a concrete pumper homicidal.
Rocks don't pump. If they did, quarries around the world would have concrete pumps in them.
A pumpable 5 sack 3/4 rock mix will most likely not pump if pea gravel is substituted. Pumping 1.5" rock thru a 5" system is easier than 3/4 , than 1/2" than 3/8....etc. CDF...2.5 sack, course sand and water and one jug of Palmolive soap per load pumps fairly good depending on conditions, pump and system. Concrete pumps because of the cementicious, water, sand and rock is in the proper blend. Pea gravel has more surface area than the larger rock. Bloodbaths are often encountered when pumping pea gravel because the rock content per yard is simply too high and not enough cement is present to provide the lubrication.
I pump a 7 sack pea gravel mix thru 2.5 rubber hose all day long. When the customer wants to lower the sack content I pull out my arrival ticket that states I require a pumpable mix....It's his money if we spend time banging hoses and doctoring mixes!
Wetting up a pea rock load that is already shy of cement is death.
I have had troubles at 6 sack. It wont do a 5 unless the hose is short.
Smaller line pumps cannot pump more than pea rock. An Olin ball valve is a perfect tool. Boom pumps can pump pea rock but going smaller is a real risk. Rocks are good in a boom pump. Stiff loads are fine provided the cement content is there.
(Structural)
30 Jan 05 22:35Navigator7...you are assuming the only criterion for a pump mix is to get it pumped and in place. Not the case. While I agree that a harsh mix is difficult to pump, there is still a need to balance the components so that pumpability and durability are not compromised. High cement content mixes alone are not the key. Proper aggregate gradation, proper cement content, and appropriate admixtures can provide excellent pumpability without just increasing the cement content. Increasing the cement content increases the water demand as well as significantly increasing the shrinkage of the mix. This leads to increased cracking and increased complaints about performance of the concrete, particularly flatwork.
(Mechanical)
30 Jan 05 23:03I don't disagree.
In my experience, however, the pumpability of the mix is not given the consideration it is due. Often the pumper is at the mercy of the inspector, the machine is in jeopardy and the placing crew at risk because the the mix won't pump.
It's true, engineers goof, but so does the batch plant and conditions can play havoc with the task.
(Structural)
30 Jan 05 23:12Navigator7...I agree with that. Most ready-mix suppliers have stock "pump" mixes that assume generalized conditions. There's a big difference between pumping 75 feet through a 2-1/2 inch hose and pumping with a boom truck, yet their mix is supposed to address both. Kind of like "One Size Fits All"....that doesn't fit anyone!
Tailor the mix to the application, whether pumped or dumped. One of my favorite sayings about concrete is:
Good concrete is made of aggregate, cement, and water.....bad concrete is made of the same ingredients.
(Mechanical)
31 Jan 05 09:03Another fun item is specing slump as it's placed or at the pump truck.I see specs call for a 4 slump. It's easy to get it at the pump truck but it's hard to cart a load off a high rise.One day we had an impossible mix that was going in the pump a 3 and coming out a 2.The heat was making it worse...a call was placed to the engineer....was that a 3 placed or at the pump truck?a 3 placed...but it can be a 4 if having troubles with heat.The inspector had me swing the boom over and tested the load.It was about a 1.5.We wet it accordingly and finished the job without an murders.
(Structural)
31 Jan 05 19:54Navigator7...age old argument...where to take the slump?!
Both places..at the discharge chute and at the end of the pump. The one for record, to check against the mix design, should be taken from the chute. For mix adjustments by ready-mix QC staff, at the end of the hose.
(Structural)
1 Feb 05 08:24Navigator 7, I'm a little surprised that adding Palmolive soap to a concrete mix seems to be standard practice in America. In Australia and Britain, adding soap/detergent to a concrete mix is banned. Am I reading your post incorrectly?
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(Coastal)
1 Feb 05 19:36Adding soap does help the mix flow better.There are many situations where concrete is pumped to fill voids etc where the strength or specs are not critical
(Structural)
2 Feb 05 05:55DaveMinter....It isn't a "standard" practice here and isn't allowed here either when it's caught. Many of our projects do not get inspection or testing, so contractors and subs fall into some bad habits. When they do an inspected job the first statement I usually hear...."I've never had to do this before"...or..."I was "allowed" to do this on the last job"....or..."I've been doing this for 20 years and no one ever made me (do)/(stop) this before". Makes our job as designers/inspectors/testing more difficult, 'cause then we have to "educate" the owner or GC as to why "WE" are slowing his job down!
(Geotechnical)
2 Feb 05 08:40Ron,
I wish I had a dime for every time I've heard one of those statements...I would be living on an island somewhere warm with a drink...
(Structural)
2 Feb 05 13:58GPT... me too! I can see those little umbrellas in the glass now! Oh well....I'll have to settle for fighting with them again tomorrow and then having a cold beer...not altogether so bad I guess!
(Mechanical)
2 Feb 05 21:08DaveMinter,
No, your are not reading incorrectly...but it was CDF...nothing structural.
Filling a swimming pool about 6 feet for example.
I soaped a slab job in 110º heat after the contractor poorly prepared an in-floor heating system. The loops were popping up and wallsin a mono pour were collapsing.
There is a time and a place for soaping loads.
I never do it when things are going well.
If it's not coming out the tip hose...what are you gonna do?
The only time I do it is with permission from the responsible party.
My Arrival Ticket clearly states pumpable mud is prerequisite for the job. When you get into an unpumpable situations it is generally not the pumper's fault but he is the guy that suffers and is usually the guy that will keep the job going. Not to mention the risk.
Now we are back to pumpable mix design and conditions.
If somebody doesn't make the proper call at the proper time, you've got a wrecked pump, the job is not accomplished and so on.
(Structural)
3 Feb 05 08:44Controlled Density Fill is not a term I'd come across working in Australia or Britain. As it is non-structural, strength loss would not be a problem, as noted.
(Mechanical)
3 Feb 05 12:29Actually Dave,The customer really wanted sand to fill the void. This would be compacted and then 4" concrete over that.The problem was transporting the sand by wheel barrow, down a flight of stairs, inside a house, into and thru impossible openings and such. Not fun or easy.My 120' of 2.5" hose thru a basement window was just too easy provided the mix pumped at all.Of course, they originally wanted me to pump just sand alone.Sand and cement is not fun to pump either as the mix builds on the product piston face. Stop too long and you are dead. The pump two-blocks and prevents stroke change which must be done manually. The addition of soap ....my guess is....keeps everything in suspension better.I pumped 2.5 sack, course sand and soap and lots of water thru 120' of 2.5 hose using a boom pump with 9" product pistons and and 83" stroke.It pumped like grease.
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Glad I live in Norway where all concrete is measured withing +- 5 kg or there about.
But we get funny mixes from time to time.
Counting out the obvious stuff like putting the recycled rock in the sand and other stupid things, I would say it's a whole lot of things to consider.
You can get 2 batches of the same mix, but one is not pumpable. Why?
Because the RM company don't do their job, got a bad batch of something or just the odd screwup.
If the chem like retarders, plasticizers... are old they don't work as supposed to, if they have sat to long and not been stirred, they can separate and don't work, if they are in an open container they can loose their effect or just a bad batch from the producer and so on.
Much to consider.
And then you have the rocks and sand, if you don't control the moist in the aggregate or get it wrong the concrete can be bad, and if you get aggregate with the wrong grading curve? (Is that the right word? My technical dictionary only goes english -> norwegian...) your in for some fun, is there is to little fine stuff or too much coarse stuff you get something that looks like a rock plug, the opposite you will get ball like plugs of fine silt. Also a propblem with recycled aggregate. And the rock don't have to be 22mm just because it passed through a 22mm mesh...
And if the plant has been producing other mixes there can be leftovers from that.
And then you have the drivers, don't know elsewhere, but here in Norway you are supposed to only rotate the drum 2-3 times pr minute when driving, then mix it up 2 min before unloading. Worst case, you get 'false air' (to fast) or nothing (no remix), either is a bitch with lines.
And when unloading a small load all the fine stuff tend to stick and all you get out is rock. Same with high slump, too fast when start to empty = rocks.
And high slump doesn't necessary good plasticity or viscocity or pumpable mud and the other way round.
There is no real good answer to this question, just a lot of if's and maybe.
It is only through years of experience you can say a batch has good pumpability or something you have to hammer through, just by looking at it.
But it's NEVER the pumpers fault, YOU DON'T MAKE THE CONCRETE, or deliver it...
But I guess thats why I love this job, you never know what you are gonna get or what the next brings.
Always learning new stuff and seeing new things is fun, although it can be hard from time to time.
Do you guys use PP-fibre?(glassfibre) The stuff that look like feathers...
If I get a rocky miz I usually throw a couple of bags and some plasticzicer in and mix it up, it makes a nice plastic pumpable mix.
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