Shotcrete Problem

[BigH]

Anyone with any experience with shotcrete. On this new job - we are using a air pump and the spec calls for 80mm slump plus/minus 30mm. Contractor can't pump it. He is trying to get a 180 to 200mm slump. Problem is that a few think it is pumpable but they could have been using a piston pump. Any thoughts??? We need 40MPa (about 6500psi) at 28-days.

 

[BigHarvey]

Check the fine content of the mix. In order to be pumpable the mix must contain at least 400 kg of fines per m3 ( easisest is 400 kg of cement ! but if yo have a filler locally available it can do ). I woulmd suggest a plasticizer to reduce the amount of water ( subject to escape under pressure through seals and other leakage sources and block the material in the ducts ). Good luck !

 

[SlideRuleEra]

BigH - I've only been involved in two shotcrete projects, and that was years ago. However, at this link

http://www.concreteconstruction.net/search/Search_Results.asp?Page=1

do a "keyword search" on "shotcrete" for 122 hits on background information; a few of these may be helpful.

http://www.SlideRuleEra.net

 

[BigH]

BigHarvey - thanks. The contractor's mix design is:
560kg of cement
375kg of 5-10 agg (pretty flaky/elongated)
1045kg of sand
230kg of water
20kg of silica fume
20kg of Ribtec Fibre (steel fibres)
5litre of Glenium T admixture
3% MEYCO SA160 accelerator at nozzle
Slump is 150mm (which is outside specs) and they say they can't "pump" it - read that easily. Pneumatic pump putting out 6m3/hr and he says he can't knock it down any more.
Any more comments?? Srength at three days on a panel of the above was 32.2MPa avg with range of 26.5 to 39.9 MPa.

 

[Ron]

BigH...a couple of things that I think are working against you....

The flaky, elongated aggregate is more harsh for pumping. That doesn't show in a slump test, but does affect the pumpability.

The silica fume increases the surface area related water demand in greater proportion than its proportion in the mix, which requires either more water or more plasticizer (Glenium T).

The Ribtec fiber, while a good choice, also makes the mix significantly more harsh. I would use the shortest fiber available for this application.

I would suggest a drop in the CA (to say, 300kg) (with an increase in FA to 1100kg), a slight increase in the cement and water (580/240, keeping the ratio), a slight increase in the super P (per mfr instructions), and a slight drop in the silica fume (to 10kg). These are rough generalizations....the absolute volumes would have to be balanced.

 

[BigH]

Thanks Ron - I've mentioned the flakiness to them but, there are other issues. First - one more bit of information - we are using a Aliva Type 285 pump which is good for dry and wet mix. Yesterday, we batched up a batch at the plant and got 240mm slump!! Didn't seem that bad but as it was practice we sprayed it anyway. In doing so, we noted that with a pneumatic pump, you want to increase the pressure a bit to make more short spurts of air to pump the mix than a larger single spurt. to make matters worse, the second batch was found also, sadly, to be 230mm slump at the plant but only 23mm at the site! No joke. Something happened. Will look into Ron's suggestions on the adjustments and as it seems the applicator has no experience on pneumatic (only piston) there will be several more "practice" sessions!!!

 

[BigH]

Ron - I've had two of your addresses at aol kick back out. Did you change? drop me a line at bohica.fries@bigfoot.com .

 

[BigHarvey]

I don't see anything wrong with the formula. Check the size of the fibers in relation with the diameter of the hose. try pumping the mix without fibers and with fibers to check if the problems comes from here. if the hose is 65 mm I wouldn't use fibers larger than 30 mm. Check also if proper equipment to inject the fibers in the mix is used ( otherwise they have a tendancy to stick together and block the line).

 

[BigHarvey]

I suppose the problem is solved. What went wrong and what did you change to make it work !

 

[BigH]

Well, it is and isn't. We had some very funny stuff happening. We ended up doing a lot of small mixer batches - if we had a litre too much water, the slump was 230 to 240mm, mix 10 minutes still the same, 10 minutes more, maybe 200mm. If a litre too little water, the slump would start out at 230 - go to 120 then go to less than 40 after 20 minute of mixing. The specs say to pump 80mm but can't be done.
So, we have let it go at 230 at the batch plant. When the Meyco160 is added at the nozzle, it sprays pretty well given that we are spraying with an excavator bucket rather than the robotic arm.
Had some good results of test panels - got our strengths. So, the contractor is spraying - while outside spec of slump, it is looking okay - but of every 5 batches or so, he loses one on the slump. Degussa guy was here and they are sometimes, if the slump on arrival is down to 150, adding 1/2 litre more of gelenium.
Ideal and to specs - not - nightmare to QA. Working okay? Seems to be with difficulty. Beams were cut of some panels and looked uniform without any appreciable sand layerings, etc.

 

[PEinc]

My biggest problems pumping shotcrete are caused by angular aggregate, low slump, concrete mixer trucks that have not been cleaned out of old, larger aggregate, and pump hoses that are too narrow or too long.

The aggregate should be rounded, 3/8 inch maximum. The slump should be 4 inches, and the mixer trucks need to be completely washed out before filling with the shotcrete mix. Tell the concrete plant that, if possible, you want only trucks that are dedicated to delivering your shotcrete. I also place a screen across the top of the pump hopper to prevent large aggregate from getting into the pump.

 

[BigH]

PEinc - you typically use piston pumps or pneumatic pumps for the shotcrete?

 

[PEinc]

Smaller piston pumps (40 to 60 CY/hour) for soil nail & shotcrete walls.

 

[BigH]

Ours was a pneumatic pump - and the "specialist" had always before used piston pumps. Still, we had some rather interesting behavioural problems with the slump.
Ciao.