A typical slab will test at 2500 to 3000 psi in 28 days. Or so I’ve been told. Shotcrete walls have been tested at 4500+ psi in 14 days.
Does anyone know what’s the composition of the mix that causes this? Is it just more portland, or something further? And I woulda thought the pressure of spraying it also added to its density and strength, but the 4500 test was from right outta the hopper and so was unaffected by the way it’s placed.
What the most accurate explanation of the source(s) of strength differences between typical poured concrete and typical shotcrete? Define tems as you see fit, be/c I’m playing kinda loose with terms like “typical,” “strength,” etc.
Replies
Typically it is the ratio of Portland cement to water. More Portland with less water is stronger concrete. In the midwest we use 4000 psi (air entrained) for sidewalks and driveways to help with the freeze thaw environment. Using superplasticizers (sp) you can get 10,000 psi concrete that will flow.
http://www.portcement.org/
When I call the batch plant, I'll tell'em that I'm aiming for 2500# or 3500# or whatever with 3/4" stone or pea stone according to use. They'll add more portland for the stonger mix.
You're right about fast and loose with variables. Temp on site and how much water I have the driver add will affect final strength.
Gunnite, being from a sand mix instead of stone aggregate, will need even more portland to achieve high strength but I think it mixes drier so that adds to strength.
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Excellence is its own reward!
Edited 10/30/2002 7:51:49 PM ET by piffin
Now, I got to thinking, the gunnite is not normally used for a slab and you do domes. What is the project you are dealing with, anyway?
I'm wondering if you need compressive strength or tensile strength? The latter comes from correct steel placement.
Excellence is its own reward!
It came up in a dome question. We don't ask for a specific psi rating for the shotcrete, but it is always 4000+ just by definition, it seems. Has 3/4 or 3/8 stone in it. Concrete for compressive and steel for tensile. Need and use both. Better compressive strength, thinner the shell we can achieve. But I just got to wondering if anything other than the amount of cement contributed to the compressive strength of the end product.
interesting thread....isnt true that concrete is constantly gains more p.s.i as time goes on. now i realize the curve over time is'nt as dramatic as lets say over the first fourteen days, with frequent wet downs and a poly cover. but with the wetting down and the change's in weather the p.s.i. very gradually goes up?? i'm sure with a lot of variable's optimum mix ratio, initial curing?? just a question i figure i'd throw out there thanks for any info..... bear
When my Dad was working on the construction of some of the hydro-electric dams in Oregon in the late 40's, early 50's, he was told the curing of the concrete dam structure would continue for 30 years. If that was the truth, we can assume they've been at full strength for the last 20 years.
ARCHYII had the info you wanted. Compressive strength is inversely related to the water-cement ratio. Here are a few quotes from "Design and control of concrete mixtures", an engineering publication from the Portland Cement Association. ISBN 0-89312-087-1.
1. The strength of the cement paste binder in concrete depends on the quality and quantity of the reacting components and on the degree to which the hydration reaction is completed. 2. Concrete becomes stronger with time as long as there is moisture available and a favorable temperature. 3. Hydration is the chemical reaction between the cement and water.
Most of us just order what we need from the plant based on what the engineering requires or on past experience or "how we've always done it". Then, when the batch gets to the work site, somebody decides the concrete needs to flow better or faster and the mix gets totally screwed up by the addition of water. The 4,000 psi that was ordered, properly mixed and shipped is now toast.
It would be interesting to see what the numbers are for Cloud's shotcrete after placement. Does the gun operator blow the mix by adding water because he thinks it would shoot better?
I took samples on a 300 yard pour last wed. today befeore coming home I broke the seven day sample.
this is a 4000 mix
1 5 in slump 4200psi
2 6 3900
3 5.5 4367
4 7 3200
5 9 2700
6 6 3560
concrete and water are the MAIN variable but there are many more, aggregate, time, temp, add mixture etc. But mainly water is what you can control
>It would be interesting to see what the numbers are for Cloud's shotcrete after placement. Does the gun operator blow the mix by adding water because he thinks it would shoot better?
In shotcrete (versus gunnite), it's just the mix in the hopper and air pressure. Not possible to mix water at the nozzle.
Ok, your operator uses the wet process. Did you ever see him add water to the hopper?
There is a dry process where a premixed blend of cement and damp aggregate is blown thru a hose with compressed air and at the nozzle water is added. That application I have not observed.
Ralph,
I've often seen a gallon and 5 gallons of water added. I've seen it in the truck so thick it won't flow down the chute. The shotcreting I've observed was with guys who'd done that for about 25 years each, and they had a good idea of what consistency was needed for their pump. I was just trying to watch and learn (and drag hoses and keep clumps out of the hydraulics and ...). The 4500 test sample was from the hopper of the pump, at which point there's no way to add further water.
The dry process is gunnite. I recall a FHB article on a gunnite wall in SF, I think. From what I've been told, it requires a much more experienced operator, but that such an operator has much more control over the quality of the mix. The equipment is more expensive, too.
Jim
Cloud,
The other posts were largely correct in stating that the water/cement ratio is the most important factor in design strength versus actual achieved strength. The best way to make sure you achieve the minimum strength required is toorder a mix with a higher design strength to allow yourself a cushion.... that said it has been my experience that most mixes surpass their design strength so long as the concrete is poured with a reasonable slump and is off the truck and in place fairly quickly. You can usually pour a 6" slump on flatwork without hurting strength much if any. However I have seen guys try to pour at a 4 or 5 " slump and temper the load 2 or 3 times not realizing that the last half or third of the load might be a 12 slump in actual water /cement ratio. Best approach is to know what you are doing, have enough competent help present to handle the quantity ordered, including enough so that a couple of guys can drop back and start finishing if it is a large pour. Get the mix workable right at the get-go and get it the heck off of the truck. Another big strength reducer is too many revolutions on the mixer drum (many state and federal codes limit the revolutions if the concrete is to be used for highways or government buildings)this goes back to getting it off the truck as fast as possible. Compaction is another issue: too stiff a mix cannot be adequately compacted by hand screeding, especially getting rid of the voids created by workers wading around in the concrete. Cylinder tests are one thing but they are not always accurately reflective of what happens once the concrete is on the ground and has been finished. I would also recommend you read the literature put out by the Portland cement Assoc. regarding hot and cold weather concreting as temperature can play a big part and won't refected by the cylinder tests. There are a lot of variables at play.
Mark
There's a decent article at http://www.thelabtexas.com/Cold_Weather.htm about cold weather concrete.
Eric
Edited 10/31/2002 8:46:39 AM ET by EBORG2