Phi angle for wet concrete? 2

[TPK06]

I'm looking at a steel sheet pile cell design that may require tremie concrete cell fill due to concerns with vessel impact.

Does anyone have any information regarding the phi angle of wet concrete? I'm trying to determine the earth pressure coefficent for the design.

Thanks!

 

[PEinc]

Check formwork hardware catalogs and design manuals for pressure formulas. The pressure from a concrete pour to the sheet piling depends on the rate of pour and the temperature of the concrete. The rate of placement, the temperature, and concrete additives (including fly ash) determine the rate of hardening and, therefore, the amount of liquid head of concrete.

 

[PEinc]

Phi could be as low as 0 degrees (Ka = 1, a liquid head).

 

[TPK06]

That makes sense. Thank you!

 

[GeoPaveTraffic]

Actually if the concrete is vibrated in-place Ka can be greater than 1 for short periods of time. I know it may not be imeadiatly how this can happen and I don't remember the explaination that I was given. But I have seen the instrumentation data from large concrete pours on Lock walls for the USACE.

 

[dgillette]

GeoPaveTraffic -

This might be related to when soils compacted against a wall give high lateral pressures because they yield during compaction, and some of the stress is locked in. The aggregate acts as a granular material that is not completely fluid.

Or maybe not.

 

[GeoPaveTraffic]

No not a backfill issue. The case I listed was instrumented with varrious instruments (pressure cells and strain gauges). The reason for the instrumentation was that the form work was deflecting out of spec during material placement. The pressures calculated to make the forms deflect to the observed extent were higher than the Ka=1 condition. Therefore no one believed them.

We placed numorous instruments in two different pours and the measurments showed higher than Ka=1 conditions. Not for a long time but the pressures did spike during placement and vibrating.

 

[msucog]

here's my opinion: i think ka goes to 1 (phi=0) once vibration is applied and then the mechanical vibration implies significant additional forces (likely harmonic wave) to the immediate area. essentially, there's additional loading from the vibrator. i'm not sure i comprehend how ka would be more than 1, but i've never really looked in to the issue before so others here are likely much more learned on this than me. i guess maybe you could translate the added forces due to vibration over to the phi (maybe "effective phi" better describes??) of the concrete--i don't know since i've never thought about it much or researched the topic.

the vibration is what i've always seen cause problems during wall placement. it depends on the things peinc mentions above. check out ACI 309 for info regarding the vibration and effects of vibration on concrete. for tremie placement, i visualize that there would be additional forces due to the pump itself pushing the concrete to displace the water....but i'm getting out of my area of knowledge so i'll leave the discussion to you folks that are more experienced than myself.

 

[civilperson]

The yield weight of concrete times the height is the pressure designed for in form work. Deflection usually controls due to straightness provisions of finish product.

 

[PEinc]

That is not always true. The pressure is not always totally triangular and fluid. Depending on the factors I mentioned above, the pressure can max out as the concrete starts to set.

 

[LCruiser]

Lateral pressure under vibration, especially with a mixture such as concrete, can indeed produce what appears to be a ka >1. Particles move into position during the low pressure phase of the wave, and when the high pressure phase hits they are essentially locking together. Repeat...

 

[rconner]

Of course one of the effects of effective vibration can be consolidation. As I suspect some would sort of define consolidation as an increase in density (and bringing dense aggregate particles closer together etc.), it just kind of seems that effective vibration resulting in an increase of fluid density would at least to some extent kind of naturally result in a higher pressure (and that eventual concrete setting/shrinkage would kind of naturally reduce the pressure)? Depending on what kind of "instruments" are used to measure pressure (or deformation of formwork etc.), if those e.g. are dial gauges or linear devices more things than pressure against the formwork could influence deformation (actual thickness, moduli, benchmarks etc. compared to assumptions). All this being said, maybe angular aggregates coming into contact (as wedges?), and/or vibration waves according to other responses may be the culprits; in any case it probably behooves one to be reasonably conservative. [BTW, I didn't notice in the case studies referred to how much more than one the k values appeared, or what was measured or assumed? for the vibrated fluid density against the sheets/walls in determinations for same?]

 

[GeoPaveTraffic]

The instruments used included static and dynamic pressure cells, static strain gauges, and thermistors.

 

[fattdad]

O.K., I'm coming in late and may be misguided. . .

If you have concrete with a slump of 6 inches, doesn't that imply that there HAS to be a friction angle? Wouldn't that also imply that the friction angle would be pretty high (again, I may be misguided)?

To me, there is a liquid component to the wet concrete, but there is also a fair amount of aggregate that will develop internal friction. With this internal friction, there should be a corresponding affect on the lateral "earth" pressure acting on the retaining wall. I've never studied this at all (so again, I may be misguided), but it does lead me to wonder. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[rconner]

fattdad, this is a good point but of course several responders have now thrown vibrators/ion into the mix (when you put a vibrator into the pile, I think it will make the apparent slump a lot more ;>) )

 

[PEinc]

Sorry if I insult anyone, but some of you sure know how to make something more complicated than it should be. You have turned a construction question into a research project. Have any of you actually designed concrete formwork? If so, how often (if ever) have you increased the design load on the concrete forms because someone used a vibrator? A vibrator may increase the concrete pressure on a limited area for a short time but I have never seen this to be a problem.

 

[fattdad]

I for one have never designed any formword and was floating a thought to help my learning. Not sure of PEinc's reaction and if it relates to my post, but if it does, I'm somewhat shocked. Forums when polite, are for learning and discourse. Shuting this down by being emphatic does not foster this intent.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i've never designed formwork but built quite a bit back in my contractor days...and when we built it and were in the middle of the pour, i was always watching (and only ever saw problems) in the immediate vicinity of the vibrator. if you've got a weak spot, the vibrator will find it. that also happens to be why my superintendent would drop whatever he was doing to run the pump hose for wall pours so that he would be next to the guy running the vibrator to make sure he didn't screw it up. so in my opinion, the vibrator is contributing "something"...maybe it's covered in the extra fluff from the rest of the design...i do no know.

i don't know the answer to the question(s) at hand but thought i'd throw in my field experience from a past life to add to the discussion. i'll leave the remainder of this discussion for those more knowledgeable than myself.

 

[muuddfun]

In my experience blowouts of the forwork usually occur at the time the vibrator is near. However I do not ever remember the effects of vibration being included in the design of the formwork. The inputs to design of the formwork were the rate of placement of the concrete to determine the horizontal load. Usually it was limited to 4 feet per hour as I recall. It has been about 10 years since I did a lot of work checking forwork design calculations so I am a bit rusty.

We should remember a little bit about what we are talking about here, for Ka to be equal to or greater than 1, what we mean is that the lateral pressure is equal to or greater than the vertical pressure. i.e. both c, and phi are = to 0, not just phi, in other words the concrete acts as a full fluid which by definition has no shear strength. For concrete to have any slump it must have some internal strength that acts agains shear forces. It would either have to be adheasion, cohesion, or surface tension to prevent it from flowing our completely to fill its container. Since we are all comfortable with the terms of phi and c that is how we are describing it but that may not really be the correct terminology to describe the forces acting within the wet concrete.

So for us to say that Ka was measured to be greater than 1 what we are saying was that the horizontal pressure measured was higher than the static unit weight of the material. i.e. if we have a concrete with a wet density of 150 pcf then the hoizontal pressure was greater than 150 psf. The only way to get such a pressure is if we have placed concrete in the formwork and as we vibrate it we are inducing the pore pressure to exceed the overburden pressure and create a quick condition where the interlocking forces of the aggregate etc. are exceeded and the aggregate are floated like we see in liquefaction. When the pore pressure is raised then we may feel a higher pressure on the load cell placed on the forwork, than we would expect in a fully static fluid condition. But we should remember that this pore pressure will act in all directions equally so the ratio of actual pressure in the region of the vibration for hoizontal to vertical would not exceed 1, so therefore Ka trully does not exceed 1, it just seems like it.

If you are concerned about the additional pressures on your forwork, than in you design just add a few feet to the fluid height you have chosen for design to add some conservatism to it. Usually the reason forwork was blowing out was that the rate of placement greatly exceeded that of the design, and so you would have much more that 4 feet of concrete pressuring the forms.

 

[PEinc]

I am sorry if I was impolite to anyone or insulted anyone, especialy you, fattdad. Your post was one of, if not the most, intertesting posts prior to my comments. The two previous comments by msucog and muuddfun are also very good comments. Some of the other comments, in my opinion, were off track.

Excessive vibrating can cause problems with concrete forms. However, I don't believe that designs should be made overly conservative just to account for any possible sloppy field procedures. In my opinion, contractor supervisors, inspectors, and quality control personnel should control improper field procedures - not designers.