Concrete boom pump - extra pressure on formwork 4

[Marc_Bloke]

Hi there,

Does anyone have any advice or experience calculating the additional pressure on formwork when concrete is being pumped out of a boom pump from a great height?

Thanks

 

[hokie66]

Concrete delivered by pump does not normally result in higher pressure on formwork.

 

[Marc_Bloke]

Thanks for your response hokie66.

If you imagine a scenario where you have a boom pump fully extended in height - for arguments sake lets say the concrete is being pumped/ falling from 10m in height. The concrete is going into a 6" wide formwork for a wall - the impact of the concrete hitting the concrete below it is surely going to momentarily increase the pressure acting on the formwork?

 

[Ron]

If the end of the hose is submerged in the wet concrete it can increase the pressure on narrow forms, but as hokie66 notes, usually not an issue.

 

[Marc_Bloke]

Okay that's interesting.

The reason I ask is because we had a blowout on a new ICF system where the plastic ties failed. Based on pressure calculations done using pgh as the maximum pressure the ties SHOULD have taken the load, however they didn't and I attributed this to an increased pressure caused by the almost ballistic impact of the concrete being pumped from a great height.

Any thoughts?

 

[SlideRuleEra]

How deep was the concrete inside the forms when the blowout happened?

The "Design Guide For Multi-Story Buildings" by Insulating Concrete Forms Manufacturers Association (ICFMA) recommends a lift height of 4 feet on page 42.

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[Marc_Bloke]

The concrete was less than 0.5m in height in the formwork when it blew, so considerably less than the 4'.

I should mention that this was a test environment, not a real construction project.

I'm wondering whether the rapid loading could have played a part? Is that usually considered in formwork calculations? Or is pgh considered to be a worst case scenario to cover this?

 

[Lomarandil]

It's normally not considered to be significant. I think your failure originates elsewhere.

----
The name is a long story -- just call me Lo.

 

[SlideRuleEra]

Agree with Lo, rapid (uniform) loading not important.

Edit: Yes, pgh is worst case... hydrostatic pressure of an (assumed) 150 lb/ft[sup]3[/sup] fluid.

Was the concrete level within the forms brought up more or less uniformly along the wall length? If only one "spot" was loaded to 0.5m, the ties at that spot could have been overloaded.

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[dik]

The increase in slump, for pumping, may have an added load effect. Not likely and issue... just noticed was only .5m high... as noted by others, failure likely from other causes.

Dik

 

[Ron]

If the hose is submerged in the concrete, you get the head pressure from the full height of the hose. This can significantly increase the lateral load on narrow forms.

 

[emmgjld]

Ron 28 Apr 18 01:26
If the hose is submerged in the concrete, you get the head pressure from the full height of the hose. This can significantly increase the lateral load on narrow forms.
BINGO! Just figure the friction of the flow through the hose and whether the pump operator has continued with pressure after the concrete stream has topped the high point of the boom. The problem becomes VERY complicated. Very localized areas of elevated pressure are possible. I usually see and encourage a small amount of freefall when pumping.

Also Remember, a true tremie operation does not occur when freefall happens. Increased pressure in the concrete column is a plus to keep the contaminated/watered down mass on the perimeter & top of the tremie placement.

 

[BridgeSmith]

I agree that with a narrow gap and a large hose there will be a local increase in pressure on the walls (forms), but to me it seems it would be a function primarily of flow rate, viscosity (slump) of the concrete, and depth of concrete above the end of the hose. The pressure would affect the flow rate, but if I understand correctly, with a pumper, that would be a function of the pressure at the pump, the difference in height from the pump to the end of the hose, and the losses due to friction in the hose. For calculation purposes, it should be independent of the height it rises to, since it's a closed system. Granted, if the high point of the system is higher, greater pressure may be required at the pump.

All that to say, in order to limit the pressure on the formwork and prevent blowouts, the flow rate and the depth of the hose into the concrete must be limited.

Presuming that there will not be any water in the forms, there really isn't any reason to have the hose in the concrete. It should not be more than a few feet above the filled depth to prevent segregation of the concrete mix, and this too would increase pressure on the formwork due to impact.

 

[dik]

HotRod... as Ron noted the inclusion of the hose end in the concrete can significantly increase pressures just due to the head of concrete; the pressure due to hydrostatic head is based on the height of the pump boom above the discharge. If you remove the hose end from the concrete mass, the head is diminished to the depth of concrete in the form.

Dik

 

[SlideRuleEra]

Agree, Ron has an answer. Slump per the "Design Guide" is high, 5" to 6", or more. Concrete that "soupy" will be a close approximation of fluid and will act like one.

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[oldestguy]

Hot rod is hitting on an important factor which is the head loss in the hose doe to a variety of things. Without pressure gauges in various places in the hose you won't know what sort of impact occurred at the discharge. It might not be anywhere near the head differential from its upper hose position above the discharge. Hell, you might even find that it takes added pressure to get it to go down the hose to the outlet in some cases.

 

[dik]

OG... but, it can be quite a bit greater than if there is an air gap.

Dik

 

[BridgeSmith]

"...the pressure due to hydrostatic head is based on the height of the pump boom above the discharge."

In an open system, that is true, but I was under the impression that a concrete pumper was a closed system. In a closed fluid system, only the pressure at the beginning, the change in elevation from the beginning to the end, and the friction losses in between, matter to the pressure at the end. Various points along the way will have different pressures depending on their elevation.

"If you remove the hose end from the concrete mass, the head is diminished to the depth of concrete in the form."

Agreed.

"...it can be quite a bit greater than if there is an air gap."

I believe the pump operators take steps to prevent this, as it could cause segregation problems. Also, that's one of the reasons the last length of the pumping line is a flexible line. If the flow rate slows, it naturally collapses due to the vacuum that is created, so that no air is introduced.

 

[dik]

maybe an oversight on the part of the nozzle handler...

Dik

 

[Marc_Bloke]

Thank you to all contributions to this post.

As suggested early on in the thread I agree the failure was likely caused by a factor other than the concrete exerting extra lateral force on the walls, so as such will get back to redesigning

thanks again