Small Centrifugal Pump Pads Doweled onto Concrete Paving

[structengr23]

2 Jul 20 00:17
For smaller centrifugal pumps, say a 50 hp 1200 rpm machine weighing 2400 lbs on a base approximately 5.7'x2.5', has anyone built a mounting pad/block doweled onto pavement approximately 6" thick using hilti HIT 200R anchor system. This is assuming you can get the 3 times mass with the combination of existing paving, new block and grouted pump base. I've done several larger pumps where we would never consider this and they were on independent blocks. This is the first small pump I've done and it seems like a composite structure with paving, block and grouted base may be an option and save on demolition. This is in an industrial plant, pulp and paper mill.

 

[HTURKAK]

We built common base for 3 pumps but the base and pavement casted together. In your case, the vibration of ( new base+ pump block+ pavement ) together should be warranted .

If the pavement is RC slab, my opinion is, the anchor rods shall extent to the existing pavement and a preloading should be applied after erection of the pump.
The preloading should provide a compression to joints , at a level equivalent to the tension capacity of concrete.

 

[SlideRuleEra]

This is in an industrial plant, pulp and paper mill.

Don't "cut corners", saw a hole in the existing slab and construct a proper independent foundation isolated from the surrounding slab.

I assume you have no details for the existing slab, why gamble that it is "good enough"?

http://www.SlideRuleEra.net

 

[structengr23]

Thanks HTURKAK! I was thinking along those lines. I do need to verify the pavement makeup regarding thickness and reinforcing. I was trying to find out if there is a precedent for this type of brownfield addition. In effect the pavement acts as a 6" seal slab/spread footing and helps spread the load on the soils below. This plant is old, so the soils have been loaded for a while. If the pavement is in good condition, it makes sense. If I were to breakout the pavement for the block, I may need to use helical piles if the soils are expansive. Then I would need to do a probing investigation to make sure there are no subsurface obstructions. Doing this composite makeup saves a lot of extra work, if it's acceptable. Attached is the detail.[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1593691888/tips/Centrifugal_Pmp_Fdn_c35kss.pdf[/url]

 

[structengr23]

That's also a good point, SlideRuleEra. That's my dilemma. I prefer the complete block, but would need to either treat the soils to reduce the Plasticity Index or use helical piles to limit potential settlement. I seemed to recall seeing this composite design, but if it's not commonplace for brownfield work, that would be my concern.

 

[structengr23]

I do have good info on the paving. It's 6" thick with double layer of steel (#4 at 12" each way top and bottom). But the surface has gotten beaten up over time (old 1950 era plant).

 

[SlideRuleEra]

...would need to either treat the soils to reduce the Plasticity Index or use helical piles to limit potential settlement.

It's 6" thick with double layer of steel (#4 at 12" each way top and bottom).

That's not very encouraging, top and bottom in a 6" slab does not have any real meaning...

Supposedly 3" concrete cover on the bottom.
Each #4 mat takes up 1" of vertical thickness.
If the two mats touch each other, that leaves 1" of concrete cover on the surface.

Something was almost certainly compromised, I'd wager the 3" bottom cover was reduced... on a very used 70 year old slab, supported by questionable soil.

For an industrial application, long-term reliability is typically more important than lowest first cost. The pump is a component in some industrial process. If that process has go off-line because a "cheap" foundation failed... you can fill in the rest of the story.

I view these issues from an Owner's (electric utility, generation) perspective, IMHO, design a proper foundation.

http://www.SlideRuleEra.net

 

[structengr23]

Good points! Thanks!

 

[HTURKAK]

I do have good info on the paving. It's 6" thick with double layer of steel (#4 at 12" each way top and bottom). But the surface has gotten beaten up over time (old 1950 era plant).

The age of concrete 70 years old slab, discouraging point. In past, i observed buried old concretes which can be hand excavated. Of course, that depends on environmental cond, soil type, bottom protection ..etc.. If you decide to use the existing slab, excavate a small pit and make sure that the concrete is in good position. Even observation of the hand excavation of the pit you will see the situation.

I agree with SlideRuleEra (Structural) ..

 

[structengr23]

That's true. The age of the concrete is a concern. A core sample that could be used for a compression test is an option. Thanks, HTURKAK. Good points.

 

[r13]

Will raised house keeping pad work?

 

[structengr23]

retired13....That’s basically what I was considering initially. I had an 8” thick pad anchored/doweled into a 6” thick pavement. For a centrifugal pump, I need concrete total mass that is two to three times mass of pump. I have two times mass if I consider composite/combined pad and pavement.

 

[r13]

2 times might just work, if taking into account of soil damping effect, or you might add a layer of vibration absorbing material in between the pad and the pavement.

 

[structengr23]

Thanks, retired13. Only real concern is the condition of the 70 year old pavement, which has got some wear and tear (cracks; rebar condition?) and soil condition below slab. They’ve had some fluid collection (spillage and rain) and not sure about potential washout of soils (clayey sand; SC and SM Classification; granular) And since there’s drain trenches also in the area, I’ve got my concerns. If it was new paving, I would have less concern. But, I agree that the composite section concept for small pumps seems reasonable if paving and soil is in good condition