Wet Concrete Pressure in Sloped Slab Overlay Application

[EastEng1012]

Project Background: The project is rehabilitation of a concrete spillway at a dam. The spillway apron is being partially removed(removal of 2-3" of shotcrete surface) and removal of a minimum of 1/4" to sound concrete into the existing 18" concrete slab. This is being done via hydro demolition. A 6" concrete overlay will then be placed over the apron with a grid of reinforcement and drilled anchors into existing slab.

Question: As a precaution, I want to verify of the strength of the existing slab and calculate the maximum concrete removal before the existing slab will not be able to support the loading from wet concrete during placement of the overlay. The overlay will be placed in 12'x18' segment. The spillway apron is sloped at maximum 34deg from vertical. My question is for calculating the pressure of the wet concrete on the existing slab - should I use the ACI equations for calculating the lateral pressure based on the head and rate of placement? The loads seem very high and wanted to make sure I was calculating it correctly.

The designer's calculations show they only assumed the load to be concrete density*6" depth even though it is sloped. Is this correct?

 

[dik]

If I understand this... you have an 18" conc slab with 2" of shotcrete. The slab is sloped. You want to remove 1/4" by hydrodemolition and place a new 6", attached, slab over it.

Is there an additional loading placed on the slab because the 6" is actually 6-1/2" or of that ilk due to the slope? the increased depth of the slab should offer a slight increase in moment resistance. Is this understanding correct?

Dik

 

[EastEng1012]

Correct, 18" concrete slab with 2" of shotcrete.... The shotcrete will be completely removed and a minimum of 1/4" into sound concrete. The issue will be if more concrete is removed than anticipated due to deterioration and the means and methods being used. The existing slab will then be loaded with wet concrete (prior to curing)and every inch removed beyond the 1/4" minimum will be replaced with new concrete (in addition to the 6" overlay). Therefore, in reality the new slab thickness will vary.

The critical load condition will be when placing the new concrete while it is still uncured. Therefore, I am determining the maximum amount of existing concrete that can be removed without overloading the slab during concrete placement.

I am unsure of what the wet concrete pressures would be on the existing sloped slab. Is it calculated similar to lateral concrete pressures on vertical formwork or is it the weight of the concrete projecting on to the face (thickness*150pcf)?

 

[cvg]

so, is this a spillway chute slab on grade? or is this some sort of cantilevered flip bucket?
not sure why a slab on grade would be limited by an additional bit of concrete (either wet or dry). Hopefully the foundation is on rock or competent foundation. the load from the flowing water is the critical load. since I assume this is a slab with a steep slope, the slump will be very low so the "lateral / head pressure" loading is low and the wet concrete will be essentially a vertical dead load.

 

[EastEng1012]

The slab is not a slab on grade. It is an overflow section of the concrete dam which is essentially hollow on the inside. The slab is simply supported and spans between buttresses.

 

[SlideRuleEra]

I am unsure of what the wet concrete pressures would be on the existing sloped slab.

Unless you will have real-time, total control of the Contractor's means and methods during the placement... keep it simple. Assume concrete is a 150 PCF fluid and calculate the pressure accordingly. There are too many variables during a concrete pour to "fine-tune" the loading calcs. For example, it is unlikely the concrete will be placed in a precise, uniform layer. There can be local "spots" that are temporarily overloaded. Also, concrete vibration during placement will cause increased short-term pressure.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

I think the loads applied by the plastic concrete are dwarfed by the loading provided by the water flow... and, if I understand this, would have little or no effect.

Dik

 

[hokie66]

At a "maximum of 34 degrees from the vertical", this is more a wall than a slab. How will you place the concrete? Formed or shotcreted?

 

[EastEng1012]

Hokie66 - that is correct that it is more of a wall then slab. The 34 degrees is at the steepest part of the spillway. The contractor is planning on using forms to place the new concrete.

 

[dhengr]

Rwalker8202:
After the initial demolition, I wonder that you mightn’t slip form this inclined wall. On an upper level final demolition and clean up and reinforcing application; and finally on a lower level concrete placement.