High water velocity over concrete 4

[flamby]

I am doing a preliminary dam design study, where water is jetting out off undersluices at a velocity of 25-30 m/sec. Too high, and I provided heavy steel lining over the concrete surface, properly anchored.

Now I am under heavy criticism and peers say, I should go for high performance/ high strength concrete. They do not seem to have any eaxmples, but feel, it is much easier done and if required, repaired. I do not contest that steel lining is difficult to erect, but I am a bit nervous with any concrete becuase of no experience with such high water velocities.

Your ideas, on this issue are most welcome.

Ciao.

 

[JedClampett]

I've got an old reference called "Durability of Concrete Structures" by ACI. It discusses abrasion, erosion and cavitation. At those velocities, I'd also be real worried about cavitation.
As far as high strength concrete, silica fume, etc., count me as a skeptic. The ACI document reviews the use of these, but I think it is just a delaying tactic. A better idea is to remove the offending process (cavitation, erosion, etc.) than to spend a lot of money on fancy mixes.

 

[cvg]

You may want to increase the size of your sluice to reduce the velocity.

Cavitation produces pressures of up to 100,000 psi. If you aerate the jet, you may be successful in reducing the amount of cavitation. You should also attempt to trap sediment before it goes through the sluiceway. Either steel or high strength concrete are recommended, however, neither one can provide full protection. I'm not sure one is better than the other. We are contemplating rubber on a dam to reduce erosion caused by sediment flow over a dam. However, I don't think rubber in your sluiceway would be recommended either. Perhaps you can use a layer of high strength steel over a layer of high strength silica fume concrete. Kind of a belt and suspenders approach.

 

[civeng80]

Interesting, so you hydraulic guys think that at this high flow velocity it would be enough to reduce the pressure down to vapour pressure and thus cause cavitation problems?

 

[cvg]

absolutely - cavitation in spillway chutes and outlet structures (especially in emergency spillways) can be severe enough to jeopardize the safety of the dam. Concrete does not stand up well to cavitation. And during an emergency overflow situation at a dam, there is no way to "shut off" the flow so you can repair the spillway. If the cavitation is severe enough it can erode right through a concrete chute in a matter of days. However, in most cases spillway geometry and outlet channel characteristics are designed to try and prevent it. An Ogee spillway may help prevent cavitation. Increasing the tailwater depth may also help. Providing air through the spillway chute or in the outlet tunnel can help to reduce both cavitation as well as increasing the flow capacity of a tunnel.

 

[Dinosaur]

I would counter your critics by asking over what volume they wanted to use this higher strength mix. One of the concerns I have heard concerning dam construction is the ability to address the heat of hydration during concrete placement and curing operations. Generally speaking, a mix with this higher strength will experience more heat of hydration. The consequences of this would be cracking within the volume of this high strength concrete, especially at the surface. At this point, you have defects in the wall of your sluce <sp?> and you can expect rapid scour from then on. I don't think a steel lined channel is over the top.

 

[cvg]

you may find some useful information here regarding controlling shrinkage cracking and other requirements.

http://www.silicafume.org/general-concrete.html

 

[civilperson]

Chromgard is a high strength stainless steel used to line rock chutes and other high abrasion locations. I like the idea of protecting the concrete with a replaceable lining made of steel. The cavitation that is discussed is real and can be lessened by clever geometry.

 

[kslee1000]

With fly ash and other additives, you can obtain high strength concrete with low heat hydration. With proper reinforcing, concrete placing/cooling and curing, the cracks can be minimized or eliminated. I have seen baffle blocks been placed on the spillway to resist the impact from the water jet, as the flow rate is reduced, so does the chance of cavitation. I personally think it is cheaper and easier to use one type of material, rather than mixing the concrete with steel lining, for which erection/fitting could be costly, and it's not easy to hold large steel plate over the concrete for long. The subsequent lining replacement/repair could be a nightmare.

 

[hokie66]

Steel fibres in concrete improves toughness of concrete surfaces markedly. We use it in workshops for tracked vehicles and in areas subject to abrasion from buckets, etc. Maybe this would be applicable in your situation.

 

[kslee1000]

hokie66: Good idea, may other types of fiber (less corrsive material) works in this case?

 

[hokie66]

I don't think plastic fibres would do any good, but I wouldn't worry about surface corrosion of the fibres. It doesn't go in very far or cause spalling. But you might check on this with fibre suppliers. I suppose ideally you might use stainless fibres, but I don't know if such a thing exists.

 

[LittleWheels]

Steel fibres degrade the concrete appearance (rust spots appear) but not the concrete.

 

[civeng80]

Agree with hokie those steel fibres really make for a hard concrete surface in industrial buildings here. I think its applied as a topping to concrete and finished hard, it may be difficult finishing it to a grade (but possible !).