Concrete Weir - Increase abrasion resistance? 3

[macjohnw]

Hi,

We are currently repairing some concrete weirs. The constant problem we are experiencing is the abrasion found on the downstream side of the crown (see picture below). The sediment has erodes the concrete, exposes the aggregate and so decreases the durability.

The flow speeds of the river are high and the weirs designed as such to prevent the settling out of the sediments, thus lower design speeds are not an option.

What options are there to increase the abrasion resistance of the concrete to counter this effect? I do not like the "topping" type products that are plastered on top of concrete as it tends to layer off with time or on impact from objects. Fibers, hardeners?

 

[macjohnw]

I am having trouble uploading the picture - will post it as soon as I can.

 

[dik]

Could the damage be caused by cavitation?

 

[macjohnw]

Dik, just spoke to the designer and he is adamant that it is not cavitation as pressures are to low. Thus sediment is the culprit.

 

[JedClampett]

If you're really concerned, you can armor plate the weir with a stainless steel or carbon steel cap, precisely shaped to the weir shape. But that's a considerable sum of money.
How long did it take for this damage to occur? Maybe you're better off with a maintenance repair every 10 years or so.

 

[Ron]

Agree with JC...stainless steel plate with drive pin anchors.

 

[Hoaokapohaku]

Did the apparent abrasion happen quickly and then slow? Or does it continue at about the same rate? If it is the first, the problem could have been caused by any of the usual list of bad finishing practices that result in high water/cement ratio at the top of the mix. If so, just ensure that best practice finishing methods are consistently employed.

If finishing is the problem, then the problem is aesthetic only and should not affect the long term performance of the concrete.

Any possibility of reactive aggregates? If so, normally you would see a typical cracking/crazing pattern on the surface. But in this application, particles dislodged on the surface could be scrubbed away by the action of the water possibly leaving a surface similar in appearance to what is shown in the picture. If this is the case, the weir will continue to disintegrate until it is gone.

In the photo, it appears that the course aggregate is intact, and the problem is the mortar around the aggregate is being removed. Therefore I'm crossing off my mental list the possibility that the aggregate itself has poor abrasion resistance. hmmm...unless the course and fine aggregate are from different source rock...

If it's not something like the above, the mortar in the mix is just too weak, either because it was not designed correctly, or not produced according to design. Redesign the mix for severe exposure conditions and step up the QA/QC to ensure you are getting what is designed. Use a maximum .45 water/cementatious materials ratio, or less, and include silica fume in the design. Given that you already have a known problem, I would go with a lower w/cm ratio, something like .38 to .42. If the area is subject to freeze/thaw, include 6 - 8% entrained air in the design. In order to ensure the surface has the maximum possible density, specify that the surface be hard trowelled.

Actually, freeze/thaw conditions alone could account for the problem if water stops flowing over concrete which has been constantly submerged during freezing conditions. Designing for severe exposure as suggested above will address this problem.

 

[cvg]

sediment basin
silica fume concrete overlay
use large size, strong aggregate
rubber or other matting that can absorb impact of gravel
increase the tailwater depth

 

[dik]

Low pressures are a source of cavitation...

Dik

 

[cvg]

dik is correct, cavitation is caused by low pressures, not high. But I agree with the original premise, it is likely the gravel is causing the erosion. Without removing the gravel or reducing the velocity, you will have continuing problems with erosion of the concrete surface. This is not an uncommon problem with instream diversion dams and weirs.

another option not previously mentioned is to construct the slab thicker to allow longer time between repairs.

Depending on the hydraulic conditions and type of sediment, steel plate may or may not be successful, but it will be extremely expensive.

http://www.usbr.gov/ssle/damsafety/Risk/BestPractices/20-Cavitation201108.pdf

 

[Hoaokapohaku]

Thinking about it further, a couple things come to mind.

First, I think I crossed an aggregate issue off the list prematurely. What kind of aggregate is it? If it is limestone, the rock is water soluble. In this application where the aggregate is constantly exposed to rushing water, a minute fraction of the rock could be dissolving right at the bond between the mortar and rock at an accelerated rate resulting in mortar and fine aggregate being washed away down stream. If so, eventually the large aggregate will also be loosened and washed away.

Second, I agree with others that cavitation is at least a component of the problem. It's a little hard to tell in the pictures, but it appears that the problem is most severe just past the high point in the weir, just where cavitation would be expected. As water spins around the vortex of the cavitation zone, wear on the concrete is increased for at least two reasons. One, the angle of attack of the water (possibly carrying abrasive sediment) is turned increasingly toward the concrete until it is at right angles to the concrete at tangent of the cavitation, then reduced again until directed away from the concrete toward the upstream end. This causes the water and sediments to be directed toward the concrete with impact, thereby increasing the damage to the concrete when compared to areas without cavitation.

Also, the cavitation accelerates the water and any sediments it contains as it circles around the center of rotation at the rate of the diameter of the rotation times pi, less reduction due to friction at the interface with the concrete. So, the water is moving at an increased rate, probably two to three times faster, as it moves around the vortex than the water does when it is freely flowing parallel with the concrete. Accelerated water + increased angle of attack = much greater rate of wear (or limestone dissolution) in the cavitation zone.

 

[Hoaokapohaku]

From

http://theconstructor.org/concrete/concrete-durability-problems/852/.

Bold for emphasis by me.

Under many circumstances, concrete surfaces are subjected to wear. Concrete wear may be caused by the sliding, scraping or impact of objects that fall onto the concrete. In hydraulic structures, the action of the abrasive materials carried by flowing water generally leads to erosion of the concrete. Another cause of damage to concrete in flowing water is cavitation .

Abrasion damage to concrete may be caused by the sliding or scraping of equipment across the concrete. Abrasion damage to concrete may also be caused by subjecting the concrete to abrasive materials (such as sand) that are carried by wind or water. Test results indicate the following facts;

- That abrasion resistance is clearly related to the compressive strength of the concrete.
- Strong concrete has more resistance than weak concrete.
- Since compressive strength depends on the water-cement ratio and adequate curing, a low water-cement ratio and proper curing of the concrete are necessary for abrasion resistance.
- Hard aggregates are more abrasion resistant than soft aggregates.
- Steel-trowelled surfaces resist abrasion more than a surface that is not trowelled.

Cavitation occurs when a high-velocity, flow of water (or any other fluid) suffers an abrupt change in direction or velocity. This change in direction or velocity in flowing water causes a zone of low pressure to occur at the surface of the concrete that is immediately downstream from the direction or velocity change. This low pressure zone may allow pockets (or cavities) of vapor to form. When leaving the low pressure zone, these pockets of vapor collapse. The collapse of these vapors causes a localized high-energy impact on the concrete surface. This localized high-energy impact leads to erosion of both the cement paste and aggregate.

Concrete that is affected by cavitation has an irregular, jagged, and pitted surface. In contrast, concrete that is eroded by water-borne solids has a smooth surface. Damage to the concrete surface does not progress steadily. After an initial period of small damage, rapid deterioration will occur. This rapid deterioration is followed by damage to the concrete at a slower rate.

 

[dik]

Can you provide a close up of the damage?

The surface damage appears to be caused by cavitation and not abrasion, although, I would have expected the damage to be more localised near the top.

Dik

 

[Ron]

Agree with dik...it looks like cavitation....and there is preferential damage near the top. Looking at the plate strip, there is preferential attack just to the right of the plate. This is likely a result of turbulent flow and the cavitation that accompanies such, yet the plate shows no particular abrasive attack.

Cavitation usually creates a negative pressure at the surface, causing localized tensile failure (popout) of the concrete surface. That creates a rough surface texture, whereas abrasion generally creates a smooth surface texture. Abrasion can create preferential paste attack, if the coarse aggregate has high abrasion resistance and the paste does not. Water alone can do either....it doesn't take sediment to cause the problem, it does; however, exacerbate the issue.

Looking at the surface, there is roughness that is not consistent with abrasion. Not sure where this is located, but the exposed coarse aggregate appears to be calcareous. Calcareous aggregates typically have lower abrasion resistance than reasonable quality cement paste.

In short, this does not appear to be an abrasion issue. More photos with closer surface texture would be helpful.

 

[dik]

You can plot an 80% or 90% gravity curve. This is a trajectory for the fluid velocity and compare it to the geometry of the weir. Areas outside of the curve may be subject to negative pressures. As Hoaok... noted, cavitation is caused from small air bubbles dissolved in water in contact with a surface, and under low pressure these expand and 'burst'... the fluid on the far side 'blasts' through the void created and impinges on the material in contact with the bubble.

If cavitation is the issue and the weir is not repaired properly, even steel plate can be subject to major pitting caused by cavitation... it can be far more agressive than abrasion... Many stainless steels are 'soft' and may be damaged.

Dik

 

[macjohnw]

Gents,

I have been away for a long weekend and could not answer sooner - thank you for all the feedback so far. Unfortunately I don't currently have other/more closeups photos, but will take some more during our next site visit and get some more info if possible. I will also have another chat to our water/hydraulics engineer and share your views with him.

John

 

[dik]

Try plotting a 0.9g trajectory against the shape of the weir using the current velocity of the water in a free fall and compare damage to any area of low pressure (where there is a flow separation)... may be a bit of a start. It's difficult to construct a weir with straight line segments and not have an area of cavitation.

Have your hydraulics engineer log on...

Dik

 

[macjohnw]

I have little hydraulic experience, but requested that the design engineer have a look at this thread and provide accurate info and answers. It is possible that I misunderstood his explanation.

 

[dik]

Did you get this resolved?

Dik

 

[macjohnw]

Dik,

The engineer is a senior with a Ph.D in water engineering - he is sure that cavitation is not the problem, maybe I am just not understanding his explanation well enough or am failing to convey it properly. I have not been able to go out to the site again as it is some distance away, but will have to as we need to conduct some other repairs on it. I will post better quality pics as soon as I can.

In the meantime I am trying to do some research on hydro-abrasion causes and remedies to educate myself more.

Thank you for the interest - I will keep you up to date as I also want to learn and try to prevent/minimize the effects.