Concrete mooring anchor lifetime?

[smokey73]

Does anyone out there have a feel as to how long a servicable lifetime one can expect for a commercially manufactured concrete mooring anchor if kept in salt water for the full year (not removed in the winter and reset in spring). The style of concrete anchor does not have a metal bar embedded that can corrode but instead has a PVC tube that goes through the body of the concrete that a chain (that can be replaced) goes through the PVC to which the heavy chain rode is attached. Does concrete itself degrade or weaken when constantly submerged in salt water?

Maybe Mainsail can comment on this?

 

[Rich Stidger]

I don'tnow the nswer to your question, but I do know that the holding power of a concrete block is much less than the same weight in an iron mushroom due to the weight of the volume of water that is displaced by the concrete. So if you need an 800# mushroom you would need a much heavier concrete block to have the same holding power.

 

[smokey73]

I don'tnow the nswer to your question, but I do know that the holding power of a concrete block is much less than the same weight in an iron mushroom due to the weight of the volume of water that is displaced by the concrete. So if you need an 800# mushroom you would need a much heavier concrete block to have the same holding power.

Understand, but we have a relatively hard bottom so a mushroom will not work. We are looking at roughtly 4000 lb concrete anchors or heavier. The mushroom also counts on some "settling" into the mud or bottom which we cannot count on here where our mooring field is located.

 

[dLj]

I also don't know the answer to your question, but I can tell you that's how I anchored a boat for 10 years with no known degradation.

I would say that if you are using a concrete formation that is designed for use in salt water, I would think the lifetime would be measured in many decades if not centuries.

dj

 

[smokey73]

I also don't know the answer to your question, but I can tell you that's how I anchored a boat for 10 years with no known degradation.

I would say that if you are using a concrete formation that is designed for use in salt water, I would think the lifetime would be measured in many decades if not centuries.

dj

For concrete anchors where there is a steel rod embedded in the concrete with a ring that the mooring tackle attaches the lifetime is probably limited to corrosion of the steel rod. Are club will be replacing several that are that design after 13 years. The remaining anchors are the type without the steel rod/ring. Just helping put together a long range maintenance plan and deciding if we need to budget for replacement of the ones without the steel rod/ring.

 

[Project_Mayhem]

Perhaps a company that builds/maintains docks could point you in the right direction (assuming they also service moorings)

 

[smokey73]

Perhaps a company that builds/maintains docks could point you in the right direction (assuming they also service moorings)

Planning on making some of those inquiries today. I imagine all will say inspect (which we plan to do) and replace as necessary since there is probably a big liability issue if they say 20 or 30 years or indefinite. I can't imagine them saying anything else, I know I would hedge my bets. I know the forum members we have here have a wealth of information on a lot of construction, materials science, electrical, chemistry and a host of other highly technical subjects plus years and years of experience in sailing. I was hoping someone had some perspective on this. We are going to budget for replacement, but the question to be answered is to use a 10, 15, 20 or 30 year replacement schedule.

 

[Scott T-Bird]

Concrete that is submerged is least affected by salt water (opposed to concrete at water line or even above water line). Without the steel to corrode, the concrete will not degenerate due to the weak bonds between steel and concrete as the steel corrodes. I suspect that you can budget for replacement about twice the period that you experience now with the steel rebar embedded. 30 years is probably not unreasonable. You can "improve" the concrete if you take measures to make it for the best durability. Low water to cement ratio is just one objective. Concrete Exposed to Seawater – Effects and Preventions

 

[Ron20324]

, I would think the lifetime would be measured in many decades if not centuries.

I think "concrete boots" come with a lifetime guarantee?

 

[PaulK]

Concrete structures designed as harbor breakwaters (Mullberries) for the Normany invasion still litter the beaches there. At 150 pounds per cubic foot, an 8000 pound block would be about 50 cubic feet: 5’x5’x2’; pretty unwieldy. And if the bottom is hard, even they might slide around. How is chafe of the chain against the concrete handled? Have helical anchors been considered?

 

[smokey73]

Concrete structures designed as harbor breakwaters (Mullberries) for the Normany invasion still litter the beaches there. At 150 pounds per cubic foot, an 8000 pound block would be about 50 cubic feet: 5’x5’x2’; pretty unwieldy. And if the bottom is hard, even they might slide around. How is chafe of the chain against the concrete handled? Have helical anchors been considered?

All understood - bottom is hard/rocky and will not accept a helical screw anchor (city marina nearby has tried and the screws don't work). There is nothing for a mushroom or other style to sinke into, which is a major part of their holding power so we are left with deadweight anchors and concrete is the general solution for 8000 lb anchors and besides that, they are already in place except for 6 that are the embedded bar type and have to be replaced. The question is just, how long can we expect a concrete anchor (without the embedded bar) to last. We are going on 14 years and we have experience that for our area that is about the lifetime of a concrete anchor with the embedded rod/eye. We want to determine if we need to budget to replace the all concrete anchors. The debate on the kind of anchor is dictated by the bottom surface.

At that size we know that they are unwieldy and will have to be placed professionally hence the need to budget for it if necessary. Thanks for the response anyway. It seems the consensus and literature seems to tend towards multiple decades if not shodily made. Thanks for the response anyway.

 

[smokey73]

Concrete that is submerged is least affected by salt water (opposed to concrete at water line or even above water line). Without the steel to corrode, the concrete will not degenerate due to the weak bonds between steel and concrete as the steel corrodes. I suspect that you can budget for replacement about twice the period that you experience now with the steel rebar embedded. 30 years is probably not unreasonable. You can "improve" the concrete if you take measures to make it for the best durability. Low water to cement ratio is just one objective. Concrete Exposed to Seawater – Effects and Preventions

Thanks for the response. This seems to tend towards many decades (if properly made). Chain chafe is the largest concern and we will be performing inspection 2 times per year on each mooring, once in the spring and once in the fall (before and after hurricane season).

 

[dLj]

Planning on making some of those inquiries today. I imagine all will say inspect (which we plan to do) and replace as necessary since there is probably a big liability issue if they say 20 or 30 years or indefinite. I can't imagine them saying anything else, I know I would hedge my bets. I know the forum members we have here have a wealth of information on a lot of construction, materials science, electrical, chemistry and a host of other highly technical subjects plus years and years of experience in sailing. I was hoping someone had some perspective on this. We are going to budget for replacement, but the question to be answered is to use a 10, 15, 20 or 30 year replacement schedule.

You won't get anyone to give you a date in all likelihood. It is too dependant upon deployment location.

In general, inspection time frames are established based upon a best guess/liability balance. Talk to your insurance company that is providing you liability insurance and see what they tell you. You will have to comply with their requirements in the end anyway.

A difficulty with concrete, there is no way to inspect it for any kind of internal defect that may be manifesting. So all you can do is inspect for external damage, and that is being done underwater. At best a dubious inspection. You should look carefully at you block design and determine your failure modes and then how you are going to inspect for them. Bring that into your insurance company. Research concrete structures in use and how those are inspected. Use that info to define a plan and see if your insurance company buys into it.

That's about all I got...

dj

 

[smokey73]

You won't get anyone to give you a date in all likelihood. It is too dependant upon deployment location.

In general, inspection time frames are established based upon a best guess/liability balance. Talk to your insurance company that is providing you liability insurance and see what they tell you. You will have to comply with their requirements in the end anyway.

A difficulty with concrete, there is no way to inspect it for any kind of internal defect that may be manifesting. So all you can do is inspect for external damage, and that is being done underwater. At best a dubious inspection. You should look carefully at you block design and determine your failure modes and then how you are going to inspect for them. Bring that into your insurance company. Research concrete structures in use and how those are inspected. Use that info to define a plan and see if your insurance company buys into it.

That's about all I got...

dj

Thanks, that is a great suggestion. See previous response on inspections and chafe. Thanks

 

[smokey73]

Concrete structures designed as harbor breakwaters (Mullberries) for the Normany invasion still litter the beaches there. At 150 pounds per cubic foot, an 8000 pound block would be about 50 cubic feet: 5’x5’x2’; pretty unwieldy. And if the bottom is hard, even they might slide around. How is chafe of the chain against the concrete handled? Have helical anchors been considered?

Good data point. The reference Scott posted indicates that the wear in a marine environment is most for concrete at and just above the waterline. Fully submerged concrete, less so. Sounds encouraging. It all depends on design, quality of materials and workmanshop I would assume.

 

[markwbird]

Rule of thumb is 2 years. That is really for the wire or chain or rope or whatever and its connection to the mooring. The concrete mooring itself could last much longer if constructed correctly.

 

[ifmdalvey]

In Maine the preferred material for a block is granite, much denser than concrete.
That said, i have a 1500 pound concrete block for my 20000 pound boat but i am in what is considered a hurricane hole, if i was out in open water i would probably want at least double that size.
I have all the hardware checked every 3 years and have only ever had to replace the top chain in the last 15 years.
I guess it depends on many local factors to come up with the right answer for you.

 

[Ron20324]

I think it would be great if they could combine a mooring device with those "reef balls" that provide habitat for fish and a place for shellfish to attach to as they grow.

 

[smokey73]

I think it would be great if they could combine a mooring device with those "reef balls" that provide habitat for fish and a place for shellfish to attach to as they grow.

They do. I can't find the actual link but here are some examples

Found the link: See the section on Conservation Moorings

https://www.maine.gov/dmr/mcp/downloads/access/moorings.pdf

 

[smokey73]

In Maine the preferred material for a block is granite, much denser than concrete.
That said, i have a 1500 pound concrete block for my 20000 pound boat but i am in what is considered a hurricane hole, if i was out in open water i would probably want at least double that size.
I have all the hardware checked every 3 years and have only ever had to replace the top chain in the last 15 years.
I guess it depends on many local factors to come up with the right answer for you.

1500 pounds seems a little light for a 37 foot 20,000 pound boat? The subermged weight of a 1500 pound concrete block is only 750 pounds. I think the "recommended" concrete anchor weight for a 37 foot boat is more like 4000 (giving a submerged equivalent weight of 2000 pounds). From your avatar, as a fellow submariner I am sure you are familiar with the concept