Concrete Forming in the intertidal zone

[sly_karma]

One note - if it was me, I'd pour without reinforcing and use a fiberglass mesh admix to the concrete (basically a bag of fibreglass fibres that added directly to the mix).

Concrete fails for lots of reasons, but the most relevant in your case would be water penetrating into the concrete and rusting the reinforcing. As steel turns to rust, it expands and creates internal stress in the concrete, causing it to crack - letting in more water, causing more rust. This is why parking garage slabs fail.

Make sure your footing has good bearing, eliminate the rebar, you eliminate the rust. If you need dowels (either to attach the footing to rock, or to connect something to the footing) - go with stainless threaded rod. If you absolutely need rebar, use fibreglass bar - they sell it at home depot now. Regular bar won't work in the long run - and epoxy coated bar doesn't work much better either.

Agree with this approach now that we know the application. Simple vertical loading, light load. Rebar not required.

Don't fuss over form material, it's temporary. Use 2x4 backers top and bottom of the form ply to reduce bulging. For simple square or rectangular shapes up to 4 ft height, corners can be simply screwed together with fairly tight spacing. 3" centres down low, easing to 6" by the top.

Discuss mix design with concrete supplier. 25 MPa is sufficient for the loading, but for the few extra bucks I'd step up to 32 MPa. More cream means a nicer finish against the form, with fewer bug holes. Appearance isn't a big deal for this job, but those bug holes are a more inviting entry point for salt damage.

To the OP, please outline your rationale for adding plasticizer. This isn't a thin section form with a lot of reinforcement. External or internal vibration will be sufficient to get a smooth cream layer up against the form, and there'll be less pressure on the form without plasticizer, and less cost. Higher slump concrete more effectively displaces any water in the form - more risk of runny concrete absorbing some of the water and weakening the mix.

 

[eriks]

Agree with this approach now that we know the application. Simple vertical loading, light load. Rebar not required.

Don't fuss over form material, it's temporary. Use 2x4 backers top and bottom of the form ply to reduce bulging. For simple square or rectangular shapes up to 4 ft height, corners can be simply screwed together with fairly tight spacing. 3" centres down low, easing to 6" by the top.

Discuss mix design with concrete supplier. 25 MPa is sufficient for the loading, but for the few extra bucks I'd step up to 32 MPa. More cream means a nicer finish against the form, with fewer bug holes. Appearance isn't a big deal for this job, but those bug holes are a more inviting entry point for salt damage.

To the OP, please outline your rationale for adding plasticizer. This isn't a thin section form with a lot of reinforcement. External or internal vibration will be sufficient to get a smooth cream layer up against the form, and there'll be less pressure on the form without plasticizer, and less cost. Higher slump concrete more effectively displaces any water in the form - more risk of runny concrete absorbing some of the water and weakening the mix.

Thanks to both of you for your super informative response. I don’t know anything about concrete and was just researching it and saw some videos of plasticizers vs concrete without platicizers when pouring into water. We have a fairly low tide Monday Feb 3rd, so I’ll be able to see what I’m up against. I have some big rocks that I can dowel stainless redo rod into, but my neighbor’s footing is 6’x6’x3’ and it seems to be stable.
I did do a pour at our cabin last summer and spoke to the line pump guy about this pour. Another issue I have is a 60’ or so elevation drop from the line pump down to the foreshore. He said he pumps plastic bags down the line to prevent aggregate separation.

 

[Max123]

No plasticizer needed (makes mix really runny - only needed if you have a ton of rebar). Ask for 32-35mpa - higher MPA means more cement in the mix (cement is the active ingredient that turns sand/gravel into concrete). Cement increases workability when you are pouring, and also sets up faster. 25mpa sets up really slowly when temperatures are cold. At this time of year, all the local plants in Vancouver should be using hot water in their mix due to cold weather - and this is typically added to your bill as a small seasonal upcharge - but worth confirming as it will help setup faster. Do not pour on top of snow/ice or frozen ground. I wouldn't bother dowelling into any rocks, unless the footing is going to be totally on top of one big rock - If you are in a boulder field, avoid the rocks - you don't want differential settlement between your footing and some adjacent rock. Excavate down to clean gravel if you can. Getting your footing embedded 18" into the ground will do more for the strength/longevity of your footing than anything else. Assuming you don't have a concrete vibrators, so when you are nearly finished pouring, give the formwork a few whacks around the perimeter to help settle and get rid of bug holes. Also, get an old broom handle and stab it into the wet concrete a few times - this will help release the interior air bubbles (sounds weird, but works).

Not sure if you have considered this - I don't know how far the footing location is from road access (obviously close enough to get a line pump) - but an alternative is to just buy two lock blocks (30"x60"x30"), bolt them together and use that as a footing. This will be bigger/heavier than your proposed footing - and avoids a lot of hassle. If you are in the lower mainland, call someone like K2 Cartage - there are always lock blocks moving around - delivery trucks have surprisingly long cranes.

I've seen lock blocks used for dock footings before - pretty common on remote cabin sites where its easier to get them dropped by a barge service than get concrete delivered (or mix on site).

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If you go for lock blocks, ask for flat top blocks (standard blocks have an X cast into the top, a flat top will make it easier to attach your dock.

 

[eriks]

No plasticizer needed (makes mix really runny - only needed if you have a ton of rebar). Ask for 32-35mpa - higher MPA means more cement in the mix (cement is the active ingredient that turns sand/gravel into concrete). Cement increases workability when you are pouring, and also sets up faster. 25mpa sets up really slowly when temperatures are cold. At this time of year, all the local plants in Vancouver should be using hot water in their mix due to cold weather - and this is typically added to your bill as a small seasonal upcharge - but worth confirming as it will help setup faster. Do not pour on top of snow/ice or frozen ground. I wouldn't bother dowelling into any rocks, unless the footing is going to be totally on top of one big rock - If you are in a boulder field, avoid the rocks - you don't want differential settlement between your footing and some adjacent rock. Excavate down to clean gravel if you can. Getting your footing embedded 18" into the ground will do more for the strength/longevity of your footing than anything else. Assuming you don't have a concrete vibrators, so when you are nearly finished pouring, give the formwork a few whacks around the perimeter to help settle and get rid of bug holes. Also, get an old broom handle and stab it into the wet concrete a few times - this will help release the interior air bubbles (sounds weird, but works).

Not sure if you have considered this - I don't know how far the footing location is from road access (obviously close enough to get a line pump) - but an alternative is to just buy two lock blocks (30"x60"x30"), bolt them together and use that as a footing. This will be bigger/heavier than your proposed footing - and avoids a lot of hassle. If you are in the lower mainland, call someone like K2 Cartage - there are always lock blocks moving around - delivery trucks have surprisingly long cranes.

I've seen lock blocks used for dock footings before - pretty common on remote cabin sites where its easier to get them dropped by a barge service than get concrete delivered (or mix on site).

For Sale: Concrete Blocks and Shipping Containers

Let us connect you with a deal! Pre-cast concrete lock blocks, shipping containers, and more. Call for a FREE QUOTE.

k2crane.com

If you go for lock blocks, ask for flat top blocks (standard blocks have an X cast into the top, a flat top will make it easier to attach your dock.

Thanks so much for your response. Bolting two lock blocks together actually may be the ticket for me.

 

[Max123]

If you are going this route, for any mounting hardware I recommend just using stainless steel rod and then drill/epoxy. As always - Hilti has a product listed for this application. The basic epoxy sold at Home Depot likely isn't going to cut it.

HY-200 in Salt Water/Tidal Area - epoxy adhesive, under water ...

I am designing an installation that can be completed in dry conditions, but will ultimately be submerged in salt water and exposed to varying water elevations due to tides. Can I use HY-200 for this application.

www.hilti.com

Buy 4ft lengths of stainless threaded rod, and then just cut yourself a bunch of 4-6" dowels for all your mounting hardware. In Vancouver, Pacific Fasteners is the best source for all stainless hardware (that's all they sell) - its across the street from Summit Tools. Use lock washers or even better is to double nut to prevent anything from loosening over time.

If you don't own or have access to one, rent a nice beefy hammer drill and get a fresh bit - it will go in like butter. It sucks to be drilling large diameter hardware with some ancient homeowner black & decker.

 

[eriks]

If you are going this route, for any mounting hardware I recommend just using stainless steel rod and then drill/epoxy. As always - Hilti has a product listed for this application. The basic epoxy sold at Home Depot likely isn't going to cut it.

HY-200 in Salt Water/Tidal Area - epoxy adhesive, under water ...

I am designing an installation that can be completed in dry conditions, but will ultimately be submerged in salt water and exposed to varying water elevations due to tides. Can I use HY-200 for this application.

www.hilti.com

Buy 4ft lengths of stainless threaded rod, and then just cut yourself a bunch of 4-6" dowels for all your mounting hardware. In Vancouver, Pacific Fasteners is the best source for all stainless hardware (that's all they sell) - its across the street from Summit Tools. Use lock washers or even better is to double nut to prevent anything from loosening over time.

If you don't own or have access to one, rent a nice beefy hammer drill and get a fresh bit - it will go in like butter. It sucks to be drilling large diameter hardware with some ancient homeowner black & decker.

Roger all that. I was in the steel biz so am very familiar with most Hilti products and have a nice big hammer drill. I’ll bolt two blocks together with galvanized 3/4” plate (I’m actually building galvanized steel stairs down to the water so am sending items to Silver City fairly regularly).
I’m very glad that I posted this up here. You all have been very informative and have helped me change the plan for the better. I’m very grateful.