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.

 

[eriks]

Another question…. So I have cleared out the location, leveled (kinda - not absolutely perfectly) it off and put down an inch or so of a product called Mud Bed Mix by Mapei. I mixed it really dry as per the recommendations and it has worked out well curing under water. I’ve inspected it at LT and am happy with how it will perform as a base for the concrete blocks.
It’s not perfectly flat, but close. I’m not worried about it not being perfectly level because I’ll use leveling nuts to plumb the column up perfectly.
The question I have relates to placing the blocks onto this pad. Should I use some product for grout, to fill any gaps and provide better bearing for the blocks? PL or Sitka non-shrink grout? I’m not sure how much time I will have before the tide comes in again.
I’m doing this on Thursday 12th June.
Thanks in advance for any input.

 

[Max123]

Another question…. So I have cleared out the location, leveled (kinda - not absolutely perfectly) it off and put down an inch or so of a product called Mud Bed Mix by Mapei. I mixed it really dry as per the recommendations and it has worked out well curing under water. I’ve inspected it at LT and am happy with how it will perform as a base for the concrete blocks.
It’s not perfectly flat, but close. I’m not worried about it not being perfectly level because I’ll use leveling nuts to plumb the column up perfectly.
The question I have relates to placing the blocks onto this pad. Should I use some product for grout, to fill any gaps and provide better bearing for the blocks? PL or Sitka non-shrink grout? I’m not sure how much time I will have before the tide comes in again.
I’m doing this on Thursday 12th June.
Thanks in advance for any input.

I don't know what your bearing surface is like - is this on a mud flat, a rocky/gravel beach, exposed bedrock?

Mud flat is going to be the worst, and I have limited advice on that. For rocky/gravel beach you can just roughly level it - i would try to dig down a bit so the block is keyed in below the surface. For bedrock, you would need to create a flat pad for the block to sit on (so the block isn't rocking around on a high spot) - likely with a grout product.

Upload a photo if you have one.

 

[seachicken]

Another question…. So I have cleared out the location, leveled (kinda - not absolutely perfectly) it off and put down an inch or so of a product called Mud Bed Mix by Mapei. I mixed it really dry as per the recommendations and it has worked out well curing under water. I’ve inspected it at LT and am happy with how it will perform as a base for the concrete blocks.
It’s not perfectly flat, but close. I’m not worried about it not being perfectly level because I’ll use leveling nuts to plumb the column up perfectly.
The question I have relates to placing the blocks onto this pad. Should I use some product for grout, to fill any gaps and provide better bearing for the blocks? PL or Sitka non-shrink grout? I’m not sure how much time I will have before the tide comes in again.
I’m doing this on Thursday 12th June.
Thanks in advance for any input.

I would be inclined to bed it into a quick setting grout or leveling compound. I'd rather not have any gravel, silt or growth under the blocks that over time could shift it somehow.

 

[macro]

If you are using rebar I recommend epoxy coated (green). Also increase your concrete cover over the rebar to 3” to help protect against the chlorides. There is specific concrete that’s should be used when exposed to salt water but I am not sure how technical you want to get.

May be a case for using fibreglass rebar.. nothing to corrode... or stainless $$$$

 

[Max123]

I'm going to assume this is on bedrock based on your description. You could place the pad on a skimcoat of non-shrink grout. If you want to fully seismic-proof the install, I'd do x2 corner brackets with rods attaching the block to the bedrock

 

[eriks]

Sorry, should have better described the existing grade. It’s very solid, it’s actually on a debris fan, with some boulders as big as cars. The bearing is great. I selectively cleared a couple big boulders, filled a few depressions with fist sized rocks and then covered everything with 3/4” clear crush. I then covered the crush (mostly to just hold it all together) with about 1-2” of Mapei Mud Bed Mix. There are a couple perfectly placed large boulders that face NW, the predominant direction from which waves come in the winter storms, protecting this pad that was poured and compacted. The blocks will be placed up against these rocks.
The pad isn’t 100% flat, and I’m just wondering if it would be best to put something down before placing two flat top lock blocks onto it.

 

[Brett83]

I would use hydraulic cement. It dries in minutes so you'll need to work quickly. It should be available and most decent building centers. Like Standard lumber or a timber mart.

 

[Max123]

I'd leave as-is. You don't want to buildup the subgrade with 3/4 so much that it could potentially wash away later on. If there is any concern that the bearing material could wash out, you could do a ring of mud/grout around the base after the blocks have been placed to lock everything underneath. But even then, I don't love grout - I'd rather just backfill it with more rock.

At this point if you are happy with your foundation, leave well enough alone, place the blocks, and then just shim your post hardware to level it all out.

Photos when you are finished - have fun out there!

 

[eriks]

I don't know what your bearing surface is like - is this on a mud flat, a rocky/gravel beach, exposed bedrock?

Mud flat is going to be the worst, and I have limited advice on that. For rocky/gravel beach you can just roughly level it - i would try to dig down a bit so the block is keyed in below the surface. For bedrock, you would need to create a flat pad for the block to sit on (so the block isn't rocking around on a high spot) - likely with a grout product.

Upload a photo if you have one.

 

[sly_karma]

Agree with @Max123: you've created a flat surface, now place the blocks. If you find the blocks have a little wobble happening, drive a couple of tapered wood shims to deal with it, then some non-shrink grout worked in around them to make it permanent.

 

[Max123]

Agree with @Max123: you've created a flat surface, now place the blocks. If you find the blocks have a little wobble happening, drive a couple of tapered wood shims to deal with it, then some non-shrink grout worked in around them to make it permanent.

100% - this is the way to go.

PS - Looks great so far!

 

[Rain City]

Can you drill a couple fiberglass rebar pins to lock it in place? Shouldn't be that hard to align. That'll keep it from sliding. Or some stainless angles after the fact.

 

[Brett83]

Agree with @Max123: you've created a flat surface, now place the blocks. If you find the blocks have a little wobble happening, drive a couple of tapered wood shims to deal with it, then some non-shrink grout worked in around them to make it permanent.

I'd do this with hydraulic cement because it will dry better and cure faster in your window. I don't believe it's cure is affected by the cold ocean. It's often used around pipes entering drainage sumps.

 

[Bifmalibu]

I know you're deep in now, but I think Helical piles may have been a good option.