Hollow Core Slabs - Application in Marine Environment 1

[STRUKT]

Hi all,

We've been working on a project of a pier slab located in a small harbor to help load and unload fishing boats.
The contractor looks for a precast solutions instead of cast in situ regarding the obvious issues around formwork and reinformcent jobs on a tide zone.
Our main concerns regards the environmental requirements of the precast hollow core system and it's application on marine and costal environments. From our point of view, as structural designers, seems that this kind of system does not suits the requirements in terms of durability imposed by the standards, namely Eurocode.

Have someone any experience on apply this kind of solution in such harsh environmental conditions? What are your thoughts about this?

Thank you,

STRUKT

 

[GeorgeTheCivilEngineer]

You need to talk to the manufacturers. They could do a special batch that meets the relevant requirements, but yes an off the shelf hollowcore slab I doubt would be suitable, they are usually indoor products.

 

[jayrod12]

Locally here, hollowcore has been used in exterior applications with varying results. Most (maybe all) of the suppliers around here highly recommend against it. They could however just provide pre-cast concrete slabs that will meet the durability/robustness requirements you're after. We quite often use pre-cast solid slabs in exterior applications.

 

[STRUKT]

Where is here jayrod12 ?

 

[phamENG]

I agree with George and jayrod. Hollowcore would not be suitable. If precast is the desired option, solid precast would be best. Depending on the site and the competency of the contractor, they could cast them on beds on site to minimize transportation costs. Once they've cured adequately (use a high early strength and you could be moving them inside a week), pick them up and put them in place. Coordinate size with the contractor's crane capabilities - weight of each piece, span for reach (both to place the slabs and to drive the piles if they'll be building it progressively), etc.

Of course, this won't be cheaper. You're trading the more common method of forming and casting in place for the speed and simplified site work of precast. This is usually necessary in unprotected harbors where keeping a barge in the surf just isn't possible. But relatively protected waters - cast-in-place is very doable and is more common because it's usually less expensive.

 

[Lomarandil]

In a tidal marine environment, I've also seen hollowcore slabs used as an in-place form made composite with the slab above.

----
just call me Lo.

 

[jayrod12]

Here is central Canada. So we aren't dealing with the nasty saltwater and surf that you are. But we still don't use hollowcore in exterior applications that much. By that I mean I've never used it in an exterior environment, but have seen it.

Small sidetrack, you can get an idea where a user is posting from by clicking on their username. If you click on mine, at the end of the bio it says (MB, CA) standing for Manitoba, Canada. For the OP I get (PT). Honestly don't know where that would be, Pakistan maybe?

 

[phamENG]

Portugal, I believe. I always Google "XX country code" and it usually steers me in the right direction. I think Pakistan is PK.

 

[STRUKT]

Thanks for the inputs.

Maybe the hollow core slabs could be used as lost formwork for the structural slab.

About the sidetrack from jayrod12 i've found this complete list of iso 3166 country codes. Hope this could enlighten about what PT stands for.

https://en.wikipedia.org/wiki/List_of_ISO_3166_country_codes

 

[jayrod12]

Thanks Pham and Strukt. And Portugal makes a heck of a lot more sense given the letters.

 

[wth]

I've seen hollowcore slabs used both as "lost formwork" and as part of a "composite slab" with a in-situ topping for piers.

They used a special form for the elements with extra cover, and dowels were placed in the channels which were also filled with concrete on site. The slab itself was design by others, but I think the channels were filled due to shear, but also to maintain cover on all sides.

Another option would be solid "formwork" elements with the bottom reinforcement prefabricated, and finished on site. Designed to handle the top pour without shoring.
As I've understood it, this solution is more common for prefabricated piers in my area.

 

[dhengr]

STRUKT:
I’d want a solid precast plank/slab too, with appropriate steel cover and conc. mix for a salt water environment. Could be regular rebar or tensioned, but the steel cover details and min. conc. cracking are the important things for steel protection and longevity. You might cast a male keyway in one edge of the plank and a female keyway in the other edge and you might have some lateral tie detail, perpendicular to the length of the planks, to hold things together.

 

[JLNJ]

I've never asked for it, but I imagine they could make "hollowcore" as a solid slab. (Obviously the extra DL is defeating the whole "hollow" concept.) They might even be able to increase the cover over the tendons to make it more suitable for exterior exposure.

 

[dik]

When I did one of the Health Science Centre parking garages about 40 years back, I used 12" HC slabs, and they have fared quite well. Deicing salts used all the time, but with Winnipeg, there is only one freeze thaw cycle each winter, it seems. I'm not sure if it was Conforce or Supercrete that I talked to back then. Have to be concerned about drainage at the ends. Due to stressing, the HC planks develop a slight upward camber.

About 50 years back I did a 6 storey apt bldg down the street and Conforce did not recommend the use of HC outside, at the time. Developer/owner used them anyway... they are still OK.

Best to talk to the precaster.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I don't think they can... the 'hollows' are a result of the Spiroll spiral extruding machines that compress the concrete against the containing part.


I've never encountered regular rebar in HC slabs, except in the joints between or in the cores, and I don't know how many thousands of sq.ft. of done using it.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Settingsun]

Pretensioned concrete has a chequered history in maritime. Some of it is due to inherent properties of the steel and some due to design choices.

Since the pretension strand is much stronger than mild steel reinforcement, there tends to be less steel. That means a given amount of corrosion is proportionally more detrimental. You may also get more corrosion due to the wire nature of the strands: if you lose 1mm around the perimeter of a 24mm bar, you lose 16%. If you lose 1mm around a 4mm wire, you lose 75% of that wire. All the wires that make up a strand are exposed so can potentially corrode. Another issue is that the expanded rust volume is less for a given loss of capacity and can expand a little into the space within the twisted strand, so you don't see evidence of the corrosion as early.

Some historical design issues came from overconfidence in the protection provided by uncracked concrete. The chlorides can still get in, so reduced cover wasn't warranted but was adopted.

Lastly, repairs to pretensioned concrete are harder than for mild steel reinforcement, particularly over water.

I don't think the hollowcore shape (hollowness) is any particular problem. The hollows are in the part of the structure that isn't relevant for durability.

 

[FE_struct1]

This may be a non-starter...are GFRP bars an option ? Using GFRP with precast planks would do away with rebar corrosion as an issue for the most part. The down side is that you'll also lose all ductility and I don't believe they can be preloaded either, so no pretensioning.

You'll still want a mix design suitable for marine environments.

 

[Settingsun]

GFRP is good for durability. The biggest drawbacks I can think of offhand are bent bars - poor availability and lower strength; and stiffness/deflection - pretensioned wins easily, need deeper concrete to match the spans using GFRP so bigger transport and installation costs.

 

[MHobbs]

I agree that the hollowness is not the durability concern as the least cover to the reinforcement will probably still remain the distance from the bottom of the slab.
Regarding the possibility of solid cores or ends or areas it is frequently done to provide an area for fixing into. The process will vary but I have seen it done by in addition to the cassette moving along the bed there is a bucket on the overhead crane. The cores are collapsed by hand and additional concrete added where required.

Due to the high compressive strength of the concrete generally used in prestressed they are often suitable for relatively onerous exposure classes but as people have mentioned above you'd want to ask your local supplier as they might not easily be able to increase the depth of the strands if this is required.

 

[JLNJ]

Is freezing a concern?