Thick Concrete Wall Crack Prevention

[Sean_14]

I am working on a project that requires a 36in thick concrete wall about 10ft tall. This wall cannot have cracks. How can I modify this design to prevent cracking? I am sure the right rebar selection is key as well as the mix. Not sure where to start.

 

[jayrod12]

You can't prevent cracking. There's no such thing as crackless concrete. You can, however, limit the crack width such that it is not visible and does not leak. For that you'd be wanting to look at the provisions in ACI 350, specifically the design of water/effluent tanks.

 

[KootK]

1) If the crack limitation is just aesthetics and control joints would be possible, that is usually the most cost effective method.

2) As jayrod12 mentioned, a common approach is to reinforce the wall so heavily that the cracks that do develop are very small and well distributed. One the order of 0.6% or more depending on the purpose of the wall (fluid retention etc).

3) In the extreme, I suppose that one might post tension the wall along its length. You'd be fighting against probable restraint at the bottom (and possibly) top of the wall though.

 

[centondollar]

There are ways to calculate stresses due to temperature gradients and autogeneous and drying shrinkage (see e.g., CIRIA thermal cracking recommendations) and the reinforcement required to minimize cracking do to those effects. Tensile strain due to restraint at the wall-floor or wall-ground interface, can be found in e.g., Eurocode. Reinforce for that, close to the wall faces, and choose cement so it contains minimum Portland cement and significant amounts of slag - this reduces the peak curing temperature and therefore restricts cracking from temperature gradient between wall core and wall faces, but it also slows down strength development. The peak temperature during curing can be found in nomograms (function of wall thickness and mix type).

You may also consider casting in two layers (18 inches at a time) if you can't make it work in any other way. However, that would require stirrups or some other form of shear keys to connect the two wall segments, so it might not be economical.

PS. The dimensions of this wall are quite unusual. 3 meters tall and 0,9 meters thick (approximately) - what is it used for?

 

[dik]

3'thick... you are also looking at mass concrete issues and likely thermocouples.

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

-Dik

 

[Sean_14]

Yea I understand the cracks are inevitable but they have to be small and not allow leakage. I know the reinforcement ratio will have to increase as well. I will have to really look into ACI 350. @centondollar, I cannot comment on what this will be used for. Only that this will be a room and will also have a 2ft thick concrete roof.
the length of the walls will be no more than 15ft so i am trying to avoid expansion joints. I am thinking that increasing the rebar ratio and finding the right mix will do the trick.

 

[JedClampett]

It's extremely unlikely that a 36-inch wall 10'-0" high will leak. There's just too much concrete for the liquid to fight through. Use a rho (total) of .005 (#7's at 6" EF horizontal) and it should hold up pretty well. Water to cement of under .45.
Also, define the wall as a water containing structure, leak test it and have the contractor fix any leaks with epoxy injection. Put the burden on them.

 

[centondollar]

Better advice could be given if you would explain the purpose of this structure beyond "required to be leakage proof".

The structure is 15ft by 15ft with 36 inch walls and 2ft roof, so in metric units approximately 4.6 meters by 4.6 meters in plan with 0.9 meter thick and 3 meter tall walls and a 0.6m thick roof. What on earth is that size and is required to be liquid proof? It certainly is not a liquid container, and presumably not a gas container (using some rule of thumbs for the mix and rebar percentage would not suffice for such structures). Not even explosion-resistant shelters in buildings are built with walls of that size. Are you sure that the structure is not grossly over-dimensioned?

 

[skeletron]

Schrödinger's Concrete Bunker

 

[dik]

The cat's dead... RIP

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

-Dik

 

[Sean_14]

@centondollar No the structure has been specifically designed with these dimensions for its use. I cannot discuss the use exactly. Only the dimensions and crack resistance is very important.

 

[LittleWheels]

centondollar has it about right IMHO. Defining your maximum allowable crack width will go a long way towards identifying your solution.

CIRIA C766 is very useful in assessing likely crack widths for various concrete mixes and specific pour dimensions to Eurocodes but if you are using imperial dimensions, you are probably using American codes. Somebody else will understand those better than me.

Out of left field - any chance of crack repairs post-pour?

 

[Sean_14]

Yes we can repair cracks post pour. The hope is to reduce those as much as possible.

 

[bones206]

I've worked on some old bunkers with thick walls and roofs, yet still have water intrusion issues. My advice is to not rely on the concrete alone if you don't have to. Take a multi-layer approach with redundant systems to prevent intrusion of water or whatever it is you are trying to keep out of the room. For example; physical waterstops at all joints (maybe also swelling type waterstops at those same joints), crack-healing crystalline concrete admixture, membrane layers, good joint detailing, high rebar ratio with closely-spaced small bars rather than large diameter bars, good exterior drainage system and proper sloping.

It's hard to comment more specifically since we don;t know if you are trying to keep out water, radiation, etc. or if it's just for aesthetics.