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Calculation of crack widths in foundation design

CJLCivilStruct

CJLCivilStruct

Structural
Aug 23, 2022
34
Just wondering what the general consensus is for calculating crack widths in RC foundation design - think ground beams, rafts and pads.

I have always calculated and limited the crack widths to 0.3mm for exposure classes XC3/4 for durability.

However, I sometimes come across examples of ground beam designs for example where crack widths aren't considered at all. Is this lazy design or generally what Engineers tend to do given its in the ground and more than likely not to be seen again?

Given that quite often for me the majority of the loading in foundation design is quasi-permanent, I find that crack control typically governs and results in an extra 20/30% area of steel required. Am I being over conservative?!

Anyway, I hope this is a good topic of discussion. Thanks in advance.
 
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I've never limited crack widths for the purpose of durability nor have I seen it referenced as a method to do so in design codes. I'm open to being corrected on that and would be interested to know if it is referenced anywhere because from a logical stand point it makes sense. However don't design codes handle durability through specifying minimum concrete strengths and cover depending on exposure class?
 
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CJLCivilStruct said:
Just wondering what the general consensus is for calculating crack widths in RC foundation design - think ground beams, rafts and pads.

I have always calculated and limited the crack widths to 0.3mm for exposure classes XC3/4 for durability.

However, I sometimes come across examples of ground beam designs for example where crack widths aren't considered at all. Is this lazy design or generally what Engineers tend to do given its in the ground and more than likely not to be seen again?

Given that quite often for me the majority of the loading in foundation design is quasi-permanent, I find that crack control typically governs and results in an extra 20/30% area of steel required. Am I being over conservative?!

Anyway, I hope this is a good topic of discussion. Thanks in advance.
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Interesting, but crack width is not something I have ever checked in foundation design. I suspect I am not alone in that regard. I don't believe it is lazy design; it is simply not a requirement of the concrete code in Canada. Class XC3/4 is part of the BS code, or so I believe. An additional 30% of rebar seems like an extreme waste to me, with no obvious advantage in durability. Concrete is a permeable material, so water can seep through it even without cracks.
 
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Not disagreeing with any of you - I can see the logic from both sides of the fence on this one. I've attached an extract from Eurocode 2 regarding control of crack widths (the code I design to in the UK).

There is no indication that I've seen to say that this does not apply to foundations. I do note there are some 'loose' clauses there such as:

"5) A limiting value, wmax, for the calculated crack width, wk, taking into account the proposed function and nature of the structure and the costs of limiting cracking, should be established."

But no principle clauses allowing you to disregard crack width when undertaking foundation design (unless i've missed it!!)
 

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  • BS EN 1992 - Crack Width Extract.png
    BS EN 1992 - Crack Width Extract.png
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There was an ACI paper that said cracks really didn't make a big difference for durability - the main factor is concrete cover
 
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According to ACI 224R-17:
The role of cracks in the corrosion of reinforcing steel is controversial (ACI 222R). One viewpoint is that cracks reduce the service life of structures by permitting more rapid penetration of carbonation and allow chloride ions, moisture, and oxygen to reach the reinforcing steel. Another point of view is that while cracks accelerate the onset of corrosion, the corrosion is localized. With time, chlorides and water penetrate uncracked concrete and initiate more widespread corrosion. Consequently, after a few years of service, there is little difference between the amount of corrosion in cracked and uncracked concrete. More important parameters for corrosion protection are concrete cover and concrete quality.
Click to expand...

For example, here in Brazil, we only require minimum concrete cover for elements in contact with the ground (there’s no specific limit for crack width). Of course, this changes when the groundwater table reaches the structure, but that’s about watertightness, not just cracking or durability. For that, I’ve been using Annex H of EN 1992-1-1:2023
 
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canwesteng said:
There was an ACI paper that said cracks really didn't make a big difference for durability - the main factor is concrete cover
Click to expand...
But isnt there an ACI doc for liquid containing structures that specifically limits crack width to address durability?
 
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I agree with canwest that cover is the main factor.

This is taken into account by the min cover for things permanently exposed to or cast against ground.

Also for corrosion to happen you need oxygen, which for buried things is hard for it to get too.
 
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There seems to be a vast difference here between, UK, EU, AUS and North America. The former three definitely do emphasise minimising crack width to ensure durability.

It seems like from the responses here that those from the US and possibly Canada have missed the memo. Now I'm not suggesting North American designs are less durable, but it could be that the emphasis is solely on other items like cover than crack width.

There is copious research showing that crack width matters and limiting it is important.

CJLCivilStruct said:
Given that quite often for me the majority of the loading in foundation design is quasi-permanent, I find that crack control typically governs and results in an extra 20/30% area of steel required. Am I being over conservative?!
Click to expand...

Probably not. Crack control for many footings dominate design in my experience. This is particularly the case in raft footings and footings where mass is required to resist uplift/overturning. Here you end up with plenty of concrete but don't need much steel for strength. Crack control becomes dominant.
 
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canwesteng said:
There was an ACI paper that said cracks really didn't make a big difference for durability - the main factor is concrete cover
Click to expand...
Interestingly, if you follow a crack width calculation process per CIRIA's guidelines, increased concrete cover makes things worse because the crack actually gets wider as cover increases, bringing you closer to whatever limit you set.

For rafts with significant hydrostatic uplift, this is a HUGE design consideration if forced to follow the procedure, it can lead to lots of rebar at really tight spacing.
 
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CJLCivilStruct said:
But no principle clauses allowing you to disregard crack width when undertaking foundation design (unless i've missed it!!)
Click to expand...
You haven't missed anything. Both EC and BS is silent for the this case.
My interpretation is , if the exposure class is XC3/4 ,the concrete indicative strength class should be C30/37 and the minimum cover is 40 mm.
Short answer for durability of the foundation,
- Always provide/spec asphalt membrane at the bottom and bitumen paint at least for the side surfaces,
- Spec minimum cement dosage ,
- Use suitable cement type for alkali - cement reaction.

My personnel observation is , if the concrete without protection exposed to XC4 and in case of aggresive soils, 20 years later the strength would drop catastrophically such that one can dig with hand tools.
 
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HTURKAK said:
You haven't missed anything. Both EC and BS is silent for the this case.
My interpretation is , if the exposure class is XC3/4 ,the concrete indicative strength class should be C30/37 and the minimum cover is 40 mm.
Short answer for durability of the foundation,
- Always provide/spec asphalt membrane at the bottom and bitumen paint at least for the side surfaces,
- Spec minimum cement dosage ,
- Use suitable cement type for alkali - cement reaction.

My personnel observation is , if the concrete without protection exposed to XC4 and in case of aggresive soils, 20 years later the strength would drop catastrophically such that one can dig with hand tools.
Click to expand...
Makes sense - would you still be sticking to the 0.3mm crack width generally? Or with the protective measure noted you'd be more comfortable taking a pragmatic approach?

Obviously this is a case by case basis. I'd be more relaxed about crack widths in a ring beam to an extension of a 1970's house than a 6 storey apartment block (I think!).
 
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CJLCivilStruct said:
Makes sense - would you still be sticking to the 0.3mm crack width generally? Or with the protective measure noted you'd be more comfortable taking a pragmatic approach?
Click to expand...
I never calculated crack width for the foundation elements . But crack width should be limited for water retaining structures and for superstructure elements for servicebility reasons.
I would suggest to spec waterproofing for XC2--4 for XDs and XSs , and at least bitumen paint for XC1. In past i did so.
 
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Interesting. Would be good to hear from people who do calculate crack widths for foundations to Eurocodes.. I can't be the only one!

I do a fair bit of water retaining concrete design so can appreciate that's a different matter altogether.
 
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I always thought crack control was important only for certain water processing and waste processing facilities.... If you're storing a material that could be corrosive or which could leak into the ground water.

Other than that, it's mostly about aesthetics, not capacity. Meaning that crack control for foundations isn't very meaningful. The general concept is to try to use smaller more closely spaced bars to prevent excessive or unsightly cracking.
 
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