Maximum clear cover

[ramihabchi]

hi
For elements under bending I know that the larger the concrete cover is the larger is the crack width,but I could not find requirements in ACI or other codes to deal with/limit large cover.What I have found in aci is requirement about skin reinforcement in beams which seems to me
based on the same principle(correct me if I am wrong)when concrete tensil crack occurs they have tendency to become larger and larger until the element fails.reinforcement will prevent this continuous increase in crack width regardeless of used diameters.
Also I have found requirement for moment frame columns imposing a maximum cover of 4 inches to the transverse reinforcement but I am not sure if it is also based on the same mentioned principle as in the commentary of the code they mention only confinement.
What do you think?and in case increase of cover and putting reinforcement to limit cracks what is the maximum spacing allowed between such reinforcements?

 

[Agent666]

I know in British standards they used to let you evaluate the crack width from a durability perspective at the minimum cover required (effectively ignoring the excess cover). Obviously from a visual perspective the visual crack width might be larger but it doesn't affect the durability. But it seemed fairly logical to me at the time. It's been a few years since I dealt with British Standards in detail, so no idea if they (or Eurocodes) contain the same recommendations or not. Hopefully someone else can confirm.

 

[r13]

Similar to Agent666 pointed out, in the US, the maximum effective cover for crack control calculation used to be 2". I don't know if it is still valid these days.

 

[Settingsun]

Why are you considering large cover? And is your concern with cracking related to durability, appearance, both, something else?

The Australian code says 100mm maximum to centre of longitudinal beam reinforcement. Say it's a 24mm bar with 12mm stirrups, that's 76mm maximum clear cover which is generally more than adequate.

 

[blihpandgeorge]

Hi steveh49 - i'm not familiar with the 100mm max dimension, can you advise which AS this is in (and potentially clause) - this would've come in handy a few times...

 

[Settingsun]

Blihpandgeorge, it's AS 3600 clause 8.6.1, item (b).

 

[HTURKAK]

The ACI 318 building code requires side face reinforcement for deep beams with a depth h > 36 in . The cracks may between the neutral axis and the tension face.

The required skin reinforcement will be uniformly distributed along both side faces of
the member for a distance d/2 .

The spacing should exceed the minimum of d/6, 300 mm, or specified at ACI 318 code 10.6.4 .

Regarding the maximum cover, ACI 318 7.7 — Concrete protection for reinforcement suggests different minimum covers
depending on the exposure class and ( 3 in minimum for Concrete cast against and permanently exposed to earth)

AASHTO LRFD Bridge Design Specifications suggests 100 mm for direct exposure to salt water.

Regarding the quote 'What do you think?and in case increase of cover and putting reinforcement to limit cracks what is the maximum spacing allowed between such reinforcements?'

I think the major concern shall be corrosion of reinforcement rather than crack width and will suggest you to look ACI 318 CHAPTER 4 — DURABILITY REQUIREMENTS.

 

[mfrad]

See ACI 117-16 Tolerances. Effective depth "d" has to be considered.

 

[blihpandgeorge]

steveh49 - thanks. i was looking in section 4 durability. interesting that is listed under beams, but a useful reference nevertheless