Concrete Wall Reinforcement Placement

[Jerehmy]

ACI Section 14.3.4 states:

14.3.4 — Walls more than 10 in. thick, except basement walls, shall have reinforcement for each direction placed in two layers parallel with faces of wall
in accordance with the following:

(a) One layer consisting of not less than one-half and not more than two-thirds of total reinforcement required for each direction shall be placed not less
than 2 in. nor more than one-third the thickness of wall from the exterior surface;

(b) The other layer, consisting of the balance of required reinforcement in that direction, shall be placed not less than 3/4 in. nor more than one-third
the thickness of wall from the interior surface.

Are these distances to center of rebar or are they clear cover. I'm assuming center of rebar?

 

[hokie66]

I'm assuming clear, based on the 3/4 inch to the inside. You would never want less cover than that.

 

[Jerehmy]

Ah, right.

I just don't understand the 2" requirement. Per Chapter 7 I can have 1-1/2" clear. But now it must be 2" per chapter 14. Was hoping I could have it at 1-1/2" clear. Well, 1-11/16" clear I guess for a number 5.

 

[hokie66]

I have never seen, and don't think you will find, a code which defines concrete cover by distance to center of bar.

In a relatively thick wall, why are you resistant to more cover? The more cover, the better protection for the steel.

Cover on the exterior of walls should depend on exposure classification, e.g. distance to salt water, and also grade of concrete. Doesn't the ACI Code have these limitations? I haven't looked in a long time.

 

[Jerehmy]

ACI handles all concrete protection in chapter 7, or so I thought. This little blurb about 2" cover for walls isn't specified in chapter 7, just randomly thrown in the wall chapter.

I have a 15ft high 400ft long site wall. 1.5" cover vs 2" cover could make some difference on reinforcement. I don't think 1-1/2" vs 2" cover makes much difference for protection when its only exposure is wind and rain. Plus chapter 7 says its OK for 1-1/2" cover while chapter 14 renegs that. I just want to understand the provision because it's slightly ambiguous. Seems it is indeed cover. I wish they'd just specify all cover requirements in chapter 7.

 

[TehMightyEngineer]

Huh, interesting. I never noticed that as well but it does seem to be prescribing a clear cover greater than chapter 7. I confirmed that ACI 318-14 still has this note in section 11.7.2.3 (the equivalent of 14.3.4).

However, if I look in ACI 350-06 is says:
14.3.4 - Walls more than 10 in. thick shall have...
(a) One layer, consisting of not less than one-half nor more than two-thirds of total reinforcement required for each direction, shall be places with not less than the concrete cover limits given in 7.7 nor more than one-third the thickness of wall from the exterior surface."

But in ACI 350-06 the min. clear cover for walls exposed to earth/weather/etc. is 2 inches so it's effectively the same limit.

I don't have a copy of ACI 318-08 to check what that says.

PCA Notes on ACI 318 doesn't clarify.

All said, it does appear that the way it's written is that 2" clear cover is required. However, this seems to not be correct to me as they have an exception of basement and retaining walls. This would mean that the concrete exposed to soil can potentially have 1-1/2" clear cover but a finished building interior wall needs 2" clear? That doesn't make sense to me either.

I wanna say that the intent was to follow the clear cover provisions in Chapter 7 but it's definitely not written that way.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

Huh, this also potentially effects me quite heavily. There's no exception for precast unlike chapter 7. Effectively this appears to prescribe that I need 2" clear cover for precast walls. That's completely ridiculous for the typical clear cover we use.

Edit: Yeah, I don't get this. In ACI 318-14, Table 20.6.1.3.3 has a column for precast concrete specifically for walls, either exposed or not exposed to weather. 11.7.2.3 appears to completely override this.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[MrHershey]

You note yours is a site wall, may want to look at ACI 318-14 which now specifically omits cantilever retaining walls from these requirements.

This clause in the code is very old. I've found it as far back as ACI 318-63. 2202(g) for the curious.

Feel like I usually just see people go by Chapter 7 and ignore this (though for bars #6 and greater they match for CIP). Perhaps also worth mentioning that 14.2.7 specifically lets you out of 14.3 requirements if structural analysis shows adequate strength and stability.

 

[Jerehmy]

By site wall I meant free-standing wall, sorry for the confusion. My boss likes to say site wall and I have gotten into the habit of calling free standing walls site walls.

TME - my excerpt was from 318-11. I'm looking at 318-08 and 318-11 at the moment. Chapter 7 requirements are the same. And section 14.3 is also the same.