Concrete Cover

[Zambo]

I am interested in board members views about concrete cover and construction joints.

Over the years if the drawings show either a column penetrating into a slab or beam or a bored pile penetrating into a pilecap I would allow the horizontal bars to sit directly onto the construction joint.

For clarity if this was a pilecap with specified 75mm cover to the bottom layer of rebar I would have the pile cut off at +75mm then place the first layer of rebar directly on the bored pile. But I am now on a project (still at design stage) where the designer is insisting that spacer blocks of minimum 50mm must be placed on the top of the cut off pile. This means that to satisfy the condition of pile cut off 75mm above the pile cap base then I must have 75 + 50mm = 125mm cover (although tbe specified cover is min 75mm).

Is there a correct way of doing this or is it variable depending on interpretation?

 

[hokie66]

I agree with you on pile caps. I think the designer on your project is naive. However, for columns, I would never allow a column to project above a beam or slab soffit.

 

[ChrisMagadiaCom]

the rebar should sit directly on the cutoff pile as there is a sufficient concrete cover provided already

thanks
chris magadia

http://www.chrismagadia.com

ChrisMagadia.Com - The Structural Engineers' Forum and Resources Website. Civilizations owe its existence to Structural Engineering. Do you Agree?

 

[valleyboy]

I agree with the above. What is the designers rationale for the extra 50mm of cover?

Another detail I have used for 2 pile caps with bar arrangements similar to a beam is to have the links sit on top of the piles. This has typically been used if the piles are a larger diameter than the link spacing. This results in 75 cover to shear links and 87 to main longitudinal bars ( assuming 12 diameter links.)

 

[BAretired]

I agree with the above. The pile makes a pretty good spacer block by itself.

BA

 

[DST148]

@Zambo> CRSI Handbook (1996) shows reinforcing placed 3" above pile cut offs. CSA Handbook (older version) shows reinforcing placed 2" above pile cut offs. Textbooks by Bowles, O'Rourke/Winter show reinforcing placed 3" above pile cut offs. I Googled many web sites where reinforcing is typically shown 3" above pile cut offs. Could it be because of the theory behind design of the pile caps? ( tied arch of CRSI or the strut-tie models of the present day codes)

 

[JAE]

The interface between concrete placements - i.e. a line of a construction joint - is not a face of concrete requiring cover.
Also - over pile caps, there are no development needs at that point so lack of competent concrete all around the bar is not necessary.

 

[TXStructural]

If the piles are steel, there should be isolation between the pile and the reinforcement. Connecting the two may induce corrosion in certain soils and exposures. CRSI Design Handbook (10th Ed, 2008) specifically states that tabulations were done using 3 inches of concrete cover between reinforcement and the top of steel piles. The CRSI handbook also specifies a 6 inch (150 mm) embedment of steel shapes into the pile cap unless a top plate is installed on the pile. For concrete piles, the minimum embedment is 4 inches (100 mm). The designs in the handbook all use 3 inches (75 mm) of cover between the top of the pile and the bottom of the reinforcement. These values are part of the design, not "extra cover".

As far as needing confinement for development and splices, these would be a consideration in certain instances, and I don't think you can make a general statement that competent concrete is not needed. One other possible consideration is that rotation of the pile could compromise the reinforcement if there is direct contact.

I'm not sure why you are concerned with having addition cover. The 75mm minimum (3" in the states) reflects it being cast against soil, because of the inherent irregularities in that process, as well as potential exposures.

 

[TLHS]

I've never thought about this before, but is there a concern with bond? If you're sitting directly on the cured pile the bottom of the bar isn't going to bond properly and you might also have confinement issues?

It would depend what the rebar is doing at that point. Maybe not a huge concern if you're just using it for shrinkage, but if it's doing something I'd look a little more closely maybe.

 

[hokie66]

TX,
So are you saying that for a concrete pile projecting 4" into a pile cap, there should be 7" cover to the soil under the bottom bars of the pile cap? I have never seen it done that way, and it seems very wasteful.