Necessity of clear Cover in Pile Cap after pile embedment 5

[NewbieInSE]

Dear Engineers,
Please see the following and provide your valuable response. Thanks.

 

[Trenno]

I'd use the pile heads as chairs for the B1 layer. In the UK it's common to have a 3" pile embedment.

 

[NewbieInSE]

Trenno, You mean you wouldn't place an additional clear cover for reinforcement. In that case, I heard there is a possibility of water seeping through the weak concrete at the pile peripheral face and pile cap interface, which can corrode rebar. I'm not sure about using that 3" cover since it reduces the rebar effective depth or costs extra concrete.

 

[HTURKAK]

This excerpt from IBC ..


(81810.3.11 Pile caps.
....The heads of all piles are to be embedded not less than 3 inches into pile caps, and the
edges of the pile caps are to extend at least 4 inches beyond the closest sides of all piles. The
degree of fixity between a pile head and the concrete cap depends on the method of
connection required to satisfy design considerations.)

It is common practice to provide 3 in embedment and lay the pile cap bottom rebar on pile heads..

 

[canwesteng]

How are you getting concrete around the bar if you sit it on the pile head to develop the bar in tension? Especially since that's a node in a S&T model. I'd normally provide 1 1/2" from pile top to bar

 

[dik]

You need 3" min cover, not 7"... in your case with the 4" embedment of the piles, you would have 4" cover.


I don't do this... it's only a very local condition and I'm not concerned about the reduced bond on the rebar, or the lack of corrosion protection.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[slickdeals]

If you look in the CRSI handbook and also the new pile cap design guide, they show 3" of concrete above the top of the pile embedment. This has been their standard detail for the longest time.

 

[STrctPono]

I provide 1-1/2" clear cover from pile top to reinforcing steel. I would like concrete under the reinforcing steel and am concerned about corrosion even if it is only localized.

 

[dik]

Sure... your choice and you have to be comfortable with it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[canwesteng]

Would localized corrosion be a concern? The chlorides still have to migrate over 3". I suppose a construction joint should be wetted so has a better bond, but if we presume that chlorides can run up that joint so we want more cover, what about the joint between pile cap and pier?

 

[NewbieInSE]

So, it's all about the engineer's choice whether he is going to provide that additional clear cover above the pile head or not?

 

[Lomarandil]

I also am happy to use the pile embedment as my chair/cover in standard low-corrosion environments.

CRSI does include the 3" cover from the pile top, but they're also set up for steel (and before, timber) piles.

----
just call me Lo.

 

[bugbus]

Common practice where I'm from is to have 50 mm pile embedment into the pile cap, then allow 75 or 80 mm total cover to the bottom face of the pile cap. This lets the concrete get in between the bottom pile cap bars and the top of the pile cut-off.

I cannot see how water could seep in above the pile cut-off. But I'd be more comfortable knowing that the pile cap bars are well encased. In some cases, I would think that there would be a benefit from the large compressive force through the piles on the bond of the pile cap bars, which would compensate for the reduced amount of concrete around the pile cap bars. However, many times we are designing for tension in piles, so I would prefer to be a bit more cautious.

 

[Lomarandil]

Just to play devil's advocate, how is a bar sitting on the pile different from a bundled bar?

(Granted, development length can be an issue in some pile caps)

----
just call me Lo.

 

[dik]

Lom... I start getting concerned when I use bars greater than 35M...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

@Lo: are you arguing that a single bar within a bundle also does not have 360 degree bond with the surrounding concrete?

 

[Lomarandil]

Right, specifically the question of bond

----
just call me Lo.

 

[NewbieInSE]

But the bundle bars in a group are considered as one, and their development length is also calculated considering the effective dia. So, maybe as a whole, the bundle gets 360 deg. concrete bonding. But in pile cap, bottom reinf. won't get at pile top. My thinking might be wrong.

 

[KootK]

I was going to make that same point Newbie. When space permits, I prefer to develop the rebar beyond the piles rather than over them. When that's the case, the (important) tie anchorage issue is obviated.

 

[HTURKAK]

NewbieInSE ,

Apparently the sketch that you have posted got from web (e.g.

https://www.designingbuildings.co.uk/Pile_cap

) and you added 4 in embedment and 3 in extra clear cover...

I think everybody will agree to provide embedment in the range of 4 in. but the approach differs for the requirement of extra 3 in. In my region, the common practice is , to provide 100 mm embedment and spacer around 25 mm above the pile.
I suspect, the extra 3 in. is provided if the pile cap is analyzed with STM . If the depth of triangular bottom node is wt, the tie depth would be wt/2 above the top of pile.

This excerpt from ( Strut-and-Tie Models for Unified Design by Chen, W.F. )

And this is excerpt from worked example ACI SP-17(14)

And the following is the same pile cap obtained from sectional design.

I will prefer to install the rebars 25 mm above the pile top and all the bars bent 90 degr.,extent to get the required development length rather than simple hook .And provide minimum cage reinf and horizontal ties for bursting reinf.