Concrete cover for piled rafts

[EngrRC]

CRSI Design Guide for Pile Caps shows a minimum embedment of pile into the pile cap by 4 inches, and then the bottom reinforcements are to have 75mm concrete cover above pile head.
Is this valid for concrete piles or is this just for steel piles? I do not see the need to have 75mm extra cover for reinforcement when the pile head is also concrete.



The CRSI handbook does have a statement about this, but it's not a very clear statement whether concrete piles need extra cover on top.



Can pile cap reinforcements be laid directly on top of the concrete pile head? If not, how much spacer is required?

 

[CJLCivilStruct]

I'm not familiar with the CRSI Guide, but in the UK it's pretty standard in my experience for piles to be embedded 75mm to 100mm (3-4") and have the bottom mat laid directly onto the piles.

 

[HTURKAK]

I have 2015 edition and copy and paste of the same statement ;
Concrete Cover: A concrete cover (measured clear between the top of the pile and the nearest reinforcing bars) is assumed
to be 3 inches in this design guide.

In your case , i assume compressive loading only on piles .
In case of moderate uplift , or bending moments or heavy uplift loads the cover would be more than 75 mm.

 

[EngrRC]

HTURKAK, yes, it is mentioned that 3inches above the pile head is assumed in CRSI. But why is this required? Considering the pile head is embedded 100mm into the raft, the total cover for the reinforcements will now be 175mm.

 

[HTURKAK]

HTURKAK, yes, it is mentioned that 3inches above the pile head is assumed in CRSI. But why is this required?

You did not mention if the piles are precast driven or bored piles and type of design loading ( compression only, heavy uplift , bending moment ..)
If your detail includes leveling/ blinding concrete 100 mm, in this case you can directly lay the main reinf. on the piles for compression only piles .
Local practice and standards may differ but a a minimum 75 mm clear cover should be provided.

 

[EngrRC]

They're bored piles for a tall building, and they're mainly in compression, but will also have some uplift. For the raft, the cover requirement is 75mm, but due to pile embedment (and if spacers are required to lift the bars from the pile head), it will more.

 

[HTURKAK]

This detail is OK for me.
Provided that ,
- the pile heads after trimming have reasonably the same elevation,
- the pile main bars extend to the pile cap ,
- use a minimum spacer 20 mm .