Concrete Pile Caps

[4wilmar]

As part of the on site QC team, I am concerned about thermal cracking in some up-coming pile cap pours which are over 10 ft thick, 15 ft wide and 20 ft long. However the designers have declared that these are not really mass concrete pours and no speacial precations need to be taken.

1. If none of the vertical faces are reinforced, does anyone know how much cracking we might expect if we start off with a concrete temp. of about 90 degrees.

2. If some of the piles are designated as tension piles, how will the tensile load transfer thru the pile cap to the column above without any vertical steel???

Any comments would be helpful as the contractor & engineer of record don't seem very concerned about any of this.

 

[youngstructural]

1. You can calculate the expected cracking based on the tensile strength of and differential temperature expected in the concrete, however this is overly conservative. There is some additional tolerance for cracking often provided by the use of large course aggregate (typically 75mm or larger on such a thick concrete pad. This also depends heavily on HOW they pour (ie: Might not be a problem at all if poured in lifts which are allowed to cure and a SSD condition cold joint permitted).

If they are pouring in one hit (which would surprise me) or not making other allowances (such as ice instead of water), you probably have a problem. There's a good resource at my office that has a formula which may be applicable; Let me know if you want me to look it up.

2. It will not, in my opinion. I am suprised you would still need tension piles with such a large cap; If the dead load is incufficient, you probably do need to insist upon reinforcement to lap through, at least enough to cause interaction of the lap zones.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[asixth]

What is the magnitude of the uplift on the pile, I calculated the weight of the pile cap to be 2,000kN (470 kips). There would have to be a very large overturning moment on the structure to result in uplift.

 

[4wilmar]

Thanks for your comments:

1 They plan to cast full height in one pour, which has me worried as well since they do not intend to take any precautions such as chilled water, ice or insulating blankets (nothing really).

2 I am not sure about the magnitude of the up lift since I was not envolved with the design, I just assumed there would be an uplifting force since they are requiring tension piles with DYWDAG bars & anchor plates at this location. However the anchors only extend about 2 ft into the bottom of the pile leaving nothing but the tensile strength of the concrete to transfer the load up to the column anchors above.

 

[4wilmar]

Sorry if the 2cd sentence in # 2 confuses the issue, I meant to say that the achors extend only 2 ft into the bottom of the pile cap.