Concret Pile Caps
Concret Pile Caps
(OP)
As part of the QC team on site I am concerned about thermal cracking on some up-coming pile cap pours which are over 10 ft thick, 15 ft wide & 20 ft long. However the designers advise that these are not really mass concrete pours and no speacial precautions need be taken.
1 If none of the vertical faces are reinforced, how much cracking might we expect if we start out with a concrete temp of 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 if there is no vertical steel in the pile cap???
Any comments would be greatly appreciated as the contractor & engineer of record don't seem very concerned about any of this.
1 If none of the vertical faces are reinforced, how much cracking might we expect if we start out with a concrete temp of 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 if there is no vertical steel in the pile cap???
Any comments would be greatly appreciated as the contractor & engineer of record don't seem very concerned about any of this.





RE: Concret Pile Caps
seems a little strange to not have any vertical reinforcement for tension piles...but i'm just a dumb ole geotech. a structural might have a better explanation.
i say document, document, document...it ultimately falls on the eor.
RE: Concret Pile Caps
RE: Concret Pile Caps
http://www.gostructural.com/pg.asp?id=20
RE: Concret Pile Caps
If there is no vertical bars in the cap tension loads must be transfered by concrete shear. How is the uplift tranfered to the pile cap from the mebers that aresupported by the pile cap?
RE: Concret Pile Caps
RE: Concret Pile Caps
Starting early is good - Late also works. If it is a cap,keep the bar and forms wet all day and pour about 3:30 4:00 so that as the heat builds, the outside temperature is dropping, hopefully. Since you don,t have a lot of exposed area, finishing should not be a big deal. Just have the finishers stay late. They could even come in late.
I would also look into retarder.
RE: Concret Pile Caps
This temperature differential can be reduced by lowering the temperature of the concrete at the time of placement and/or by lowering the heat of hydration.
Your team aim to reduce the temperature at the time of placement by casting early in the morning. It would be worth monitoring the temperature of the aggregates in case spraying with water the night before can further reduce their temperature (btw casting late in the day, rather than early morning, only reduces the ambient temperature, the aggregates, cement and water will still be hot from the heat of the day).
You also have concrete with fly ash to reduce the heat of hydration. What grade of concrete are you using? From previous test results does it seem possible to reduce the cement content and still reach the required compressive strength.
My feeling is that if you are using a 30N/mm2 concrete there will not be a problem. If using 80N/mm2 concrete there will be cracking. Other grades in between could be a bit worrying.
RE: Concret Pile Caps
RE: Concret Pile Caps
RE: Concret Pile Caps
Just to let you know, the pile cap was poured with a mix design which included 25% flyash, but no addition of any ice (despite my concerns) to control the initial concrete temp. which went in at about 90 degrees. Day time temps were runing about 100 degrees and night time temps were dropping to around 80 degrees at that time. No exterior insulation was provided to control the temp. differential and when we removed the forms 3 days later, we had a horizontal crack about 10 to 15 mils wide located about 4 to 5 ft from the top, running around the entire pile cap. Crack width increased for another day or two to around 20 to 25 mils and then subsided to around 15 to 20 mils (slighltly over ACIs recommendations but acceptable to the Engs. as there was no rebar in the vertical faces to cause a corrosion problem). Several vertical cracks were also present but these were less than 10 mil wide.
Aside from the flyash, no speacial precautions were taken to control concrete temp. or the temp. differential in very hot weather.
Thanks to all for your comments & advice; hope this info might be helpful to some.
RE: Concret Pile Caps