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Re: Prestressed Double T Camber

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Wontae,

There was a theory many years ago that the Youngs Modulus does not increase as quickly as compressive strength, especially in cases like steam curing. I remember they were talking about a Maturity Coefficient. Have not heard of it since but it would explain your dilemma. With the rapid strength increase under steam curing, the modulus is probably lagging. Because you are leaving these planks for 3 days instead of 8 hours there may be a large increase in Ec over that time. The computer program would not be predicting that.


At 10:07 PM 25/07/2008, you wrote:
Thank you for your responses!
 
I did not know that late stripping of prestressed members from the mold would cause severe camber difference.
Though, this deviation is within product tolerance (0.25"/10ft , max 0.75"/10ft) according to PCI Design Handbook;
 I got 2" while 0.75x60'/10'=.45", which is very big number!
 
I understand camber is related with E and E is eventually affected by f'c.
In my case, when I ran a program to compare how f'c affects camber using 3500psi (@stripping)/5000psi(@28-day)
 and 6000psi(@stripping)/8000psi(@28-day), cambers were only 0.5" difference.
I am still puzzled at what caused 2" camber difference.
In most precast plants, steam curing is popular and I think concrete strength does not change much between 12 hours
 and 36 hours after concrete placement
 because like my company weekend concrete mix may be used and steam is not provided during weekend.
 
Here is my conclusion:
 we have to cut strands if prestressed members stay in the mold for extended hours in order to prevent strand failure and/or concrete spalling
 and excessive camber variance.
I did not know second part was so important.
 
Thanks!
 
PS: Jim G., I appreciate your remedial advice!
 
 

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Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 84 003 163 586)
5 Cameron Street Beenleigh Qld 4207 Australia
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