huangyhg

thick concrete slab crack control joint
the footing slab of the 40-ft underground concrete tank i am working on will need to withstand uplift pressure (almost 35'). it does act as a ballast. can i do 90' x60' x 12' thick slab in one pour with no joints assuming the contractor can do it..?
aci 224 talks about 24 to 36 times the slab thickness for control joints but does not address thick slabs beyond 10". any recommendations to reduce the heat during placement or just use ice..?
thank you.

that volume of concrete deserves a proper thermal analysis.
see aci 207.2 guideleines on mass concrete.
also, talk to your concrete supplier. there are many ways to adjust the concrete mix to reduce the heat during placement for thick concrete sections. use of fly ash to replace cement content is one.
the important thing is to limit the differential tempatures between different areas in a mass concrete structure. if the exterior surface(s) is(are) allowed to cool too quickly, the tempature gradient between a hot core and cool exterior surface can lead to proplems. consider use of insulated forms and insulation at any exposed surface to allow the placement to cool uniformly.
jmho.
concrete that doesn't shrink? where do you find that.....? yes you will need control joints.
that's ridiculous - you don't put control joints in a 12 ft. thick mat of concrete.
it isn't a "slab-on-grade". it is a foundation mat.
jae - if it was a large foundation mat (supporting columns and walls) that was not the base of a tank, would you still say no joints?
agree with jae--a foundation mat should never have control joints, because if it did, it would not be able to resist bending and shear.
daveatkins
thanks to all. nobody said anything about "none shrinking concrete". i agree with dave & jae. the other problem with putting any joints is that the joints have to continue vertically in the walls. you cannot put joints on slab and not in the walls. then, the job will cost more to form. the idea is to do one slab pour and do horizontal skirt pours 4 to 5 feet high. horizontal joints do not need forms. soil freezing method will be used for construction. the slab will be 40 feet down (temp is almost controled).
agree with jae. if shrinkage stresses are the concern add reinforcement to resist. the only reason to put a joint in a mat foundation is to limit the pour for the contractor.
i would expect to see joints in your walls along the 90' section.
although the temperature of the ground may be somewhat controlled at 40 feet deep, the temperature of the concrete will not be controlled at that depth unless specific measures are taken to control it.
lkjh345 and daveminter are correct with regards to concrete temperature. a thermal analysis should be done as daveminter pointed out. significant thermal stresses can develop resulting in cracks which affect the structural capacity of the mat. thermal cracking is one of the primary concerns for placing mass concrete and should not be taken lightly.
the soil freezing method you mention will further complicate this issue. i'm assuming lin will be used to freeze the ground. with lin temps around -385 degrees f, a significant temperature gradient will occur between the interior and exterior of the mat.

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