huangyhg

chilling experience
okay fellow engineers. after 20 years in the business and thinking that i have seen it all, guess what? i have not.
a client called me to investigate a 18 kip liquid nitrogen concrete pad. they accidentally spilled ln2 on the footing and it is cracking big time in one corner. the boiling temperature of ln2 is -273 degrees f.
i have few questions:
1.how does the concrete react to the temperature shock?
2.how do the rebars react to the temperature shock?
3.how deep and far in plan are the thermal effects on the concrete?
4.should the pad be repaired or should it be replaced?
here is what i am planning on what to do and please let me know your thoughts:
1.take at least three core samples (astm c-42) and test for compressive strength
2.if the samples meet the original strength (3000 psi) it should be okay.
3.if not, perform petro-graphical analysis and see how bad the micro cracks are and how deep they are.
4.if strength is acceptable, repair the cracks in the footing. if strength is bad and the cracks are too many, then replace the entire pad.
your input is most welcome.
lutfi
this almost sounds like an urban legend.
a small ln2 spill on concrete would immediately boil and run off the pad. the only way the pad could be seriously cracked would be by total immersion in ln2.
was it a small spill or a complete loss of ln2 from the tank?
i would guess that it's more likely that someone dropped a filled, portable ln2 tank on the corner and cracked it simply from the mechanical shock damage.
ttfn
lutfi...concrete is a pretty good insulator and its permeability is such that ln2 would not enter as a liquid..it would dissipate first. therefore the effects should be rather superficial unless the "spill" was very large and immersed the concrete as irstuff stated.
your investigative approach is correct.
would it be cheaper to just replace the damaged section, instead of all the coring and testing? just playing devils advocate.
the aci has a publication, aci 350.2r-97 entitled,"concrete structures for containment of hazardous materials". it presents recommendations for structural design, materials, and construction of structures commonly used for hazardous materials containment. perhaps this would be of use to you.
i want to thank the gang for their insightful responses. the amount on ln2 is unknown, however, the concrete pad developed large cracks (like 0.375 inch and larger). i am not sure if it was immersed or not. i will find out during my interview with the involved players. i am about to commence the investigation and report next week.
thanks everyone for your input.
lutfi
the cracks are simply due to the thermal effects of a cold surface and a warm interior. fill the cracks with epoxy (injection ports, etc.) and move to the next job. a one time event like this won't cause much problems. if it is a cyclic event, then the concrete will eventually fall apart.
just a thought here but wouldn't you want to have the necessary testing data done in one fell swoop? you may not want to wait to assess the crack propagation. concerns that must be addressed run from strength to durability.
for everones information.... we tested the concrete and the stength was so detrioated. it was in the order of few hundred psi. i can crush it with my bare hand and the core broke within the footing due to lack of strength.
needless to say, the footing will be repalced.
tahnks everyone for yoyur help.
lutfi

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