environmental precast structures

[RMPE]

We've been seeing more precast box structures for small wastewater plant applications. Most are two or three piece stacked similar to manholes. Concern is very thin walls for buried application (2 3/4" is the smallest!) and wire mesh reinforcing. Design is by yield line analysis for two way panels and they "comply" with the ACI 318 "building code" only. Does Crack control requirements of 10.6.4 apply and would 10.6.5 not imply that ACI 350 should be used (as these tanks must be watertight)? In my opinion the 350 requirements should be adhered to, minimum 2" cover for all panels, compliance with reduced z values and use of 1.3 durability factor. Any opinion?

 

[ishvaaag]

I think these Sewage structures should be checked for sanitary structures maximum allowd crack width. However it soon appears that these things and sewage pipes have joints, used as excuse for noncompliance like here. Both joints and sewage ducts of any kind should meet the respective specifications for crack width and maximum loss of liquid.

 

[rethornton]

I agree that ACI 350 is a valid requirement for such structures but it should be specified in the contract documents, otherwise I don;t believe the precaster is obligated to apply that standard. 2" cover is not really necessary for precast elements, however due to the low w/c ratio and higher compressive strengths associated with precast. The additional bar cover makes it more difficult to acheive the lower "Z" values associated with ACI 350.