Crack Control for Rectangular Concrete Tanks
Crack Control for Rectangular Concrete Tanks
(OP)
I have been using the PCA Rectangular Concrete Tanks book to design some WWTP tanks, as well as, other tanks. I have developed some questions based on the procedure. I am looking for others experience to get a feel for the final design.
My concern is with the Crack Control req'd steel. I am required to maintain 3 5/8" cover on the steel when I compute the stress in the steel. This is due to the DeNeef Swellseal bead to protect the steel at the cold joint and make the tank water tight. The bead requires 3" of cover and the bead is to be 5/8" in order to maintain a waterproof joint. Because of this, my max spacing for the rebar to control cracking ends up being less than 4" (unreasonable). The only way I can see to fix this problem is to increase the size of my wall or reduce the cover on the steel. My tank is a small 12'L x 8'W x 12'H tank. I have been trying to maintain a 12" wall. It just seems excessive to increase the wall thickness to 1.5' just to increase the steel spacing.
The other problem I have is the buoyant force. The WWTP is located on the side of a hill, so I don't see how the soil could become saturated long enough to lift the tank, but the calculation shows that my FS is less than 1 and I have to increase the walls and slab to be huge to get the F.S. to even be 1. I don't see any other way to make the tank heavy enough to resist the buoyant force.
So, I guess my question, on this small of tank, do you go to the extent of installing helical piers or is there other justification for resisting the buoyant force? Also, is my crack control spacing out of control? I just need a feel for good practice to know whether I am not trying to make the tank too small, and should be looking at thicker walls, slabs, etc. Mass concrete or economy?
Thanks for your comment/sorry for the long explanation!
My concern is with the Crack Control req'd steel. I am required to maintain 3 5/8" cover on the steel when I compute the stress in the steel. This is due to the DeNeef Swellseal bead to protect the steel at the cold joint and make the tank water tight. The bead requires 3" of cover and the bead is to be 5/8" in order to maintain a waterproof joint. Because of this, my max spacing for the rebar to control cracking ends up being less than 4" (unreasonable). The only way I can see to fix this problem is to increase the size of my wall or reduce the cover on the steel. My tank is a small 12'L x 8'W x 12'H tank. I have been trying to maintain a 12" wall. It just seems excessive to increase the wall thickness to 1.5' just to increase the steel spacing.
The other problem I have is the buoyant force. The WWTP is located on the side of a hill, so I don't see how the soil could become saturated long enough to lift the tank, but the calculation shows that my FS is less than 1 and I have to increase the walls and slab to be huge to get the F.S. to even be 1. I don't see any other way to make the tank heavy enough to resist the buoyant force.
So, I guess my question, on this small of tank, do you go to the extent of installing helical piers or is there other justification for resisting the buoyant force? Also, is my crack control spacing out of control? I just need a feel for good practice to know whether I am not trying to make the tank too small, and should be looking at thicker walls, slabs, etc. Mass concrete or economy?
Thanks for your comment/sorry for the long explanation!






RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
Perhaps helical anchors could be used to resist buoyant forces? Cost is ~$1500 each.
Additional weight of concrete might end up being more economical though.
RE: Crack Control for Rectangular Concrete Tanks
For civilperson, I would like to put the swellseal bead in front of the steel to protect the steel at the cold joint at the bottom of the wall. However, maybe I am being over conservative. I am assuming that the wastewater from the tank would leak into the cold joint and corrode the steel away. Am I correct in my thinking?
RE: Crack Control for Rectangular Concrete Tanks
Yes, that was our practice (for water retention projects). I remember there was a paper discuss that, unfortunately, I don't have a copy on hand.
But one argument I remember well is that with all steel at the same stress (tension) level, so the concrete around the steel, then how could the extreme fiber of a concrete section with more than 2" clear cover experiences same level of, or higher stress (that causing concrete to crack) than a section with 2" cover only (note both cases have the same lever arm "d", the same load capacity, but different depth "h")? I hope you can see clearly the logic behind this argument.
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
For a section with constant d, As, constant bending moment, in the elastic range, which one would crack first - with 2" cover or 6"?
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
RE: Crack Control for Rectangular Concrete Tanks
thicker sections requires greater shrinkage steel. But shrinkage crack control is so dependent on site control designing to the required accuracy for 0.2mm crack widths seems a bit bizarre, to me anyway.
RE: Crack Control for Rectangular Concrete Tanks