Good Questions Gumpmaster,
And also, good information from JedClampett. As someone who has designed hundreds of wire-wrapped prestressed concrete tanks (both Type II and Type III), a former owner of a prestressed concrete tank firm in the southeastern US and a current owner of a nationwide prestressed concrete tank firm, this one is right down my alley.
In the interest of full disclosure, I'm a proponent of any wire-wrapped tank with a steel shell diaphragm (Type II, III and IV). The steel shell diaphragm provides a positive waterstop in the wall which guarantees watertight performance assuming construction is performed properly. Yes, Type I does contain vertical prestressing which should help to produce a more watertight tank structure but there is no positive waterstop like the steel shell diaphragm. I've seen it work fantastically thousands of times over my career.
To start off with, despite what most industry suppliers will tell you, structurally, all these tanks are the same. They are all designed using the same equations for plates and shells by Timoshenko, equations for design of seismic developed by GW Housner in the 1950’s and typical reinforced concrete design. Yes, the diaphragm location is different and requires slightly different design methodology as it is typically used for temperature and shrinkage steel on either the inside or outside face depending on Type but as any structural engineer knows, that is only semantics.
The real difference is in means and methods of construction and thus diaphragm location. Type II tanks are constructed of shotcrete and the diaphragm is located on the inside face of the wall with a minimum of 1" of cover. Shotcrete is a cement rich grout mix comprised of fine aggregate, water and cement. The cement content is very high in the mix (1:3 against the diaphragm) thus producing excellent corrosion resistance and long term durability. Any problem with a Type II tank comes from construction quality (just like all other structures). When shotcrete is applied in accordance with the guidelines of ACI 506 by an experienced nozzelman the result is excellent encasement of the diaphragm and long term durability. When it is not, hollows can develop between the shotcrete and diaphragm with the result being corrosion issues and poor durability. That said, when good quality control measures are followed, the result will always be an excellent tank structure. I have personally performed destructive inspections many Type II tanks that have been in service for decades and have found shiny steel diaphragm on the tank when good quality shotcrete has been applied. I have also seen the opposite in very, very limited cases. The problem can be exacerbated in cold climates where ice lensing occurs and has served to give Type II a bad reputation in some instances where the real blame was due to poor quality.
Type III tanks are constructed by casting the corewall in a tilt-up panel. You should know that is primarily done for ease and economy of construction as these tanks were first constructed in areas with short construction seasons and high labor rates. This construction technique has the added benefit of locating the steel shell diaphragm behind a minimum of 4” of cast concrete. You might say, problem solved; however, these tank Types have other details that require excellent attention to quality for proper construction and particular for watertightness. With good quality construction techniques, these are also great structures and should provide many years of durable service. Again, I have also personally seen the opposite in limited cases where construction quality was poor.
I could go on and on, but, in short, either tank Type will work great for most purposes if you have experienced personnel designing and constructing the product. Quality control is the key on these projects, yes, just like all other field constructed products. Historically, industry suppliers have long tried to fool good engineers into specifying one Type of tank over the other solely in the interest of reducing competition. Unfortunately, up until recently, no one has come along to challenge this notion. For example, why would a Type II tank be an excellent choice in Sunny Florida and not in Sunny Texas or Sunny California? Conversely, why could a Type III tank not be constructed in any of those three locations as well with excellent durability? Hum... let me try to think of a good reason.
Be wary of other tricks of the industry as well such as unachievable experience requirements to “qualify” for construction.
K. Ryan Harvey, PE
Vice President - PCT Division
Wire-Wrapped Prestressed Concrete Tanks
Caldwell Tanks, Inc.
