I believe “DR 26” and “DR 35” are really two somewhat different sort of animals, produced to different ASTM specifications. I suspect the DR 26 you are talking about is probably meant to refer to pvc pipe per ASTM D2241, and the DR 35 is e.g. per ASTM D3034 standard. The former is alleged to be pressure rated for water (many years ago quite commonly applied as “rural water pressure pipe”), whereas the latter
for “sewer”. To understand at least minimally what one is getting in the way of pvc, it would probably be advisable to at least glance at both of these standards. I believe D2241 requires compounding/manufacture etc. that is intended to result in some level of long-term or sustained pressure containment etc. strength, whereas D3034 pipe on the other hand has been compounded primarily for minimal short-term pipe (ring crushing) stiffness.
I guess some might argue one unplasticized pvc pipe looks basically like another unplasticized pvc pipe. However, the ASTM standards for pvc pipes in general do not put many limits on the amount of “fillers” or additives that can or cannot be incorporated in the pipes (other than what is necessary to meet the minimal physical requirements of the
standards). In practice some spot analyses our laboratories have performed over the years have indicated at least some pvc manufacturers are more likely to put a whole lot more of cheapening fillers e.g. calcium carbonate (like limestone or “talc”) into the walls of the pvc solid and profile-walled gravity pipes, and often at least some less in the pressure-rated pipes. While such fillers can in fact enhance short-term ring stiffness while at the same time minimizing manufacturing cost/(maximizing at least price competitiveness or at least short-term profit?), some references indicate that can be at the expense of some other, and maybe particularly long-term properties. See the discussion also on filler loading of pvc pipe walls under “Importance of Volume Cost to the Plastics formulator” at the site
To attempt to “put a number” on inferences others have made on this thread related to buried performance, I guess one can readily look at least at the theoretical, relative short-term pipe stiffnesses (PS) of pipes. Per ASTM plastic pipe standards,
PS = (E*I)/(.149*r^3)
Therefore, the short-term, calculated “pipe stiffness” (PS, sometimes used or considered in evaluations of installation, bedding, external loading, buckling/collapse scenarios etc.) e.g. of a 8” nominal size DR35 upvc pipe per ASTM D3034, with incidentally 8.40`” OD and 0.24” wall thickness, is in effect:
~ (400,000 lb/in^2) (0.24 in.)^3/12/((.149) (8.4-0.24 in)/2)^3) = 46 lb/in^2
In some contrast, PS of a 8” nominal size DR26 upvc pipe per ASTM D2241 with 8.625” OD and 0.332” wall thickness may be in effect:
~ (400,000 lb/in^2) (0.332 in.)^3/12/((.149) (8.625-0.332 in)/2)^3) = 115 lb/in^2, or this DR26 pvc pipe is about two and a half times as stiff as the DR35 pvc pipe.
Now as to 8” Special Thickness Class 52 ductile iron pipe (DIP) also mentioned in the OP, if one were to calculate the theoretical pipe stiffness in similar fashion,
PSDIP ~ (24,000,000 lb/in^2)(0.33 in.)^3/12/((.149)( 9.05-0.33 in)/2)^3) ~ 5,820 lb/in^2, or this class DIP in this size is more than 50 times as stiff than as the stiffer DR26 upvc pipe. It may not be hard to figure why this Authority prefers ductile iron pipe for shallow/potentially heavy-trafficked? sewer, and why some others prefer it also for quite deep and often nasty sewer constructions and
inexorable/unrelenting heavy earth loads there as well (where
differences in pipe cost are also often relatively quite insignificant in terms of total installed/constructed project cost). While I guess time will tell if the less stiff pvc is in the long-term adequate for what lies between, I have noticed “ageing” (perhaps non-obvious to some) has been reported in some European pvc sewers (e.g. see Table 2 at
), that
generally have at least a little more age on them than pvc sewers in the New World.
Everyone have a good weekend!
