Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Joe writes:
It appears that the typical transition in an eccentric riser for a manhole is from 48-inch ID to 24-inch ID. Can a larger ID (say 26 inches) be provided at the top without any cost increase? Also, what is the typical height of a 48-inch ID to a 24-inch ID eccentric riser?
NPCA Technical Services engineers answered:
You are correct that most manhole transition sections are 48-inches-by-24-inches, either using a cone reducer or flat-top reducer. Cone sections can be concentric, but the majority are eccentric to facilitate a straight, uniform line of steps and easier ingress and egress. The 24-inch opening seems to be a size the sewer industry has settled upon for decades. Consequently, many of the casting frames specified are also for a 24-inch opening size.
The answer to your cost question is “possibly.” However, some increase in expense to the precaster will likely be passed on to the purchaser. There has been a small trend to increase the 24-inch opening, perhaps because attempting to enter a manhole and casting with the necessary personal protection equipment required today necessitates a larger opening than the traditional 24 inches.
For a flat-top reducer, the added expense of a 26-inch or 28-inch diameter opening would be minimal as it is simply a matter of placing larger diameter blockouts in the flat form. A cone reducer is more complicated. A lot of the added cost would be related to the type of forms the precaster is using to pour its cone reducers. Revising the inside diameter of the opening would require a modification to the form core and jacket.
If this revision to a larger opening will be a permanent new standard, the precaster may wish to purchase a new form fabricated specifically for the new geometry and also keep the existing 24-inch-diameter opening form for those jurisdictions within its market still using the 24-inch-diameter opening.
The answer regarding typical heights for these items, unfortunately, is not straightforward. They too may vary between different precast companies depending on the forms they own. It is best to check with local manufacturers to determine what the available form heights are.
Rahul writes:
Is there a strength benefit to using one type of spacer over another? Or using spacers made out of one material versus another (concrete, plastic, metal)?
NPCA Technical Services engineers answered:
Spacers are not intended to provide any sort of compressive strength or structural integrity to the product. Spacers are used to maintain appropriate reinforcement positioning within the formwork before, during and after concrete placement, maintain the designed concrete cover over the reinforcement, and help maintain the rigidity of the reinforcement assembly.
One thing to be mindful of when using any spacer is the porosity of the microstructure at the surface of the spacer. Any time you introduce a foreign object into fresh concrete – whether it’s a spacer, anchor, reinforcing or aggregates – the microstructure of the paste at the interface of the concrete and the foreign object can be somewhat compromised with higher degrees of porosity. This can result in a preferential path for aggressive materials to get into and move through the concrete. This is why it’s important to make sure the spacers are free of dirt, oils and especially form release. Also, an increase in surface roughness of the spacer material can slow down any migration along its surface. The bonding capacity of the spacer to the concrete will depend less on the type of material and more on the geometry, much like deformed rebar bonds better with concrete than smooth rebar.
If the structure is expected to be exposed to an aggressive environment or considerable temperature variations, then spacer type and application should be analyzed much more closely.
Leave a Reply