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For years precast concrete manholes have been the default choice in sanitary and storm water applications.  Over the years, though, Packaged Fiberglass Manholes have begun to displace concrete manholes for a number of reasons.

Fiberglass manholes are:

  • Light weight (1/10th the weight of concrete)
  • Corrosion-resistant
  • Single, piece
  • Watertight
  • Able to integrate a wide variety of shapes and structures
  • Extremely robust and possessing long service lives

Fabricating Fiberglass Manholes

Fiberglass manholes are typically fabricated as a manhole barrel with a top (domed fiberglass, aluminum hatch, or fiberglass H-20 highway loading reducer) into which a structure is molded.  Integrated structures include:  flumes, weirs, grinder channels, diversion / bypass channels, storm water separation units, and energy absorbing baffles

A key advantage that fiberglass manhole have over concrete ones is that the structure to be integrated does not have to fit inside the footprint of the manhole barrel.  Portions of the structure and stick out of the manhole and extend upstream / downstream as needed. 

During the fabrication process, these structures are provided with fiberglass covers, which are laminated to the structure and then back to the fiberglass manhole barrel – forming a single watertight, corrosion resistant structure that is not susceptible to infiltration or seal degradation.  With no seams between either manhole sections or the manhole barrel and the integrated structure, penetration and damage by tree roots is virtually eliminated!

Monolithic vs. Segmented Construction

Concrete manholes come in segments for field assembly.  These segments require watertight seal materials to be applied in the field at the base and between each successive manhole sections.  All of this takes time and introduces the chance of seal failure. 

In the corrosion environment found in sanitary sewers, even initially well-sealed concrete manholes will fail over time.  The failure of a concrete manhole near a body of water (stream, creek, etc.) can lead to wide ranging environmental damage.

Fiberglass manholes are almost always monolithic in nature – although they may be fabricated from a variety of shapes and sections, by the time they leave the factory they are single-piece, watertight units.  From base to top, there are not seams or sections to leak or degrade over time.

Materials Handling

Fiberglass manholes are typically 1/10th the weight of similarly sized concrete units.  This lower weight directly translates into fewer personnel required to install at unit and smaller / less costly equipment to offload and move the manhole. 

Installation Time

Since fiberglass manholes typically come as single-piece, monolithic units, installation is a breeze…requiring little more than digging a hole, dropping the manhole in, and backfilling.

A Visual Guide to Installing a Fiberglass Manhole shows how quickly and straightforward an installation can be. 

fiberglass metering manhole on a concrete mounting slab

Once the installation site has been excavated and the sewer line exposed a base-mounting pad is poured.  Flow through the line is temporarily stopped or diverted and the pipe is cut.  The fiberglass manhole is moved into position and flexible coupling are used to secure the fiberglass manhole to the sewer line.  Even before the manhole is attached to the mounting pad, flow can resume once the flexible couplings are in place.

Alternately, a precast slab can be attached to the base of the fiberglass manhole and the whole unit transported to the job site, making an already quick installation even quicker!

For industrial plants, this limits installation down time to a bare minimum!  Perfect for weekend or holiday plant shutdowns!

As a monolithic unit, there are no segments to seal or coat for water tightness and with off-site attachment of a precast slab, there is not even a requirement to pour concrete on the job site.  For jails and prisons, the is reduction in personnel and equipment to clear through security alone can be worth the cost!

Corrosion Resistance

Fiberglass manholes are unmatched in resistance to the corrosive effects of sanitary sewage.  Where concrete is susceptible to biogenic / Microbial Induced Concrete Corrosion (MICC) as hydrogen sulfide is converted into sulfuric acid by microbes, which then in turn converts the calcium hydroxide in concrete into calcium sulfate – destroying the polymeric nature of calcium hydroxide in concrete and introducing a large molecule into the matrix.

corroded concrete manhole housing a Parshall flume monitoring bakery effluent

The resin rich interior surface of a fiberglass manhole acts as another barrier to corrosion, in addition to what the manhole barrel itself already posses. 

Biogenic / MICC is a primary reason why reliners (usually fiberglass when installed as a rigid unit) and spray coatings have been developed over the years for concrete manholes.  Skip the need to reline in the future with fiberglass and install fiberglass in the first place!

Fiberglass does not suffer from MICC and other forms of corrosion in normal sewer environments and as such fiberglass manholes have much longer service lives that concrete manholes.  Many of the fiberglass manholes originally installed over thirty years ago by Owens Corning are still in operation through the country.

Cost

While fiberglass manholes exceed against concrete manholes in almost every respect, they do cost more to buy. As we have seen, though, when installation costs for fiberglass manholes are lower and maintenance is virtually eliminated. 

Combine these with the watertight, monolithic nature of fiberglass and the ability to integrate a wide variety of structures and sizes into a fiberglass manhole and the choice is clear:  fiberglass wins hand down!

 

learn more about Fiberglass manholes

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