QGPC
Left: inside a QGPC manhole base. Top right: the polymer concrete itself, stone aggregate held in polyester resin, with no cement. Bottom right: the finished base. QGPC photos.
- In sewers, bacteria turn hydrogen sulfide gas into sulfuric acid on the manhole wall.
- That acid attacks cement. Research reports concrete losses of 1–5 mm a year.
- Published modeling for a sewer system in Qatar anticipated hydrogen sulfide of up to 900 ppm.
- In an independent acid test, Portland cement concrete lost roughly 20% of its mass; polymer concrete's loss was insignificant.
- QGPC data: 0% corrosion in 20% sulfuric acid, against 99% for reinforced concrete.
- QGPC manholes contain no cement and no steel reinforcement, and need no coating or liner.
A manhole spends its whole life in the one place a city would rather not look. Below the cover, the air can carry hydrogen sulfide, and the walls are often damp.
On the wall above the water line, bacteria quietly turn that gas into sulfuric acid, and the acid goes looking for cement. For ordinary concrete, that is the start of a slow loss of material. For a manhole made without cement, the acid finds no cement to attack.
What happens inside a sewer manhole?
Concrete corrosion in sewers is driven by microbes. Researchers describe it as a three-stage process1; Diagram 1 shows it as four steps. The first two happen in any sewer. The last two describe what the acid does to cement.
- 1Gas forms. In sewage with little or no oxygen, sulfate-reducing bacteria produce sulfide. Hydrogen sulfide (H₂S) gas escapes into the air, especially where the flow is turbulent, such as at drop manholes.1,2
- 2Acid forms. On the damp wall above the flow, sulfur-oxidizing bacteria turn H₂S into sulfuric acid.1
- 3The pH falls. On concrete, the surface pH drops from about 12–13 to 9–10, then 4–5, and finally to about 2.1
- 4The cement fails. The acid turns cement into gypsum, then ettringite, which expands, cracks the concrete from within and strips it away layer by layer.1
- ✓No cement to attack. In QGPC polymer concrete the binder is polyester resin, not cement, so step 4 has no cement to work on.
Research reports corrosion rates of about 1–5 mm a year1. In one field exposure, Portland cement mortar samples placed in a heavily corroded sewer pipe corroded at 5.7 mm a year1.
Why does heat matter?
Ontario's sewer design guidelines (Canada) are direct:
“Hydrogen sulphide emissions are increased at lower pH and higher temperature conditions.”2
Sulfide production also depends on temperature2. Laboratory studies of this corrosion speed it up by giving the bacteria favorable conditions, including warmth1.
The same guidelines draw the practical conclusion for manholes: where corrosive conditions are anticipated, “corrosion protection on the interior of the manholes should be provided”2. The guidelines do not prescribe the form. In practice it is either a coating or liner added to concrete, or a manhole material that resists the acid itself.
What do engineers in Qatar plan for?
A paper presented at Qatar University's CIC 2023 conference gives two figures3:
- Modeling for a sewer conveyance system in Qatar anticipated up to 900 ppm (by volume) of hydrogen sulfide in the air. Short-term peaks were modeled at up to 1,700 ppm.
- At the raw-sewage handling units of a treatment works in Qatar, the paper reports the highest average levels of 100–500 ppm (by volume).
These are the conditions engineers in Qatar design for, and they are why the material of a manhole matters.
What does an independent test show?
The US Army Engineer Research and Development Center (ERDC) exposed Portland cement concrete and polymer concrete to a film of about 5–10% sulfuric acid4. Exposure ran nine hours a day, five days a week, for four weeks.
The Portland cement concrete lost roughly 20% of its mass. The report describes the mass loss of the polymer concrete samples as “insignificant”.a
This was a severe, accelerated laboratory test on small samples (two of polymer concrete, one of Portland cement concrete), not a field measurement, and not a test of a QGPC product. Its value is the comparison: the same acid, the same time, two very different results. Across its acid tests, the report concludes that “polymer concrete will resist biogenic sulfide corrosion much better than regular Portland cement concrete”4.
QGPC's own data show the same pattern, from a different test with its own method and measure: in 20% sulfuric acid, QGPC polymer resin concrete showed 0% corrosion, against 99% for reinforced concrete.
Why does polymer concrete resist the acid?
The answer is in what the material is made of. A QGPC manhole is made of silica, aggregate and polyester resin. The binder is resin, not cement. The cement compounds that sulfuric acid turns into gypsum and ettringite are not there, so step 4 has no cement to work on. There is no steel reinforcement either.
That is why a QGPC manhole needs no coating and no liner. Its resistance to acid and corrosion is in the material itself, not in a layer applied on top.
What does the standard cover?
QGPC manholes are made to BS EN 14636-2, the European standard for polymer concrete manholes and inspection chambers. Beyond strength and dimensions, the standard itself covers the leak-tightness of a manhole and its joints (Annex F), and includes a test of long-term crushing strength, including the effects of media attack, with a 50-year evaluation point (Annex G)5.
What does it mean on a project?
Warmer sewers release more hydrogen sulfide, the gas that starts the attack on cement. A QGPC manhole has no cement for that attack, and no coating to maintain.
| Binder | Polyester resin, no cement |
|---|---|
| Steel reinforcement | None |
| Coating or liner | Not needed, so no recoating or relining later |
| Expected service life | More than 100 yearsb |
| Single piece | Up to 3 m tall |
| Approved in | Qatar, Saudi Arabia, Oman, the UAE, Jordan and Kuwait |
| Made in | Qatar, since 2011 |
Common questions
Is QGPC polymer concrete strong enough?
According to QGPC data, its flexural and tensile strengths are three to four times those of ordinary concrete, and its compressive strength is about three times. It reaches 80 MPa in 24 hours, and third-party testing has confirmed a compressive strength of 126 MPa.
Does a QGPC manhole need a coating or liner in a corrosive sewer?
No. The resistance comes from the material, so there is no coating to apply on site and no liner to renew.
Notes
- In the ERDC test, both polymer concrete samples lost less than 0.03% of their mass (ERDC/CERL TR-19-4, Table 3).
- Based on third-party long-term durability testing (QGPC data).
References
- Chaudhari, B., Panda, B., Šavija, B., & Paul, S. C. (2022). Microbiologically Induced Concrete Corrosion: A Concise Review of Assessment Methods, Effects, and Corrosion-Resistant Coating Materials. Materials, 15(12), 4279. pmc.ncbi.nlm.nih.gov/articles/PMC9228145
- Ontario Ministry of the Environment (Canada). Design Guidelines for Sewage Works, Odour Control and Design of Sewers (sections 4.2, 4.3.2.4, 5.6 and 5.9.9). ontario.ca
- Trikis, S., Sumant, V., Arshad, M., Olliver, A., Abushereeda, M. J., & Brown, J. (2023). Implementation of Odour Control Systems for Nuisance-free and Public Friendly Environment in Qatar. International Conference on Civil Infrastructure and Construction (CIC 2023), Qatar University, 1324–1332. doi:10.29117/cic.2023.0164. journals.qu.edu.qa
- Wilson, C. A., Edwards, J. S., Lamkin, S. L., Delgado-Connor, A., & Simonton, P. C. (2019, February). Demonstration and Validation of Polymer Concrete Piping for Corrosive Environments: Final Report on Project F14-AR07. ERDC/CERL TR-19-4, US Army Engineer Research and Development Center. apps.dtic.mil
- EN 14636-2:2009 (published in the UK as BS EN 14636-2): Plastics piping systems for non-pressure drainage and sewerage — Polyester resin concrete (PRC) — Part 2: Manholes and inspection chambers. Table of contents of DIN EN 14636-2:2010-04. din.de
Specifying manholes for a sewer network? See the QGPC manhole range, or ask the team for technical data at [email protected].
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