Structural pipe lining is required when a host pipe has deteriorated so far that it can no longer carry its own loads, so the liner must carry them instead. Engineers call this fully structural, or Class IV, rehabilitation: the liner is designed as a standalone pipe that resists all internal and external loads on its own. Insituform, the inventor of CIPP and a worldwide manufacturer of CIPP liner systems, produces several liners built to that structural class. Getting the class right is the whole game when specifying trenchless pipe rehabilitation on a critical asset.
What Class IV Rehabilitation Actually Means
Rehabilitation sits on a spectrum, and Class IV is the fully structural end of it. At this class, the liner is designed to carry every internal and external load on its own, as though the host pipe was no longer there.
That is a different design basis from semi-structural rehabilitation, where the host is assumed to keep contributing strength for the life of the repair. The dividing line is set by the condition of the host and the design standard applied to it. For full structural work, specifying engineers design to the fully deteriorated criteria in ASTM F1216, which sizes the liner to stand alone under soil, groundwater and live loads.
When a Pipe Condition Requires Class IV Rehabilitation
Class IV is a design decision driven by condition data, and a CCTV survey is where that data comes from. Certain findings point firmly towards fully structural rehabilitation:
- extensive circumferential cracking or fractured pipe
- a missing crown or invert
- heavy corrosion loss in metal pipes
- cracked concrete with visible reinforcement
- ovalised or deformed sections
None of these is a default trigger on its own. The right structural class comes from reading the condition data as a whole. Underspecify at this stage and the liner can fail early. Overspecify and the programme carries wall thickness and cost it never needed. Precise condition assessment is what keeps the design honest.
Why Fibre-Reinforced and Composite Systems Matter at This Structural Class
Standard felt CIPP performs well across a huge range of work, but as structural demand climbs, and especially where large diameter meets non-circular geometry, wall thickness alone starts to work against you. A thicker felt liner adds strength, yet it also eats into the bore and the flow the asset was built to carry.
Fibre reinforcement changes that balance. iPlus Composite uses a sandwich laminate structure, so load is distributed across the reinforced wall rather than carried by the resin alone. That lets a fully structural liner reach the required strength at about half the wall thickness of conventional CIPP, which protects flow capacity in the finished pipe. Getting that laminate to perform depends on clean, controlled resin impregnation during manufacture, which is one reason buying from the manufacturer matters at this class.
Which Insituform Products Deliver Fully Structural Performance
Three products in the Insituform range are specified for fully structural work, each suited to a different set of conditions.
iPlus Composite is the fibre-reinforced structural workhorse. It covers 600mm to 2,400mm, handles a fully deteriorated host and non-circular sections, and is where many engineers land for large or awkward gravity assets. It is also the product behind the shift to composite systems for tough pipe repairs that felt alone would struggle with.
InsituMain Pressure takes fully structural CIPP into force mains and water pipelines. It runs from 200mm to 1,200mm, carries an internal pressure rating of 250 psi and above, and its mechanical properties exceed ASTM F1216 and ASTM F1743, which matters when the liner has to stand up to pressure as well as ground loads.
Insituform CIPP Felt supports fully structural design too, at the appropriate wall thickness, and covers the widest diameter range at 70mm to 2,500mm across any host material. Every one of these liners is manufactured in Wellingborough under ISO 9001, with 28 quality control checks for every single CIPP liner.
What to Specify When Writing Class IV into a Rehabilitation Programme
When Class IV goes into a specification, three requirements protect both the asset owner and the design engineer.
Ask for evidence of independent testing at the exact diameter and structural class you are specifying, so the published numbers apply directly to the pipe in front of you. Require wall thickness designed to the fully deteriorated criteria in ASTM F1216, so the liner is sized to stand alone. And call for ISO 9001 certified manufacturing, so the product that arrives on site matches the one that was tested. Written in plainly, those three points hold every shortlisted supplier to the same standard.
Specifying Structural CIPP With Confidence
Class IV is the point where product choice, manufacturing quality and design assumptions all have to line up. The manufacturer that can answer for all three at the diameter and structural class in front of you is the one worth writing into the specification.
For fully structural work, Insituform manufactures and supplies iPlus Composite and its wider structural range worldwide, each backed by 28 quality control checks for every single CIPP liner. To pressure-test a specific Class IV design with our technical team, get in touch.
