What is a Blistering of Liner

Blistering of liner is a defect in pipe rehabilitation where raised bubbles, bulges or dome-shaped areas develop on the surface of a cured lining. These irregularities can form when air, moisture, gases or other materials become trapped during installation or curing. Depending on their size, location and internal structure, blisters may affect the liner’s surface finish, hydraulic performance, watertightness or structural integrity.

Blistering is associated with certain cured-in-place pipe (CIPP) systems, resin-based coatings and other rehabilitation materials. In CIPP installations, a blister may indicate a local irregularity within the cured composite, while in a bonded coating it may result from separation between the coating and the original pipe surface. Although the defects can appear similar during CCTV inspection, their causes and consequences are not necessarily the same.

A small surface blister does not automatically mean that the entire lining has failed. However, pronounced bulges, repeated blistering or defects accompanied by cracking may require further investigation. The assessment must consider the lining technology, the position of the defect and whether the affected area still meets the intended performance requirements.

How Blisters Form During Pipe Lining and Curing

Blisters can develop at different stages of the rehabilitation process. Some originate during resin impregnation or liner installation, while others become apparent as the material cures and changes from a flexible or liquid state into a hardened structure. The underlying mechanism depends on the lining material, installation pressure, curing process and condition of the host pipe.

In a typical CIPP installation, a flexible liner containing a thermosetting resin is inserted into the existing pipe. The liner is expanded against the internal pipe wall using an appropriate installation method, and the resin is cured through a specified process. Depending on the system, curing may involve hot water, steam, ultraviolet light or another approved technique.

During impregnation, air can become trapped within the resin-saturated material if the process does not adequately remove it. If these pockets remain during curing, they may produce internal voids or localised defects. Their appearance depends on the liner construction, resin distribution and position of the trapped air.

Moisture presents a different problem. Some resin systems are sensitive to unwanted water during application or curing, while elevated temperatures can affect moisture trapped within the lining arrangement. Under certain conditions, vapour formation or expansion may contribute to localised bubbles or internal voids.

Blister-like defects can also result from irregular contact between the liner and host pipe. If the liner bridges a depression or encounters a local obstruction, the finished surface may develop an irregular profile. However, these geometric defects should not automatically be classified as blisters because their formation mechanism may be different.

Potential causes include:

• Trapped air during resin impregnation, leaving voids or localised areas of incomplete saturation.

• Unwanted moisture entering the lining system before or during curing.

• Incorrect resin preparation or handling that affects curing behaviour.

• Inadequate liner expansion or localised contact with the host pipe.

• Temperature conditions outside the specified curing requirements.

• Groundwater infiltration through defective joints or cracks during installation.

• Contamination remaining on the substrate where a bonded lining or coating is used.

Not every bubble visible after curing results from the same cause. For example, a raised area formed by trapped gas within a composite is different from a coating blister caused by pressure acting behind an adhered layer. Establishing the actual mechanism is important because it determines whether local repair is possible and whether the surrounding lining may also be affected.

Blisters, Wrinkles, Delamination and Other Liner Defects

Blistering is sometimes confused with other irregularities found in rehabilitated drainage pipes. CCTV footage can show a raised feature without revealing whether it contains trapped air, represents folded material or results from separation between layers. Correct identification is essential because different defects have different implications for the liner.

A blister generally describes a localised raised or bubble-like feature. A wrinkle is a fold or ridge in the lining material, often resulting from the way a flexible liner conforms to the existing pipe geometry during installation. Delamination describes separation between layers of a composite or between materials that are intended to remain bonded.

These distinctions are particularly relevant to CIPP systems, which may contain multiple material layers, protective films and resin-impregnated reinforcement. An irregularity involving an internal film may have different consequences from a defect within the structural composite itself. The role of each layer must be understood before determining whether the observed condition compromises the lining.

A raised feature may involve more than one defect category. For example, a wrinkle may contain a void, while a blistered coating may also show adhesion failure. The visible shape alone does not establish the internal condition.

The distinction between blistering and adhesion failure is especially important. A bonded coating can blister when water or other pressure acts between the coating and substrate. A structural CIPP liner, however, may not depend on continuous adhesion to the host pipe, so an interface gap does not necessarily indicate a failure of its structural function.

Wrinkles also require separate consideration. A longitudinal wrinkle may create a persistent internal ridge, whereas a localised blister may form a smaller obstruction. Both can affect the internal profile, but their structural significance must be assessed according to the liner design and the extent of the defect.

Effects of Blistering on Drainage Performance

The consequences of liner blistering depend on the geometry and material condition of the affected area. A shallow surface irregularity may have little measurable influence on flow, while a large inward-projecting blister can reduce the effective cross-sectional area. In a gravity sewer, this can create a local obstruction that affects the movement of wastewater and suspended solids.

The position of the blister is important. A defect located near the pipe invert may interfere with the transport of sediment and other material moving along the bottom of the drain. A similar feature near the crown may have less hydraulic impact during normal partially filled flow, although it can still affect structural performance.

For a circular pipe flowing full, cross-sectional area is proportional to the square of its internal diameter. A reduction in diameter therefore has a greater effect on area than the same percentage change might suggest. However, the hydraulic effect of an isolated blister cannot be calculated accurately from its maximum projection alone because its length, shape and location also matter.

Blistering can affect wastewater movement in several ways:

1. A raised surface can reduce the local area available for flow.

2. An irregular projection may catch wipes, fibrous material or other debris.

3. A damaged blister may create rough edges that encourage further accumulation.

4. Cracking or rupture can expose underlying lining material to wastewater.

5. Internal voids may compromise local structural properties if they affect load-bearing layers.

A blister can also interfere with future drainage work. CCTV cameras, cleaning tools and other equipment may encounter restrictions where the liner projects into the pipe. Mechanical contact with a fragile raised area could damage the material further.

Structural consequences require particular care. A cured liner is intended to satisfy specified mechanical and geometric requirements, and an internal void may affect the distribution of stresses within the composite. The significance depends on the location, dimensions and relationship of the void to the structural layers.

A surface defect is not necessarily evidence that the full wall thickness is compromised. Conversely, a relatively small visible imperfection may be associated with a larger internal defect that cannot be determined from CCTV images alone. Where structural performance is uncertain, additional examination or testing may be required.

Watertightness is another consideration. An intact blister may not create a direct leakage path, but cracking or associated material separation could undermine the sealing function of some rehabilitation systems. The condition of joints and lateral connections should also be considered because leakage may originate from these locations rather than the blister itself.

Identifying Blistering and Assessing Its Severity

CCTV inspection is commonly used to identify visible liner irregularities after rehabilitation. A camera can document raised surfaces, changes in profile, cracks and areas where material appears detached or distorted. Inspection immediately after installation also provides a baseline for distinguishing construction defects from deterioration that develops later.

The appearance of a blister can vary with camera position and lighting. A smooth raised area may be difficult to distinguish from a wrinkle, particularly when the pipe is wet or partially submerged. Shadows can exaggerate the apparent size of a defect, while reflections from glossy liner surfaces may conceal its boundaries.

A reliable assessment should establish the defect’s position along the pipe, its circumferential location and the extent of the visible deformation. Measurements are particularly useful where the blister projects into the flow path or where several defects occur close together. Specialist inspection equipment may be required if the relevant dimensions cannot be determined from conventional CCTV footage.

The lining records can provide additional evidence. Useful information may include the resin system, impregnation records, installation conditions and curing data. For systems using controlled curing procedures, the recorded temperature or exposure information can help assess whether the specified process was followed.

Where a defect is suspected, the investigation should address several questions:

• Is the feature a blister, wrinkle, detached film or another type of irregularity?

• Does it affect the surface layer only or extend into the structural composite?

• Is the material cracked, soft, detached or otherwise visibly deteriorated?

• Does the feature significantly obstruct wastewater flow?

• Are similar defects present elsewhere along the installation?

• Was the irregularity recorded during the original post-installation inspection?

• Does the observed condition fall outside the relevant product and project acceptance requirements?

CCTV cannot normally determine the internal composition of a blister. It may identify a raised feature but cannot confirm whether the underlying cavity contains air, moisture or another material. Destructive examination or specialist testing may be necessary where the internal condition directly affects the repair decision.

There is no universal blister size that automatically determines acceptance or rejection for every lining technology. The relevant project specification, manufacturer’s requirements and applicable rehabilitation standards must be considered. For CIPP renovation of underground non-pressure drainage and sewerage networks, BS EN ISO 11296-4 is a relevant product-system standard, but the complete installation requirements and acceptance criteria must be established for the particular project.

Where several blisters are distributed along the same lining, their pattern may suggest a broader installation problem rather than isolated local damage. This possibility becomes more significant if the defects coincide with unusual curing conditions or other recorded irregularities. Affected sections may then require more extensive assessment than a single visible bubble would suggest.

Repair Methods and Prevention During Liner Installation

The appropriate treatment for a blister depends on whether the lining remains structurally sound and whether the defect affects drainage operation. Some minor surface irregularities may be acceptable under the applicable specification, while pronounced or damaged blisters may require corrective work. Repair should not begin until the defect has been identified sufficiently to select a compatible method.

Localised corrective work may be possible where the defect is limited in extent and the surrounding liner is sound. Depending on the system, an approved repair procedure might involve removing defective material and reinstating the affected area with a compatible product. Such work must preserve the required wall thickness, watertightness and structural performance.

Grinding or cutting away a raised area without checking the liner’s construction can be unsafe for its intended function. A blister may involve part of the structural composite rather than only a disposable film or excess surface material. Removing it indiscriminately could reduce wall thickness or expose reinforcement.

Widespread blistering may require a different response. Where investigation identifies extensive incomplete curing, resin defects or substantial internal voids, a local repair may not restore the specified performance. Depending on the condition of the existing lining and available pipe diameter, rehabilitation or replacement of the affected section may be necessary.

Preventing blistering begins before the liner enters the pipe. The installation team must select a rehabilitation system compatible with the existing pipe, identify active infiltration and prepare the host pipe according to the specified method. The condition of the original drainage structure can influence whether the liner expands and cures as intended.

For resin-impregnated systems, controlled material preparation is important. Impregnation procedures should achieve the required resin distribution and minimise retained air in accordance with the manufacturer’s process. Storage conditions, resin working time and installation timing must also remain within the specified limits.

During curing, the process should follow the requirements of the selected technology. Temperature, exposure, pressure and curing duration may all be relevant, but their roles differ between steam-cured, hot-water-cured and UV-cured systems. Parameters suitable for one resin and liner combination should not be transferred to another without technical justification.

Post-installation inspection is especially important because some irregularities may not be evident until the liner has fully cured and the installation equipment has been removed. Recording the condition before the pipe returns to normal service helps determine whether later observations represent original installation defects or subsequent changes.

For a cured pipe liner, the critical issue is not simply whether a raised bubble is visible. The assessment must establish which material layer is affected, whether the defect changes the internal flow profile and whether the lining continues to provide its specified structural and sealing performance. These findings determine whether the blister can remain in service, requires local treatment or indicates a more extensive failure of the rehabilitation installation.