Quality controls are key to successful CIPP lateral lining applications

From the building owner’s perspective, the problem is inadequate sewage flow, service calls and worst case back-up onto the property. From the wastewater system operator’s perspective the problems are service calls and increased sewage volumes to handle from ground water infiltration at laterals. From both perspectives, the problem is caused by laterals that no longer provide a structural and leak free barrier to external factors. The lateral pipe may be broken, cracked or fractured. Lateral pipe joints may be separated and open. These problems allow debris and tree roots into the pipe along with infiltration. Without a smooth flow surface, sewage debris can hang up leading to flow blockage. When ground water is sufficient, infiltration occurs at problem locations. A further class of problems involves lateral pipe material that is prematurely deteriorating from the inside leading to blistering and collapsing such as with "black" pipe.

A successful lateral lining application provides the building owner with a long-term, trouble free solution to the sewage flow and back-up problem. As well, a successful lateral liner application must prevent infiltration into the lateral helping to solve system wide capacity problems. The infiltration issue is complicated by private versus public ownership of sections of the lateral and its connection to the main line sewer. Nevertheless, a successful lateral liner application must prevent infiltration over the location where it is installed.

A successful lateral lining application provides the building owner with a long-term, trouble free solution to the sewage flow and back-up problem. As well, a successful lateral liner application must prevent infiltration into the lateral helping to solve system wide capacity problems. The infiltration issue is complicated by private versus public ownership of sections of the lateral and its connection to the main line sewer. Nevertheless, a successful lateral liner application must prevent infiltration over the location where it is installed.

The principles and history to CIPP lining are well established. Nevertheless CIPP lateral lining requires quality control of design, sizing, materials, curing and testing to assure successful CIPP lateral lining implementation.

CIPP lateral lining products are also employed for rehabilitating many other types of pipelines other than laterals. Examples include interior drain lines, building stacks and swimming pools service pipes. Many of the quality controls applicable to successful lateral lining also apply to these other uses.

CIPP lateral liner - descriptions and components
Cured-in-Place Pipe (CIPP) uses a thermosetting plastic resin as its central material concept. Prior to thermoset, the CIPP liner is in a soft, pliable, state. The irreversible thermoset reaction converts the material into a hard rigid material. The lateral liner is installed into the lateral in its pre-thermoset state. The pliable and flexible state is well suited to lateral lining as it facilitates installation into and expansion within laterals, small pipes that often include inline bends and size changes. In its pre-thermoset soft pliable state, the liner can be made to conform to the varying shapes of the existing pipe before the thermoset action is initiated. This results in a liner that, when cured to hardness, should fit well to the variations in diameter, shape and turns encountered in typical laterals.

Some CIPP products have a reinforced carrier containing a structural reinforcement that imparts strength to the CIPP. For non-reinforced CIPP, the carrier material does not impart any strength, serving solely a carrier to hold the resin in place in the lateral. Each approach has its advantages and disadvantages. Reinforced liners have increased strength properties that can result in a thinner liner. Non-reinforced liners can provide superior expansion and form fitting properties for laterals with inline bends, shape and size changes. In both approaches, liquid catalyzed resin is worked into the carrier before the liner is place inside the existing lateral. The process of adding the resin is called wet-out or impregnation and is critical step in quality control.

The thermosetting reaction is initiated by catalysts (also called hardeners) and can be either a heat cure or an ambient cure. Heat cure systems use catalyst formulations that do not initiate thermoset until the liner temperature is elevated by a heat source typically hot water or steam, introduced into the interior of the liner. Heat cure systems remain stable until the temperature is raised to around 110°F. Heat cure provides long working times, measured in days and shorter cure times. Ambient cure systems use catalyst formulations that initiate the cure over time, starting at mix-up and cure at the temperature expected in the lateral. No additional heat source is required. Ambient cure systems have short working times, measured in a few hours, take 3-6 hours to cure on their own and provide no control over the curing process once mixed up.

Installation into the lateral is accomplished either by inversion or pull-in. In inversion, the liner is prepared with the inside out then inverted to reverse the surfaces using water, steam or air pressure. For pull-in, the liner is prepared without turning it inside out, pulled in and expanded to fit. Expansion may be done with a re-useable bladder or other means.

Quality controls for successful lateral lining
Successful means that the lateral liner is:

• Strong enough to resist buckling under ground water pressure.
• Strong enough to resist soil loads & traffic loads – where required
• Strong enough to resist the intrusion of tree roots.
• Leak proof against both infiltration and exfiltration.
• Smooth and impediment free inside to keep sewage flowing away.
• A tight fit to the inside of the existing lateral
• Free of fins, lumps and flow impediments especially at inline bends and size changes

Six key quality control areas identified for discussion are 1) Design, 2) Carrier Sizing, 3) Carrier Material, 4) Resin Quantity & Impregnation, 5) Curing and 6) Testing for Cured Properties. These are discussed further below.

Quality design - control
As underground pipes, lateral liners are subject to external loads. Depending on the lateral’s condition, design loads can be only groundwater or may also include soil and traffic loads. ASTM F1216 provides a widely used method for design of CIPP liners for external loads. It should be verified in design that the lateral liner being installed provides sufficient resistance to the loads expected. The external loading situation is commonly based on the F1216 classifications of:

Partially Deteriorated Condition – Groundwater hydrostatic load only.
Fully Deteriorated Condition – All loads from groundwater, soil, traffic and other sources.

Determination of deterioration condition requires assessing the condition over the life of the rehabilitation, not just the current state of the lateral. A lateral assessed in Partially Deteriorated condition for liner design must be expected to remain in this condition over the life of the lateral liner. Parameters needed for ASTM F1216 Appendix X1 design shown in the table below.

Design quality control requires that:

1. The lateral condition is correctly identified as Partially or Fully Deteriorated.
2. The design parameters are correctly determined for both the lateral and the liner.
3. Required liner thickness is checked by an accepted design calculation, such as F1216.

Reinforced versus Non-Reinforced Liners.

Reinforced liners tend to have much higher strength and modulus than non-reinforced. However the design relationship between strength and thickness is not linear and twice the strength does not translate into half the thickness. Quality control of liner thickness requires starting with an engineered thickness design for external loads.

Liner Thickness Required for Installation Compensation

Load design thickness is required in place after installation. Installation factors contribute to loss of thickness. Infiltration can wash away some resin before cure. Pulled-in liners may encounter scrape away or loss of resin quantity. Installation bladder, inversion or cure pressures can compress down the thickness. Stretching at bends and size changes can produce localized thinning. Quality control must take into account these factors and not base liner thickness selection solely on load design. Quality control must make sure that the liner supplier has included installation compensation in its materials and recommendations so that in place thickness meets requirements throughout the installation.

Quality control - Sizing
Proper sizing of the lateral liner impacts not only fit and finish but also structural performance. Sizing involves the carrier material circumference and thickness. Sizing of carriers is complex and requires a detailed understanding of carrier material behavior during the installation. The carrier materials have stretch and compression characteristics that must be taken into account when fabricating the carrier. If the carrier is to small, it may stretch too much, thinning out the wall. If it does not stretch enough annular gaps may occur behind the liner.

Significance of annulus gap between the liner and the lateral

1. The structural capacity is degraded by increasing annulus space. Loose fitting liners must be thicker/stronger than tight fitting liners.
2. Channeling of ground water through annulus space increases when liner is not tight fitting. Tighter fit gives less annulus flow meaning less final infiltration at exit point.
3. Increasing annulus gap allows more space for tree routs to penetrate behind the liner in search of moisture.

When carrier sizing is too large, excess material can cause wrinkling, fining and donuts. Liners that are oversized or do not stretch properly will not negotiate or conform well to bends in the lateral These occurrences degrade flow capacity leading to increased maintenance or back-ups.

Proper quality control steps in sizing the liner are key to both satisfactory fit and finish and to structural performance. The effect of poor fit on structural performance should not be underestimated.

Quality control - Carrier material
The carrier is the tube, sleeve or other configuration that is used to transport the resin into the lateral. Some carriers contain reinforcement that is integral to liner strength properties. The carrier controls the finished thickness of the liner by controlling the amount of resin that can be carried and by the thickness of its other elements including fibers and reinforcement. To obtain reliable and repeatable cured-in-place liner thickness, the carrier must provide reliable resin adsorption and material thickness. This requires quality controlling the carrier manufacturing process to provide uniform properties.

Fit to bends and size changes in laterals

Laterals typically have inline bends and size changes typically not encountered in main line sewers. Most importantly lateral liner carrier properties must address the need to line through inline bends and size changes while maintaining fit, finish and wall thickness. A carrier material suitable for straight runs may provide unsatisfactory results for lateral lining. Quality control must match the liner carrier to the lining requirements. Typically a lateral liner system will require more than one type of carrier tube due to different lateral geometries. Quality control steps must be in place to make sure the correct carrier tube product for the lateral has been selected. Otherwise quality control will be required for open cut replacement of the liner. Liner suppliers and manufacturers should be able to provide technical guidance to installers regarding the correct carrier tube for the installation.

Figure 11 shows three different types of lateral liner carrier materials from one supplier. Choice of carrier material is matched to lateral situation such as need to line through bends or size changes.
Good quality control requires knowing which carrier material to use for different lateral situations

Carrier thickness versus compression

Liner installation results in internal pressure on the carrier material. This compresses the carrier material reducing the CIPP wall thickness. Initial carrier thickness must take into account this compression otherwise in place liner thickness will be less than required.

For example, a carrier that is to produce a minimum 3 mm in lateral liner cured thickness will normally have to be more than 3 mm thick. The degree of over thickness depends on the compression characteristics of the carrier and the required installation pressure. Quality control requires good knowledge of the carrier material properties by the carrier manufacturer and quality control of these properties.

Carrier thickness versus stretch out to size

Initial carrier thickness must also take into account how much the carrier is undersized versus the lateral inside diameter. This is a complex consideration requiring considerable knowledge of the how the carrier material behaves during stretch out to size under installation pressures whether from an expanding packer, membrane or inversion head. As the carrier stretches in circumference to fit the lateral, the wall thickness will reduce. First the initial thickness must be sufficient to compensate for the reduction under stretch out to fit. Second the carrier material must stretch evenly around the circumference to prevent excessive localized wall thinning will result.

Quality control steps are required to assure the initial carrier thickness sufficiently compensates for these thinning effects and delivers the required final thickness in the pipe after installation.

Quality control - Resin quantity and impregnation
Resin quantity is key to determining cured-in-place physical properties and thickness. An accurate method is required to determine the volume of resin needed. Too little resin will result on liners that are too thin and may result in liner wall voids. Inadequate resin reduces cured physical properties substantially resulting in a liner with insufficient structural strength. Resin voids are also sources for infiltration leaks through the liner.

Determination of required resin quantity is related to the thickness of the carrier, the air space characteristics of the carrier, shrinkage characteristics of the resin and specification requirements for excess resin. The initial carrier thickness determines the resin quantity because the carrier tube must be impregnated with sufficient resin to fill all the free space in the carrier material.

Key Quality Control Steps for Resin Quantity

Proper quality control for resin quantity requires first an engineered calculation of the volume required and second a verification that the required quantity was used. Inadequate resin quantity can result in inadequate liner thickness and poor quality physical properties.

Verifying CIPP lateral liner impregnation

The carrier material must be completely impregnated with resin, meaning that no void spaces (air spaces) remain in the carrier material after impregnation (wet-out). Before resin impregnation the carrier material contains air space to be replaced by resin. The goal of impregnation is to fill all the air space with resin, expelling all the air. If all the air is not exchanged for resin, the cured liner will have voids, air pockets. These air pocket voids degrade the liner physical properties and can allow leakage through the cured liner. Quality control procedures include vacuum assisted impregnation and resin distribution rollers.

Vacuum Assisted Impregnation: Exchanging all the air for resin can be difficult. Simply spreading resin onto the carrier does not provide certainty that the impregnation is complete, that all the air has been replaced with resin. Vacuum assisted impregnation offers the best method for assuring that the impregnation is complete. Vacuum assisted impregnation withdraws the air from the carrier material ahead of the advancing resin wet-out face. With vacuum assisted impregnation, the carrier material is seen to collapse as the air is withdrawn then expand as the resin is takes its place.

Resin Distribution Rollers: Obtaining good quality CIPP liners requires the resin to be distributed uniformly throughout the carrier material. Without uniform distribution, the cured liner will have varying thickness and physical properties. This is best achieved by controlling the thickness of the impregnated liner after all the air has been expelled and the resin added. Squeeze rollers, properly used, are effective for this purpose as long as sufficient quantity of resin remains at the leading edge of the rollers. Determining roller gap is a function of theoretical calculation and the characteristics of the roller mechanism. Roller gap calibration based on resin quantity actually used is required as roller gaps based on theoretical calculations rarely yield the right amount of resin.

Without procedures and equipment that produce reliable, repeatable resin impregnation, distribution and quantity, the quality of the cured-in-place lateral liner will be variable. Liner thickness may not be adequate throughout the liner. Minimum required physical properties of flexural strength and flexural modulus may not be achieved throughout the liner.

Quality control - Curing
Good quality curing to achieve required physical properties is essential for a successful lateral liner application. A reliable cure method and its quality control are essential because obtaining cured samples for testing is difficult for lateral liners. Curing methods that provide higher reliability, such as heat cure, are preferred when sampling is difficult or infrequent as with laterals.

Quality Control Steps – Heat and Ambient Cures

Generally, heat cure produces better control of liner properties than ambient cure. To achieve heat cure results using ambient cure, a much higher level of quality control is needed for the ambient method. With a suitable level of quality control, ambient cures can approach the reliability of heat cures. However, this requires incorporating a higher level of quality control than may be typically present in some ambient cure installations.

Quality control - verifying cured properties
Quality control in design requires input of verified, reliable liner properties of flexural modulus and strength. Periodic testing of short-term values is required to verify design inputs. Field samples from lateral liners should be tested for flexural strength and flexural modulus according to ASTM D790 and for thickness. The D790 values of strength and modulus to be used as design inputs should be the values that are reliably obtained over several tests, not the maximum values obtained from any one test. An ongoing quality control testing program is required.

Experience shows that when field installation samples have been tested, results are significantly less than laboratory samples. If laboratory samples are the only basis for liner properties, values taken for the liner should be significantly reduced from the laboratory sample values. True field samples may be 25-35% less than controlled laboratory samples especially for ambient cure process.

Good quality control dictates that unless there is a reliable history of test results from field samples then strength and modulus values used in the design of the lateral liner should be chosen very conservatively.

Summary

A successful CIPP lateral lining application requires quality control over design, sizing, carrier material, resin quantity/impregnation, cure and sample testing.

• Design quality controls should start with using an accepted approach with reliable procedures in place for correctly identifying the design inputs for field and liner parameters
• Carrier sizing quality control starts with understanding carrier material behavior then matching it to lateral geometry and structural design requirements.
• Carrier material quality control starts with knowing field behavior of the material, controlling the properties and matching them to lateral requirements.
• Resin quantity/impregnation quality control starts with a rational basis for determining required resin volume, verifying volume used and employing methods to ensure thorough void free wetout.
• Cure quality control starts with understanding the strengths and weaknesses of different methods and understanding the importance of obtaining design physical properties.
• Sample testing quality control starts with realizing how the liner physical properties relate to liner quality, understanding that laboratory sample results are usually optimistic and instituting a regular ongoing testing program to verify both liner quality and design inputs.

Photographs in this article are courtesy of: PipeFlo Contracting Corp., D.M. Robichaud Contracting Inc., Applied Felts, Inc.