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A cured-in-place pipe (CIPP) is a trenchless rehabilitation method used to repair existing pipelines. CIPP is a jointless, seamless, pipe lining within an existing pipe ranging in diameter from 0.1 to 2.8 meters (2–110 inches). As one of the most widely used rehabilitation methods, CIPP has applications in sewer, water, gas, and chemical pipelines.

Process
A resin-saturated felt tube made of polyester, fiberglass cloth or any of a number of other materials suitable for resin impregnation, is inserted or pulled into a damaged pipe. It is usually done from the upstream access point (manhole or excavation). It is possible to insert the liner upstream (e.g. from the downstream access point) but this carries greater risk. It is possible to install a liner from the downstream access point, upstream to a blind end; however, this carries the highest risk of all the CIPP installation methods. CIPP is considered a trenchless technology. Little to no digging is involved in this trenchless process, making for a potentially more cost-effective and less disruptive method than traditional "dig and replace" pipe repair methods. The liner can be inserted using water or air pressure. The pressure required for insertion can be generated using pressure vessels, scaffolds or a "chip unit". Hot water or steam can be used to accelerate the curing rate of the resin. If a fiberglass tube is used, the curing of the resin can be triggered though the use of UV light introduced into the tube. As the resin cures, it forms a tight-fitting, joint less and corrosion-resistant replacement pipe. Service laterals are restored internally with robotically controlled cutting devices called cutters in the larger-diameter pipe. Smaller diameters (100 mm) can be opened remotely using smaller cutters designed for small diameter pipe. The service lateral connection can be sealed with specially designed CIPP materials, sometime referred to as a 'top-hat'. The resin used is typically polyester for mainline lining and epoxy for lateral lines. Since all resins shrink (epoxy resins shrink far less than poly and vinyl ester versions) and it is impossible to bond to a sewer that has fats, oils, and grease an annular space exists between the new CIPP liner and the host pipe. The annular space exists in all installations just some are larger than others and depending on the severity may need additional rehabilitation. There are multiple ways to prevent water from tracking in the annular space and entering back into the waste stream including: water swelling material (hydrophilic), lining of the entire connection and host pipe with a continuous repair (YT repair) gaskets, and point repairs placed at the ends of the host pipe.

History
In 1971, Eric Wood implemented the first cured-in-place pipe technology in London, England. He called the CIPP process insitu form, derived from the Latin meaning "form in place". Wood applied for U.S. patent no. 4009063 on January 29, 1975. The patent was granted February 22, 1977, and was commercialized by Insituform Technologies until it entered the public domain on February 22, 1994.

The process began to be used in residential and commercial applications in Japan and Europe in the 1970s and for residential application in the United States in the 1980s.

Advantages
As a trenchless technology, CIPP does not require excavation to rehabilitate a pipeline that is either leaking or structurally unsound. Depending upon design considerations an excavation may be made, but the liner is often installed through a manhole or other existing access point. Anything larger than 60 inches must be excavated in order to install. Liner is installed as it is wet out on site in these instances. In the case of sewerlines, lateral connections are also restored without excavation via a remote controlled device that drills a hole in the liner at the point of the lateral connection. If larger than 24" and it is safe to do so someone will reinstate laterals by hand. CIPP has a smooth interior and no joints. While CIPP can repair a pipe with bends, special design considerations must be taken into account to prevent wrinkling and stretching. CIPP can effectively reduce infiltration and leaks in pipeline systems without digging.

Disadvantages and limitations
Except for very common sizes, liners are not usually stocked and must be made specifically for each project. CIPP requires bypass of the flow in the existing pipeline while the liner is being installed. The curing may take from one hour to 30 hours depending on pipe diameter and curing system (steam, water or uv) and must be carefully monitored, inspected, and tested. Obstructions in the existing pipeline, such as protruding laterals, must be removed prior to installation. Cost should be compared with similar methods such as Shotcrete, thermoformed pipe, close-fit pipe, spiral wound pipe and sliplining as these other methods can provide a similar design solution for similar or less cost in certain situations. CIPP must also be carefully monitored for release of chemical agents utilized in the reaction process for contamination downstream of rehabilitated pipes. The liner material used for common sizes is normally a felted fabric (non-woven) and does not go around bends well without wrinkling and going out of round on corners. Liners used for pipes with bends (particularly 100 mm pipes) are made from a woven fabric allowing it to go around bends with minimal wrinkling. The more flexible the liner, the more care needs to be taken during inversion to ensure the liner remains on the correct alignment. Once a line is repaired with the CIPP method, that line can no longer be cabled or snaked with a machine, it must be cleaned by method of Hydro jetting (AKA high-pressure water blasting)

Quality assurance and quality control
Testing of CIPP installations is required to confirm that the materials used comply with the site and engineering requirements. Since ground and ambient installation conditions as well as crew skills can affect the success or failure of a cure cycle, testing is performed by 3rd party laboratories in normal cases and should be requested by the owner.

Samples should be representative of the installation environment since the liner is installed in the ground. Wet sandbags should be used around the restraint where the test sample will be extracted from. As with any specimen preparation for a materials test, it is important to not affect the material properties during the specimen preparation process. Research has shown that test specimen selection can have a significant effect on the CIPP flexural testing results. A technical presentation at the CERIU INFRA 2012 Infrastructures Municipales Conference in Montreal outlined the results of a research project which examined the effects of test specimen preparation on measured flexural properties. Test specimens for ASTM D790 flexural testing must meet the dimensional tolerances of ASTM D790.

The North American CIPP industry has standardized around the standard ASTM F1216 which uses test specimens oriented parallel with the pipe axis, while Europe uses the standard EN ISO 11296-4 with test specimens oriented in the hoop direction. Research has shown that flexural testing results from the same liner material are usually lower when determined using EN ISO 11296-4 as compared to ASTM F1216.

Environmental, public health, and infrastructure incidents
Testing conducted by the Virginia Department of Transportation and university researchers from 2011 to 2013 showed that some CIPP installations can cause aquatic toxicity. A list of environmental, public health, and infrastructure incidents caused by CIPP installations as of 2013 was published by the Journal of Environmental Engineering. In 2014, university researchers published a more detailed study in Environmental Science & Technology that examined CIPP condensate chemical and aquatic toxicity as well as chemical leaching from stormwater culvert CIPP installations in Alabama. In this new report additional water and air environmental contamination incidents were reported not previously described elsewhere.

In 2017, CALTRANS backed university researchers examined water impacts caused by CIPPs used for stormwater culvert repairs.

In April 2018, a study funded by six state transportation agencies (1) compiled and reviewed CIPP-related surface water contamination incidents from publicly reported data; (2) analyzed CIPP water quality impacts; (3) evaluated current construction practices for CIPP installations as reported by US state transportation agencies; and (4) reviewed current standards, textbooks, and guideline documents. In 2019, another study funded by these agencies identified actions to reduce chemical release from ultraviolet light (UV) CIPP manufacturing sites. .

With proper engineering design specifications, contractor installation procedures, and construction oversight many of these problems can likely be prevented.

Worker and public safety concerns
On July 26, 2017, Purdue University researchers published a peer-reviewed study in the American Chemical Society’s journal Environmental Science & Technology Letters about material emissions collected and analyzed from steam cured CIPP installations in Indiana and California. To further make the study accessible to the public and CIPP worker community, the study authors setup a website and made their publication open-access, freely available for download. Purdue University professors also commented on their study and called for changes to the process to better protect workers, the public, and environment from harm.

On August 25, 2017, the National Association of Sewer Service Companies, Incorporated (NASSCO), which is a (501c6) nonprofit dedicated to "improving the success rate of everyone involved in the pipeline rehabilitation industry through education, technical resources, and industry advocacy", posted a document on its website bringing up several important concerns and unanswered questions regarding the study, and its messaging.

On September 22, 2017, NASSCO announced it would fund and coordinate an assessment of previous data and studies, and an additional study and analysis of possible risks related to the CIPP installation and curing process.

On September 25, 2017, a document was issued by the Purdue University study authors that clarified incorrect assertions put forward by others about their CIPP study.

On September 26, 2017, the US Centers for Disease Control and Prevention (CDC) National Institute for Occupational Safety and Health (NIOSH) published a Science Blog contribution regarding Inhalation and Dermal Exposure Risks Associated with Sanitary Sewer, Storm Sewer, and Drinking Water Pipe Repairs.

In September 2017, the NASSCO posted a request for proposals to “review of recent publication(s) that propose the presence of organic chemicals and other available literature relating to emissions associated with the CIPP installation process, and a scope of services for additional sampling and analysis of emissions during the field installation of CIPP using the steam cure process.”

In September 2017, the California Department of Public Health issued a notice to municipalities and health officials about CIPP installations. One of several statements in this document was that "municipalities, engineers, and contractors should not tell residents the exposures are safe."

On October 5, 2017, the National Environmental Health Association sponsored a webinar about the hazards involved for workers and residents associated with cured-in-place pipe repair. The video can be found here. Several questions about the webinar, and the study have been raised, and feedback noted by industry members.

On October 25, 2017, a 22-year old CIPP worker died at a sanitary sewer worksite in Streamwood, Illinois. The Occupational Safety and Health Administration (OSHA) completed their investigation April 2018 and issued the company a penalty. Chemical exposure was a contributing factor in the worker fatality.

In April 2018, NASSCO issued a request for proposals to conduct a study on chemical emissions from 6 CIPP installations.