EDITION 62 | DEC 2020 | NASSCO.ORG forces associated with the installation process. Resin to glass ratios of these liners range from 40% to 60% glass creating an extremely strong composite structure. Flexural modulus values of 1.5 to 2.4 million psi or higher and flexural strength values of 25,000 to 50,000 psi and higher are common for these fiber reinforced materials. Hybrid fiber reinforced felt liners were originally developed to target pressure pipe applications. The controlling design value is tensile strength for CIPP in pressure pipe applications; the fiber reinforcement is used to increase the tensile properties. The earliest iterations used chopped strand mat fiberglass attached or needled into the felt material. Properties of these types of materials are more isotropic and any improvement in flexural modulus may be minimal dependent upon placement of the reinforcing layer within the composite construction. Many of these hybrid liners now take advantage of continuous fiber mats which allow a higher percentage of the reinforcing fiber to be placed in the hoop or triaxial direction. The added customization has allowed for the development of CIPP for pressure pipe applications that have significantly higher tensile strength, increasing the technical envelope to accommodate a wider range of pressure applications. Fiber reinforced liners are now finding increased usage in standard gravity applications for large diameter pipe rehabilitation. The required wall thickness of gravity CIPP is typically controlled by the flexural modulus of the cured liner, although it may be controlled by flexural strength in structures with flat wall sections like box culverts and some inverted egg shaped pipes. The flexural modulus was initially increased by using resin enhancers or fillers. The addition of fillers to the resin increased the flexural modulus allowing for a reduction in the wall thickness of the CIPP. Construction has evolved to incorporating fiber reinforced layers to increase both the flexural modulus and flexural strength of the composite. The fiber reinforced layers are typically added to the outer and inner layers of the liner with the middle or sandwich layers made with the standard felt. This construction creates an I-beam structure that has significantly higher flexural modulus and flexural strength. These high modulus composites allow for thinner CIPP design which reduces resin consumption saving the customer money with the added benefit of increasing flow capacity. The lower weight also helps with material handling and increases the length of liner that can be transported to the job site. In addition, fiber reinforced liners typically have higher creep retention properties than standard felt products. The higher creep retention can be used to further reduce the design thickness of the installed liner. While there remains some level of education that is required for the design engineering community on potential opportunities and uses for fiber reinforced composites, these products are now fairly widely accepted. As the demand for gravity and pressure pipe rehabilitation continues to grow, manufacturers and contractors will continue to work together to further advance the use of these high tech, fiber reinforced products throughout the industry. TECH TIPS IS A BI-MONthly article on trends, best practices and industry advice from NASSCO's Trenchless Technology membership PROFESSIONALS Follow us on social media or visit NASSCO.orghttp://www.NASSCO.ORG http://www.NASSCO.org