Well-Designed FRP VS. Other FRP
Service life and cost performance of glass fiber reinforced plastic (FRP) composites depend on good design and judicious selection of materials and processes. In FRP composites, glass fiber reinforcements control structural properties; whereas, resin type controls corrosion resistance and service temperature suitability.
Can structural properties differ in FRP materials with similar weight & appearance?
YES. Strength and stiffness of FRP composites are controlled by type, content, and alignment of its glass fiber reinforcements. To optimize structural properties, glass fibers must be in the right place, oriented in the right direction, and in the right amount.
- The most effective type of reinforcements for load transfer within the composite are unidirectional continuous, non-woven fibers located along the load path.
- Short, chopped strand fibers (Fig. A) are the least effective since load transfer from one fiber to other randomly oriented fibers relies heavily on the lower strength resin matrix. The random orientation can provide good shear strength and bulk, but reinforcing content is limited to approximately 25% for an FRP building panel.
Fig. A Chopped Strand: Inefficient load transfer
Approx. 25% reinforcing content (by wt.)
3. Woven fibers provide reinforcement on both perpendicular axes; however, strength of fibers is weakened at intersections of woven patterns due to stress concentration (Fig. B).
Fig. B Woven Roving: Weak at intersections
Approx. 40% reinforcing content (by wt.)
4. To optimize performance, reinforcement design requires a combination of unidirectional, non-woven continuous fibers oriented in longitudinal and transverse directions with a minimal amount of chopped fibers (Fig. C).
With this design, reinforcing fibers in fiberglass corrugated panels can efficiently make up 48 to 52% of the material weight. In structural (FRP) fiberglass beams, utilizing pultrusion process, glass fiber content can exceed 60% of the material weight.
Fig. C Unidirectional + Chopped: Optimum properties
Approx. 50% reinforcing content (by wt.)
What is the effect of reinforcement alignment for FRP Building Panels?
Strength and Stiffness are dependent on quantity of reinforcements. Amount of glass fiber reinforcing is dependent on the reinforcing alignment.
Continuous aligned fibers offer significantly higher glass content and structural properties than chopped strand or woven reinforcing. 50 times stronger than resin, glass fiber content and alignment controls strength and stiffness.
Deflection under load is dependent on quantity and alignment of reinforcements. The data shown in Fig. E indicates deflection for the continuous glass reinforced panel is 30% less than the woven panel and over than half less than the chopped strand pane.
Can corrosion resistance differ in new FRP materials with similar appearance?
YES. Type of resin is the primary control factor for corrosion resistance and temperature suitability. Polyester resins are predominantly used for construction applications due to cost and balance of properties including mechanical, chemical, and ease of processing. This discussion offers recommendations for isophthalic (iso-polyester), orthophthalic (ortho-polyester), and vinyl ester resins.
- Of the two polyesters, isophthalic resins are much better suited for corrosive environments and elevated temperatures. Iso-polyesters provide exceptional corrosion protection against splash and spill chemicals and suitability for moderate operating temperatures (up to 150 deg F). Offering significantly better chemical resistance, as well as better thermal and mechanical properties, iso-polyesters should be selected over ortho-polyesters (general purpose) for industrial applications with chemical exposures or wet operating conditions.
- Vinyl ester resins combine performance advantages of epoxy resins with the ease of handling and processability typical of unsaturated polyester resins. Vinyl esters have better mechanical properties than polyesters and exhibit better strength retention at elevated temperatures. As a result, vinyl ester can be used at higher operating temperatures (up to 225 deg F).
Can UV Protection differ in new FRP materials with similar appearance?
YES. For effective UV protection, UV stabilizers must be incorporated into the resin mix. UV stabilizers can be Ultraviolet Light Absorbers (UVA) or Hindered Amine Light Stabilizers (HALS), used individually or as blends. The additives are necessary to protect against UV breakdown or fiber blooming at the surface of fiberglass corrugated panels. In addition, a protective surface coating will provide extended color retention for opaque FRP materials and retention of light transmitting properties for translucent roofing panels and siding.
What requirements should be included in project specifications to help ensure higher quality FRP plus fair competition?
- Structural performance criteria for FRP materials must be included and can be similar to criteria used for metal components.
- Criteria should include all applicable design loads, such as positive, negative, collateral, and dead loads to ensure products are suitable for the required load conditions.
- Criteria should include appropriate deflection limits and factor of safety, as determined by the project’s engineer, for each type of component to ensure product suitability.
- For corrugated fiberglass roofing or siding panels, it is recommended requirements include minimum deflection limit of L/60 (max. allowable). More conservative deflection limits up to L/180 will provide more rigidity and better performance.
- Factor of safety should be 2.5 minimum for roof positive loads and 1.88 minimum for siding, negative loads, and fastener pullover.
- For secondary FRP structural members, such as roof purlins or wall girts, deflection limit of L/120 to L/180 is suggested. Recommended minimum factors of safety are 2.5 for roof purlins and 1.88 for wall girts.
- For primary FRP structural members, deflection limit of L/180 or L/240 is suggested. Minimum factor of safety of 3.0 is suggested for FRP primary members.
Product compliance will prevent excessive deflection that can lead to panel buffeting, fastener pullover, or damaged panels.
Please note: The recommended, minimum deflection limit of L/60 for FRP roofing and siding is the same as the requirement for metal panels as listed in ICC IBC-2021, 1604.3a. The allowable limits in the dated ASTM standard for Glass-Fiber-Reinforced Plastic Panels are not recommended due to the excessive deflection limits listed of L/20 for fiberglass siding and L/40 for fiberglass roofing panels. This ASTM criteria was developed over 30 years ago primarily by manufacturers of low performing, chopped strand, fiberglass corrugated panels. Most manufacturers, today, offer higher performing continuous glass reinforced panels, which are suitable for more conservative and appropriate deflection limits (L/60 min) that should be specified.
- To ensure materials of suitable quality are furnished, “Materials” specification should include these requirements.
- Resin type shall be: UV Stabilized, Iso-polyester____ UV Stabilized, Vinyl Ester____ Specifications should indicate one or the other, dependent on application and service temperature requirements.
- Exterior roofing and siding panels shall have a finished surface coat of Acrylic Polymer, .4 mil dry film thickness (min) for additional UV protection.
For additional information and assistance, please contact Enduro Composites, Inc. Our technical staff will be pleased to help with identifying appropriate materials and specifications for your project.