Pultruded GRP Profiles: A Technical Guide for Engineers and Specifiers
Pultruded GRP profiles have moved from specialist composite applications into mainstream structural and infrastructure specification over the past two decades. The combination of structural performance, corrosion immunity, non-conductive properties, and low lifecycle cost that characterises the material has made it a credible alternative to steel and aluminium across a wide range of sectors. For engineers and specifiers evaluating pultruded GRP for the first time, or reviewing its suitability for a specific project application, understanding the manufacturing process, the applicable standards, the mechanical performance data, and the design principles that govern its use is the foundation for confident specification.
What Is the Pultrusion Process and Why Does It Matter?
Pultrusion is the manufacturing process that gives pultruded GRP profiles their consistent mechanical properties and dimensional accuracy. Continuous glass fibre rovings and mat reinforcements are pulled through a resin impregnation system and then through a heated die that cures the composite into a profile of constant cross-section. The process is continuous, producing profiles in lengths up to 11 metres with consistent properties throughout. The term pultrusion derives from the pulling action that drives the process, distinguishing it from extrusion where material is pushed through a die. The pulling action allows continuous glass fibre reinforcement to run the full length of the profile, which is what delivers the high longitudinal tensile and compressive strength that makes pultruded profiles suitable for structural applications.
The glass content of a well-manufactured pultruded profile is typically 60 percent or more by weight, with continuous rovings providing longitudinal strength and continuous filament mat providing transverse properties and interlaminar shear resistance. The resin matrix binds the fibre architecture together, transfers loads between fibres, provides chemical resistance, and determines fire performance. Isophthalic polyester is the standard resin for most industrial and infrastructure applications. Vinyl ester is specified for aggressive chemical environments. Phenolic resin is used where enhanced fire performance is required for rail, marine, and tunnel applications.
BS EN 13706: The Standard That Governs Structural Pultruded GRP
BS EN 13706 is the fundamental European standard for pultruded GRP profiles and the reference point for structural specification in the UK. Published in three parts covering designation, test methods, and specific requirements, it establishes the grade-based classification system that enables unambiguous specification of mechanical performance. Engineers specifying pultruded GRP profiles should always reference the grade, not just the material, to ensure that what is supplied meets the structural requirements of the application.
Grade E23 is the standard structural grade and the grade to which Engineered Composites manufactures and supplies pultruded profiles. E23 requires a minimum longitudinal flexural strength of 240 MPa, longitudinal flexural modulus of 23 GPa, longitudinal tensile strength of 240 MPa, tensile modulus of 23 GPa, interlaminar shear strength of 25 MPa, and pin bearing strength of 150 MPa longitudinally and 70 MPa transversely. Minimum glass content is 60 percent by weight and water absorption must not exceed 1.0 percent at 24 hours. These are minimum values. Well-manufactured E23 profiles routinely exceed them.
Grade E17 is a lower performance grade used in non-structural or lightly loaded secondary applications. For primary structural elements, access platforms, walkway frames, secondary structural members in infrastructure, and load-bearing applications of any kind, E23 is the appropriate and required grade. Any supplier unable to confirm BS EN 13706 E23 compliance with supporting test certificates should not be specified for structural applications.
Profile Range and Section Properties
The standard range of pultruded GRP structural profiles covers I-beams, channel sections, angle sections, box sections, flat bar, and round and square tube. I-beam sections are available from 76x38mm through to 356x127mm with wall thicknesses from 6.4mm to 12.5mm. A 152x76mm I-beam at 8mm wall thickness has a second moment of area of 920 cm⁴ and a section modulus of 121 cm³, weighing 3.9 kg per metre compared to the equivalent steel section at approximately 16 kg per metre. That weight saving of around 75 percent is consistent across the profile range and has direct implications for installation cost, handling, and structural loading on supporting frameworks.
GRP box sections are widely specified for secondary structural members, frame construction, and applications where a closed section is required for torsional stiffness or aesthetic reasons. GRP angle sections are used for framing, bracing, and connection details. The full range also includes flat strip, rod, tube, and specialist profiles including handrail sections, toe rails, and embedment angles. Fire retardant profiles are available across the standard section range for applications where fire classification is a specification requirement.
Key Design Considerations for Pultruded GRP
Designing with pultruded GRP requires adjustment from steel design practice in two principal areas. The first is the anisotropic nature of the material. Because the continuous fibre reinforcement runs predominantly in the longitudinal direction, GRP profiles have higher strength and stiffness along their length than across it. Transverse tensile strength is typically 50 MPa at E23 grade compared to 240 MPa longitudinally. Connection details, bolt spacing, and edge distances must be designed for GRP rather than assumed from steel practice, and load tables provided by the manufacturer are specific to the profile and span configuration tested.
The second consideration is that GRP has a modulus of elasticity of approximately 23 GPa at E23 grade compared to steel’s 210 GPa. Deflection under load will therefore be greater for an equivalent section, and serviceability limits rather than strength limits will typically govern the design of spanning members. Engineers compensate by selecting deeper sections or closer support spacings. The lower modulus is offset by the significantly lower self-weight of GRP, which reduces dead load contributions and frequently allows a more slender structure overall than would be achievable in steel when the total loading picture is considered.
Where Pultruded GRP Profiles Are Specified
The sectors where pultruded GRP profiles are most consistently specified are those where steel corrosion is a persistent operational problem and where the non-conductive and non-magnetic properties of GRP provide additional functional advantages. Water and wastewater treatment infrastructure, chemical and process plants, offshore and marine structures, rail infrastructure, electrical substations, and data centres all present conditions where GRP profiles outperform steel on a whole-life basis.
In water treatment and wastewater facilities, pultruded GRP framing for access platforms, handrail stanchions, equipment supports, and secondary structural elements eliminates the corrosion maintenance cycles that steel accumulates in H2S-rich and chemical dosing environments. In electrical substations, the non-conductive properties of GRP are a safety requirement rather than a preference. ECL holds RISQS accreditation for the rail supply chain, making its pultruded profiles suitable for specification on Network Rail infrastructure programmes including current CP7 and major enhancement schemes. In data centres, the electromagnetic neutrality of GRP is increasingly valued alongside its corrosion resistance in humidity-controlled environments.
Physical Properties That Influence Specification
Beyond mechanical strength and modulus, several physical properties of pultruded GRP influence specification decisions. Density is typically 1,850 kg per cubic metre, approximately 25 percent of equivalent steel sections. Thermal conductivity is 0.35 W per metre kelvin, compared to steel’s 50 W per metre kelvin, making GRP profiles the correct choice wherever thermal bridging through structural elements is a concern in insulated facades, balcony connections, or building envelope applications. The coefficient of thermal expansion longitudinally is approximately 8 x 10⁻⁶ per degree celsius, which is closely matched to concrete and significantly lower than steel, reducing differential thermal movement at interfaces with concrete structures.
GRP is non-magnetic, which makes pultruded profiles the correct structural material for MRI facilities, scientific installations, and sensitive defence environments where ferromagnetic materials create operational problems. It is non-conductive, which eliminates earthing requirements and electrical safety risks near live infrastructure. UV resistance is provided by the polyester surface veil incorporated during manufacture, maintaining surface quality and colour stability in external exposure without the maintenance painting required by steel.
Specify with Confidence
Engineered Composites supplies pultruded GRP profiles to BS EN 13706 E23 grade with ISO 9001 quality management certification and next-day delivery nationwide on stocked sections. Our full range includes GRP profiles, GRP box sections, GRP I-beams, GRP angle sections, and fire retardant profiles. Contact our technical team for section properties, load tables, and specification support.
