GRP Square Tubes: Smart Engineering for Modern Construction
Product Overview
GRP (Glass Reinforced Plastic) square tubes are composite structural profiles manufactured through continuous pultrusion, combining continuous glass fiber reinforcement (50-65% by weight) with thermoset polymer resin systems. These hollow-section profiles provide corrosion resistance, favorable strength-to-weight ratio, and electrical insulation properties for industrial applications.
Primary Applications: Chemical processing plants, wastewater treatment facilities, electrical substations, marine structures, cooling towers, offshore platforms, and environments requiring corrosion-resistant, non-conductive structural components.
Manufacturing Process: Continuous pultrusion creates consistent fiber alignment, dimensional accuracy, and uniform mechanical properties along the profile length. For more information on our production equipment, see our pultrusion machines and pultrusion dies.
Technical Specifications
Mechanical Properties (Typical Values)
Property
Longitudinal
Transverse
Test Standard
Tensile Strength
240-450 MPa
80-120 MPa
ASTM D638
Tensile Modulus
20-25 GPa
6-9 GPa
ASTM D638
Flexural Strength
300-500 MPa
–
ASTM D790
Flexural Modulus
17-24 GPa
–
ASTM D790
Compressive Strength
200-350 MPa
–
ASTM D695
Compressive Modulus
18-22 GPa
–
ASTM D695
Interlaminar Shear Strength
20-35 MPa
–
ASTM D2344
Barcol Hardness
40-60
–
ASTM D2583
Values are typical ranges and may vary based on fiber content, resin system, and manufacturing conditions. Project-specific testing is recommended.
Physical & Thermal Properties
Property
Value
Test Standard
Density
1.65-1.95 g/cm³
ASTM D792
Water Absorption (24hr)
<0.5% by weight
ASTM D570
Coefficient of Thermal Expansion (Longitudinal)
8-12 × 10⁻⁶/°C
ASTM D696
Coefficient of Thermal Expansion (Transverse)
20-30 × 10⁻⁶/°C
ASTM D696
Thermal Conductivity
0.3-0.4 W/m·K
ASTM C177
Glass Transition Temperature (Tg)
110-130°C (Polyester)140-160°C (Vinyl Ester)
ASTM D3418
Continuous Service Temperature
-40°C to +90°C (Polyester)-40°C to +120°C (Vinyl Ester)
–
Electrical Properties
Property
Value
Test Standard
Dielectric Strength
15-20 kV/mm
ASTM D149
Dielectric Constant (1 MHz)
4.0-5.0
ASTM D150
Volume Resistivity
10¹²-10¹⁴ Ω·cm
ASTM D257
Surface Resistivity
10¹¹-10¹³ Ω
ASTM D257
Fire Performance
Property
Standard Grade
Fire-Retardant Grade
Test Standard
Flame Spread Index
<75
<25 (Class 1)
ASTM E84
Smoke Developed Index
<450
<50
ASTM E84
Oxygen Index
26-32%
35-45%
ASTM D2863
UL94 Rating
HB
V-0 or V-1
UL94
Engineering Design Data
Design Safety Factors (Recommended)
Static loads: 3:1 to 4:1
Dynamic/cyclic loads: 5:1 to 6:1
Impact loads: 8:1 to 10:1
Long-term sustained loads: Apply creep factor (multiply deflection by 2.0-3.0)
Allowable Stress Design Values (FOS = 4:1)
Tension: 60-90 MPa
Compression: 50-75 MPa
Bending: 75-125 MPa
Interlaminar Shear: 5-8 MPa
Maximum Deflection Guidelines
Structural beams: L/180 to L/240
Walkways and platforms: L/240 to L/360
Handrails: 25mm maximum under 1.0 kN/m lateral load
Section Properties – Standard Sizes
Size (mm)
Wall (mm)
Weight (kg/m)
Area (cm²)
Ix = Iy (cm⁴)
Zx = Zy (cm³)
Rx = Ry (cm)
25 × 25
3.0
0.28
1.52
1.89
1.51
1.01
38 × 38
3.2
0.45
2.43
5.89
3.10
1.52
50 × 50
4.0
0.75
4.20
15.6
6.24
2.01
60 × 60
5.0
1.15
6.40
32.4
10.8
2.40
75 × 75
6.0
1.75
9.60
71.5
19.1
3.01
100 × 100
6.5
2.55
14.2
202
40.4
4.05
100 × 100
8.0
3.10
17.2
235
47.0
4.01
150 × 150
8.0
4.80
26.8
762
102
6.03
150 × 150
10.0
5.95
33.0
920
123
6.01
200 × 200
10.0
8.05
44.7
2150
215
8.04
Properties based on density ρ = 1.80 g/cm³. Values are indicative; confirm with project-specific calculations.
Weight Comparison vs. Metals (100×100×6mm, 1 meter)
Material
Weight (kg)
Relative Weight
GRP
2.55
Baseline
Mild Steel
11.9
367% heavier
Stainless Steel 304
12.4
386% heavier
Aluminum 6061
4.1
61% heavier
Material Composition & Performance
Standard Compositions
Polyester Grade (Standard):
Glass fiber: 50-60% by weight (E-glass continuous roving + CFM)
Resin: Isophthalic or orthophthalic polyester
Surfacing veil: Optional (50-80 g/m² for UV protection)
Vinyl Ester Grade (Enhanced Chemical Resistance):
Glass fiber: 55-65% by weight
Resin: Vinyl ester or epoxy-vinyl ester
Higher Tg and improved chemical resistance
Corrosion Resistance – Chemical Compatibility
Good to Very Good Resistance (many common chemicals):
Mineral acids: HCl, H₂SO₄, HNO₃ (moderate concentrations)
Alkalies: NaOH, KOH up to 50%
Salt solutions: Seawater, brine, chlorides
Hydrocarbons: Diesel, gasoline, crude oil
Wastewater and sewage environments
Moderate Resistance (confirm suitability for specific conditions):
Concentrated acids
Strong alkalies (>50%)
Hypochlorite solutions (bleach)
Limited Resistance (use vinyl ester or epoxy):
Ketones (acetone, MEK)
Chlorinated solvents
Phenols and cresols
Detailed chemical resistance charts are available on request.
UV Resistance
Standard Grade (No surfacing veil):
Indoor applications
Short-term outdoor exposure
UV-Protected Grade (With surfacing veil):
Extended outdoor service life
Recommended for outdoor applications
Actual UV performance depends on exposure conditions and geographic location. Project-specific testing is recommended.
Connection & Installation Guidelines
Mechanical Fastening Specifications
Parameter
Specification
Bolt Size
M10 to M20 (application dependent)
Hole Clearance
Bolt diameter + 2mm
Edge Distance
Minimum 2.5 × bolt diameter
Fastener Spacing
4-6 × bolt diameter
Washer Size
Minimum 3 × bolt diameter, 3mm thick
Hardware Material
Stainless steel 316 or HDG recommended
Maximum Torque Values:
M10: 15-20 Nm
M12: 25-30 Nm
M16: 40-50 Nm
M20: 60-70 Nm
Important: Do not overtighten – crushing can reduce bearing strength significantly.
Adhesive Bonding
Recommended Adhesives:
Methacrylate adhesives
Two-part epoxy
Polyurethane adhesives
Bond Specifications:
Surface preparation: Abrade with 80-grit, clean with solvent
Minimum overlap: 25mm (optimal: 40-50mm)
Cure time: 24-48 hours at 20°C before loading
Shear strength: 10-15 MPa (properly prepared)
Cutting & Machining
Cutting Tools:
Abrasive cut-off wheel (1.0-1.5mm thin wheels)
Carbide-tipped circular saw (60-80 teeth)
Diamond blade (precision cuts)
Drilling Parameters:
Drill bits: HSS or carbide, 118° point angle
Speed: 500-1500 RPM
Always back up material to prevent splintering
Use appropriate PPE – glass fiber dust hazard
Available Configurations
Standard Profile Sizes
Category
Sizes Available
Wall Thickness Range
Small Sections
25×25, 38×38, 50×50mm
2.5-5.0mm
Medium Sections
60×60, 75×75, 100×100mm
4.0-10.0mm
Large Sections
120×120, 150×150, 200×200mm
6.0-15.0mm
Custom sizes available; minimum order quantities may apply.
Resin System Options
Resin Type
Max Service Temp
Chemical Resistance
Best For
Orthophthalic Polyester
80°C
Good
General purpose, economical
Isophthalic Polyester
90°C
Very Good
Improved water resistance
Vinyl Ester
120°C
Excellent
Chemical plants, harsh environments
Epoxy
140°C
Excellent
Maximum strength and heat resistance
Fire-Retardant Polyester
80°C
Good
Indoor, Class 1 fire rating required
Color Options
Standard Colors (Stock):
Safety Yellow (RAL 1003)
Gray (RAL 7035)
Green (RAL 6011)
Black (RAL 9005)
Custom colors available for qualifying orders. Color is pigmented throughout the profile, not a surface coating.
Surface Finishes
Standard Smooth: General applications
UV-Protected (Surfacing Veil): Outdoor exposure (recommended)
Fire-Rated Surface: Class 1 flame spread (ASTM E84)
Standard Lengths
Stock lengths: 6m, 12m
Custom cut-to-length: Available
Maximum continuous length: Up to 12m
Standards & Quality
Manufactured to Comply With:
ASTM D3917 – Dimensional Tolerance of Pultruded Shapes
BS EN 13706 – Reinforced Plastics Composites – Pultruded Profiles (Parts 1-3)
ASTM E84 – Surface Burning Characteristics (fire-rated grades)
ISO 14706 – Petroleum and Natural Gas Industries – GRP Systems
Quality Control:
Batch testing per ASTM standards
Material Test Reports (MTR) available upon request
Dimensional tolerance: ±1.0mm (standard), ±0.5mm (precision grade)
Technical Documentation & Support
Available Technical Resources:
CAD drawings (STEP, DWG, DXF formats)
Chemical resistance charts
Load tables and span charts
Installation manuals
Material test certificates
Fire test reports (for FR grades)
Engineering Support Services:
Structural load calculations
Connection design assistance
Material selection guidance
Project-specific specifications
Custom fabrication capabilities
Related Capabilities
Pultruded FRP profiles
FRP rebar
Pultrusion machines
Pultrusion dies
Need GRP square tubes for your project? Contact our team for a quote and technical assistance.
Frequently Asked Questions
What are GRP square tubes used for?
GRP square tubes are used for lightweight structural frames, platforms, handrails, equipment supports, wastewater treatment structures, electrical substations, marine structures, and chemical processing environments where corrosion resistance and low weight are important.
What sizes of GRP square tubes are available?
GRP square tubes are available in common sizes such as 25×25 mm, 50×50 mm, 75×75 mm, 100×100 mm, 150×150 mm, and 200×200 mm. Wall thickness, length, color, resin system, and surface finish can be customized based on project requirements.
Which resin systems are available for GRP square tubes?
Common resin systems include orthophthalic polyester, isophthalic polyester, vinyl ester, epoxy, and fire-retardant polyester. Vinyl ester is usually selected for stronger chemical resistance, while fire-retardant systems are used where flame spread performance is required.
Can GRP square tubes be cut, drilled, or fastened on site?
Yes. GRP square tubes can be cut with abrasive wheels, carbide-tipped saw blades, or diamond blades, and they can be drilled with HSS or carbide drill bits. Stainless steel fasteners, proper washer sizes, and controlled torque are recommended to avoid crushing the composite wall.
How do I request a quote for custom GRP square tubes?
To request a quote, provide the profile size, wall thickness, total length, resin system, color, surface finish, quantity, and any cutting, drilling, or packaging requirements. Drawings or application details help confirm the correct material grade and production lead time.
