What Makes an Adhesive "Structural"?
A structural adhesive is one designed to carry sustained loads in a structural assembly. Unlike general-purpose adhesives that primarily seal or bond non-load-bearing elements, structural epoxy adhesives must meet specific performance criteria for tensile strength, shear strength, creep resistance, and durability under environmental exposure.
Our structural adhesive range is specifically formulated for construction applications where the adhesive bond is a critical load path in the structural system.
Our Structural Adhesive Range
Steel Bonding Adhesive (XQ-JG)
Designed for bonding steel plates to concrete surfaces for flexural and shear strengthening.- Shear strength (steel-to-concrete): ≥ 14 MPa
- Tensile strength: ≥ 30 MPa
- Bond line thickness: 1-3mm
- Thixotropic (non-sag) for vertical and overhead application
CFRP Impregnation Resin (XQ-TJ)
A low-viscosity epoxy system for saturating carbon fiber fabric in wet lay-up applications.- Viscosity: 500-1,500 mPa·s (mixed)
- Tensile strength: ≥ 50 MPa
- Elongation at break: ≥ 2.5%
- Pot life at 25°C: 45 minutes
CFRP Plate Bonding Adhesive (XQ-PB)
Thixotropic paste adhesive for bonding pultruded CFRP plates to concrete or steel substrates.- Shear strength: ≥ 18 MPa
- Tensile strength: ≥ 35 MPa
- Bond line thickness: 1-5mm
- Non-sag on vertical surfaces
Metal Bonding Epoxy (XQ-MBA)
General-purpose structural epoxy for metal-to-metal and metal-to-concrete connections.- Shear strength (steel-to-steel): ≥ 20 MPa
- Temperature resistance: -40°C to +80°C
- Chemical resistance: Resistant to fuels, oils, and most solvents
Surface Preparation: The Most Critical Step
In our experience supporting over 1,000 structural bonding projects, the single most common cause of bond failure is inadequate surface preparation. The adhesive itself rarely fails — the interface does.
Concrete surface preparation:- Remove laitance, paint, coatings, and loose material by grinding or sandblasting
- Expose aggregate to create a rough, open-pored surface
- Clean with compressed air to remove dust
- Surface must be dry (moisture content below 4%)
- Apply primer if specified in the system datasheet
- Remove mill scale, rust, paint, and contamination
- Blast clean to SA 2.5 (near-white metal) standard
- Apply adhesive within 4 hours of blasting (before re-oxidation)
- Surface roughness Ra 50-75 μm is optimal for bond strength
- No preparation needed for fresh wet lay-up (bond is formed during cure)
- For secondary bonding to cured CFRP: light abrasion with 80-grit sandpaper, solvent wipe
Application Guidelines
Structural adhesives are typically applied using either cartridge guns (for small to medium areas) or bulk dispensing systems (for large projects). Key points:
- Mix components thoroughly according to the specified ratio (our products use 2:1 or 3:1 by volume)
- Apply within the pot life — do not extend by adding solvent
- Maintain minimum bond line thickness (usually 1mm) using spacer beads or shims
- Apply continuous pressure or clamping during cure to prevent void formation
- Do not load the bond until full cure is achieved (typically 7 days at 25°C for design strength)
Choosing Between Our Structural Adhesives
For steel plate bonding to concrete: Use XQ-JG (Steel Bonding Adhesive). It is specifically formulated for this application with the right rheology, bond strength, and cure characteristics.
For CFRP fabric application: Use XQ-TJ (Impregnation Resin). Its low viscosity ensures complete fiber saturation, and its elongation matches the carbon fiber system requirements.
For CFRP plate bonding: Use XQ-PB (Plate Bonding Adhesive). Its thixotropic nature allows application on vertical and overhead surfaces without sagging, and it maintains even bond line thickness.
For general metal connections: Use XQ-MBA (Metal Bonding Epoxy). It provides high shear strength across different metal combinations and resists harsh chemical environments.
Structural Epoxy vs Polyester vs Vinylester: Which Resin System?
Not all structural adhesives use the same resin chemistry. Three resin families dominate construction adhesive formulations, and choosing the wrong one for your application can lead to premature bond failure.
| Property | Structural Epoxy | Polyester Resin | Vinylester Resin |
|---|---|---|---|
| Tensile Strength | 30–50 MPa | 15–25 MPa | 20–35 MPa |
| Shear Strength (steel-concrete) | 14–20 MPa | 5–8 MPa | 8–12 MPa |
| Elongation at Break | 1.5–3.0% | 1.0–2.0% | 3.0–5.0% |
| Chemical Resistance | Excellent | Moderate | Good |
| Shrinkage During Cure | 0.1–0.3% | 5–8% | 1–3% |
| Cure Time (25°C) | 12–24 hours | 2–4 hours | 4–8 hours |
| Service Temperature | −40 to +80°C | −20 to +60°C | −30 to +100°C |
| Substrate Moisture Tolerance | Low (standard) | Very Low | Moderate |
| Typical Cost (per kg) | $15–$30 | $5–$10 | $10–$20 |
| Best For | Load-bearing structural bonds | Non-structural lamination | Chemical plant anchoring |
Application Scenarios: Where Structural Epoxy Is Essential
Bridge Girder Strengthening with CFRP Plates
The most common structural epoxy application in infrastructure. CFRP plates are bonded to the soffit of concrete bridge girders using our XQ-PB plate bonding adhesive (shear strength ≥18 MPa). The structural epoxy transfers the full flexural load from the concrete into the CFRP plate — any weakness in the adhesive line means the entire strengthening system fails. See our CFRP bridge strengthening guide for project details.Steel Plate Bonding for Beam Shear Strengthening
Steel plates (typically 6–10 mm thick) are bonded to the sides of concrete beams to increase shear capacity. Our structural adhesive XQ-JG fills the 1–3 mm gap between the steel and concrete while transferring shear forces of 14+ MPa. This technique is used in building structure strengthening where adding new concrete is impractical.Seismic Retrofit Connections
Post-earthquake or seismic upgrade projects often require bonding steel angles, brackets, or collar elements to existing concrete columns and beams. Structural epoxy is the only adhesive type that provides both the shear and peel strength needed under cyclic loading. Read more in our seismic retrofit guide.Industrial Equipment Base Plate Bonding
Heavy equipment in factories may need chemical anchoring in addition to mechanical bolting. Structural epoxy fills the gap between the base plate and the foundation, providing uniform load transfer and vibration dampening. For larger gaps, epoxy grout is used instead.Related Resources
- Two-Part Structural Epoxy Adhesive: Types, Mix Ratios & 40 MPa Data — Understanding structural epoxy chemistry and curing mechanisms.
- Carbon Fiber Beam Strengthening: Design & Methods — CFRP plate bonding and wet lay-up for beam strengthening.
- Epoxy Anchors for Concrete Guide — When structural adhesive anchoring is needed for post-installed connections.
- Cementitious Grout vs Epoxy Grout: Selection Guide — When structural epoxy is needed for grouting vs cementitious alternatives.
- Concrete Repair Mortar: Types & Strength Comparison — Surface repair and patch materials often used alongside structural adhesive bonding.
- Structural Adhesive for Concrete: Bonding & Repair — Broader guide to adhesive selection for construction applications.
- Resin Injection for Concrete Crack Repair — Low-viscosity structural epoxy injection for concrete crack sealing.
Frequently Asked Questions
What is structural epoxy?
Structural epoxy is a two-component reactive adhesive engineered to carry sustained mechanical loads in structural assemblies. Unlike general-purpose adhesives that primarily seal or bond non-load-bearing elements, structural epoxy must meet specific performance criteria: tensile strength of 30 to 50 MPa, shear strength of 14 to 20 MPa, and creep resistance under permanent loading. Structural epoxy is used in construction for bonding steel plates to concrete beams, attaching CFRP carbon fiber reinforcement, connecting precast elements, and anchoring metal fixtures to concrete or masonry substrates. It is the highest-performance adhesive class available for construction, surpassing polyester and vinylester resin systems in bond strength and dimensional stability.
What is the difference between structural epoxy and regular epoxy?
Regular epoxy (also called general-purpose epoxy) is designed for sealing, filling, and light bonding with tensile strength typically between 10 and 20 MPa. Structural epoxy is formulated with specialized hardeners and fillers to achieve tensile strength of 30 to 50 MPa and shear strength exceeding 14 MPa, with engineered rheology (thixotropic paste for vertical application or flowable for gap filling). Structural epoxy also undergoes quality testing per standards such as GB 50728-2011 and ASTM D1002 for lap shear strength, while regular epoxy has no structural performance standard. In construction, using regular epoxy instead of structural epoxy on load-bearing bonds is a code violation and a safety risk.
Can structural epoxy bond metal to concrete?
Yes. Structural epoxy metal-to-concrete bonding is one of the most common applications in structural strengthening. Our structural adhesive XQ-JGN achieves shear bond strength of 18+ MPa between steel and concrete — exceeding the tensile strength of the concrete substrate itself. The key to successful bonding is surface preparation: the concrete must be ground to expose aggregate (CSP 3 to 5), and the metal must be cleaned to Sa 2.5 standard with all rust, oil, and mill scale removed.
What temperature can structural epoxy withstand?
Standard structural epoxy maintains full performance from minus 40 to plus 80 degrees Celsius in service. The critical parameter is the glass transition temperature (Tg), which ranges from 50 to 80 degrees Celsius for standard formulations. Above Tg, the epoxy softens and loses load-bearing capacity. For applications requiring higher temperature resistance, our high-temperature structural adhesive XQ-JGN-HT has a Tg of 100 degrees Celsius, maintaining structural performance up to 80 degrees Celsius continuous service.
How long does structural epoxy take to cure?
At 25 degrees Celsius, structural epoxy typically reaches handling strength (approximately 10 MPa) in 8 to 12 hours and full design strength in 7 days. Cure time is highly temperature-dependent: at 15 degrees Celsius, double the cure time; at 35 degrees Celsius, cure is approximately 40 percent faster. Our rapid-cure option reaches handling strength in 4 hours at 25 degrees Celsius for time-critical applications such as bridge CFRP plate bonding where lane closures must be minimized.
Is structural epoxy waterproof?
Cured structural epoxy is highly water-resistant and impermeable. However, standard structural epoxy must be applied to dry substrates — moisture on the bonding surface during application can reduce bond strength by up to 50%. For damp or wet conditions, our moisture-tolerant structural epoxy XQ-JGN-W or vinylester alternatives are formulated to bond to moisture-saturated concrete. Once fully cured, all our structural epoxies withstand continuous water exposure without degradation.
Browse all structural adhesives or request technical data for your specific application.