Epoxy vs Vinyl Ester Injection Anchors: Cure Time, Loads and Approvals

Injection mortars for bonded anchors and post-installed rebar fall mainly into two families: epoxy and vinyl ester (including methacrylate-based and so-called hybrid systems). Both fix threaded rods, internally threaded sockets and reinforcing bars into holes drilled in hardened concrete. They behave differently on site, and the choice affects programme, installation risk and what the design may assume.

In Europe, a product's permitted use is set by its European Technical Assessment (ETA), issued against a European Assessment Document (EAD). Projects designed to ACI 318 use a different approval route, covered below.

What are the typical differences between epoxy and vinyl ester?

The table shows typical tendencies of each family. Individual products can sit well outside them, so confirm every value against the product's ETA and installation instructions.

What are epoxy and vinyl ester mortars made of?

Both are two-component mortars: the resin and the hardener sit in separate chambers of the cartridge and mix in the nozzle. A typical make-up, as declared in product safety data sheets (ranges vary between products):

The chemistry explains the site behaviour. Epoxy cures by a slower reaction between the resin and the amine hardener, so working and curing times are longer. The methacrylate cures by a fast chain reaction started by the peroxide, which gives short working times and lets curing continue at lower temperatures. The cement and water in the hybrid mortar are what the manufacturer term hybrid refers to.

How do working time, cure time and temperature compare?

Manufacturers publish two times against base material temperature:

• Working time: how long the mixed mortar can be injected and the rod or bar inserted and adjusted. (Some manufacturers call this gel time; strictly, gel time is when the resin starts to set, which ends the working time.)
• Curing time: how long before the anchor may be loaded.

The temperature that governs is the concrete at the hole, not the air. The cartridge also has its own temperature range for mixing and flow, and a stated shelf life and storage temperature.

Below the stated minimum temperature the mortar may not cure properly at all.

What changes in hot climates?

In the Gulf and similar climates, heat is usually the bigger site risk:

• Working time shrinks. In hot concrete a vinyl ester can start to gel before a long bar is fully inserted. Check the working time at the actual concrete temperature, not at +20 °C.
• Maximum temperature. ETA cure tables usually stop at a stated maximum base material temperature. Installing above it is outside the assessment.
• Sun-exposed concrete can be much hotter than the air. Measure it, and where possible install in the cooler part of the day.
• Cartridges must be stored out of direct sun and within the manufacturer's temperature range.
• Cure-time gap narrows. Epoxy cures faster in heat, so its programme disadvantage is smaller than in temperate climates.

Service temperature is a separate check. ETAs state long-term and short-term service temperature ranges. Anchors on sun-exposed roofs and façades or near hot equipment need a product assessed for the temperatures they will actually reach.

Does the resin change the load capacity?

There are two different design situations.

Bonded anchors (for example a steel base plate on threaded rods) are designed to EN 1992-4 (Design of fastenings for use in concrete), using parameters from an ETA based on EAD 330499 (bonded fasteners for use in concrete). The design checks steel failure, concrete cone, combined pull-out and concrete failure, splitting, concrete edge failure in shear and pry-out. Spacing and edge distance enter most of these checks. The resin mainly affects the combined pull-out and concrete check; the other concrete checks depend largely on the concrete, geometry and embedment.

Post-installed rebar (for example extending a slab or adding a wall) is designed to EN 1992-1-1 (Eurocode 2: Design of concrete structures), for both end anchorage and lap splices, with the bond values in the product's ETA (EAD 330087, the successor to the EOTA TR 023 assessment). For end anchorage and moment-resisting concrete-to-concrete connections, EOTA TR 069 offers an alternative method for products assessed to EAD 332402.

Some design inputs are easy to miss:

• Concrete strength. ETAs cover a stated concrete strength range, commonly C20/25 to C50/60. Weaker concrete is outside the assessment unless the product covers it.
• Sustained load. ETAs for bonded anchors give a factor for sustained load, which can reduce bond resistance. Some products are also assessed for a 100-year working life.
• Fatigue and repeated dynamic load (for example crane rails or vibrating machinery) are not covered by a standard ETA for static loads. They need a separate product assessment.

Long embedment needs enough working time: a 16 mm bar set 500 mm deep takes time to inject and insert correctly, and hot concrete shortens that time further. Check that the product's working time at site temperature covers the longest bar. Our guide to post-installed rebar design covers the design approach in more detail.

Which approvals apply: ETA or ICC-ES?

ETAs are the approval route for design to EN 1992-4 and EN 1992-1-1. Projects designed to ACI 318 use a different route: adhesive anchors qualified to ACI 355.4, usually shown by an ICC-ES evaluation report (ESR). The Saudi Building Code SBC 304 is based on ACI 318, so Saudi specifications often ask for this route. Check which approval the specification requires; an ETA alone may not satisfy an ACI-based specification, and the reverse also applies.

How do wet holes and drilling method affect bond?

Hole condition and installation quality have a large effect on bond, whichever resin is used. The ETA states which hole conditions and drilling methods are covered, and may reduce resistance for some of them.

• Dry, wet and flooded holes. Many products are assessed for dry and water-saturated concrete. Fewer are assessed for water-filled (flooded) holes, and some have reduced design values in wet conditions. Underwater installation needs a product specifically assessed for it.
• Drilling method. Hammer drilling is the reference method. Diamond core drilling leaves a smooth hole that reduces bond unless the product is assessed for it. Some ETAs accept hollow drill bits with vacuum extraction as a cleaning method.
• Cleaning. Dust left in the hole acts as a release layer. Follow the specified sequence of blowing and brushing, with the correct brush diameter and compressed air where required. Poor cleaning is a common cause of reduced capacity in the field.
• Mixing. Use the manufacturer's mixer nozzle, and discard the first strokes until the mortar is an even colour. Unmixed mortar may never cure fully.
• Overhead and horizontal holes. Use mixer extensions and resin stoppers as specified to avoid air voids, especially for long embedment.
• Handling. Resin mortars are hazardous chemicals; follow the safety data sheet and use the specified protective equipment.

Installer training and site supervision matter as much as product choice. Where the design relies on high bond values, the engineer may specify confirmation testing on site.

What about seismic and fire performance?

Seismic and fire suitability are product-specific assessments, not properties of the resin family. For anchors, ETAs can include seismic performance categories C1 and C2 for use under EN 1992-4, often with separate design values and sometimes restricted diameters. For post-installed rebar, ETAs under EAD 330087 can include bond resistance at elevated temperature for fire design. All resin mortars lose strength at high temperature, so fire-exposed connections need the product's fire data, not an assumption.

How do you compare two products?

Start from the design requirement and site conditions. Shortlist the products whose approvals cover them, then use design resistance, programme and cost to choose.

Then confirm the installation method, cleaning procedure and any site testing with the engineer. The anchoring systems catalogue for the products SHM360 supplies is on our technical documents page.

How SHM360 helps

AnchorDesign360^™ is SHM360's tool for post-installed connections in existing concrete. It works in three steps: design, compare, choose.

1. Design. Model the connection and check it to the right document: base plate anchors to EN 1992-4, post-installed rebar end anchorage and lap splices to EN 1992-1-1 with EOTA TR 023, and end anchorage and moment-resisting connections to EOTA TR 069 where the product is assessed for it. Each check uses the product's own ETA data.
2. Compare. Run the same design across epoxy and vinyl ester mortars from different manufacturers and see side by side which products work for your case and how they perform.
3. Choose. Select the product that suits the job, with the design basis behind the choice.