Anchor Theory vs Reinforcing Bar Theory: When AS 5216 and AS 3600 Apply

The two design frameworks

Australia has two Standards that apply to post-installed connections in concrete. AS 5216:2021 covers post-installed anchors using the Concrete Capacity Design (CCD) method. AS 3600:2018 covers post-installed reinforcing bar using development length and bond stress concepts from reinforced concrete theory. The correct Standard depends on how the connection transfers load, not simply on whether the element is a threaded rod or a deformed bar.

When anchor theory governs

AS 5216 treats the post-installed element as a discrete fastener subject to three independent failure modes: concrete cone breakout, bond failure along the adhesive-substrate interface, and steel failure in the anchor rod. Cone breakout is the defining consideration, the embedment depth is relatively shallow (typically less than 20 times the anchor diameter), and the surrounding concrete is assumed to lack sufficient reinforcement to restrain the cone.

The cone concept drives edge distance requirements, anchor spacing rules, and the minimum member thickness specified in the manufacturer's European Technical Assessment (ETA). If your connection relies on the concrete tensile strength to resist pull-out, which most baseplate-to-concrete connections do, anchor theory applies.

When reinforcing bar theory governs

AS 3600 treats the post-installed element as bonded reinforcement that develops capacity through bond stress along its embedment length. Under this theory, cone breakout is inhibited because local or global reinforcement creates compressive struts that resist the breakout force. The classic example is a post-installed starter bar lapping with existing reinforcement in a concrete slab, the tensile load transfers from the new bar to the lapping bar through the concrete, and no cone forms.

The key assumption is that the surrounding reinforcement is fully developed and has sufficient tensile capacity to resist the applied load. If this assumption is not satisfied, the connection may behave as an anchor rather than as reinforcement, and AS 5216 should govern.

Why it matters for testing

The design theory directly determines the test configuration. Connections designed under anchor theory should generally be tested with an unconfined setup, the reaction frame must span beyond the cone diameter so the full breakout mechanism can develop. This produces conservative results that match the in-service loading condition.

Connections designed under reinforcing bar theory can often be tested with a confined setup, the reaction frame bears close to the bar, which simulates the compressive restraint provided by the surrounding reinforcement in service. Confined testing produces higher apparent capacities because breakout is restrained.

Specifying confined testing for an anchor-theory connection will produce non-conservative results. The test will show higher capacity than the anchor will deliver in service, because the test rig is restraining a failure mode that the structure does not restrain.

Practical guidance

• Review the structural drawings and engineering calculations to determine which design theory has been applied
• If the connection transfers load through a baseplate, bracket, or steel plate to the concrete surface, it is almost certainly an anchor-theory connection
• If the connection is a post-installed starter bar or dowel that laps with existing reinforcement, it may be a reinforcing-bar-theory connection, but only if the lapping reinforcement is fully developed
• When in doubt, test unconfined, it is conservative and will detect all failure modes
• The test specifier should document the design theory and corresponding test configuration in the test specification before testing commences

References

• AS 5216:2021 Design of Post-Installed and Cast-In Fastenings in Concrete
• AS 3600:2018 Concrete Structures
• AEFAC TN05 Volume 2: Proof Tests (Guidelines for Site Testing of Anchors)
• BS 8539:2012+A1:2021 Code of Practice for the Selection and Installation of Post-Installed Anchors in Concrete and Masonry