Double-Lap Shear Behaviour of Epoxy-Bonded Timber–Concrete Composite Joints: A Four-Group Comparison of Adherend Arrangement and Pre-Bond Curing Regime

Direct adhesive bonding could simplify prefabricated timber–concrete composite (TCC) construction, but the measured capacity of small bonded joints can depend on both substrate condition and specimen arrangement. This study reanalyses 32 traceable nominal shear-strength observations from epoxy-bonded radiata pine–concrete double-lap specimens arranged as concrete–wood–concrete (CWC) or wood–concrete–wood (WCW) and bonded after nominal 7-day or 28-day pre-bond concrete curing regimes. Mean strengths were 1.36 and 1.53 MPa for CWC-7D and CWC-28D and 1.10 and 0.61 MPa for WCW-7D and WCW-28D, respectively. A heteroscedasticity-robust two-factor analysis identified a configuration × curing-regime interaction (p = 0.016): the CWC–WCW contrast was modest at 7D but pronounced at 28D. Post-test visual inspection predominantly showed removal of the concrete surface layer rather than clean adhesive–concrete separation. The experimental archive does not contain companion age-specific concrete-strength measurements, quantitative surface-moisture measurements, specimen-level fabrication dates, local bond-line slip, or specimen-level post-cure bond-line thickness measurements, and fabrication was not fully randomised. Accordingly, the curing regime is treated as a process label and the interaction as an empirical feature of the four tested groups rather than an isolated concrete-maturity effect. The results underline the need to report adherend arrangement, pre-bond substrate condition and fabrication history together when interpreting direct adhesive TCC shear tests.