Adhesively bonded timber-concrete elements: Effect of concrete surface condition and maturity on the shear performance

With growing interest in adhesively bonded timber-concrete composite (TCC) elements, the question of practical manufacturability has become central. Factories must account for how fabrication variables affect shear transfer across the adhesive interface. As the continuous epoxy joints are stiff, their performance highly depends, among others, on the concrete surface and the age when bonding. This study isolates these two production-related parameters and investigates their effect on short-term shear capacity. The experimental programme, based on the Vertical Slip-Block test (VSB), examines four scenarios: abrasion of steel formwork, application of formwork oil, use of synthetic plastic foil, and the influence of the concrete maturity at bonding and loading. In addition, correlation analyses are conducted using surface morphology data from light microscopy, contact-angle measurements, and surface roughness. The obtained results yield three practice-oriented conclusions: (i) Reliable bonding requires minimum surface roughness but open porosity, (ii) Bonding to concrete younger than one week is discouraged; insufficient hydration leaves a weak boundary layer that triggers cohesive failure, and (iii) Beyond a threshold maturity, further ageing offers negligible gains, enabling shorter production cycles without sacrificing load capacity. The derived quantitative thresholds provide immediate criteria for off-site quality-control checks and support wider adoption of bonded TCC technology. • Correctly dosed formwork oil shows no penalty, overdosing adds hydrophobic residues and scatter. • Open porosity with minimal roughness is essential, steel-formwork surfaces perform well. • Bonding to <1-week concrete forms weak boundary layers and sharply reduces shear. • Synthetic foils seal the laitance, sanding/surface activation is needed for reliability.