Assessment of sulfate resistance in fresh-to-fresh hybrid systems: a comparative study

Composite systems (CSs) incorporating various concrete types have been introduced to enhance the performance of conventional concretes, while optimising the economic applicability of modern concretes. Systems consisting of layers of high-performance fibre-reinforced concrete (HPFRC) in tension combined with compressive self-consolidating concrete (SCC) layers were evaluated in a severe sulfate environment. A variety of HPFRC materials were utilised, including ultrahigh-performance fibre-reinforced concrete (UHPFRC) and engineered cementitious composites reinforced with steel and polyvinyl alcohol (PVA) fibres. The samples were cyclically exposed to a concentrated sodium sulfate solution and physicomechanical changes and microstructural variations in the bonded layers, particularly the interfacial properties, were evaluated at 90 and 180 days. Exposed CSs were examined for visual appearance, mass gain, flexural and compressive strength, and tensile bond strength. Compared with monolithic SCC, CSs demonstrated superior chemical resistance under prolonged exposure to an extreme sulfate environment. Based on scanning electron microscopy and energy dispersive X-ray spectroscopy, the UHPFRC-based system demonstrated higher resistance to deterioration. Interfacial fibres had a varying effect, with PVA fibres showing slightly better residual retention. Interfacial zones of CSs were also affected by high concentrations of reaction products from SCC, although the effects were less pronounced for UHPFRC layers.