Study of Alkali- Aggregate Reactivity of Aggregates from the Seti River and Its Downstream Confluences

Abstract

Concrete is the most widely used construction material in the world and aggregates constitute approximately 70-80% of its volume. In Nepal, river sourced alluvial aggregates are predominantly used in construction due to their availability and cost-effectiveness. The long-term durability of concrete structures can be compromised by alkali-aggregate reaction (AAR), a chemical reaction between alkalis (primary source cement) and reactive minerals in aggregates. AAR leads to internal expansion, cracking, increased permeability, and eventual deterioration of concrete. In Nepal, systematic evaluation of AAR has been limited and no comprehensive study has been conducted covering whole river system. This study evaluates the potential for alkali-aggregate reactivity of aggregates obtained from the Seti River and its downstream confluences located at central-western Nepal. Five sampling locations were selected, namely, Seti River (Pokhara), Seti River (Kotre), Seti-Madi Confluence (Jhaputar), Trishuli River (Bhateri) and Narayani River (Rajahar). The samples were tested by accelerated mortar bar method (AMBT) as per ASTM C1260 to determine reactivity. Later, the effectiveness of Class F fly ash was assessed according to ASTM C1567. The study finds that alkali-aggregate reactivity varies significantly along the Seti River and its downstream confluences which may be attributed to differences in local geology and mineral composition. The 14-day expansion values vary from 0.095% to 0.264%. Fly ash is found to be effective in mitigating AAR with higher fly ash replacement being more effective. At 20% fly ash replacement, 14-day expansion ranged from 0.033% to 0.058% (suppression of 65.3% to 81.9%) which further reduced to 0.010% to 0.024% (suppression of 83.2% to 96%) at 35% fly ash replacement. Further studies with long-term concrete prism tests and evaluation of effectiveness of other locally available supplementary cementitious materials are recommended.