Experimental research on bond behavior of high-strength reinforcement in unconfined fly ash-based geopolymer concrete

Geopolymer concrete (GPC) has emerged as an eco-friendly ‌alternative‌ to ordinary Portland cement (OPC) concrete in ‌recent years‌, due to its ‌superior‌ mechanical properties and durability. A series of 48 pull-out tests investigated the bond behavior of high-strength reinforcement in unconfined fly ash-based GPC considering factors such as bond length, rebar size, concrete strength and cover thickness, and the results were compared with OPC concrete under identical conditions. A formula of ultimate bond strength and a bond stress-slip model were presented for unconfined GPC specimens that failed by concrete splitting. Test results showed that the ultimate bond strength of steel bars in unconfined GPC specimens increased with higher concrete compressive strength and thicker covers, but declined with longer bond lengths and larger rebar diameters. The OPC concrete specimens with a bond length of five times the rebar diameter happened in a pull-out failure mode, while the GPC specimens with the same experimental parameters failed by concrete splitting. When the steel bars had not yield until failure, the ultimate bond strengths of GPC specimens were higher than those of OPC concrete. Furthermore, increasing bond length altered the bond stress distribution and reduced the average ultimate bond strength between steel bars and GPC. The presented formula of ultimate bond strength and constitutive model can predict the experimental results of unconfined GPC specimens well with a certain safety margin.