Adhesive Bond Strength of Steel Bars Embedded in Fly Ash-GGBS-based Geopolymer Concrete

The bond behaviour of plain rebars embedded in ambient cured geopolymer concrete (GPC) prepared with a mix of fly ash and ground granulated blast furnace slag (4:1 by mass) was studied to evaluate the adhesive bond between these rebars and concrete. As GPC is an inorganic polymer concrete, the adhesive bond strength of concrete is evaluated for its suitability to reinforced concrete applications. Pull-out tests were conducted to measure the slip of the applied load. Diameters of the embedded bars were varied as 12, 16 and 20 mm, providing compressive strengths of 39 to 68 MPa. Chemical adhesion occurs between the bar and concrete as the bar surface is polished. We observed a gain in adhesive bond stress as bar diameter is increased. The bond stress-slip curves of GPC and ordinary Portland cement concrete were different and the peak bond stress was about 70% higher in GPC. Regression analysis showed that the adhesive bond strength in GPC is about one-fourth of the bond strength of the deformed rebars. Higher bond strength in GPC is due to chemical bonding and formation of both sodium aluminosilicate hydrate and calcium aluminosilicate hydrate gels besides the denser interface as seen from scanning electron microscopy and the ordering of silicon and aluminium as identified in MAS-NMR.