Mechanical properties and bond behaviour of steel rebar and fly ash-slag based geopolymer concrete in pull-out test

The behaviour of the bond between reinforcing bar and the blended fly ash (FA) and ground granulated blast furnace slag (GGBS) geopolymer concrete is investigated in this research. A single bar, each with a diameter (d(b)) of 16 and 20 mm, was pulled out from the ambient-cured M40 grade geopolymer concrete in a 4:1 (GPC 1), 2.33:1 (GPC 2), 1.5:1 (GPC 3) and 1:1 (GPC 4) blend ratio of FA and GGBS. Sodium hydroxide (SH) and sodium silicate (SS) in liquid form were used as the alkali activator solution (AAS). The fresh and hardened properties of GPC mixes were also studied. To study the pull-out behaviour of GPC mixes, concrete cubes having the cover thickness to diameter of the bar (c/d(b)) of 3.25 (20 mm) and 4.18 (16 mm) along with an embedment length (l(d)) of 150 mm were used. The l(d)/d(b) ratio for 20 and 16 mm diameter of bars was taken as 7.5 and 9.3. A single pull-out test as per RILEM recommendations was carried out, and the corresponding bond stress against slip behaviour was plotted. A maximum pull-out load of 125 kN with ultimate bond stress of 13.27 MPa was attained by GPC 4 mix with 20 mm diameter bars in comparison with other specimens. The ultimate bond stress values from experimental findings were compared with the predicted bond stress of concrete from equations proposed by several researchers.