## 1 Introduction
This finding agreed with other studies carried out by [10; 15], where the bond strength in FAGC with various diameters of steel bars ranging from 12-24 mm was higher by far. Besides, [PERSON] [21] had also confirmed that the bond strength of reinforcing bar embedded in geopolymer concrete is higher than the bond strength of rebar and conventional concrete. Recently, [PERSON] [22] has been using beam end tests and statistical hypothesis tests to evaluate bond behavior of geopolymer concrete and compare with traditional Portland cement concrete. This research reported that the bond behavior of reinforced geopolymer concrete is significantly different with that of OPC. In summary, it has been shown that reinforced FAGC possesses favorable properties for its potential use as construction materials. The sufficient bonding between the rebar and concrete has a crucial effect on designing of reinforced concrete members. Thus, it is vital to have more knowledge about the bonding behavior in reinforced geopolymer concrete to apply it as a substitute material in conventional structural concrete. It is worthwhile to emphasize that while those studies provide a quick means to define the bonding performance, the preparation at the stage of casting must be noticed to ensure interfacial consistency and minimize the appearance of air void around the reinforcing bars. Along with the above-mentioned experimental studies, several researchers have also utilized numerical modelling methods to evaluate the bonding behavior of reinforced concrete [23; 24; 25]. In the studies by [PERSON] [26] and [PERSON] [27], the finite element method (FEM) was applied to simulate the bonding between concrete and several types of reinforcement, including steel bars and fiber reinforced polymer bars. It was indicated that FEM had a great potential to evaluate the bond stress and splitting failures with good agreements to experimental test results. An implementation of the FEM was also used to assess the bonding behavior of FAGC with glass fiber reinforced polymer (GFRP) bars in the work of [PERSON] [28]. The surface-base cohesive behavior framework was employed to represent the bond between GFRP bars and concrete. The bond stress distribution along the length of GFRP bars obtained from the analysis showed a similar trend with the results from the pullout test. The numerical analysis was also employed to evaluate the bonding performance of metakaolin-based geopolymer concrete with ribbed steel rods [29], fiber reinforced geopolymer concrete [30; 31], and steel-reinforced FAGC [32; 33]. However, the investigation on the effects of reinforcement size and strength of concrete on the bonding behavior in reinforced geopolymer concrete has not yet been considered thus far. Based on this background, this paper investigates the effects of geopolymer concrete compressive strength and reinforcing bar sizes on the bond performance of FAGC using both experimental and FEM. A pullout test was conducted to determine the bond stress and slip in reinforced FAGC comprising ribbed steel bars with diameters of 12, 16, and 20 mm and FAGC with various compressive strengths 20, 30, and 40 MPa. Numerical analysis by FEM was applied to simulate the bonding of the steel bars and FAGC. The bond-slip relation determined from simulation results was evaluated and compared with the experimental work. Up to now, there is no standard for geopolymer concrete. Thus, the proposed modelling in ABAQUS would be validated with experimental data. Eventually, the stress-slip curves of rebar in fly ash-based geopolymer concrete are also derived using non-linear regression for various experimental conditions. This proposed formulation and modelling could predict the bond-slip behavior of reinforced fly ash-based geopolymer concrete for practical purposes.