Debonding of the fiber-reinforced polymer (FRP) reinforcement due to shear stresses is a very significant issue in design of concrete structures. Several experimental and theoretical investigations have been carried out to produce a relationship between the shear bond strength and the governing variables. However, existing empirical models do not provide an accurate prediction due to the complexity of the debonding process. In the present study, group method of data handling (GMDH) network as a novel machine learning approach was employed to predict the externally bond strength between FRP composites and concrete structures. The GMDH model was developed based on a reliable database including 342 experimental tests obtained from literature. The GMDH results were compared to the most common existing equations and also to the regression approaches developed in this study through statistical error parameters. Furthermore, some correction factors for four well-known equations were suggested based on regression approaches to improve their accuracy. Results indicated that the developed GMDH model outperformed the existing equations and also the developed regression-based equations in terms of both accuracy and safety aspects. Finally, parametric and sensitivity analyses were performed for further verification of the developed GMDH model in capturing the underlying physical behaviors of bond strength.

In this study, we made examinations by means of experimental and analytical methods for the purpose of grasping the shear characteristics of RC walls with corroded reinforcement and verified the validity of analytical models and constitutive laws by making comparisons between the analysis results and the test results. First of all, we conducted two kinds of element tests on the bond strength of reinforcement greatly affected by corroded reinforcement and tension stiffening of concrete around the reinforcement. Regardless of the diameter of the reinforcement, we found that the bond strength could be assessed using past equations and, for tension stiffening, proposed an equation in consideration of the effect of corrosion weight loss by obtaining data of constitutive laws for FEM analyses. Then, we conducted in-plane shearing tests on the RC walls with corroded reinforcement and found that the effect of the corrosion weight loss of reinforcement on the shear strength of the walls was low. Finally, we found that the analyses almost follow the test results through FEM analyses of the RC shear walls with corroded reinforcement using the constitutive laws ob-tained through element tests and past constitutive laws.

The durability performance of palm oil fuel ash engineered alkali-activated cementitious composite (POFA-EACC) mortar exposed to different acid solutions is assessed in this study. 50 mm cubic specimens used for the study were prepared from 100% POFA, alkali-activator (Na₂SiO_3(aq)/NaOH_(aq)) ratios of 2.5, different molarities (10, 12 and 14 M) of NaOH_(aq) and 2% volume fraction of polyvinyl alcohol (PVA) fibres. Specimens were exposed to 10% H₂SO_4(aq), 10% HNO_3(aq) and 10% HCl_(aq) at pH of 0.56, 0.52 and 0.42 respectively for 3, 6 and 9 months, with unexposed specimens as control. Small changes in compressive strength were identified with POFA mortar specimens during exposure to H₂SO_4(aq), while exposure to HNO_3(aq) and HCl_(aq) greatly reduced the strength of the POFA mortar specimens. The results were supported through microstructural examinations using SEM, while the characterization was done using XRD and FTIR. The high resistance of POFA-EACC mortar to H₂SO_4(aq) is the contribution received through the formation of gypsum, which hinders the infiltration of more acids into the matrix microstructure.

In order to investigate the influence of crack through section, which is often generated under cyclic loading, on the shear behavior and strength of RC beam, an experimental method was attempted to introduce a 0.5 mm and 1.0 mm-wide pre-crack, respectively, in beam cross section. The two pre-cracked beams were tested and the shear behav-iors were compared with a non-cracked one. As the experimental result, it was found that the section pre-cracks led to a significant degradation of shear strength. Secondly, the different shear behaviors between pre-cracked and non-cracked beams were simulated by 3-D RBSM, and the developments of shear resistances were decomposed into the shear con-tributions provided by beam action and arch action. Based on the decomposition result, it became clear that the arch actions governed the grades of shear strengths and the section pre-cracks played a primary role in inducing extra crack behaviors and obstructing the longitudinal compressive stress transfer in concrete.

Supplementary cementitious materials, such as fly ash (FA) and ground granulated blast furnace slag (BFS), are usually used to replace cement for improving the properties of concrete and achieving environment-friendly concrete. In this paper, the time-dependent fluidity of fresh concrete with mineral admixture was discussed by the time-dependent Discrete Element Method (td-DEM). The effects of FA and BFS on the time-dependence were taken up. A gravity-induced funnel flow test of fresh mortar, in which FA or BFS replaced part of Portland cement, and the numerical funnel flow simulation using the td-DEM, were conducted respectively. The agreement of the numerical and experimental results indicates that the td-DEM is applied to the prediction of the change of fluidity of fresh concrete containing FA or BFS. The numerical and experimental results also show that the use of mineral admixture would decrease the rest time-dependence of fresh concrete.