Solver-free artificial intelligence framework for stress-strain prediction and rapid field visualisation of fibre-reinforced polymer-confined recycled aggregate concrete

Accurately predicting the stress-strain behaviour of fibre-reinforced polymer (FRP)-confined recycled aggregate concrete (FRCRAC) remains challenging due to the complex mechanics introduced by recycled aggregate. This study presents a novel two-stage machine-learning (ML) framework with a mechanics-inspired visualisation module (MIVM) to predict and visualise the stress-strain behaviour of FRCRAC. First, Optuna-optimised Categorical Boosting (CATO) models are used to predict ultimate axial strength, axial strain, and hoop strain. These predictions are then integrated into Long Short-Term Memory (LSTMO) models to construct full axial and hoop stress-strain curves, forming a CATO-LSTMO framework. Trained via ten-fold cross-validation on a combination of 194 experimental and 600 synthetic datasets, CATO-LSTMO significantly outperforms conventional analytical and hybrid models with a coefficient of determination, R2, above 98 %. Secondly, a solver-free MIVM that replicates finite element (FE) behaviour is proposed to enhance the physical interpretation of ML models. Three dedicated Categorical Boosting regressors are trained on over 350,000 nodal field outputs from Abaqus simulations to predict the three-dimensional stress, strain, and displacement distributions across 21 loading frames. These predictions are then scaled using the previously ML-generated stress-strain curves to reconstruct the frame-wise three-dimensional contour plots. The MIVM visualisations achieve results comparable to Abaqus outputs, while being about 500 times faster. This study contributes to artificial intelligence through novel hybrid ML frameworks and solver-free surrogate visualisation. The integrated CATO-LSTMO-MIVM framework is deployed as an interactive web application, and it offers practical use in engineering applications for rapidly estimating and visualising the stress-strain behaviour of FRCRAC.