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This paper investigates the effects of pozzolanic substitutions for ordinary Portland cement (OPC) with silica fume (SF) and metakaolin (MK) on the mechanical and toughness performances of steel fiber reinforced concrete (SFRC). Initially, a reference concrete mix with a water‐to‐binder ratio of 0.4 is blended with different volume fractions of steel fibers with varying geometry: crimped steel (CS) and straight steel (SS), both individually and in combination, to examine their mechanical properties. In the subsequent phase, the study investigates the impact of combining macro‐ and microsteel fibers on flexural toughness, to determine potential synergy for suitable combinations. Also, the possible influence of pozzolans in the variation of flexural toughness of hybrid steel fiber reinforced concrete (Hy‐SFRC) was evaluated. Hybridization of steel fibers was found effective in improving the workability of the concrete mix up to 11%. SFRC mixes containing pozzolans exhibited a significant enhancement in compressive strength, modulus of rupture, and modulus of elasticity compared to non‐pozzolanic SFRC. The hybrid combination of CS 1.5% and SS 0.5% was considered the best in terms of mechanical properties. Additionally, the results of synergy assessment showed that hybridization of steel fibers in the pozzolanic concrete mix was particularly effective in the post‐cracking stages with a positive 14% compared to a negative 8% in the pre‐cracking stage. The pozzolanic addition improved the flexural toughness of Hy‐SFRC to about 10%–20%. Blending of SF and MK in Hy‐SFRC was found effective in enhancing the toughness mechanism of concrete compared to Hy‐SFRC mixes containing binary SF and MK, indicating a stronger bond between the fibers and the matrix resulting from the pore refinement and hydration products developed at the interface. Hy‐SFRC containing a ternary pozzolanic mix of SF 10% and MK 10% gave the best results in flexural toughness, and the corresponding synergy values were found to be the maximum. The results were consistent with the morphology analysis, which revealed an increase in hydration products at the interface between the aggregate and concrete matrix, as well as between the steel fiber and concrete matrix, due to the ternary blending of SF and MK. [ABSTRACT FROM AUTHOR]

Fiber‐reinforced polymer (FRP)‐confined double‐skin tubular columns (DSTCs) are an innovative type of hybrid columns that consist of an outer tube made of FRP, an inner circular steel tube, and a concrete core sandwiched between them. Available literature focuses on hollow DSTCs with limited research on DSTCs made with inner steel tubes filled with concrete. Overall, DSTCs have many applications, highlighting the importance of studying the effects of concrete filling and strength on the composite system. To address this gap, finite element models (FEMs) and both traditional and innovative machine learning (ML) techniques were used to develop accurate models for predicting load‐bearing capacity and confined ultimate strain under axial loads. A comprehensive database of 60 experimental tests and 45 FEMs simulations of columns was analyzed, with five parameters selected as input variables for ML‐based models. New techniques like gradient boosting (GB), random forest (RF), convolutional neural networks, and long short‐term memory are compared with established algorithms like multiple linear regression, support vector regression (SVR), and empirical mode decomposition (EMD)‐SVR. Regression error characteristics curve, Shapley Additive Explanation analysis, and statistical metrics are used to assess the performance of these models using a database containing 105 FEMs test results that cover a range of input variables. While EMD‐SVR and GB perform well for confined ultimate strain, the suggested EMD‐SVR, GB, and RF models show superior predictive accuracy for confined ultimate load. To be more precise, for confined ultimate load prediction, EMD‐SVR, GB, and RF obtain R2$$ {R}^2 $$ values of 0.99, 0.989, and 0.960, respectively. The R2$$ {R}^2 $$ values for GB and EMD‐SVR at confined ultimate strain are 0.690 and 0.99, respectively. However, design engineers are limited by the “black‐box” nature of ML. In order to solve this, the study presents an open‐source GUI based on GB, which gives engineers the ability to precisely estimate confined ultimate load and strain under various test conditions, enabling them to make well‐informed decisions about mix proportion. [ABSTRACT FROM AUTHOR]

To study the performance of PVC-confined concrete columns under concentric and eccentric axial compression loads, a numerical simulation using ABAQUS software was conducted. The correctness of the model was validated with the existing test results of the stress-strain curve and failure modes of specimens. Then, the compression load was applied at 20, 30, 40, and 50 mm eccentricity. The mechanical behavior, concerning the influence of different loading conditions, the diameter of specimens, the height of specimens, and the strength of unconfined concrete were investigated. The FEM and experimental results of loads and strains had AAE of 6.46 and 11.28%, respectively. In addition, the analytical and experimental strain values had an AAE of 36.24%. The COV was 7.57 and 12.26% between experimental and FEM results for loads and strain, respectively. An analytical model was developed to predict the ultimate stress and strain of PVC-confined concrete columns, which was compared with the existing models available in the literature with AAE of 13.4 and 29.5% for loads and strain respectively. Due to small F-values between FEM, analytical, and experimental values, indicating closer to their mean values, the analytical model can be adopted in the design of these short columns. [ABSTRACT FROM AUTHOR]

When estimating the rectangular stress block of a reinforced concrete flexural member, it is important to take into account different behaviours due to load eccentricity, the ratio and type of fibre-reinforced polymer strengthening, specimen height and confinement ratio. To investigate the impact of these key variables on a rectangular stress block, an extensive analysis of an experimental database of 84 specimens reported by several independent research groups was undertaken. The accuracy and conservativeness of several models under axial compression and flexural loading were first assessed. Comparisons revealed overestimations for specimens strengthened with carbon-fibre-reinforced polymer and underestimations for those reinforced with glass-fibre-reinforced polymer. Inaccurate predictions were also obtained even after more accurate models of concentrically loaded tests were analysed using existing concrete design codes. Expressions for columns strengthened with fibre-reinforced polymer under pure axial compression and modifications for the coefficients of the rectangular stress block were developed. The results indicate a good and safe correlation of predicted strength values. [ABSTRACT FROM AUTHOR]

Waste Glass (WGs) and Coir Fiber (CF) are not widely utilized, even though their silica and cellulose content can be used to create construction materials. This study aimed to optimize mortar compressive strength using Response Surface Methodology (RSM). The Central Composite Design (CCD) was applied to determine the optimization of WGs and CF addition to the mortar compressive strength. Compressive strength and microstructure testing with Scanning Electron Microscope (SEM), Fourier-transform Infrared Spectroscopy (FT-IR), and X-Ray Diffraction (XRD) were conducted to specify the mechanical ability and bonding between the matrix, CF, and WGs. The results showed that the chemical treatment of CF produced 49.15% cellulose, with an average particle size of 1521 µm. The regression of a second-order polynomial model yielded an optimum composition consisting of 12.776% WGs and 2.344% CF with a predicted compressive strength of 19.1023 MPa. C-S-H gels were identified in the mortars due to the dissolving of SiO2 in WGs and cement. The silica from WGs increased the C-S-H phase. CF plays a role in preventing, bridging, and branching micro-cracks before reaching maximum stress. WGs aggregates and chemically treated CF are suitable to be composited in mortar to increase compressive strength. [ABSTRACT FROM AUTHOR]