Aggregate Size Ratio Research Articles

Explainable machine learning model for predicting punching shear strength of FRC flat slabs

Reinforced concrete slabs are vulnerable to punching shear failure at the slab-column joint, which can initiate catastrophic progressive collapse. The addition of steel fibers in the concrete matrix has emerged as an effective strategy to mitigate such progressive failure. However, the effects of the diverse mixture proportions of the concrete matrix with different types and dosages of fibers have made the accurate prediction of the punching shear strength (PSS) of the fiber-reinforced concrete (FRC) flat slabs a complex task, where the existing mechanical models have several limitations. Therefore, this study proposes an explainable XGBoost model for predicting PSS of flat slabs made with different types of FRC based on a newly established comprehensive database of 251 flat slabs including normal strength FRC slabs, high-performance FRC slabs, and ultra-high-performance FRC slabs. A customized procedure was proposed to establish the XGBoost model considering data preparation, feature selection, hyperparameter tuning and model validation. The performance of the XGBoost model was then compared with that of existing mechanical models. Finally, sensitivity analysis and SHapley Additive exPlanations (SHAP) analysis were applied to identify the most influential parameters on the prediction of PSS. Results show that the proposed feature selection method is effective in identifying six influential parameters from the eleven parameters related to the PSS of FRC flat slabs. The developed XGBoost model yielded highest prediction accuracy and lowest variation, which outperformed the other mechanical models. Sensitivity analysis also indicated similar trends of parameters in both the XGBoost model and the mechanical models. The PSS of FRC flat slabs can be improved by increasing the concrete compressive strength, reinforcement ratio, and fiber volume, and by decreasing the column width-to-depth ratio, water-to-binder ratio, and aggregate size ratio. The proposed XGBoost model could enhance the understanding of PSS of FRC flat slabs and guide future pertinent design code provisions.

Influence of the interfacial transition zone and microcracking on the diffusivity, permeability and sorptivity of cement-based materials after drying

Available experimental investigations on the effect of the aggregate–paste ‘interfacial transition zone' (ITZ) on mass transport properties of cement-based materials appear to be ambiguous. While some studies have found a deleterious effect of the ITZ, results from others seem to suggest otherwise. The present study examines the relative influences of ITZ and microcracking on the oxygen diffusivity, oxygen permeability and water sorptivity, to further enhance understanding of the links between microstructure and transport properties. Specimens of several neat cement pastes, mortars and concretes were prepared and tested. Variables include water/cement (w/c) ratio (0·3 and 0·5), binder type (CEM I and CEM II with 8% silica fume), curing period (3 and 90 days), aggregate volume fraction (0–70%) and preconditioning temperature (50°C and 105°C). 105°C drying was adapted to induce microcracking. Backscattered electron microscopy and image analysis were applied to characterise the microstructure, in particular the microcracks. It was observed that in all cases, the transport properties of mortars decreased with increasing ITZ fraction. Permeability was far more sensitive to the presence of microcracking and changes in total porosity, compared with diffusivity or sorptivity. However, no critical threshold sand content linked to an ITZ percolation effect was found, even in the deliberately damaged samples. Concretes have about the same diffusivity and sorptivity, but significantly higher permeability than mortars with the same volume fraction of aggregate, despite a lower ITZ fraction. It is argued that the higher permeability of concrete is attributable to more microcracking, and because the microcracks and paste are less tortuous than the equivalent mortar. It is also shown that conventional permeability testing is affected by the sample thickness-to-maximum aggregate size ratio, which may be another factor explaining the disparity between the measured permeability of concretes and mortars.

The influence of the permeability of concrete cover on reinforcement corrosion

Surface permeability tests were performed on near-surface concrete, which was either directly over or remote from reinforcing bars. Test specimens were 150 mm concrete cubes with either 8 mm or 16 mm diameter bars placed in either the corner or the side faces of the specimens. Permeability of cover concrete over a bar was found to be greater than that remote from a bar. Cover to maximum aggregate size ratio was identified as the most important variable in the identification of the point when there will be no significant difference between the permeability of cover concrete over a bar and that remote from a bar. Aggregate size, bar location, method of compaction and cover thickness were also found to influence near-surface concrete permeability.

Size effects in concrete fracture: Part I, experimental setup and observations

This paper presents an experimental investigation on the influence of microstructural parameters, such as aggregate size, and macroscopic parameters, such as specimen dimensions, on brittle fracture. Maximum aggregate size was used as a representative parameter of aggregate distribution in agreement with ASTM C 136 standards. Six groups of geometrically similar concrete specimens with various dimensions and aggregate sizes were prepared. Similarity of the specimens was strictly maintained by scaling the specimen dimensions from one group to another by a factor of two starting from a specimen size of (width × total depth × thickness) 105×105×12.5 mm to 1680×1680×200 mm. Two separate sets of removable pre-cast notches were designed to determine the effect of initial notch size. A considerable effort was devoted to the design of the loading fixture to have a reproducible crack initiation and controlled crack growth. Several loading fixtures were evaluated prior to selection of the one used in the experimental program. Quasi-static splitting cyclic loading in edge cleavage configuration was applied. A servo-hydraulic Instron machine was used for testing. The fracture process was monitored by optical and acoustic imaging techniques. Three forms of comparisons of the test results with respect to the specimen and aggregate sizes were adopted. The first corresponded to the various specimen sizes cast with the same maximum aggregate size. The second comparison was based on the geometrically identical specimens cast with various maximum aggregate sizes. The third form of comparison dealt with complete geometrical similarity, i.e., all dimensionless geometrical characteristics including specimen thickness to maximum aggregate size ratio were identical. Results from this study indicated that as the specimen size decreases, the envelope becomes larger within the first and third forms of comparison. In the second form of comparison, i.e., geometrically identical specimens cast with various maximum aggregate sizes, the area under the envelope was greater as the maximum aggregate size increased. The existence of a trend in dimensionless critical load-CMOD envelopes despite the apparent geometrical and physical similarity of the test conditions is the direct indication of a scale effect, i.e., the modified fracture energy, $$\overline {G_F } $$ indicates the existence of a strong scale effect: $$\overline {G_F } $$ increases with the specimen dimensions as well as maximum aggregate size.