The poor abrasion resistance of recycled aggregate pervious concrete (RAPC) limits its application. Granulated blast furnace slag (GBFS) and copper slag (CS) were used as mineral admixtures to replace cement in RAPC for improving the abrasion resistance. At the 10% GBFS replacement level, 5% to 25% of CS was added in RAPC respectively, which reduced the consumption of the cement. The effects of GBFS and CS on paste fluidity, porosity, permeability, compressive strength and abrasion resistance were evaluated. RAPC exhibited an excellent abrasion resistance after adding GBFS and CS. In this case, 10% blended amount was the optimal amount of GBFS and CS. Compared with the single cement group, the abrasion resistance increased by 38.78%, which is mainly because the fayalite in CS supplies good abrasion resistance, whereas the GBFS also plays an important role. The mechanism was studied via X-ray diffraction analysis and microhardness. The porosity and permeability of the RAPC increased with the CS replacement level. Moreover, the leaching concentration of heavy metal ions in the prepared RAPC was significantly low, indicating that RAPC could be applied safely. The results not only provides a feasible technical means for the large-scale utilisation of CS but also presents a theoretical basis for the improvement of RAPC abrasion resistance.
Microcracking induced by mechanical damage and moisture conditions both significantly modify the transport paths of aggressive species in cementitious materials. This study proposes a microscopic model to evaluate the coupling effects of microcracks and moisture conditions on gas permeability in cementitious materials accounting for its heterogeneous microstructure. The 3D microstructure of hydrating cement paste was simulated using a voxel-based hydration model, based on which the fracture process of cement paste under the uniaxial tensile loading was simulated using a finite element model. Subsequently, treating the damaged cement paste with various damage degrees as input, a lattice Boltzmann modelling framework was present to mimic the gas permeability of partially saturated cement paste considering the moisture distribution in its pore structure. Results indicate that gas permeability of cement paste with a lower water-to-cement ratio is more sensitive to the damage and the relative gas permeability is increased with the increase of damage degree. With the increasing water saturation level, the permeation paths for gas in cement paste are declined, while the air-filled microcracks as permeation paths are not significantly influenced. At a given damage degree, the increasing water saturation level leads to an increase in the relative gas permeability.
The mortar flow and flow speed of self-compacting geopolymer mortar (SCGM) were determined by considering the various influencing parameters for the normal self-compacting mortar (SCM) and geopolymer mortar (GM). The test results showed that the effect of volume of water to volume of powder on the relative flow area (Gm) and relative flow speed (Rm) was different compared to normal SCM due to the high viscosity of matrix. The flow properties of SCGM were analyzed in terms of volume of water to volume of powder ratio, and superplasticizer to powder ratio, powder to sand ratio, viscosity of alkaline liquid and the water to geopolymer solid ratio. According to the analysis, flow prediction models for SCGM were formulated based on the parameters of both physical and chemical point of view. Moreover, a mix design of SCGM including a flow chart and mathematical expression was also proposed. This indicates that the typical volume ratio of SCGM was similar to SCM considering the volume of alkaline liquid with water in SCGM was equivalent to the volume of water in SCM.
The objective of the study is to develop a model that will help in understanding the process of carbonation. This study investigates the influence of different physical, chemical and environmental parameters on the carbonation process in concrete. The model incorporates the influence of change in porosity on carbonation and the variations in the internal relative humidity of concrete due to drying on the rate of carbonation. The pore saturation and porosity were found to control the rate of carbonation in concrete. The model demonstrates the preconditioning duration and amount of water released on carbonation have an insignificant effect on the extent of carbonation in the long-term. Increased tortuosity and higher alkalinity could help in improving the carbonation resistance of system.
For the purpose of clarifying the dissolution characteristics of shotcrete used for tunnel lining of radioactive waste repositories, the cement was mixed with a quick setting admixture, and immersion tests were conducted. The quick setting paste was ground after hardening and contacted with pure water in a few months of testing. As a result, it was confirmed that more ettringite was produced in the cement paste with the quick setting admixture, which consisted mainly of calcium sulfoaluminate, than in the cement paste without the quick setting admixture. Immersion of the cement paste in water was also confirmed to result in the dissolution of Ca(OH)2 and monosulfate and further formation of ettringite. Furthermore, it was clarified that Calcium ions, Sulfate ions, and Aluminium ions are being consumed for the production of ettringite, and these ions do not readily leach to surrounding water. Based on these test results, a dissolution equilibrium model for shotcrete was developed in order to evaluate the durability of the barrier system or pH plume for the underground repositories.
The study aims to solve the problem that the concrete suitable for three-dimensional (3D) printing achieves more working performances, while the mutual influence, crossover, and contradiction among the working performances. In this study, a multi-index evaluated, gradual-optimized test based working performance is proposed. First, the concrete proportions satisfying 3D printing were obtained through orthogonal test and then gradually optimized systematically to the proportions determined by conducting an orthogonal experiment. The correlation between assessment indexes was analyzed. The test results show that fluidity and thixotropic sensitivity can be combined to evaluate comprehensively the work performance of 3D-printed concrete. The flow rate can further optimize the mix ratio, and the correlation between stacking height and number of extruded layers is weak. Moreover, the building performance of 3D-printed concrete can be evaluated from the number of layer and deformation of concrete to optimize the concrete ratio further. The suitable fluid range for 3D-printed concrete is 17.7-20.0 cm, and the corresponding stacking height is 45-57 mm. The successfully printed building components from the optimized concrete proportion further verify that the test method used in this study is reasonable and scientific, and that it can be extended to prepare other concretes satisfying more performance.
The out-of-panel flexural properties of composite slabs of reinforced concrete (RC) and cross laminated timber (CLT) joined by shear connectors (lag screws) were studied. Firstly, a push-out test was performed on the join surface of composite slabs. As a result, lag screws showed good deformation capacity properties in the push-out test. Then, a bending test was performed on composite slabs. Higher flexural properties were obtained with the deeper insertion of lag screws into CLT. Finally, the γ-method was applied to calculate initial stiffness and yield load, important factors in evaluating flexural properties of composite slabs. As a result, the calculated values showed a good correlation with experimental values.