This paper introduces the development of mesoscale modeling approach with Rigid Body Spring Model (RBSM) for concrete with damages for the multi-scale modeling of concrete and concrete structures with damages. The development works have been conducted by the team, including the authors, at Hokkaido University. The mesoscale modeling is with the generic model applicable for various damage/deterioration. The main concept for the mesoscale modeling is explained first with the literature review to show the necessity of the mesoscale modeling. As the simulation results, macroscale stress-strain relationships and crack propagation in compression and tension under static, sustained, and cyclic loadings, mesoscale damage evolution under freeze thaw cycles (FTCs), and FTCs combined with fatigue loadings and the resulting macroscale stress-strain relationships under compression and tension, and macroscale chloride diffusion of concrete with frost damages are presented. Using the numerical parametric study with the mesoscale simulation, the macroscale stress-strain relationships in compression and tension of concrete with frost damages are presented and applied to the macroscale FEM analysis for concrete beam with frost damage. The analytical results show the applicability of this multi-scale modeling approach. Meanwhile, the remaining tasks for the developed mesoscale modeling approach with RBSM are summarized, which are development of 3D models, extension of generic mesoscale model to other damage/deterioration cases, interface model between concrete and reinforcement, model for materials other than cement concrete, microscale modeling of materials, multi-field analysis, modification of the inaccuracy with modeling by RBSM as well as the experimentation in mesoscale.
Loop reinforcements are commonly used to connect precast concrete elements. However, the strength of the connection in the case of short overlapping lengths and mortar joints has not been discussed extensively in previous studies. Moreover, proposed equations tend to overestimate the flexural capacity of the connection. In this study, six specimens with vertical loop steel bars and mortar joints were tested. The variables were the overlapping length, bending diameter, and distance between the loop bars in a pair. The findings indicated that reducing the overlapping length and bending diameter decreased the flexural strength of the loop joint. Further, the distance between the loop bars had a negligible effect on the bearing capacity but a significant impact on the ductile performance. Three-dimensional finite element analyses were conducted to predict the mechanical behaviors of the loop connection. The numerical simulation corroborated the experiment results in terms of the load-deflection behavior and reinforcing bar strains. Finally, empirical equations were proposed to predict the flexural strength of multiple vertical loop connections.
This paper presents an experimental campaign and a numerical model that describe a potential application of magnetic nanoparticles (MNP) in oil well cementing pastes. MNP, when subjected to an oscillating magnetic field, can generate a heating effect, which accelerates the cement hydration reaction through thermal activation. This, in turn, reduces the downtime during expensive oil well cementing operations. An experimental campaign was conducted to verify the heating potential of MNP in cement pastes. The obtained results were used to calibrate and validate a numerical model implemented in an optimized computer code, which takes into account the thermo-chemical coupling of the hydration reaction. The early application of the numerical model successfully reproduced the thermal behavior of MNP-blended cement pastes exposed to inductive heating. Furthermore, it has the potential to be extended for simulating the behavior of cement pastes in oil wells during the early ages when the effects of hydration are crucial to consider.
The setting time of ternesite-calcium sulfoaluminate (TCSA) is shorter while the fluidity and mechanical strength are lower. Herein, the method of synergistic regulation of polycarboxylate superplasticizer and β-cyclodextrin was adopted to solve this. The results showed that, compared to the reference sample, when 0.25% polycarboxylate superplasticizer compounded with 0.05% β-cyclodextrin, the setting time of TCSA was prolonged, the fluidity was increased by 118% (201 mm). And the net pastes compressive strength reached 73.9 MPa and 103.8 MPa for 7 days and 28 days, respectively. In addition, under the synergistic effect of polycarboxylate superplasticizer and β-cyclodextrin, the early hydration heat rate of TCSA was decreased significantly, which delayed the formation of ettringite. Meanwhile, the density of the product was improved due to the reduction of water consumption, thus achieving the effect of normal setting time, suitable fluidity and high mechanical strength. Moreover, the cement has good frost resistance.
The fundamental reason to the unsatisfied performance of recycled aggregate concrete (RCA) is the existence of attached mortar in recycled concrete aggregate. A new physical method to remove the attached mortar is proposed in this study, which is a multiple process based on freeze-thaw cycles (FTCs). This study adopts the freeze-thaw modification technology and explores its efficiency in treating RCA with different strength and water to cement ratio. The effectiveness of such modification method is also evaluated, where the results show that for w/c=0.68, 10-15 freeze-thaw cycles are needed, for w/c=0.54, 15-20 FTCs are required and for w/c=0.38, 20-25 cycles are necessary. The mechanical properties of unmodified recycled aggregate concrete (RAC) are related not only to the strength of the parent concrete, but also to the content of attached mortar of recycled concrete aggregate and its durability which is further related to the water-cement ratio (w/c). On the other hand, modified recycled aggregate concrete (mRAC) could achieve satisfied mechanical properties and long-term properties as using natural aggregate concrete (NAC).