Deterioration Mechanism Of Recycled Aggregate Concrete (RAC) Based On Interface Parameters And The Application Of RAC

It is of vital importance to recycle the large amount of waste concrete, to ensure the sustainable development of the construction industry. Compared with natural aggregate concrete (NAC) structures, however, recycled aggregate concrete (RAC) structures are generally inferior in their structural performance and durability. The predominant reason for such inferiority lies on the more types and larger amount of interface transition zones (ITZs) in RAC than in NAC, which presents the essential difference between the two categories of concrete on the micro scale, as well. Till now, no integrated interface parameters which are able to reflect multi interface properties of RAC have been reported yet, meanwhile the relation between the selected interface parameters and the material properties and structural performance of RAC has not been described quantitatively, either. Besides, more effective techniques need to be explored, in order to improve the properties of RAC; the application of RAC in real projects acquires more trials, as well, whose feasibility needs more first-hand data from in situ monitoring of RAC projects.Therefore, this study focuses on how the interface properties of NAC and RAC on the micro scale can influence the material properties and structural performance on the macro scale. It is also the main concern of this research to reveal the geometric and mechanical features of different types of ITZs contained in RAC and NAC, and based on which to select out the proper interface parameters which are physically reasonable and meanwhile can reflect multiple interface properties comprehensively. Based on the selected multiple interface parameters, this study aims to establish the degradation models of RAC’s properties on both the material scale and the member scale, and moreover, to reveal the mechanism of the differences in the degradation process of reinforced RAC and NAC structural performance, under the coupled effects of loads and Cl- diffusion. The main work of this research can be summarized as follows:1) Three groups of RAC with different replacement ratios of recycled coarse aggregates (RA) were prepared. The mechanical properties and durability of these RAC groups at different ages, on the macro scale, were experimentally studied, meanwhile the properties of the diverse types of ITZs contained in RAC were tested by the nanoindentation technique, on the micro scale. The development patterns of RAC’s material properties were explained by revealing the characteristic features of the different types of ITZs contained, meanwhile the mechanism of RAC’s inferiority to NAC in material properties was also revealed, qualitatively, based on the comparisons between RAC and NAC in their ITZ properties.2) Three groups of RAC beams with different RA replacement ratios were designed to bear both sustained loads by the paired anchorage and the Cl- diffusion by cyclic wetting-drying in NaCl solution, for a long period of time in laboratory. The coupled effects of loads and environmental influence on these beams mainly imitated the serving conditions of concrete memebers in splash zones in marine envimonent. The cracking patterns on the surface of these beams as well as those inside the concrete of these beams were monitored and recored at different ages through the long period of coupled damages, based on which the durability properties of these reinforced RAC beams were compared and analyzed. The deterioration of these injuried RAC beams’flexural performance was experimentally studied, and the mechanism of such deterioration was revealed.3) Three basic requirements for assessing the rationality of the integrated ITZ parameters of RAC were suggested. Based on these requirements, the effective integrated interface parameters which can comprehensively reflect both the geometrical and the mechanical properties of all the different types of ITZs, meanwhile are able to correlate well with RAC’s performance on both the material scale and the member scale, were raised and compared. Based on the design, assessment and selection of the integrated ITZ parameters, the mechanism of the degradation of RAC’s material properties and memeber performance were revealed.4) RA was surface treated by two groups of strengthening slurries containing different types and dosages of nano-materials. The modification effects of such treatment on RAC material properties were experimentally studied, by taking use of the SEM and the nanoindentation techniques. The mechanism of the improvement of RAC by such treatment using these two nano-slurries were thereby revealed, from the angle of the enhancement of ITZs and the old adhering mortar.5) RAC was applied in three real projects, with the application levels improving gradually. The material properties of these RAC materials were tested in laboratory, while the long-term structural performance of the targed members were monitored on construction site. By comparing the performance of RAC with that of NAC applied in the same structure, the feasibility of RAC’s application was verified, from the angle of both mechanical strength of concrete materials and anti-cracking properties of reinforced members.The output of this project will not only help reveal the degradation models of RAC materials and structures and the corresponding mechanism, but also contribute to the supplement of the theory system of concrete study. Moreover, the exploration in the modifying techniques by using nano-materials may help promote the high-level utilization of RAC in the construction industry. Additionally, the trials of applying RAC in different members in real projects and the in-situ performance monitoring will provide the first-hand data and valuable guidance to RAC’s application and construction.