Influence Of Steam-curing System On The Performance Of High Strength Concrete

High strength concrete (HSC) is the mainstream development of concrete technology in the 21th century, and has found wide application in various concrete projects, especially in the large-scale key fundamental facilities. Steam curing could improve the early strength of concrete, accelerate the circulation of moulds and increase the productivity, making it a common technique to produce concrete products. Steam curing exerts some influence on the physical and mechanical properties, volume stability and durability of high strength concrete whose evolvement in property differs slightly compared to the standard cured concrete. Based on the steam curing-related parameters (rest period, heating-up rate, thermostatic period and thermostatic temperature), this thesis undertook a systematic study on their influence on the properties of HSC, characterized the evolvement in HSC's properties, mastered the key technology of HSC's preparation and application, all of which underpinned the theoretical and technological foundation of the design, preparation and application of HSC. The main research activities and achievements were as follows:The paper put forward the evaluation methods of the adaptability of steam curing of HSC and investigated the influence of steam curing-related parameters on the physical and mechanical properties of HSC. Based on the adaptability of main components of HSC (high range water reducing admixture and mineral admixture) for steam curing, it was noted that improved Naphthalene-based water reducing admixture could increase the early and late compressive strength; fly ash and slag both demonstrated good adaptability for steam curing and slag micropowder could have a more effective pozzolanic activity, resulting a coordinated development in the compressive strength of HSC during every period.The influence of steam curing-related parameters on durability (chloride permeability, carbonation, etc) of HSC was systematic studied and it was noted that the durability of HSC could be improved with a longer rest period, a shorter thermostatic period, a lower heating-up rate and thermostatic temperature. The Grey Theory was adopted to analyze the significance of the influential steam curing-related parameters, results indicated that the influence of heating-up rate and thermostatic temperature on HSC's durability was markedly prominent, followed by rest period and thermostatic period. The evaluation model of chloride permeability and carbonation of HSC was established, which reflected the influence of steam curing-related parameters on durability of HSC and offered a guide for the service life prediction, choose of the thickness of protective layer, durability design and curing technique of HSC, base on which the model of service life prediction of reinforced concrete was established which could predict and evaluate the service life of steam-cured high strength reinforced concrete subjected to chloride ingress environment.The paper undertook an in-depth investigation into the evolvement of internal humidity, self desiccation and conductivity of steam-cured HSC and put forward an easy, reliable testing method for the internal humidity and self desiccation of HSC and established a model of self desiccation predication, which provided a theoretical foundation for the design of steam curing of HSC. It was noted that steam curing accelerated the decreasing gradient of internal humidity and increased the self desiccation of HSC, and the introduction of humidity-compensation materials to retard the decrease in internal humidity was envisaged, which could decrease the self desiccation remarkably and provide a new method to improve the volume stability of steam-cure HSC.Some sophisticated testing instruments (XRD, MIP and SEM, ere) were taken to investigate the hydration process, the profile of microstructure and pore structure. Results indicated that more hydrating products, poor crystallization, uneven distribution and impaired pore structure were the fundamental reason for its evolvement in property.