A new multi-scale approach to characterize concrete creep and its implications in long-term durability

最近对71座混凝土桥梁近20年的监测表明，徐变挠度不断增大，远远高估了几个国家规范的预测值。然后敲响了警钟，声明目前的国家设计建议和规范“不仅在理论上过时，而且站不住脚”。提出的研究计划旨在更好地了解和表征混凝土的长期徐变，并最终改进现有的预测长期行为的设计方法。 利用短期徐变试验(如3-12个月)估算混凝土的长期徐变(如30-50年)一直是土木工程中不可避免的问题。在这种情况下，新颖的微压痕技术提供了突破性的解决方案，因为它们允许在几分钟内表征长期蠕变的对数性质。从设计的角度来看，现代模型代码FIB 2010最近采用了一个对数函数来预测长期蠕变。这对于拟议的研究计划非常有吸引力，该计划旨在开发微压痕技术来快速估计用于预测混凝土长期徐变的对数函数的参数。 这项研究计划的具体目标是： (1)在微观结构层面，通过微压痕试验，充分表征和更好地了解温度和可能的损伤对水泥浆体蠕变行为的影响； (2)将在水泥浆体水平上测量的性能提升到混凝土水平； (3)实施一个数值模型，以预测结构在不同环境条件(例如温度)下的长期变形，以及可能因耐久性问题而造成的损坏； (4)改进现有的长期徐变预测设计方法； (5)发展国际卓越人才网络，培养学生高度专业化的能力。 这一创新的研究方向将为土木工程做出突出贡献，例如： (I)促进目前对混凝土长期蠕变的理解，重点是其对温度和损伤的依赖； (2)开发用于表征水泥浆体长期蠕变的超快速微压痕技术； (3)开发工程工具，以便更好地预测桥梁等混凝土结构的长期徐变； (4)为改进设计建议作出贡献，以便更好地预测混凝土结构的长期徐变，并确保加拿大未来基础设施的预期耐久性。

A recent survey on 71 concrete bridges monitored over a period of about 20 years showed that the creep deflection increases incessantly and it overestimates by far the predictions of several national codes. A wake-up call was then launched stating that the current national design recommendations and codes “are not only theoretically obsolete, but also indefensible”. The proposed research program aims at better understanding and characterizing the long term creep of concrete, and finally, to enhance the current design methods for predicting the long term behavior. Estimating the long-term creep of concrete (e.g., 30-50 years) using short term creep tests (e.g., 3-12 months) has been an unavoidable crux in civil engineering. In this context, novel microindentation techniques have been provided a breakthrough solution as they allow characterizing the logarithmic nature of the long term creep in few minutes. From a design point of view, the modern model code fib 2010 has recently adopted a logarithmic function for predicting the long term creep. This is extremely appealing for the proposed research program which intends to develop micro-indentation techniques to rapidly estimate the parameters of such logarithmic function for predicting the long term creep of concrete. The specific objectives of this research program are: (1) At the microstructure level, to fully characterize and better understand the effect of the temperature and possible damage on the creep behaviour of a cement paste by means of microindentation tests; (2) Upscale the properties measured at the level of a cement paste to the concrete level; (3) To implement a numerical model for predicting the long term deformation of a structure at different environmental conditions (e.g., temperature) and with possible damage due to durability issues; (4) To improve the current design methods on the prediction of the long term creep; (5) To develop an international network of excellence and train students in highly specialized competences. This innovative research direction will provide outstanding contributions to civil engineering, such as: (i) Fostering the current understanding on the long term creep of concrete with emphasis on its dependence on temperature and damage; (ii) Developing of ultra-rapid microindentation techniques for characterizing the long term creep of cement paste; (iii) Developing engineering tools for better predicting the long term creep of a concrete structures, such as bridges; (iv) Contribution to enhanced design recommendations for better predicting the long term creep of concrete structures and assuring the desired durability of the future Canadian infrastructure.