A new multi-scale approach to characterize concrete creep - Phase 2: drying effects, application to emerging concretes and micro-to-macro validation

最近对71座混凝土桥梁进行了近20年的监测，结果表明，桥梁的徐变挠度不断增大，远远超出了一些国家规范的预测。随后敲响了警钟，指出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理解和表征混凝土的长期徐变，最后，提高目前预测长期行为的设计方法。原因是混凝土徐变取决于许多因素，因此发表的结果往往存在争议。从几个月的压缩蠕变试验中预测几十年后结构的蠕变挠度是一个巨大且容易出错的挑战。目前表征混凝土徐变的方法耗时长，在实践中应用较少(3)。此外，关于新兴现代混凝土的蠕变数据的可用性有限，例如超高性能混凝土（UHPCs），它们在促进设计寿命和降低维护成本方面具有巨大潜力，从而实现更可持续的基础设施系统(4)。通过开拓一个创新的新研究方向，该研究项目旨在建立一个开创性的多尺度方法，以增强当前在表征方法，设计精度和先进材料开发方面对混凝土徐变的了解。本研究计划的具体目标有六个方面：(1)通过创新的微力学测试，促进对环境变化下混凝土徐变机制的认识；(2)开发一种多尺度方法，将测量的性能从水泥浆体水平提升到混凝土水平；(3)验证所提出的双尺度方法（2SM），以表征不同水灰比下的水泥浆、砂浆和混凝土，重点是现代混凝土和新兴的UHPC；(4)为加拿大混凝土结构长期挠度预测设计做出贡献，改进现行做法；(5)建立混凝土细观力学的国际卓越网络；(6)培养创新产业所需的高度专业化能力。

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. The reason is that concrete creep depends on many factors so that published results are often controversial. Predicting the creep deflection of structure after decades from experimental compression creep tests lasting few months is a tremendous and error-prone challenge. Current methods to characterize concrete creep are time consuming and rarely applied in practice (3). Furthermore, there is a limited availability of creep data on emerging modern concretes, such as ultra-high performance concretes (UHPCs), which have enormous potentials for fostering the design lifetime and reduce the maintenance cost towards a more sustainable infrastructure system (4). By pioneering an innovative new research direction, this research program aims setting-up a groundbreaking multi-scale approach for enhancing current knowledge on concrete creep in terms of characterization methods, design accuracy and advanced material development. The specific objective of this research program is six-fold: (1) Foster the knowledge on the mechanisms of concrete creep under environmental changes by innovative micro-mechanical testis; (2) Develop a multiscale method to upscale the measured properties from the cement paste level to the concrete level; (3) Validate the proposed two-scale method (2SM) for characterizing cement paste, mortars and concrete at different water-to-cement ratios with emphasis on modern concretes and emerging UHPC; (4) Contribute to the Canadian design for predicting long-term deflection of concrete structures and improve the current practise; (5) Develop an international network of excellence on concrete micromechanics; (6) Train students in highly specialized competences requested for innovating industry.