Actively controlled temperature induction to strengthen reinforced concrete structures

许多既有钢筋混凝土结构需要加固。这样做的典型动机是针对增加的荷载进行升级，例如由于交通原因而出现在桥梁上的荷载或由于额外服务荷载而发生在建筑物上的荷载。额外加固的关键问题和基本限制是，如果不临时提升或施加预应力，额外加固对自重是无效的。因此，只有使用荷载按比例分配给现有结构和新放置的钢筋。特别是对于自重高达70%左右的钢筋混凝土结构，提出了一种全新的解决方法，即在局部温度感应过程中加装钢筋。这种方法对梁和板都适用。它以快硬高性能混凝土(HPC)为复合材料，通过在缝隙中插入额外的钢筋来提高抗弯承载能力，从而补充现有的钢筋，其核心思想是通过系统加热将现有钢筋的预应变诱导到额外的钢筋，从而使两者的应变相等。然后，这两个钢筋作为一个真正的单位共同作用，提高了承载能力，限制了裂缝宽度。在当地，热输入必须严格限制在高性能混凝土预拉伸和快速硬化所需的槽内。相比之下，加热整个横截面将减少额外钢筋的拉伸，因此是不可取的。因此，感应需要精确的时间和空间控制。在该项目中，温度感应是在建模(数字孪生)和相应的实验之间以通用和交互的方式开发的。这是在规模(实验)和方法扩展(模型建立)方面连续进行的。建模从描述数学系统和执行器以及传感器布置开始，并由此发展了对热固化及其热力耦合的控制和优化。结果是一种基于模型的顺序控制概念，带有能够实现瞬时温度和应变分布的观测器。在材料层面上，确定了必要的热参数，并对高性能混凝土的快速硬化和复合材料的蠕变进行了实验研究。在部件层面上，确定了热垫和红外辐射器的净温度输入，并研究了热力耦合，以实现必要的预应变并减少不必要的二次裂纹。最后，将模型和实验相结合，形成了一个全仪表化的大型演示台。为此，应对典型的钢筋混凝土板进行加固，以承受两倍于其使用荷载的荷载。在这个演示器上，所有的发展都得到了真正的实施和评估。

Many existing reinforced concrete structures need to be strengthened. The typical motivation for this is an upgrade against increased loads such as those that occur on bridges due to traffic or on buildings due to additional service loads.The key issue and fundamental limitation of additional reinforcement is that it is not effective for self-weight without temporary lifting or pre-stressing. Thus, only service loads are proportionately distributed to the existing structure and the newly placed reinforcement. This is in particular detrimental with respect to reinforced concrete structures, in which the self-weight usually amounts up to around 70%.Here, a completely new method is proposed to solve this problem, namely installing of additional reinforcement during local temperature induction. This method works for both, beams and plates. It increases the bending load bearing capacity by additional reinforcement inserted into slots with fast hardening high-performance concrete (HPC) as a composite and thus supplements the existing reinforcement.The core idea is to induce the pre-strain of the existing to the additional reinforcement by systematic heating, and thus to equalize both strains. Then, the two reinforcements act together as a real unit regarding both, increased load-bearing capacity and limited crack widths. Locally, the heat input must be strictly limited to the slots where it is necessary for pre-stretching and rapid hardening of the HPC. By contrast, heating of the total cross-section would reduce the stretching of the additional reinforcement and is hence undesirable. Induction therefore requires precise temporal and spatial control. In the project, temperature induction is developed in a generic and interactive way between modelling (digital twin) and corresponding experiments. This is performed consecutively with respect to size scale (experiment) and methodological extension (model building). Modelling starts from the description of the mathematical system and the actuator as well as sensor placements and develops the control and optimization of the thermal curing and its thermo-mechanical coupling from this. Outcome is a model-based concept of sequential control with an observer that is able to realize transient temperature and strain profiles. On material level necessary thermal parameters are identified while rapid hardening of HPC and creeping of the composite is investigated experimentally. On component level, the net temperature input from thermal mats and infrared radiators is determined and the thermo-mechanical coupling is investigated to achieve the necessary pre-strain and to reduce undesired secondary cracks. Finally, model and experiment are combined to a fully instrumented large-scale demonstrator. For this purpose, a typical reinforced concrete slab shall be strengthened to carry twice its service load. On this demonstrator all developments are implemented and evaluated true to scale.