The cover thickness design of concrete structures subjected to chloride ingress from RBDO solution technique

Abstract: Diffusion is the principal transport mechanism of chloride ions into concrete pores. The chlorides trigger the reinforcements’ depassivation when its concentration at the concrete/reinforcement interface reaches the threshold level. Thus, the depassivation defines the initiation stage end and the propagation stage start. The structural safety reduces widely during the propagation stage because of the various deleterious mechanisms triggered by reinforcement’s corrosion. Therefore, the engineers should accurately predict and prevent the propagation stage start. The literature describes several models for evaluating the end of the initiation stage. However, few of them applies the Boundary Element Method (BEM) for this purpose, despite its known accuracy. Besides, enormous randomness affect the phenomenon. Thus, it is adequately handled solely in the probabilistic context. Optimisation techniques may be coupled in the problem modelling to propose adequate cover thickness values accounting for probabilities of failure. This study presents a Reliability-Based Design Optimisation (RBDO) approach for designing accurately the cover thickness of concrete structures subjected to chloride ingress. The BEM handles the diffusion modelling whereas the Monte Carlo simulation assesses the probabilities of failure. The RBDO is formulated in the context of Weighted Average Simulation Method (WASM), which requires only one assessment of the reliability analysis. It leads to a reliable and computationally efficient solution technique. The problem formulation and the implemented solution scheme are described herein. Moreover, one application is presented, in which the design results are interpreted properly.

摘要：氯离子侵入混凝土孔隙的主要传输机制为扩散作用。当氯离子在混凝土与钢筋界面处的浓度达到临界阈值时，会触发钢筋的钝化脱钝。由此，钢筋钝化脱钝标志着初始阶段的结束与扩展阶段的开端。在扩展阶段，钢筋腐蚀引发的多种有害机制会导致结构安全性能大幅下降。因此，工程人员需精准预测并阻滞扩展阶段的起始。现有文献已提出多种用于评估初始阶段结束节点的模型。然而，尽管边界元法（Boundary Element Method, BEM）已被证实具有较高精度，但采用该方法开展此类评估的模型却寥寥无几。此外，该过程受极强随机性影响，唯有通过概率分析框架才能对其进行恰当处理。可将优化技术耦合至问题建模中，结合失效概率推导合理的混凝土保护层厚度取值。本研究提出一种基于可靠性的设计优化（Reliability-Based Design Optimisation, RBDO）方法，用于精准设计受氯离子侵入影响的混凝土结构保护层厚度。该方法采用BEM开展扩散建模，并通过蒙特卡洛模拟评估失效概率。本研究在加权平均模拟法（Weighted Average Simulation Method, WASM）的框架下构建RBDO模型，该框架仅需进行一次可靠性分析评估，由此得到的求解方法兼具可靠性与计算高效性。本文详细阐述了问题建模与所采用的求解方案，此外还给出一个应用案例，并对其设计结果进行了合理阐释。