基於目標使用年限之混凝土設計規範架構研究

Abstract

台灣混凝土設計規範目前的耐久性設計多屬於基於規範設計方法（Prescriptive Design），由於缺乏與材料實際耐久性能連結的驗證機制，導致結構物在使用年限內之耐久性及維護管理面臨挑戰。相較之下，性能設計方法（Performance-based Design）可針對結構物所處環境條件及目標使用年限，透過性能驗證程序評估實際耐久表現，因而逐漸成為國際主流。 因此，本研究以混凝土結構物為對象，回顧國外規範（如 Fib Model Code、Eurocode、JSCE、ACI 等）中以設計使用年限為目標的性能設計方法並針對台灣規範提出改進建議。本研究同時探討台灣規範未涵蓋的驗證方法，如部分係數法與全機率法等耐久性劣化模型之運作方式，並結合台灣本土環境與材料參數，進行使用年限之評估分析；進而提出一套以設計使用年限為導向的耐久性設計流程圖。 分析結果顯示，混凝土水膠比越高，使用年限越低；保護層厚度越小，使用年限亦越低。雖然台灣規範針對不同曝露環境訂有限制最大水膠比，但在相同水膠比條件下，混凝土配比的差異仍會顯著影響使用年限評估結果，顯示以水膠比作為設計控制指標仍有其局限性。因此，本研究依據此前所提出之耐久性設計流程圖中鹽害環境下的氯離子擴散係數進行補充，進一步使用多種擴散模型推估在不同條件下各配比的氯離子擴散係數容許值，彌補台灣現行規範未能提供評估依據之缺口。 本研究亦根據模型推估之氯離子擴散係數容許值，結合台灣本土實驗室之材料試驗數據，提出耐久性指數（Durability Index, DI），作為綜合評估指標。同時建立氯離子擴散係數與設計使用年限之關係圖，提供工程人員於設計初期即可根據目標使用年限選擇合適之混凝土配比與材料組成。

Durability design in Taiwan’s current concrete design code primarily follows the prescriptive design approach, which lacks verification mechanisms linking actual material durability performance. This results in challenges for ensuring structural durability and maintenance throughout the design service life. In contrast, performance-based design has become a global trend, as it evaluates the actual durability performance of structures based on environmental conditions and target service life through rational verification procedures. This study focuses on concrete structures and reviews international codes (e.g., Fib Model Code, Eurocode, JSCE, ACI) that adopt performance-based design principles centered on target service life. Recommendations for revising Taiwan’s code are proposed. In addition, this study explores durability models not yet covered by local codes, such as the partial factor method and full probabilistic method, incorporating Taiwan’s local environmental and material parameters to assess design service life. A durability design flowchart based on service life is developed accordingly. Analysis results show that higher water–binder ratios lead to shorter service life, while smaller cover thicknesses similarly reduce durability. Although Taiwan’s code sets limits on the maximum water–binder ratio for different exposure conditions, variations in mix proportions under the same ratio still significantly affect service life predictions. This suggests that using the water–binder ratio as a durability control parameter is insufficient.Accordingly, this study refers to the previously proposed service life-based durability design flowchart and identifies a key missing element in Taiwan's current code: the allowable chloride diffusion coefficient under salt exposure conditions. To address this gap, this study adopts multiple durability models to calculate the maximum allowable chloride diffusion coefficients under various mix conditions A Durability Index (DI) is introduced based on model-derived allowable diffusion coefficients, and compared with local material data to validate applicability. Diagrams showing the relationship between service life and chloride diffusion coefficients are presented. These provide engineers with practical tools to select acceptable durability performance based on service life requirements, offering references for materials selection, mix proportioning, and specification control in durability-oriented design.