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Concrete In Australia : December 2014
Concrete in Australia Vol 40 No 4 43 concentration near the surface varies over time of exposure due to wetting and drying action. For that reason, an adaption of the diffusion based model became necessary. According to Gehlen (1), the limit state equation linked to chloride induced depassivation is given by Equation 1. (1) Ccrit. : critical corrosion-inducing chloride content at depth of reinforcement [wt.- %/cem.] CS,Δx: substitute chloride content at depth Δx (intermitting chloride impact) [wt.- %/cem.] dc: concrete cover depth [mm] Δx: depth of convection zone layer [mm] DRCM,0: rapid chloride migration coefficient [mm2/a] ke: factor considering temperature impact on effective chloride diffusion coefficient [-] kt: transfer variable (test method) [-] t0: time of reference for testing, here: 28d [a] a: ageing exponent [-] t: time of exposure up to the target service life Tsl [a] The dimensionless correction factor ke accounts for the influence on the actual onsite temperature and is given by (2) where Treal is the onsite temperature, T ref is the reference temperature, and be is a regression variable (all in [K]). 3.0 BIRTH CERTIFICATE 3.1 General The birth certificate forms an important first step for the durability and quality control of newly completed structures. Although the term itself is borrowed from the field of medicine, birth certificate in this context is not limited to the mere documentation of the state of the structure directly after its completion (“birth”), but it also includes both the optimisation of the structure during design and planning and the quality control during construction and after completion. The approach followed in this paper is schematically illustrated in Figure 2. 3.2 Design stage In contrast to load design, durability design has traditionally been carried out according to merely empirical “deemed to Figure 2: Stages in the development of a birth certificate. 42-49 - Gehlen.indd 43 42-49 - Gehlen.indd 43 21/10/14 2:10 PM 21/10/14 2:10 PM