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Concrete In Australia : December 2014
52 Concrete in Australia Vol 40 No 4 FEATURE: DURABILITY construction quality is quantified and expressed by the obtained probability of corrosion. During operation of the structure, durability analyses are further applied as a basis for the future condition assessment and preventive maintenance. For each new condition assessment, the probability of corrosion is then calculated with new input parameters based on data from the real chloride ingress being observed. Before this probability of corrosion becomes too high, appropriate protective measures should be implemented. In the following, a case study is shown demonstrating how the above calculations of corrosion probability can be applied as a basis for the durability design of a new concrete harbour structure. The overall durability requirement to the structure was based on a 120-year service period before 10% probability of corrosion would be reached; the marine environment had a quite severe chloride loading of 5.5% by weight of cement observed on similar concrete structures in similar environments, and a high annual temperature of 20 °C. In order to select a proper combination of concrete quality and concrete cover which would meet the above durability requirement, two steps of durability analyses were carried out, the first of which was carried out in order to evaluate the effect of various concrete qualities, while the other was carried out in order to evaluate the effect of various concrete covers on the probability of corrosion. 2.2 .1 Effect of concrete quality In order to evaluate the effect of concrete quality, four different concrete qualities were considered for which the 28-day chloride diffusivities (D28) were determined by Rapid Chloride Migration (RCM). Apart from the type of binder system, all of these concrete mixtures were identical and fulfilled the minimum durability requirements according to the current concrete standards for a 100-year service life, including a water/binder ratio ≤ 0.40 and a binder content ≥ 330 kg/m3. The various binder systems included four different types of commercial cements in combination with 10% silica fume (CSF) by weight of cement; one high-performance Portland cement (Type 1), one fly ash cement with 20% fly ash (Type 2) and two blast-furnace slag cements with 34 and 70% slag (Types 3 and 4), respectively (Table 1). Based on the obtained 28-day chloride diffusivities (D28), a nominal concrete cover of 70 mm (XC) with a standard deviation of 6 mm and estimated values both for the time dependence of the chloride diffusivities (α) and the critical chloride content (CCR) as shown in Table 1, durability analyses were carried out. For the other input parameters, the estimated values both for environmental loading CS (5.5;1.4%), age at chloride loading t’ (28 days) and temperature T (20 °C) were kept constant for all analyses. Table 1: Input parameters for analysing the effect of concrete quality. Concrete quality Input parameter D28 (m2/s x 10-12) α CCR (% by wt. of binder) Type 1 (CEM I 52.5 LA + 10% CSF) N1( 6.0;0.64) N(0.40;0.08) N(0.4;0.10) Type 2 (CEMII/A–V42.5R+10%CSF) N(7.0;1.09) N(0.60;0.12) Type 3 (CEMII/B-S42.5RNA+10%CSF) N(1.9;0.08) N(0.5;0.10 Type 4 (CEM III/B 42.5 LH HS + 10% CSF) N(1.8;0.15) Input parameter Average value Standard deviation Comments D28 6.0 0. 64 Chloride diffusivity (m2/s x 10-12) α 0.40 0.08 Time dependence factor CCR 0.40 0.10 Critical chloride content (% by wt. of binder) XC 70 6 Nominal concrete cover (mm) 90 6 120 6 Table 2: Input parameters for analysing the effect of concrete cover. 1 Normal distribution with average value and standard deviation. 50-56 - Gjorv.indd 52 50-56 - Gjorv.indd 52 21/10/14 2:24 PM 21/10/14 2:24 PM