by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : December 2011
Concrete in Australia Vol 37 No 4 33 on several fronts. e process is described in detail in 300 Year Design Life for Second Gateway Bridge 6 and summarised as follows: • Define the characteristics of the environment. • Identify the potential deterioration mechanisms in that environment. • Determine the likely rate of deterioration. • Assess the material life. • Define the required material performance. • Take a probabilistic approach to the variability of the relevant parameters. • Assess and define the need for further protection. ere was a range of exposure conditions, from the submerged, tidal, and splash zone of the river piers, to potential acid sulphate or chloride infused soil at the northern approach piers, to air exposed concrete. Chloride ingress and carbonation were the main mechanisms which would lead to corrosion. A comprehensive literature review was undertaken to ensure that all the available relevant research was considered. e research is somewhat inconsistent, and a reasonably conservative approach was taken. Testing of core samples from pile caps on EGB indicated surface chloride concentrations of 0.4% to 0.5% by weight of concrete. A worst case surface chloride concentration of 0.65% by weight of concrete was used in the analysis and is similar to published data for marine splash environments. For the river pilecaps with the ternary concrete mix, testing in accordance with NordTest NTBuild 443 was performed on trial mixes and indicated that the chloride diffusion coefficient was 1.2-1.4 x 10-12 m2/s at 56 days. A conservative initial diffusion coefficient of 2 x 10-12 m2/s was selected for the modelling. is decreases with time at a rate dependent on the concrete mix proportions. e time weighted average diffusion coefficient was calculated to be 1.1 x 10-13 m2/s. e corrosion threshold chloride concentrations for ordinary black reinforcing steel and three grades of stainless steel are shown in Figure 7. ese concentrations were allowed to be reached after 280 years, leaving 20 years for corrosion to progress before the design life is achieved. e plot of the predicted chloride concentration profile with depth shows that the ternary mix including blast furnace slag is superior to the mix with flyash only, but also that black steel at a cover of 75 mm is beginning to corrode. A further probabilistic analysis was undertaken considering possible variations to concrete diffusion properties, surface chloride concentration, and depth of cover. To achieve a 90% probability of no corrosion initiation before 280 years, ordinary reinforcement required a cover of 150 mm. e pilecap was dimensioned and the structural design carried out on this basis. Such a large cover would lead to wide surface cracks. To control such cracking, a relatively light mat of stainless steel reinforcing was placed at 75 mm cover. Carbonation is the predominant corrosion mechanism expected for air exposed concrete. e flyash which is provided in all mixes to avert alkali silica reaction tends to increase the carbonation rate. Based on an atmospheric CO2 concentration of 0.04% and a carbonation rate not greater than 3.0 mm/yr 0.5, cover of 55 mm was selected for 50 MPa concrete with a maximum water cement ratio of 0.4 and minimum cementitious content of 450 kg/m3, as shown in Figure 8. It was recommended to monitor the structure for carbonation Figure 7. Chloride profiles at 280 years and corrosion threshold concentrations. DTWA = 1.1 x 10-13 m2/s; 30% FA; 21% BFS; t = 280 years DTWA = 2.6 x 10-13 m2/s; 25% FA; t = 280 years 316 LN LDX 2101 Pickled LDX 2101 Unpickled Black Steel Chloride Concentration (% by wt. concrete) Depth of Cover (mm) 0102030405060708090100110120130140150 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0