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Concrete In Australia : December 2008
TECHNICAL existing concrete structures. In fact amongst all test methods the VPV test method (AS 1012.21) is the only one which has proven its repeatability (ie. ±3%, VPV value of 0.3), reproducibility (ie. results between the different laboratories is within ±4%, VPV value of 0.4)4 and versatility. This compares favourably with the reproducibility of compressive strength, which is of the order of ±10%. Test methods such as chloride diffusion, water permeability, rapid chloride permeability, sorptivity etc do not have any proven record in relation to repeatability and reproducibility. In addition, the VPV has the ability to provide information as to the uniformity of a particular concrete in relation to whether it is a bleeding mix, whether it is harsh or relatively bony or a well graded mix (refers to compactibility and ability to densify compared to a coarse mix). The VPV has the ability to minimise the variability of various ingredients in concrete mixes with particular emphasis on the control of the total water in the actual concrete mix. The VPV classifi cation limits and their effectiveness (refer table 1) were developed based on the assessment of extensive data generated over the past 18 years at least, by calibrating these limits to the optimum performance of laboratory concrete mixes, as infl uenced by W/C ratio; cementitious material type and content, concrete grade and strength; curing regimes; type and adequacy of compaction (ie. vibration, rodding); testing of concrete cores extracted from in-situ structures (and precast components) and correlating with cylinders procured from site, as well as comparing with the condition of previously diagnostically investigated concrete structures and comparisons with other research work. Table 1. VPV Limits for various concrete grades6,7 Maximum VPV values at 28 days (%)1 Concrete grade VR330/32 VR400/40 VR450/50 VR470/55 Test cylinders (compacted by vibration) 14 13 12 11 Test cylinders (compacted by rodding) 15 14 13 12 Test cores 17 16 15 14 Note 1: Test results with a VPV value of equal to or less than 0.5% higher than the maximum allowable for the corresponding concrete grade, may be rounded down to the nearest whole number. It should also be emphasised that in contrast to other test methods such as chloride diffusion, water permeability, rapid chloride permeability, sorptivity etc, the VPV criteria offer an advantage in that they allow for the different type of compaction utilised to produce the concrete specimens and an allowance is also made for samples procured from the in-situ structure. The whole thrust of the discussion in relation to the advantages of adopting the VPV as a durability test is that if good quality high performance concrete (using SCMs) is used together with the VPV for prequalifi cation of the concrete mixes, and subsequently as a quality control test in a similar manner to strength (either at the general sampling frequency 28 Concrete in Australia Vol 34 No 4 for strength or at some lower frequency), then no special admixtures such as corrosion inhibitors etc are required. According to research undertaken in the US (Whiting 1988) into various durability techniques, test results for VPV show reasonable correlation with more time consuming permeability tests such as helium porosimetry, rapid chloride permeability test and the 90 day chloride ponding test1,2 . The ASTM C642 (AS 1012.21 in Australia) standard test method is quite commonly used in shotcrete specifications and quality control testing, particularly in North America as a means of quantifying the quality of the in-situ shotcrete. Performance indicators of shotcrete quality related to the main parameters of total absorption and volume of permeable voids have been developed on the basis of many fi eld observations and several large research studies conducted in Canada. The fi rst published reference to its use for assessing the quality of shotcrete was in the 1970s by the then Chairman of the ACI 506 Committee, P Seabrook. Since the early 1980s the method has been used on a routine basis as a quality control measure for shotcrete and other transportation applications, by many government agencies in Canada such as the British Columbia Rail and Hydro, the Ministry of Transportation and Highways, Alberta Transport and others. Major projects where this method has been specifi ed include major railway tunnels constructed through the Rocky Mountains, for British Columbia Rail1,2 This method has also been specifi ed in a number of marine . structures in the Persian Gulf. Performance criteria for marine exposure have also been specifi ed here as well. Chloride diffusion (including the Nordtest method) The use of the chloride diffusion parameter as a criterion for determining the long term quality of concrete and any protection measures that may be used is very questionable indeed. The test method is underpinned by theoretical assumptions, which render the recommended or specified chloride diffusion coeffi cient and its application to determine the quality of concrete very questionable indeed. In simple terms it is considered that the chloride diffusion method determines a very conservative diffusion coeffi cient (D) which in effect would force the design and manufacture of a more conservative and much higher performance concrete (more diffi cult to achieve/replicate in practice either at the concrete batching plant or on-site due to the inherent difficulties in adequacy of quality control) than would otherwise be required if more realistic parameters were used. The diffusion behaviour of the chloride ions in concrete is a more complex and complicated transport process than what can be described by the Ficks Law of diffusion. The different drift velocities and movement of the various ions and chloride ions in the solution, the chemical binding, along with other factors, interfere with the transport of the chloride ions. The effect of this ionic interaction signifi cantly reduces the chemical potential and thus the driving force of the diffusing species. However, the application of Fick’s Law assumes diffusion only from one side and straight forward without any other interactions and it therefore determines a much lower chloride diffusion coefficient (D) than what it would otherwise be the case. As such the model is not strictly correct as it does not give consideration to factors such as the effects of carbonation,