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Concrete In Australia : December 2008
TECHNICAL Chloride and the other in Sodium Hydroxide. The full test would take about three days to complete, with six hours required for the actual test. The total electric charge passed (coulombs) is a measure of the chloride ions migrating through the concrete under the prescribed conditions. However, this method has been criticised as not really measuring permeability (movement of chloride ions) but resistivity of the water saturated sample. In addition, ASTM C1202 states that the rapid test should not be used unless proper correlations are made with long term ponding tests (such as the 90 day chloride ponding test, AASHTO T259). The RCPT may also suffer from possible interferences (as indicated in ASTM C1202) and should be considered in the interpretation of results. Interferences affect the resistivity (conductivity) of the concrete and in turn, the coulomb value. Interferences may include the degree of moisture in the specimens, presence of reinforcing steel or ions other than chlorides, use of SCMs and other admixtures etc. Another more serious problem associated with the method is the potential overheating of test samples, particularly with younger concrete, which would result in damage and therefore distortion of the test data. At younger ages high performance concrete is so conductive, it would not survive the RCPT due to overheating. For quality assurance, this test is normally conducted when the concrete is one to three months old. The method may have to be modifi ed with lower voltages to enable accurate testing for younger concrete. It should also be noted that as part of the SHRP program in the USA (high performance concrete project), a more rapid and less expensive resistivity method (ie. AC Impedance Test) was investigated as an alternative to the RCPT for the reasons mentioned above. It was also suggested that RCPT initial current (amps), may be a better indication of permeability than charge passed (Coulombs) because it avoids possible distortion of the data caused by overheating of the specimen. Sorptivity test It is considered that the sorptivity test method has the inability to be used as an effective quality control tool due to the duration of the test. The turnaround of results would be something of the order of 8 to 10 weeks. As such it has the inability to really infl uence the ongoing quality of concrete during manufacture and construction and thus it is unable to provide some continuous relationship to quality control and the original design of the concrete mix. However, the greatest concern with this method is related to the non-uniform conditioning of samples. After demoulding following the required curing, specimens are dry conditioned in a room or chamber (23 o C and 50 ± 5%) for a minimum period ranging from 21 to 35 days depending on the exposure conditions. This type of conditioning is considered unsatisfactory because it cannot assure the uniform and constant conditioning of all samples (ie. constant mass preconditioning for all samples). Considering that the moisture content of the test specimens strongly infl uences their sorptivity values, the test tends to discriminate in favour of higher strength concretes as they would dry more slowly. In addition, the method has the inability to really distinguish the quality of various concrete grades above about 40MPa. 30 Concrete in Australia Vol 34 No 4 The increased rate of moisture loss in lower strength or poorly cured concretes would tend to cause greater micro-cracking which would increase the measured sorptivity. Higher strength concrete specimens on the other hand would tend to produce less micro cracking at the start of testing due to the reduced rate of moisture loss. This is despite the fact that on-fi eld higher strength concrete may be characterised by greater micro cracking due to increased thermal and shrinkage stresses. The increased rate of moisture loss in lower strength concretes would tend to cause greater micro cracking, which would increase the measured sorptivity. Neither chloride diffusion nor sorptivity have any proven record in relation to repeatability and reproducibility (comparison of results between different laboratories). The use of sorptivity testing to determine the effectiveness and length of time of curing regimes which may have the potential to reduce the amount of curing or use less effective curing methods in-situ is not recommended. Initial surface absorption test (ISAT) This test is described by the British Standards in BS188111 and can be used on laboratory or fi eld concrete. The test measures the rate at which water is absorbed into the surface of the concrete. It does not measure the bulk permeability (penetrability) of concrete. Field samples can only be tested if no water has fallen onto the test surface in the previous 48 hours. This test has been chosen by a number of designers for compliance testing in the Persian Gulf. The test has a record of being used in the UK for the purpose of assessing durability for concrete paving fl ags and kerbs. Porosity tests Such tests include helium and mercury porossimetry and some RILEM tests. The small and inert helium molecules are used to penetrate cementitious systems which have restricted pore openings. This method is based on Boyle’s Law for isothermic gas expansion. Higher pressures and oven drying of specimens are required. The mercury intrusion process (MIP) involves the injection of mercury into the concrete under pressure and the measurement of the mercury ingress. Some variation in results has occurred during testing. Gas permeability This uses gases such as nitrogen and requires elaborate apparatus and conditioning of specimens, although it is a much quicker test. Problems are encountered in this test as moisture in the capillaries acts as a barrier to the gas. In addition, most contaminants are introduced into the concrete as water borne substances and most deterioration reactions can only take place in substantially moist environments. General comments on durable construction In general it is considered that a relatively dense and impermeable concrete cover based on high performance SCM concrete in conjunction with a multi-level protection approach consisting of various durability provisions, and good quality control for both concrete supply and construction, is what is required to deliver a long lasting structure to satisfy the