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Concrete In Australia : June 2008
Table 6. Summary of precision data published for the RCPT. Method edition 1 operator CoV T277-83 – C1202-91 C1202-94 C1202-97 C1202-05 12.3% 12.3% Max. variation between 2 samples, 1 person 19.5% 35% 42% Table 7. Chloride permeability based on charge passed, from Reference. Charge Passed (C) >4,000 Chloride permeability High 2,000 – 4,000 1,000 – 2,000 100 – 1,000 <100 Moderate Low Very Low Negligible disconcerting, as it not only indicates that the ASTM version is signifi cantly less repeatable than the original version, but that acceptance of variation between test results has become more liberal over time. A recent Middle East study quotes a CoV of 18% for a single grade of production concrete tested through one laboratory, which is comparable with the values presented in Table 6. A further concern with this test method is its applicability and interpretation with respect to modern Australian concretes. The original development of the test criteria was based on ordinary Portland cement bridge deck concrete of w/c 0.4-0.6, along with latex modifi ed, polymer impregnated (methylmethacrylate polymerised in-situ in pores), “internally sealed” (concrete batched with wax beads, heat treated after hardening to melt wax into pores), and polymer concrete (cement binder replaced by synthetic resin). The original classifi cation criteria from AASHTO T277-83 are reproduced in Table 7, the last column was not transferred to the ASTM versions. The results obtained from the T277/C1202 test method are directly related to the conductivity of the sample. Therefore, anything that changes the inherent conductivity of the pore water will affect the result, regardless of the inherent “quality” of the concrete. It has been argued by Feldman et al that the initial current reading, or a resistivity measurement, are as effective as the charge passage in classifying concrete for potential chloride ingress resistance. It is also known that admixed corrosion inhibitors will increase charge passage, yielding apparently higher chloride permeability, and the authors have measured a two to three fold increase in charge passed when dosage of high range water reducer is increased to raise slump values. Wee et al have shown that for constant cementitious content and w/c ratio, RCPT results are not only affected by the rate of addition, but also by the fi neness of the added pozzolan. This is potentially of particular concern with respect to slag blended Multi-laboratory CoV – 18.0% 18.0% Max. variation between 2 samples, 2 labs – 51% 51% Max. variation between means of 3 samples, 2 labs – 29% 42% Typical concrete High water/cement ratio, conventional (>0.6) PCC. Moderate water/cement ratio, conventional (0.4-0.5) PCC. Low water/cement ratio, conventional (<0.4) PCC. Latex modifi ed concrete. Internally sealed concrete. Polymer impregnated concrete. Polymer concrete. cements, because the fi neness of grind is routinely altered during cement manufacture to control strength characteristics. Water permeability DIN 1048 does not provide any information on the repeatability of the test method, however a recently published study from the Middle East using a single grade of production concrete recorded a single laboratory CoV of 125%, against 7.6% for corresponding compressive strength samples. The authors of the study also indicate that the penetration front is only measurable to about 5mm, making specification of low penetration values redundant, and they consider the test unsuitable as a quality control test. The method colloquially known in much of Australia and some areas overseas as the “Taywood Method” or “TEL Method”, and latterly the “GHD Method” since the acquisition of TEL’s Perth laboratory by GHD Pty Ltd in 2001, is a uniaxial outfl ow method. It is similar in operation to many other methods described in the literature, the nomenclature being a historical issue of its development, marketing and availability. During 1994 TEL’s Perth laboratory conducted repeatability testing on samples of two concretes, 400GP/40BFS/40CSF @ w/c 0.32 and 440SR @ w/c 0.32, measuring flow rates daily up to 15 days for 18 replicates. A mean CoV of 16.8% was returned, with a minimum of 5.7%. Where high CoVs were obtained, >40%, these generally resulted from one or two outlier values in the data pool. CoVs for void content by weight gain were 10.0% and 5.2% for the two concretes. Inter- laboratory testing conducted between the TEL and MRWA methods yielded comparable results for the two methods. Additional work during 1999-2000 confi rmed that water permeability testing is highly sensitive to sample preparation, with systematic differences identifi ed from compaction method Concrete in Australia Vol 34 No 2 55