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Concrete In Australia : December 2014
54 Concrete in Australia Vol 40 No 4 FEATURE: DURABILITY problems that later on developed during service could be ascribed due to weaknesses and deficiencies in the achieved construction quality (5,24,25). The damage was mostly related to low concrete cover, but low cover is also often difficult to compact, giving concrete covers of reduced quality. As a result of the durability design, requirements to both the chloride diffusivity of the concrete and the concrete cover are established, both of which must be properly controlled, which is achieved through ongoing verification and documentation during concrete construction. When any unacceptable deviation occurs, immediate actions for correction must be taken. 3.2 Chloride diffusivity For the specifying and testing of chloride diffusivity of the concrete, several test methods exist, a thorough review and discussion of which are given elsewhere (5,26,27). Although all of these test methods are accelerated methods giving different values for the chloride diffusivity, they all show a good correlation. In order to be suitable for a regular control and quality assurance during concrete construction, however, the RCM method (28) was adopted. This is the only method which can be applied for testing the chloride diffusivity very rapidly, independent of concrete age. For most durability analyses, the 28-day chloride diffusivity is normally used as an input parameter, although the chloride diffusivity obtained after longer curing periods can also be applied. It may be argued that both from a transport mechanism and theoretical point of view, such a strongly accelerated test method as the RCM method is questionable (21,29). It should be noted, however, that the above 28-day chloride diffusivity only is a simple, relative index reflecting the density, permeability and the general mobility of ions in the pore system and hence, both the resistance to chloride ingress as well as the general durability properties of the concrete. Thus, the 28-day chloride diffusivity may be comparable to that of the 28-day compressive strength, which is also only a very simple, relative index primarily reflecting the compressive strength but also reflecting the general mechanical properties of the concrete. For the above durability design, the 28-day chloride diffusivity of the concrete is as important as the 28-day compressive strength is for the structural design. Although the RCM method is a very rapid test method which provides data on the chloride diffusivity already within a few days, this is not good enough for the regular quality control during concrete construction. However, based on the Nernst- Einstein equation expressing the general relationship between diffusivity and electrical resistivity of all porous materials (30), a calibration curve as that shown in Figure 3 is established. Such a calibration curve relating the two tests must therefore be established for the given concrete before concrete construction starts. Measurements of the electrical resistivity are then carried out as a quick, non-destructive test on the same concrete specimens as that being used for the regular control of the 28-day compressive strength (31). All control measurements of the electrical resistivity are carried out immediately before the compressive strength testing, either by use of the two-electrode or the four-electrode method (Wenner) (5). 3.3 Concrete cover In severe environments, the concrete cover is normally very thick, and the reinforcement is often highly congested, making it difficult to measure cover thickness very accurately based on conventional cover meters. The use of stainless steel reinforcement may further complicate the measurements, although cover meters based on pulse induction can then be used. However, more sophisticated systems for control of achieved concrete cover based on image scanning exist. With such equipment available, there should be little excuse for allowing low cover in new concrete structures, yet this seems to continue to be a problem on many construction sites. 4.0 ACHIEVED CONSTRUCTION QUALITY 4.1 General Upon completion of the concrete construction work, the obtained data from the quality control of both chloride diffusivity and concrete cover are used as input parameters to new durability analyses for documenting the achieved construction quality, which is then expressed in the form of the obtained corrosion probability. Since the specified chloride diffusivity is based on small and separately produced concrete specimens cured in the laboratory for only 28 days, this chloride diffusivity may be quite different from that obtained on the construction site during concrete construction. Therefore, some additional documentation of achieved chloride diffusivity on the construction site during the construction period must be provided. Since neither the 28-day chloride diffusivity from the laboratory nor the achieved chloride diffusivity on the construction site during the construction period reflects the potential chloride diffusivity of the concrete, additional documentation of the long-term diffusivity must further be provided. For the owner of the structure, proper documentation of the achieved construction quality should be very important since it may have implications both for the future operation and expected service life of the structure. 4.2 Compliance with specified durability As a result of the durability design, a required service period with a probability for corrosion of less than 10% is specified. Figure 3: Typical calibration curve for control of chloride diffusivity based on electrical resistivity measurements. 50-56 - Gjorv.indd 54 50-56 - Gjorv.indd 54 21/10/14 2:24 PM 21/10/14 2:24 PM