During the construction of Ayia Napa Marine in Cyprus we collaborated with C.D. Contantinidis SA. construction company for the the optimization of concrete mix design using two different standard methods/tests, ASTM C1202 and NORDTEST NT UILD 492, in order to examine the chloride permeability and therefore the durability of the concrete mix. These standard methods are analyzed in Table 1.
The improvement in durability is a key parameter to ensure the service life of concrete structures. Replacement of the conventional components with new supplementary materials in concrete, is one of the recently focused areas for the construction of structures that provide superior properties compared to those usually used in concrete technology.
Some primary factors that affect the durability of the mix design of concrete is water cement ratio, water/binder ratio, temperature, age and curing of concrete.
In this matter, new types of concrete are made with an inferior water/binder (w/b) ratio and elevated cement contents and additions. The material obtained has high mechanical performance features and a compact microstructure. By lowering the w/b, the mechanical and durability properties of concrete enhance due to the generation of a denser matrix [1-4].
Although a low water/binder ratio also reflects a low porosity and a high resistance to chloride ingress, extensive experience demonstrates that selecting a proper binder system may be much more important for obtaining a high resistance to the chloride ingress than selecting a low water/binder ratio.
For example, if the water/binder ratio is reduced from 0.50 to 0.40 for a concrete based on pure Portland cement, the chloride diffusivity can be reduced by a factor of 2 to 3, while incorporation of various types of supplementary cementitious materials such as blast-furnace slag, fly ash or silica fume at the same water/binder ratio can reduce the chloride diffusivity by a factor of up to 20.
Also, while a reduced water/binder ratio from 0.45 to 0.35 for a concrete based on pure Portland cement may only reduce the chloride diffusivity by a factor of 2, a replacement of the Portland cement by a proper blast-furnace slag cement may reduce the chloride diffusivity by a factor of up to 50. By also combining the blast-furnace slag cement with silica fume, extremely low chloride diffusivity can be obtained and then, a concrete with an extremely high resistance to chloride ingress can be produced. As a result, a basis for the durability design and suitable concrete mix design should be made, in order to obtain a best possible control of the chloride penetration during the initiation period and before any corrosion starts .
In general, the incorporation mainly of silica fume in concrete improves concretes’ strength and durability characteristics. It has been also successfully used to produce chemically resistant concrete with very high strength and low permeability . Because of its ultra-fine particles, the large pores can be divided into small pores and as a consequence changing the cement paste microstructure .
The partial replacement of cement with silica fume and/or fly ash in the concrete improves the overall property of the concrete, gives a way for the reuse of the supplementary material, while giving back a cleaner environment. The interface between the molecules in the concrete improves with these pozzolanic materials. Researches, have concluded that the fly ash blended concrete mix produces high resistance to alkali Silica reaction, increment in bond strength, reduces chloride ion penetration and sulphate attack .
Among some others, there are two main methods that are used from many researchers, in order to predict the chloride ion penetration in different concrete mix designs. Both of them are non-steady state tests: Rapid Chloride Permeability Test (RCPT, according to ASTM C1202) and Rapid Migration Test (RMT, according to NORDTEST NT BUILT 492). The methods are briefly analyzed and compared in the Table 1 below.
|Main Features||Method ASTM C1202||Method NORDTEST NT BUILD 492|
|Description||Rapid Chloride Permeability Test- RCPT (Non-Steady state test)||Rapid Chloride Migration Test-RMT (Non-Steady state test)|
|Specimens||Cylinders 100mm (diameter) x50mm (thickness)||Cylinders 100mm (diameter) x50mm (thickness)|
|Pre-conditioning||Vacuum saturation with de-aerated water||Vacuum saturation with saturated lime-water|
|Testing||Imposing a 60V external potential across the specimen, with the test surface exposing in 3% NaCl solution and the oppose surface in 0.3M NaOH solution.||Imposing a 30V external initial potential across the specimen, with the test surface exposing in 10% NaCl solution and the oppose surface in 0.3M NaOH solution.|
|Test duration||6 hours||6 to 96 hours (usually 24h, depending on the initial current that will come from the initial potential, then the test voltage has to be be chosen in order to calculate the test duration)|
|Test results||Measurement of the amount of charge (in Coulombs) passing through the specimen by recording the current as a function of time. The higher the total charge passed, the greater the chloride permeability of the concrete.||Split of each specimen and spray with AgNO3, then measurement of the chloride penetration depth and finally calculation of non-steady state migration coefficient (Dnssm) from the average penetration depth.|
|Precision||The charge passed is related not only to chloride ions, but to all the ions present in the pore solution (e.g. Cl–, OH–, SO42–, Na+, Ca2+, etc.) whose concentration may vary in different concretes. The high voltage applied (60 V) leads to an increase in temperature, which affects the amount of charge measured in the test.||The coefficient of variation (COV) of repeatability is 9% and the COV of reproducibility is 13% for Portland cement concrete or for concrete mixed with slag cement.|
|Where to use the methods||In existing and newly constructed concrete bridges, pavements, marine structures, etc.) with the use of e.g Proove it System. Quick and easy diagnosis of the resistance of concrete to chloride ion penetration.||Important for evaluating the long-term durability of roadway bridge decks, piers, and other concrete structures affected by chlorides and other oxidizing agents. The method can be used to identify weaker areas of concrete, which can then be flagged and repaired during construction if necessary.|
|Where not to use the methods||At concrete mixtures that include steel fibers or admixtures that influence the conductivity of the pore solution (e.g. corrosion inhibitor) because the presence of either of these will increase the current, giving artificially high results.||At steel fiber reinforced concrete or concrete that contains conductive solutions (e.g. corrosion inhibitor).|
- J.M. Gandía-Romero, J.E. Ramón-Zamora, I. Gasch, M. Valcuende, V. Calvet, R. Calabuig, P. Serna (2018). Absorption, porosity, capillarity and chloride diffusion in Ultra High Performance Concretes. Sixth International Conference on Durability of Concrete Structures- ICDCS2018: ICC-P03
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