I agree with answers provided by Jon Tanks

(Paul Uno - Adjunct Senior Lecturer, University of Sydney, Australia)

You can check that the pH of the concrete is 10 or higher by using solutions such as Phenolphthalein. If the concrete has carbonated around the reinforcement then you are more than likely going to get corrosion occurring earlier rather than later in the life of the structure. A dense good quality concrete will slow down the carbon dioxide diffusion process thereby extending the life of the reinforced concrete element.

I agree with answers provided by Jon Tanks

(Paul Uno - Adjunct Senior Lecturer, University of Sydney, Australia)

You can check that the pH of the concrete is 10 or higher by using solutions such as Phenolphthalein. If the concrete has carbonated around the reinforcement then you are more than likely going to get corrosion occurring earlier rather than later in the life of the structure. A dense good quality concrete will slow down the carbon dioxide diffusion process thereby extending the life of the reinforced concrete element.

From a theoretical point of view there could be another effect of a less alkaline cement on corrosion if it is initiated by chlorides. We usually believe that the chloride threshold level depends on the pH, with a lower threshold level for a lower pH. Consequently, the time to reach corrosion initiation in a chloride environment will be shorter if you have a less alkaline cement, keeping all other parameters constant.

First, as you said, less alkaline may influence the microstructure and strength, denser microstructure means higher electrical resistance and lower corrosion rate. Second, less alkaline means lower PH value, under which the passive film formed on the surface of rebar may be less stable. Third, the depassivation chloride concentration in marine environment is related to OH concentration, say, it is [Cl]/[OH] rather than [Cl] which de-passivate steel bar surface.

A recent paper by Bu and Weiss may provide some valuable information:

The Influence of Alkali Content on the Electrical Resistivity and Transport Properties of Cementitious Materials

Original Research Article

In Press, Accepted Manuscript, Available online 28 March 2014

Yiwen Bu, Jason Weiss

available at:

http://www.sciencedirect.com/science/journal/aip/09589465

Please check the Pourbaix diagram for steel. An alkaline environment is essential for formation of paasive layer around the steel bars. Thus in less alkaline cement there is higher risk of corrosion of steel bars.

In Australia we use supplementary cementitious materials such as flyash, slag and silica fume as replacements for general purpose cement. The replacement levels for these SCM materials can be anywhere between 0 and 15% (SF),0 to 35% (FA), 0 to 65% (GGBFS). These materials are not as alkaline as cement but the alkalinity of blended cement concrete mixes is still high enough to provide a passivating protective layer of ferric oxide on steel reinforcement. With correct mix design, placement, cover, vibration and curing these materials provide reductions in permeability/porosity thereby reducing carbon dioxide diffusion into concrete. To date, our blended cement concrete mixes in Australia have performed well and have resulted in improved resistance to potential reinforcement corrosion.

It is also very much associated with carbonation of concrete. Carbonation of concrete is associated with the corrosion of steel reinforcement and with shrinkage. However, it also increases both the compressive and tensile strength of concrete, so not all of its effects on concrete are bad.

Carbonation is the result of the dissolution of CO2 in the concrete pore fluid and this reacts with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3). Aragonite may form in hot conditions.

Within a few hours, or a day or two at most, the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process penetrates deeper into the concrete at a rate proportional to the square root of time. After a year or so it may typically have reached a depth of perhaps 1 mm for dense concrete of low permeability made with a low water/cement ratio, or up to 5 mm or more for more porous and permeable concrete made using a high water/cement ratio.

The affected depth from the concrete surface can be readily shown by the use of phenolphthalein indicator solution. This is available from chemical suppliers. Phenolphthalein is a white or pale yellow crystalline material. For use as an indicator it is dissolved in a suitable solvent such as isopropyl alcohol (isopropanol) in a 1% solution.

An alkaline environment favorite the formation of passive layer around the steel bars. While, in less alkaline cement there is higher risk of corrosion of steel bars.

It is true that elevated resistance involves a low porosity and low permeability accordingly. This last is a good indicator of the durability of concrete because the permeability reduces the penetration of aggressive agents within the concrete mass.

Decrease in alkalinity will reduce the value of pH of concrete and will help in provoking corrosion at earlier ages. However, as the use of pozzolans will also reduce porosity, especially capillary porosity, it may be helpful in reducing moisture and air movement and give some resistance to corrosion. There are lots of articles on the web in this regard.

It is important to note that porosity and permeability are interrelated. Basically, permeability in non-stressed concrete is due to capillary porosity. In stressed concrete, tiny cracks (even microscopic) may appear and connect some of the non capillaries to  become capillaries and increase water movement inside concrete. 

Dear Kunal. Do not be afraid to use pozzolans because the overall durability will be enhanced. The lower strength will be attained later on.

Steel rebar used in RCC construction is protected from corrosion due to high pH environment (12.5 - 13.5) where steel becomes passive. However, due to carbonation or use of NaCl for ice-breaking, Cl- ion in coastal region may bring down the pH level significantly affecting passivity of steel and it starts corroding.

Similarly, use of less alkaline mixture/ concrete will decrease the pH value of the environment and steel will corrode at faster rate.

You may find details in the attached paper.

Article Protection of Steel Reinforcement for Concrete - A Review