The objective of this investigation was to study the effect of carbonation on two reinforced concrete structures of 42 and 60 years old, located on two different environments: urban and rural. Samples of concrete were taken, the carbonation depth was measured and density, absorption and porosity tests were performed. The carbonation process can be modeled by the equation of the square root of time. From that model, the carbonation constant for each sample was calculated, the progress of the carbonation depth versus time was plotted and the correlation between porosity and depth of carbonation was analyzed. It was verified the existence of different microclimates surrounding structural elements. The moisture content of the surrounding surface of each structural element is the most important factor affecting the progress of carbonation.