chapter  4
18 Pages

FUNDAMENTAL CONCRETE CARBONATION MODEL AND APPLICATION TO DURABILITY OF REINFORCED CONCRETE

ByJ.H.BROWN

Reinforcement corrosion is the most important durability problem of reinforced concrete structures today. Reinforcing bars are protected from corrosion by a thin ferrous oxide layer which forms on their surface due to the high alkalinity (pH≈12.5) of their environment. If the pH-value of this environment drops below 9, or if chloride ions penetrate up to the surface of the reinforcement, the oxide layer is destroyed, and steel corrosion starts. The high alkalinity of the concrete mass is due to the (OH)− ions in the pore water, provided by the dissolution of Ca(OH)2 from the solid phase of cement gel into the pore water. A very small concentration of Ca(OH)2 in this solid phase is enough for its dissolution in the aqueous phase of the pores at the equilibrium concentration of (OH)−, which in turn guarantees a pH-value equal to 12.5. Carbonation of concrete is the reaction of the Ca(OH)2 which is dissolved in the pore water with the atmospheric CO2, and its conversion into CaCO3, which gives to the pore-water an equilibrium pH-value around 8.3, and therefore signals the onset of steel corrosion. Due to the importance of the emerging problem of concrete durability, a lot of research has been devoted to carbonation, mainly during the last decade, but also earlier (Hamada, 1969; Schiessl, 1976; Tuutti, 1982; Nagataki et al., 1988; Richardson, 1988). Some of this work has led to empirical models of the evolution of carbonation. Being empirical, these models cannot cover adequately the entire spectrum of conditions and all combinations of parameters that affect carbonation, and hence cannot serve as the basis of a comprehensive quantitative design process against carbonation-initiated corrosion. This work aims at filling exactly this gap, by developing a fundamental yet simple model of the carbonation process and applying it for the selection of design parameters, such as concrete cover of the reinforcement, concrete composition, etc.