EVALUATION OF CARBONATION PROCESS IN CONCRETE WITH AN ANALYSIS OF CARBON ISOTOPES

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  • 同位体分析による実構造物中のコンクリートの中性化進行評価の試み

Abstract

Radiocarbon method has been applied to concrete in order to estimate progression and age of carbonation in concrete. Concrete cores from a building constructed in 1967 on the campus of Nagoya University, Japan, were collected in 2008, and were investigated for 14C, δ13C, and carbon content. The amount of carbon dioxide was 8％ at the concrete surface where the concrete is highly carbonated, decreasing with depth, and was 1％ at the depth where carbonation is hardly observed. The measured 14C values were 144 to 148pMC at the carbonated surface and 71 to 82pMC at the depth of less carbonation, decreasing from the surface down to the depth. These profiles can be interpreted as the following：（1）The age of carbonation in the concrete can be estimated by the variation of 14C and carbon concentrations, coupled with atmospheric 14C value at 1967（~170pMC：Hua and Barbetti, 2004）and the present one（~100pMC）. Most part of CO2 in the surface concrete is estimated to be absorbed and fixed in ten years after construction.（2）The deeper part of the concrete also contains small amount of atmospheric CO2, which is 43 to 78％ of CO2 in the deeper part. However, δ13CPDB value in the deeper part is -25 to -21‰, and is significantly lower than those of atmospheric CO2（δ13C≅8‰）and marine sedimentary limestones with geological ages（δ13C≅0‰）. Carbon isotope compositions, 14C and δ13C, in the deeper part of concrete cannot be explained by a simple mixing of the two components. There are two possible causes of the lowest δ13C value：material originating from cements and/or concrete, and direct absorption of atmospheric CO2 by high-pH cement solutions with kinetic effects in the high-pH zone.

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