An applied mineralogical investigation of concrete degradation in a major concrete road bridge

Abstract

A core of concrete taken from a major road bridge in the Strathclyde Region, Scotland, has been subjected to an applied mineralogical investigation, which involved stable isotope analysis, petrography, X-ray diffraction and scanning electron microscopy.The structure is actively undergoing severe degradation due to mineral growth which is related to chemical reactions between the concrete and pore fluid. The physical growth of minerals causes disfigurement and structural weakening.Pyrite and pyrrhotine hosted by dolerite aggregate appear to have been oxidized, providing sulphate for the deposition of ettringite and minor gypsum, in spheroidal cavities within the cement paste. The rainwater which passes through the structure mobilising sulphate from original gypsum in the paste and oxidizing the iron sulphides is also involved in the further leaching of elements from the cement paste and in the deposition of calcite. The isotopic values of calcites forming a crust on the concrete and a stalactite under the bridge are similar with ~13C= -19%o PDB and 618%~= +16%o SMOW. We suggest that atmospheric carbon dioxide was the carbon source. The carbon isotopic fractionation of -12%v from atmospheric carbon dioxide of 6t3C = -7%0, (O'Neil and Barnes, 1971) can best be explained as due to a kinetic fractionation related to the hyper-basicity of the pore water. The equilibrium formation temperature of about 45~ calculated from the oxygen isotope values and assuming a 61SO value of meteoric water of -8%0 SMOW, is considered unreasonable. The exceptionally low 6180 values are attributed mainly to reaction kinetics and the calcite inheriting its oxygen, two-thirds from atmospheric carbon dioxide and one third from the meteoric formation water (O'Neil and Barnes, 1971). A 61sO value of atmospheric carbon dioxide of +41%o SMOW and a 8180 value of meteoric water of -8%0 SMOW, lead to a calculated 61So value for the calcites of +10%o SMOW. The calcites analysed have a value of +16%o and this may be due to partial reequilibration towards a calculated value of +21%o for calcite in equilibrium with the meteoric water at 20~ K E V W O R D S: concrete deterioration, pet rography, ettringite, calcite, stable isotopes.

Mortar Dating Using AMS 14C and Sequential Dissolution: Examples from Medieval, Non-Hydraulic Lime Mortars from the Åland Islands, SW Finland

“…Calcitic crusts and stalactite-like formations on the mortar surface and in voids are typical for modern (MacLeod et al 1990(MacLeod et al , 1991 and ancient (hydraulic) concrete (Lindroos 2005), but sometimes they occur in somewhat non-hydraulic mortars. They form when percolating acidic water comes into contact with alkaline inclusions.…”

“…Sometimes the mortar is subjected to efflorescent carbonate growth and partial dissolution and recrystallization due to the interaction of acidic waters with alkaline inclusions (e.g. Charola and Lewin 1979;MacLeod et al 1990). Younger calcites usually dissolve readily and affect only the first dissolution increments.…”

“…We have not isolated recrystallizations for AMS or MS measurements, but for the first increments there seems to be a relation: the large dissolution relates to constant, young 14 C ages and highly negative δ 13 C values. MacLeod et al (1990) have measured the stable isotopes in calcite crusts and stalactite-like formations in modern concrete and have found δ 13 C values from -18.5‰ to -19.2‰ (n = 2) corresponding to calcite growth and strong kinetic fractionation in a super-alkaline milieu.…”

Implications of Pyrite Oxidation for Engineering Works

“…Concrete degradation in a Scottish bridge in Strathclyde was reported by Macleod et al (1990). Pitting and discolouration of the concrete was observed, in addition to a coating of mineral precipitates.…”

Petrography and Geochemical Analysis for the Forensic Assessment of Concrete Damage

“…The resulting rebar corrosion is colloquially known as concrete rot, but has its origin in alteration of the cement paste (e.g. MacLeod et al 1990). …”