The influence of ageing on the salt-frost resistance of concrete

Abstract: Abstract This thesis presents the results of an investigation of the influence of ageing on the salt-frost resistance of concrete. The ageing mechanisms investigated were hydration, drying and carbonation. Specimens of ‘micro’-concrete, either with OPC alone as the binder or with silica or slag as part of the binder, were freeze/thaw tested at different ages after casting. Prior to testing, the specimens were conditioned for 7 days in climate chambers specially built for the purpose, creating climates with increased CO2 (~1 vol-%) and with no CO2 at all, both at 65 % RH and +20 °C. Water-cured, never dried, specimens were tested as well. The results show that ageing has a strong influence on the salt-frost resistance, and that carbonation is the most dominant ageing mechanism. The effect differs, however, for concretes with different binder types/-combinations. For concrete made with OPC alone, or with silica (up to 10 %) or slag (up to about 30 %), carbonation leads to a significantly improved salt-frost resistance. For concrete with high slag contents (over about 50 %) carbonation leads to a markedly poorer salt-frost resistance. The properties of the pore structure of carbonated and uncarbonated mortars were further investigated by capillary suction and by measuring the freezable water content. Results show that carbonation leads to substantial changes of the pore structure and therefore of the freezable water content. It is proposed that, although different for different materials, it is this substantial change in pore structural properties that is the primary cause of the observed influence of carbonation on the salt-frost resistance. A limited XRD investigation of carbonated materials with and without slag as part of the binder showed no marked difference in carbonate phases formed. For both materials calcite was the dominant phase. As a complement to the laboratory investigations, field investigations were carried out at three exposure sites; two situated in saline environments (marine and highway), and one in a salt-free environment. A comparison shows that results from the laboratory investigations correspond well with the experience from up to seven years exposure at the exposure sites.