Abstract:

Recent advancements in microscopy have expanded opportunities for research on frost action of concrete. Few studies have examined the microstructure of entrained air voids and their effects on frost resistance. While the presence of air-entrained voids have been associated with decreased cracking observed during freeze-thaw cycling, the mechanisms underlying this phenomena are the object of much debate.

The present study explores this issue through examination of entrained air void surfaces with an Environmental Scanning Electron Microscope (ESEM). Observations of ice penetration into air-entrained voids were made using a directional cooling stage mounted onto an optical microscope.

Three different surfactants were used to produce air voids in cement paste samples: sodium oleate (SO), sodium dodecyl sulfate (SDS), and sodium dodecyl benzene sulfonate (SDBS). The fractured surfaces of the specimens were examined at 1 and 7 days using ESEM. Upon examination of hardened cement pastes using ESEM, it was evident that a distinct shell formed around most air voids, regardless of the surfactant type. Hydration products were observed, and needle-like crystals appeared inside the air voids.

The kinetics of ice formation was studied using a directional cooling stage for calcium hydroxide (CH) solution with and without SDBS, low-alkali pore cement solution (KOH and NaOH) with and without SDBS, various water/cement ratio cement pastes samples, and de-icing salt solutions. The directional cooling stage was mounted onto an optical microscope to view samples, and images were taken using a digital camera. Samples using the directional cooling stage revealed that ice penetrated air-entrained voids as the ice-solution interface passed the void. The shape of the air-solution interfaces was easily manipulated during freezing of calcium hydroxide solution; in contrast, the air-solution interfaces formed in the low-alkali cement solutions were significantly less malleable. As the ice-solution interface moved across the air-entrained void of a cement paste, observations indicated that pore solution entered into the void and then froze. De-icing salt solutions produced brine pockets in the ice phase. Migration of brine pockets and grain boundaries toward higher-temperature regions was observed. An analysis of the brine pocket migration was conducted using an imageJ software plugin.