In this work, the effects caused by both the amount of air-entraining agent (AEA) and freeze-thaw cycles on microstructure of high-strength concrete have been analyzed. For this purpose, five series of concrete specimens have been manufactured, each of them containing a different amount of AEA. Then, all series have been subjected to up to 300 freeze-thaw cycles. In addition, the specimens have been analyzed using a computed tomography (CT) scan device at pre-defined freeze-thaw cycles and all data have been processed with digital image processing (DIP) software.
The results reveal, on the one hand, that the quantity of AEA has a greater influence on pore structure, and additionally the freeze-thaw action only slightly modifies the pore structure. As AEA increases, a progressive rise of the porosity and the number of pores is observed up to a maximum value. Next, a decrease is noticed. Moreover, there is not a linear relation between porosity and AEA. Furthermore, as AEA increases, a variation of its size and shape is observed. Alternatively, the effect of freeze-thaw cycles is more complex and does not show a monotonous tendency. The results reveal that the first 50 freeze-thaw cycles have the strongest influence on pore structure, observing a decrease in porosity. For the rest of the cycles, the porosity increases progressively resulting, after 300 freeze-thaw cycles, in a slightly lower porosity in almost all series than in those presented at the beginning. Hydration of unhydrated cement particles alongside with microcracking act as opposite performances during the freeze-thaw cycles. Therefore, this can suggest that, under these conditions, freeze-thaw action is not able to damage significantly the microstructure of concrete.
The results show that the series with a lower AEA content show a better behavior under freeze-thaw cycles. In this case, the specimens exhibit a lower porosity and a higher level of small pores, and the pores evince a more elongated shape. All these features lead to a more impermeable concrete and, therefore, with a better performance under freeze-thaw cycles.
