Hydrated Cement Paste and Freeze-Thaw Damage

Traditionally, the durability of HCP under a freeze-thaw environment has been closely linked to the size and volume of the entrained air bubbles in the concrete (ACI 2016; Kosmatka and Wilson 2016). These air bubbles are purposely formed in the fresh concrete during mixing through the addition of an air-entraining admixture (AEA) conforming to AASHTO M 154 or ASTM C260/C260 M. Most entrained air bubbles range in size from 10 to 100 μm (Kosmatka and Wilson 2016) and, ideally, are uniformly dispersed throughout the HCP, as shown in figure 2. The air void system required to protect concrete from freeze-thaw damage is commonly linked to exposure conditions, the most severe of which is when deicers are present (ACI 2016; Kosmatka and Wilson 2016). Specific requirements regarding the characteristics of the air-void system needed to protect the HCP from freeze-thaw damage typically include air contents ranging from 5 to 8 percent.

In addition to the entrained air-void system, a maximum water-to-cementitious materials ratio (w/cm) is also typically specified. This is a recognition that the overall porosity of the HCP decreases as w/cm decreases, resulting in a decrease in permeability and an increase in strength. ACI (2016) recommends a maximum w/cm of 0.45 (for plain concrete) if freeze-thaw conditions are to be encountered. Furthermore, if the pavement is to be hand finished, the supplementary cementitious materials (SCM) content is limited to a maximum of 25 percent fly ash or 50 percent slag cement by mass of total cementitious materials. ACI (2016) recognizes that formed and machine-finished surfaces, such as slip-formed pavement surfaces, are not greatly at risk of scaling and these SCM limits are not applied.

In summary, traditional freeze-thaw damage in the HCP is mitigated primarily through the creation of an effective entrained air-void system in the concrete, in which the spherical air voids are spaced closely enough to relieve the stress generated as the pore solution freezes. Current guidance suggests that the total volume of entrained air should be increased and the maximum allowable w/cm should be reduced as environmental conditions become more severe (i.e., with increasing availability of water and, especially, the presence of chemical deicers). Historically, this strategy has been largely effective as long as the desired air-void system parameters were achieved and traditional deicing practices were used.