3.3 Autogenous Deformation and Drying Shrinkage

The curves for drying shrinkage microstrain vs. time are provided in Figure 2. For both mixtures with internal curing and SRA, less drying shrinkage is observed, despite their higher overall water content and greater mass loss during drying. As shown in Figure 3, for each of the three mortar mixtures, a nearly linear relationship was observed between measured mass losses and measured drying shrinkage microstrains. For the two mixtures with internal curing and SRA, the slope of the drying shrinkage microstrain vs. mass loss curve is less, as indicated in Table 3. The SRA reduces the surface tension of the pore solution [13, 14], leading to a reduced saturation level and reduced strain within the three-dimensional microstructure when exposed to a constant (reduced) relative humidity drying environment. Because there is a greater quantity of total water in mixtures 2 and 3 relative to mixture 1, they do exhibit a greater mass loss during drying but with less accompanying shrinkage. In terms of the early (< 7 d) and later (> 7 d) age shrinkage, as indicated in Table 3 and Figure 2, mixture 2 with FLAIR delivery exhibits more shrinkage at early ages, but less subsequent shrinkage at later ages when compared to mixture 3 with conventional delivery of the SRA. This would be expected as the SRA would be released (drawn out) from the prewetted fine LWA during the first few days of hydration under sealed conditions. As some of the SRA may be absorbed by the cement hydration products, delaying its introduction into the hydrating cement paste system may potentially improve its efficiency at later ages. This is the second potential benefit of FLAIR vs. conventional delivery of chemical admixtures. Because the FLAIR-delivered admixtures are not initially distributed throughout the cement paste component of the fresh concrete, it is envisioned that the FLAIR technology will be most readily applicable to admixtures that influence the properties of the hardened concrete, such as SRAs, corrosion inhibitors, and admixtures employed to mitigate alkali-silica reactions. Further research is needed to evaluate the FLAIR technology in such systems.

Figure 1- Measured (average of two samples) autogenous deformations vs. time for the three mortar mixtures. Error bars on lower curve indicate range of values for the two mixture 1 specimens.

Figure 2- Measured drying shrinkage microstrain vs. time (average of four samples) for the three mortar mixtures. Error bars on upper curve indicate ± one standard deviation for the four mixture 1 specimens.

Figure 3− Measured drying shrinkage microstrain vs. measured mass loss (average of four samples) for the three mortar mixtures.