The measured evaporative water mass losses vs. time are provided in Figs. 4, 5, and 6, for the 1 h, 2 h, and 3 h applications, respectively. Both the timing of the solution addition and its composition have significant influences on the subsequent evaporative water loss. These results are quite consistent with the previous observation that SRA addition accelerates evaporation from bulk solutions, but retards evaporation from cement pastes and mortars.7 Thus, following the 1 h application (results shown in Fig. 4), a "layer" of the curing solution remains on the top surface of the specimens, and the specimens sprayed with the SRA-containing solutions initially lose mass at a faster rate than the specimens sprayed with only distilled water. For example, 2 h after the application of the curing solutions, the specimens to which water was applied have lost only 8.6 g of their mass (relative to their 1 h value), while those with the 10 % and 20 % SRA solutions applied have lost 12.9 g and 12.3 g, respectively. However, as this layer of solution "recedes" into the porous mortar microstructure, the subsequent rate of mass loss is diminished in the specimens sprayed with the SRA solutions, such that by 24 h, the overall mass loss is greatest from the specimens sprayed with only water. After 24 h, the total water mass losses for the three types of specimens (including the 10 g of mass of the added solutions) were about 79 g, 73 g, and 67 g, for the water, 10 % SRA, and 20 % SRA solutions, respectively.
For the 2 h application, as shown in Fig. 5, the specimens with the SRA have a lower mass loss from the time of the solution application forward. After this first 2 h of exposure to the 50 % RH environment, the specimens have all lost sufficient mass (about 10 g) such that their top surfaces are visually "dry". Thus, the sprayed-on solutions will immediately penetrate into the top layer of the porous mortar and the subsequent drying kinetics will be those characteristic of a (saturated) porous material, and not of bulk solutions. After 24 h, the cumulative mass losses for this case were about 76 g, 67 g, and 60 g, respectively. In comparison to the 1 h application, the 2 h application is thus seen to result in less cumulative mass loss after a 24 h exposure. The benefits of the SRA addition to the curing solution are also more clearly seen for these 2 h application results relative to the 1 h application results presented in Fig. 4.
Fig. 4: Evaporative water loss vs. time when the additional liquids are applied 1 h after casting. Two samples were evaluated for each of the three different solutions. Error bars for water results indicate +/− one standard deviation.
Fig. 5: Evaporative water loss vs. time when the additional liquids are applied 2 h after casting. One sample was evaluated for each of the three different solutions.
For the 3 h application, as shown in Fig. 6, following application of the curing solutions, the influence of the SRA on the drying kinetics is once again that which occurs in a porous material and not in a bulk solution. However, in terms of the 24 h mass losses, the 3 h application is seen to be inferior to a 2 h application, as the higher mass loss prior to the application of the curing solutions contributes to a larger overall mass loss after 24 h. Thus, for these particular drying conditions, sample geometries, and addition rates, a 2 h delay before applying the liquid solutions by spraying appears to be optimum.
These results demonstrate that the timing of the "curing solution" application will be critical to its successful performance. Too early of an application will result in a more rapid evaporation of the (bulk) solution ponded on the top surface and would also likely lead to additional "runoff" under actual field conditions. It would seem that the optimum time to apply the curing solution would be when the top surface of the concrete first appears "dry" and free of a layer of surface water, much the same as the criteria currently employed for the application of curing compounds. The exact time when this condition occurs will naturally depend on the concrete mixture proportions and the environmental exposure conditions in the field. It must be emphasized that the application of the SRA solutions clearly do not eliminate the evaporative water loss, but do reduce it significantly relative to the application of water only in this study. Their usage is thus seen as just one more tool available to the contractor or field engineer in their curing toolbox. This observation of the reduced evaporation rates due to the surface tension reduction provided by SRAs may lead to modifications to existing practices as well as new strategies for curing.
Fig. 6: Evaporative water loss vs. time when the additional liquids are applied 3 h after casting. Error bars for water results indicate +/− one standard deviation between three samples.
The measured degrees of hydration of the mixtures treated 2 h after casting with the 0 %, 10 %, and 20 % SRA solutions, after 6 d of curing at 23 ºC and 50 % RH, were 0.25, 0.32, and 0.32, respectively. Thus, treatment with the SRA solutions resulted in greater than a 25 % increase in the achieved degree of hydration under these rather severe curing conditions. Relative to the mixtures treated with only distilled water, for the mixtures treated 3 h after casting with the 10 % SRA and 20 % SRA solutions, the measured degrees of hydration after curing for 14 d at 23 ºC and 50 % RH were increased by 22 % and 33 %, respectively. For the mixtures treated 1 h after casting with the 10 % SRA and 20 % SRA solutions, the measured degrees of hydration after 28 d (6 d exposed to 23 ºC and 50 % RH followed by 22 d sealed in a plastic bag at 23 ºC) were increased by 18 % and 41 %, respectively. The increase in both evaporable and non-evaporable water contents for the smaller cuvette specimens with the applied SRA solutions is demonstrated in Fig. 7. The application of a solution containing SRA to the top of these specimens clearly results in an increased free water content and an increased amount of hydration (as indicated by the non-evaporable water content) in both the top and bottom portions of the specimens. After 6 d of curing, the measured cumulative mass losses of the specimens were 6.3 %, 3.7 %, and 2.0 % of the initial as-cast masses, for the water, 10 % SRA, and 20 % SRA solutions, respectively.
Fig. 7: Water contents of small cuvette specimens after 6 d exposure to 23 ºC, 50 % RH environment following immediate (after casting) application of the "curing" solutions. The labels of the x-axis indicate first the applied solution and then whether the water contents were measured on the top (exposed) or bottom half (bot) of the broken cuvette specimens.