# Effect of Dune Sand on Drying Shrinkage Cracking of Fly Ash Concrete
## 2 Experimental Plan and Method
### Test Method  
Compressive strength and split tensile strength were tested with three specimens at ages of 3, 7, 14, and 28 days according to KS F 2405 [20] (ASTM C 39 [21]) and KS F 2423 [22] (ASTM C 496 [23]).  
The ring-type restrained shrinkage cracking test proposed by ASTM C 1581 [24] was conducted to evaluate the drying shrinkage cracking characteristics of concrete. To fabricate specimens, the mixed concrete was sieved to remove CA that exceeded 13 mm. The concrete was poured into a test mold in two layers, and each layer was compacted with tamping rods. Each fabricated specimen was moved into a constant temperature and humidity chamber at a temperature of 23 \(\pm\) 2 \({}^{\circ}\)C and a relative humidity of 50 \(\pm\) 4%. Regarding the inner ring of the specimen, the bolt was removed so that the ring could not be fixed to the bottom plate, and the strain gauge attached to the ring was connected to a data logger. The specimen was covered with polyethylene film to prevent drying. After one day, the polyethylene film and outer ring mold were removed, and the drying was initiated. Two specimens of restrained drying shrinkage cracking test were made for each mixture.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture Identity**} & \multirow{2}{*}{\
\begin{tabular}{c} **Slump** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{**W/B**} & \multirow{2}{*}{**S/a (\({}^{\text{G}}\))**} & \multicolumn{6}{c}{**Unit Weight (kg)**} & \multirow{2}{*}{
\begin{tabular}{c} **AD (\({}^{\text{th}}\)**)** \\ **(bw\%)** \\ \end{tabular} } \\  & & & & **W (\({}^{\text{th}}\)**)** & **C (\({}^{\text{th}}\)**)** & **F (\({}^{\text{th}}\)**)** & **CS (\({}^{\text{th}}\)**)** & **DS (\({  
ASTM C 1581 proposes the net time to cracking and stress rate as measures to evaluate the cracking risk of concrete mixture. The net time to cracking represents the time difference between the onset of specimen drying and the time of cracking. The time of cracking refers to a point at which a strain that has slowly increased suddenly drops. The stress rate was calculated using Equations (1) [24] and (2) [24]. In this instance, the strain rate factor in Equation (1) was calculated based on the regression analysis of the measured data. In this study, regression analysis was conducted using the nonlinear regression analysis program (NLREG) software.  
\[\varepsilon_{net}=\alpha\sqrt{t}+k \tag{1}\]  
where \(\varepsilon_{net}\) is the net strain (m/m), \(\alpha\) is the strain rate factor for each strain gauge ((m/m)/day\({}^{1/2}\)), \(t\) is the elapsed time (days), and \(k\) is the regression constant.  
\[q=\frac{G\big{|}\alpha_{avg}\big{|}}{2\sqrt{t_{r}}} \tag{2}\]  
where \(q\) is the stress rate in each test specimen (MPa/day), \(G\) is 72.2 GPa, \(\mid\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!  
In the mixture conditions of this study, the DS/FA ratio of 10% was optimal for compressive strength. Furthermore, when the DS/FA ratio was more than 10%, the decrease in bond strength between DS and the cement paste could have a greater effect than that of the compactness of the aggregate.  
Figure 5 shows the histories of split tensile strength development. Similar to the compressive strength, the D10 mixture showed the highest tensile strength regardless of unit water content. The highest tensile strengths were 30.1% and 21.6% higher than the lowest in the W170 mixtures and W160 mixtures, respectively.