# On the Feasibility of Using Industrial Urea to Mitigate Thermal and Shrinkage Cracking in Concrete
## 3 Experimental Design
### Mixture Design  
Table 3 shows the mixture proportions of the concrete used in the experiments. The water/binder ratio (W/B) was fixed at 0.5 for all mixtures, and three urea mixture ratios (5%, 10%, and 15% mass) were used based on the mass of the binder. Due to the strong water-soluble property of urea, a higher urea mixing ratio increased the volume (mixing water + urea) of the aqueous solution proportionally. Therefore, as shown in Table 3, the increase in aqueous urea volume with increasing urea mixture proportion must be considered carefully. Because the urea that dissolves and exists in water does not impact cement hydration, as a rule, it does not affect the W/B (weight proportion of water required for hydration of a binder) of concrete. However, when the amount of aqueous solution is increased while keeping the amount of binder fixed, the surplus amount that does not affect hydration increases, and this could result in decreased strength of cement composites after hardening [2,4]. In particular, increasing the amount of aqueous solution in unhardened concrete is expected to strongly influence flow properties such as the slump. In this study, two solutions are suggested for resolving the issue of excessive increase in the amount of unhardened concrete in which more than 15% of urea is mixed. One solution is to adjust the viscosity of cement paste by adding a viscosity agent (U15V in Table 3). This method maintains the initially designed mixture proportion and controls the segregation that is likely to occur owing to the surplus amount by making the paste more viscous. The other solution is to reduce the unit water content to \(\sim\)89% compared to the standard (U15W in Table 3). The volume will reduce upon lowering the unit water content; to counteract this feature, the overall volume of concrete is fixed by increasing the ratio of fine aggregates and coarse aggregates. In the second solution (i.e., U15W), namely, reducing the amount of water, the W/B ratio will be lower than the initially set value of 0.5; therefore, the mechanical properties should be evaluated accordingly. Both solutions were tested through experiments to identify the one that is more advantageous in terms of the engineering performance of concrete. Meanwhile, a control group with different types of binders was prepared to evaluate comparatively the reduction in the cement hydration heat, which is the main purpose of mixing. Low-heat binders are often mixed in the existing mass of concrete, as investigated herein through Cases 1 and 2. Case 1 is ternary blended cement in which FA and GGBS, both of which are representative supplementary cementitious materials (SCM), are converted to 20% and 30%, respectively. In Case 2, OPC was completely replaced by LPC and then FA was applied after 20% conversion. These two types of blended cement are very effective in reducing the cement hydration heat because they significantly increase the proportion of SiO\({}_{2}\) among the components of a binder. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Binder** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **CaO** & **MgO** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **TiO\({}_{2}\)** & **SO\({}_{3}\)** & **LOI 1** \\ \hline OPC 2 & 21.2 & 6.5 & 2.8 & 62.5 & 3.3 & 0.57 & 0.32 & 0.3 & 2.2 & 0.31 \\ GGBS 3 & 32.5 & 15.5 & 0.44 & 42.1 & 5.7 & 0.4 & 0.18 & 0.9 & 2.08 & 0.2 \\ FA 4 & 54.7 & 27.3 & 7.8 & 6.4 & 1.3 & 0.83 & 0.54 & - & 0.82 & 0.31 \\ LHC 5 & 25.3 & 2.5 & 3.6 & 62.5 & 2.0 & - & 0.2 & - & 1.9 & 2.0 \\ \hline \hline \end{tabular}  
* Loss on ignition. 2 Ordinary Portland cement. 3 Ground granulated blast-furnace slag. 4 Fly ash. 5 Low-heat Portland cement (ASTM C150 type IV).  
\end{table} 
Table 2: Chemical composition of cementitious materials (mass%).