## 3 Experimental Results
However, the target strength of 123 MPa was achieved by d 7. Additionally, up to 56 d of age, the compressive strength of VG20-70 remained at least equal to that of VG20-20 and continued to increase over time. Compared to SF10, the compressive strength of SF10 was initially higher at 1 d of age, but after 7 d, the strength gain trend shifted. Previous studies have reported that steam curing of concrete without admixtures can lead to lower long-term strength compared to non-steam curing [40,41] and that high temperatures combined with prolonged curing times may further reduce long-term strength [42,43]. However, Si and Al components, which are abundant in pozzolanic materials such as fly ash, ground granulated blast furnace slag, and SF, react with Ca(OH)\({}_{2}\) in pore water to form a pozzolanic reaction layer around the particles. This reaction mitigates the typical strength reduction associated with steam curing [44, 45, 46, 47]. VG has also been reported to exhibit excellent pozzolanic reactivity [48], suggesting that it provides similar long-term strength benefits to conventional pozzolanic materials. In this experiment, steam curing was conducted in a small curing tank. However, for large products such as precast concrete piles, steam curing is typically performed in a covered curing tank, which could measure, for example, 20 m \(\times\) 3 m \(\times\) 3 m. In addition, the product can be moved along with the formwork using a crane. Therefore, the 70 \({}^{\circ}\)C, 8-h curing conditions proposed in this study are fully applicable to actual manufacturing processes. Moreover, incorporating VG as an admixture in precast concrete piles is expected to facilitate additional moisture absorption from groundwater in the soil, potentially contributing to a minor increase in compressive strength over time. Figure 18 presents the estimated Young's modulus curve [32], calculated using Equation (1) for the material and strength properties of OPC and VG concrete mixtures. Actual measurements for VG20 and SF10 are also plotted. OPC concrete specimens could not be produced due to the high cement content, which resulted in a highly viscous concrete  
Figure 17: Relation between curing age and compressive strength of VG20-70 concrete. mix that was challenging to handle with the available concrete mixer. Consequently, the density of OPC concrete was estimated as \(\gamma=2.50\) g/cm\({}^{3}\). \[E_{c}=k_{1}\cdot k_{2}\cdot 3.35\times 10^{4}\cdot\Big{(}\frac{\gamma}{2.4}\Big{)}^{ 2}\cdot\Big{(}\frac{\sigma}{60}\Big{)}^{\frac{1}{3}} \tag{1}\]  
In Equation (1), \(E_{c}\) is the Young's modulus (MPa), \(k_{1}\) and \(k_{2}\) are the coefficients for the coarse aggregate and admixture type, respectively, \(\gamma\) is the density of concrete (g/cm\({}^{3}\)), and \(\sigma\) is the compressive strength of concrete (MPa). In this experiment, the correction factor \(k_{1}\), determined by the type of coarse aggregate, was set at 1.0, and the measured density of the VG concrete was \(\gamma=2.49\) g/cm\({}^{3}\). By substituting the measured values of compressive strength and Young's modulus into Equation (1) and back-calculating the coefficient for the VG admixture type, we obtained \(k_{2}=0.88\). Substituting this coefficient back into Equation (1) to fit the relationship curve between compressive strength and [PERSON]'s modulus, the estimated curve generally matched the measured values. This coefficient for VG is smaller than that for SF (\(k_{2}=0.95\)), indicating that for concrete of equivalent strength, the Young's modulus of VG-mixed concrete is lower than that of SF-mixed concrete. Previous studies have suggested that VG results in a higher C-S-H density than SF [26]. Since C-S-H density is strongly correlated with strength development in concrete, this would typically imply an increase in Young's modulus. However, the results of this experiment showed the opposite trend, suggesting that further research is needed to understand this behavior.