## 3 Results and Discussions  
### Development of Compressive Strength of HVFA UHPC  
The results in Figures 3 and 4 show that the development of the compressive strength with time of UHPC with different _W_/_B_ ratios and different FA contents from 0% to 70% by mass of binder, in which the SF content was fixed at 10% by mass of binder under the standard curing condition and the heat curing, respectively.  
Figure 3: \(Cont.\)  
Figure 4: _Cont._  
Figure 3: Effect of fly ash content on compressive strength of UHPC with time at different \(W/B\) ratios from 0.12 to 0.18 under standard curing condition.  
Figure 4: Effect of fly ash content on compressive strength of UHPC with time at different \(W/B\) ratios from 0.12 to 0.18 under heat curing condition.  
Under the standard curing condition, the compressive strength of UHPC developed slowly during the first 7 days (particularly the UHPC using 70% FA) and then increased highly at the age of 28 days. Meanwhile, if the heat curing condition was applied, the compressive strength could be developed quickly even at early ages, and the ultimate compressive strength could be achieved just after 2 days heat curing.  
It can be observed that under standard curing, the addition of FA decreased the compressive strength of UHPC at early ages, i.e., 3 days and 7 days, especially when the FA content increased to 50% and 70%. The compressive strength of UHPC was significantly reduced, from 25% to 75% depending on the _W_/_B_ ratio and the FA content, compared to that of the control specimen. It should be noted that when cement was replaced with 20%-30% FA, compressive strength of UHPC reached the maximum value at a later age. Besides, with the same FA content, when the _W_/_B_ ratio decreased from 0.18 to 0.12, the compressive strength of UHPC increased 9%-57% depending on the FA content, in which the higher FA content gave a higher positive effect. The interesting point is that the incorporation of 70% FA increased the compressive strength of the UHPC specimens sharply from 50% to 57%, in comparison with that of specimens using 20%-50% FA (in which the compressive strength was improved only about 9%-23%).  
It can be observed that the similar tendency occurred with the development of the compressive strength of UHPC when adding FA under the heat curing condition. However, the compressive strength of UHPC increased rapidly at early ages, i.e., 3 days and 7 days, reaching 92% to 99% compared to that of UHPC at 28 days, respectively. It should be noted that at later ages, i.e., 90 and 180 days, the compressive strength of UHPC specimens is similar. As discussed above, the water to binder of UHPC is very low, normally less than 0.24. Therefore, minimal water is available for cement hydration and the resulting pozzolanic reaction can occur after 90 days. This means that the 90-day and 180-day strengths are very close to each other. This is different from normal strength concrete as the amount of water is still available, thus the hydration is still kept going and the strength keeps gaining up to a year.  
Moreover, when the percentage of cement replacement by FA increased to 30%, the compressive strength of UHPC was increased, but it reduced with an increase of the replacement content up to 50% and 70% and this reduction depended significantly on the _W_/_B_ ratio. For example, when the _W_/_B_ ratio decreased from 0.18 to 0.12, the compressive strength of concrete increased with all FA contents. However, the compressive strength of UHPC was not increased significantly, i.e., about 10% with adding 20% and 30% FA, but increased very rapidly, i.e., from 45 to 65%, with the cement replacement content from 50% to 70% FA when the _W_/_B_ ratio reduced from 0.18 to 0.12.  
It is clear from these experimental results that, for the UHPC using high-volume FA, the very low _W_/_B_ ratio in combination with the 48-h heat curing gave the highest efficiency in improving the compressive strength of UHPC.