# Effect of Pouring Time on the Integrity of Pulverized Fuel Ash Concrete
## 3 Results
### Compressive Strength  
Studies have shown that faster pulse velocities represent higher densities and compressive strengths [26]. In order to further confirm the conclusion in Section 3.1, PFA concrete samples taken from different positions of the concrete slab were tested for compressive strength. Figure 4 shows the compressive strengths of concrete samples with 0.45 and 0.55 water-cement ratios at different positions after 28 and 56 days of curing, respectively. When the water-cement ratio was 0.55, after curing for 28 days and 56 days, the strengths of the concrete samples increased first, then decreased, and then increased again as the sampling position lowered. Among sampling positions, the lowest value of compressive strength appeared at position 3, the highest value was at position 5, and the maximum difference reached 15 MPa. The lowest value of compressive strength occurred at position 3, at approximately 46.5 MPa, the test block with the highest compressive strength (approximately 60 MPa) was from position 5, and the largest difference reached 15 MPa. This was because position 3 was at the junction of the two layers of concrete, where there were more pores, and the bonding of the two layers of concrete caused a weak area, thereby reducing the strength of the concrete. In addition, the compressive strength of position 1 was lower compared to the strengths of PFA concrete samples from the other four positions, while the sample from position 5 had higher compressive strength. This may be due to the fact that after the vibration, the concrete aggregate sank, so that the concrete at the top contained more paste, resulting in reduced strength, and the concrete at the bottom was more compact, resulting in increased strength. In addition, it can be clearly seen that under different curing times, the compressive strength at position 3 decreased as the pouring time interval between the two layers of concrete became longer. This shows that the pouring time had a significant impact on the integrity of the concrete. This result is consistent with results of the Pundits test.  
Figure 4: Compressive strengths of concrete samples at different positions: (**a**) 0.55 w/c, 28 d; (**b**) 0.55 w/c, 56 d; (**c**) 0.45 w/c, 28 d; and (**d**) 0.45 w/c, 56 d.  
When the water-cement ratio was 0.45, the compressive strength of concrete samples taken from different locations showed a similar development trend to samples with a water-cement ratio of 0.55. The concrete at position 3 exhibited the lowest compressive strength, and compressive strength decreased as the pouring time interval increased. It is worth noting that the pouring time interval had a more pronounced effect on the strength of PFA concrete samples with a 0.45 w/c compared to samples with a water-cement ratio of 0.55. When the curing age was 28 d and the pouring time interval was 60 min, the compressive strength of position 3 was reduced by approximately 10 MPa compared with concrete samples with a 30 min interval time. When the curing age increased to 56 days, the difference in compressive strength decreased slightly, but still reached 8 MPa. This shows that when the water-cement ratio of concrete was relatively low, the pouring time interval had a greater impact on the integrity of the concrete.  
In addition, according to the results of compressive strength and pulse velocity tests, when the concrete pouring time interval was less than 1 h, although the two-layer concrete interface had a lower strength, it did not have characteristics of the cold joint described in the literature [14]. Therefore, subsequent experiments are required, with testing over a longer period to determine if the junction will become a cold joint in a few years.