# Effect of Pouring Time on the Integrity of Pulverized Fuel Ash Concrete
## 3 Results  
### The Pundits Test  
The Pundits test method is suitable for assessing the uniformity and relative quality of concrete, as it can indicate the presence of voids, cracks, and changes in the properties of concrete. In this experiment, the bond strength of slab samples was tested to identify  
Figure 1: (**a**) Schematic diagrams of ultrasound pulse test; (**b**) concrete sampling positions. if there was an effect of the pouring time on the integrity of PFA concrete. Concrete slabs with a water-cement ratio of 0.55 were selected as the main analytical object. Figure 2 shows the pulse velocity comparison of the five selected positions in the concrete when the water-cement ratio was 0.55. It can be seen in Figure 2a-c that as the pouring time intervals progressed from 30 to 45 min, and then to 60 min, the sampling position decreased, and the pulse velocity of the PFA concrete sample showed a wave shape at different curing ages; the pulse velocity of position 3 was always the lowest among the five selected positions. This shows that the time interval between pouring the two layers of concrete had an adverse effect on the integrity of the concrete. The main reason for this was that position 3 was at the junction of the two layers of concrete, where there were more internal pores, which reduced the density of the concrete and led to a decrease in pulse velocity. More seriously, air and water could enter the pores between layers, which would accelerate the cracking of the concrete and the corrosion of the internal steel structure. In addition, another of the five positions with a lower pulse velocity was position 1. This was because position 1 was at the top of the concrete slab. After the vibration, the aggregate at the top of the concrete partially sank, resulting in more slurry at the top; therefore, the density of the concrete at the top was reduced, which made the pulse velocity slower. In addition, while pulse velocities at positions 2 and 4 were similar, it is worth noting that the concrete sample at position 5 had the fastest pulse velocity in most results. This may be due to the fact that after the vibration, the bottom of the concrete slab was denser and had fewer internal pores. Figure 3 shows the variation in pulse velocity at position 3 with different curing times and pouring time intervals. Although the pulse velocity at position 3 increased with the increase in curing age under different water-cement ratio conditions, the length of the two-layer concrete pouring time had a significant impact on the integrity of the interface when the curing age was the same. It can be observed in Figure 3a,b that the pulse velocity at position 3 decreased with the increase in the pouring time interval of the PFA concrete under different water-cement ratio conditions. This result is consistent with the related research of [PERSON] et al. [24]. This phenomenon proves that, in consideration of the cost and difficulty of on-site construction, the pouring time interval of two-layer concrete should be shortened as much as possible to improve the integrity of the concrete. It is worth considering that the fresh concrete was stored in a large mixer before transfer. Also, the concrete did not have a wet cloth cover to simulate the actual cast-in conditions of delayed placement, such as a large placement area or lack of workers. Therefore, when the pouring time interval was long, the moisture in the fresh concrete evaporated because there was no protection. This resulted in a slight decrease in the w/c of the concrete and a slight increase in its strength, which compensated for a portion of the strength loss caused by the long pouring time interval [25]. In this experiment, this uncertainty was not taken into account. Figure 2: _Cont._  
Comparing Figure 3a,b, it can be seen that when the curing age was the same, the pulse velocity of the sample with a 0.45 w/c was always higher than that of the sample with a 0.55 w/c. This was due to the higher density and strength of concrete samples with a lower water-cement ratio.