# Effect of Pouring Time on the Integrity of Pulverized Fuel Ash Concrete
## 1 Introduction  
Pulverized fuel ash (PFA) is an industrial by-product collected from coal-fired power plants, and is used as a supplementary cementitious material (SCM) in the production of concrete [1]. Pulverized fuel ash exhibits pozzolanic activity, leading to a secondary hydration reaction with calcium hydroxide, a hydration product of cement, resulting in the formation of C-A-S-H gel [2]. Additionally, the small particle size of PFA enables it to serve as nucleation sites for the formation of C-S-H gel [3]. The filling effect of PFA particles, along with their pozzolanic activity and promotion of cement hydration, contribute to a denser concrete matrix. As a result, replacing a portion of cement with PFA significantly enhances the long-term strength of concrete, refines its pore structure, fills voids to improve its impermeability [4, 5], reduces water loss during drying, and noticeably decreases drying shrinkage in PFA concrete [3, 6]. In freshly mixed PFA concrete, fine particles of PFA can effectively envelop and lubricate aggregate particles, resulting in a significant improvement in both setting time and workability [7, 8]. The full utilization of fly ash helps reduce cement consumption, save energy, reduce carbon dioxide emissions, and promote the development of green buildings [9, 10]. For large concrete structures, such as bridges and tunnel structures, delayed placement can result in incomplete concrete structures when all concrete cannot be placed on the entire structural support or the process is interrupted part way through [11, 12]. A cold joint is an undesirable discontinuity between layers of concrete that occurs when one layer of concrete hardens before the rest of the concrete is poured [13]. It has been reported that incomplete concrete structures such as cold joints not only cause a decrease in shear strength, but also accelerate the penetration rate of chloride ions and the rate of carbonization reactions,thereby reducing the durability of concrete [12; 14; 15]. In addition, in concrete structures with high seismic or impermeability requirements, incomplete structures can lead to brittle joints and permeable belts in concrete, which greatly increase maintenance costs in the long run [16; 17]. Ultrasonic pulse velocity (UPV) technology is one of the efficient methods for evaluating concrete performance. As the ultrasonic pulse velocity technique is a non-destructive test method, it is often used during a project to predict the development of the compressive strength of the concrete. This estimation method is based on the empirical relationship between the compressive strength and ultrasonic pulse velocity of concrete. It is well known that many factors, such as mix proportion, aggregate type, water content, and the use of mineral admixtures, all affect the strength development of concrete, and these factors also affect ultrasonic pulse velocity [18]. Studies have shown that the content of aggregates affects ultrasonic pulse velocity. When the strength is the same, more aggregate content in the concrete may cause the ultrasonic pulse velocity to decrease [19]. Another factor affecting concrete strength is the water-cement ratio. It is well known that the higher the water-cement ratio, the lower the concrete strength [20]. However, studies have found that higher water content increases ultrasonic pulse velocity [20]. Therefore, the contradictory nature of this interaction leads to inaccurate results in the analysis of ultrasonic pulse velocity results of different types of concrete under different conditions. Hence, the evaluation method needs to be calibrated for different types of concrete. In this study, PFA was used to replace a portion of the cement to improve concrete's setting time and long-term strength development. In addition, a series of tests were conducted to investigate the effect of the pouring time interval on the integrity of the concrete, particularly the bond between the two concrete casting layers. A series of tests was conducted to investigate the quality of concrete.