Prepacked aggregate concrete is a distinctive type of concrete made by placing and packing appropriately graded coarse aggregates into formwork, filling the gaps between these aggregates with grout. This method, also known as "two-stage concrete," facilitates construction in challenging environments and is aligned with sustainable building practices. The PAC approach, pioneered by Xiaojun Zhou and colleagues, supports the development of concrete with reduced environmental impact and superior mechanical properties. PAC has found applications in underwater concrete, large-volume constructions, and the reinforcement of bridges and tunnels, providing economic and environmental benefits by minimizing cement usage and enhancing concrete durability.
Current research, highlighted in their recent study, focuses on the interaction between grouting material properties, pouring methods, and aggregate size. The study, published in Scientific Reports, reveals that the efficiency of PAC is significantly influenced by the fluidity of the grout and the method of pouring. Gravity pouring, relying on the material’s weight to fill voids, is effective only with highly fluid grouts. In contrast, pump injection, which uses pressure to force grout into the aggregate voids, is more versatile, accommodating less fluid grouts and larger aggregate sizes. This method is particularly beneficial in reducing the brittleness and shrinkage of concrete.
The researchers employed various testing methods, including ultrasonic pulse velocity tests and core sampling, to evaluate PAC's compactness and mechanical properties. The study found that a fluidity level of 18 ± 2 seconds is optimal for achieving a density comparable to conventional concrete, regardless of the pouring method. Additionally, the study demonstrated that PAC's mechanical properties, such as ductility and shrinkage, are significantly enhanced when combined with steel tubes. This combination addresses the brittleness and shrinkage issues typical of PAC, enhancing the structural integrity and longevity of concrete components.
Materials used in the study included Ordinary Portland cement, fly ash, silica fume, and various admixtures like expansive agents and superplasticizers. The aggregates varied in size, with the maximum size of manufactured sand being 2.36 mm. The coarse aggregates used in the experiments had a particle size range of 10 to 25 mm and a porosity of 36.7%, with a bulk density of 1.684 g/cm³. The study’s experimental program also utilized calcium oxide as an expansion agent and superplasticizers with a 25% water reduction rate. These materials were crucial in preparing the different grout mixtures used in the PAC specimens.
The experiments explored five different grout types with varying fluidities, testing their flow times according to GB/T 50448-2015 standards. The researchers adjusted the grouting mix ratios and tested the PAC's performance through formulas, ultrasonic testing, and core sampling. These tests aimed to optimize the flowability and compactness of the grout, enhancing the mechanical properties of the PAC and its application in steel tube concrete. The study concluded that the optimal grout mix and pouring method significantly improve the mechanical properties of PAC-based steel tube concrete, making it a viable option for advanced construction projects.
The findings of this research underline the potential of PAC in revolutionizing concrete construction, particularly in structural applications where enhanced durability and reduced environmental impact are critical. The combination of PAC with steel tubes could pave the way for more resilient, sustainable, and cost-effective construction solutions, supporting the development of advanced building technologies and infrastructure projects worldwide.
