Study on the Size Effect of PCCP Protective Layer Mortar and the Influence of Admixtures

The thickness of the protective layer mortar for prestressed concrete cylinder pipe (PCCP) is usually only 25 mm, whereas the laboratory standard mortar specimen sizes are 70.7 mm or 40 mm. To establish an effective correlation between experimental results and the actual performance of components, it is crucial to systematically investigate the influence of size effects. To address this issue, this study produces cubic specimens of three sizes, 70.7 mm, 40 mm, and 25 mm, by varying the contents of fly ash (FA) and nano silica (NS), in order to explore the impact of these supplementary materials on the performance of PCCP protective layer mortar and to reveal the regularity of strength variation with specimen size. The main conclusions are as follows: Firstly, the optimal combined dosage of FA (20%) and NS (3%) is determined, which shows the best 28-day mechanical performance and dimensional stability. Secondly, the mortar strength exhibits a clear size effect: as the specimen size decreases from 70.7 mm to 40 mm, the strength increases, but further reduction to 25 mm led to a decrease in strength. Finally, the established size conversion equation indicates that the conversion coefficient first increases and then decreases with the admixture content and is dependent on the dosage. This provides a theoretical basis for accurately predicting the actual performance of thin-layer PCCP mortar.