Systematic Evaluation of Concrete Placement Reliability in Heavily Reinforced Nuclear Structures

Concrete placement in nuclear power plant (NPP) structures is challenged by severe reinforcement congestion and complex internal geometries, where conventional slump-based workability control may not reliably predict placement performance. This study investigates the relationship between polycarboxylate ether-based admixture (PCE) adsorption, rheological behavior, and placement performance through a multiscale experimental program. Conventional workability tests (slump, L-box, and U-box), PCE adsorption analysis using total organic carbon (TOC), rheological characterization, and custom-fabricated placement simulation tests were performed. The results showed that mixtures with similar slump values exhibited significantly different passing behavior due to differences in PCE adsorption and resulting rheological properties. A clear relationship between yield stress and blockage risk was identified using a vertical drop test, indicating that mixtures with paste yield stress exceeding approximately 0.3 Pa exhibited reduced discharge and a high risk of blockage under reinforced congestion. A horizontal filling test further demonstrated that horizontal placement represents a more severe condition than vertical passing, as even highly flowable concrete could not achieve complete void-free filling along extended horizontal paths under self-weight.