Ultra-High-Performance Concrete (UHPC) demonstrates substantial potential in civil engineering applications owing to its superior mechanical characteristics and enhanced durability. The material's exceptional workability and structural adaptability make it particularly effective for thin-layer strengthening of aging concrete structures. Pumping technology has become prevalent in construction operations due to operational efficiency and site adaptability. Existing investigations have documented performance variations in conventional concrete materials under diverse pumping conditions. These studies indicate material-specific response patterns, with post-pumping mechanical properties exhibiting notable disparities across concrete categories. However, the mechanical behavior evolution of UHPC under pumping conditions remains insufficiently explored, particularly given its distinct material composition compared to conventional concrete. This study conducts a controlled experimental investigation to evaluate the mechanical performance variation of UHPC subjected to three transport-casting conditions: non-transported baseline, screw pump transportation, and piston pump transportation. Through systematic comparison of pre- and post-pumping mechanical parameters, the research aims to clarify the pumping-induced performance modifications in UHPC.

With transport methods (direct casting without transport, screw pump transport, piston pump transport), casting directions (random casting, parallel casting, perpendicular casting; Here parallel casting denotes casting along the long axis of prismatic specimens, while perpendicular casting denotes casting perpendicular to this axis), and steel fiber volume fractions (0%, 1%, 2%) as test parameters, 11 specimen groups were fabricated. Each group comprised 4 prismatic specimens for flexural strength testing and 4 cubic specimens (cut from the prisms) for cubic compressive strength testing. During flexural testing, electrical resistance strain gauges with a 20 mm gauge length were bonded to the tensile face of specimens to quantify longitudinal strain development within the pure bending zone prior to crack initiation. High-precision displacement transducers were vertically aligned at mid-span to quantify flexural deflection, complemented by dual auxiliary sensors longitudinally mounted on opposing sides of the specimen's tensile surface to monitor elongation within the pure bending zone. Following mechanical testing, prismatic specimens were longitudinally divided into four sections. Each exposed surface was digitally imaged using high-resolution photographic equipment. Subsequent computational analysis quantified the angular deviation between steel fiber orientations and corresponding cross-sectional normals through dedicated image processing algorithms.

Screw pump transport casting significantly influenced fiber orientation in specimens. Compared with directly cast specimens, at a 1% fiber content, the fiber orientation angle of screw-pumped parallel-cast specimens decreased by 9.8° across the full section, with the orientation coefficient increasing from 0.454 to 0.607; at 2% fiber content, the orientation angle decreased by 10.6°, and the coefficient increased from 0.461 to 0.637. Which indicates that screw pump transport with parallel casting significantly enhances fiber orientation in the mixture, with similar orientation effects across different fiber contents. Piston pump transport had no significant impact on fiber orientation. Compared to direct casting, at 1% fiber content, the piston-pumped parallel and perpendicular casting methods reduced fiber orientation angles by 1.9° and 0.2°, respectively, and increased the orientation coefficients by 5.7% and 0.4%, indicating no obvious fiber orientation occurred during piston pump transport. A significant wall effect was observed in fiber distribution across specimen sections: fiber orientation coefficients at the edges were higher than those in the middle for all specimens, with notable differences across casting methods. The difference in orientation coefficients between edge and middle sections reached 30%-35% for directly cast specimens; this was reduced to 5%-10% for screw-pumped parallel-cast specimens but expanded to 35%-45% for perpendicular-cast specimens; piston-pumped specimens exhibited edge-middle differences of 15%-20%. Compression tests showed that UHPC compressive strength decreased by 2.6%-8% after transport, though the reduction was not significantly affected by pumping method, casting direction, or fiber content—primarily attributed to disturbance of the UHPC pore structure during pumping. Flexural tests revealed that at 1% and 2% steel fiber contents, screw pump transport with parallel casting increased flexural strength by 19.9% and 32.1%, respectively, compared to non-pumped direct casting. Flexural strength of piston-pumped specimens remained similar to direct casting, with differences within 6%.

Based on the test results, the recommended values for the steel fiber orientation coefficient in UHPC under different transport and casting methods are as follows: 0.5 for UHPC with direct casting or piston pump transport-casting, 0.62 for UHPC with screw pump parallel casting, and 0.38 for UHPC with screw pump perpendicular casting. When screw pumps are employed, material placement direction should align with the structure's principal stress direction. No directional constraints are required for piston pump applications. The flexural strength of UHPC under different transport methods and casting directions was predicted using the proposed fiber orientation coefficients, and the predicted results closedly matched the experimental measurements.