Investigation of early-age cracking experiments in concrete and research on improvement methods

The Chinese standard GB/T 50082-2009 specifies early-age cracking tests for concrete but lacks detailed control over fan-induced airflow, resulting in poor flow uniformity and poor comparability of results. This study investigates how blade diameter, shaft position, and lateral or rear obstacles affect airflow distribution above the concrete specimen. Based on the findings, a gas rectification cover is designed and optimized using ANSYS Fluent simulations. Experimental and numerical results show that using a blade diameter equal to or larger than the specimen's long edge (recommended ≥ 800 mm for an 800 mm‑long specimen), positioning the shaft below the specimen surface, and maintaining obstacle distances over 700 mm minimize external disturbances. The optimal cover dimensions are inlet length 300 mm, outlet length 1500 mm, specimen-to-outlet distance 500 mm, transition length 400 mm, inlet diameter 600 mm, and outlet diameter 900 mm. At a fan speed of 10 m/s, the optimized configuration achieves 90% coverage of the target wind speed range with 4.5-5.5 m/s over the specimen surface. This study fills the technical gap in airflow control within the current standard, providing a quantifiable and practical solution that significantly improves test repeatability. The proposed approach is also applicable to other standards requiring uniform airflow distribution.