Manual Casting Practices and In Situ Concrete Strength in Sudanese RC Structures: Evidence from Large-Scale Core Tests, Cube Tests, and Field Survey

Concrete remains the most widely used construction material worldwide due to its versatility and durability, but ensuring consistent quality remains a challenge, particularly in contexts with limited quality control. This study investigates the compressive strength of reinforced concrete (RC) members in Sudan using 629 core tests, 6,435 cube tests, and 1,014 survey responses. Results show that while most cube specimens exceeded the 25 MPa threshold (class C25), 34% failed to achieve grade C25, whereas nearly 60% of the core samples were under strength, with RC columns performing most poorly. Box-plot analyses revealed that column medians and interquartile ranges consistently fell below C25, and in 2020 dropped entirely below C20. Survey data confirmed that 74% of columns are manually casted, and Bayesian probability analysis demonstrated that such columns have an 88.3% probability—nearly a nine-in-ten chance—of failing to meet grade C25. These findings demonstrate that manual casting is a key driver of inadequate structural performance. The study highlights the urgent need to reduce reliance on manual casting, expand the use of ready-mix and mechanized placement, and strengthen quality control measures. Although focused on Sudan, the findings carry broader implications for regions that still rely heavily on manual construction methods.