Research on Cracking Causes of Industrial Roof Slabs and Optimized CFRP Strengthening Measures

Large-scale irregular cracks developed on the second-floor roof slabs of an industrial plant shortly after formwork removal. To identify the cracking mechanism and optimize the strengthening scheme, field inspection, X-ray diffraction (XRD) phase analysis, restrained-shrinkage stress calculation, and nonlinear finite element (FE) simulation were combined in a closed-loop study. The concrete strength, reinforcement arrangement, slab thickness, and flexural capacity all met the specification requirements, and XRD detected no crystalline sulfate-bearing phases within the 1–2 wt.% detection limit, ruling out structural overloading and sulfate-type chemical attack. The cracking was attributed mainly to excessive restrained drying-shrinkage stress caused by mix-proportion defects (a high water-to-binder ratio, excessive fly ash, and a high mud content in the fine aggregate); the calculated maximum restrained shrinkage stress of 3.19 MPa exceeded the concrete tensile strength. Preferential microcrack initiation in the aggregate–mortar interfacial transition zone (ITZ) is presented as a proposed mesoscale mechanism, because direct microstructural (SEM) observation was not performed. Under identical total CFRP consumption, a four-dimensional evaluation (bearing capacity, material utilization, crack suppression, and relative interfacial slip) showed that the grid-shaped layout outperformed the cross- and hash-shaped layouts, and a ±10% sensitivity analysis kept all predicted crack-width variations within ±5%. A composite remediation of low-pressure grouting plus grid-shaped CFRP was then applied; after 12 months of monitoring, the maximum crack width decreased from 0.32 to 0.04 mm, demonstrating the synergistic effect of the composite system rather than CFRP alone. This case provides a reference for the diagnosis and treatment of shrinkage cracking in comparable industrial roof slabs.