Chemical prestressing is a method of introducing stresses into concrete elements by the controlled expansion of the concrete itself. Providing a proper amount of expansive admixture and sufficient restraint, for example in form of internal reinforcement, prestressed elements can be produced. As concrete expansion takes place in all directions, chemical prestressing might be a particularly good solution for surface elements with biaxial reinforcement. Even more advantageous it is for structures with carbon and textile reinforcement, where conventional prestressing technologies are challenging, as well as for shell structures with varying curvatures, which are currently being developed within the project CRC/TRR 280 “Design Strategies for Material-Minimised Carbon Reinforced Concrete Structures— Principles of a New Approach to Construction”. However, standard measurements of expansion and methods for calculating the corresponding chemical prestress are performed on unidirectional, prismatic specimens. Results of expansion of specimens with biaxial restraint are also scarce. Within the work presented here, thin concrete slabs with dimensions of 2000 x 1000 x 30 mm³ and biaxial, symmetrical carbon textile reinforcement were produced and chemically prestressed. During setting and hardening, the expansion was measured with distributed fibre optic sensors (DFOS) installed on the carbon reinforcement and in concrete in the longitudinal and transverse directions. A sensor gauge length was set as 10 mm what allowed for distributed presentation of strain development in both directions. The measured strains were then compared with results obtained in standard uniaxial restrained expansion tests (ASTM C848). The thorough understanding of the bi-directional influences between reinforcement, expansion and chemical prestress is a key issue for further development and application of this technology to elements with more complex geometry, such as the shell concrete structures, which are being designed within the subproject C01 within the CRC/TRR 280.