An expansion joint (1) for a concrete slab formwork system, said joint comprising first and second longitudinal elements (2, 3), arranged side-by-side in the same horizontal plane, and intended to each form an integral part with an upper and horizontal edge of a concrete slab, the first and second longitudinal elements (2, 3) comprise arrangements (5, 6, 7) opening on their upper horizontal walls and enabling at least a portion (2a, 3a) of each of the first and second longitudinal elements (2, 3) to be withdrawn for replacement thereof. The first longitudinal element (2) comprises a stack of three layers of material (2a, 2b, 2c), with an intermediate layer (2b) having a smaller width than that of the two other layers (2a, 2c) such that the first longitudinal element (2) has a U-shaped cross-section, and the second longitudinal element (3) comprises a stack of three complementary layers of material (3a, 3b, 3c), with an intermediate layer (3b) having a larger width than that of the two other layers (3a, 3c) so as to form a transverse projection that is able to fit in the recess of the U-shape of the first longitudinal element (2).