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Many models exist to calculate slab curling but sometimes with conflicting results. Ytterberg (6) compared different approaches and found that:
•temperature and/or moisture gradients can cause curling in slabs at grade. The most critical time for road pavements is when the bottom of the slab is relatively wetter and warmer than the surface;
•a critical slab length can be defined as that length beyond which warping stresses do not increase with a further increase in slab length. This critical length is between 7 m and 12 m, depending on moisture and temperature gradient, slab support and slab thickness;
•on softer slab support the slab tends to sink in the middle leaving a shorter slab end that shows lift off.
It is clear from the work reported by Ytterberg (6), that slab curling in a slab with 4,5 m joints will not result in critical slab stress, but upward movement of slab ends will create a gap between the slab support and the slab. This gap can be regarded as the initial phase of voids developing between the slab and the subbase especially if an erodible subbase is used.