Microcracking in the form of drying shrinkage, or freeze-thaw cracks is common in most of the cores as vertical surface cracks extending 10 mm to 20 mm into the core. These cracks may be found both exclusively in the cement paste, and also through the paste and coarse aggregate. Cracks along the vibration trails appear to be drying shrinkage cracks resulting from the greater shrinkage potential of the mortar-rich vibration trail regions and the bulk concrete. The pattern cracking observed on the surface may result from similar phenomena. Cores through vibration trails exhibit a lower entrapped air volume, poor air void distribution in upper half, and appear to be enriched in the mortar fraction
Bakharev and Struble  examined microstructural features associated with freeze-thaw deterioration. An early manifestation was microcracking extending vertically several mm into the specimen. Additional freeze-thaw cycles resulted in a horizontal cracking that, when intersecting the vertical cracks, resulted in spalling. They considered the crack development to result from localized expansive shear stresses from hydraulic pressure and localized stresses from differential volume changes. They postulate as the top layer of the concrete is frozen, water is expelled to the surface and to the interior causing hydraulic pressure. As the permeability of the frozen surface layer is low, water moves inward. Horizontal crack development may be due to development of shear forces at some distance from the surface due to differential volume change of saturated and unsaturated layers, that is, the surface layers are expanding while unsaturated interior layers may be contracting.