Delamination in Paving Units
Delamination occurs in certain types of paving stones, especially those than have been riven, or split from a natural grain in the material, where they have been formed over millions of years, layer upon layer, compressed into a compact form under enormous pressure. Examples of such types are sandstone and slate, which are split from larger pieces of stone, into thinner ‘slices’ to suit the project in hand.
Sandstone and slate are the two most common types of riven stone used in paving. The process of creating the ‘slabs’ is either by cropping or shearing mechanically, leaving a fettled edge to the slab. In this ‘natural’ state, they are unlikely to split, or delaminate once set into a mortar bed, although slight spalling or flaking of the surface is quite natural, and should not present any problems after a few months of weathering.
If a design calls for precise dimensions, and a sawn edge is required, it is advisable to dress or fettle the edges with a comb hammer or mason’s pitcher to replace/repair the sawn edge. If the slab is laid with an unfettled sawn edge, there is a likelihood that water may penetrate the slab, allowing frost freeze/thaw action that will split or delaminate the paving across the natural layers.
The effect on a project can be troublesome, with a number of slabs shedding their upper layer, leaving a recess in the pattern and causing further problems with standing water and frost action.
