Crazy pavers – why pavers go white

We’ve all seen them somewhere – pavers coated with patches of white salt. This unsightly problem is easy to avoid, though harder to fix.

If you’ve ever sat on the grass at a picnic and stood up with a damp seat, you’ve experienced capillary action first hand. Capillary action is the ability of very closely spaced solids to lift water on account of the surface tension of the water. A sheet of paper on a kitchen bench shows capillary action if you drip water at its edge.

So long as there is an unbroken length of closely spaced particles, soil moisture will be conducted to the surface of whatever is laid on the ground. Like pavers.

All soils have salts in them, as a result of natural weathering, deposition of trace amounts of sea salt in rain, and fertiliser application. Salts, being dissolved in the soil water, will therefore also rise to the surface, where the water evaporates, leaving them behind.

In the “good old days”, pavers were always laid in a bed of sharp river sand, which used to be easy to come by and cheap. The sand grains, being of nearly uniform size, have few points of contact with each other, and so capillary action is hindered. This is the crucial point: without capillary action, the soil water can’t rise very far. Salt stains on pavers were uncommon.

Today, sharp sand is expensive, so landscapers look for cheaper alternatives. Crusher dust is the dust left over from blue metal crushing for gravel. As a bed for laying pavers, it has excellent compaction. Unlike sharp sand, however, the particle size is continuous, not uniform. This means that the dust particles compact with a lot of surface area contact, and therefore very good capillary conduction. So the route from the soil to the paver surface is continuous, and the soil water travels all the way to the top, as the diagram shows:

To avoid this problem, you will either need to use the more expensive sharp sand or consider laying a moisture barrier, such as plastic sheeting (which will increase runoff at the edges of the paved area). Alternatively, gap-graded crusher fines (small gravel particles of uniform size) will do the same job.

To remedy a problem that already exists, you will need to lift the pavers and replace the bedding material.

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