The preparation and application of the wearing surface require much care if satisfactory results are to be obtained. The most evident service of this layer is to withstand wear, and it should therefore be made of rich Portland cement mortar. With a sand consisting principally of quartz particles, it is found that a mortar composed of equal parts cement and sand gives about the best results in tests of abrasion. If the mortar is used richer than this, it is likely to check or crackle in setting, marring the appearance of the walk. Mortar containing two parts cement to three parts sand gives nearly as good results, and two parts sand or fine crushed granite to one of Portland cement is usually satisfactory. The sand for the mortar should be quartz if possible, or crushed granite or trap. It should be screened through a quarter inch mesh, and there should not be a large proportion of very fine particles.
The thickness of the layer of top dressing is usually about one inch, and this is probably the maximum thickness ever required. One-half inch of top dressing is believed to be sufficient when the wear is not excessive, provided the base has been carefully leveled.