Nicolock paving stone are best installed over recycled concrete base. Just like any other paving materials, Nicolock pavers need drainage. If water sits under the paving stones because is trapped by the base or the wrong base preparation the colors will get affected and more weeds are more likely to grow because of the dampness that is in between the Nicolock paver joints. It is ok to install Nicolock on poured concrete base as long as drainage it created; there are few ways to make drainage in concrete base, by drilling holes in the concrete slab the water will be able to escape down in to the ground. Or by installing the outer edge of the concrete slab with rca extending the base for the finished patio this will allow the water to filtrate in to the ground. The long island Nicolock expert company would create the right installation for you. For more information please contact Gappsi at 631 543 1177 or visit our web site at www.gappsi.com