For the past couple of days the thread has been discussing the reasons why a concrete base is not suitable for natural stone paving. The blog must have been thought provoking because we have received a phone call today from a paving contractor who is in the process of installing a Yorkstone paving patio. The project architect had specified that the Yorkstone patio is to be laid on top of a concrete base. It had all the hallmarks of over kill by an over zealous architect, until we found out that there was a valid reason for specification of the concrete base. It turned out that underneath the proposed paved area were natural springs. The architect was concerned that the natural springs could potentially erode and undermine a standard MOT base. The advice we gave to the contractor was that if he was intending to lay natural stone on a concrete base, in order to prevent moisture from leaching through the slabs the underside of the paving would need to be sealed.
If a situation does develop when it is necessary to seal the underside of stone paving it is important to use a bond bridge. A bond bridge is a mixture of sbr and water, painted onto the underside of the paving. Sealing the underside of paving will affect the adhesion between paving and bedding layer and a bond bridge is essential to aid adhesion.