Conservation Methodology

Resetting on/in existing base
Unsecured stones in existing bases require re-setting. Generally the base should be reset level and aligned with adjacent stones. Pins should be removed if present. The stone can then be re-set level and plumb in the existing slot.

Re-set stone on a full bed of modified lime (or hydraulic lime) mortar. Historically ratios of 1 part cement, 4 parts lime and 8 parts fine sand have been used with reasonable results. This mix is generally considered to be a soft mortar. Some conservation recommendations have specified ratios as high as 3 parts cement, 2 parts lime and 8 parts sand. The increased cement and reduced lime content has the effect of increasing the strength and adhesion of the mortar. In theory this would tend to make the mortar last longer than the traditional mix. The negative aspect is that the higher cement ratio produces a harder joint which induces a compression stress on the stone as the stone swells with varying weather conditions.

Our recommendation is to use 2 parts cement, 4 parts lime and 8 parts fine sand which, increases the strength somewhat while still retaining some of the softer properties to help reduce stress on the stone.

 

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Resetting into new cast concrete base
There are several situations where a new cast base will be required. Usually tablets which are broken near grade level or have been cut years earlier and set into bases that have failed are typical examples of when a new base is needed. Bases can be set above grade or below depending on the stone, aesthetics or other factors. Bases can be cast on site or pre-cast and set in place on a level bed of gravel and sand.

Cast concrete bases are typically made with a slot that is ½” wider and thicker than the stone and is recessed 3”-4”. Depending on the size of the stone the base is usually 8”-12” deep, 8”-12” greater thickness and 6”-8” wider than the stone. This method is fine when resetting stones with a square bottom.