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fieldstone foundation

fieldstone foundation

(OP)
Problem, local church went through flood, building set on fieldstone foundation, field trip discovered no apparent shifting or settlement of existing foundation, but some areas of foundation appear to be missing alot of mortar. Checked all of the walls in the building and appear to be fairly plumb. What is the best way to repair the existing foundation? I thought about placing forms and filling with self consolidated concrete or mortar, which eventually could "pop-out" or just have re-pointed? Of course the building could be jacked and place a new foundation, but that must be quite expensive. PCC may be the better answer for future flooding problems.Thanks for any help.

RE: fieldstone foundation

The short term solution (and perhaps for the long term as well) would be to repair the damaged areas, to their original condition.

Do you have any photos? A picture can be a great help.

The question of jacking the structure and replacing the foundation depends on the likelihood of future flooding. If you're in a floodplain and this is a common occurence, probably makes sense. If flooding was out of the ordinary, it comes down to that Dirty Harry question: "Are you feeling lucky today?"

RE: fieldstone foundation

Get in with the local historic preservation community to get info on the type of construction used, to be sure you're putting things back accurately. You're likely looking at lime mortar, so the worst thing you could do is come back in with Portland cement.

Check the NPS preservation briefs: http://www.nps.gov/tps/how-to-preserve/briefs.htm (#2 jumps out)

Good luck!

RE: fieldstone foundation

You may have a bit of a problem. You have to check the existing mortar joints, both inside and outside with a screwdriver or something hard and pointy. Carefully dig down a couple of feet on the outside of the foundation wall in an area where the mortar on the inside is soft.

With many old ashlar (fieldstone) walls, the mason had to source the lime he would use to make the mortar. If the old lime had few pozzolans, the mortar has poor hydraulic properties, ie., it doesn't stay hard and retain strength over time. Water leaches the lime away, leaving essentially sand.

You may have to remove samples of the existing mortar and have them petrographically or chemically tested for composition.

If this has happened, it is essential that a new foundation wall be constructed. It can replace the existing or alternatively encapsulate it. The entire church foundation may be a problem. If the damage is small and only local, it is possible to patch the mortar using a high lime with masonry cement (Similar to N type mortar) to effect a repair. The strength of the mortar should be kept as low as possible to improve the plasticity of the repair. Too many renovations to historic masonry have been undertaken using high strength mortars, only to have the masonry units fail.

Dik

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