Masonry Wall Conservation

EXISTING CONDITION

In the fall of 1990, Anrep Associates Ltd. were employed by the parish of Sainte-Anne d'Ottawa to investigate the cause of a serious bulge on the exterior face of the west transept wall.  Upon investigation of the wall it was discovered that, on the inside face of the wall at the location of the bulge, there was a recess in the wall measuring 6' X 7' x 14' dp. approximately.  Additional investigation revealed the same recess at the opposite end of the west transept wail and two similar recesses on the east transept wall.  In one of the recesses in the east transept wall, part of the timber lintel over the opening had deteriorated due to moisture and failed.  This resulted in a large amount of the rubble, between the interior and exterior stone wythes, falling into the recess.

The walls themselves consisted of two wythes of stone, with rubble and mortar in the centre.  The walls were about 26" thick.  Over time, the mortar in the centre had been reduced to sand.  With constant weathering, freeze thaw cycles due to moisture penetration into the walls, and vibrations, the loose rubble worked its way down the wall, slowly acting like a wedge to pry the two stone wythes apart. This action eventually leads to the wall no longer acting as a single unit, but has two separate wythes acting independently.  This results in a substantial weakening of the wall and eventual failure.  It was our conclusion that this, in combination with the weakening effect of the recess, was the cause of the bulge. We recommended that the client shore the wall immediately until remedial work would be carried out. In addition, we were also concerned that the potential was high for the same condition to occur on the east wall.

SOLUTION

In consultation with the client, it was agreed that we would not try to remove the bulge in the west transept wall as the cost to shore and stabilize the wall above, while rebuilding the lower section of wall, would be excessive.  In addition, in order to strengthen the transepts it was agreed that the interior recesses be built up in order to increase the wall capacities at the location of the recesses. These recesses were covered by the drywall finish on the interior face of the walls, so were not exposed.   Structurally, our concern was that due to window placement in the wall, most of the wall self weight was transmitted to the foundations by way of the wall at the location of each recess.

We proposed to repair the walls as follows in order to restore their structural and historical integrity.

1. Rebuild the four recesses (two in each transept wall) using a similar limestone, with a lime base mortar.
2. Anchor the two wythes of stone together using a CINTEC anchoring system.
3. Pressure grout the voids in the wall using a cementitious grout.
4. Rake out the existing joint mortar to a depth of approximately 55mm and repoint the wall using a Lime, White Non-Staining Portland Cement, sand mortar mix. At the client’s request, the joints were retooled to match the profile of the mortar joints on the remainder of the structure.
5. At the corner where the bulge existed, the exterior wythe of masonry was stitched using CINTEC stitching anchors. The anchors were installed in holes drilled parallel to the exterior wall face in two directions, staggered, and spaced at 300mm on centre. These anchors were 3000mm long in one direction and 1200mm long in the other direction.
6. Below grade, where the mortar in the joints had deteriorated to sand, the joints were regrouted as above grade, the face of the stone was parged, and a protective coating of a waterproof material was applied to the face of the parging.

ANCHORING SYSTEM

The CINTEC anchors, noted above, originated in England and provided a much better anchoring system than expansion anchors, for the conditions in question.  The anchors consisted of a hollow structural steel section which is surrounded by a fabric sock.   Grout is pumped into the tube and extrudes through a hole at the inner end of the steel section and fills the fabric sock from the inside out.  When the moisture in the grout mix seeps through the fabric on the exterior hole face, the installer knows that the fabric sock has filled the void, and the desired pressure has been achieved.   This moisture also seeps into the surrounding parent material and forms a cementitious bond.

The CINTEC anchors were chosen on this project because of their adaptability to be designed for the specific project and the compatibility of the anchoring system with the parent wall material.  The stress between the interface of the anchor and the parent material is low due to the large area of the interface. This makes the anchor ideal for use in old/heritage type buildings, where the strength of the parent material is generally low.  The lower the strength of the parent material, the larger the diameter of the hole drilled in the wail and the larger the diameter of the fabric sock.

The typical wail anchors noted in item #2 above were 55mm long.  They were spaced at 900mm on centre horizontally and 450mm on center vertically.  The outer end of the anchors were recessed 25mm from the face of the wall and covered with the lime based mortar.

SUMMARY

Anrep Associates Ltd. worked closely with the client, with representatives of the Heritage department of the City of Ottawa, with the general contractor, Lariviere Construction Ltee. from Hull, Quebec, and with CINTEC representatives from England, to successfully stabilize and restore the condition of the transept walls, while matching the wall finish with the remaining walls of the church.  Though the mortar joint was brighter than that of the mortar on the other walls, it was felt that the brightness would fade over time due to weathering of the mortar joint.  The restoration permits the transept walls to perform in the manner they were intended and thus prolong the life of the structure.