Needle Joint Masonry
By Craig E. Barnes, P.E., SECB
Needle joints are defined simply as thin mortar joints, less than 3/8”. The biggest problems in dealing with needle joint masonry are the techniques of joint excavation and repacking (tuck pointing). Over many years of restoration work in Back Bay and on Beacon Hill structures where needle joint masonry abounds, we have come to appreciate that the best of specifications can result in the poorest of product unless the masons use appropriate techniques, have an understanding of the mission, and are monitored frequently. Most often the tools in the mason’s quiver are fine for a 3/8″ standard joint but not appropriate for needle joint work. Nonetheless, masons will, more often than not, use their standard tools and when done, from a distance the joints may look fine but the overall product is lacking in quality because standard tools are too large in size to properly repair these thin mortar joints.
In many cases we choose not to go the general contractor route with masonry subcontractors quite simply because of the loss of control. Normally, the pre-project study authorized by the client results in a fairly comprehensive understanding of how the original construction is assembled and what efforts had been made through the years in maintaining the masonry of older structures. We suggest selecting three (3) contractors who are given detailed access to that study and an outline of the demands that would be placed upon them during the construction process. On one occasion a contractor took it upon themselves to follow the initial study with a site test panel used for joint cutting in an effort to determine the most efficient excavation process.
One may pre-qualify the mechanics in the techniques of excavation and pointing. In advance of a full process field test panel, the contractor may prepare a mock-up, which is a replication of the needle joints that they would be dealing with in the field. The purpose for the mock-up is to determine an efficient way to pack the joints. The contractor may use traditional tool tuck pointing, sponge packing, grout gun, and mortar bag techniques. Mortar gun and mason bag are usually judged to be unsuitable. Why go through the process of testing a procedure one knows will be unsuccessful simply because the contractor may not know what the result will be?
Once the project is awarded, mechanics qualifying for the process are instructed by CBI and observed through the cutting operations and the pointing. Depending on the needs of the project, one of the mechanics may use the traditional tool tuck pointing process while the other mechanic will use the sponge packing process. Wash down of the completed product is simpler with the traditional tooling than it is with the sponge process. The sponge packing process requires that the contractor undertake two (2) wash down steps. Masonry cleaning procedures are reviewed on the test panel and with some tuning, wash down of the standard pointing process and of the sponge application process are acceptable.
Packing a joint in two (2) 3/8″ layers to achieve the minimum 3/4″ depth is a standard requirement for a normal repointing project. We have found in needle joint work that a one-shot application will work satisfactorily with the right installation techniques. The contractor is allowed to test both techniques with their crew in order to judge the acceptability of the one-shot approach.
Can a pre-mixed and bagged product be used? The answer is yes if rigorous testing in the field is found to be acceptable. The contractor may be authorized to use either the pre-mixed or the on-site component mix but would not be allowed to use both processes at the same time. Color and texture mismatch are a concern with a mixed application.
A process has been set-up where the contractor will complete excavations in a given area, which will then be inspected before the joints are packed. A second inspection will be done during or at the conclusion of the pointing process as time permits, and a final inspection following wash down.