Mortar is commonly used on most masonry structures, including stone arch bridges. Over time mortar deteriorates. This deterioration weakens the structure and allows for water infiltration. Repointing is a common means to fix deteriorated mortar joints. However, one needs to be aware of the limitations of repointing.
What Does Repointing Do?
Though repointing the mortar joints is primarily superficial, this does not mean that repointing is not beneficial. On the contrary, it is quite useful. After all, it seals back up the mortar joints, preventing water infiltration. And, because of this, the repointing works to protect the interior mortar. This is significant, because the interior mortar, which can be quite important structurally, is not so easily repaired. Failure of the interior mortar can significantly weaken a structure, depending on how it is built; see Understanding Mortar in Old Bridges for more information. In short, regular repointing of the masonry joints works to protect the interior mortar, ensuring the stability of the structure in question for years to come.
The exterior mortar joints also help prevent plants from taking root between stones. Hence, it follows that repointing will, in turn, help minimize plant-related damage to a structure. This is important as plants tend to hold moisture into masonry speeding up deterioration of the external faces of the stone and mortar, while their roots tend to work away at the interior stone and mortar. Trees in particular can do massive amounts of damage, dislodging enormous amounts of stone and even destroying a entire structure, so keeping the joints sealed from plant life is highly beneficial.
How Often Should it Be Done?
The mortar does not need to be repointed if it is in good condition. However, numerous cracks and significant patches of missing mortar show that a complete repointing of the structure is probably needed.
If most or all of the external joints appear to have lost their mortar, be aware that repointing may not be adequate to repair the masonry back to its original strength and condition; the interior mortar may have been lost. (This problem can, however, be remedied, as we will see in part 2 of this series.) As far as planning goes, the mortar should be investigated with an eye towards repointing about every 15 years. Obviously, if the mortar is found to still be in good condition, repointing is not required, though it wouldn’t hurt to touch up an occasional bad joint should any be found. Modern mortars do tend to last considerably longer than many old-style mortars, requiring less maintenance, though be aware modern mortars are also harder than old-time mortar, and may not be suitable for a stone arch bridge. (See our post on The Different Kinds of Mortar.)
To conclude, the external mortar is actually a relatively short-lived item requiring regular maintenance. Keeping the external mortar joints in good shape can go a long way in keeping an old bridge in good condition.
Stone Arch Bridges 