Mortar is a definite maintenance item in old stone bridges, provided, of course that the bridges were originally mortared. Different types of stonework and construction practices rely on mortar differently and to different degrees, so it helps in bridge maintenance to understand the properties and uses of the mortar in various old bridges.
Though the Romans had high-quality cement (arguably better than modern cement mixtures), they often built their bridges mortarless. The quality of most if not all their work was very high, which is why mortar was not needed. In the cases where mortar was used, the stones were precisely cut to allow a very thin layer of mortar. Roman cement mixes tend to become stronger with age, and are quite enduring.
In medieval times, stone bridges usually relied heavily on mortar. The build in many medieval bridges was of a far lower quality than the Roman bridges. Often, a medieval bridge was laid with stones that fit moderately well, and heavy doses of lime mortar were used to fill up the extensive gaps. In arches, the mortar was often poured in. The lime mortar used was soft and elastic, allowing it to accommodate some shifting of the structure. However, the mortar had a tendency to leach out over time, depending on its composition. Most of the arch stones in a medieval bridge will likely stay in even if the mortar is leached out, for the stones were still placed where they would fit, even if they were crudely trimmed or even left unaltered. However, the vibration of modern traffic can cause arch stones to work loose if the mortar is gone. Therefore, medieval structures will need some sympathetic mortar repointing or even injection with a lime-based mortar from time to time. That said, there are still some quality-built medieval bridges that are not as particular about their mortar.
Later Stone Bridges
By the 1700s and certainly the 1800s, premium stone bridges appeared across Europe. These bridges usually featured stonework of first-class quality. They used mortar, but relied heavily on the tight fitting of the stones as well. In America, high-quality stone bridges were built for railroads beginning in the 1800s. Again, these bridges did not rely heavily on mortar, and were built to stand, mortar or no mortar. For road bridges, mortarless construction was common in the New England area. The mortared American road bridges of this time often were of fair quality. That said, the mortar still can be important in preventing some of the lower-quality bridges from being vibrated apart.
There may be other cases where the mortar is more heavily relied on than meets the eye. Only in-depth research can determine this. As an example, Kansas builder Walter Sharp relied on mortar to actually make the arch angles, as he himself stated. Hence, the arch is very dependent on the mortar. The arch faces were cut to a tolerable joint tightness, which can give the wrong impression of how the inside of the arch is built.
How dependent the bridge is on mortar appears to vary, both due to local stone considerations and the date the bridge was built, but as a whole mortar condition is an important factor in the durability of a Walter Sharp bridge, particularly his later ones. Hence, the mortar needs to be kept in sound condition. Furthermore, Walter Sharp also used a harder mortar mix than the traditional lime mortars in order to ensure strength. Thus, a lime mortar such as is often used for historic structures may be inappropriate. A case-by-case examination and research may be necessary to determine how important the mortar is to a bridge. It is worth noting that there were several builders copying Walter Sharp’s methods in the Kansas area, and Walter Sharp himself built stone bridges in other states as well.
Things to Look For
The dependence of a structure on its mortar can often become clear upon observation. A close look at the joints between stones in the bridges often give clues on what to expect. Rather wide gaps or mortar joints between stones indicate that the structure depends largely on mortar for its survival.
Even if the joints are filled with stone fragments, mortar may be important provided the bridge was not originally intended to be a drystack structure.
In arches, the bottom joints may fit tightly, but still rely on mortar heavily up above; it is sometimes hard to tell with arches unless the end joints are obviously loose or access to the top joints in the arch is available. On the other hand, tight-fitting joints between stones show that the structure is high-quality and not terribly dependent on mortar.
The arch often ends up being the weakest link in most stone bridges; hence it is usually safe to assume that the arch is no better than the surrounding masonry. As the arch is the primary structural element in a stone bridge, its mortar condition will also be more important than that of any other part of the structure.
Obviously, mortared structures should receive mortar maintenance as needed. How urgent this is depends on the quality of the structure, so lower quality structures will need priority. In the end, though if it was built with mortar, periodic re-pointing should be carried out.