Sometimes, a stone arch bridge will show significantly deteriorated stones. If the deterioration is manifest throughout the bridge, it can be a rather tricky problem to deal with — the stones make up the entire fabric of the structure. Unless a significant impact broke the stones, deteriorated stonework is almost always a sign of water infiltration into the structure.
The Water Problem
Ironically, water is one of the worst enemies of a stone arch bridge. Besides being perfectly capable of undermining a bridge, water can play havoc with softer stones such as limestone and some types of sandstone. Stones that have numerous thin strata are the worst. Water seeps into the strata. When it freezes, it breaks the stone into its numerous thin layers, turning it into rubble. On limestone bridges, it is not uncommon to see the stones cracked and spalled (i.e., having fragments broken off) at the waterline.
One common fix for these waterline problems seems to be pouring a concrete apron around the affected portion of the bridge. The apron acts as a scour barrier and also prevents direct water contact with the stones. The apron also fills in voids and holds already broken stones together.
While there may be legitimate concerns about having concrete in direct contact with stonework, evidence seems to show that it is better to pour a concrete apron around the affected portion of the bridge than to do nothing.
Another waterline problem that occurs on stone arch bridges made from very soft rock is erosion. Again, protecting the affected portion from the water’s direct action is a good solution.
Common Methods Used to Repair Deteriorated Stones
When it comes to patching stones, mortars are manufactured that fill in and bind together cracks.
Epoxy is occasionally used to do the same thing.
Extreme cases may require stones to be replaced.
Anything can be repaired; in the worst case a “repair” may actually be more like a “rebuild.” Regardless of how the bridge is to be fixed, it is highly advisable to determine why the stonework was deteriorated in the first place.
The cause of waterline problems is usually obvious; less obvious is deteriorated stonework in the upper portions of the arch or spandrel walls. Sometimes the problem is as simple as poor-quality stones. Stones with numerous strata or really soft chalky stones tend to crumble apart in winter if they are not kept perfectly dry.
Another problem is hard mortar or concrete smeared indiscriminately over the stonework. It is interesting to note that concrete can either protect stonework or deteriorate it, depending on how it is applied.
Determining the Cause
The best way to solve the deteriorated stone problem is to think about where the water is lingering. If the stones are dry, it is unlikely that they will be deteriorating. Vines festooned over the stonework can cause problems by holding in moisture, as can a layer of concrete or hard mortar smeared over the stones that prevents them from “breathing” and thus drying out when they are wet.
One of the most common problems leading to deteriorated stones is waterlogged fill. Wet fill holds water against the entire bridge, keeping the stones soaked. The solution for this is either to allow the fill to drain by adding some form of weep holes or to simply replace the fill with something that will not become waterlogged. (Ironically, replacing a dirt fill with concrete fill can be helpful if done right.)
Once it is determined where the water is lingering, diverting it away from where it tends to accumulate and/or draining the water from its present course is the solution. An ounce of prevention is worth a pound of cure.
In the town of Marion, Kansas, in the 1800s, water-related damage to a bridge was feared. The problem was simple: A water main that ran through the bridge had a leak and was perpetually soaking the bridge. The solution was, of course, to stop the leak. (Or, as they ended up doing, to rerout the water main.)
The bottom line: Determine why water is being held against the stonework, and then determine how to prevent it from accumulating on (and in) the stones.