As the years have gone by, the methods used to rehabilitate stone arch bridges have seen a distinct improvement. Not so very long ago, the general method of “strengthening” or otherwise stabilizing a shaky stone arch bridge seemed to consist primarily of coating part or all of the structure with concrete in some form. Happily, there are many other, better ways to rehab a stone arch bridge, that both keep the structure stable for the long term and maintain its historic appearance. Though there are many suitable repairs for stone arch bridges, there are some tried-and-true methods that are more commonly used than others, and we will list them here.
Concrete aprons/collars are primarily used for scour, but can also be used as an extra support for a settling structure with some careful planning. Concrete is poured around piers or abutments, creating a barrier for water. This prevents water from directly undermining a bridge, greatly reducing the scour threat. Based on our observations, concrete aprons also help prevent waterline deterioration of the stone when used with limestone bridges. The key is to make the apron wide enough to protect the structure from scour without obstructing the stream.
When used as part of the support of a settling structure, micro-piles are usually added under the apron with the apron on top of the piles. The apron will need to be bonded to the stonework in some fashion.
Many stone arch bridges could use a boost for modern traffic loads, and concrete arch saddles help greatly. Essentially, the arch saddle is a reinforced concrete arch built into the bridge directly above the existing arch. From the outside, the bridge looks the same. But on the inside, the arch thickness is greatly increased, stabilizing the bridge and allowing for heavier traffic loads. Concrete arch saddles have been found to be impressively effective for stabilizing a stone bridge.
Concrete Slab Deck
The concrete slab deck is often used to distribute loads over a bridge. The slab can significantly increase the load capacity of a stone bridge. There are other benefits to the slab, however. For one thing, the slab gives the bridge a solid, distributed dead load that helps keep down vibration. This, in turn, helps prevent a stone bridge that is feeling its age from deteriorating. Furthermore, the concrete slab also helps reduce the amount of rainwater that seeps into the structure, which will reduce stone and mortar deterioration.
The slab can be cantilevered to widen the bridge, if appearance is not a major concern, and can also be equipped with modern crash-proof barriers for further safety.
Repointing is an important part of the maintenance of a mortared bridge. Weak mortar joints can allow moisture to seep into the structure, which helps advance the deterioration of mortar within the bridge. Some stone arch bridges rely rather heavily on the mortar (see Arch Building Simplified: Walter Sharp’s Secret to Building Stone Arch Bridges for more details) and therefore it is important to keep the outside mortar sound to protect the interior mortar.
Happily for bridges where the interior mortar has largely leached out, there are ways to pressure-inject grout into the structure, ensuring solidity once again.
Sometimes, the best repair for a bridge with badly battered and missing masonry is to rebuild and replace the stones as needed. For very historic bridges, the stonework of existing portions of the bridge is examined to achieve a pattern that best matches missing stonework. Where battered masonry is to be rebuilt precisely, photos are taken before tearing out the old masonry to allow for accurate recreation of it. Stones should should be numbered as well. For less historically important work, as long as the stone used is the same type as the original, and the same type of bond that the rest of the bridge uses is followed, the end result should match up nicely. In the photo of the Wilson Bridge above, the top layer of stones was added not too long before the photo was taken. Though the stones added are not perfect fits, they still match the character of the original work well, and certainly look better than concrete would have.
While we have mentioned these various solutions in the past, it is worthwhile listing them all together, as they have proven to be remarkably effective in revitalizing a weak stone arch bridge.