Studying stone arch bridges that have been built using different construction methods reveals some interesting facts. Basically, the life of a stone arch bridge is directly related to how exactly the arch was built. Obviously, for any structure to last at all the foundations must be solid; but, that aside, how much of a role do the different methods of “turning the arch” play in bridge life expectancy?
We have observed numerous bridges around Butler and Cowley counties in Kansas — including bridges built by various builders using various building methods. We have also viewed one bridge in Kay County, Oklahoma, which was built to precision standards. Our research has revealed some interesting information. There are certain factors that play a role when it comes to how long stone arch bridges last, what needs to be done to maintain them, structure longevity, and ways to increase weight ratings, among other areas of interest.
The Foundations
Before we go any further, let us just state here that the foundations of the bridge are the most important factor. This should go without saying; no foundation — no bridge. Most of the bridges we’ve studied include concrete aprons and concrete pontoons around the substructure. This largely helps prevent failure, though not infallibly so.
Generally speaking, the stones at the waterline of a limestone bridge seem to deteriorate quite badly over time (probably frost action), and protecting them from direct exposure to water seems to go a long way toward preserving the bridge. The concrete aprons and pontoons seem to help protect the foundations from this sort of deterioration as well, at least to some degree.
The Construction
Assuming the foundations are secure, the quality of construction obviously plays an enormous role in bridge life. There is a consistent connection between bridge condition and the method originally used to build it.
The Wolf Creek Bridge in Kay County, Oklahoma, which is no longer on the road network, shows precision workmanship with tight gaps between the stones, including the arch.
Mortar was used, but was obviously not a significant factor in the bridge construction. The bridge’s fantastic condition is a tribute to the durability of its design — especially considering that the bridge goes underwater regularly, the fill is washed out on the top and at the ends, and the bridge is no longer being maintained. In Butler County, Kansas, bridges constructed by C. C. Jamison use more precise cutting, though each stone was not necessarily cut to a precise “brick” shape. His bridges also seem to hold out well over time.
On the other hand, even stone bridges that are still on the road network designed with the arches “turned” with the aid of mortar seem to frequently to have deteriorated.
The problem is not with the use of mortar in the arch angles, per se, but with the fact that mortar leaches out over time. This in turn starts loosening the arch stones and can quickly cause problems. As can be seen in the Walz Ford Bridge photograph above, little, if any, trace of mortar remains in the arch of the bridge. A detailed examination of the Walz Ford bridge reveals that there appears to be no mortar left in the structure! Incidentally there is significant variation in this type of construction. Jerry Hammond used a large amount of stone fragments, which, even though the mortar leaches out, still help keep the arch tight.
Walter Sharp stated in 1922 that he cut the faces of the arch fairly precisely, while he poured grout into the joints of the center stones of the arch, creating the necessary angles. Another method of construction seen occasionally consists of incredibly loose arch stones with a heavy amount of mortar and stone fragments to make the angle.
Repairing Mortar
So what do you do to preserve a stone arch bridge when mortar was used to help hold the arch together? The best solution is to keep the bridge pointed. This surface treatment helps preserve the interior mortar joints, which, in the method of construction used by Walter Sharp, are highly important to holding the bridge together.
First, material cannot fall out (being held back by the pointing) and, as such, cannot very well be washed out by water.
Also, keeping the load limits relatively low helps to some degree, as vibration can, at times, actually vibrate out the material holding the arch together. This is particularly a problem in mortarless constructions where stone chips are used to help hold everything together.
The Deck
This in turn brings up another interesting solution that not only helps protect the bridge from vibration, but can also be used to increase the weight limit of the bridge. Increasing the fill height can do both, but increasing the fill height on an existing bridge is not always an option. What can often be done is to pour a reinforced concrete slab atop the bridge. This slab helps distribute weight and vibration evenly and provides the bridge with a good, solid, dead load, which not only helps the bridge, but also can allow for an increased weight limit.
In the area where we have photographed bridges extensively, we note that the several stone arch bridges treated in this fashion seem to hold up much better to traffic. The Black Crook Creek Bridge near Winfield, Kansas, is a good example. Despite being built in a fashion similar to the poorly built arch shown above, the Black Crook Creek Bridge is still in good shape. This bridge features a cantilevered concrete top to widen it — on a road that is fairly well traveled. The concrete slab is a huge aid in protecting the stonework below, and also makes the mortar last longer.
Another example of a stone arch bridge with a concrete top is a double-arch bridge over the Walnut River near Cassoday in Butler County. This bridge carries a two-lane blacktop and is also in good repair. To increase the life of a stone arch bridge, then, keeping the foundations secure, keeping the mortared joints pointed, and, if needed, adding a concrete deck on top can go a very long way.
Stone Arch Bridges 