Stone arch bridges made of limestone suffer greatly from deterioration of the stonework at the waterline. Limestone is a soft, sedimentary rock that is always a favorite with stone masons. It is reasonably hard, reasonably easy to shape, and is usually enduring. However, one of its worst faults is that it deteriorates easily in water.
Damage from Acid and Erosion
Limestone is easily acted upon by acid. Any slight acid content in water dissolves the stones. Obviously, if the water is highly acidic, the limestone will dissolve more rapidly. A likely clue to this type of damage is discolored stones.
Erosion affects limestone in the same way as acid.
In the case of a stone arch bridge, the end result of both erosion and acid damage is a visible “jut out” above the average waterline. The masonry at and below the waterline vanishes, leaving the masonry above the waterline sticking out further than what is supporting it below.
In both cases, the speed of the damage depends on the character of the stones. Chalky limestone is the most vulnerable, while dolomite has a well-earned reputation for durability.
Eventually, of course, with this type of damage a point is reached where the bridge cannot support itself, let alone vehicles.
Freeze-thaw damage is a great risk to limestone bridges.
Water finds its way into cracks and crevices in the stone and then freezes, splitting the stone. In cases of heavy deterioration, the stones at the waterline become a disjointed mass of rubble upon which the arch is precariously supported.
Quite a bit of damage can be tolerated, but the never-ending accumulation of broken stones and stone pieces will cause the bridge to eventually fail. Freeze-thaw damage does not have to occur only at the foundations of the bridge. It is possible to find bridges where the upper stonework is fractured. However, water is usually up against the waterline masonry, and therefore this is the most likely part of the bridge to be damaged.
Protecting the Foundations
A way to repair all waterline woes is to pour a concrete apron around the bridge. This protects the base of the bridge from all water action (including scour). It also appears to be effective at stabilizing existing damage at the foot of the bridge.
The main trick is to pour the apron before the damage becomes very serious. The definition of “very serious” is a judgment call. In general, if foundational damage is visible, it is never too early to take action. There is no need to panic over a light amount of superficial spalling. As long as existing damage to the masonry of a stone bridge is regularly monitored, it is improbable that any serious developments will go unnoticed. That said, if the stones at the waterline show signs of damage, it is almost certain that sooner or later corrective action will need to be taken, for the damage will keep worsening over the years.
In Kansas, Cowley County’s Andes Bridge is a testimony to the effectiveness of the concrete apron. The large aprons protecting the foundation of this bridge were added by Walter Sharp nearly a century ago, yet still seem to be doing their job well.
The basic idea is to keep the water from directly contacting the foundations of the bridge. Occasionally a stone arch bridge will be found where soil still prevents water from directly touching the stone foundations. This soil does a surprisingly good job of keeping the base of the bridge safe, and such a bridge is unlikely to have deteriorating stonework at the foundations. If the stream the bridge spans is usually dry, waterline damage is improbable.
Designing the Foundations to Prevent Damage
In design, there is an easy solution for avoiding future waterline problems: Don’t use limestone at the waterline.
In the Stone Arch Bridge that spans the Mississippi in Minneapolis, the brilliant engineers used granite for the water-level masonry. They then used high-quality dolomite for the external stonework, and local limestone for the interior masonry. The result is that the bridge is more or less impervious to the weather, and yet did not cost more than necessary.
In Butler County, Kansas, this waterline trouble had, apparently, come to light at an early date, so that by 1908 stone arch bridges were being built with concrete under the arch up to the waterline as opposed to traditional masonry. This solution allowed for the use of local stone and local labor in bridge construction, with the added advantage of concrete’s better freeze-thaw handling.
An early bridge built this way is C. C. Jamison’s 1908 Walnut River Bridge near Cassoday. Concrete aprons appear to have been added, obscuring this feature. The concrete was likely placed as scour protection in this instance.
A good example of a stone arch bridge built with concrete at the waterline is C. C Jamison’s 1910 Henry Creek Bridge. The arch rests on concrete work that is plainly visible and quite old. This method seems to work well, for it appears that this bridge has needed few repairs over the years.