There are some distinct advantages to both mortared and mortarless masonry construction. In this post, we will compare and contrast these two methods of construction.
Pros and Cons of Mortared Construction
Mortared masonry is easy to build. The mortar fills in nooks and crannies between stones, creating a solid structure with a good dose of friction between the elements. Unfortunately, the stiffness of a mortared structure can lead to cracks in the joints with a resultant entrance for water. This can, however, be reduced by using a mortar high in lime, as lime mortars are more elastic. The waterproofing nature of mortar can potentially lead to some other problems unless a little planning is implemented. Drainage and/or waterproofing of the fill need to be provided to prevent water from accumulating in the fill. Otherwise, once the fill is waterlogged, all manner of damage can occur ranging from stone deterioration to dramatic failure of spandrel walls.
Choosing the right type of mortar is also important. If the mortar is too hard and impervious compared to the stones used, the stones will deteriorate, while the mortar remains. In order to prevent this, soft, porous mortar can be used. This works because a soft, porous mortar will tend to deteriorate instead of the stones. This does mean that the mortar will regularly need repairs, though these repairs should not be needed too often, and, at any rate, repointing mortar is far easier than replacing stones. Be aware that, because the mortar does deteriorate over time, the stones will still need to be fitted in at least reasonably well during construction without too much reliance on the mortar. Otherwise, the bridge may disintegrate as the mortar fails.
Despite these potential pitfalls to be aware of, mortared masonry is very strong, and, thanks to the mortar, the stones are quite hard to knock or vibrate free. Frost action is not a concern as long as the mortar is sound, and mortared construction is inherently forgiving. Especially when small stones are used, mortared masonry can far outperform its mortarless counterparts, and there are plenty of mortared stone bridges that are quite old to testify to this.
The Pros and Cons of Mortarless Construction
Mortarless construction, when properly built, is far less vulnerable to water action, won’t need regular repointing (obviously), and is remarkably accommodating to external forces. And, of course, mortarless masonry, not requiring mortar, can be far more cost-effective than a mortared structure would be.
Good mortarless masonry can be challenging to build, and can be easily dislodged, especially if small stones are used. Properly capping a mortarless wall is important for stability. Vibration, unless planned for, can very easily take a toll on the structure as well. Mortarless masonry often uses stone chips and fragments for stability, which can be vibrated out, particularly if the main stone blocks are not meticulously shimmed and fitted in. Heavy water flows can wash small stone chips out of a bridge, which is why stones that fit reasonably well in the first place are important. The stones also must be carefully laid to prevent frost from working them loose over time as well.
In the final tally, mortarless masonry requires more skill to successfully execute than mortared masonry. However, if massive, well-cut stones are used, mortarless masonry becomes quite rugged. The ancient Romans built many stone bridges in this fashion, a sizable number of which are still in use, despite being two millennia (give or take) old!
Stone Arch Bridges 