Thursday, February 16, 2012

Mortar, reconsidered

Mortar is used to glue masonry units together in a wall or structure.  As noted in an earlier entry, “it keeps blocks together and it keeps them apart.”  It combines masonry units into a consolidated structure.  It also keeps individual masonry units from touching one another.

Mortar typically has similar qualities to the masonry units it joins together: it is brittle and hard.  This quality of mortar does not allow it to act as a damper to blunt forces between blocks, like a fluid or ductile material would.  Mortar is effective at stopping cracks only insofar as it creates a boundary between blocks to blunt the energy focused at a crack tip.  Intimate bonding of mortar with block (what good joinery strives for) reduces the boundary effect of crack blunting. 

If a ductile material is used instead of mortar between masonry units, then a more toughened masonry structure can be built.  If the ductile material acts like a gasket and completely absorbs forces between blocks it will dampen and dissipate energy in a structure much more effectively than brittle mortar. 

In domes and arches, gravity acts to force the blocks together.  Masonry units can be assembled in a dome or arch without mortar.  The cementing or gluing effect of mortar is not needed to assemble a dome or arch; this is achieved by gravity acting on the masonry units, forcing them together.   The masonry units in a dome or sphere can also be held together by a tensile web of cable, or rope, or wire which is woven between blocks in a criss-crossing of great circle arcs into a net of tension, as I'll describe in my next entry.  If this tensile web is included, then the structure does not rely on gravity alone to hold it together.

Triangular block are particularly effective at distributing or dampening any applied force.  An individual block distributes any load or force to its 3 adjacent neighbors; these 3 distribute to their 6 adjacent neighbors; etc., rapidly and effectively dampening and reducing any applied load or force.  If this inherent dampening feature of triangular masonry units is coupled with a ductile, shock-absorbing, gasket-like material used between masonry units, then a very tough and robust structure results.

A ductile gasket material also make it easy for a domed or arched structure to relieve stress through conjugate shearing, as described here and here.

Currently I am investigating the possibility of using recycled rubber from tires as a material for gaskets to replace mortar in domes and arched structures. 

The two roles of mortar are to keep blocks together and to keep them apart.  If gravity is used to keep blocks together and gasket material is used to keep them apart, then high performance toughened masonry structures can be built without brittle mortar.