R&D in Concrete Block Masonry

Manufactured concrete block represents a great success story of the 20^th century.  An entire global industry has developed and evolved into a high state of efficiency and economy, all based on manufactured concrete block.  This technology thrives in virtually every country on earth: the traditional, rectangular concrete masonry unit (CMU) is produced inexpensively and with an engineering knowledge which is well understood and successfully put into practice by block producers globally.  The result is something we all tend to take for granted: high-strength, consistently dimensioned, inexpensive, rapidly produced CMU’s which are suitable for vertical walls in virtually any type of building, including residential, commercial, public buildings and infrastructure.  With such a successful model of production, distribution, assembly and availability already well established and in place, what –if any- new developments can research and development (R&D) add to this existing industry and practice?

My own work as a masonry designer has addressed this question for 25 years now.  I will attempt to summarize the areas of potential future growth, development and design which this robust industry has left essentially unfulfilled.  A look at current areas of research conducted by various segments of the scientific and engineering world indicate areas which stand to benefit and develop rapidly from the existing engineering practices of the concrete block industry.  The research and development proposed here hold the potential to transform the concrete block industry’s offerings into an entirely new realm of products which will provide better building systems at a lower cost on a global basis.  A modest effort in research and development will reap huge benefits for humanity; it will grow the concrete block industry and make superior, affordable, beautiful and holistic construction available for all.

One specific area of current research which has garnered significant attention from scientists, engineers, designers and practitioners is the idea of topological interlocking structures.  “Topological” refers to “Topology (from the Greek τόπος, "place", and λόγος, "study") [which] is the mathematical study of shapes and topological spaces. It is an area of mathematics concerned with the properties of space that are preserved under continuous deformations including stretching and bending, but not tearing or gluing. This includes such properties as connectedness continuity and boundary.  Topology developed as a field of study out of geometry and set theory, through analysis of such concepts as space, dimension, and transformation. Such ideas go back to Leibniz, who in the 17th century envisioned the geometria situs (Greek-Latin for "geometry of place") and analysis situs (Greek-Latin for "picking apart of place"). The term topology was introduced by Johann Benedict Listing in the 19th century, although it was not until the first decades of the 20th century that the idea of a topological Space was developed. By the middle of the 20th century, topology had become a major branch of mathematics” (taken from Wikipedia).

Currently, concrete block design and practice do not provide for topological construction.  The standard rectangular concrete block designs (with which we are so familiar) can only be used to create straight vertical walls and square corners.  A few designs allow for a slightly curving wall, which have found use mainly in retaining walls and landscaping applications.  Other novel designs allow for slight variations to the basic idea of a vertical wall, including corners which occur at 45 degrees and so on.  Current concrete block designs are far from providing a full expression of topology.  Curving walls –such as those provided by landscaping applications- only curve in one dimension, like a cylindrical surface, and do not allow curvature in two dimensions, like a spherical surface.  A form of concrete block known as “articulated block” (shown above and below) does some interesting work as an erosion-arresting embankment material.  Articulated block do not interlock in the plane being assembled; blocks can slide in and out of the assembly.   There are some great articulated block designs being developed though.

The design of CMU’s which allow for full topological expression provides the ability to use block to make roofs and complete curved structures (e.g. complete spheres, ovals, elliptical, catenary and other designs).  The design ability which can create a full expression of topology allows the use of high strength, affordable, rapidly produced building components which provide all the benefits of concrete block, including: fire resistance, termite resistance, rot resistance, building longevity, resale value, solidity, appearance, and the ability to withstand extreme weather events (hurricanes, tornadoes, typhoons, storm surges, tsunamis, etc.).    The creation of CMU designs which allow for full topological expression will create an entirely new architectural vocabulary for building with concrete block, and will create entirely new markets for concrete block.

If an interlocking aspect is included in the masonry unit then the topology in masonry is made particularly more effective.  One striking example (which has fueled much of the current research) is the failure of thermal tiles on the space shuttle Columbia.  Because they did not interlock, these topological tiles (designed to wrap around the shuttle: topologically) were free to move and dislodge themselves from their protective positions since they were held in place only by adhesive, leaving the shuttle vulnerable to catastrophic reentry into the earth’s atmosphere.  Researchers were quick to realize that if an interlocking aspect of each masonry unit (or tile) were incorporated, then the geometry of the individual masonry units would have helped keep them in their proper location (anchored by adjacent masonry units) and prevented them from being removed.  Furthermore, researchers have realized that topological interlocking masonry units (or tiles) would not suffer complete, systemic failure if one of these masonry units were damaged: the other adjacent and surrounding tiles would stay in place, even if one tile broke or was removed.  By including the interlocking feature into the masonry unit itself, a separate independent connector is not required.

While this idea of the beneficial nature of interlocking masonry units is illustrated by the Columbia tragedy, it holds great significance for the less exotic application of buildings here on terra firma.  To fully understand this, we will look at the current state-of-the-art for masonry engineering analysis.  Examining a masonry arch, the current engineering model makes 3 assumptions: 1. Masonry units have infinite compressive strength; 2. Masonry units have no tensile strength; 3. Masonry units never slide against each other (they remain in their fixed position).  We will concern ourselves here with the third assumption, the idea that masonry units in an arch (known as voussoirs) never move relative to one another.

In reality and in practice, voussoirs are known to move against each other in a masonry arch.  When this occurs, the arch can be significantly weakened and this movement of voussoirs can result in failure and collapse of the arch.  A catenary thrust line is an imaginary line of force which exists in the wall thickness of the arch.  Catenary (from Latin “catena” or chain) is the shape of a hanging chain or cable under gravity; if this shape of a hanging chain is inverted, then a catenary thrust line is generated.  As long as this imaginary thrust line does not touch or exit the arch wall thickness, the arch will remain standing and stable.  If the imaginary catenary thrust line touches or exits either the inner surface (intrados) or the outer surface (extrados) of the arch, then a hinge will form at that location.  Several hinges allow a mechanism for movement of the arch, resulting in a buckling or folding of the arch about these hinge locations, leading to failure and collapse of the arch.  However, if voussoirs possess an interlocking feature such that they are not free to move relative to any adjacent (interconnected) voussoirs, then the catenary thrust line will not touch or exit either the intrados or extrados of the arch due to movement.  Thus interlocking masonry units in an arch are fundamentally much stronger, more robust and more stable than masonry units which do not interlock.

The creation of an effective interlocking feature on a topological masonry unit produced on a standard conventional block machine is a very real challenge for the masonry designer.  Interlocking features are actually commonplace in standard (non-topological) blocks: the ‘top’ and ‘bottom’ of the concrete masonry unit can readily incorporate interlocking features.  A wide variety of designs is possible if the interlocking feature does not include topological arrangements, but the designer is still limited to building straight vertical walls.  In order to provide an interlocking feature for a topological masonry unit, the sides of the block must be used (not just the ‘top’ and ‘bottom’ of the block) as sites of interlock.  The difficulty here is that a block mold must be readily stripped from the block without any undercut, or draft, or negative angle.  In other words, an interlocking feature on a topological block will create undercuts: an interlocking topological block simply will not release from a mold.  This contradiction can be overcome by symmetry and design.

Another difficulty in creating an interlocking topological block on a block machine is the ability of the mold cavity to be filled completely, evenly and homogeneously.  If a section of the mold near the ‘bottom’ of the block has an overhanging feature (steel mold above it) then it will not fill as readily as an open cavity which allows the concrete mix to flow into it, unimpeded.  A section of mold cavity which has an overhanging feature will impede the flow of concrete into the cavity, resulting in segregation of aggregate.  This segregation of aggregate will typically result in a weakened section of the block where larger aggregate is prevented from filling as easily as in an open mold cavity.  Lack of larger aggregate in a filled mold section creates a weaker section of concrete as a result.

In addition to sections of mold being less than ideally filled due to overhanging mold parts, there is another problem where a section of mold cavity at the ‘top’ of the mold has an open space below it (at the ‘bottom’).  This will create an overhanging projection of block, which is unsupported from underneath (at the ‘bottom’).  These cantilevered features of block are prone to cracking and breaking, especially upon handling as the un-cured block leaves the block-making machine. 

How can a topological interlocking masonry unit be created in a manner that provides adequately filled mold cavities at the ‘bottom’ of the mold, while also not creating weak cantilevered sections at the ‘top’ of the mold?  This is a very interesting design challenge; one which I hope will attract the efforts and solutions of other designers.

Catenary thrust line analysis of masonry domes is another area of current research.  Computer models which digitally process the applied stress and the resulting strain as hinge mechanisms are used to develop visual models.  Catenary thrust line analysis is also used to digitally analyze a computer 3D model as a tool for designing buildings.

Biomimicry/Biological Design as a source of masonry design is ripe with potential.  “Nature’s masons” include single-celled radiolarian and foraminifera, coral, sea anemones, sea horses, turtles and tortoises, Thor’shero shrew, and an endless array of life’s other innovative design solutions.

Anisotropy in manufactured concrete block has not been fully utilized with current block designs.  Vertical block walls are made with the weaker axis of the block facing horizontally, to the outside.  It is possible to orient the block so that the high strength axis faces outside, resulting in a significantly stronger building.

Robotic assembly is still in its early stages regarding masonry, but real progress continues in this field.  Robots may play an important role in the future of masonry.  Robotic assembly may have an early adaptation for situations that might endanger a human mason, such as radiation or other hazardous materials.  Construction Robotics is one company that is currently successfully developing robotic masonry.

3D Printing is also in its early stages, but is expected to develop with time.  3D printing should find early use in masonry applications which require a unique masonry piece, such as at the intersection of two arches, or to allow conduit or openings, etc.   In this role it will be cost effective fairly soon.