Manufactured concrete block represents a great success story
of the 20th 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.
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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.