Walls are independent architectural elements with the inherent capability to enclose and define space. In the book Constructing Architecture (2013), Andrea Deplazes elaborates on Eugene Viollet-Le-Duc and Gottfried Semper’s architecture theory, identifying two distinct archetypal construction systems: “filigree” and “solid.” ³ The filigreeconstruction consists of frame-like structures with slender members assembled to create a usually lightweight planar lattice with a separation between load-bearing and protecting functions. The solidconstruction is characterized by massive, three-dimensional load-bearing walls made by layering stones or bricks ⁴ or by casting material into a mold that solidifies after curing.⁵ These walls bear the weight of the structure above them, transferring it to the foundation while providing enclosure to the building. The “filigree” and the “solid” construction systems have been used since the origins of human civilizations and have responded primarily to the availability of local materials.

During the Roman Empire, Roman walls were constructed using dressed stones and a solid core.⁶ Walls were classified under the term opus, Latin for work, followed by a second term that defined each particular technique, exerting the distinctive character of the different wall typologies. Opus craticium combined timber frame with masonry infills, common in ancient Rome’s multistory dwellings.⁷ Opus quadratum, a technique rooted in Greek construction, used regular squared stone blocks stacked typically without mortar. Opus caementicium, also known as Roman concrete, was the predominant wall type in the architecture of the late Republic and a testament to the Roman innovative use of materials in architecture.⁸ The use of local materials played a vital role in the discovery of concrete. In many parts of western Italy, the sand used in construction came from volcanic deposits with a high silica and aluminum oxide content, known as pozzolana. The resulting mix offered superior compressive strength when combined with lime and water. Opus caementicium consisted of lumps of small stone aggregates (caementa) or rubble mixed with high-strength mortar, acting as a filler. 

Roman concrete became an efficient and economical substitute for traditional construction materials such as stone and timber and more suitable for constructing complex geometries such as vaults, domes, and curvilinear walls. Roman wall construction offered high flexibility on the facings that varied according to the available materials, labor, and different trends. Opus incertum employed random interlocking patchwork of irregular stones of different sizes, opus reticulatum consisted of a netlike pattern of small square-shaped “tufa” set diagonally, and opus testaceum used courses of brick or tiles cut in triangles with one edge forming the facing of the wall. Opus mixtum combined the methods of opus reticulatum with opus testaceum.⁹ These walls were built from the outside to the center, using brickwork or stones to shape and contain the inner wythe, allowing the builders more precision, reduced timber shuttering use, and the need for high-skilled labor. However, regardless of the facing of the wall, opus caementicium formed the structural core, carrying the weight down to the foundations. 

Load-bearing masonry served as the most fundamental and substantial structural component for durable secular and religious buildings in Western civilizations until the late eighteenth century. However, during the nineteenth century, technical advances accelerated by industrialization enabled the separation of structural frames from non-structural enclosures, challenging the role of the load-bearing wall as the primary enclosure and structural component of buildings. Today, load-bearing walls are common in low-rise buildings and are usually present as flat surfaces made primarily of masonry (bricks and CMU), wood (platform framing), and concrete (cast in place, precast, and tilt-up). In multistory buildings, load-bearing walls almost exclusively serve as shear walls for lateral bracing rather than as an expressive, performative building enclosure.

Concrete owes its shape to the mold in which it is cast. This reliance on a temporary armature brings mold-making to the forefront of the possibilities of concrete. The use of formwork for concrete construction dates back to Roman times, with the introduction of concrete walls (opus caementicium) cast between timber shuttering with vertical supports on the inside.¹³ This system, known as terre pisé or rammed earth, was a technique used by ancient civilizations that later became a precursor of modern concrete.¹⁴ During the latter half of the nineteenth century, as the use of concrete increased, building contractors realized the high incidence of the cost of the formwork in concrete construction. The terre pisé system was improved and standardized for economic purposes, with formwork variations patented in France, England, and other European countries.¹⁵ Formwork development has paralleled the growth of concrete construction throughout the twentieth century,with its economic implications becoming increasingly significant.¹⁶ Despite concrete becoming widespread recently, formwork carpentry has remained a costly part of the system, often accounting for a substantial portion of the total cost of the project. In the construction industry, the cost of formwork often ranges from 35-60 percent or more of the total cost of the concrete structure.¹⁷

Static Formwork

“Static formwork,” a temporary structure of preeminent importance in concrete construction, is used to contain poured concrete, mold it to the required dimensions and shape, and provide support until it can be self-supportive. It should provide the face contact material and the bearers directly supporting the fluid mix until it cures.¹⁸ Some formwork requires an additional framework called “falsework,” which includes any temporary structure used to support the concrete inside the molds until the material gains self-supporting strength. Static formwork must be robust, capable of supporting the weight and pressure of the wet concrete, and rigid to maintain its position. It should also be tight enough to prevent leaks, and designed for easy removal, ensuring the safety and stability of the concrete structure.¹⁹

Wood is the most typical material in cast-in-place practices and precast plants, given its availability, low cost, and ease of working with simple tools. Builders commonly use plywood for sheathing and dimensional lumber to stiffen and brace the formwork components.²⁰ While wood formwork can be reusable, it does deteriorate when in contact with water, requiring replacement. In contrast, metal formwork, though more expensive, offers distinct advantages. It is a reliable choice for constructing large structures and when repetition is necessary. They last longer and are more economically viable in the long run. Materials like fiber composites, rubber, and plastics also find applications in formwork construction. 

Dynamic Formwork

Dynamic formwork is characterized by its ability to move. It allows for continuous concrete casting into a formwork that slides at a pace determined by the hydration rate of the material. This results in a self-supporting concrete structure upon release from the formwork.²¹ Slipformingisa process that utilizes forms that move horizontally or vertically during concrete casting. The form acts as a moving die, shaping the concrete in an extrusion process, typically moving vertically at a rate of 6 to 12 in. [15 to 30 cm] per hour. This method is particularly beneficial for constructing concrete cores on tall buildings, towers, and stacks, offering a cost-effective solution.²² In fact, despite a century of operations, slipform remains the fastest construction method for designated vertical structures that are typically at least 75 ft. [25 m] tall for economic reasons.²³

Jumpforming, a similar construction technique, involves casting concrete into a form that is then raised vertically for succeeding concrete lifts. The lower concrete segment supports the ones above as the concrete hardens and cures, ensuring the stability of the structure. Once an entire lift is placed and the concrete is partially hardened, the form moves upwards, completing one cycle.²⁴

Sustainability and Life Cycle of Concrete

Reinforced concrete has revolutionized architecture and engineering projects with its strength, versatility, durability, and affordability. However, cement, the “binder” in the concrete mix, bears a stigma due to the high energy required for its manufacturing and the high CO₂ emissions. In standard concrete, ninety percent of the carbon footprint is from Portland cement,²⁵ and cement manufacturing accounts for about seven percent of the anthropogenic CO₂ emissions.²⁶ Cement production increased exponentially during the second half of the twentieth century. Moreover, the cement market is projected to grow significantly in the coming years, particularly in developing countries.²⁷

Several measures have been implemented to lower the environmental impact of Portland cement production. For example, the introduction of blended cements reduces the need for Portland cement clinker by replacing it with byproducts of other industries, such as fly ash, slag, silica fume, and other pozzolanic materials called Supplementary Cementitious Materials (SCMs).²⁸ By replacing cement content with SCM, concrete acquires higher compressive strength while reducing the amount of cement needed and lowering the carbon footprint of the resulting concrete mix. The increased compressive strength allows the design of more efficient structures, decreasing the overall material usage. Using alternative low-carbon fuels in cement manufacturing, such as hydrogen or biomass, as substitutes for fossil fuels is another alternative to lowering emissions. Other tactics to minimize the environmental impact of concrete include optimization of total aggregate gradations, use of superplasticizers and water –reducing admixtures, and CO₂ sequestration and injection, among other strategies. Public agencies and corporations can encourage and mandate further reductions through incentives, regulation, and legislation.

Shortages in limestone – the primary raw material for Portland cement – coupled with the ongoing improvement in efficiencies in the chemistry of concretes and admixtures, as well as tighter environmental regulations, are set to catalyze a shift from the traditional “cement industry” to the “hydraulic binder industry.” This transition promises to lead to a more sustainable, high-tech, and economical concrete in terms of performance, offering a compelling future for the cement industry.²⁹

The United Nations defines sustainability as “meeting the needs of the present without compromising the ability of future generations to meet their own needs.” ³⁰ In the context of concrete production, this translates to using materials and methods that minimize environmental impact, conserve resources, and ensure the longevity and performance of the material. Hence, to comprehensively assess the concept of sustainability, it is imperative to address the material’s “life cycle.” For this reason, it is vital to consider the short and long-term consequences of the material selection for a specific project. Since most buildings are designed to last several decades, the long-term impacts become preeminent in determining the environmental implications of the material selection.³¹ Concrete can deliver durable structures, envelopes, and other building components that are flexible in design, affordable, low maintenance, and can perform efficiently through their lifetime. Therefore, the long-term impact of concrete buildings is considerably lower than that of other standard construction alternatives when considering the longevity and performance of the material over its extended “life cycle.”

High-Performance Concrete and Ultra High-Performance Concrete

Modern concrete is increasingly becoming a sophisticated chemical material, incorporating mineral components, mineral admixtures, and amorphous products.³² This evolution has led to the emergence of High-Performance Concrete (HPC) and Ultra High-Performance Concrete (UHPC), which boast exceptional compressive strength. These types of concrete enable the construction of structures with reduced cross-sections, thereby minimizing the required material and steel reinforcing volume. UHPC results from combining Portland cement, supplementary cementitious materials, reactive powders, limestone, quartz flour, fine sand, high-range water reducers, and water. The material formula can provide compressive strengths over 29,000 pounds per sq. in. (psi) [200 MPa] while standard concrete ranges between 2,500 to 5,000 psi [17 to 35 MPa]. Adding high-carbon steel, PVA, glass, and carbon fibers further enhances its performance to meet specific requirements.³³

Furthermore, the high flow and self-compacting characteristics of UHPC make it highly workable and easy to mold. Another outstanding characteristic of HPC and UHPC is the remarkable increase in durability. The potential for a millennium-long lifespan building façades with minimal to no maintenance and environmental impact over time is a massive paradigm shift in sustainable infrastructure.³⁴ The life cycle of HPC is estimated to be two to three times that of standard concrete, and it can be recycled two or three times as coarse aggregates for other concretes before reaching its end life, further enhancing its sustainability properties.³⁵

Opus Versatilium

This method involves using either small-size 3D printed molds or handmade plywood frames with exchangeable inserts made out of rubber, 3D printed, or any other mold materials. The molds are highly reusable and generate virtually no waste. The molds are arranged in horizontal rows and locked with small spring clamps. The construction of the wall allows the placement of steel reinforcing rebars as the casting progresses. The concrete is poured as each row of molds is set and locked. While the mix is curing, the next row of molds can be set on top for the last row to cast the forthcoming segment. This innovative methodology is systematic, efficient, and straightforward to cast on-site. It offers a compelling alternative wall assembly compared to traditional concrete casting systems, providing a robust and resilient building enclosure system with reusable formwork. The simplicity of the process facilitates its execution by unskilled labor. 

The casting methodology capitalizes on the plasticity and self-consolidating nature of UHPC. This type of concrete eliminates the need to compact or vibrate the batch, allowing the use of very economical spring clamps to lock the molds in place temporarily. The fluidity of the UHPC can quickly fill intricate mold geometries and textures while delivering high compressive and considerable tensile strength envelopes, which reduces the overall thickness, material usage, and weight of the structure. UHPC can produce building enclosures that are inherently resilient against extreme weather conditions (hurricanes and tornadoes), fire, earthquakes, insects, flood, moisture, and mold, including human-caused phenomena (blasts, force protection, and acoustic mitigation). In addition, UHPC offers much more durable concrete structures that require no maintenance while significantly extending the life cycle compared to standard concrete. 

As a result, the proposed method delivers load-bearing building enclosures that integrate structures and skins with non-conventional, highly customizable geometries with variable cross-sections; hence, the name of the wall system: opus versatilium. The term refers to a “versatile casting system” that allows subsequent placement of “customizable molds” to produce variations in the anatomy of the wall while offering multiple surface textures. OV provides flexibility in design for concrete load-bearing wall construction with simple and complex enclosures that are low-cost and are not dependent on skilled labor for their execution. This versatility opens up a broad range of creative possibilities in the design of concrete load-bearing walls. The main advantages of OV include: 

(1) small size of molds

(2) high reusability of molds

(3) mass customization of affordable 3D-printed molds

(4) design of non-standard structures and geometries

(5) ability to provide openings, cantilevers, and tapering profiles

(6) simplicity of the casting process (non-skilled labor)

(7) zero formwork waste

(8) functional gradation of the material

(9) low cost compared to conventional formwork

(10) safety

(11) minimal or no need for crane and scaffolding

(12) feasibility as a precast and cast-in-place method

As a contemporary evolution of opus caementicium, the essence of the Roman wall, opus versatilium advances concrete construction into a broader spectrum of morphological configurations in low and high-rise architectural applications. The ultimate goal of this methodology is to make load-bearing concrete walls more accessible to communities, offering design flexibility while using affordable and local resources. This approach is not limited to specific geographical regions, making it a viable solution for diverse construction needs (Fig. 2). 

Ultra High-Performance Concrete (UHPC) was the casting material for all prototypes. This self-consolidating concrete offered maximum flowability and no vibration requirement, proving its effectiveness, especially when casting concrete into molds with intricate geometries. The UHPC used yielded a compressive strength of 15,000 psi [100 MPa] at twenty-eight days, three times higher than standard ~5,000 psi concrete [35 MPa]. This higher compressive strength allowed for a reduction of wall thickness and, consequently, less material use compared to standard concrete. The construction of the prototypes used 4 x 4 in. [10 cm x 10 cm] plywood frames with interchangeable plug-in inserts, offering morphological versatility and ease of use. Several different inserts – 3D printed and made with urethane rubber – were employed for each proposed option. The overall size of each mockup was 32 in. [81 cm] wide and 40 in. [102 cm] high with variable thicknesses (Fig. 4).

Bird Blind

The Audubon Center is located within the 3,700 acre [1,497 ha] Riverlands Migratory Bird Sanctuary, strategically positioned at the core of the Mississippi Flyway along the banks of the Mississippi River, near its confluence with the Missouri River. More than 300 avian species rely on the flyway for sustenance, refuge, and safe passage, rendering it one of the most extraordinary migration corridors on the planet. The Audubon Center at Riverlands is committed to fostering a deep connection between humans and the splendor and significance of the Mississippi River and the Great Rivers confluence to inspire the conservation of the river’s diverse avian population, wildlife, and other natural assets.³⁷

Site 

The Army Corps of Engineers commissioned the author to design and construct a bird blind along the banks of the Mississippi River near the Audubon Center. The selected site is located next to a parking pad with views to Heron Pond at the starting point of the Paul E. Bauer Memorial Trail. The off-the-grid location lacks access to power, potable water, and sewer systems. 

Design

The design of a single 124 ft. [37.8 m] long spiral wall encloses a ramp with a constant 1:12 slope. The resulting accessible promenade leads the visitor into a platform with a seating area elevated 6 ft. [1.8 m] above grade. The height of the concrete load-bearing wall, remaining at handrail level along the sloping ramp, rises 8 ft. [2.4 m] above the platform to support a concrete slab that provides shelter to the seating area. However, the downward sloping of the wetlands toward the pond increases the effective height of the viewing platform to a total of 15 ft. [4.5 m]. Biologists at the Army Corps of Engineers determined this optimal height difference. The goal was to obtain panoramic views of the ecosystems from an elevated point (Fig. 5).

The spiral wall serves both aesthetic and functional purposes. It features silhouettes of the most common local birds using a halftone reprographic technique to create patterns of small circular openings or “dots” of varying sizes and depths. The taller portions of the wall can accommodate larger number of dots, allowing more elaborate images to be reproduced. (Fig. 7).

Materiality

The Audubon Center’s need to provide a durable and resilient structure able to withstand harsh climatic conditions – including flooding events that regularly occur at the site – weighed on the material selection for the project. Concrete’s strength, durability, and maintenance-free qualities made it an ideal choice for this type of outdoor public space. 

The compressive strength of concrete depends on the mixing ratios of the cement, aggregates, and water. For instance, a standard-grade concrete M25, with a compressive strength of 3,625 psi (25 MPa), typically requires a mix ratio of 1:1:2 (one part of cement, one part of sand, and two parts of coarse aggregates) and a maximum W/C (water-to-cement) ratio of 0.45. For the construction of the bird blind, High-Performance Concrete (HPC) was employed, using a mix ratio of 1:2:2 (one part of cement, two parts of river sand, and two parts of 3/8 in. [10 mm] limestone chips as coarse aggregate). The mix incorporated 2.5% of the cement weight in AR glass fiber filaments to enhance its flexural strength. The concrete mix included 3 percent of the cement weight of Tec-10 GFRC Admix made by Trinic. 

This concrete admixture powder increases the workability of the mix and mitigates shrinkage cracking. To reduce the W/C ratio, a proprietary blend of a powdered plasticizer with dispersing and wetting agents was included in the mix (one percent of cement weight). This product complies with ASTM C-494 A and F High-Range Water Reducers. The superplasticizer enables a low W/C ratio of 0.33, resulting in much lower porosity and higher compressive strength. Concrete cylinders – 4 x 8 in. [10 x 20 cm] – were tested at fifteen days, yielding an average compressive strength of 8,500 psi [60 MPa] and 9,250 psi [64 MPa] at twenty-eight days. Additionally, HPC demonstrates superior durability, with a life cycle of two to three times that of standard concrete. A local quarry along the Missouri River, located only four miles from the site, supplied the river sand and the limestone gravel. The use of white cement instead of regular grey Portland cement yielded a light tan color mix showcasing the local nature of the concrete aggregates

Mold Making

The Bird Blind was designed with a 6 x 6 in. [15 x 15 cm] modular dimension, a choice based on the experience of constructing the prototypes. This dimension provided a suitable height to support a taller concrete row than in the prior tests and ensured that the size was compatible with most standard desktop 3D printers, enabling domestic and affordable mold production.

The surface of the spiral wall is characterized by three conditions: solid, indented dots, and perforated dots. Two types of formwork were developed to materialize these conditions. The solid segments of the wall were cast using HDO plywood frames. The frames were faced with a melamine resin laminate. This highly hydrophobic surface works effectively for demolding while providing a very sturdy set of molds able to withstand multiple casts. The geometry of the segments follows the four different radii of the corresponding spiral sections. The intentional use of plywood frames of different lengths (one, two, three, four, and six modules) allowed for different arrangements of the molds, resulting in a random seam pattern on the surface of the concrete wall. In addition, these modules fit within a 6 x 6 in. [15 x 15 cm] grid throughout the entire surface of the spiral wall, facilitating the precise location of each dot as part of the halftone system.

The dots were constructed using 3D printed molds, each containing a conical pin in the center of a box-shaped mold with an open back side for clamping. The pins partially or fully penetrate the wall thickness with four different diameters: 1-1/2 in. [3.8 cm], 2-1/2 in. [6.3 cm], 3-1/2 in. [8.9 cm] and 4.5 inches [11.5 cm]. These molds were 3D printed with an infill density of 30 percent, yielding robust molds that support multiple casts. The partial penetrations create indentations that produce shades of darker dots, while the full penetrations create perforations, which, in addition to creating shades, also allow the user to see through. The perforations were cast using two molds paired up on both sides of the wall. Each pair contains a three-inch [7.5 cm] long pin that fits together with a tongue and groove connection, preventing mold displacement while casting concrete. One pair of 3D-printed molds formed each perforation, with each half of the pair forming one side of the wall. The indentations were cast using only the exterior side molds of the wall with a pin protruding inside the wall and an end cap to obtain a smooth, rounded end (Fig. 9). 

The casting process follows the opus versatilium technique, the same procedure used to construct the wall prototypes. The system is highly adaptable to various geometries and wall configurations. A small concrete mixer was employed during the casting, allowing for batches of approximately one cubic foot. Once a row of molds is cast, the next layer is clamped on top, ready for the next cast. This process repeats for every subsequent row (Fig. 11). The spiral wall contains 5,288 modules. On a typical working day, about sixty modules can be cast by a single person working on site, including demolding, cleanup, reinforcing, mold set up and cast. Therefore, completing the spiral wall will take about ninety working days. The project is currently under construction, and the wall is about thirty percent completed. It is estimated to be finalized during the early fall of 2024.

Vernacular construction provides a repertoire of methods and traditions with the potential to be combined with innovative approaches to respond to contemporary needs. Opus versatilium’s innovation relies on the hybridization of the Roman method of building load-bearing walls (opus caementicium) with a novel formwork methodology (SCCS). This approach harnesses the power of digital technologies (3D printed and digitally fabricated molds) and the latest technological advances in concrete (HPC and UHPC). The self-consolidating property of HPC, a crucial characteristic of the concrete used, allows to cast small intricate spaces without vibrating or compacting the concrete. 

This research aims to leverage the global availability and affordability of concrete to provide a practical, cost-effective, and reusable formwork system that is particularly convenient for communities in need. OV has demonstrated remarkable design flexibility by offering an innovative armature to support non-standard load-bearing concrete wall morphologies. Its simplicity enables high- and low-tech applications to be adapted to various scales and conditions. The versatility of the method was further proven in constructing the bird blind – a low-budget, off-the-grid project without requiring skilled labor, instilling confidence in its practicality and effectiveness. Although emphasizing the life cycle of concrete was instrumental in commissioning the project, additional feasibility studies on traditional construction methods might be necessary to support the scope of future applications.

The ongoing research seeks to engage future practices focusing on processes rather than products. By recognizing current needs and demands, OV has the potential to adapt to local cultural and environmental contexts with the latest material and technological advances. OV’s innovative approach advances load-bearing walls into new architectural anatomies by taking advantage of the fluidity and resiliency of concrete. It offers promise in developing formwork as an active and accessible design tool and fertile ground for innovation in building envelopes.