Concrete is the most used construction material worldwide. There is an equivalent of forty tons of concrete for every person on the planet, and one ton per person is added annually.¹ The material’s ability to phase change from liquid to solid allows it to adopt virtually any shape, making it exceptionally versatile. Concrete is commonly used in a wide range of applications, including building foundations and structural elements, roads and pavements, sidewalks and driveways, bridges, dams and water retention structures, retaining walls, tanks and silos, interior flooring, furniture, and many other applications. Its strength, durability, low cost, low maintenance, wide availability, thermal mass, soundproofing qualities, resiliency, and design flexibility make it an ideal material for constructing robust building enclosures. In the building industry, four primary concrete wall construction methods are commonly employed: concrete masonry units (CMUs), cast-in-place, precast, and tilt-up systems. 

Concrete masonry units (CMUs) are standardized, precast rectangular blocks used in masonry walls. They are frequently used in low-rise residential buildings, educational, commercial, and industrial facilities. CMUs can be reinforced vertically and horizontally to meet structural demands, offering flexibility in design and construction.² As a mass-produced element, CMUs are widely available and can reduce labor and material costs, providing a cost-effective solution for many building types (Fig. 1a). Cast-in-place concrete involves pouring concrete directly into formwork constructed on site. It is especially effective for large-scale or complex structures, such as high-rise buildings. This method allows for seamless, monolithic structures with high structural integrity. The material and labor costs associated with formwork execution can account for 35–60% of the total concrete construction cost ³ (Fig. 1b). 

Precast concrete refers to concrete elements cast off site, typically in a precast plant, and transported to the construction site for assembly.⁴ Precast concrete members are cast under a controlled environment enhancing quality and consistency. The system allows faster on-site assembly, significantly reducing construction time and labor expenses. Precast plants are usually located near the project site to make it economically viable (Fig. 1c). A tilt-up concrete system consists of pouring concrete panels on site, laid flat on a floor slab and lifted to an upright position by rotation on one edge using a mobile crane after the concrete achieves sufficient structural strength to support manipulation to its final position (Fig. 1d). 

The speed of construction is a critical benefit of tilt-up concrete, the panels can be cast and erected more quickly than other conventional methods, minimizing delays often associated with traditional construction. The system is cost-effective, making it a more economical choice for larger projects by reducing labor costs and allowing for quick construction timelines. Since the panels are cast on site, their size is not limited by transportation restrictions and cost. Additionally, the method requires no vertical formwork or scaffolding, further streamlining the process. The tilt-up system is suitable for a range of building types such as warehouses, manufacturing facilities, schools, and commercial structures. Tilt-up walls offer some level of customization in terms of wall panel geometries, sizes, and finishes. 

While versatile, there are some design limitations compared to cast-in-place. The site needs sufficient space for laying out and tilting the panels, which might not be available at certain sites, particularly in urban areas. However, panels can be cast stacked on top of each other if space is limited. Lifting and positioning large concrete panels requires heavy machinery, which can increase project complexity and costs. While the overall labor costs may be lower, the technique requires skilled labor for proper panel production and installation, which can be a limiting factor in some areas. Although it can save money in the long run, the upfront costs associated with equipment and materials may be higher compared to traditional construction methods. Tilt-up construction is vulnerable to weather conditions; rain, snow, or freezing temperatures can delay the process or affect the quality of the finished product. 

Concrete construction has a long history that can be traced back to ancient civilizations. Roman architecture gradually evolved from modest Greek origins to a more grandiose Imperial architecture. Such evolution relied, to a great extent, on a single technological advancement: the development of opus caementicium [Roman concrete]. This innovative material had a profound impact that influenced subsequent European architecture and remains present in contemporary construction.⁷ Roman concrete consisted of lumps of small stone aggregates [caementa] or rubble mixed with high-strength mortar that acts as a filler with the possibility of becoming a moldable material. Roman concrete became a more 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. 

The Roman wall construction offered high flexibility on the facings that varied according to the available materials, labor, and different trends. Walls were cataloged under the term opus, Latin for “work,” followed by a second term that defined each particular technique, exerting the distinctive character of different wall typologies. During the late third century BCE, the Romans employed a random interlocking patchwork of opus incertum, using irregular stones of different sizes. The checkboard pattern of opus reticulatum became common during the late second century BCE. The wall facing consisted of small square-shaped tufa [tuff] set diagonally, forming a netlike pattern. Opus testaceum, or opus latericium, was a common facing wall during the Imperial age, which consisted of courses of brick or tiles cut into triangles with one edge forming the facing of the wall. Opus mixtum combined the methods of opus reticulatum with opus testaceum; as a result, pyramidal tufa blocks interlocked with bricks at regular intervals.⁸ The walls were built from the outside to the center, using brickwork or small stones to shape and contain the inner wythe. However, despite the facing of the wall, opus caementicium formed the structural core, carrying the weight down to the foundations. The method allowed the builders to make the process more precise and minimize the use of timber shuttering and the need for high-skilled labor.⁹ 

The fundamental principles of working with concrete, including the use of formwork, have largely remained consistent through time. Roman times mark the beginning of the use of formwork for concrete construction. Modern concrete practices evolved from ancient techniques developed by the Romans over two millennia ago. Among the many wall typologies coined by the Romans, they constructed concrete walls [opus caementicium] cast between timber shuttering with vertical supports on the inside.¹⁰ This system, based on the terre pisé [rammed earth] technique, was used by ancient civilizations which later became a precursor of modern concrete.¹¹ During the second half of the nineteenth century, with the increasing use of concrete, building contractors began to recognize the significant impact of the cost of formwork in the concrete construction. The terre pisé system was improved and standardized for economic purposes. Builders patented variations of formworks in France, England, and other European countries. 

While technology and materials have advanced, the basic processes for constructing structures with cast-in-place concrete still reflect the ingenuity of Roman engineering. The formwork systems that we utilize today have evolved from the ancient methods the Romans perfected, demonstrating a continuity of knowledge and craftsmanship that has shaped the way we build today. This legacy highlights both the durability of Roman techniques and their enduring influence on contemporary construction practices. 

The tilt-up concrete construction method emerged in the early 1900s, offering an innovative technique that transformed the way buildings were designed and constructed. The technique became prominent in the United States during the 1940s and 1950s with the introduction of the mobile crane ¹² and ready-mix concrete trucks. Initially, tilt-up construction was used for simple, one-story structures, often for warehouses and industrial buildings. 

In the post-World War II era, the demand for quick and economical building solutions soared, leading to the widespread adoption of tilt-up construction. The technique enabled the construction of many commercial and industrial buildings in a relatively short time frame. Today, tilt-up construction is used for a diverse range of building types, including “big box”buildings, retail spaces, schools, residential units, and even multi-story structures. This versatility demonstrates its continued relevance in modern construction practices. One of the most remarkable features of the tilt-up system is that the wall panels are cast on site horizontally on a floor slab on grade and tilted up to a vertical position, forming a strong building envelope. The method drastically eliminates the use of traditional formwork, as the floor slab serves as the primary mold. The only temporary formwork required to cast the panels are the edges, typically made with two-inch [5.08 cm] nominal lumber or steel angles and channels. 

Despite its numerous benefits, tilt-up construction typically delivers planar wall panels, with the possibility of adding formliners to create textured surfaces and other wall treatments. However, it is possible to challenge the two-dimensional limitation by incorporating a third dimension to the commonly flat tilt-up concrete panel using earth (dirt) as formwork. Earth, the most primitive and fundamental construction material, can support an efficient mold-making system that is environmentally sensitive. This innovative approach utilizes an accessible, economical, and reusable local material allowing the configuration of molds with double-curvature surfaces. These surfaces can provide additional structural rigidity and stability to tilt-up concrete panels, enhancing their structural performance. The term opus luteum is rooted in the Latin word lutum, which has two meanings: “modeling/building” and “dirt/mud/clay.” Opus luteum refers to the technique [opus] that uses dirt as the main modeling material [luteum] for mold-making. 

Double-curvature surfaces have distinctive structural properties that contribute to their strength and stability. Unlike flat surfaces, which can be susceptible to bending and deformation under loads, double-curvature surfaces distribute stresses more effectively. This can lead to a reduction in thickness and material usage.¹⁸ Structures with such geometries can withstand large forces with minimal thickness and take advantage of the “form follows forces” principle,¹⁹ allowing for a significant reduction of the weight while making them material efficient. Double-curvature surfaces are able to handle forces from multiple directions, making them particularly advantageous in architecture and engineering applications. For example, domes and shells utilize double curvature to create strong structures that can span large areas. Double-curvature surfaces are often self-supporting due to the way the curvature distributes forces. When subjected to loads, double-curvature surfaces can act like membranes, transmitting forces directly to their supports without significant bending. This ability is especially useful in thin-walled structures where bending moments are minimized. 

From a structural standpoint, tilt-up wall panels can be considered rigid surface structures. Therefore, their geometry, structural performance, and construction methods are interdependent.²⁰ As a result, the configuration of the concrete panels can significantly impact the spatial quality of the space they enclose. Despite their structural advantages, double-curvature surfaces can be challenging to construct becoming a highly skilled endeavor. Traditional building construction methods are usually not equipped to produce complex curves, becoming particularly cumbersome for fabricating non-developable surfaces (Gaussian curvatures). 

Given the fluidity and plasticity in its mixing stage, concrete is an ideal option to materialize Gaussian surfaces. The reintroduction of concrete with steel reinforcing (reinforced concrete) in the 1910s enabled the emergence of rigid structural surfaces such as shells and folded plates as the most efficient and cost-effective solution for long-span structures.²¹ In concrete construction, double-curvature surfaces require complex formwork and very intensive highly skilled labor, resulting in higher cost. The concrete industry typically relies on linear and flat products to build formwork. However, opus luteum, which was implemented in the construction of the tilt-up prototypes, employs compacted earth as an affordable, simple, and much less labor-intensive alternative to build complex double-curvature molds. The method is well suited to perform as a malleable and yet robust mold with challenging geometries as a substrate for concrete casting. By using locally sourced earth – the most primitive, economical, and accessible of all construction materials – the proposed approach takes advantage of an abundant resource that requires no chemical process, promoting a more ecological casting method.²² Earth can be reused and reshaped endless times as the primary formwork material, further reducing the environmental footprint of the casting process.

The curves of the panel’s edges were drawn on the plywood frames. The frame provided a rigid boundary able to support the confined earth within its interior space. The dirt was placed inside the frame, wetted, shaped, and compacted following approximately the proposed double-curved geometry. The resulting concave surface was covered with local volcanic rocks, reinforced and the concrete cast on top to form an 8 in. [20 cm] thick wall panel. The top surface was troweled to obtain a smooth finish surface. Upon demolding, the rocks remained exposed on the convex side of the panel (Fig. 4). The geometry of the double curvature allowed the panel to stand by itself without the need to brace it as typically do flat tilt-up panels. The casting of the mockup was successful, leading to approval to proceed with the casting of two tilt-up prototypes for the TCA Expo. 

As part of the main feature of the exhibition, two full-scale innovative prototypes were cast on site to showcase the potential of tilt-up construction. The panels were erected with their convex side facing each other, creating an occupiable interior space, while leaving a small 6 in. [15 cm] gap between them. This gap captures sunlight while offering a glimpse of Phoenix’s typical clear blue sky. Two small openings were incorporated into one of the panels to explore small penetrations as a source of natural light, visual connections, and other design possibilities. 

The panels were reinforced with standard vertical and horizontal #4 [Ø13 mm] steel rebars placed 8 in. [20 cm] O.C. (On Center) in each direction, following the geometry of the mold surface (Fig. 7 left). After completing the reinforcing, a total of eight lifting inserts were placed for panel demolding and erection. The concrete mix design included fly ash, allowing a considerable reduction of cement use, achieving a compressive strength of 5,000 psi [35 MPa] at 28 days. The mix had a slump of 4.5 in. [11 cm], providing a moderately fluid consistency. This was essential for a controlled flow, preventing excessive material accumulation down the slope and ensuring an even distribution of concrete. Maintaining the right balance between flowability and stability is crucial for applications that require precise and consistent overall thickness. The concrete was carefully poured to minimize the displacement of the loose black lava rocks. The concrete mix contained a light chrome green pigment resembling Phoenix’s vibrant vegetation. The exterior surface of the panels was troweled (Fig. 7 right) and lightly sandblasted after curing to achieve a smooth and uniform exterior shell surface, contrasting with the coarse interior rocks on the inside face.

The convex sides of the panels face each other, creating a small interior space of 110 sq. ft. [10 m²] (Fig. 9 left). The entire surface of the interior side of the panels features exposed black lava rocks (Fig. 9 middle). The dark and rough surface of the interior sharply contrasts with the green and smooth outside face of the panels, creating a distinct visual impact. The panels are separated by a narrow gap of only 6 in. [15 cm], which enables the infiltration of natural light while offering a glimpse of the bright Phoenix sky. Additionally, one of the panels includes two openings with small apertures on the exterior face that gradually widen toward the interior, creating a larger opening inside (Fig. 9 right).

The emergence of ultra high performance concrete (UHPC) is one of the most significant advancements in the concrete industry in recent decades. This type of concrete has achieved an unprecedented level of compressive strength, thereby reducing the material usage and weight while simultaneously improving its durability and integrity. UHPC is typically formulated by combining Portland cement with supplementary cementitious materials, reactive powders, limestone, quartz flour, fine sand, high-range water reducers, and water. This concrete has a characteristic compressive strength of at least 21,750 psi [150 MPa]. The absence of coarse aggregate also characterizes the material.²⁵ Fibers such as high carbon steel, PVA, glass and carbon among others, are generally included in the mixture to ensure ductile behavior under tension and to achieve other specific requirements.²⁶ 

UHPC is 10% denser than conventional concrete due to the optimized gradation of the raw material components. The presence of nanometer-sized, non-connected pores within the cementitious matrix contributes to its imperviousness and durability against adverse conditions and aggressive agents.²⁷ UHPC performs exceptionally well in terms of abrasion, chemical resistance, freeze-thaw, carbonation, and chloride ion penetration. Predictive modeling of ion transportation indicates that it would take a thousand years for UHPC to reach the same chloride penetration level that high-performance concrete would achieve in less than a hundred years. The potential for building façades with a millennium-long design lifespan, along with little to no maintenance and less environmental impact over time, is a huge paradigm shift from the way sustainable infrastructure is viewed today.²⁸ The use of UHPC in tilt-up wall panels can considerably reduce the use of conventional steel reinforcing, the overall thickness, and weight, while maintaining and even enhancing its structural properties. 

The contractor estimated a 20% increase in material and labor costs to produce the two prototype panels of this study compared to conventional tilt-up panels. The rise is primarily due to the additional time required for manufacturing the frame to contain the earth, as well as the compacting and finishing processes. However, larger projects with repetitive panels could enable the reuse of earth forms, reducing the cost significantly. Additionally, a more detailed structural analysis, along with optimized panel geometry and the use of UHPC, could minimize enclosure thickness and reinforcing needs, leading to further savings. 

Opus luteum has proven to be an effective, structurally strong, and simple casting system for double-curvature tilt-up concrete panels using compacted earth as the primary material for mold-making. The compacted material was able to withstand the pouring and weight of the concrete mix with minor imperfections, demonstrating the effectiveness of the method. Since the design of the panels required exposed small rocks on the concave (interior) side, the rocks were laid loose on top of the compacted earth prior to pouring the concrete. Some rock displacement happened during the casting process. This was evident in the casting of the mockup and somehow minimized when casting the full-size panels. The 4.5 in. [11 cm] slump of the concrete required vibration for proper compaction and elimination of air pockets. During the casting of the wall panels, deep and excessive vibration in some areas led to some displacement of the compacted dirt and became visible after demolding. 

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