Reinforcement in concrete 3D printing

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The reinforcement of 3D printed concrete is a mechanism where the ductility and tensile strength of printed concrete are improved using various reinforcing techniques, including reinforcing bars, meshes, fibers, or cables. The reinforcement of 3D printed concrete is important for the large-scale use of the new technology, like in the case of ordinary concrete. With a multitude of additive manufacturing application in the concrete construction industry—specifically the use of additively constructed concrete in the manufacture of structural concrete elements [1] —the reinforcement and anchorage technologies vary significantly. Even for non-structural elements, the use of non-structural reinforcement such as fiber reinforcement is not uncommon. [2] The lack of formwork in most 3D printed concrete makes the installation of reinforcement complicated. Early phases of research in concrete 3D printing primarily focused on developing the material technologies of the cementitious/concrete mixes. These causes combined with the non-existence of codal provisions on reinforcement and anchorage for printed elements speak for the limited awareness and the usage of the various reinforcement techniques in additive manufacturing. [3] [4] The material extrusion-based printing of concrete is currently favorable both in terms of availability of technology and of the cost-effectiveness. Therefore, most of the reinforcement techniques developed or currently under development are suitable to the extrusion-based 3D printing technology. [5]

Contents

Types of reinforcement

The reinforcement in concrete 3D printing, much like that in conventional concrete, can be classified based either on the method of placement or the method of action. The methods of placement of reinforcement are preinstallation, co-installation, and post-installation. [6] The examples of each are pre-installed meshes, fibers mixed with concrete, and post-tensioning cables, respectively. The classification based on the structural action is once again the same as that in conventional concrete. Examples of active and passive reinforcement in 3D printed concrete are reinforcement bars and post-tensioning cables used to prestress segmental elements, respectively. The majority of the reinforcement in concrete has conventionally been steel and continues to be even in 3D printed concrete. Alternate composite materials such as FRPs and fibers of glass, basalt etc., in the mix have gained considerable prominence. [7]

Some common reinforcements in 3D printing

Reinforcing steel bars

The high availability and popularity of deformed bars or rebars as a passive structural reinforcement in conventional concrete systems make it sought after in printed concrete. They are welded together to form trusses laid between layers to form a very effective co-installed reinforcement strategy without the use of formworks. [8] [9] They are erected to reinforce cages around which concrete is printed to form wall and beam elements, making rebars an effective pre-installment strategy. [10]

A bunch of rebars A bunch of rebar.jpg
A bunch of rebars

The rebar-based formative skeletal structure can also act as a core on which printable concrete is shotcreted in a new method developed at TU Braunschweig. [11]

The rebar cages can also be installed inside printed concrete formworks in non-structural members, and the holes are filled with grout. This method of post-installed reinforcement has proven to be cost-effective; however, it requires attention to the interface between steel and the printed concrete. [12] The use of printed concrete as formwork requires higher tensile hoop strength of the concrete, which could be provided by the use of fibers in the mix. [13]

Smart Dynamic Casting

Smart Dynamic Casting (SDC), a new printing technology being developed in ETH Zurich, combines slipforming and printing material technologies to produce varied cross-sections and complex geometries using very little formwork. [7] Reinforcement bars are pre-installed, just like in the case of conventionally cast concrete, and the rheology of the concrete is adapted to retain the shape of the slipforming formwork before concrete hydrates enough to sustain self-weight. [14] Concrete facade mullions of varying cross-sections are produced for a DFAB house [15] in Switzerland.

Reinforcement meshes

Similar to the use of rebars, reinforcement meshes are also used popularly as a passive reinforcement technique. The welded wire meshes are laid in-between printed layers of slabs without requiring any formwork. They can also be used to print wall elements that are fabricated laterally and erected in place. In a method unlike with rebars, spools of meshes are unwound simultaneously ahead of the printer nozzle to provide both horizontal and vertical reinforcement to the printed elements. This method not only acts as reinforcement in the hardened state of concrete but also compensates for the lack of formwork in the fresh state of concrete. [16]

A building worker is spraying shotcrete on welded wire mesh Bauarbeiten ostliches Domumfeld-Kolner Dom-Spritzbeton-8306.jpg
A building worker is spraying shotcrete on welded wire mesh

Cables

High-strength galvanised steel cables provide effective reinforcement in printed concrete elements where sufficient cover concrete cannot be provided owing to the complexity of the shape. [3] The cables can either be laid in-between layers or extruded simultaneously like the meshes. The bond between high-strength steel cables and concrete needs special attention. [17]

Continus yarn or Flow-based Pultrusion

Continuous yarn in Glass, Basalt, High-performance Polymer or carbon can also effectively be used as reinforcement for 3D-printed concrete without needing additional motors. [18] The technique takes advantage of the extruded concrete consistency to passively pultrude numerous continuous yarns. The obtained material is a unidirectional cementitious composite with an increase in strength and ductility in the extrusion direction depending on the proportion of fiber. Thanks to the small diameter of the yarn used their bond with the matrix is usually great. Furthermore, the process takes advantage of the small bending stiffness of the yarn to ensure the same geometric freedom with extended buildability possibility thanks to the early traction strength provided by the yarn during the printing. This feature comes with a more complex extrusion nozzle and the use of a specific device for handling the numerous yarns.

Post-tensioning cables

The automated fabrication of elements realises its true potential when printed segmental elements are fit in place using post-tensioning. The concrete segments are printed, leaving holes for the post-tensioning cables that not only act as an active reinforcement but also help in connecting the segmental elements to form a load-bearing structure. The holes left behind for the cables are filled with grout post the tensioning of the cables. [19] A bicycle bridge has been constructed in TU Eindhoven by printing segments that are post-tensioned using high-strength cables running perpendicular to the printing direction. [20] [21] The post-tensioning technology has a lot of potential as a reinforcement strategy in additively manufactured concrete systems. [20]

Fiber reinforcement

The use of fibers in the mix has several advantages like in the case of conventional concrete. The higher cement content and faster hydration rate requirements of printed concrete make it susceptible to shrinkage cracking and thermal stresses. The use of fibers (structural or non-structural) can counter these significantly. [22] Fiber reinforcements are also useful in printing shell structures as the tensile membrane action required to convert bending moment into axial force is possible only with tough and high stiffness concrete. [13] Fibers, when aligned can provide this required higher toughness and stiffness. [23] The flexural tensile strength is also improved with the addition of structural steel or PVA fibers. [24] These properties make the fiber-reinforced concrete a suitable material for printing formworks. The cohesiveness of concrete in the fresh state, which is crucial for printing, can be improved by using non-structural fibers such as polypropylene or basalt. The use of fiber reinforcement in 3D printing creates a much-needed segway into the fields of ultra-high performance concretes with enhanced strengths and durability, crucial in aesthetic slender elements. [22]

External anchor connectors

Anchor connectors are installed in truss elements with the aim of connecting them to similar units using exposed threaded bars. This reinforcing technique has the advantage of faster fabrication of lightweight units that can be arranged in a free-form manner on-site, depending on the requirement. [25] The exposed reinforcement might face corrosion issues when installed in outdoor environments. Topologically optimised truss shapes with force-follows-form can be created and used to save material and, in turn, the construction costs. The anchors can be connected both by in-plane and out-of-plane threaded rebars to create elements beyond simple beams and arches. [25]

Bamboo Reinforcement

Bamboo reinforcement, including bamboo wrapped in steel wires has been proposed as reinforcement for traditional concrete elements as early as 2005, [26] with recent studies suggesting possible applications in 3D-printed concrete. This technique has the advantage of producing potentially 50 times less carbon emissions than traditional steel reinforcement techniques. One drawback of this method is potential durability issues, as the organic nature of bamboo makes it vulnerable to pests and decomposition. Proper treating of the material can circumvent this issue, and can preserve the bamboo reinforcement for as long as 15 years. [27]

Other less common reinforcement techniques

Interface ties and staples are sometimes used to improve the bonding between printed layers. [28] Ladder wire is used to reinforce printed elements to improve horizontal bending. Print stabilisers are used to prevent the elastic buckling of printed layers during the printing process. Welded/printed reinforcement is a technology being developed at TU Braunschweig where the steel reinforcements are simultaneously printed using gas metal arc welding. [29]

Hybrid solutions

Each reinforcement technology is usually more effective when used in conjuncture with another reinforcing technology, leaving a lot of scope for research and development. The mesh mould technology can be combined with SDC to produce highly automated elements faster. The printable Fiber Reinforced Concrete (FRC) technology can be combined with most other reinforcement techniques seamlessly to produce a highly durable concrete structure. Fiber-reinforced concrete, when used to print formwork, has a higher resistance to hoop stresses owing to higher filament strengths. The meshes and bar cages are almost always combined in the usage of large-scale construction projects. [3]

See also

Related Research Articles

<span class="mw-page-title-main">Concrete</span> Composite construction material

Concrete is a composite material composed of aggregate bonded together with a fluid cement that cures to a solid over time. Concrete is the second-most-used substance in the world after water, and is the most widely used building material. Its usage worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminium combined.

<span class="mw-page-title-main">Reinforced concrete</span> Concrete with rebar

Reinforced concrete, also called ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel bars (rebar) and is usually embedded passively in the concrete before the concrete sets. However, post-tensioning is also employed as a technique to reinforce the concrete. In terms of volume used annually, it is one of the most common engineering materials. In corrosion engineering terms, when designed correctly, the alkalinity of the concrete protects the steel rebar from corrosion.

<span class="mw-page-title-main">Rebar</span> Steel reinforcement

Rebar, known when massed as reinforcing steel or steel reinforcement, is a steel bar used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has low tensile strength. Rebar significantly increases the tensile strength of the structure. Rebar's surface features a continuous series of ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.

This page is a list of construction topics.

<span class="mw-page-title-main">Ferrocement</span> System of reinforced mortar or plaster

Ferrocement or ferro-cement is a system of construction using reinforced mortar or plaster applied over an "armature" of metal mesh, woven, expanded metal, or metal-fibers, and closely spaced thin steel rods such as rebar. The metal commonly used is iron or some type of steel, and the mesh is made with wire with a diameter between 0.5 mm and 1 mm. The cement is typically a very rich mix of sand and cement in a 3:1 ratio; when used for making boards, no gravel is used, so that the material is not concrete.

Basalt fibers are produced from basalt rocks by melting them and converting the melt into fibers. Basalts are rocks of igneous origin. The main energy consumption for the preparation of basalt raw materials to produce of fibers is made in natural conditions. Basalt fibers are classified into 3 types: Basalt continuous fibers (BCF), used for the production of reinforcing materials and composite products, fabrics, and non-woven materials; Basalt staple fibers, for the production of thermal insulation materials; and Basalt superthin fibers (BSTF), for the production of high quality heat- and sound-insulating and fireproof materials.

<span class="mw-page-title-main">Insulating concrete form</span> Construction material

Insulating concrete form or insulated concrete form (ICF) is a system of formwork for reinforced concrete usually made with a rigid thermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs. The forms are interlocking modular units that are dry-stacked and filled with concrete. The units lock together somewhat like Lego bricks and create a form for the structural walls or floors of a building. ICF construction has become commonplace for both low rise commercial and high performance residential construction as more stringent energy efficiency and natural disaster resistant building codes are adopted.

<span class="mw-page-title-main">Shotcrete</span> Concrete or mortar building material

Shotcrete, gunite, or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface. This construction technique was invented by Carl Akeley and first used in 1907. The concrete is typically reinforced by conventional steel rods, steel mesh, or fibers.

<span class="mw-page-title-main">Concrete slab</span> Flat, horizontal concrete element of modern buildings

A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-reinforced slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ceilings, while thinner mud slabs may be used for exterior paving.

<span class="mw-page-title-main">Formwork</span> Molds for cast

Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.

Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete. The concrete cover depth can be measured with a cover meter. The purpose of concrete cover is to protect the reinforcement from corrosion, fire, and other potential damage.

Fiber-reinforced concrete or fibre-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers – each of which lend varying properties to the concrete. In addition, the character of fiber-reinforced concrete changes with varying concretes, fiber materials, geometries, distribution, orientation, and densities.

<span class="mw-page-title-main">Rebar spacer</span> Component of reinforced concrete construction


A rebar spacer is a short, rod-like device used to secure reinforcing steel bars, or rebar, within cast assemblies for reinforced concrete structures. The rebar spacers are fixed before the concrete is poured and remain within the structure.

<span class="mw-page-title-main">Textile-reinforced concrete</span> Type of reinforced concrete

Textile-reinforced concrete is a type of reinforced concrete in which the usual steel reinforcing bars are replaced by textile materials. Instead of using a metal cage inside the concrete, this technique uses a fabric cage inside the same.

Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

<span class="mw-page-title-main">Types of concrete</span> Building material consisting of aggregates cemented by a binder

Concrete is produced in a variety of compositions, finishes and performance characteristics to meet a wide range of needs.

Concrete has relatively high compressive strength, but significantly lower tensile strength. The compressive strength is typically controlled with the ratio of water to cement when forming the concrete, and tensile strength is increased by additives, typically steel, to create reinforced concrete. In other words we can say concrete is made up of sand, ballast, cement and water.

Construction 3D Printing (c3Dp) or 3D construction Printing (3DCP) refers to various technologies that use 3D printing as a core method to fabricate buildings or construction components. Alternative terms for this process include "additive construction." "3D Concrete" refers to concrete extrusion technologies whereas Autonomous Robotic Construction System (ARCS), large-scale additive manufacturing (LSAM), and freeform construction (FC) refer to other sub-groups.

<span class="mw-page-title-main">3D concrete printing</span> Additive manufacturing process using concrete

3D concrete printing, or simply concrete printing, refers to digital fabrication processes for cementitious materials based on one of several different 3D printing technologies. 3D-printed concrete eliminates the need for formwork, reducing material waste and allowing for greater geometric freedom in complex structures. With recent developments in mix design and 3D printing technology over the last decade, 3D concrete printing has grown exponentially since its emergence in the 1990s. Architectural and structural applications of 3D-printed concrete include the production of building blocks, building modules, street furniture, pedestrian bridges, and low-rise residential structures.

References

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