February 18, 2009

Cement Jet Printed Buildings on the Moon and Mega-scale Fabrication

Contour Crafting, cement-jet printing of buildings, has been studied for use on the moon. Contour Crafting is the only layered fabrication technology which is suitable for large scale fabrication. CC is also capable of using a variety of materials with large aggregates and additives such as reinforcement fibre. Due to its speed and its ability to use in-situ materials, Contour Crafting has the potential for immediate application in low income housing and emergency shelter construction. One of the ultimate goals of humans is building habitats on other planets for long term occupancy. The CC approach has direct application to extraterrestrial construction. A NASA supported project was aimed at studying the applicability of CC for Lunar construction (Khoshnevis et al., 2005). The development path is to first make single household houses, then multi-unit apartments and offices and then whole communities and towns.

Contour crafting (CC) is a method of layered manufacturing (LM) process that uses polymer, ceramic slurry, cement, and a variety of other materials and mixes to build large scale objects with smooth surface finish (Khoshnevis, 1998). Other key advantages of CC are faster fabrication speed and possibility of integration with other robotics methods for installing internal components such as pipes, electrical conductors, and reinforcement modules to enhance mechanical property (Kwon, 2002).

Contour Crafting technology has the potential to build safe, reliable, and affordable lunar and Martian structures, habitats, laboratories, and other facilities before the arrival of human beings. Contour Crafting construction systems are being developed that exploit in situ resources and can utilize lunar regolith as construction material. These structures can include integrated radiation shielding, plumbing, electrical, and sensor networks.

Contour Crafting (CC) is the only fabrication technology capable of building objects with large layer heights while maintaining near-perfect surface quality. CC uses computer control to exploit the superior surface-forming capability of troweling to create smooth and accurate planar and free-form surfaces out of extruded materials. With its relatively large nozzle orifice, CC offers important advantages, including better surface quality, higher fabrication speed, and wider choice of materials/additives.

Lunar Construction
The concept development and trade study of the mobile gantry robot design for CC lunar construction includes the study of mobile platform design, deployable gantry, interface with nozzle and material delivery, leveling using stabilizing legs, passive compliant joints and locking, sensor-based motion and mobility control, and supervisory autonomous control. A lunar mobile gantry robot can be viewed as two rover platforms connected by a crossbeam. Two mobile rovers must be able to move without damaging the rovers and crossbeam. Passive compliant joints and position sensors will be utilized for safe and reliable mobility control. The mobile platforms will most likely to have four wheels with steering and drive motors for each wheel. The mobile platforms will also be equipped with CPU’s as well as various sensors such as inertial measurements units (IMU’s), laser sources, and cameras.

In Situ Lunar Materials
Cementitious materials such as concrete consist of cement, water, and aggregates, and are produced by curing mixed material in molds. Cement can be produced by lunar Anorthite and glass by means of sintering and crushing processes, while water can be made by reducing lunar oxides with hydrogen. Hence, hydrogen would be the only required material to be transported from Earth. Some lunar materials like lunar glass, basalt, and anorthite contain 9 to 19% by weight of CaO and are considered potential raw materials for cement production. By comparison, Portland cement generally consists of CaO content of around 64% by weight and is contained in a relatively small area of the CaO-Al2O3-SiO2 phase diagram. From a mineralogical standpoint, lunar materials are classified as pyroxene, olivine, plagioclase feldspar, ilmenite, and spinel. Among these, pyroxene and plagioclase feldspars are possible sources for calcium oxide. Table 1 shows the mineral compounds of lunar cement compared to that of Portland cement. The most important compounds contributing to the strength of hydrated cement paste are C3S and C2S. However, research shows that this is not a problem in developing the required compressive strength of lunar concrete. In fact, a tested lunar mortar mix has been shown to be capable of developing 3,176 psi compressive strength without gypsum being added to the mix and 5,627 psi with gypsum.

Even though concrete appears to be well suited for lunar structures, the production of concrete on the Moon poses many challenges. The mixing and placing of concrete in a microgravity environment is not well understood and has been the subject of recent research. The mixing and placing of concrete on earth is largely dependant on gravity for uniform consolidation of cement particles and proper hydration. It is seen that traditional wet mix methods are not adequate for a lunar environment especially due to the water constraint. Three methods of producing concrete in a microgravity environment are considered in the research paper. The traditional wet-mix method, the Dry- Mix/Steam Injection (DMSI) method, and the waterless sulfur/regolith mix (WSRM) method.

The dry mix and the sulfur concrete options seem to be more promising.

Fiberglass and glass rods are ideal candidates for use in lunar construction. Fiberglass can be used in a number of applications including weaving cylindrical pressure vessels, electrical insulation, braided cables and reinforcement in structural materials. Glass rods can be utilized as struts, compression members in tensegrity elements and structural reinforcement. Both are prime candidate in-situ materials for use as reinforcement in lunar concrete structures.

Mega-scale Fabrication

Mega-scale fabrication with contour crafting pdf.

Commercial concrete construction processing consists of mixing, transporting, and
placing of the concrete. Truck-mixing is used in most construction fields. Concrete is transported with belt conveyers, buckets, and chutes depending on the construction field site and structure design. After the concrete is placed, it is compacted within the forms to remove lumps or voids. Hand tools or mechanical vibrators are used to guarantee a uniform dense structure.

In the contour crafting process, placing concrete requires different procedures. A batch of concrete is poured to a height of 5 inch (13 cm) and a second batch is poured on top of the first batch after one hour. A one hour delay batch is sufficient to control the lateral pressure of the concrete by allowing it to partially cure and harden. Even using one hour delay, it is possible to erect 10 foot high concrete walls in a day. The time delay needs to be adjusted depending on the concrete hardening rate. Accelerators chemicals may be added when higher concrete placing rates are needed.

Some structures cannot be constructed without sacrificial support. For example,
depending on the material properties (tensile strength and interlayer adhesiveness) and arch geometry (span, rise and thickness), certain arches will collapse during the
contour crafting construction process without external support. However, once completed (with the equivalence of keystone inserted) arches are very stable load-bearing structures. Sacrificial supports contribute to construction waste, and they may need to be manually removed after construction. The goal of this reasoning module is to design sacrificial supports that

• reduce stresses that cause breakages and deformations
• minimise the material and time used to construct the sacrificial supports.

Building on top of off-the-shelf CAD systems, such as ArchiCAD, we can provide tools to allow architects to design complex vaulted structures by composing them from primitive design elements. From a palette of primitive design elements, the architect can select and drag multiple design elements on to canvas. These design elements can be resized and repositioned as needed. Then, these design elements are combined using Boolean operations similar to Constructive Solid Geometry (CSG), such as union, intersection and difference.

There are two advantages in using this CSG-like representation. One advantage is that
it can be used to provide quick feedback on the structural appropriateness of the design. Of course, detailed structural properties of complex compound vaulted structures may be difficult to predict without resorting to finite element analysis. However, we can perform quick calculations and simple checks on the design base on knowledge of the primitive vaulted elements. For example, the common column shared by two adjacent arches has to support twice as much downward force.

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