Natural disasters and emergencies can devastate the communities they hit, and the speed of a response can be crucially important. Crisis management after natural and non-natural disasters, such as earthquakes, floods, droughts, bushfires, refugees, raids, and even wars is one of the significant concerns of governments, where fast decision-making is an essential element of an effective crisis management system. When a large number of houses have suffered damages and become unusable, causing a high number of homeless people, rapid housing-reconstruction programs play a decisive role in disaster recovery and providing temporary housing is a crucial step of these programs. Experts estimate that, on average, it can take 5 to 10 [1
] years for communities to recover from the effects of a major seismic event, which highlights the severity of the disaster and the importance of rapidly assembled buildings as an effective postdisaster housing system. In addition to residential accommodation, rapid-assembly buildings can be employed in several other applications, such as field hospitals, storehouses, and other temporary and semipermanent facilities. Some rapidly assembled systems have the potential to be used as temporary structures as well as provide long-term serviceability. Abulnour [3
] defined temporary dwellings as a step toward permanent houses in a disaster recovery and reconstruction plan, and classified them into two distinct categories: (i) Temporary shelter, used to incubate people immediately after a disaster; and (ii) temporary housing, allowing the return to normal daily activities, e.g., work, school, cooking at home, and shopping.
Mobile and rapidly assembled structures play a major role in postdisaster management through building temporary accommodation and shelters. These types of structures are also of primary importance in many military and civilian service applications and are widely used for rescue and maintenance services. Air-liftable origami-inspired deployable systems, pliable structural systems with rigid couplings for parallel leaf-springs, scissor systems, elastic grid shell systems, and structural panels are some popular types of mobile and rapidly assembled structures [4
]. Some successful attempts on employing paper tube arches for temporary structures have also been discussed by Preston and Bank [5
A wise selection of rapid-assembly building systems has an impact on their performance in an effective crisis management system. For instance, while use of big precast structural elements is very common for postdisaster housing, as the dimension of precast elements increases, some significant construction problems will appear in transportation and erection phases. A temporary accommodation building can be any class of building as defined under the National Construction Code (NCC) [6
]: Class 1b (boarding house, guest house, hostel, or the like), class 2 (residential units), or class 3 (motel) building, depending on its configuration [7
]. Among the existing systems, air-liftable origami-inspired deployable systems, pliable structural systems with rigid couplings for parallel leaf-springs, scissor systems [8
], elastic grid shell systems [9
], and structural panels are some popular types of mobile and rapidly assembled structures [10
]. However, most of these rapidly assembled structural systems suffer from low tolerance in the making and erection phases and need skilled labors for installation that will result in an increase in total construction costs and lower efficiency.
Lightweight structural panels are one of the most popular types of mobile and rapidly-assembled structures. Rapidly assembled panels are a form of modular construction, commonly used in residential buildings as well as industrial structures [12
]. A wide range of these panels is made from new lightweight components such as foams. Many types of foams are on the market and polyurethane (PU) foams are the most popular types [15
]. Low self-weight and relatively high stiffness and durability have increased the demand for this type of composite structures [16
]. Foam-filled sandwich construction, characterized by two relatively thin and stiff faces and a relatively thick and lightweight foam-core, is becoming an interesting solution for prefabricated building wall and floor systems.
With regard to the literature, a wide range of studies on the foam-filled composite panels are on those made of PU foam-core [17
]. The results of these studies indicate that the stiffness and strength of a majority of conventional foam-filled sandwich panels hardly meet the structural requirements for use in building floors or walls, at least for standard spans and loads, mainly due to different failure modes, such as delamination of the skins from the core, buckling or wrinkling of the compression skin, flatwise crushing of the core, or rupture of the tension skin. The main weaknesses of these panels stem from the low stiffness and strength of the core, and the skin’s susceptibility to delamination and buckling, owing to the local mismatch in stiffness and the lack of reinforcements bridging the core and the skins [18
]. The use of stitches for connecting the two side skins [19
] or use of reinforcing ribs [20
] are two popular strengthening techniques being employed for improving the mechanical performance of standard sandwich panels. Despite their very competitive costs, conventional foam-filled sandwich panels are susceptible to some different failure modes. Delamination of the skins from the core, buckling or wrinkling of the compression skin, flatwise crushing of the core, and rupture of the tension skin are some of the very common types of failure.
In this study, in order to enhance the properties of the foam-filled sandwich panels with regard to such failure modes for application in semi-temporary housing, a new sandwich system is proposed, in which 3D high-density polyethylene (HDPE) sheets with 2 mm thickness are used as the skins, and high-density PU foam is used as the core, as illustrated in Figure 1
, with a total thickness of 100 mm. The system is cast in a pneumatic fabric formwork, which is used to accelerate installation and simplify the transposition process. Using the HDPE sheets, manufactured with approximately 1200 studs per square meter, higher pullout, and delamination strength, as well as better stress distribution and buckling performance can be achieved. The studs also improve the resistance of the face sheets and foam-core from debonding and increasing the interface strength between the foam-core and the face sheets.
The fabrication of these sandwich panels takes place in a single step. Therefore, the face sheets and foam-core are integrated into one construction in the fabric formwork. Rapid assembly, lightweight and easy transportation, durability, and a wide range of applications are some merits of this new design. Given that the introduction of a new design typically brings new challenges to designers to utilize the new properties of the materials and geometry, the main goal of this research work is to investigate some structural properties of the newly-developed sandwich system.
3. Edgewise and Flatwise Compressive Behavior
Edgewise compressive strength of sandwich construction is important as it provides the basis for the assessment of load-carrying capacity [33
]. The compressive properties of the sandwich composite along the direction parallel to the plane of the sandwich face skin were evaluated through edgewise compression tests on 100 mm × 200 mm × 300 mm samples using a test rig (universal testing machine) in accordance with the ASTM C364 standard [34
]. This test method consists of subjecting a sandwich panel to monotonically increasing compressive force parallel to the plane of its faces. The force is transmitted to the panel through either clamped or bonded end supports. Stress and strength are reported in terms of the nominal cross-sectional area of the two face sheets, rather than total sandwich panel thickness, although alternate stress calculations may be optionally specified.
For design purposes, the nonlinear behavior of the stress–strain relationship can be approximated by two linear behaviors with different stiffness. The initial portion can be used to determine the initial elastic modulus using regression analysis of the data up to 2% strain. Due to the significant nonlinear behavior observed beyond the strain level of 2%, the second slope, conservatively representing the reduced elastic modulus can be determined approximately based on the data measured between strains of 4% up to failure strain. These two calculated slopes are extended between 2% and 4% strain until they intersect each other in order to obtain the full approximation of the compressive edgewise behavior (Figure 6
). Specific geometric factors that affect edgewise compressive strength of sandwich panels include face-sheet fiber waviness, core cell geometry (shape, density, orientation), core thickness, and specimen shape (L/W ratio).
The compressive strength of the composite was also assessed through the flatwise compressive tests [35
] of small sandwich cubes. Four specimens were tested to determine flatwise compressive strength and elastic modulus for the sandwich core’s structural design properties, using a universal testing machine and following the ASTM C365. Deformation data are obtained and, from a complete force versus deformation curve, it is possible to compute the compressive stress at any applied force (such as compressive stress at proportional limit force or compressive strength at the maximum force) and to compute the effective modulus of the core. Flatwise compressive tests were performed until the load–displacement curve indicated a collapsed structure, i.e., with significantly high deformation of specimens. The results, shown in Figure 7
, indicate that the flatwise compressive behavior of the specimens is governed by rigid foam behavior, and the composite specimens show similar behavior to the foam specimens. That is, experiment results confirmed that, although a separation between the core and the skin is observed at the failure load, the possible local ruptures in the foam, due to the increased stress on the studs’ tips, do not influence the flatwise compressive behavior of the sandwich composite panel.
In Figure 7
, E is the flatwise compressive modulus, P0.001
are forces, carried by test specimens at 0.1 % and 0.2% linear variable differential transformer (LVDT) deflection, respectively, B is the corrected zero displacement point (δ = 0.000) from which all displacements must be measured, and δs
are corresponding LVDT deflections.
4. Flexural and Shear Behavior
The flexural stiffness of sandwich beams/panels, which can be calculated using First-order Shear Deformation Theory (FSDT) [37
], is used to estimate the shear stiffness of each sandwich beam type by fitting the results collected from four-point flexural tests. A perfect bond must be assumed to exist between the core and the facings. The bending stiffness can be computed accounting for the deflection components that are associated with bending and shear deformations [41
]. This study examined the core shear properties of introduced PU infill-foam composite panels subjected to flexure in such a manner that the applied moments produce curvature of the sandwich facing planes. Also, in this regard, core shear ultimate stress, facing bending stress, transverse shear rigidity and core shear modulus of introduced sandwich panels are calculated based on ASTM C393/C393M [43
] and ASTM D7250/D7250M [22
] using six medium-scale sandwich specimens with 45 cm length, 20 cm width, and 10 cm as total thickness of composite section. This test method consists of subjecting a beam of sandwich construction to a bending moment normal to the plane of the sandwich. Forces versus deflection measurements are recorded. The applied force versus crosshead displacement and midspan deflection are shown in Figure 8
, and transverse shear rigidity calculated based on ten load-deflection selective steps is shown in Table 2
Effects of Cold Joints
One of the most important construction problems of foam-made panels is cold joints, which are also known as seams. When the placing of foam in the panels is delayed or interrupted for some reasons, the foam that has already been placed starts to condense, producing a kind of construction joint (seam), called a cold joint, between it and newly placed foam. A seam is a plane under mixed materials, or a fold that is developed within the rising foam mass, which appears as a line on the foam surface or section [22
]. Such joints between new and old portions of foam that are formed when the new foam is placed adjacent to the foam that has hardened or has started to harden, may have negative effects on the strength of rigid foam panel. Hence, attention must be paid to the position and direction of the joints, and the effects on structural behavior. For experimental investigation, three series of bending tests were carried out on two types of panelized specimens. Two types of 1500 × 1000 × 100 mm3
rigid PU panels were used: Type S (seamless) and type TS (with transverse seams) specimens, as shown in Figure 9
. The expansion rate of this type of foam is 3.0, and the average weight of both types of panels is 29.0 kg.
A comparison between the results of the tests shows that casting at the end of gel time, instead of the end of tack free time, resulted in an 80% increase in the tensile strength of the seams. Also, casting at about 20 s before the end of tack free time (120th s), increased the tensile strength of the seams by 60%. The seamed section exhibited about 33.1% of the maximum tensile strength of an intact section. In addition, seamless panels showed a larger deflection capacity—20% more than that of TS panels.
5. Integrated Connections
Connections represent major challenges in the design of composite structures, mainly because they entail discontinuities in the geometry of the structure and material properties, and introduce high local stress concentrations. Despite some constructability complications, integrated connections could be a reliable solution. For the composite sections in this study, the connections between the panels are constructed by continued foam casting to achieve better integrity. The primary function of these connections is to guarantee the transfer of lateral (seismic and wind) loads between the composite panels, as well as between panels and roof in rapid-assembly postdisaster buildings. In addition, this connection accounts for restricting the rotation, i.e., the maximum deflections along the span. This is a significant factor because, in practice, the maximum allowable deformation is usually the governing factor in the design of lightweight composite sandwich panels.
For the experimental investigation, six L-shape specimens, representing the connections between adjacent sandwich panels, are tested. In order to better study the composite performance and compare the results with noncomposite behavior, three of the specimens were made of composite sections, while three of them were foam-only sections; all of them were manufactured by a one-shot casting method in wooden formworks and were cut out of actual adjacent sandwich panels. The composite connections comprised of 2 mm thick 3D HDPE face sheets enclosing a 96 mm thick core of rigid PU foam. The test specimens were supported in a cantilever configuration test rig, and a point load was applied at 40 mm of the free edge, as illustrated in Figure 10
As presented in Table 3
, the overall mechanical response, stress distributions, failure modes, moment resistance, initial rotational stiffness, and rotational capacity of the connections were studied. The experimental test results indicated that in composite sections the bending ultimate strength increases by 25% compared to foam-only connections. The composite connections also show 2.2% greater rigidity and an increased rotational stiffness of 85%. With regard to the relative ultimate cantilever deflection, i.e., bending stiffness, composite connections presented a better performance by 12% in comparison with foam-only connections.
6. Concluding Remarks
A new foam-filled sandwich panel and its integrated connections were developed at the Centre for Infrastructure Engineering of Western Sydney University, as a rapid-assembly system for postdisaster housing and semipermanent accommodations. It is composed of 3D HDPE sheet skins with 2 mm thickness, and high-density PU foam-core with a total thickness as 100 mm, incorporated into a pneumatic fabric formwork. This paper investigated the structural performance of the panel and integrated connections with respect to the material properties, edgewise and flatwise compressive behavior, flexural and shear behavior, and the effect of cold joints (seams). The findings for each criterion indicate that the system fully complies with the relevant standards for semipermanent and temporary accommodations, and meets their requirements for postdisaster housing. In this regard, the following conclusions are achieved:
The used rigid foam is in accordance with ASTM E1730 Type 4, for which carrying out a thermal conductivity test is not required. A 100 mm thick layer of rigid PU provides a U-value of 0.04, which demonstrates high insulation performance of PU foam.
Barrateen was selected from the mentioned list of seven potential candidates as the best pneumatic formwork candidate for foam-filled structural composite panels. The type of lateral pressure of foam on this fabric formwork with thickness of 100 mm is hydrostatic.
The failure mode of specimens under the edgewise compression was local buckling (wrinkling) of the HDPE sheets between two edge studs, resulting in a local delamination and debonding between the face and core.
Results indicate that under flexure, the foam-core and skins displacement are in sync, which demonstrate well-integrated and ductile behavior of the introduced composite panel.
Further research on the constructional and architectural aspects, such as the integration of windows and doors, and onsite foam-casting methods are in progress.