buildings Article
Development of an Innovative Modular Foam-Filled Panelized System for Rapidly Assembled Postdisaster Housing P. Sharafi *
ID
, S. Nemati, B. Samali and M. Ghodrat
Centre for Infrastructure Engineering, Western Sydney University, Kingswood, NSW 2747, Australia;
[email protected] (S.N.);
[email protected] (B.S.);
[email protected] (M.G.) * Correspondence:
[email protected] Received: 21 June 2018; Accepted: 28 July 2018; Published: 30 July 2018
Abstract: In this paper, the development process of a deployable modular sandwich panelized system for rapid-assembly building construction is presented, and its structural performance under some different action effects is investigated. This system, which includes an innovative sandwich panel and its integrated connections, can be used as structural walls and floors in quickly-assembled postdisaster housing, as well as load-bearing panels for prefabricated modular construction and semipermanent buildings. Panels and connections are composed of a pneumatic fabric formwork, and two 3D high-density polyethylene (HDPE) sheets as the skins, filled with high-density rigid polyurethane (PU) foam as the core. HDPE sheets manufactured with a studded surface considerably enhance stress distribution, buckling performance, and delamination strength of the sandwich panel under various loading conditions. The load-carrying behavior of the system in accordance with some American Society for Testing and Materials (ASTM) standards is presented here. The results show the system satisfies the codes’ criteria regarding semipermanent housing. Keywords: postdisaster housing; rapid assembly systems; foam-filled sandwiches; modular construction
1. Introduction 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 [1,2] 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.
Buildings 2018, 8, 97; doi:10.3390/buildings8080097
www.mdpi.com/journal/buildings
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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 [10,11]. 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 [12–14]. 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
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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 performance achieved. studs also the and buckling performanceand canbuckling be achieved. The studscan alsobeimprove theThe resistance of theimprove face sheets resistance of the facedebonding sheets and foam-core from and increasing the foam-core interface strength and foam-core from and increasing the debonding interface strength between the and the between the foam-core and the face sheets. face sheets.
Figure 1. Sandwich with 3D high-density polyethylene (HDPE) skins, polyurethane (PU) foam-core, Figure 1. Sandwich with 3D high-density polyethylene (HDPE) skins, polyurethane (PU) foam-core, and pneumatic formwork. and pneumatic formwork.
The fabrication of these sandwich panels takes place in a single step. Therefore, the face sheets The fabrication these sandwich panels takes place in single step. formwork. Therefore, the face sheets and and foam-core are ofintegrated into one construction ina the fabric Rapid assembly, foam-core are integrated into one construction in the fabric formwork. Rapid assembly, lightweight and lightweight and easy transportation, durability, and a wide range of applications are some merits of easy transportation, durability, and a wide range applications are some brings merits of thischallenges new design. this new design. Given that the introduction of of a new design typically new to Given thatto the introduction a new design typically brings challenges to designers to utilize the designers utilize the newofproperties of the materials and new geometry, the main goal of this research new of the some materials and geometry, goal of this research work system. is to investigate workproperties is to investigate structural propertiesthe of main the newly-developed sandwich some structural properties of the newly-developed sandwich system. 2. Materials and Methods 2. Materials and Methods To evaluate basic material properties, in addition to using manufacturers’ data, some To evaluate basic material properties, in addition to using manufacturers’ data, some experimental experimental tests were performed. tests were performed. 2.1. Foam-Core 2.1. Foam-Core Rigid foam foam systems systems are are energy-efficient, energy-efficient, versatile, versatile, high-performance high-performance systems, systems, where where liquid liquid Rigid components are mixed together and expand and harden on curing. Rigid PU foam is one of the most components are mixed together and expand and harden on curing. Rigid PU foam is one of the most efficient, high-performance high-performanceinsulation insulation materials, enabling very effective with efficient, materials, enabling very effective energy energy savings savings with minimal minimal occupation of space. A 100 mm thick layer of rigid PU provides a U-value of 0.04, which occupation of space. A 100 mm thick layer of rigid PU provides a U-value of 0.04, which demonstrates demonstrates insulation performance of PU foam [21].how U-values measure how effective a high insulationhigh performance of PU foam [21]. U-values measure effective a material is an insulator. material is an insulator. The lower the U-value is, the better the material is as a heat insulator. The lower the U-value is, the better the material is as a heat insulator. The popular popular type type of of PU PU foam foam being being used used for for thermal thermal insulation, insulation, refrigeration, refrigeration, and and water water heater heater The systems is is made made of of aa 100:100–110 100:100–110 weight weight ratio ratio mixture mixture of of AUSTHANE AUSTHANE POLYOL POLYOL AUW763 AUW763 and and systems AUSTHANE MDI [22]. This foam is formulated using a zero Ozone Depletion Potential (ODP), zero AUSTHANE MDI [22]. This foam is formulated using a zero Ozone Depletion Potential (ODP), Global Warming Potential (GWP), andand Volatile Organic Compound zero Global Warming Potential (GWP), Volatile Organic Compound(VOC) (VOC)exempt exemptblowing blowingagents. agents. 3 In this study, high-density rigid PU foam with a density of 192 kg/m was selected for the core
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In this study, high-density rigid PU foam with a density of 192 kg/m3 was selected for the core material Buildingsto 2018, x FOR PEER 4 of 15 according the8, results of REVIEW the preliminary finite element models. Table 1 and Figure 2 show the PU foam’s manufacturing and mechanical properties, provided by the manufacturer and validated in the material according to the results of the preliminary finite element models. Table 1 and Figure 2 show laboratory according to the ASTM 1730 [23] standard specification for rigid foam for use in structural the PU foam’s manufacturing and mechanical properties, provided by the manufacturer and sandwich panel cores. It also meets the thermal conductivity, dimensional stability, and flame resistance validated in the laboratory according to the ASTM 1730 [23] standard specification for rigid foam for requirements of ASTM E1730panel [23]. cores. In Table 1, “cream time” is a conductivity, measure of the beginning of the foam use in structural sandwich It also meets the thermal dimensional stability, reaction, characterized by a change in the liquid’s color as it begins to rise; “gel time” is the time and flame resistance requirements of ASTM E1730 [23]. In Table 1, “cream time” is a measure of the when the foam has developed strength toby beadimensionally stable; color “track time”toisrise; the time beginning of the foamenough reaction,gel characterized change in the liquid’s as free it begins between the beginning thethe foam pour the point at which the outer of the foam loses its “gel time” is the time of when foam has and developed enough gel strength to be skin dimensionally stable; “track free the time between the weight beginning of unit the foam pourof and thefoam pointthat at which thefree outerrise or stickiness; andtime” “freeisrise density” is the per volume the can be skininto of the foam loses stickiness; and “freeisrise density” is the weighttoper unit volume of the packed a mold. Theits compressive force to be applied parallel the rise direction offoam the foam, that can be free rise or packed into a mold. The compressive force is to be applied parallel to the riseD1621, and the compressive strength is determined in accordance with Procedure A of Test Method direction of the foam, and the compressive strength is determined in accordance with Procedure A or by Test Method C165. The tensile force is to be applied parallel to the rise direction of the foam, of Test Method D1621, or by Test Method C165. The tensile force is to be applied parallel to the rise and the tensile strength is determined in accordance with Test Method D1623. Shear force is applied direction of the foam, and the tensile strength is determined in accordance with Test Method D1623. perpendicular rise direction of the to foam, shear strength determined accordance Shear forcetoisthe applied perpendicular the and rise direction of thewas foam, and shear in strength was with Test determined Method C273/C273M. in accordance with Test Method C273/C273M. Table 1. Mechanical propertiesofof selected foam. Table 1. Mechanicaland andmanufacturing manufacturing properties thethe selected PU PU rigidrigid foam. Mechanical Properties PUFoam Foam Mechanical Properties of of the the PU Compressive yield strength Tensile yield strength 3) 3) Compressive yield strength (MPa) Tensile yield strength (MPa) Density (kg/m Density (kg/m (MPa) (MPa) 192 3.51 1.896 192 3.51 1.896 Manufacturing properties of AUW763 Manufacturing properties of AUW763 Cream timetime Gel Track free free time Cream Geltime time Track time 35-40 s 94 115 ± 35-40 s 94±±44ss 115 ±5s
Shear yield strength (MPa) 1.034 1.034
Shear yield strength (MPa)
Freedensity rise density Free rise 3 3 280–300 kg/m 280–300 kg/m
45 40 35
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∆L/L % Figure 2. Results of the uniaxial load test on PU foam. Figure 2. Results of the uniaxial load test on PU foam.
yieldingbehavior behavior can explained by theby buckling of the foam’s internal walls.internal Scanning walls. TheThe yielding canbebe explained the buckling of the foam’s Electron Microscopic (SEM, JEOL, Sydney, Australia) images, provided before and after and Scanning Electron Microscopic (SEM, JEOL, Sydney, Australia) images, provided before test,test, substantiate such behavior. A long and ratherand flat rather plateauflat wasplateau followed. Then, a aftercompression compression substantiate such behavior. A long was followed. densification (hardening) region was created by gradual stress increase when the cell walls were Then, a densification (hardening) region was created by gradual stress increase when the cell walls were stacked prior to final densification. In this range of loading, no visible signs of failure were observed. stacked prior to final densification. In this range of loading, no visible signs of failure were observed. Residual displacement of the collapsed foam, however, occurs once the unloading stage was Residual displacement of the collapsed foam, however, occurs once the unloading stage was complete. complete.
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2.2. Fabric Formwork For the selection of a formwork system, some criteria are usually taken into account: Quality (strength, rigidity, position, and dimensions); safety (both of workers and the concrete structure); efficiency (in operation, handling, erection and dismantling, and number of repetitions); and economy (life cycle cost to be consistent with quality and safety). Fabric formworks offer lower weight (approximately 1/300th that of a conventional rigid form), lower material cost, lower labor cost (no cost of stripping, placing, erection, and waterproofing), and better constructability (adaptable to uneven ground conditions, easier infill protection, and stakeless system). These make the use of fabric a viable option, especially for rapid-assembly construction [24]. Although fabrics have been used as formwork for many years [25], thanks to recent advances in the textile material science, durable and low-cost fabrics are becoming increasingly available for construction purposes. Using fabric formwork as a mold in concrete structure, it is possible to cast architecturally interesting, structurally optimized nonprismatic structures that use up to 40% less concrete in comparison with an equivalent prismatic section [24], offering potentially significant embodied energy savings [26], and, subsequently, a striking reduction in CO2 emissions [27] can be achieved. There are two general types of fabric formworks: Slack-sheet mold and energized (tensioned) formwork sheets [28]. Each type of fabric distributes force slightly differently depending on the material it is made of and the nature of its internal structures. This study will focus on pneumatic fabric formworks, in which pneumatic force is used for the erection of the flexible fabric formwork. The critical aspect of fabric formwork for achieving desirable performance is the selection of the fabric itself. Although a wide range of woven fabrics can be used as formwork for fabric formwork, tensile strengths in both warp and weft directions must be sufficient to hold the infill material (which is PU in this research) and a low creep modulus is desirable to limit formwork deformations during casting and curing/hardening. For the selection of the fabric, criteria such as aesthetics, permeability, sewability or weldability, relative cost, durability, and strength were considered [29]. Conducting some experimental tests to determine the selection indicators for criteria like durability and strength for each candidate, we selected Barrateen as the most suitable pneumatic formwork candidate for foam-filled structural composite panels. Barrateen fabric is an HDPE-coated unbalance woven textile. The coating material is low-density polyethylene and well inflatable. In addition, its tensile strengths in the warp and weft directions are not the same. The result of tensile tests on 10 cm wide and 20 cm long specimens according to D5034–09 [30] showed that the module of elasticity of the principal direction is higher, but, in strain about 270%, it can have a sudden brittle rupture (Figure 3). The test is designed for determining the breaking strength and elongation of textile fabrics, and is applicable to woven, nonwoven, and felted fabrics. A series of weldability tests were also conducted on the fabric; the tensile-bearing capacity of heat-welded connections can reach up to 13% of the average strength of the material. In addition, the maximum strain was measured as 90% at the failure point.
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Figure 3. Barrateen fabric fabric tensile tensile behavior behavior in main main and Figure Figure 3. 3.Barrateen behaviorin main and and transverse transverse directions. directions.
Maximum lateral deflection (mm)
With performance of fabric formwork, some experimental tests With regard regard to the structural performance of the thethe fabric formwork, some experimental testswere were With regardto tothe thestructural structural performance of fabric formwork, some experimental tests conducted. Formwork should be designed for the ultimate as well as serviceability limit states. conducted. Formwork should be designed for for thethe ultimate asaswell were conducted. Formwork should be designed ultimate wellasasserviceability serviceabilitylimit limit states. states. Low-yield the lateral pressure on the forms to Low-yield stress stress and plastic viscosity of filling material increase the lateral pressure onon thethe forms to Low-yield stress and andplastic plasticviscosity viscosityof offilling fillingmaterial materialincrease increase the lateral pressure forms ato as high as the hydrostatic pressure. That is, formwork pressure exists as long as filling a degree degree as high as the hydrostatic pressure. That is, formwork pressure exists as long as filling a degree as high as the hydrostatic pressure. That is, formwork pressure exists as long as filling material material is is in in aaa plastic plastic state, state, and and its its rate rate of of decay decay is is related related to to the the rate rate of of the the stiffening stiffening of of filling filling material is in plastic state, and its rate of decay is related to the rate of the stiffening of filling material [31]. Figure 4 depicts the effects of formwork thickness and foam density on the maximum material [31]. Figure 4 depicts the effects of formwork thickness and foam density on the maximum material [31]. Figure 4 depicts the effects of formwork thickness and foam density on the maximum lateral lateral deflection deflection of of the the fabric fabric formwork. formwork. ItIt shows higher higher thickness thickness will will result result in in higher higher effects effects of of lateral deflection of the fabric formwork. It shows shows higher thickness will result in higher effects of density density of of the the maximum maximum lateral lateral deflection. deflection. density of the maximum lateral deflection. 11 11 10 10 99 88 77 66 55 44 33 22 11 00 100 100
10 10 99
150 150kg/qubic kg/qubicmeters meters 200 200kg/qubic kg/qubicmeters meters 250 250kg/qubic kg/qubicmeters meters
200 200 Samples' Samples'Depth Depth(mm) (mm)
300 300
Figure 4. Effects of samples’ thickness and foam density on the maximum lateral deflection. Figure Figure 4. 4.Effects Effects of ofsamples’ samples’ thickness thickness and and foam foam density density on on the the maximum maximum lateral lateraldeflection. deflection.
2.3. Skin Sheets 2.3. 2.3. Skin Skin Sheets Sheets The face sheets of the thesandwich sandwichpanels panelsare aremade madeof of3D 3DHDPE HDPEsheets sheetsprimarily primarilyproduced produced The as aa The face face sheets sheets of of the sandwich panels are made of 3D HDPE sheets primarily produced asas a concrete embedment liner to provide protection from mechanical damage and a corrosive and concrete concrete embedment embedment liner liner to to provide provide protection protection from from mechanical mechanical damage damage and and aa corrosive corrosive and and erosive environment. In addition to resistance to chemical and environmental threats, its relatively erosive erosive environment. environment. In In addition addition to to resistance resistance to to chemical chemical and and environmental environmental threats, threats, its its relatively relatively
high high strength, strength, and, and, in in particular, particular, its its 3D 3D studded studded face face with with approximately approximately 1200 1200 studs studs per per square square meter, meter, can can effectively effectively contribute contribute to to the the sandwich sandwich composites’ composites’ structural structural performance performance by by providing providing
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high and, in minimum particular,lateral its 3Dmovement studded face with approximately 1200 studs perdifferent square high strength, pullout strength, of the skin, and stronger bonding. Four meter, can effectively contribute to the sandwich composites’ structural performance by providing thicknesses of the sheets were initially investigated (2 mm, 3 mm, 4 mm, and 5 mm), and, at the end, high pulloutwith strength, minimum lateral movement thethe skin, and stronger bonding. Four different the sheets 2 mm tackiness were selected offor sandwich composite. Figure 5 shows thicknesses of the sheets were initially investigated (2 mm, 3 mm, 4 mm, and 5 mm), and, at the end, mechanical properties of the selected sheet provided by the manufacturer and validated by the sheets with 2 mm tackiness were selected for the sandwich composite. Figure 5 shows mechanical experimental tests in the laboratory, in accordance with the ASTM D5199, ASTM D1505, and ASTM properties of the selected provided by5the manufacturer and bystructural experimental tests in D6693 provisions [32] at sheet a loading rate of mm/min. In order to validated identify the behavior of the laboratory, in accordance with the ASTM D5199, ASTM D1505, and ASTM D6693 provisions [32] the skin, inplane tensile tests were conducted on two principal perpendicular directions (lengthwise at a loading rate of of 5the mm/min. In order to identify the structural behavior the skin,according inplane tensile and crosswise) HDPE sheets, using a universal hydraulic testing of machine, to the tests were conducted on two principal perpendicular directions (lengthwise and crosswise) of the ASTM D6693 standard [32]. Figure 5 shows the coupon test results. HDPE sheets, using a universal hydraulic testing machine, according to the ASTM D6693 standard [32]. Figure 5 shows the coupon test results.
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Figure 5. HDPE tension test results in the lengthwise and crosswise directions. Figure 5. HDPE tension test results in the lengthwise and crosswise directions.
3. Edgewise and Flatwise Compressive Behavior 3. Edgewise and Flatwise Compressive Behavior Edgewise compressive strength of sandwich construction is important as it provides the basis for Edgewise compressive strength of sandwich construction is important as it provides the basis the assessment of load-carrying capacity [33]. The compressive properties of the sandwich composite for the assessment of load-carrying capacity [33]. The compressive properties of the sandwich along the direction parallel to the plane of the sandwich face skin were evaluated through edgewise composite along the direction parallel to the plane of the sandwich face skin were evaluated through compression tests on 100 mm × 200 mm × 300 mm samples using a test rig (universal testing machine) edgewise compression tests on 100 mm × 200 mm × 300 mm samples using a test rig (universal in accordance with the ASTM C364 standard [34]. This test method consists of subjecting a sandwich 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
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panel to monotonically increasing compressive force parallel to the plane of its faces. The force is Buildings 2018, x FOR PEER through REVIEW either clamped or bonded end supports. Stress and strength 8 of 15 transmitted to 8,the panel are reported in terms of the nominal cross-sectional area of the two face sheets, rather than total sandwich and strength are reported in terms of the nominal cross-sectional area of the two face sheets, rather panel thickness, although alternate stress calculations may be optionally specified. than total sandwich panel thickness, although alternate stress calculations may be optionally For design purposes, the nonlinear behavior of the stress–strain relationship can be approximated specified. by two linear behaviors with different stiffness. The initial can be used to determine For design purposes, the nonlinear behavior of portion the stress–strain relationship can the be initial elastic modulus using regression analysis of the data up to 2% strain. Due to the significant nonlinear approximated by two linear behaviors with different stiffness. The initial portion can be used to behavior observed beyond strainusing levelregression of 2%, theanalysis secondofslope, conservatively determine the initial elasticthe modulus the data up to 2% strain.representing Due to the the significant observed beyond the strain based level on of the 2%, data the measured second slope, reduced elasticnonlinear modulusbehavior can be determined approximately between conservatively the reduced elastic modulusslopes can be are determined approximately on strain strains of 4% up torepresenting failure strain. These two calculated extended between 2%based and 4% the data measured between strains of 4% up to failure strain. These two calculated slopes are until they intersect each other in order to obtain the full approximation of the compressive edgewise extended between 2% andgeometric 4% strain factors until they each other in order tostrength obtain the full behavior (Figure 6). Specific thatintersect affect edgewise compressive of sandwich approximation of the compressive edgewise behavior (Figure 6). Specific geometric factors that panels include face-sheet fiber waviness, core cell geometry (shape, density, orientation), core thickness, affect edgewise compressive strength of sandwich panels include face-sheet fiber waviness, core cell and specimen shape (L/W ratio). geometry (shape, density, orientation), core thickness, and specimen shape (L/W ratio).
Figure 6. Test setupsetup and design elastoplastic diagram of composite panel under compression, Figure 6. Test and design elastoplastic diagram of composite panel edgewise under edgewise and theof determination of module of elasticity andcompression, the determination module of elasticity
The compressive strength of the composite was also assessed through the flatwise compressive
The compressive strength of the composite was also assessed through the flatwise compressive tests [35,36] of small sandwich cubes. Four specimens were tested to determine flatwise compressive testsstrength [35,36] and of small cubes. Four specimens were tested toproperties, determineusing flatwise compressive elasticsandwich modulus for the sandwich core’s structural design a universal 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 testing machine and following the ASTM C365. Deformation data are obtained and, from complete force versus deformation curve, it is possible to compute the compressive stress at any applied a force (such as compressive at it proportional force or the compressive strength at at the maximum force versus deformationstress curve, is possible limit to compute compressive stress any applied force force) and to compute the at effective moduluslimit of the core.orFlatwise compressive testsatwere performed force) (such as compressive stress proportional force compressive strength the maximum the load–displacement curve indicated a collapsed i.e., with high until anduntil to compute the effective modulus of the core. Flatwisestructure, compressive testssignificantly were performed deformation of specimens. The results, shown in Figure 7, indicate that the flatwise compressive the load–displacement curve indicated a collapsed structure, i.e., with significantly high deformation behavior of the specimens is governed by rigid foam behavior, and the composite specimens show of specimens. The results, shown in Figure 7, indicate that the flatwise compressive behavior of the similar behavior to the foam specimens. That is, experiment results confirmed that, although a specimens is governed by rigid foam behavior, and the composite specimens show similar behavior to separation between the core and the skin is observed at the failure load, the possible local ruptures in the foam specimens. That is, experiment results confirmed that, although separation between the the foam, due to the increased stress on the studs’ tips, do not influence the a flatwise compressive corebehavior and theofskin is observed at the failure load, the possible local ruptures in the foam, due to the the sandwich composite panel. increased stress on the studs’ tips, do not influence the flatwise compressive behavior of the sandwich composite panel.
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B = 0.46 mm δ0.001 = 1.46 mm δ0.003 = 3.46 mm P0.001 = 13.16 kN P0.003 = 39.31 kN E = 130.8 MPa
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Dispacement (mm) Figure 7. Description experiments calculation. Figure 7. Descriptionofofflatwise flatwise compressive compressive experiments calculation.
In Figure is the flatwise compressivemodulus, modulus, P and P0.003 are forces, carried by test In Figure 7, E7,isEthe flatwise compressive P0.001 0.001 and P0.003 are forces, carried by test specimens at 0.1 % and 0.2% linear variable differential transformer (LVDT) deflection, respectively, 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 B is the corrected zero displacement point (δ = 0.000) from which all displacements must be measured, measured, and δs are corresponding LVDT deflections. and δs are corresponding LVDT deflections. 4. Flexural and Shear Behavior
4. Flexural and Shear Behavior
The flexural stiffness of sandwich beams/panels, which can be calculated using First-order
The stiffness of sandwich beams/panels, can calculated First-order Shearflexural Deformation Theory (FSDT) [37–40], is used to which estimate thebeshear stiffnessusing of each sandwichShear Deformation Theory (FSDT) is used to estimate the shear stiffness each sandwich beam beam type by fitting the [37–40], results collected from four-point flexural tests. of A perfect bond must be type assumed to existcollected betweenfrom the core and the facings. TheAbending stiffness canbebeassumed computed by fitting the results four-point flexural tests. perfect bond must to exist accounting for the deflection components that are associated with bending and shear deformations between the core and the facings. The bending stiffness can be computed accounting for the deflection [41,42]. This examined with the core shearand properties of introduced [41,42]. PU infill-foam composite components that study are associated bending shear deformations This study examined panels subjected to flexure in such a manner that the applied moments produce curvature of the the core shear properties of introduced PU infill-foam composite panels subjected to flexure in such sandwich facing planes. Also, in this regard, core shear ultimate stress, facing bending stress, a manner that the applied moments produce curvature of the sandwich facing planes. Also, in this transverse shear rigidity and core shear modulus of introduced sandwich panels are calculated regard, core shear ultimate stress, facing bending stress, transverse shear rigidity and core shear based on ASTM C393/C393M [43] and ASTM D7250/D7250M [44] using six medium-scale sandwich modulus of introduced sandwich panels are calculated ASTM C393/C393M [43] and ASTM specimens with 45 cm length, 20 cm width, and 10 cm based as totalon thickness of composite section. This D7250/D7250M [22] using six medium-scale sandwich specimens with 45 cm length, 20 cm width, test method consists of subjecting a beam of sandwich construction to a bending moment normal to and 10 cm as total thickness of composite section. This test method consists of subjecting a the plane of the sandwich. Forces versus deflection measurements are recorded. The applied forcebeam versus crosshead displacement and midspan deflection in Figure and transverse shear of sandwich construction to a bending moment normalare to shown the plane of the8,sandwich. Forces versus rigidity calculated based ten load-deflection selective is shown in Table 2. deflection measurements areon recorded. The applied force steps 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.
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Figure Four-point quarter-span quarter-span loading flexural testtest results. Figure 8.8.Four-point loading flexural results. Table 2. The least maximum applied forces and their related midspan deflections.
Table 2. The least maximum applied forces and their related midspan deflections. 4-Point Quarter Span Loading max (kN) ∆midspan (mm) 4-Point PQuarter Span Loading Tests Specimen 1 76.19 21.9 ∆midspan (mm) Pmax (kN) Specimen 2 79.72 22.4 Specimen 1 76.19 21.9 Specimen 3 71.95 (minimum) 19.1 Specimen 2 79.7275.95 22.4 Average 21.1 Specimen 3 71.95 (minimum) 19.1 Standard deviation 3.89 1.8 Average 75.95 21.1 CV (%) 8.42 Standard deviation 3.89 5.12 1.8 CV (%) 5.12 8.42 Tests
3-Point Mid Span Loading Pmax (kN) 3-Point Mid Span ∆midspan (mm) Loading 56.23 24.9 ∆midspan (mm) Pmax (kN) 52.54 (minimum) 23.8 56.23 53.98 24.2 24.9 52.54 (minimum) 54.25 24.3 23.8 53.98 24.2 1.86 0.56 54.25 24.3 3.51.86 2.3 0.56 3.5 2.3
Effects of Cold Joints the most important construction problems of foam-made panels is cold joints, which are Effects of One ColdofJoints also known as seams. When the placing of foam in the panels is delayed or interrupted for some One of the mostthat important construction problems foam-made panels is cold joints, which are reasons, the foam has already been placed starts to of condense, producing a kind of construction also joint known as seams. When the placing of foam in the panels is delayed or interrupted for some (seam), called a cold joint, between it and newly placed foam. A seam is a plane under mixed reasons, the foam been placed to condense, producing a kindas of aconstruction materials, or athat foldhas thatalready is developed within starts the rising foam mass, which appears line on the joint foam surface or section Such it joints new and old portions thatunder are formed (seam), called a cold joint, [45]. between andbetween newly placed foam. A seam of is foam a plane mixedwhen materials, the new is placed adjacent to the foam thatmass, has hardened or has started to harden, may have or a fold thatfoam is developed within the rising foam which appears as a line on the foam surface or negative effectsjoints on the strengthnew of rigid panel. Hence, attention must be paid to the section [22]. Such between andfoam old portions of foam that are formed when theposition new foam is and direction of the joints, and the effects on structural behavior. For experimental investigation, placed adjacent to the foam that has hardened or has started to harden, may have negative effects on three series of bending tests were carried out on two types of panelized specimens. Two types of the strength of rigid foam panel. Hence, attention must be paid to the position and direction of the 1500 × 1000 × 100 mm3 rigid PU panels were used: Type S (seamless) and type TS (with transverse joints, and the effects on structural behavior. For experimental investigation, three series of bending seams) specimens, as shown in Figure 9. The expansion rate of this type of foam is 3.0, and the 3 testsaverage were carried out on two types of panelized weight of both types of panels is 29.0 kg. specimens. Two types of 1500 × 1000 × 100 mm
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.
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Figure 9. Casting schedule of seams at specimens with transverse seams (TS). Locations of transverse Figure 9. Casting schedule of seams at specimens with transverse seams (TS). Locations of seams. transverse seams.
A comparison between the results of the tests shows that casting at the end of gel time, instead comparison between the results theincrease tests shows thattensile castingstrength at the end of gel time, Also, instead of of the endAof tack free time, resulted in an of 80% in the of the seams. the end of tack free time, resulted in an 80% increase in the tensile strength of the seams. Also, casting casting at about 20 s before the end of tack free time (120th s), increased the tensile strength of the at by about 20The s before the section end of tack free time (120th s),ofincreased the tensile strength of the seams seams 60%. seamed exhibited about 33.1% the maximum tensile strength of an by 60%. The seamed section exhibited about 33.1% of the maximum tensile strength of an intact section. intact section. In addition, seamless panels showed a larger deflection capacity—20% more than that addition, seamless panels showed a larger deflection capacity—20% more than that of TS panels. of TSInpanels. 5. Integrated Connections 5. Integrated Connections Connections represent major challenges in the design of composite structures, mainly because 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 they entail discontinuities in the geometry of the structure and material properties, and introduce local stress concentrations. Despite some constructability complications, integrated connections could high local stress concentrations. Despite some constructability complications, integrated connections be a reliable solution. For the composite sections in this study, the connections between the panels could be a reliable solution. For the composite sections in this study, the connections between the are constructed by continued foam casting to achieve better integrity. The primary function of these panels are constructed by continued foam casting to achieve better integrity. The primary function of connections is to guarantee the transfer of lateral (seismic and wind) loads between the composite these connections is to guarantee the transfer of lateral (seismic and wind) loads between the panels, as well as between panels and roof in rapid-assembly postdisaster buildings. In addition, this composite panels, as well as between panels and roof in rapid-assembly postdisaster buildings. In connection accounts for restricting the rotation, i.e., the maximum deflections along the span. This is addition, this connection accounts for restricting the rotation, i.e., the maximum deflections along a significant factor because, in practice, the maximum allowable deformation is usually the governing the span. This is a significant factor because, in practice, the maximum allowable deformation is factor in the design of lightweight composite sandwich panels. usually the governing factor in the design of lightweight composite sandwich panels. For the experimental investigation, six L-shape specimens, representing the connections between For the experimental investigation, six L-shape specimens, representing the connections adjacent sandwich panels, are tested. In order to better study the composite performance and compare between adjacent sandwich panels, are tested. In order to better study the composite performance the results with noncomposite behavior, three of the specimens were made of composite sections, 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
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while three of them were foam-only sections; all of them were manufactured by a one-shot casting Buildings 2018, 8, x FOR PEER REVIEW of 15 method in wooden formworks and were cut out of actual adjacent sandwich panels. The 12 composite connections comprised of 2 mm thick 3D HDPE face sheets enclosing a 96 mm thick core of rigid panels. The composite connections comprised of 2 mm thick 3D HDPE face sheets enclosing a 96 mm PU foam. Theof test specimens supported a cantilever test rig, and atest point thick core rigid PU foam. were The test specimensinwere supportedconfiguration in a cantilever configuration rig, load was applied at 40 mm of the free edge, as illustrated in Figure 10. and a point load was applied at 40 mm of the free edge, as illustrated in Figure 10.
Figure 10. The ultimate deflection and the mechanism of collapse and dimensions of the connection.
Figure 10. The ultimate deflection and the mechanism of collapse and dimensions of the connection.
As presented in Table 3, the overall mechanical response, stress distributions, failure modes, moment resistance, initial3, rotational stiffness, and rotational of the connections were As presented in Table the overall mechanical response,capacity stress distributions, failure modes, studied. The experimental test results indicated that in composite the bending were ultimate moment resistance, initial rotational stiffness, and rotational capacitysections of the connections studied. strength increases 25% compared foam-only connections. The the composite connections The experimental testby results indicatedtothat in composite sections bending ultimate also strength show 2.2% greater rigidity and an increased rotational stiffness of 85%. With regard to increases by 25% compared to foam-only connections. The composite connections the alsorelative show 2.2% ultimate cantilever deflection, i.e., bending stiffness, composite connections presented a better greater rigidity and an increased rotational stiffness of 85%. With regard to the relative ultimate performance by 12% in comparison with foam-only connections.
cantilever deflection, i.e., bending stiffness, composite connections presented a better performance by 12% in comparison foam-only connections. Table 3. with Summary of the experimental carried tests for both simple and composite systems. Ultimate Load Ultimate Displacement Ultimate Rotation Table 3. Summary Test Details of the experimental carried tests for both simple and composite systems. Specimen 1 Test Details Specimen 2 Foam-only Specimen 3 Specimens Specimen 1 Average Specimen 2 CV (%) Foam-only Specimens Specimen 3 Specimen 1 Average SpecimenCV 2 (%) Composite Specimen 3 Specimen 1 Specimens Average Specimen 2 Composite Specimens Specimen 3 CV (%)
6. Concluding Remarks
Average CV (%)
(N) 7991 Ultimate Load 7172 (N) 7870 7991 7678 7172 5.8 7870 9299 7678 5.8 9602 9926 9299 9609 9602 9926 3.3 9609 3.3
(mm) 42.0 Ultimate Displacement 44.0 (mm) 52.0 42.0 46.0 44.0 11.5 52.0 44.0 46.0 11.5 41.0 38.0 44.0 41.0 41.0 38.0 7.3 41.0 7.3
(Degree) 5.0 Ultimate Rotation 6.0 (Degree) 8.0 5.0 6.3 6.0 1.8 8.0 4.0 6.3 1.8 6.0 3.0 4.0 4.3 6.0 3.0 1.8 4.3 1.8
A new Remarks foam-filled sandwich panel and its integrated connections were developed at the Centre 6. Concluding
for Infrastructure Engineering of Western Sydney University, as a rapid-assembly system for A new foam-filled panel and accommodations. its integrated connections wereofdeveloped the skins Centre for postdisaster housingsandwich and semipermanent It is composed 3D HDPE at sheet Infrastructure Western Sydney University, a rapid-assembly for postdisaster with 2 mmEngineering thickness, andofhigh-density PU foam-core withas a total thickness as 100system mm, incorporated into and a pneumatic fabric formwork. This paper investigated the structural performance of thewith panel2 mm housing semipermanent accommodations. It is composed of 3D HDPE sheet skins and integrated connections respect with to thea total material properties, edgewise and flatwise into thickness, and high-density PUwith foam-core thickness as 100 mm, incorporated compressive behavior, flexural andpaper shear behavior, and the of cold performance joints (seams). The findings a pneumatic fabric formwork. This investigated theeffect structural of the panel and for each criterion indicate that the system fully complies with the relevant standards for
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
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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. Author Contributions: The paper, which is a part of S.N.’s PhD research project, was written through contributions of all authors. The experimental and numerical parts were carried out by S.N., under the B.S. and PS supervision. M.G. had contribution to the data collection and consulting. Funding: This research received no external funding Conflicts of Interest: The authors declare no conflict of interest.
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