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Composite structural insulated panels (CSIPs) have been developed for structural floor applications instead of traditional structural insulated panels (SIPs). However, the load bearing capacity of CSIPs is low due to the debonding between the top face sheet and the core when they are used for floors. To overcome this drawback, an improved composite structural insulated panel (ICSIP) was proposed and analyzed in this paper. In ICSIPs, a thick layer of concrete is used as the top face sheet instead of glass-fiber-reinforced polymer (GFRP) in CSIPs to increase the stiffness of the top compression face sheet. However, the bottom GFRP face sheet and EPS cores in CSIPs are preserved to reduce the weight of the structure and act as a template for the top concrete panels. Full-scale experimental testing and finite-element analysis were conducted to predict the flexural strength and deflection of the ICSIP floor member. Good agreement has been observed between the numerical results and experimental response up to the failure. The cause of failure of ICSIPs is the crushing of concrete face sheet rather than debonding. Moreover, the calculation formula for the ultimate bearing load and deflection was also developed based on the classical sandwich theory. The theoretical predictions reflect well the linear flexural response of the ICSIPs, while deviate as the load increases up to failure due to the theory limitations.

The sandwich panels named structural insulated panels (SIPs) were proposed in 1935 by Forest Products Laboratory (FPL) in the United States [

To overcome the shortcomings of traditional SIPs, a new composite structural insulated panel (CSIP) was proposed in 2010 [

However, two drawbacks of CSIPs are still noticeable when they are used as floors or large span roofs. One is the deflection of CSIPs is large relatively in despite of the high strength [

An improved composite structural insulated panel (ICSIP) was proposed to expand the application of CSIPs in this study. ICSIPs retains the bottom GFRP face sheet and EPS core in the CSIPs, but recycled aggregate concrete was used as the top face sheet instead of the top GFRP face sheet. The recycled aggregate concrete face sheet has higher compression performance than the GFRP face sheet. Therefore, ICSIPs effectively overcome the problem of large structural displacement and low bearing capacity due to the low compressive performance of the top face sheet in CSIPs. Meanwhile, the recycled aggregate concrete has lower cost than the GFRP, which makes the ICSIPs have more advantageous in terms of production cost than the CSIPs. Similarly, the cost of ICSIPs is still attractive compared to traditional SIP while maintaining the same thickness of EPS core.

The characteristic and manufacturing of ICSIPs were studied in this paper and flexural behaviors of full-scale ICSIP floor members were investigated. In order to understand the load bearing capacity and deflection of the ICSIP floor member, the experimental testing, finite-element analysis, and theoretical evaluation were performed.

The concept of ICSIPs is still based on a sandwich structure, in which a soft light-weight thick core is sandwiched between two strong face sheets. The ICSIPs that are developed and evaluated in this research are composed of low-cost orthotropic thermoplastic GFRP laminate as the bottom face sheet, expanded polystyrene (EPS) foam as the core, and recycled aggregate concrete as the top face sheet (Figure

Schematic of the ICSIPs (unit: mm).

The top face sheet in ICSIPs carries the compressive stress, and the bottom face sheet in ICSIPs carries the tensile stress. However, the core stabilizes the face sheets against buckling and increases the stiffness by holding the face sheets apart. Therefore, structural layout of ICSIPs is reasonable for GFRP as the bottom face sheet provides high tensile strength, high durability, and fire resistance; EPS as the core are characterized by light weight, thermal insulation, and excellent impact properties, and recycled aggregate concrete as the top face sheet provides high compressive strength and low cost. The assembly of the bottom GFRP face sheet and EPS core can serve as a template for the top concrete face sheet. The application of recycled aggregate concrete reduces the construction waste materials effectively and achieves the purpose of environmental protection. Moreover, ICSIPs have another advantage related to corrosion, for the bottom GFRP face sheet, and the EPS core can prevent moisture entering the cracks in concrete.

Figure

Manufacturing of ICSIPs: (a) the EPS core was bonded on GFRP; (b) steel mesh was setup; (c) the recycled aggregate concrete was cast; (d) a piece of ICSIP was made.

The thermoplastic composite bottom face sheet used for ICSIPs consists of 70% bidirectional glass-fibers impregnated with polypropylene resin. The production of GFRP is directly obtained from the manufacturer with mechanical properties listed in Table

Mechanical properties of materials.

Items | Glass-PP face sheet | EPS foam core | Recycled aggregate concrete face sheet | Reinforced steel |
---|---|---|---|---|

Nominal thickness, |
3.04 mm | 70 mm | 70 mm | / |

Density, _{f} |
980 kg/m^{3} |
36 kg/m^{3} |
2500 kg/m^{3} |
7850 kg/m^{3} |

Weight percentage of glass-fiber | 70% | / | / | / |

Longitudinal modulus, |
15,169 MPa | 75 MPa | 26 GPa | 206 GPa |

Transverse modulus, |
15,169 MPa | 75 MPa | 26 GPa | 206 GPa |

Thickness direction modulus, |
1050 MPa | 75 MPa | 26 GPa | 206 GPa |

Poisson’s ratio (_{xy}, _{yz}, _{xz}) |
0.11, 0.22, 0.22 | 0.25, 0.25, 0.25 | 0.2, 0.2, 0.2 | 0.3, 0.3, 0.3 |

Shear modulus (_{xy}_{yz}_{xz} |
1800 MPa, 1800 MPa, 750 MPa | 50 MPa, 50 MPa, 50 MPa | 10.4 GPa, 10.4 GPa, 10.4 GPa, | 79 GPa, 79 GPa, 79 GPa, |

Tensile strength (MPa) | 690 MPa | 0.6 MPa | 1.20 MPa | 235 MPa |

Compression strength (MPa) | 317 MPa | 0.8 MPa | 20.4 MPa | 235 MPa |

Recycled aggregate concrete is used as the top face sheet. The use of recycled aggregate concrete can not only reduce the construction waste but also decrease the cost. Ordinary Portland cement with a strength of 32.5 MPa and natural river sand fine aggregate with a fineness modulus of 2.67 are used in the recycled aggregate concrete. The recycled coarse aggregate is a commercial concrete with a strength of C20∼C40 discarded by a testing station. The concrete was crushed using a small jaw crusher, which produced aggregates with a maximum nominal size of 30 mm. After that, the aggregates were separated according to their dimension, by mechanical sieving, and only the diameter between 5 mm and 20 mm was used. After determining the basic performance indexes such as void ratio, water absorption rate, and apparent density of recycled coarse aggregate, the mix design was determined (Table

Recycled aggregate concrete composition.

Items | Composition |
---|---|

P.O 32.5 cement (kg/m^{3}) |
476 |

River sand (kg/m^{3}) |
742.5 |

Coarse aggregate (kg/m^{3}) |
1090.5 |

Water (l/m^{3}) |
191 |

W/C ratio | 0.40 |

It is well known that debonding between the core and face sheet is the main failure mode of sandwich composite structural. According to the pull-off testing of the three kinds of adhesives for EPS core and GFRP face sheet, epoxy spray adhesive was the most cost-effective candidate adhesive [

Performance specifications of the epoxy adhesive.

Items | Parameters |
---|---|

Viscosity (cps) | 103000 |

Curing time (h) | 24 |

Shore hardness | 75 ± 2 |

Glass transition temperature (°C) | 170 |

Shear strength (MPa) | ≥250 |

Compressive strength (MPa) | ≥5 |

Tensile strength (MPa) | ≥2.5 |

Five specimens were produced in the laboratory and the production process is shown in Figure

The experimental testing was performed according to the ASTM E-72-5 standard [

Experimental setup for the test.

The load-deflection relationship is meaningful to reflect the flexural bearing capacity of ICISPs. The experimental results of the five test pieces are basically the same. And the average of the results of the five test pieces was calculated to evaluate the mechanical properties of the ICISPs. As seen from Figure

Load-deflection in the midspan position.

Figure

Typical failure mode of the ICSIPs.

The tensile and compressive strains are recorded from the strain gauges attached to the ICSIPs specimen, as plotted in Section

The ICSIPs in this paper were analyzed using the universal finite-element software ANSYS [

The properties of all materials are listed in Table

Stress-strain relationship of recycled aggregate concrete.

Based on the previously described finite-element model, numerical calculations with ANSYS were conducted. The experimental limit load 27.7 kN is averaged and applied at the loading points in addition to structural dead weight. The results of linear static analysis are summarized in Figure

Results of static analysis: (a) structural displacement; (b) stress in the recycled aggregate concrete layer; (c) stress in the EPS core; (d) stress in the GFRP skin.

The maximum vertical displacement of the ICSIP is 7.76 mm. It is about 1/309 the span and can satisfy the requirement of codes with surplus. If only gravity is considered, the displacement is 0.09 mm. This proportion in the total displacement is about 1.2%. It shows that the structural self-weight is light relatively.

From the side-by-side comparison of stresses in three components of ICSIPs (Figure

Nonlinear buckling analysis can satisfy the real-world situation such as large deflection, initial imperfection, and cracks better [

The results obtained from finite-element models are summarized in Figure

Results of nonlinear analysis: (a) crack appears; (b) cracks layout at limit state; (c) structural deformation.

Subsequently, cracks expand as the load increases, and these cracks extend out from the initial one. Displacements of the panel begin to increase at a higher rate with the increase of the load. The characteristic of nonlinear is obvious from the load-displacement curve. At 29.4 kN, one crack has reached the top of the panel and failure is soon to follow. At last, a few severe cracks throughout the entire thickness arise, and the compressive stress in concrete has been beyond the ultimate strength. The panel can no longer bear the additional load as indicated by an insurmountable convergence failure. In the last stage, it shows the crushing of recycled aggregate concrete. The maximum displacement at midspan is 8.28 mm.

Figures _{1} = 70 mm, the depth of the EPS core is _{2} = 70 mm, and the depth of the GFRP layer is _{3} = 3.04 mm. The distance between the recycled aggregate concrete of the top layer and the core is _{1} = 70 mm, and the distance between the center of the core and the bottom layer is _{2} = 36.52 mm.

Force diagram: (a) schematic illustration the loading; (b) forces for cross section.

The failure mode of ICSIPs is the crack of the recycled aggregate concrete top skin when the applied load reaches the ultimate strength. The classical theory of sandwich panels provides the calculation formula corresponding to this failure mode [

And the flexural stiffness of ICSIPs was calculated using the sum of the flexural stiffness of the constituent parts about the center axis of the entire section. It can be obtained by the following equation:

In ICSIPs, the shear stiffness of EPS core is very low compared with the face sheets, so the shear deformation should be accounted in the total deflection. And the total deformation of the ICSIPs can be predicted as the sum of bending and the shear deflections:

As we know, the deflection at the midspan of the beam is the maximum for a simply supported beam under four-point static bending. So, the total deflection can be calculated by the following equation, which is based on the principle of virtual work from the classical structural analysis:

Therefore, the deflection can be calculated by integrating the deflection along the beam as follows:

For the specimen with a rectangular section, the shear correction factor

The analysis of results comparison between the theoretical prediction, numerical simulations, and experimental testing is discussed in this section.

The comparison of the theoretical analytical, numerical and experimental loads, and midspan deflection curves for ICSIPs tested under four-point bending is shown in Figure

The load-deflection relation curves of the experimental (mean ± SD,

The maximum load of experiment is 27.7 kN. However, the theoretical peak value of the load is 32.3 kN which is 16.6% larger than the experimental results. Correspondingly, the maximum deflection of theoretical results is 6.72 mm, which is 13.4% smaller than the experimental results. The reason is owing to the assumption of linear elastic behavior of materials.

The FE analysis considering the double nonlinear behavior of material and geometry provides the process of failure in good agreement with the experiments. The numerical value of the maximum load is 29.4 kN, which is a little larger than that of the experimental result. And the corresponding deflection of midspan is 8.28 mm, which is a little smaller than that of the experimental result. The small divergence between the experimental and numerical results is attributed to two reasons. One is the effect of the debonding phenomenon was not considered in the FE analysis, and the other is the errors during the manufacture and test of specimen.

Figure

Load-strain relationships. The bar graphs are presented as mean ± SD (

The experimental results showed that the failure mode of the specimen under four-point bending test is the compressive failure of the top recycled aggregate concrete skin. The failure mechanism analysis from the FE analysis is shown in Figure

Failure modes of ICSIP and CSIP: (a) crush of recycled aggregate concrete for ICSIP; (b) debonding of skin for CSIP.

The research results of the improved CSIPs in this paper show three obvious differences from CSIPs [^{2} except of self-weight when they are used in floors with the span 2.4 m.

The second is that the maximum deflection of ISCIP (8.8 mm) is about 6 times smaller than that of CSIP (52 mm). The deformation of 1/273 of the span shows that it can satisfy the requirement of stiffness when they are used in floors or roofs.

The third is that the failure mode of ICSIP is different from that of CSIP (Figure

If only the recycled aggregate concrete top layer is kept while the EPS core and the bottom GFRP skin are moved away, the limit load is 2.8 kN. Even if enough reinforced steel bars are set inside to abide by the building codes for panels, its ultimate load is no more than 12.5 kN. It shows the ICSIP is much stronger than the recycled aggregate concrete panel with nearly the same self-weight. The sandwich structure of ICSIP has much larger moment resistance stiffness by enhancing thickness of the panel. And advantages of compressive concrete and tensile GFRP materials are made full use. The sandwich structure of ICSIP can also be used for reference to strengthen the damaged concrete floors or roofs in service.

New improved composite structural insulated panels (ICSIPs) were presented for the structural floor of roof applications in this paper. A full-scale experimental test of four-point bending was investigated. The FE analysis was conducted based on the ANSYS program. And theoretical formulas were also developed to calculate the ultimate bearing load and deflection. The main conclusions from this study can be summarized as follows:

The ICSIPs are applicable for building floors or roofs with enough structural strength and stiffness in comparison with CSIPs. The ICSIPs can overcome the two shortcomings of CSIPs; one is the large deflection, and the other is the low bearing capacity. It is especially suitable for large-span roofs owing to excellent thermomechanical properties and superior durability except of high strength.

The general mode of failure of ICSIPs was the crush of the top recycled aggregate concrete layer at the midspan.

The nonlinear FE analysis is effective in the engineering design of ICSIPs in engineering practice. And the theoretical predictions reflect well the linear flexural response of the ICSIPs, while deviates as the load increases up to failure due to the theory limitations.

ICSIPs are strong enough to bear heavy loads. From the analysis of stiffness, it is still the requirement of deflection that controls the design of ICSIPs. Therefore, the strength of the recycled aggregate concrete is feasible as the top face sheet in ICSIPs, and it is conducive to save resources and to protect the environment.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conﬂicts of interest.

QL designed and performed the experiments, as well as drafted the manuscript. WD have carried the literature study and participated in experiments design and result discussions. NU provided some advices on the manuscript. ZZ performed the experimental works. All authors read and approved the final manuscript.

The authors would like to express sincere thanks to Yun Sun., Zhihao Du., and Shuailiang Zhang., for supporting the fabrication of test specimens. This research was supported by the National Science Foundation (NSF) (CMMI-0825938) and National Science Foundation of China (NSFC) (U1704141).