Keywords

1 Introduction

Sandwich sections consisting of two stiff panels connected by mechanical headed studs and separated by a lightweight material [1], are being used widely in many applications. Most commonly used are Steel–concrete-steel sections. SCS sandwich structures exhibit excellent performance, with high integrity, impact resistance, crack control and leakage prevention, which offer the advantages of shortened construction time, saving formworks and promoting construction efficiency [1, 2]. Apart from serving as shielding structures for nuclear power plants, popular applications of the SCS sandwich structures include tunnels, bridges, coastal, and offshore applications [3].

However, high density steel is not ideal for offshore applications due to lack of corrosion resistance and strength-to-weight ratio [4]. An alternative known as Glass Fibre-Reinforced Polymer, GFRP, is being researched for its promising potential. GFRP is a flexible, corrosion-free material having adequate strength and stiffness compared to steel. “GFRPs are preferred over steel for their small dimensions, low weight, high strength and superior durability properties” [5].

Engineered Cementitious Composite (ECC), also called Strain Hardening Cement-based Composites (SHCC) or more popularly as bendable concrete, is an easily moulded mortar-based composite reinforced with fibres [6]. Engineered Cementitious Composites (ECC) with high strength and tensile stain hardening behaviour prior to crushing failure will be incorporated in the sandwich composite systems. The term strain hardening refers to the fact that after first cracking, the material will continue to gain strength with increasing strain [7]. Besides, high mechanical properties [8, 9], ECC also exhibits an excellent durability characteristic [10].

This research proposes to study the incorporating GFRP composites as facing plates in order to enhance the ultimate strength and deformation capacity in the sandwich composite systems of ECC core.

2 Experimental Program

Three 400 mm × 1500 mm slabs were fabricated as GFRP-ECC sandwich sections in this study. Engineered Cementitious Composites (ECC) is incorporated as core between facing plates. The configurations of the slabs are the slabs with one face GFRP plate at the bottom, and with two faces of GFRP plates at top and bottom. The geometry and configurations of the slabs are shown in Table 1, and Fig. 1. Moreover, the mixture design of ECC and characteristics of GFRP plates used in this study are shown in Tables 2 and 3, respectively. Bolted shear connectors of normal mild steel was used in this study as shown in Fig. 2, The bolts have a diameter of 8 mm with a longitudinal spacing of 100 mm and transverse spacing of 200 mm. Three-point bending tests were carried out on the specimens using the setup illustrated in Fig. 3.

Table 1 Specimen dimensions
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Fig. 1
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2-face composite section GFRP-ECC-GFRP

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Table 2 ECC mixture design
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Table 3 Characteristics of GFRP plates according to manufacturer
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Fig. 2
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Preparation of slabs

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Fig. 3
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Test set-up

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3 Results and Discussion

3.1 Mechanical Properties of ECC Core

The results of compressive and tensile tests for the three slabs are shown in Table 4. As stated in Table 4, the range of compressive strength for ECC used in the three slabs is between 48 and 53 MPa, the tensile strength of specimens are between 3.2 and 3.6 MPa.

Table 4 The results of compressive and tensile tests for ECC patches
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3.2 Slabs

Figure 4 shows the ultimate loads and ultimate displacements for the tested specimens. It is clear that slab 1 and slab 3 exhibited the same behaviour although slab 1 did not have a skin plate in compression zone while slab 2 had. This might be explained as the GFRP plate did not contribute to the compressive strength of the slabs. Another possibility for this similarity is the weak composite action contributed by the shear connectors due to the large spacing to the depth used for theses slabs. Crack development for slab 1 and slab 3, shown in Tables 5 and 7, support the weak composite action hypothesis as the initial cracks at early loading stages developed between the bottom skin plate and the concrete core. In addition, excessive slippage occurred at the final loading stages, which means that early debonding of tension plate occurred leading to the deterioration of section.

Fig. 4
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Load–displacement of slabs

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Table 5 Crack development with the increase of loads for slab 1
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As shown in Fig. 4 and Table 6, slab 2 exhibited a larger capacity with a better composite action due to the low shear connectors spacing to depth ratio. The initial cracks started to occur late at 20 kN between the bottom skin plate and the concrete core. The load-deformation relationship showed elastic behaviour of the specimens until the load value of 30 kN.

Table 6 Crack development with the increase of loads for slab 2
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Table 7 Crack development with the increase of loads for slab 3
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4 Conclusions

Sandwich composites have been used widely in the industry for its advantages. The use of Steel plates is the most common plate material implemented in those composites. However, Steel is unsustainable and requires intensive maintenance. Glass Fibre Reinforced Polymer is a better alternative in some cases such as offshore applications. Its considered advantageous due to its corrosion free nature. In this paper, two 1-face and one 2-face slabs have been constructed to investigate the influence of parameters such as span-to-depth ratio, mechanical connectors, and the effectiveness of multiple plates.

Load versus deflection responses of the slabs shows that the thicker the core concrete, the more load can be resisted. However, on the contrary, deflection decreases. Although the high-performance concrete used was relatively low in strength (to compensate for low strength GFRP plates), it showed tremendous ductility in all slabs which is considered an advantage in failure cases.

The use of GFRP plate in the compression section influenced little to no difference in the failure behavior. GFRP is not suitable to be used in compression side.

The ratio between the longitudinal spacing of shear connectors and the effective depth of section is very crucial to the behaviour of the sections; the increase of this ratio leads to the decrease of the degree of the composite action and accordingly the capacity of the section.