Impact behaviour of crushed-brick lightweight RC sandwich slabs

In recent construction practice, sandwich panels have been increasingly adopted due to the fact that they are lightweight, energy efficient, aesthetically attractive, and easy to handle and erect. This study examines the impact behaviour of lightweight reinforced concrete (RC) sandwich slabs produced from local lightweight coarse aggregate. The experimental program included testing two slabs produced with normal coarse aggregate (solid slab + sandwich slab) and six concrete sandwich slabs (CSSs) produced using crushed clay bricks as lightweight coarse aggregate with various shear connecter configurations and types and steel fibre volume fractions (VF%). The impact load was applied at the midpoint of each slab using an 8 kg drop-weight from a height of 2 m. The test results showed CSS behaviour variation with different VF% and the optimum layout and location of shear connectors.


Introduction
Concrete sandwich slab (CSS) construction is an advanced modern construction method involving the use of two reinforced concrete (RC) layers separated by an insulating material such as polystyrene. Many different shear connectors have been used to hold such concrete layers together, though steel truss-shaped shear connectors are generally the most powerful shear connectors. The essential functions of such shear connectors are to transmit shear to ensure that the faces of slabs do not slip over each other when the panel is bent, and to link the concrete layers to enable them to operate as a single unit. Each part of a composite is relatively weak and flexible, though together they offer opportunities to create extremely stiff, sturdy, and lightweight structures. The main advantages of such systems include high bearing capacity, useful insulation, simpler handling due to lightness of weight, and reduced material and labour costs. Other important advantages of these CSSs include low self-weight and high thermoacoustic adequacy, facilitating applications in residential, commercial, and industrial buildings in North America and Europe [1][2][3][4]. The structural behaviours of CSS differ according to the IOP Publishing doi: 10.1088/1757-899X/1067/1/012057 2 stiffness and strength of the shear connectors, while the configuration and spacing of the shear connectors differs according to the loads applied, the extent of the composite action, the length of the shear connectors, and their materials [5]. Currently, there are no specific rules, guidelines or design codes to determine the number or arrangement of connectors required [6]. While the bounding layer between the insulation core and concrete layers has been found to provide shear transfer, this reduces in capacity over time and does not retain the strength of the shear connectors over the panel's lifespan [7][8][9].
Previous research has shown the possibility of using crushed bricks as a structural material based on the resulting mechanical properties and structural behaviours [10].Low density is one of the main properties of crushed bricks, which offer about 40 % lower density than normal coarse aggregate [9]. Panels using crushed brick as coarse lightweight aggregates exhibit better structural performance than normal aggregate panels [11], and lower density also offers advantages in terms of reducing labour cost, reducing construction time, and supporting the philosophy of creating green buildings [12,13].

Experimental Work
The experimental programme in this work included the testing of one solid slab, one normal concrete sandwich specimen, and six lightweight concrete slabs with two layers. All specimens were designed with one-way action and with the same dimensions of 1,100mm total length × 400 mm width × 90 mm thickness. The CSSs consisted of 30 mm RC with a 30 mm thick layer of polystyrene at its core. The two concrete layers were reinforced with steel wire mesh with spacing c / c of 150 mm and a wire diameter of 6 mm. The reinforcement cover was 12 mm deep. The steel truss cage connectors were linked to the two concrete layers with a spacing of 150 mm, and the diameter of the shear connectors' deformed steel bar was 4 mm. These steel bars were bent to form continuous w-shaped zigzags (see Plate 1), with the angle of each bend being 45°. Table 1 shows the description of the slabs and Plate 2 shows details of the reinforcement of the slabs.

Concrete Mix
The structural lightweight aggregate concrete was a concrete with an oven dry density less than 2000 kg/m 3 [15], as shown in Figure 1.

Experimental results
The results recorded for the slabs, including the impact force and corresponding reaction force, midpoint displacement, and crack propagation, are shown in Table 3. Four variables were assessed based on the results of the tests:

Replacement of traditional solid slabs with CSS.
The main benefits of using the sandwich principle include minimisation of the overall weight by about 30.6% and a load reduction of about 33.72% as compared with the solid slab. Figures   2 and 3 show the load-time and deflection-time histories for SS and SN.
Generally, the behaviours of SBC-1% are very similar to those of the SS, as shown in Figure  12.

Cracking patterns
All cracks in the tested slabs exhibited behaviour consistent with the different parameters studied, as Plate 7 shows. SS had only one crack at the bottom, with no cracks appearing at the top, but that was a very large crack width as compared to those in the CSSs. In general, the use of continuous truss shear connectors leads to a better distribution of load, thus increasing the number of cracks, while the addition of steel fibre in different percentages significantly affects both the number of cracks and their widths. With an increase in the steel fibre percentage, the number of cracks and their widths in CSSs decrease significantly, due to the addition of steel fibre making the concrete more homogeneous and isotropic, converting it from a brittle material to a more ductile material. The randomly oriented fibres also arrest the micro cracking mechanism when concrete cracks, limiting the propagation of cracks, and thus improving strength and ductility.

1.
The behaviours under impact load of concrete sandwich slabs produced using crushed clay bricks as lightweight coarse aggregate were similar to those of concrete sandwich slabs produced using normal coarse aggregate. 2. The use of discontinuous truss shear connectors decreased the load slightly, by about 2.5 to 5.3%, in exchange for a reduction in steel reinforcement by about 4.92%. 3. The addition of steel fibres has a positive effect on CSS behaviour, as the sandwich behaviour becomes more similar to the solid behaviour with increases in the percentage of addition. 4. The deflection and the number of cracks in CSSs decrease significantly with increases in the steel fibre percentage. 5. Excellent CSS behaviour was recorded for the SBC-1%, which behaved in a very similar manner to SS, reducing the response load by about 14.6%.