*2.5. Test Methods*

#### 2.5.1. Aggregate Tests

The particle size distribution of artificial LWA was determined. The tests for the physical properties of the artificial LWA were conducted in terms of specific gravity, bulk density, and water absorption in accordance with ASTM 127. The crushing strength test was performed in accordance with BS 812 part 110 by placing the pellet between two parallel plates and applying a direct load until failure, an average of 10 pellets was taken as the crushing strength. Furthermore, mineralogy and microstructural analyses were performed on the manufactured pellets by using scanning electron microscopy (SEM).

2.5.2. Concrete Tests

Compressive strength was determined using cubic samples according to the ASTM C39-18. The splitting tensile strength was calculated using ASTM C496-17 and Equation (1):

$$f\_s = \frac{2P}{nDL} \tag{1}$$

where *fs* = splitting tensile strength (MPa), *P* = maximum load (N), *D* = specimen diameter (mm), and *L* = specimen length (mm).

The fracture energy was established in accordance with RILEM 50-FMC Technical Committee [46]. Figure 2 shows a beam with notches during the test. The notch-to-depth ratio was 0.4. An LVT transducer measured midspan deflection (LVDT). The area under the load–displacement curve was used to compute the fracture energy by using Equation (2):

$$G\_{\overline{F}} = \frac{\mathcal{W}\_0 + mg \frac{S}{\mathcal{U}} \delta\_{\overline{\mathcal{U}}}}{B(\mathcal{W} - a)}\tag{2}$$

where *GF* is the fracture energy (N/mm), *W*<sup>0</sup> is the area under load–displacement curve (N.mm), *m* is the specimen mass (kg), *g* is acceleration of gravity (m/s2), *S* is the distance between support (mm), *U* is the specimen length (mm), *δ<sup>s</sup>* is the specimen deflection (mm), *B* is the specimen width (mm), *W* is the specimen depth (mm), and *a* is the notch depth (mm).

**Figure 2.** A notch beam under fracture energy test.

Also, the notched beams were used to determine the net flexural strength in accordance with ASTM C1609, 2005 by using Equation (3) [47]:

$$f\_{flex} = \frac{3P\_{max}S}{2B\left(W - a\right)^2} \tag{3}$$

where *f flex* is the failure load (MPa) and *Pmax* is the maximum load (N).

Furthermore, to measure the characteristic length to indicate the brittleness of each specimen, Equation (4) was used: [48]

$$d\_{ch} = \frac{EG\_F}{f\_s^2} \tag{4}$$

where *lch* is the characteristic length (mm) and *E* is the static modulus of elasticity (GPa).

RILEM RC6 requires 150 × 150 × 150 mm cube specimens with embedded reinforcement bars with a 16 mm diameter at the center of the sample to determine the bond strength between the steel and the lightweight aggregate [49]; Equation (5) is used to measure the bond strength:

$$
\pi = \frac{P}{n d\_b L\_b} \tag{5}
$$

where *τ* is bond strength (MPa), *db* is the diameter of the embedded steel bar (16 mm), and *Lb* is the embedded length of the steel bar (150 mm).

#### **3. Results and Discussion**

#### *3.1. Results of Aggregate Tests*

The particle size distribution of artificial LWA is shown in Figure 3. The physical and mechanical properties are displayed in Table 3. It is noted that the GGBFSA recorded the highest crush strength value and minimum water absorption rate. The microstructure for each type of artificial LWA aggregate is shown in Figure 4. The minor differences between the artificial LWAs are that the QPA has sharp corners and the presence of voids in FAA is less than in GGBFSA and QPA. The FAA particles exhibited a denser surface structure when compared to the other types of aggregate. The core of the aggregate particles showed a porous structure that gives the lighter weight and the high water absorption, leading to a reduction in the strength and stiffness of artificial aggregate particles.

**Figure 3.** Particle size distribution of artificial LWAs.

**Figure 4.** The microstructure of artificial LWAs (**a**) FAA, (**b**) GGBFSA, and (**c**) QPA.

#### *3.2. Results of Concrete Tests*

The results of compression, splitting tensile, flexure, bonding, and modulus of elasticity tests are discussed in detail below. Furthermore, the photographic views and graphical illustrations were added for a clear and detailed discussion and evaluation.

#### 3.2.1. Compressive Strength

The average compressive strength results of the cube samples which were treated for 28 days are illustrated in Figure 5.

**Figure 5.** The compressive strength of LWA concretes.

All of the mixtures generated similar results when the pressures ranged between 40 MPa and 55 MPa, compared with the natural normal-weight concrete (NWC). According to these results, the artificial aggregate used up to 60% of the traditional coarse aggregate and may still offer appropriate compressive strength. The experimental results indicate that artificial LWA concrete requires a lower water-to-cement ratio than regular aggregate concrete. A higher cement percentage is needed to compensate for the artificial aggregate's increased porosity and softness and decreased hardness [50].

The maximum compressive strength was attained at a 40% volume replacement ratio for the mixture comprising quartz artificial aggregate in this experiment. Although the lowest value was obtained by replacing 60% of the natural aggregate with GGBFS artificial aggregate, it was still 1% higher than for the regular aggregate. However, when the artificial aggregate ratios were increased, compressive strength was slightly reduced. There is a reasonable amount of strength in the concrete formed using artificial aggregates, although it is not as strong as traditional concrete. According to the previous research, adding sintered FA LWA decreases the compressive strength by 10% to 13%. The results of this experiment confirm those of Nadesan [45].

#### 3.2.2. Splitting Tensile Strength

A splitting tensile test assesses the concrete's tensile strength. It is a method through which a fracture occurs due to applied stress attempting to locate weaker paths. The average tensile strength was calculated using three 100 \* 200 mm cylindrical samples. In this study, the tensile strength of the concrete with artificial LWA is equivalent to that of sound concrete, as shown in Table 2 and Figure 6. It is estimated that the performance of lightweight concrete created using artificial aggregate is comparable to that of control concrete prepared with ordinary coarse aggregates. Artificial aggregate has a reduced strength, round shape, and poor anchoring potential with cured cement paste, which are the causes of limited improvement for the splitting tensile strength. For these reasons and because of artificial aggregates' moderate tensile strength, artificial LWA could not enhance the tensile strengths to levels higher than for ordinary concrete. Tensile strength is dependent on the shear of the interfaces zone between the cement paste and aggregate particles and this can be improved by the curing time and condition, for example, steam curing can improve the tensile strength more than water curing. Concrete is used for several purposes; however, the splitting tenacity results of concrete, including artificial aggregate, were satisfactory.

**Figure 6.** The splitting tensile strength of lightweight aggregate concretes.
