THE EFFECTS OF ADDITIVES TO LIGHTWEIGHT AGGREGATE ON THE MECHANICAL PROPERTIES OF STRUCTURAL LIGHTWEIGHT AGGREGATE CONCRETE

In the paper, the effects of different percentages of additives (perlite, LECA, pumice) on the mechanical properties of structural lightweight aggregate concrete were tested and evaluated. For the research, 14 mixing designs with different amounts of aggregate, water, and cement were made. Experimental results showed that the specific gravity of lightweight structural concrete made from a mixture of LECA, pumice, and perlite aggregates could be 25-30% lighter than conventional concrete. Lightweight structural concrete with a standard specific gravity can be achieved by using a combination of light LECA with perlite lightweight aggregates (LA) and pumice with perlite in concrete. The results indicated that LECA lightweight aggregates show more effective behavior in the concrete sample. Also, the amount of cement had a direct effect on increasing the strength regardless of the composition of LAs. The amount of cement causes compressive strength to increase. Furthermore, the stability of different experimental models increased from 156 to 345 3 kg m while increasing the amount of cement from 300 to 400 3 kg m in the mixing designs of LECA and perlite for W/C ratios of 0.3, 0.35, and 0.4. For a fixed amount of cement equal to 300 kg, the compressive strength is reduced by 4% by changing the water to cement ratio from 0.5 to 0.4. The compression ratios of strength for 7 to 28 days obtained in this study for lightweight concrete were between 0.67-0.8. Based on the rate of tensile strength to compressive strength of ordinary concretes, which is approximately 10, this ratio is about 13.5 to-17.8 in selected and optimal lightweight concretes in this research, which can be considered good indirect tensile strength for structural lightweight concretes. bond in CFST filled with lightweight and conventional concrete.


INTRODUCTION
In recent years, many experts have studied the durability of reinforced concrete structures, especially in corrosive areas, and most concluded that strength alone cannot meet all the required properties of concrete, especially its durability. In designing concrete for different applications, its reliability and durability should be considered in addition to the strength and load-bearing requirements during its operation. The use of lightweight natural and artificial materials is considered an effective solution for reducing the dimensions of the structure and minimizing the seismic force acting on the concrete, ultimately increasing the speed of implementation and reducing costs. Khaloo (1994) used industrial waste as coarse aggregate in concrete; three types of clinker bricks were crushed from 13 clinker brick manufacturers and then tested for grading, unit weight, water absorption, and abrasion resistance. The results were compared with those of ordinary crushed stone materials. Their research included experiments on concrete cylinders under uniaxial compression and splitting tension, and concrete beams under ductility [9]. Aydın and Baradan (2007) concluded that thermal stability is an essential factor in selecting materials for high heat, and the lightweight concrete should have low thermal conductivity for increased resistance against fire. They concluded that an important feature of lightweight structural concrete compared to normal concrete is the fire resistance factor [4]. Khaloo et al. (2008) evaluated the possibilities of using particles and flexible rubber as aggregates in concrete. The width of a crack and its progression rate in rubber concrete is less than in standard concrete [10]. Zaetang et al. (2013) examined light and heavy recycled concrete and reported that recycled concrete has less compressive strength [12]. Bogas et al. (2013) evaluated the compressive strength of structural-lightweight-concrete using non-destructive ultrasonic pulse velocity [6]. Approximately 84 different compounds were tested, and results indicated that normal and lightweight concretes are affected by mixture parameters. Some studies have also shown that lightweight structural concrete can be used for construction purposes as insulation, Nguyen et al. (2014) [13]. The experimental results indicated that adding steel fibers greatly improves the flexural-strength, tensile-strength, and stroke resistance of concrete models, and adding steel fibers has little effect on the compressive strength, Huang et al.  [11]. Yu et al. (2015) The available studies on lightweight concrete indicate significant variations in mechanical and thermal properties under the influence of ingredients and mixing ratios [15]. Bogas and Gomes (2015) evaluated the mechanical behavior and long-term durability of structural lightweight concrete produced with natural scoria aggregate. They concluded that long-term contraction increased by replacing the total normal weight with LWA, which was higher when they used coarse and fine scoria. The LWC with scoria can be stable and medium-resistance LWCs carbonationinduced corrosion may not be associated [5]. Shafigh  (2019) examined the specification of lightweight concrete to shell aggregate in the concrete core. Lightweight structural shell aggregates (CSAs) were produced through cold bonding by encapsulating an expanded perlite particle (as a nuclear structure) into a shell matrix consisting of cement, fly ash, and developed perlite powder. The effect of different sintering regimes on the mechanical and microstructural properties of the CSA was studied. The properties of lightweight concrete made of CSA or expanded clay aggregate (ECA) were closely compared in terms of their potential economic and environmental benefits in response to the high energy consumption associated with ECA production. The results showed that cooking at 99% relative humidity is the most suitable cooking method for CSA [20]. Regin  used, it was found that nano-silica after 28 days of curing has little or no effect on concrete shrinkage [25]. Ibrahim et al. (2020) investigated the durability of lightweight structural concrete containing expanded perlite aggregate. Their results showed that the unit weight of concrete is reduced by 20 to 30% compared to concrete with normal weight. The compressive strength of the samples tested showed that they are sufficient for use as structural concrete, especially mixtures containing 10% and 15% perlite aggregates. The durability of LWC is comparable to NWC in terms of the chloride emission and concrete corrosion resistance of reinforcing steel [24]. Salim (2020) studied the effect of fracture of artificial materials and the permeability of the concrete illustrates how the use of volcanic ash aggregates and synthetic slag materials affects the functional properties of concrete [14]. Oghabi and Khoshvatan (2020) studied the effect of the amount and length of plastic fibers on the compressive and tensile strength of SCC. They examined 13 concrete mixing designs with a w/c ratio of 0.4. They selected 1 fiber-free design as a reference model and made 12 self-compacting concrete designs containing plastic fibers with dimensions of 1, 2, and 3 cm and 100, 250, 500, and 1000 3 gr m in the laboratory. Based on the experimental results, increasing the amount and length of the fibers leads to a decrease in flowability and permeability and a sign cant, increasing the splitting strength in the model. The compressive strength increased by about 8 percent and the tensile strength by about 22 percent after adding plastic fibers, with this increase depending more on the number of fibers, whereas changing the length of the fibers did not have much effect [28]. Yang (2021) studied the shear friction response of lightweight concrete using floor ash aggregates and air foams. Experimental and analytical results showed that the addition of air foam slightly reduces the friction angle of integrated interfaces but has little effect on smooth structural joints. Therefore, the effect of air foam up to 20% by volume on the cohesion and friction angle of concrete is marginal [29]. Natalli et al. (2021) introduced a new method for the analysis of steel-concrete bonds in CFST filled with light and ordinary concrete. The results showed that the low modulus of elasticity of lightweight concrete helps to increase the trapping and microblock effect, and the extensive additive improves the adhesion performance of the filler core. Also, the new method communicates well with other tests and is easy to use [30].

Material
The aggregates used in this research consisted of sand passed through a 12.5 mm sieve and residue on a 0.5 mm sieve. The density of aggregates according to the experiments was 2560 3 kg m . Table 1 shows the chemical analysis of cement type and comparison with the existing standard. Table 2 shows the chemical analysis of micro-silica used in the present study. In this research, in order to achieve a higher amount of solidity, a mixture of two volumes of fine-grained perlite and a volume of coarse-grained perlite and LECA was used in the concrete mixing design. The density of used perlite varied according to experiments in a range of 32-400 3 kg m . The coarse-grained perlite was of a 3-5 mm diameter and fine-grained perlite, 1-3 mm diameter. 5% rock powder was used to reduce air bubbles in the concrete samples. Figure 1 shows the rock powder used in the present study. Figure 2 shows the LECA, pumice, and perlite used in this study.  Table 3. gives the mixing designs of perlite, LECA, and mineral pumice. The water to cement ratios were considered as 0.3, 0.35, and 0.4. The amount of perlite and LECA used in the concrete models varied according to the percentage. The weight of perlite and LECA varied from 244 to 368, and cement amounts of 350-400 kg were used in the models.

RESULTS
In this study, 14 different mixing designs with reference models and samples with LECA and perlite light aggregates were tested and evaluated. The behavioral results of different samples with light aggregates are given below.

The results of compressive strength and indirect tensile strength
Hardened concrete tests include compressive and tensile strength tests and tests of elastic modulus [1]. Table 4. gives the results of the elastic modulus test of perlite and LECA mixtures.   Figure 3. shows the deformation of other loaded models.   [2][3][4][5][6][7][8]. The results of compressive strength of the optimal and selected designs indicated that the requirements of ASTM C 330 for the construction of lightweight structural concretes were met in all lightweight concrete mixing designs [3]. Based on the obtained results, the ratio of compressive strength of 7-days to 28-days lightweight concretes was between 0.67 and 0.81. Considering that the ratio of tensile strength to compressive strength of ordinary concretes is about 10, this ratio in the selected and optimal lightweight concretes in this study was 13.55-17.79, which is related to the good indirect tensile strength of structural lightweight concretes. Figure 4. illustrates the results of compressive strength tests for 1 to 7 mixing designs.

The effect of la on the specific gravity of concrete
The results related to the impact of LA aggregate on the specific gravity of concrete are given in this section. Figure 9. shows the effect of specific gravity on the compressive strength of models with a mix of LECA and perlite and models with a mix of perlite and pumice. The compressive strength increased by increasing the specific gravity of the models for those with a mix of LECA and perlite. The lowest compressive strength was obtained for models with lightweight perlite and pumice with a specific gravity of 1450. The highest compressive strength was obtained for models with lightweight LECA and perlite aggregates and a specific gravity of about 1750. As shown, the positive and direct effect of increasing specific gravity on increasing compressive strength was confirmed.

The effect of la on the tensile strength of concrete
The increase of indirect tensile strength in the selected lightweight concretes is shown in Figure 10. The results indicated that the compressive strength increases by increasing specific gravity. In addition, tensile strength increased due to its relationship with compressive strength. Figure 10 displays the relationship between indirect tensile strength and the 28-day compressive strength of concrete.   Figure 13. shows an increase in compressive strength of lightweight concrete made for both groups comparatively. As shown, the compressive strength increases in approximately equal proportions through increasing the amount of cement in all designs. Figure 13. displays the effect of increasing the amount of cement on the process of increasing the compressive strength of group one concretes.   Figure 14. shows the increasing trend of compressive strength with different mixtures of perlite + LECA and perlite + pumice, with a water to cement ratio of 0.3.

3.7
The effect of w/c ratio on the compressive strength of lightweight concrete Figure 13. shows the compressive strength for a mixture of 40% LECA and 15% perlite in different cement contents. The compressive strength decreases when the ratio of water to cement increases. As observed, this reduction in strength is greater in concretes with a mixture of pumice and perlite aggregates than in LECA and perlite aggregates, which can be related to the absorption of more water pumice LAs. The effect of the water to cement ratio on the compressive strength of concrete with LA for the mixture of 40% LECA and 15% perlite in the water to cement ratio of 0.  kg cm . It is, therefore, apparent that the compressive strength is reduced by 4% for a fixed amount of cement (300 kg) while changing the water to cement ratio from 0.5 to 0.4. Figure 15. shows the compressive strength in a mixture of 40% pumice and 15% perlite in different cement grades.

The effect of la on the shrinkage percentage in structural lightweight concrete
If the specific gravity of normal concrete is considered to be 2400 3 kg m , the shrinkage percentage of specific gravity for the constructed lightweight concrete is shown in Figure 16. aggregates was more than for LECA and perlite aggregates. The highest compressive strength in lightweight concrete was for 40% LECA and 15% perlite in a water to cement ratio of 0.3 and 400