THE RELATIONSHIPS BETWEEN COMPRESSIVE STRENGTH AND DENSITY OF POLYSTYRENE LIGHTWEIGHT CONCRETE AND THEIR COMPONENT RATIOS

The research deals with the compressive strength and density of lightweight concrete, made from polystyrene, cement, sand and water. This type of lightweight concrete is characterized by good thermal insulation, low density and good compressive strength and it is used as a lining material in inclination surfaces and in the production of unloaded building units and elements. The aim of this paper is to suggest mathematical relationships between compressive strength, density and mix proportions. An intensive experimental program has been implemented for this purpose. The investigated variables were the mix proportions, namely, polystyrene to cement ratio (P/C), sand to cement ratio (S/C) and water to cement ratio (W/C). Sixty mixes were produced and tested using five polystyrene to cement ratios (P/C), 0.02, 0.03, 0.04, 0.05, 0.06 four sand to cement ratios (S/C), 2.5, 3, 3.5, 4 , and three water to cement ratios (W/C), 0.35, 0.40, 0.45. From the test results, mathematical relationships were suggested that can be of use in determining mix proportions of polystyrene lightweight concrete based on the required compressive strength or based on the required dry density.


INTRODUCTION
Lightweight concrete is considered one of the alternatives that has a great importance in modern buildings. It is one of the most prominent developments in concrete technology and has a wide use in the construction works because of its characteristics that traditional concrete does not provide. It is more economical because of its low density, ranging between (240-1840) kg/m 3 [1] and [2], reducing dead loads and resulting in smaller structural elements. In addition, polystyrene is available locally with low cost, and its waste can be used in abundance, which contributes to reducing environmental pollution. Also, it provides a good thermal insulation, saving the energy needed for cooling and heating, especially, the polystyrene is classified as first-class thermal insulation material. Kohling [3] conducted a research and showed the possibility of using polystyrene as aggregate to manufacture concrete with high thermal insulation. Maura [4] found that the compression strength of polystyrene concrete with densities ranging between (220-460) kg/m 3 is in the range of (0.7-2.3) Mpa, while the modulus of rupture ranges between Mpa (0.3-0.36). Parton and barbary [5] observed that the properties of polystyrene concrete such as density, compressive strength, modulus of rupture, and thermal insulation mainly depend on the Polystyrene to cement ratio. Sabaa and Ravindrarajah [6] carried out the experimental program using the expanded polystyrene waste in a granular form as lightweight aggregate to produce lightweight structural concrete with the unit weight varying from 1600 to 2000 kg/m 3 . The coarse aggregate replacements levels used were 30, 50 and 70%. The results showed that drying shrinkage and creep of polystyrene aggregate concrete increased, whereas compressive strength and modulus of elasticity decreased with decrease in the density of concrete. The compressive strength was found to be more sensitive to the density than the modulus of elasticity. The ultimate creep coefficient for polystyrene aggregate concrete in the density range of 1600 to 2000kg/m 3 was between 1.90 and 2.37 for the concrete with the cement content of 410 kg/m 3 . Kuhail and Shihada [7] suggested a recommended way in preparing lightweight polystyrene concrete mixes and proportions for producing homogeneous mixes with compressive strengths ranging between 17 MPa to 25 MPa and densities ranging from 1650 kg/m 3 to 1830 kg/m 3 . Kan and Demirbogab [8] observed that, with the increase of cement to EPS beads ratio in the mixes, both density and compressive strength of concretes increased up to 4 and 350 times respectively, when compared to the control sample. Hardened concrete densities and compressive strength varied from 464 to 1940 kg/m 3 , and 0.11 to 38.50 MPa, respectively. Slump values of the mixes were changed from 8 to 43 mm. Herki et al. [9] used waste polystyrene as a lightweight aggregate (LWA) to produce lightweight aggregate concrete (LWAC). In this study the WP was shredded to different sizes. Four concrete mixtures in total with varying WP content ratios of 0, 20, 40 and 60% as partial replacement of natural coarse aggregate by equivalent volume was prepared. The water/cement (W/C) ratio of 0.5 was kept constant for all concrete mixtures. Mechanical and durability properties including compressive strength, water absorption (WA) and density was determined for the various concrete mixtures at the ages of 7 and 28 days of T water curing. According to the results obtained, the utilization of WP in concrete production is possible. Abd et al. [10] carried out an investigation to study the properties, such as compressive strength and tensile strengths of light weight concrete containing Expanded Polystyrene (EPS) beads. The results showed that the amount of polystyrene beads incorporated in concrete influences the properties of hardened concrete. Since the compressive strength less than a certain percentage depending on the amount of replacement. At 28 days, it was found that compressive strength of 5%, 15%, 20%, EPS based concretes compared to control concrete were 41%, 38 %, 25%, respectively. Minh and Phuong et al [11] investigated the effect of properties and volume of the matrix on the properties of lightweight concrete. The results show that properties of structural polystyrene concrete, such as workability and compressive strength, depend on the aggregate size of the matrix. It also shows that decreasing aggregate size of the matrix is the effective way to increase workability and compressive strength of lightweight concrete. Bedekovic et al. [12] showed based on their test results on waste expanded polystyrene lightweight concrete a dependence of the component ratio on the bulk density ranging from 360 kg/m 3 to 915 kg/m 3 and compressive strength ranging from 0.385 MPa to 2.538 MPa. From the previous studies, it is clear that the researches dealing with the mix design of polystyrene lightweight concrete are very limited. Kuhail and Shihada [7] suggested a procedure for mix design based on the required compressive strength and density. This paper represents a contribution in this field by suggesting number of mathematical relationships between compressive strength and density of polystyrene lightweight concrete and their mix proportions namely, sand to cement ratio (S/C), water to cement ratio (W/C) and polystyrene to cement ratio (P/C), which can be of use to suggest the mix proportions based on the required compressive strength or based on the required dry density. Ubi, Stanley et al. [13] carried an experimental program to develop a mathematical model that will optimize the compressive strength of polystyrene lightweight concrete. The materials used for this study were, ordinary Portland cement, Water, Sand, coarse aggregate and Expanded Polystyrene beads. The study adopted the Scheffe's simplex lattice design for both pseudo and component proportion models to generate their respective mixes. The study showed that using the Scheffe's Pseudo component model, an optimized compressive strength value of 27.920 N/mm 2 can be obtained from a water, cement, sand and coarse aggregate (at 12% partial replacement with polystyrene aggregates) mix ratio of 0.455, 1, 1.820 and 2.980 respectively

Mix proportions
An experimental program was implemented including several laboratory tests on compressive strength and dry density of lightweight concrete using sand and polystyrene as aggregate. The investigated variables were the mix ratios of their components. These mixes were accomplished by using three water to cement ratios (W/C) of 0.35, 0.40 and 0.45, five polystyrene to cement ratios (P/C), 0.02, 0.03, 0.04, 0.05, and 0.06, four sand to cement ratios, 2.5, 3, 3.5 and 4. As a result, sixty mixes were produced and tested. Material weights were calculated per cubic meter using the volumetric equation to be as shown in Table 1

Materials
Cement: It is a Portland cement per specification ASTM C 150 [14]. Sand: It is the locally available sand from Zliten area -Libya. The sieve analysis was carried out and the results showed that it was within the permissible limits according to BS882: 1992 [15] as shown in Table 2. Polystyrene: It is a lightweight material that is easy to carry and transport with a white color in the form of small beads with a density ranging between (10-27) kg/m 3 . The density of polystyrene used was 22.4 kg/m 3 and Fig.1 shows a sample from polystyrene beads used. Mixing and curing water: Drinking water was used free of organic matter, salts and impurities.

Mixing, Casting and testing of concrete specimens
Sand and cement were placed in the mixing bowl, and they were mixed manually in dry condition until the mix became homogeneous. Then the polystyrene was added with continuous mixing, and with gradual addition of water. The mixing was done manually to ensure that the polystyrene beads do not fly out. For compressive strength and dry density tests, (100 x 100 x 100) mm cubes were used. After completion of concrete pouring in the cubes as shown in Fig 2,

RESULTS AND DISCUSSION
Test results of both compressive strength and dry density for specimens cast with different ratios and after the curing period 28 days were determined and are presented in Table 3 as well as in Figures 5 to 8.

Compressive strength
Figs 5 and 6 represent the relationship between compressive strength and polystyrene content for different (S/C) ratio, and (W/C) ratio. The compressive strength decreases as the (P/C) ratio increases. For all mixes the maximum decrease occurred when the (P/C) ratio was increased from 0.02 to 0.03, with average reduction of 37.55%. While, when the (P/C) ratio was increased from 0.03 to 0.04, from 0.04 to 0.05 and from 0.05 to 0.06, the average reduction was 21.21%, 14.59% and 8.19%, respectively. Also it was found that compressive strength is more sensitive to the polystyrene content than dry density as the reduction in compressive strength reached about 82% in comparison with 36% for dry density when the (P/C) ratio was increased from 0.02 to 0.06. This may be attributed to the weak bond between polystyrene beads and cement paste, and the weakness of the bead themselves.
Compressive strength decreases as the (S/C) ratio increases for specified (W/C) and (P/C) ratios in the mixture. As an example, for (W/C) ratio equal to 0.35, the increase of (S/C) ratio from 2.5 to 3 resulted in reductions of 28.05%, 16.34%, 10.37%, 23.17%, and 6.78% for (P/C) ratios 0.02, 0.03, 0.04, 0.05 and 0.06, respectively, and the increase of (S/A) ratio from 3 to 3.5 caused reductions of -7.03% (increasing), 12.07%, 19.00%, 17.46%, and 23.63% for (P/C) ratios 0.02, 0.03 and 0.04 0.05 and 0.06, respectively, while the change of (S/C) ratio from 3.5 to 4, resulted in reductions of 61.19%, 42.48%, 37.76%, -3.80%, and 21.43% for (P/C) ratios 0.02, 0.03, 0.04 0.05 and 0.06, respectively. A considerable number of mixes showed an increase in the compressive strength when the (W/C) ratio was increased in addition the obtained maximum measured value of was at (W/C) ratio equal to 0.45. This may be attributed to the good compaction achieved in this mixture.  Figure 7 and Table 3 show the expected increasing and decreasing trends for dry density with the increase of (S/C) ratio and the decrease of (P/C) ratio in the mixture, respectively. The density of the concrete depends upon mix proportions, compaction and curing conditions. For the tested mixtures, the results show that dry density values are ranging from 777.0 kg/m 3 to 1553.3 kg/m 3 and the reduction values in the dry density due the variation of mix proportions are ranging from 7.62% to 46.18%.
Polystyrene to cement ratio (P/C) Density (kg/m3)  Fig. 8 shows the correlation between dry density and polystyrene to cement ratio for mixtures with different sand and water contents. A Considerably high relationships were obtained between dry densities and (P/C) ratio for poly-styrene lightweight concrete as the regression analysis provided an average correlation coefficient (R 2 ) of 0.9401. So, they can be used to determine the proportion of the mixture components based on the required dry density.
Density (kg/m3) Polystyrene to cement ratio (P/C)  Compressive strength is more sensitive to the polystyrene content than dry density as the reduction in compressive strength reached about 82% in comparison with 36% for dry density when the (P/C) ratio was increased from 0.02 to 0.06. 3. Some mixtures showed an increase in the compressive strength as the (W/C) ratio was increased and this may be attributed to the good compaction achieved in these mixes. 4. Considerably high relationships were obtained between compressive strength and (P/C) ratio as the regression analysis provided an average correlation coefficient (R 2 ) of 0.9751, so, they can be of use in determining mix proportions of polystyrene lightweight concrete based on the required compressive strength. 5. Dry density increases with the increase of (S/C) ratio and decreases of (P/C) ratio in the mixture. A consideable decrease in the dry density values was observed as the (P/C) ratio in the mixture was increased from 0.02 to 0.06. 6. A Considerably high relationships were obtained between dry densities and (P/C) ratio of the polystyrene lightweight concrete as the regression analysis provided an average correlation coefficient (R 2 ) of 0.9401, so, they can be of use to determine the proportion of the mixtures based on the required dry density.