مطالعه آزمایشگاهی اثر اندازه ی دانه های پلی‌استایرن منبسط شده بر مقاومت چندمحوری بتن‌ سبک

Concrete as the most widely used building material suffers from the inherent weakness of having high density which results in heavy structures and consequently large inertia forces during seismic excitations. A common practice to reduce the density of normal-weight concrete is to partially replace sand and gravel with natural or synthetic light-weight particles. Among them, ultra-lightweight non-absorbent closed-cell expanded polystyrene (EPS) beads can be effectively used to produce a wide range of light-weight structural and non-structural concretes with appealing physical and mechanical properties. Many empirical researches have been already conducted to reveal the key properties of this class of material such as the tensile and compressive strengths, sound and heat isolations, and durability. New findings show that, in addition to the volume content of EPS beads, the bead size can significantly alter the strengths of concrete samples. To be more precise, the bigger the size of EPS beads, the lower the strength of concrete at a constant EPS volume content. Moreover, the existing studies have shown that this size effect fades at higher EPS contents. An overview of the literature reveals that the effect of EPS bead size and volume content have not been addressed under triaxial loading conditions. Therefore, in this study, twelve different concrete mixes that differ in terms of EPS bead diameter and volume content have been prepared and tested under uniaxial compression, splitting tension, and triaxial loading conditions. Three different EPS sizes (2.25 mm, 2.75 mm, and 3.25 mm), and four EPS volume contents of 0% (witness samples), 5%, 10% and 20% are considered. The constituent materials of tested concrete samples are water, cement, river sand, superplasticizer, and EPS beads. The ratio of sand and water to cement is 2 and 0.55, respectively. The BCB23 superplasticizer is used whose weight content is 0.6% of that of the cement. Samples are all cylinders with 5 cm in diameter and 10 cm in height and are treated in water for 28 days. The results show that increasing the volume of EPS beads would reduce the compressive and tensile strengths of concrete. Moreover, the results of uniaxial compressive tests show clear dependency to the size of EPS beads which is consistent with the results reported by other researchers. Splitting tensile test results are found different as the classical splitting mode of failure has changed to crushing mode beneath the narrow linear loading region for higher EPS contents. This phenomenon cancelled the validity of splitting test for the extraction of uniaxial tensile strength as the hypotheses of the elasticity solution for the Brazilian splitting test is not valid anymore. The failure points at the compression meridian of failure surface have been measured from the triaxial tests. The results confirm that confined strength of EPS concrete is also depends on the size of EPS beads, yet this dependency fades at higher confining stresses. This observation can be interpreted by the change of failure mechanism from local discrete cracking mode to distributed crushing mode which is also the reason behind the fade of size dependency in uniaxial compressive test at higher EPS volume contents.