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Over the course of time into the 21th century, concrete has been known as one of the most high usage materials in the construction industry. As a consequence, trying to produce light concrete is an active and developing area within the new field of construction science. This technology consists of lowering the whole weight of structure by using new bulding techniques, new materials and optimizing ways of manufacture. Lowering the weight not only economizes on the expenses, time and energy but also decreases the damages of earthquakes. Furthermore, it keeps the constructions safe and minimizes the damages resulting from the overweight of the structure during different waves of shocks and aftershocks. In spite of considerable amount of compressive strength, low tensility strength and relatively high fragility of the concrete, there are limitations in using it in some parts which are partially or fully under forces of tention in different parts of structures. This fundamental defect of concrete in practice can be eliminated by reinforcing it through using steel tabs in the direction of traction forces. Having in mind that the armature just constitues a small part of the whole cross section of the structure, it will not be correct to conceive of the cross section of concrete as an isotropic and homogeneous surface. In recent decades, in order to come up with the isotopic condition and decrease the fragility, weakness and retrogression of concrete new techniques and trends of applying slender fibers running through the internal section of the bulk of concrete has become prevalent and common practice. The concrete containing nano materials compared with the normal concrete affected by nono chemical materials with cement particles and clcium hydroxide crystals which exist in cement, has a severe effect on the performance of concrete composites while such mixtures come into each others’ contact.
In this study,we examined the effect of Nano-silica and polypropylene fibres on mechanical properties and durability of normal and light weight concretes. In the design of light weight concrete, lecalight weight aggregates were used. More than 384 cubic and cylindrical samples were made based on ASTM standards and compressive strenght, indirect tensile strength, ultrasonic and electrical resistance experiments were done.
The results of the experiments showed considerable increase in mechanical characteristics and durability of normal and light weight concretes. Nano-silica contributes to the proper spread of the fibers. Compressive strength, indirect tensile strength, and the dynamic elasticity module of the ordinary concrete were higher than those of the light weight concrete, while the electrical resistance of the light weight concrete was higher compared to the corresponding samples.
Compressive strength and indirect tensile strength increased to 71 and 55 percent in normal concrete and to 43 and 47 percent in light weight concrete respectively. Considerable increase in electrical resistance indicates high durability of these kinds of concretes. Of course, economic considerations of using nano-silica and polypropylene fibers require special attention. Finally, the right amount of utilization of the polypropylene fibers and nano silica were determined in order to achieve normal concrete and light weight concrete with optimal properties.

Keywords: Nano-silica, polypropylene fibers, mechanical properties, durability, light weight concrete

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