Effect of Dune Sand as Sand Replacement on the Mechanical Properties of the Hybrid Fiber Reinforced Concrete

This paper presents an experimental work to study the effect of dune sand as an alternative to sand on the mechanical properties of hybrid fiber reinforced concrete. 24 concrete mixes were prepared, 6 of them without fibers and containing different proportions of dune sand, and 18 concrete mixes with the same proportions of dune sand but containing long and short single and hybrid steel fibers. For the purpose of studying the properties of hybrid concrete, including measuring the workability of fresh concrete, examining the compressive strength, dry density, split tensile strength, Flexural strength, ultrasonic pulse velocity testing, water absorption of hardened concrete. The main parameters were the proportions of dune sand (20 %, 40 %, 60 %, 80 %, and 100 %) in addition to the ratio 0 % without dune sand. Two types of steel fibers were used: straight steel fibers (short) with a length of 13 mm and hook end steel fibers (long) with a length of 50 mm, It is added to concrete (single or hybrid) with a total volume ratio of 1 % of the concrete volume. The tests that were conducted showed that sand dunes affect the compressive strength, especially when the percentage increased by more than 40 %, where the compressive strength of concrete decreases by 39 % when the replacement ratio reaches 100 %, but it improves with the addition of single or hybrid fibers. Also, the workability from high to medium level decreases by 30 % with increasing dune sand content, the addition of steel fibers along with dune sand significantly reduced the workability. The tensile and flexural strength decreases with the increase in the sand dune rate, but at a lower rate than the compressive strength, which was 29.3 %, 21.1 % for splitting and flexural tensile strength, respectively. The addition of steel fibers improved the mechanical properties of the hybrid concrete, which can compensate for most of the lost strength due to the increase in sand dunes ratios. The study also showed that the use of long fibers (hook end) gives better results than straight fibers. But in the case of using hybrid fibers, we can get an improvement in the mechanical properties more than using one type of fiber.