Structural Behavior of Reinforced Concrete Beams enhanced with Hybrid steel-polypropylene fiber under Impact Load

103  ABSTRACT Current research involves study the structural behavior of reinforced concrete beams enhanced with steel and polypropylene fibers under the influence of impact load. It has casting concrete cubes of dimensions (150 150 150) mm, concrete prisms of dimensions (100 100 400) mm, concrete cylinders of dimensions (150 60) mm, and reinforced concrete beams of dimensions (150 15

Fiber reinforcement is commonly used to provide toughness and ductility to brittle cementations matrices. Reinforcement of concrete with a single type of fiber may improve the desired properties to a limited level. A composite is termed as hybrid, if two or more types of fibers are rationally combined to produce a composite that derives benefits from each of the individual fibers and exhibits a synergetic response.
The polypropylene fiber has a low modulus of elasticity, however. Consequently, they cannot prevent the formation and propagation of the cracks at high stress level. Neither can they bridge large cracks. Steel fibers have a considerably larger length and higher modulus of elasticity as compared to the PP fiber. This leads to an improved potential for crack control.
However, volumetric density is high, and steel is conductive in electric and magnetic fields.
Therefore, steel fiber content has to be reduced to below a certain level in structures such as tunnels and continuous slabs for high-speed railway systems, where 1-To provide a system in which one type of fiber, which is stronger and stiffer, improves the first crack stress and ultimate strength, and the second type of fiber, which is more flexible and ductile, leads to improved toughness and strain capacity in the post-cracking zone.

2-
To provide a hybrid reinforcement, in which one type of fiber is smaller, so that it bridges micro cracks of which growth can be controlled. This leads to a higher tensile strength of the composite. The second type of fiber is larger, so that it can arrest the propagating macro cracks and can substantially improve the toughness of the composite. Using the super plasticizer as a high range water reducing admixture (HRWRA) in concrete has become of great and beneficial important due to its high performance in reducing water cement ratio (w/c) of concrete, thereby increasing strength and improving its workability. On the other hand, the use of hybrid polypropylene and steel fibers in concrete with optimum dosage of (HRWRA) and (SF) admixture is most likely to ensure optimized

Fibers:
A-High quality monofilament micro short 12 mm polypropylene fiber, shown in Figure.(1) , was used in this investigation. Table.(6), indicates the physical properties of polypropylene fibers used in this work. Figure.(1), have a 'trough' shape with hooks at both ends, had been used in this work. The ZP305 is a cold drawn wire fiber, with hooked ends, and glued in bundles. Steel fibers were 30 mm long And 0.55 mm in diameter (aspect ratio of 55) and possessed a tensile strength of 1100 N/mm 2 the glue dissolved in the water during mixing,thus dispersing the fibers in the mix Specifications of Steel fiber (SF) used in this investigation are shown in Table.(7).   Each volume percentage of FRC contains three mixes. As an example, for (0.5%) Volume fraction percentage (V f ), Hybrid steel-polypropylene fiber reinforced concrete mix had been achieved. That is mean three FRC mixes were introduced for these three volume percentages as described in Table.10, in addition to the reference mixes.  Figure.(2). with capacity of 5000 kN. The load deflection has been drawn according to ASTM C1018-97.

B-Steel fibers manufactured by Bekaert -Dramix ® ZP305 as shown in
The test were conducted at ages of (28) days and three specimens was tested for every case.

Dynamic response tests for FRC beams:
This test had been carried out for (150×150×1000) mm, RC and FRC beams specimens, the reinforcement of beams shown in Figure. Table.(14), show ultimate toughness index which mean the value of ultimate toughness divided by first crack toughness. Figure.(7) shows the p-d diagram for all beams groups. It can clearly shows that the occurring of fibers increase toughness of RC beams and give a significantly increasing with high volume fraction.     Table.(15), and plotted in Figure.(8&9). The addition of HRWRA and SF to control mix increases the impact resistance strength at first crack, and ultimate impact resistance strength.
This mixes improvement is attributed to the pozzolanic reactivity which chemically reacts with the calcium hydroxide liberated during the hydration of cement and contributes to the densification of the concrete matrix, there by strengthening the transition zone and reducing the micro cracking leading to a significant increase in the energy absorption.
Demonstrate considerably the impact strength resistance at first crack and ultimate failure for fiber reinforced concrete specimens. It had been seen that the impact resistance increases with increasing the fiber volume fraction for both types of hybrid polypropylene and steel together. This is mainly due to the high capacity of polypropylene and steel fiber to absorb large amounts of energy prior to failure. The concrete specimens containing no fibers were cracked and failed in brittle condition when they reached the ultimate resistance. Cracks started at the center of the top face of impact rings specimens and then propagated outwards and toward the outside circumference as the number of blows increased. The number of blows was so closed between the first crack and ultimate stages for the concrete mix without fiber.
However, fiber reinforced concrete impact rings specimens also cracked at ultimate resistance, but it is capable of carrying the repeated impact load well after the crack developed on the concrete, This indicates that the fiber reinforced concrete has the ability to hold on the crack of the concrete and preventing the concrete specimens to fall apart.
Hybrid RC Beams under impact load: In adopted oscillatory system, which has 1 degree of freedom (1-DOF) (the number of independent coordinates required to describe the motion), all the impact test results are demonstrated in Table.(16) and (17), and Figure.(8&9).
In order to evaluate the natural frequency of damped oscillatory ( ) (which are properties of Where, and are the first and last amplitude (displacement) respectively.
By substituting for the damped period, The expression for the logarithmic decrement becomes: ---------- (8) Where, and are the damping and critical damping coefficients respectively.
√ ----------(9) The whole calculations that had been made for all tested beams with and without fiber are stated in Table.(17). It can be seen from the Table.(16,17), and Figure.    4.CONCLUSIONS 1. In order to mention the same range of workability for all mixes, it's found that the use of (10% by weight of cement) silica fume tended to require more dosages of super plasticizer (3% by weight of cement) which was the optimum dosage.
2. All concrete mixes shows improvement in the compressive strength test results for (7 days), and this due to the effect of (HRWRA) and (SF) on early strength properties.
3.Modulus of rupture increases as the fiber's volume fraction was increased up to 1.5%.

4.Results
showed that reference concrete (without fiber) could not sustain any load and failed quickly when the first crack was developed and hence the toughness for these specimens was very low compared to other of FRC. For hybrid fibers reinforced concrete beams, the ratio of (1.5% Vf) has high toughness index result, this may due to the large contribution (by volume fraction) belong to steel as well as polypropylene fibers.