Shear performance of polypropylene fiber reinforced high-strength self-compacting concrete beams

This paper consists of two parts, the first one look for producing self-compacting polypropylene fibrous concrete (PFSCC) with reasonable passing, filling and flow abilities and segregation resistance. Limestone powder with particle size less than 75μm was used to increase the fines fraction in the mix. The second part of this study, deals with the effect of adding polypropylene fibers and the shear strength of self-compacted reinforced concrete beams. Eighteen beams were cast which were 1.8 m long, 150 mm wide, and with overall depth of 200 mm. Four volume percentages of polypropylene fibers were used, 0, 0.1, 0.2 and 0.3 percent of cementitious materials weight. The test results showed that the polypropylene fibers have not serious adverse effect on the fresh properties and this effect was reduced by using plasticizer. Moreover, although the polypropylene fibers improved the tension properties of the hardened concrete, don`t have impact on compression properties. The test results showed that the presence of polypropylene fibers increased the cracking shear stress and the shear strength and this increase depends on the volume fraction of the fibers.


Introduction
Numerous experimental and analytical studies on structure members have been reported [1][2][3] [4] .Concrete is a material for building construction which is made from Portland-cement, water, fine and coarse aggregates.Beside good compressive strength, the concrete material is very weak in tensile strength.To increase the tensile strength, usually the concrete was added by fiber materials.Fiber reinforced concrete FRC which is developed more than sixty years ago is a concrete reinforced with discrete short fibers with tensile strength, flexural strength, flexural toughness, cracking resistance, and strain capacity.Several researches have been conducted dealing with the impact of steel fibers on the shear strength of reinforced concrete beams [5][6][7] .Prediction equations for the shear strength of steel fiber reinforced concrete beams were proposed in some studies [8][9] .It was shown that steel fibers improve the mechanical properties of concrete especially in the modulus of rupture and the splitting tensile strength [10] .However, steel fibers have disadvantages such as easily to corrode, higher weight, easily to damage the mixer, magnetic interference, and higher price [11] .Therefore, another fiber material was developed and used in concrete such as steel fibers, glass fibers, carbon fibers, and polypropylene fibers.Investigation of fiber concrete using polypropylene fiber was already done by researchers [12][13] [14] .It was shown that polypropylene can improved the tensile strength of concretes.Investigation on self-compacting concrete was comprehensively carried out in several decades ago.However, study on self-compacting concrete using polypropylene is still rare.Therefore, study on high-strength self-compacting concrete using polypropylene is still needed.

Experimental program 2.1. Materials
Main materials of concrete (Portland-cement, water, aggregates) were taken from local material.Table 1 shows the mix proportions used.Micro silica fume and Superplasticizer, were added in the concrete with the proportion 10 and 1.5 percent of Portland-cement weight, respectively.Limestone powder passing sieve No. 200 (75 microns) was added with the proportion fifty percent of total weight of the cement in the concrete mixture.To decrease friction between the mix constituents and increase the flow ability, the limestone powder was used to act as filler.Finally, micro monofilament polypropylene fibers with the four different proportion of concrete weight as presented in table 1 were used in this study.
The length of fibers was 12 mm and the tensile strength of fiber was about 350 MPa.The mix proportions were chosen to produce a nominal cube compressive strength of 80 and 90 MPa explain above.

Mix
No.

Specimens Description
To get the properties of fresh self-compacting concrete, three type properties were tested.The testing of fresh selfcompacting concrete is following the EFNARC and the properties to be tested were: Viscosity using T50 slump-flow test Passing ability using L-shape box Air-void stability in fresh self-compacting Concretes To get the compressive strength of high-strength self-compacting concrete, 24 cubic specimens with the size of (100 mm × 100 mm × 100 mm) were made.While to get the tensile strength 8 cylindrical specimens with the size of (150 mm×300 mm) were made.Sixteen beam specimens were cast and tested in this study.

Mix
No. The result shows that the compressive strength and the splitting tensile strength of self-compacting polypropylene fiber concrete was lower and higher than the self-compacting non fiber concrete, respectively.The decreasing compressive strength and the increasing splitting tensile strength of self-compacting polypropylene fiber concrete compare to selfcompacting non fiber concrete showed in table 2. The increasing splitting tensile strength of self-compacting polypropylene fiber concrete compare to self-compacting non fiber concrete was higher for specimens with higher compressive strength.The increasing splitting tensile strength of self-compacting polypropylene fiber concrete was 11 % and 14% for specimens with compressive strength of 80 and 90MPa, respectively.The applied load, mid-span deflection and strains of vertical and horizontal stirrups in the constant shear force region were measured by instrumenting the test specimens.A load was given by the actuator with the capacity of 50 kN through the transfer rigid beam, so the beam specimen was subjected two points loading as depicted in Figure 3.A linear variable differential transducer (LVDT) was used to measure deflection of the specimen.The LVDT was placed at the middle of the specimen to measure the displacement in the vertical direction.Measured data of load and deflection were read through a computer driven data acquisition system using data logger.In addition; two strain gages were placed each side on position L/4 for linear displacement.The propagation of cracks was marked along the sides of the specimens.Cubic and cylindrical specimens were tested on the same day of testing beams to determine the mean compressive and tensile strength of the concrete respectively.The load carrying capacity-deflection curve of the beam specimens can be seen in Figure 4 and 5. Firstly, the curve of load-carrying capacity-deflection relationship in the lower load is increasing linearly.Then, after initial cracks, the curve is increasing nonlinearly up to a value than the curve increasing almost horizontally up to failure.Toughness is the ability of a material to absorb energy and plastically deform without fracturing.Integrating the stressstrain curve determines toughness of materials.While percentage of polypropylene fibers were increased, the area under the load-strain curve was heightened too.

Passing ability
The maximum and relative deflection of mid-point of the beam specimens is presented in table [3] .The maximum deflection of self-compacted reinforced concrete beams was increased from 5 to 36%.The increasing maximum deflection for beams with specified compressive strength of concrete of 80 MPa was higher than those with 90 MPa.The shear strain of beams was compared and illustrated in figure 8.The shear strain for beams with higher compressive strength was significantly higher than others.

Discussion and Conclusion
Base on the experimental program, the following conclusion can be drawn: The compressive strength of self-compacting polypropylene fiber concrete was 2 to 3% lower than self-compacting non fiber concrete.The decreasing compressive strength is reduced by increasing specified compressive strength of concrete.
The splitting tensile strength of self-compacting polypropylene fiber concrete was 4 to 14% higher than self-compacting non fiber concrete.The increase of polypropylene fiber percentage causes higher splitting tensile strength.
The polypropylene fibers lead increasing the cracking shear stress of self-compacted reinforced concrete beams from 4 to 11%.
The polypropylene fibers increased the maximum deflection of self-compacted reinforced concrete beams from 5 to 36%.
The increasing maximum deflection is heightening by reducing specified compressive strength of concrete.
Flexural cracks in the beams with polypropylene fibers are shorter and on the other hand, shear cracks appear longer in the beams with more polypropylene fibers.
Linear stage of stress-strain curves related to beams with same specified compressive strength of concrete are similar.
But those curves growth more in nonlinear stage for beams with more polypropylene fibers.
Beams with polypropylene fibers absorb more energy than beams without fibers.

Figure 2 .
Figure 2. Beams dimensions and specifications

Figure 4 .Figure 5 .
Figure 4. comparison of deflection of the mid-point of beams with different amount of fibers Several line chart for comparison of strain vs. load is presented and curves for different amount of fibers and specified compressive strength of concrete are compared.In linear stage of load-strain curves, tests show similar results for beams with same specified compressive strength of concrete.But after initial cracks curves growth to nonlinear stage and beams with more polypropylene fibers carry more load which result in more strains.

Figure 6 . 11 Figure 7 .
Figure 6.Comparison of flexural strain of the mid-point of beams

Figure 8 . 11 Figure 9
Figure 8. Relative shear strain of beams by percentage of polypropylene fibers

Figure 9 .Figure 10 .
Figure 9. Appearance of crack for the beam subjected to ultimate loads (f`c=80 MPa)

Table 2 .
The result of fresh concrete tests

Table 3 .
Maximum and relative deflection of beams