BEHAVIOR OF HYBRID FIBER REINFORCED

This study aims to understand the effect of hybridization of steel and synthetic fibers on the bending-shear behavior of prestressed concrete (PSC) beams. The first phase of the test program consists of evaluating the efficiency of individual fibers, i.e. Vfd = the contribution of the steel/synthetic fiber shear reinforcement VRd3 = the design shear resistance of a section of a beam.

Vwd = contribution of shear reinforcement due to stirrups and inclined bars α = angle of shear reinforcement with respect to the longitudinal axis.

Introduction

General

This mandate arises from the fact that current ACI provisions are primarily based on experimental studies of non-prestressed concrete beams cast from concrete with a cylinder compressive strength of less than 42 MPa. However, in a prestressed concrete beam, the beneficial effect of prestressing forces could further reduce the minimum required fiber volume fraction and make the use of SFRC more economical. Thus, the focus of this research is to study the effect of steel fibers on the cracking and ductility behavior of prestressed concrete beams under bending/bending-shear.

Why add fibers to the concrete?

Different types of fibers
Mechanical properties of fiber reinforced concrete

Steel fibers (Figure 1.2) reduce the crack width of the concrete by bridging the cracked surfaces in the serviceability regime. The addition of steel fibers was found to significantly improve the strength and stiffness degradation in the post-peak region with better ductility (Figure 1.6a). On the other hand, the FRC specimen shows a similar large crack at failure, but the crack opening was delayed due to the presence of fibers in the section (Figure 1.6b).

The CMOD curves (Figure 1.6c) show that the fibers restrain the crack opening more efficiently than that of the specimen without fibers.

Figure 1.2: Steel fibers (Source: Dramix, Bekaert)

Need for hybrid fiber addition

Fibers of longer lengths bridge macrocracks and improve the overall ductility of concrete. c) Combination of fibers with different durability properties: Highly durable fibers ensure long-term preservation of strength and toughness, while fibers with lower durability control the short-term performance (transportation + installation) of the concrete composite. The effect of hybridization of steel and synthetic fibers on the bending-shear behavior is also presented.

Literature Review

Effect of shear span to depth ratio on the behavior of reinforced concrete (RC) members
Behavior of fiber reinforced concrete under Different Loads

Studies on steel fiber reinforced concrete
Studies on synthetic fiber reinforced concrete
Studies on hybrid fiber reinforced concrete

The behavior of fiber reinforced prestressed concrete members
Knowledge gaps from Literature Review

Harajli [13] studied the bond behavior of steel fiber reinforced concrete under static and cyclic loading. 23] studied the influence of concrete strength on the crack development of steel fiber reinforced concrete elements (SFRC) and found that the use of fibers led to a smaller average crack distance in high strength concrete (HSC) compared to normal strength concrete (NSC). . Antonio Conforti [28] studied the shear behavior of prestressed double T-sections with self-compacting polypropylene fiber reinforced concrete.

In the next chapter, the importance and goals of the present research program on the bending-shear behavior of hybrid fiber-reinforced prestressed concrete beams were presented.

Figure 2.2: Behavior of reinforced concrete beams at different shear span to depth (a/d) ratios

Objectives and scope

Background and research significance

Various factors such as shear span to depth ratio (a/d), compressive and tensile strength of concrete and fiber dosage affect the performance of fiber reinforced concrete elements. Although the studies of Kan [7] are limited to reinforced concrete beams, only limited works in the past have focused on the effect of different a/d ratio on the behavior of prestressed concrete beams. The present work is part of a large research program on the effect of different a/d ratio and fiber influence on the behavior of prestressed concrete beams.

In the present case, a higher a/d ratio of five is considered to study the influence of steel and synthetic fibers on the bending-shear behavior.

Objectives of the study

The casting process of fiber-reinforced prestressed concrete beams is shown in Figure 4.2. a) Mold preparation b) Strand tension measurement .. during prestressing c) Concrete installation. For the current example of a fiber-reinforced prestressed concrete beam (Figure 6.3), the crack widths are calculated in the post-crack region at the serviceability regime. It can be seen from Figure 6.5 that the crack width of steel fiber reinforced samples is in the range of 0 to 2 mm.

While for fiber reinforced samples the difference between the curves is significant (Figure 6.8). a) Effect of Moffat's factor on the control samples.

Figure 3.2: Brief overview of research work

Experimental program

Material properties

Fibers
Concrete
Internal reinforcement-prestressing steel strand

The hook point steel fibers and macro-synthetic polyolefin fibers have been used in the development of fiber reinforced concrete mixtures. Mixed coarse aggregates of size 10 mm and 20 mm along with fine aggregates were used to obtain a uniform mixture. After the 28 days of water curing, concrete blocks were tested to evaluate the compressive strength of concrete, and the results are reported in Table 4.3.

Concrete cylinders were tested (Figure 4.1a) to obtain the stress-strain curves with and without steel fibers. The addition of steel fibers was found to significantly improve the strength and stiffness degradation in the post-peak region with better ductility (Figure 4.1b). In the coming sections, the nomenclature used for different specimens is: 'Control' indicates the specimens.

Similarly, HB70 hybrid fiber reinforced (SF+PO) samples with a fiber dosage of 0.7% and PO100 polyolefin (synthetic) fiber reinforced samples with a fiber dosage of 1.0%. To prestress the beams, two numbers of half-inch (12.7 mm diameter) strands with seven low relaxation wires with an effective area of 200 mm2 were used. The ultimate tensile strength and elastic modulus were found to be 1860 MPa and 196.5 GPa, respectively.

A lifting force is applied to each of the strands to subject them to an initial tension of 0.004.

Table 4.1: Properties of Steel and macro-synthetic fibers

Specimen preparation

All test samples were grouped into four different series, namely control series (plain concrete without fibers), steel fiber (SF), macrosynthetic polyolefin (PO) and hybrid fiber (HB) reinforced series containing a combination of steel and macrosynthetic materials. fibres.

Test setup and Instrumentation

Fracture tests on fiber reinforced concrete specimens
Full-scale tests on fiber reinforced prestressed concrete beams
Digital Image Correlation (DIC) technique

The shear resistance is mainly due to three components (Figure 6.2), namely (i) the shear resistance of the concrete member without shear reinforcement (𝑉𝑐𝑑), which includes the resistance due to the uncracked concrete section (𝑉𝑐𝑑1), the aggregate interlock (𝑉𝑐𝑐𝑑2) and the dowel resistance (𝑉𝑐𝑑3). ). , (ii) contribution of steel/synthetic fibers (𝑉𝑓𝑑) and (iii) contribution of shear reinforcement due to stirrups or inclined bars (𝑉𝑤𝑑). It can be seen from Figure 6.6 that in the serviceability range, the crack width varies in the range of 0-3 mm for all fiber-reinforced samples.

Test results and Discussions

General

Fracture behavior of fiber reinforced concrete

The sample with 0.7% steel fiber dose (Figure 5.6) showed better ductility than the sample with a 0.35% fiber dose. In Figure 5.11a-c, a sudden jump in the strain contour indicates the presence of a crack in that region. Flexural dominant behavior (Figure 5.23) was observed for 1.0% hybrid fiber reinforced specimens (cracking load = 80 kN and ultimate load = 141.7 kN).

Compared to HB35 and HB70, specimens with 1.0% hybrid fibers showed better ductility (Figure 5.24) (ultimate displacement = 96 mm).

Figure 5.1: Typical Load Vs. CMOD Curves of FRC [3]

Analytical Study

General

This chapter describes the load capacity calculations of fiber reinforced prestressed concrete beams of the current test program using the RILEM recommendation. In the next section, the parameters affecting the cracking behavior such as crack width and spacing are also studied using the RILEM TC-162-TDF [17], the CEB-FIP modal code [35] and the Moffat crack spacing model [ 36].

Capacity calculation using RILEM recommendations

Flexural moment capacity
Shear capacity
Capacity comparison

The tensile stress-strain curve of fiber-reinforced concrete under tension was obtained indirectly from the fracture test results (Figure 5.1). Initially, the compressive deformation of the upper fiber is assumed, and the depth of the neutral axis is repeated to satisfy the force balance. The RILEM recommendations are used to calculate the design shear capacity of fiber-reinforced prestressed concrete beams.

The shear strength of FRC is higher than that of the control sample (without fibers). The shear strength increase is due to controlled crack propagation in the fracture process zone (FPZ) in fiber-reinforced concrete. The crack bridging effect also improves the aggregate interlocking due to which the shear capacity is improved in the case of fiber reinforced concrete specimen.

The following equations (6.6) to (6.16) are used in the shear capacity calculations of prestressed fiber reinforced concrete beams. Vfd = Shear contribution of the steel/synthetic fiber shear reinforcement VRd3 = the design shear resistance of a section of a beam. Vwd = Displacement contribution of the shear reinforcement due to stirrups and/or inclined bars α = angle of the shear reinforcement with the longitudinal axis.

The experimentally observed capacity of fiber reinforced prestressed concrete beams is compared with the analytical capacity obtained from the RILEM recommendation [17] and is presented in Table 6.1. The use of RILEM recommendations generally resulted in a conservative estimate of the flexural capacity of hybrid fiber-reinforced prestressed concrete beams.

Figure 6.1: Flexural capacity calculation

Analysis of crack width and crack spacing in fiber reinforced concrete beams

Introduction
Crack spacing model based on RILEM TC162-TDF
Crack spacing model based on CEB-FIP modal code
Modified equations based on Moffat's crack spacing model
Effect of modification factor in the formulae of RILEM and CEB-FIP code

The main motivation for using fiber-reinforced hybrid concrete is to achieve maximum crack resistance at the lowest fiber dosage without compromising fresh workability. This change in failure mode occurred at 0.7% fiber dosage in the synthetic fiber reinforced beams due to the lower modulus of elasticity of the synthetic fibers. The post-cracking behavior of hybrid fiber-reinforced specimens is intermediate between that of synthetic fiber-reinforced specimens.

The crack widths calculated using the recommendations of the Euro code and the CEB-FIP (1993) code for steel fiber reinforced prestressed concrete specimens reduced with the increase in fiber dosage. Prakash, "Mechanical behavior of sustainable hybrid synthetic fiber reinforced cellular lightweight concrete for masonry structural applications." Banthia, “Flexural response of steel fiber reinforced concrete beams: Effects of strength, fiber content and strain rate,” Cem.

Harajli, “Bond Behavior in Steel Fiber Reinforced Concrete Areas Under Static and Cyclic Loading: Experimental Evaluations and Analytical Modeling,” J. Mirza, “Mechanical Properties of Polypropylene Fiber Reinforced Concrete and Effects of Pozzolanic Materials,” Cem. Serna, “Shear behavior of prestressed precast beams made of self-compacting fiber-reinforced concrete,” Konstr.

Serna, “Failure Modes and Shear Design of Prestressed Hollow Core Slabs Made of Fiber Reinforced Concrete,” Compos. Ramaswamy, “Flexural strength predictions of steel fiber reinforced high strength concrete in fully/partially prestressed beam specimens,” Cem.