Comparison of Properties of Concretes with Different Types and Dosages of Fibers Comparison of Properties of Concretes with Different Types and Dosages of Fibers

Concretes with PP fibers 12 mm, construction polymer fibers 25 mm, 3D steel fibers 25 mm, and steel microfibers 12 mm were prepared in dosages 0.5 and 1%. The mechanical 10 properties (compressive strength, bending strength, fracture properties, and modulus elasticity) and the frost resistance of these concretes were tested and they are discussed. 12 The behavior of these concretes is also discussed using graphs load vs. deflection. As 13 bad results of frost resistance are sometimes recorded for concrete with fibers, this 14 property is also evaluated. As was expected, mechanical properties are enhanced with 15 the addition of suitable fibers. Frost resistance is usually comparative with concrete 16 without fibers, but in the case of concrete with 1% of steel fibers, it is reduced.

mm are recommended [4]. But the subtle constructions have smaller dimensions. Yhang et al.
[3] used 100 × 100 mm prisms for the testing of FRC with microwires. In this paper, prisms with the cross-section 80 × 80 mm are used because they correspond to produced element dimensions [1,5]. In Ref.
[4], a four-point bending test is recommended on an unnotched prism whose volume is nearly 16 L. In this paper, notched prisms 80 × 80 × 480 mm (volume 3.1 L) are used with the central notch and they are tested in a three-point bending. As expected, the results are different from that of Ref. [4] and not objective from the point of view of concrete properties, but they are more similar to a practical situation from the point of view of the arrangement of fibers.
10 The structural design of these subtle constructions is often very difficult and the best way to evaluate the properties are tests in a 1:1 scale, see also Refs. [5,6].
12 There are different kinds of fibers for different purposes. For example, steel wires or structural 13 synthetic fibers are designed for similar purposes, but it is clear that their performance will be 14 different. The possibility to substitute different types of fibers with other ones and optimize 15 their dosage was the main purpose of this paper. 16 2. Experimental procedure 17 2.1. Materials 18 The concrete was designed as a high-performance concrete for class C70/85 XF1 (see EN 206). 19 Ordinary Portland cement CEM I 42.5 R produced in cement plant Mokra was used. 20 Metakaolin was added for enhancing the workability of the mixture and also for enhancing strengths. Commercial polycarboxylether superplasticizer, drinkable water, and commonly 22 produced aggregates-sand 0/4 and crushed granite (Litice nad Orlici quarry)-were also 23 used. Water to binder ratio w/(c+m) = 0.27.
24 Different types of fibers were used-polypropylene 12 mm long fibers in dosage 0.5% vol , so-25 called structural synthetic polymer 25 mm long fibers-0.5 and 1.0% vol , 3D steel fibers 30 mm 26 long 0.5, and 1.0% vol and steel microfibers 13 mm long 0.25 and 0.5% vol . Properties of these 27 fibers are presented in Table 1 Concretes were mixed in a laboratory mixer; the volume of each batch was 30 L. Workability was measured using reverse Abrams cone. After mixing the concrete was placed into steel molds. After demolding at the age of 22-24 hours, specimens were stored in water t = (20 ± 2)°C.

Specimens
Cubes 100 mm were used for compressive strengths tests at the age of 28 days. Prisms 80 × 80 × 480 mm were made for the testing of fracture properties and for the testing of frost resistance. A notch to depth cca 28 mm (1/3 of high) was cut into the beams 220 mm from the one end of the beam at the age of 28 days. Fracture tests in accordance with the Karihaloo and Nallathambi effective crack model [7] were performed on the notched beam (span 400 mm).
10 The fracture toughness K IC is the main result of these tests. Toughness G c was also computed.
Fracture work W F was computed from the load-deflection curve in accordance with RILEM 12 recommendation [8,9]. The modulus of elasticity in a three-point bending on notched beam E 13 as well as the modulus of rupture f r (flexural strength on the notched beam) are the partial 14 results of these tests. After the fracture test, a part of the broken beam, whose length is 15 approximately 260 mm, was used for the test of flexural strength f b (span 220 mm). Other 16 beams 80 × 80 × 480 mm were exposed to 125 freezing and thawing cycles (FT-cycles) in 17 accordance with Czech norm CSN 73 1322-frost resistance of concrete. One cycle represents 18 4 hours in the freezer at the temperature −20°C and 2 hours in water +20°C. After the cycles, 19 values of flexural strength, modulus of rupture, and modulus of elasticity were measured and 20 compared with the values at the age of 28 days. Activity indexes I b , I r , and I E were calculated as a ratio value of values comparative beams/values of frosted beams.  Table 2. The first property is workability. It is evident that 25 the concrete which was obtained with the cone flow cca 350 mm was completely different from 26 the originally very flowable, self-compacting concrete with the cone flow 850 mm. This was the 27 case of PP fibers, but the dosage of these fibers was five times higher than normally 28 recommended. This experiment was performed for the comparison and the information about 29 the impact on workability in concretes with enhanced fire resistance, thanks to PP application. 30 In all these cases, the workability can be regulated with a higher dosage of superplasticizers but this method was not used in this case. 32 The compressive strength f c is the highest for the content of fiber 0.5% or 0.25 for M-fibers. 35 The flexural strengths f b and f r show different courses. The differences are demonstrated in 36 Figures 1-3. In Figure 1, the load-deflection curve for the reference concrete and the concrete 37 with 0.5% of S-fibers is shown. The maximum value of load is nearly the same, but the 38 postpeak regime is different-there is some residual strength for FRC. The load-deflection curve for the content of 1% S-fibers is very similar, only the residual strength is higher-it does not show here. The highest values of load are reached for steel fibers, especially for the steel fibers with a special shape-hooked ends-3D fibers (see Figure 2). These fibers hold in the matrix very well and they enhance f b and f r . D-fibers are long enough, which means that the  concrete shows a significant residual strength after the first peak. The results for 0.5 and 1% differ especially in the course curve after the first peak. These results are very similar to those of Ref.
[2]. Micro steel fibers (Figure 3) work well for the big amount; the improvement of f b and f r is the most conspicuous and the dosage of fibers is 1/2 of the previous-only 0.25 and 0.5%, but they are short and they are pulled out early.
The values of the modulus of elasticity are nearly the same-with respect to the standard deviation. Also fracture toughness K IC and toughness Gc is similar for all of the mixtures. The load from the first peak was considered the value of the maximum recorded load. The effect of fibers on the improvement of concrete without microcracks is considered in this case.   The best values are recorded for the concrete without fibers. All FRC fulfill the requirements of CSN 73 1322 − I > 75%, except the concrete with 1% of D-fibers in terms of f b . Concrete M 0.5 shows I b on the boundary of acceptable value. The experience proves that FRC with wires has worse frost resistance. In terms of f r and E the frost resistance is good. This probably means that the surface layer of the concrete is affected by the water which penetrates around the wires into the concrete and freezes later. This result-significant reduction of frost resistance-can be a consequence of the small size of the specimen, but it is especially important for the subtle or 10 thin construction from FRC.    4. The frost resistance of concretes with steel fibers can be reduced-probably thanks to the penetration of water along the fibers. This is especially important for the subtle crosssections. This can also be one of the reasons why to perform tests using small size specimens, too.

5.
On the basis of the published results, the concrete with 0.5 of D-fibers was selected as the best possibility for the production of subtle concrete elements. Tests in a 1:1 scale confirmed rightness of this choice [6].
This investigation is being continued with the target to compare the results recorded in small size specimens with those of preliminary norm requirements [4].