The term fiber reinforced concrete (FRC) can be defined as a concrete containing dispersed randomly oriented fibers. Fibers include: steel fibers, glass fibers, carbon fibers, synthetic fibers, natural fibers and others. Some of these fibers are used as structural reinforcement (fibers with high modulus of elasticity), while others (fibers with low modulus of elasticity) help to decrease the plastic shrinkage cracks. The fibers are often described by a parameter called “aspect ratio”. The aspect ratio of the fiber is the ratio of its length l to its diameter d. Typical aspect ratio ranges from 30 to 150. Fiber reinforcement is used to improve the brittle nature of cementitious materials. Fibers enhance the mechanical properties of the composite such as toughness, ductility, fatigue resistance, impact resistance, flexural strength; they reduce creep, shrinkage cracking and permeability [1, 2, 3, 4, 5, 6]. The influence of fibers on concrete properties depends on the fiber geometry (aspect ratio), size, type and the volume fraction of fibers. Polypropylene fibers reduce the plastic shrinkage crack area due to their flexibility and ability to conform to form. The addition of 0.1% by volume of such fibers is effective in reducing the extent of cracking. Cracks play an important role as they increase the permeability of concrete structures and consequently the risk of corrosion rises. Cracks reduce the quality of concrete and can make it aesthetically unacceptable.
But the decrease in the workability of fresh concrete is often the disadvantage of fibers addition [7, 8, 9, 10, 11, 12, 13]. The reduction in the workability of concrete depends on many parameters such as maximum aggregate size, fiber volume, fiber type, fiber geometry, fiber inclusion to concrete [14, 15, 16, 17]. This effect increases with the concentration and aspect ratio of the fibers . The specific surface of fiber per unit volume of mixture is influenced by the combination of fiber aspect ratio and fiber length .
The general disadvantage of Self Compacting Concrete (SCC) properties with fiber addition is the lower workability. Therefore the effect of fibers on the properties of fresh and hardened concrete should be well recognized. To utilize fibers effectively, the properties of material in fresh state must be controlled. There is a critical concentration of fibers above which the fresh concrete stops to flow, even if the fibers were added into the fluid suspension such as SCC, because above this concentration fibers tend to form clumps or balls and mechanical interlock of fibers occurs [19, 20]. Analysis of the influence of fibers on the workability and mechanical properties of SCC is one of the new fields in investigations of cement composites [11, 21, 22, 23, 24]. For instance Ponikiewski  proved that when 1.5% content of steel fibers in the mixture were used the self compactibility of concrete mixtures decreased. Despite the deterioration of workability, the self compactibility of the mixtures with the addition of steel fibers was maintained and the mechanical properties of concrete were satisfactory. In this study the threshold value of fibers was 2%.
Adequate rheological properties of the concrete mix are one of the basic technological requirements to ensure high quality of reinforced concrete structures. Over time, usually after 30-90 minutes (depending on the efficiency of the water reducing admixture), the rheological properties of concrete mixtures change, which results in a deterioration of its workability, making it difficult to properly construct the structure.
Standard workability tests may be useful, on construction site for instance. But it is important and purposeful to undertake research to identify the workability of cement matrix mixtures reinforced with fibers in rheological terms. Previous investigations showed no correlation between the results of one-point and rheological tests, which means that the information obtained by one-point tests was less reliable .
Rheological studies of concentrated suspensions of solid particles in a continuous liquid medium are commonly used to evaluate the characteristics of different materials, including construction materials. They provide an understanding of how these materials behave in the fresh (plastic) state and monitor the formation of the structure, which demonstrates the development of mechanical properties . The flow behavior of the concentrated suspensions depends on the contact surface between solid particles (dispersed phase) and intermolecular forces such as van der Waals forces and steric forces . For cementitious composites, rheological parameters help to describe the ease with which they can be operated in the fresh state during mixing, placing, pumping, compacting and finishing.
In order to apply rheological tests to cementitious suspensions, it is important to understand the influences of experimental procedure and to select the adequate rheological model of the flow characteristics. There are different models which describe the rheological characteristics of cementitious systems. There is considerable evidence that the rheology of cement-based materials conforms to the Bingham model [28, 29]:
where: τ0 – the yield stress, describing the stress needed to initiate flow; ηpl – the plastic viscosity, which is a resistance of the material to flow; τ – the shear stress;
The material is an elastic solid at shear stress τ < τ0, at higher stresses begins to flow. The yield stress is a result of interparticle forces and can be determined by a direct measurement in an appropriate device (coaxial rheometer). At least two measurements at considerably different shear rates, or shear stresses are required to characterize a Bingham material . Single point tests, such as slump or slump-flow, are inadequate because there is an infinite number of combinations of yield stress and plastic viscosity that can give the same result.
There is a need for suitable methods for measuring rheological characteristics of mortars and concretes as workability properties are increasingly important in modern concrete technology. Coaxial cylinder rheometers cannot be used for coarsely dispersed systems such as mortar or concrete. The cell geometry and the equipment in this case would be very large, since the gap width required for the measurement depends on the maximum particle size. For this reason devices that are based on rotational rheometry are often used . Typically they are based on the measurement of the torque of an agitator of variable geometry.
In this case changes of the rheological properties can be displayed qualitatively, but the results cannot be converted to the fundamental rheological units as it would for instance be possible with a coaxial cylinder rheometer. When applying a Bingham curve on the results according to the Equation (1) with
In this investigation the rheological behavior of fresh fiber reinforced cement mortar was evaluated using Viskomat XL. In this instrument, when the cylindrical sample container rotates (Figure 2a), the mortar flows through the blades of the impeller (Figure 2b) and exerts a torque which is measured by a transducer. A series of data points of torque (M) and rotation speed (N) are recorded. Because the principle of measurement in rotational viscometers differs from coaxial viscometers, Bingham’s equation was modified by Tattersall and Banfill :
where: M – the torque; N – the rotation speed of the probe or the cylindrical vessel; g and h – constants. In this equation g (the intercept) is proportional to the yield stress τ0 and h (the gradient) is proportional to the plastic viscosity ηpl of the material. The yield stress value g and plastic viscosity h, called the rheological parameters, are material constants. They govern the rheological properties of the mixture. Once the stress exceeds the yield stress value, the mixture starts to flow with the speed proportional to the plastic viscosity. The lower the plastic viscosity of the mixture, the higher is the speed of flow at a given load. But also the risk of segregation increases with the decrease of this parameter.
In the SCC mix design each, even slight, change of components content (quality and quantity) can result in changing the properties of fresh mix. Fluidity and ability to flow between the reinforcing bars as well as the resistance to segregation, are the main parameters which can be altered.
There are numerous studies on the influence of the steel fibers [11, 20, 24, 32] or carbon fibers  on the rheology of fresh cementitious materials. Polymer fibers are very commonly used and their influence on the performance parameters of hardened cementitious composites is well described [16, 33, 34, 35]. However, there is still a need to evaluate their influence on the rheological parameters of cement based mixes. For this reason the author focused on the investigation of cementitious composites reinforced with different synthetic fibers in their plastic state. Three commercially available fibers were used in the experiment. The presented research was performed on cement mortars, which more adequately reflect fresh concrete rheological behavior than cement paste. Mortars exhibit behavior intermediate between those of concrete and of cement paste. They undergo structural breakdown and the measured data are sensitive to the previous shear history of the sample, but the equilibrium flow curve conforms to the Bingham model .
2 Experimental program
The experimental program focused on the investigation of the rheological properties of cement mortars containing three different synthetic fibers at two different volume fractions. The rotational viskometer Viskomat XL was used to measure the rheological parameters (yield stress and plastic viscosity). This allows to evaluate the rheological behavior of fresh mortars and concrete mixes, which in practice has more significance than measurements of cement grout. This is especially important in case of compositions reinforced with fibers, since aggregate has an influence on proper fibrillation of fibers, their distribution and packing in the matrix .
2.1 Materials, mortar mix proportions
Specimens were prepared with Portland cement CEM I 42.5R conforming to the standard PN-EN 197-1. A quartz sand of maximum particle size 4 mm was used as an aggregate. The grain size distribution of the sand is given in Figure 3. Sulfonated polynaphthalene based high range water reducing admixture (HRWRA - superplasticizer SP) was used in all mixes at 1.9% (by weight of cement).
Three types of synthetic fibers were used in this experimental program (Figure 4). They differ in length, structure and type of polymer. The choice of the fiber types was based on the analysis of the stock commercially available:
- –Fib1 - a synthetic fiber made of isotactic polypropylene, used to improve: resistance to plastic shrinkage of fresh concrete, resistance to microcracks and cracks in hardened concrete.
- –Fib2 - an upgraded, bundled, fibrillated high performance fiber serving as a structural concrete reinforcement;
- –Fib3 - a synthetic fiber made with a mix of two raw materials, used to improve ductility after shrinkage, resistance to impact, resistance to shrinkage and bleeding.
Properties and dosing rates of the particular fibers are given in Table 1.
Characteristics of the applied fibers.
|Material||Polypropylene PP||Refined polypropylen||Mix of polypropylene and polyethylene|
|Tensile strength, N/mm2||300÷400||400||650|
|Modulus of elasticity, N/mm2||–||4900||5000|
|Melting point, °C||>165||>150||>160|
|Dosing, kg per 1 m3||0.6 and 0.9||1.0 and 4.0||1.0 and 4.0|
Fiber aspect ratio was not used as an explicit variable in the programme, but it can be noted that it increases with fiber length.
A cement mortar with sand 0/4 mm was used in all tests using water/cement (w/c) ratio of 0.4. The mortar composition is given in Table 2. Fiber lengths and fiber volumes varied depending on the fiber type. Fib1 was dosed at 0.6 and 0.9 kg per 1 m3 of mortar, while Fib2 and Fib3 were dosed at 1.0 and 4.0 kg per 1 m3 of mortar. The dosage was based on the producers’ technical recommendations.
|Component||Mass per 1 m3 [kg]|
|Cement CEM I 42.5R||695|
|Fine aggregate (0/4 mm)||1325|
|SP (1.9% c.m.)||13.25|
2.2 Experimental procedure
The components were placed in the bowl of mortar mixer (conforming to the standard PN-EN 196) in the following order: water with the appropriate quantity of superplasticizer, cement, sand and fibers. Mixing time with rotation speed 140 min−1 was 3-4 minutes.
The resulting mortar was subsequently tested in the Viskomat XL (Figure 2). This apparatus is designed to test the rheological parameters of concrete mixtures and cement mortars. A measuring cylinder with an internal diameter of 165 mm and a height of 175 mm allows the measurement of a mixture of 3 dm3. The rotation speed can be adjusted between 0.001 and 80 min−1. During the test, the measuring cylinder was located in an outer cylinder filled with water flowing through the thermostat, which allowed to keep the constant temperature during the whole test. In this study the constant temperature of 20 ± 1°C was kept during the tests.
The times at which the test started were 10, 40, and 80 min. counting from the moment of mixing the constituent materials. In this way the changes of the FRFM rheological parameters over time were investigated. In practice, parameters of fresh mixes after 60-80 minutes (when concrete mix is placed) are more important than directly after mixing. The procedure consisted of the premixing at low rotation speed (duration 240 s), the rest (duration 180 s) and the ramp decreasing rotation speed test (duration 96 s). The total duration of the test was 516 s. The first mixing cycle lasted two minutes and consisted of a steady increase in rotation speed to 0.167 s−1 (10 min−1) in the first 60 seconds. Then, for another 60 seconds, the speed was lowered until the rotation completely stopped. The second mixing cycle had the same time intervals, except that the maximum rotation speed was (0.083 s−1 (5 min−1). The first part of the protocol simulated the mixing process in a drum mixer of a transportation vehicle. The rheological parameters g and h were calculated from the final down-curve. The nine steps setup with rotation speeds from 0.450 to 0.050 s−1 (from 27 to 3 min−1) with 10 s each step is given in Figure 5. The first step was two seconds longer (12 s), however, the data was not taken into account in the first two seconds. The Bingham curve was generated from all collected data in the last step (90 seconds).
As a result of the measurement, a set of data expressing the resistance of the mix subjected to the stresses induced by the rotation of the impeller was obtained. By linear approximation of the obtained results from the down-curve, the flow line of the mixture is obtained and compared with the modified Bingham Equation (2), which allows to determine the rheological parameters of the tested mortars.
3 Results and discussion
Figure 6 shows a torque–rotation speed relationship for the mortar with w/c=0.40 containing 12 mm PP fibers (Fib1) at a dosage of 0.6 kg per 1 m3 measured after 10 min. It can be clearly seen that the mortar conforms to the Bingham model with the down-curve approximating to a straight line where flow is fully developed, although it is not superimposed on the up-curve and this would confirm that some structural breakdown during the rest period has occurred. The best straight line can be fitted to Equation (2) through the points on the down-curve after removing the highest values - the first 2 seconds after the highest rotation speed (0.450 s−1) was achieved (Figure 7). Values of g = 27.7 N⋅mm and h = 488 N⋅mm⋅s, with a correlation coefficient of 0.9935 (based on 85 points), were obtained for this particular measurement.
The same procedure of drawing the down-curve and linear approximation of the experimental results was performed for all measurements. The test results for all three fibers at two different doses for three times of measurement are summarized in Table ??. All results are presented in terms of g and h values obtained directly from the experimental data. The changes of the yield stress (parameter g) over time depending on the type and content of the fibers are presented in Figure 8.
Rheological test results.
Yield stress is a property of the fluid at shear rate zero, mainly influenced by attractions and physical networking of the components. It is, hence, mainly affected by attraction forces and mechanical entangelments. The component in the form of dispersed fibers may affect these entangelments. With time elapsed, the variations of the first hydration products of cement increase attraction forces.
Initially, short PP fibers (Fib1) at the low dosage and 50 mm Fib3 regardless the dosage do not influence the yield stress value. High increase of yield value for fresh mortars with polypropylene fibrillated 19 mm fibers (Fib2) was observed, compared to the reference fresh mortar. Changes of yield value are consequence of fiber volume fraction in case of Fib1 and Fib2. High inclusion of PP fibers (0.9 kg per 1 m3) affected significantly the yield stress of fresh mortar. The main reason for this increase in the yield stress at higher dosage is that the cement matrix is reinforced with a large amount of dispersed fibers. This reinforcing by creating a spatial mesh increases the resistance of the mixture against shear stresses.
Influence of fiber length was not observed when 12 mm and 19 mm fibers were incorporated. At approximately the same content (Fib1 - 0.9 kg/m3and Fib2 -1 kg/m3) the values of yield stress are similar for both fiber lengths.
Yield stress slowly increased with elapsed time for the reference mortar sample and mortars containing Fib1 and Fib2. This is due to the diminishing effect of superplasticizer and the formation of the structure. However, in case of long fibers (Fib3) the growth of yield stress is much more evident. The fibrillation (splitting) of the fiber ends takes place during mixing, which increases the contact surface with the cement grout and determines the homogeneity of the composite. Evidently, the initial mixing during mortar preparation was too short, or not intense enough for full fibrillation and dispersion. During the first test (after 10 min) the additional mixing was performed, which could cause additional fibrillation, better homogeneity of fibers in the matrix and increase in contact surface which resulted in much higher yield stress measured after 40 min. Increasing dosage of fibers Fib3 had no effect on the value of the parameter g (Figure 7), because this type of fiber does not form a spatial mesh such as Fib1 or Fib2. He et al.  came to the similar conclusions that in the case of identical fiber volume fraction, the workability of fresh SCC reinforced with polypropylene monofilament fiber is better than that with fibrillated polypropylene network fiber.
The rheological properties of FRFM with PP fibers at lower content (0.6 kg/m3) are better than for FRFM with other types of fibers and even the reference mortar.
SCC is designed to flow under its own mass, resist segregation and meet other requirements. Compared to conventional concrete, SCC should exhibit significantly lower yield stress (near zero), which allows concrete to flow under its own mass. As it can be seen from Figure 8, fibers, especially fibrillated fibers, influence the yield stress and this should be taken into account during designing mix composition. In order to overcome this detrimental effect of fiber addition the proper preventive measures should be considered, such as increasing the dosage of superplasticizing admixture.
Plastic viscosity is a property of fluids already in motion. It is mainly determined by dynamic friction between moving elements. Plastic viscosity must not be too high or too low. When it is too high, fresh mix is sticky and difficult to pump and place, whereas when it is too low, fresh mix is susceptible to segregation. Thus, the control of this parameter is very important to maintain the proper performance of SCC.
The change in the value of parameter h (plastic viscosity) for the control fresh mortar is linear, i.e. viscosity increases systematically, but slightly over time (Figure 9). The influence of primary hydration products and the degree of SP adsorption on the dynamic friction seem to be minor. The presence of Fib1 and Fib2 at lower dosage at the beginning slightly increased the plastic viscosity compared to the control mortar. Over time, however, the situation changed. After 40 minutes, fiber addition insignificantly reduced the h parameter compared to the control mortar. The viscosity increase over time in the mortars with fibers was slower than that recorded for the fiber-free fresh mortar. High incorporation of Fib2 into mortar (4 kg/m3) significantly influenced its plastic viscosity starting from the first measurement. There was no increase of this property value following 40 minutes.
For Fib3 even low dosage of fibers to the mortar caused the significant increase in plastic viscosity compared to the fiber-free mortar. In case of higher content, the plastic viscosity increased over time more intensely comparing to other fresh mortars. It is in a correlation with the increase in the yield stress of this composition. The only exception in the behavior exhibited FRFM with low content of Fib3 - the plastic viscosity decreased, but still, after 40 and 80 minutes, it was considerably higher than for the reference mortar as well as for mortars with shorter fibrillated fibers.
One of the effects of incorporation of profiled, or fibrillated fibers at higher volume friction into SCC could be the control of the mix viscosity, which would result in better stability and higher resistance to segregation.
In this study the effect of fibrillated fibers Fib1 and Fib2, and profiled fibers Fib3 was examined. It has been shown that in the investigated range of composition factors of FRFM the addition of fibers to the fresh mortar changed the composite rheology. The type of fibers and their volume fraction had the highest impact on the yield stress increase of the tested FRFM. In case of the long profiled fibers (Fib3) also the quality of mixing process had the significant influence on this feature. The length of fibers did not have a significant influence on the yield value and plastic viscosity of the tested FRFM, which is in accordance with the results presented in . The fibrillated PP fibers (Fib1) at low content (0.6 kg per 1 m3) did not affect the yield stress or plastic viscosity. The increase in the plastic viscosity of FRFM was caused mainly by the increase of the fibers content. There was no influence of fibers on the plastic viscosity of the mixtures when fibers content did not exceed 1 kg per 1 m3 of the mortar. The efficiency of SP over time was better when fibers were incorporated, because the loss of fluidity (increase of yield stress) is slower compared to the fiber-free fresh mortar.
The effect of two types of polypropylene fibrillated fibers and one type of copolymer profiled fibers on the rheological properties of fresh cement mortars was investigated. Analysis of the obtained rheological parameters for the FRFM showed that:
- –The type of fiber and fiber volume fraction significantly influenced both rheological parameters of the FRFM.
- –Considering the effect of the fibers on the yield stress (g parameter), it was noted that the fibrillated fibers, developing their spatial structure during mixing, increase this feature as the dosage increases.
- –Increasing the dosage of profiled fibers (Fib3) did not affect the change in the yield stress. However, the increase of this parameter was much higher over time compared to other tested mortars.
- –The viscosity increase over time in the mortars with fibrillated fibers was slower than that recorded for the fiber-free fresh mortar.
- –Even low dosage of the profiled fibers (Fib3) caused the significant increase in the plastic viscosity compared to the fiber-free mortar. In case of higher content, the plastic viscosity increased over time more intensely comparing to other fresh mortars.
- –The length of fibers did not have a significant influence on the yield value and plastic viscosity of the tested FRFM.
- –The efficiency of SP over time was higher in FRFM, i.e. the loss of fluidity (increase of yield stress) was slower compared to the fiber-free fresh mortar.
The research was supported by the projects N° S/WBiIS/1/2016, and it was financially supported by the Ministry of Science and Higher Education, Poland.
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