Effect of different geometric polypropylene fibers on plastic shrinkage cracking of cement mortars

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1 Materials and Structures/Matériaux et Constructions, Vol. 35, April 2002, pp Effect of different geometric polypropylene fibers on plastic shrinkage cracking of cement mortars Y. Ma, M. Tan and K. Wu State Key Laboratory of Concrete Materials Research, Tongji University, Shanghai , China SCIENTIFIC REPORTS Paper received: October 27, 2000; Paper accepted: July 31, 2001 A B S T R A C T Three kinds of different geometric (cross-section) polypropylene fibers were used to investigate their effects on plastic shrinkage cracking of cement mortars. The test results showed that polypropylene fiber made in drawingwire technique with circular cross-section (abbreviated as DW-PP fiber) had a more pronounced effect on the resistance to plastic shrinkage cracking than the polypropylene fiber made in a fibrillated film technique with rectangular cross-section (abbreviated as FF-PP fiber). Polypropylene fiber with Y-shape cross-section had a little more effective than that of DW-PP fiber. For the three kinds of polypropylene fibers, the plastic shrinkage cracking resistance for cement mortar increased with fiber addition changing from 0.05% to 0.15%. It was observed that there was almost no plastic shrinkage cracking even when the volume fraction of DW-PP fiber was as low as 0.10%. The mechanism of the resistance effect of different geometric polypropylene fibers on the plastic shrinkage cracking of cement mortars is discussed. R É S U M É Cet article présente l influence des fibres de polypropylène aux différentes formes géometriques sur la fissuration sous retrait plastique du mortier de ciment. Les résultats expérimentaux indiquent que la largeur de fissure et la valeur pondérée de fissuration globale des mortiers de ciment diminuent clairement lorsque l on incorpore un certain volume de fibres de polypropylène. Dans la résistance à la fissuration sous retrait plastique du mortier, la fibre fabriquée par le tirage de fil est plus forte que celle des fibres fibrillées, mais elle est un peu moins forte que la fibre en forme d Y. En fonction du volume de fibres, la résistance à la fissuration sous retrait plastique des mortiers fibrés en volume de 0,05% à 0,15% augmente. La fissuration du mortier incorporé en volume de 0,10% de fibres, fabriquées par le tirage de fil, a disparu en substance. L article présente enfin la recherche sur le mécanisme permettant d atténuer la fissuration des mortiers par la fibre de polypropylène en différentes formes. 1. INTRODUCTION Concrete is probably the most widely used building material in the world. However it does have some limitations. One of them is plastic shrinkage cracking produced in a large plain concrete slab cast on a dry, windy day in the first 4 to 6 hours after casting. This is caused by tensile stresses developed due to restraints provided at the edges of the slab, and at its bottom. These plastic shrinkage cracks will affect not only the appearance of concrete but also its mechanical and physical properties. It was found in recent years that the addition of a small quantity of polypropylene fibers, as small as 0.6 kg/m 3, is effective in reducing this plastic cracking [1, 2]. However, the effect of different geometric polypropylene fibers on plastic shrinkage cracking has not been fully reported yet. In this paper the effects of different geometries of polypropylene fibers on the resistance of plastic shrinkage cracking are reported. Almost all the plastic shrinkage cracks are produced in the mortar fraction of the concrete [3]. Therefore, a test for direct measurement of the drying shrinkage cracks, based on a thin mortar plate specimen, was used in this investigation. This test method can maximize the cracking potential of the test specimen and provides numerical results of the test within a few hours /02 RILEM 165

FF-PP I width 0.32 thickness 0.032 FF-PP II width 0.32 thickness 0.032 DW-PP 0.043 Y-PP 0.085 2. MATERIALS AND EXPERIMENTAL METHODS 15 8 400 8 0.

They are polypropylene fibers made in the drawing-wire technique (abbreviated as DW-PP fiber), polypropylene fibers made in two different fibrillated film techniques (abbreviated as FF-PP I fiber and

The physical and mechanical properties of polypropylene fibers are given in Table 1. Fig. 1 presents the morphologies of these fibers as seen in SEM.

2 Materials and Structures/Matériaux et Constructions, Vol. 35, April 2002 Table 1 Physical and mechanical properties of polypropylene fibers Fiber Diameter Length Elongation Tensile Elastic Density type strength modulus (mm) (mm) (%) (MPa) (GPa) (g/cm 3 ) FF-PP I width 0.32 thickness FF-PP II width 0.32 thickness DW-PP Y-PP MATERIALS AND EXPERIMENTAL METHODS In all the specimens, the same ordinary Portland cement and sand were used. Polypropylene fibers were produced by the Chinese Textile University. They are polypropylene fibers made in the drawing-wire technique (abbreviated as DW-PP fiber), polypropylene fibers made in two different fibrillated film techniques (abbreviated as FF-PP I fiber and FF-PP II fiber, the difference between them is the different interlinking state between their elements) and polypropylene fiber made with Y shape (abbreviated as Y-PP fiber), respectively. The physical and mechanical properties of polypropylene fibers are given in Table 1. Fig. 1 presents the morphologies of these fibers as seen in SEM. The test specimen form was mm, in which a steel wire frame (8 mm in diameter and mm in size) was installed just inside the perimeter of the form. This frame restrains the movement caused by drying shrinkage. Polyethylene film was placed in the form before casting of the mortar to prevent the bottom surface from absorbing water. The mortar mixture was mixed in a normal mixer. The mix composition was cement:sand:water = 1:1.5:0.5 (by weight). Fibers were added at the required volume fraction. After 3 minutes of mixing, the mixture was placed into the form, then screened and troweled to produce a smooth surface. Immediately after casting, the test specimens were exposed to a wind with velocity of 5 m/s, induced by a fan. At the same time, the specimens were heated by lamps (1000 W) placed 1.5 m above the samples. The heating was maintained for 4 hours, while the wind was applied during 24 hours. The measurement of the cracks was started at (a) FF-PP I fiber (c) DW-PP fiber (b) FF-PP II fiber (d) Y-PP fiber Fig. 1 Morphologies of polypropylene fibers. 166

3 Ma, Tan, Wu Table 2 Weighted values of cracking widths Crack width d (mm) Weighted value (A i ) Large (d 3) 3 Medium (3 > d 2) 2 Small (2 > d 1) 1 Hairline (d < 1) hours after casting. Crack lengths and widths were recorded for each specimen. Crack widths were divided into four categories, each of which was assigned a weighted value (A i ) as shown in Table 2. For each crack, the weighted value was multiplied by the crack length in centimeters (l i ) to get the weighted average value (W i ). The sum of the weighted average values of all cracks in a specimen was called the total cracking weighted value (W) for the specimen. This value represented the cracking potential of the specimen. 3. RESULTS AND DISCUSSIONS 3.1 Effect of different geometry of polypropylene fibers on drying shrinkage cracking of cement mortar Table 3 shows the test results for total cracking weighted values of the three kinds of polypropylene fiber modified cement mortars and the normal cement mortar for comparison. The data indicate that the four polypropylene fibers could all reduce the total cracking shrinkage values except for the FF-PP I fiber at 0.05% fiber content. However there were considerable differences in reducing drying shrinkage cracking. It can be seen from Table 3 that DW-PP fibers have the best effect on the dry shrinkage resistance characteristic of cement mortar. As compared to the control mortar, the total cracking weighted value can be decreased by 52.5% when only 0.05% DW-PP fiber is added, whereas for FF-PP II fiber and Y-PP fiber the reduction reaches only 36.6% and 44.7% respectively at the same fiber content. Especially for the FF-PP I fiber, there is no beneficial effect when the volume fraction of fiber is as low as 0.05%. The results imply that although all the fibers are of polypropylene type, they have different Table 3 Cracking potentials of panels containing four kinds of polypropylene fibers Fiber kind Total cracking weighted value (cm) / Relative total cracking weighted value (%) Vf = 0% Vf = 0.05% Vf = 0.10% Vf = 0.15% FF-PP I / / / / 48.7 FF-PP II / / / / 37.2 Y-PP / / / / 12.3 DW-PP / / / 0 0 / effects on drying shrinkage cracking because of their different geometric shapes (cross-section). From Table 3 it can also be seen that the total cracking weighted values decrease with increasing fiber content for all kinds of polypropylene fibers. It is easy to find out that the order of reducing is FF-PP I fiber < FF-PP II fiber < Y-PP fiber < DW-PP fiber. Especially when the content of DW-PP fibers increases to 0.10% or higher, and under the experimental conditions, no cracks at all could be found by naked eyes. The reasons for the above results will be discussed hereafter. 3.2 Effect of different geometry of polypropylene fibers on distribution of crack width in cement mortars Fig. 2 shows distributions of crack length in terms of crack widths for different geometries of polypropylene fibers. It can be seen that all the polypropylene fibers can reduce the crack length of 3 mm > d 2 mm cracks except for Y-PP fiber at 0.05% fiber content. The medium cracks (2 mm > d 1 mm) can also be diminished by all the polypropylene fibers except for FF-PP I fiber at the 0.05% fiber content. For d < 1 mm cracks, Y-PP fiber and DW-PP fiber show better effects than the two kinds of FF-PP fibers except for DW-PP fiber at the 0.05% fiber content. In general, it can be concluded that all the polypropylene fibers have the effect of preventing cracks from widening, and the order of the capacity for all the polypropylene fibers in preventing cracks from widening is as follows: FF-PP I fiber < FF-PP II fiber < Y-PP fiber < DW-PP fiber. 3.3 Mechanism of plastic shrinkage crack reduction for different geometries of polypropylene fibers After concrete is cast, and if the rate of water evaporation is higher than that of the migration of water from the inside of the fresh concrete to the outer surface, the capillary suction will produce shrinkage, that induces a tensile stress into the fresh concrete. Since the tensile strength of fresh concrete f p at early age is very low, the tensile stress σ t will exceed the tensile strength of fresh concrete, and as a consequence, the substantial cracking known as plastic shrinkage cracking will occur. When small quantities of polypropylene fibers are mixed into the cement matrix, the interlocking network characteristics of the fibers will prevent the large particles from sedimentation. The quantity of bleed water may be reduced. On the other hand, because of the cohesion between the fiber and the cement matrix the tensile strength of the fresh concrete will increase. If the tensile

4 Materials and Structures/Matériaux et Constructions, Vol. 35, April 2002 Fig. 2 Crack width distributions of different geometric polypropylene fibers cement mortars. strength of the fresh concrete f p is larger than the tensile stress σ t, the plastic shrinkage cracking will decrease or will even be eliminated. It was reported [4] that the tensile strength of normal concrete during the first 4 hours is as low as 0.02 MPa. If the cracking stress of 0.02 MPa is sustained by 0.1% (V f ) of random fibers, the fiber stress can reach 40 MPa. Compared to the tensile strength of polypropylene fibers (shown in Table 1), that fiber stress is a very low value. It means that if the bond between fibers and matrix in the fresh concrete is good, it is possible that the plastic shrinkage cracks could be limited by polypropylene fibers. It is assumed that the effect of polypropylene fiber reinforcement on the plastic shrinkage cracking is somewhat similar to the effect of reinforcement on cracking induced by loading of hardened cement concrete. According to the fiber spacing theory of fiber cement composites [5], the average fiber spacing (S), for random fibers, which is defined as the distance between two fibers, is: S = 13.8d(1/V f ) 1/2 (1) Equation (1) indicates that S will increase with the increase in fiber diameter d and decrease with the increase in fiber volume fraction V f. Since the diameters (or equivalent diameters) of the FF-PP I fiber, FF-PP II fiber, Y-PP fiber and DW-PP fiber are mm, mm, mm and mm respectively, and based on Equation (1), it can be found that for the same fiber content, the DW-PP fiber has the smallest S value since it has the smallest diameter. Hence it will have the best effect amongst the four fibers. This is consistent with the research results. As to the difference between the two FF-PP fibers, the reason may be due to the difference of interlinking states of the fiber elements. When the fiber volume fraction increases, the plastic shrinkage cracking will be diminished for each kind of polypropylene fiber since the S value decreases. If one uses a reduction of about 50% for the total weighted crack as a reference criterion, it can be found from Table 3 that such criterion is met at the fiber volume fractions of about 0.15%, 0.10%, 0.05% and 0.05% for FF-PP I fiber, FF-PP II fiber, Y-PP fiber and DW-PP fiber respectively. From this comparison, it can be concluded that the effects of the latter two fibers on drying shrinkage cracking are approximately 2 ~ 3 times higher than the former two fibers. From comparison between the latter two fibers, it can be seen that although the diameter of the Y-PP fiber is about 2 times that of DW-PP fiber, under the above criterion, the Y-PP fiber can reach the same effect as the DW-PP fiber at almost the same fiber content. That means that the geometrical shape of a Y-PP fiber is more effective than that of a DW-PP fiber because of its larger interfacial bonding to the mortar matrix. 168

5 Ma, Tan, Wu 4. CONCLUSION (1) When polypropylene fibers of different geometries (cross-section) are mixed into cement mortars, the total cracking weighted values can be reduced as compared to that of plain cement mortar, except for the FF-PP I fiber at 0.05% fiber content. (2) With the increase in fiber volume, the total cracking weighted values are reduced more and the crack width becomes finer. (3) The finer the fibers, the greater the reduction of the total weighted crack values for the same fiber content. (4) The geometry (cross-section) shape of Y-PP fiber has a better effect on the drying shrinkage cracking than that of DW-PP fiber. In the present study, the order of reduction in drying shrinkage cracking is FF-PP I fiber < FF-PP II fiber < Y-PP fiber < DW-PP fiber. Especially when 0.10% or more DW-PP fibers are added into cement mortar, drying shrinkage cracking can be avoided. ACKNOWLEDGMENTS The authors are grateful for the financial support from The Key Major Program of Shanghai Education Bureau of China. REFERENCES [1] Soroushian, P., Mirza, F. and Alhozaimy, A., Plastic shrinkage cracking of polypropylene fiber reinforced concrete, ACI Materials Journal 92 (5) (1995) [2] Grzybowski, M. and Shah, S. P., Shrinkage cracking of fiber reinforced concrete, ACI Materials Journal 87 (2), (1990) [3] Kraii, P. P., Proposed test to determine the cracking potential due to drying shrinkage of concrete, Concrete Construction 30 (9), (1985) [4] Hannant, D. J., Fibre reinforcement in the cement and concrete industry, Materials Science and Technology 11 (9), (1995) [5] Shen, R. X., Fiber reinforced cement composite, Fiber Cement Products (2) (1984) (only available in Chinese). 169

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