reducing the amount and size of the shrinkage cracking, the dimen- sions of the fiber .... durally, after a crack was observed, up to four points were marked along ...
ACI MATERIALS JOURNAL
TECHNICAL PAPER
Title no. 97-M50
Shrinkage Cracking in Polyolefin Fiber-Reinforced Concrete by Nemkumar Banthia and Cheng Yan The effectiveness of fibers in controlling plastic shrinkage and thermal cracking in cement-based materials is well recognized. For any cement-based composite, however, the type of fiber and its dimensions are the two most important factors controlling such cracking. In this investigation, four types of polyolefin fibers (Types 19/15, 25/15, 25/38, and 50/63, where in l/d, l is the length of the fiber and d is the equivalent diameter; Type 19/15, for example is 19 mm long and 0.15 mm in equivalent diameter) were investigated. A newly developed technique was employed for this purpose. In this technique, fiber-reinforced concrete to be tested is laid on top of a fully hardened base concrete that provides the bottom restraint and this results in cracking in the freshly placed overlay. Cracking in the overlay is then monitored as a function of time. It was noted that while the polyolefin fibers are generally effective in reducing the amount and size of the shrinkage cracking, the dimensions of the fiber have a decisive influence on the results. For example, cracks widths that exceeded 1 mm in plain concrete specimens were reduced to less than 0.40 mm with 0.7% by volume of the 50/63 fiber, but were completely eliminated at the same volume fraction of Type 19/15 fiber. For a fiber, the specific fiber surface (a parameter defined as the fiber surface area in a unit volume of the composite) appears to be of critical importance. Along with a description of the test procedure, analysis scheme, and the results, this study also provides recommendations for future work. Keywords: crack; fiber-reinforced concrete; shrinkage.
INTRODUCTION When cement paste is in the plastic state, it undergoes a volumetric contraction as high as 1% of the absolute volume of dry cement. This is due to both the evaporation of mixing water and the autogenous process of concrete hydration. If restrained, this contraction can cause strains far in excess of those needed to cause cracking in young pastes with poorly developed strength. In spite of every effort, plastic shrinkage cracking still remains a real concern, particularly in large surface area placements like slabs-on-grade, thin surface repairs, patching, and shotcrete tunnel linings. In these applications, the exposed surface area per unit volume of the overlay material is high and the old concrete substrate or the rock surface offers a high degree of restraint. Among the different solutions proposed for controlling shrinkage cracking in such applications, the most promising is the use of randomly distributed fibers of steel and polypropylene, among others, which provide bridging forces across cracks and thus prevents them from growing.1,2 There exists several techniques for studying shrinkage induced cracking in cement-based materials. These include, for example, a ring type specimen,3 a linear specimen with anchored ends,2 a linear specimen held between a movable and a fixed grip such that a complete restraint and one-dimensional fixity are achieved by returning the movable grip to the original position after shrinkage,4 and a plate type specimen where the restrain is provided in two orthogonal directions.5 While effective for laboratory measurements, most of these 432
techniques produce stress fields in the specimen that are different from those occurring in practical applications. A technique producing realistic shrinkage conditions was recently developed.6,7 In this method, a layer of fresh concrete is placed directly on a fully hardened substrate. This old substrate is given an exposed aggregate finish that enhances its roughness and, in turn, imposes a uniform restraint on the still shrinking overlay. The whole assembly is then subjected to a drying environment to induce cracking in the overlay. The objective of this study was to further develop this technique and to investigate the effect of polyolefin fiber on restrained shrinkage cracking in concrete. RESEARCH SIGNIFICANCE Shrinkage induced cracking in concrete is believed to be one of the primary causes of lack of adequate durability in concrete construction, and fiber reinforcement is believed to be one of the most effective ways of controlling such cracking. Unfortunately, however, there are no standardized techniques of conducting shrinkage tests on concrete with a volumetric restraint, and this has led to an inability to rationally quantify the usefulness of fibers in preventing shrinkage induced cracking in a drying environment. Need exists, therefore, to develop rational test methods of conducting restrained shrinkage tests on concrete and to produce useful and representative information for various fiber types under variable conditions of drying. Such an attempt was made herein. EXPERIMENTAL PROCEDURES Materials CSA Type 10 (ASTM Type I) normal portland cement was used. The fine aggregate was a clean river sand with a fineness modulus of approximately 2.3, and the coarse aggregate was pea gravel, with a maximum size of 10 mm. To produce high strength substrate bases, silica fume was added at a dosage rate of 10% by weight of cement. Four polyolefin fiber types, 19/15, 25/15, 25/38, and 50/63 (called Type A, B, C, and D, respectively), were investigated at several volume fractions. Detailed properties of these fibers are given in Appendix A. For comparative purposes, one mixture each of polypropylene and steel fiber was also investigated at producer recommended dosage rates. Preparation of substrate bases Substrate bases were prepared at least 28 days prior to the actual test to allow them to achieve sufficient strength. The bases were made from high-strength concrete and cured in ACI Materials Journal, V. 97, No. 4, July-August 2000. MS No. 99-110 received June 6, 1999, and reviewed under Institute publication policies. Copyright 2000, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the May-June 2001 ACI Materials Journal if received by February 1, 2001.
ACI Materials Journal/July-August 2000
ACI member Nemkumar Banthia is a professor of civil engineering at the University of British Columbia in Vancouver, Canada. He serves on various ACI and RILEM committees and chairs the Materials Division of the Canadian Society for Civil Engineering. In 1997, he received the American Concrete Institute’s Wason Medal for Materials Research. Cheng Yan is a research associate in the Department of Civil Engineering at the University of British Columbia, where he received his PhD in engineering materials. His research interests include high-performance cementitious materials and their dynamic behavior.
water for a period of at least 28 days. The mixture proportions are given in Table 1. To reduce the chances of breakage during handling and to enhance the linear stiffness, reinforcement in the form of steel bars (2 nos; 15 mm ∅) was provided along the length of the substrate. The bases were 40 x 95 x 980 mm in dimension. They were deliberately left somewhat smaller than the actual size of the overlay to be placed afterwards, to avoid curling-up at the ends. This way, the specimen was able to wrap over the bases and hence was restrained from upward curling. When the substrate concrete was still fresh, 25 ± 5 mm rounded aggregates were placed on the surface so that approximately half the aggregate remained exposed. The approximate density of aggregate on the surface was 12 aggregates over a 100 cm2 area. The bases were demolded 24 h later and placed in water at approximately 25 C for the following 28 days. Before the day of the test, these bases were dried overnight in air at approximately 22 C. Shrinkage test assembly The Plexiglas molds for the samples were assembled and oiled, and the air-dried substrate base was placed in the mold. A 60 mm deep layer of the mixture to be investigated was poured over the substrate base (Fig. 1). A vibrating table was used to compact the overlay and the surface was leveled using a trough. The entire assembly was then transferred to the environmental chamber. The Plexiglas environmental chamber measuring 1740 x 350 x 380 mm circulates hot air from one end of the chamber to the other at approximately 340 cfm. The chamber is equipped with humidity and temperature sensors [±1 C; ±2% relative humidity (RH)]. Signals from these sensors are sent to a personal computer that not only keeps a record of these parameters during a test but also controls the power supply to the fan and the heater as necessary. A constant temperature of 38 C and a constant RH of 5% were chosen. Under these conditions, an approximate rate of surface evaporation of 0.8 kg/m2/h was measured. This measurement was carried out as follows. A tray of clean tap water with an exposed surface area of 220 x 310 mm was weighed and then placed in the chamber. After approximately 2.5 hours, the tray was removed and weighed again to determine the loss of water. This loss was then directly related to the rate of surface evaporation per unit area per unit time. One hour and fifteen minutes after preparing the sample, the mold was removed very carefully. This is the earliest that demolding can occur without a possible specimen collapse. The process requires an extreme care because the concrete is still very weak. The total surface exposed to the drying environment greatly increases after demolding, and hence the time of demolding is critical and affects the amount and rate of shrinkage cracking. Choice of matrix for fiber-reinforced concrete mixtures Most commonly used shrinkage test techniques produce inconsistent cracking. Under identical conditions of environACI Materials Journal/July-August 2000
Table 1—Mixture proportions of substrate bases Ingredient Cement
Proportion 0.9
kg/m3 535.5
Silica fume
0.1
59.5
Water
0.28
166.6
Sand
1.36
809.2
Aggregate
1.36
809.2
Superplasticizer
3 (mL/kg cement)
1.6 L/m3
Fig. 1—Shrinkage test setup. ment and restraint, the same mixture may either crack substantially or not crack at all. Comparing materials, fiber types, and restraint conditions with such shrinkage test setups is, therefore, difficult. One of the objectives in this investigations was to devise mixtures that give consistent and reproducible cracking. This way alone, one could compare various fiber types and determine the fiber characteristics that are of importance. The concrete mixture should also be realistic, that is, it should be a concrete mixture that is used in practice, and should be compactible using traditional methods. It is also critical that the mixture does not segregate or bleed. Mixture segregation may result in an uneven distribution of fibers in the sample. After a lengthy trial and error process, a satisfactory mixture was obtained. The mixture had a water-cement ratio (w/c) of 0.48, a sand-cement ratio (s/c) and aggregate-cement ratio of unity and a silica fume content of 5% mass of cement (Table 2). The addition of silica fume minimized segregation and produced a cohesive mixture. Crack observations A continuous recording of crack widths and lengths commences once the first crack appears on the surface of the specimen. All cracks are marked in the order they appear and their widths and lengths are measured every hour until they are fully stabilized, that is, they stop from growing. Given the irregular nature of cracks and the rate at which they appear, measurements of widths and lengths are generally not very straightforward. Cracks appear very quickly and in a random order. The time elapsed between the first appearance of the crack and its growth to the maximum length and width is generally only approximately 10 min. Very often, when the first time a crack is observed, it may have already grown to approximately 80% of its final width. Also, cracks usually appear rapidly one after another, all in a matter of a few minutes. The width of a crack varies along the length and measuring the width of a crack at various locations along the length is a challenge. Given the wide variation, although the 10X microscope was capable of measuring the crack widths to an accuracy of 10 µm (0.01 mm), widths in this investigation were measured only to the nearest 50 µm (0.05 mm). Proce433
Table 2—Mixture proportions for fiber-reinforced concrete overlay mixtures Ingredient
Proportion
Cement
1.00
Silica fume Water
0.05 0.48
Sand Aggregate
Type A Type B Type C Type D ST 50/50
477
12
36
143
170
250
mm2
Lc
750
100
215
470
620
780
mm
Wmax
1.1
0.15
0.26
0.60
0.60
0.70
mm
1.00
Wc
4.78
0.20
0.67
1.83
2.78
3.62
mm
1.00
n
12
2
5
17
30
12
—
Type A Type B Type C Type D
nful
5
0
0
0
0
2
—
nful /n
0.417
0.000
0.000
0.000
0.000
0.17
—
PP 55
Ac
477
250
210
300
280
238
mm2
Lc
750
830
690
795
640
528
mm
Wmax
1.1
0.90
0.55
0.90
1.10
0.83
mm
Wc
4.78
3.15
2.16
3.06
3.03
2.52
n
12
23
15
18
13
8
nful
5
2
1
2
3
nful /n
0.417
0.087
0.067
0.111
0.231
Table 6—Results for 0.7% fiber volume None
Type A
Type B
Type C
Type D
Ac
477
20.5
57.1
109
mm2
mm
Lc
750
75
255
475
mm
—
Wmax
1.1
3
—
Wc
4.78
0.375
—
Table 4—Results for 0.3% fiber volume None
Type A
Type B
Type C
Type D
Ac
477
122.5
95.2
180
136.7
mm2
Lc
750
430
530
515
490
mm
Wmax
1.1
0.65
0.45
0.50
0.50
mm
Wc
4.78
1.71
1.42
2.30
2.26
mm
n nful
12
10
29
21
19
—
5
2
1
0
1
—
nful /n
0.417
0.200
0.034
0.000
0.053
—
durally, after a crack was observed, up to four points were marked along its length and the widths at these four points were monitored until all the cracks on the sample had stopped growing. The lengths of the cracks were measured with the aid of a piece of string and a measuring tape. The irregular and tortuous nature of the cracks meant that there existed a large margin of error in these measurements and the lengths were measured to the closest 5 mm only. Other parameters recorded were the total number of cracks n, and the number of cracks that fully developed over the entire width of the specimen nful. These are the actual number of cracks seen on the surface. RESULTS AND DISCUSSION Results for the various fiber types are given in Table 3, 4, 5, and 6 for 0.1, 0.3, 0.5, and 0.7% fiber volume fractions, respectively. Notice that in Table 3 comparative data for the 55 mm long fibrillated polypropylene fiber (each monofilament 0.03 mm in diameter, fiber denoted as PP 55) are also given. Similarly, in Table 5, the data for the 50 mm long hooked-end steel fiber (diameter of 0.5 mm, fiber denoted as ST 50/50) are given for a comparison. Values reported in Table 3 to 6 are averages taken for 2 or 3 specimens. A high coefficient of variation of up to 25% was noted. The definitions of the symbols used are as follows: Ac = total cracked area of the sample measured on the top surface of the specimen only. It is the summation of all individual crack areas calculated as average width x lengths. Lc = cumulative length of all cracks wider than 0.1 mm; = cumulative width of all cracks wider than 0.1 mm; Wc 434
None Ac
Table 3—Results for 0.1% fiber volume None
Table 5—Results for 0.5% fiber volume
No cracks
0.35
0.35
0.40
mm
0.65
0.97
2.09
mm
n
8
2
15
10
—
nful
5
0
0
1
—
nful /n
0.625
0.000
0.000
0.100
—
Wmax n
= =
width of the widest crack observed; total number of cracks observed physically on the surface of the specimen; and nful = number of cracks spanning the entire width of the specimen. Notice that there is a general decrease in total crack area Ac and in the width of the widest crack observed Wmax due to fiber reinforcement. There is also a general decrease in the cumulative crack length Lc due to fibers. This is related to the crack arrest properties of the fibers, which is also evident from the lesser number of cracks spanning the entire width nful noted in the fiber-reinforced specimens. To clearly understand the influence of fiber type on shrinkage cracking, it is necessary to relate the observed cracking in the specimens to some fiber geometrical parameter. One such parameter is the fiber aspect ratio (l/d). In Fig. 2, Ac is plotted against the fiber aspect ratio l/d for all fiber types at the various fiber volume fractions. Because the aspect ratio for polypropylene fiber is not clearly defined, the data for polypropylene fiber is not plotted in Fig. 2. While one notices a general decrease in the total crack area Ac, a definite and clear trend is not seen. Also, when the maximum crack width Wmax is plotted as a function of l/d, as in Fig. 3, once again, only a weak trend emerges. Fiber aspect ratio is only a geometrical parameter and does not explicitly involve the fiber volume fraction present in the mixture. The influence of fiber volume fraction may be introduced by considering the number of fibers crossing a crack. The idea of fiber count was previously proposed by Zollo.8 In Fig. 4, the total crack area Ac is plotted against the number of fibers per unit area N for all fiber types and fiber volume fractions. Note that the parameter N explicitly indicates the volume fraction of the fiber present in the mixture. Similarly, the maximum crack width Wmax is plotted as a function of N in Fig. 5. Notice that while the correlation is significantly improved over the parameter l/d, it is still not entirely satisfactory. Given that multiple cracking occurs in fiber-reinforced concrete during restrained shrinkage, it is the reinforcing ability of a fiber over a certain volume of the composite that is important rather than its stress transfer capability at an isolated crack only. With this mind, another interesting parameter to ACI Materials Journal/July-August 2000
Fig. 2—Total crack area versus fiber aspect ratio (l/d).
Fig. 5—Maximum crack width versus number of fibers per unit area (N).
Fig. 3—Maximum crack width versus fiber aspect ratio (l/d). Fig. 6—Total crack area versus specific fiber surface (Sf).
Fig. 4—Total crack area versus number of fibers per unit area (N).
Fig. 7—Maximum crack width versus specific fiber surface (Sf).
consider is the specific fiber surface Sf , which is defined as the fiber surface area in a unit volume of the composite. This parameter is calculated for the various fibers in Appendix A. Notice that like the Parameter N and the Sf is a parameter dependent not only on the fiber geometry but also on the fiber volume fraction in the mixture. Unlike N, however, Sf also indirectly involves the length of the fiber as the calculations of Sf are valid only over a large enough composite volume involving multiples of fiber length. This parameter therefore represents the combined influence of fiber length
and fiber diameter, as well the volume fraction, and is therefore a better way of representing the reinforcing action of fibers in a composite under shrinkage induced strains. Indeed, in Fig. 6, when the total crack area Ac is plotted against the Sf, a clear trend emerges for the polyolefin fiber. Similarly, when the maximum crack width Wmax is plotted as a function of Sf in Fig. 7, once again, a relatively better defined trend emerges. Converting fiber number to fiber surface apparently better represents the internal conditions in a material subjected to drying and shrinkage induced strains.
ACI Materials Journal/July-August 2000
435
CONCLUSIONS AND RECOMMENDATIONS Tests were performed to investigate the effectiveness of polyolefin fibers in controlling restrained shrinkage cracking in concrete. It was found that the fibers were very effective in reducing the extent of shrinkage cracking and in reducing the crack widths. While the fibers with a higher aspect ratio are generally more effective in controlling cracking, introduction of new parameter Sf appears to be a better indication of fiber performance in controlling shrinkage cracking. Based on this analysis, fiber Types 19/15 and 25/15 are more effective than Types 25/38 and 50/63. Only one concrete mixture and one set of environmental conditions were investigated in this study. For future work, it is recommended that additional data be generated with variations in the environmental conditions (wind velocity, and rate of surface evaporation) and changes in the mixture parameters. Clearly, the ultimate goal is to produce design charts where one could relate environmental conditions to the type and volume fraction of fibers required to produce crack-free concrete. There are some interesting trends evident in the data that should be further investigated through micromechanical fracture modeling. Such an effort will not only improve our understanding of the reinforcing mechanisms in these composites in the early stages of concrete hydration, but also help improve fiber geometry and other characteristics for a better performance. APPENDIX A Polyolefin fibers of different geometry were investigated. In this Appendix, the physical properties of these fibers and the different ways in which the geometrical features were characterized and quantified are described.
Fig. A1—Group of 10 fibrillated fibers. Table A1—Typical physical properties of polyolefin fibers Property
Results
Specific gravity (bulk relative density) Tensile strength
0.91 275 MPa
Modulus of elasticity Elongation at break
2647 MPa 15%
Ignition point Melt point
593 C (1100 F) 160 C (320 F)
Electrical conductivity
Low
Table A2—Dimension of fibers
Physical properties The physical properties of the Polyolefin fibers as given by the manufacturer are as given in Table A1.
Fiber Polyolefin Type A (19/15)
Length, mm 19
Average diameter, mm 0.15
Polyolefin Type B (25/15) Polyolefin Type C (25/38)
25 25
0.15 0.38
Polyolefin Type D (50/63) Polypropylene (PP
55*)
Steel (ST 50/50†)
Geometry Four fibers geometries: Type A 19/15, Type B 25/15, Type C 25/38, and Type D 50/63 were investigated. The designation of XX/XX was given by the manufacturer; the first number represents the length of the fiber in mm, and the second number is the effective diameter of the fiber in mm × 100. Fibers A and B are cylindrical in cross section with a diameter of 0.15 mm, while Fibers C and D are oval in a cross section. Their detailed properties are given in Table A2. In Table A2, the properties of the polypropylene and steel fibers investigated for a comparative evaluation are also given. Specific fiber surface Sf The specific fiber surface Sf is defined as the total surface area of all fibers within a unit volume of composite. The end faces of the fibers are neglected and the effect of fiber orientation is ignored. The total fiber surface in a unit volume of composite is given by Vf V 4V - ⋅ P ⋅ l = -----f ⋅ P f = --------f S f = ---------Af ⋅ l f Af d
where d is the diameter of the fiber. For fibrillated polypropylene fibers, where fibers are aggregated in a bunch of 10 or so filaments, the specific fiber surface may be calculated as follows (Fig. A1) 436
* †
50
0.63
55
0.03
50
0.50
Fibrillated polypropylene fiber. Hooked-end steel fiber.
Table A3—Specific fiber surface Sf at various fiber volumes Specific fiber surface (mm2/mm2 x m) Fiber volume fraction Fiber type Type 19/15 Type 25/15
0.1%
0.3%
0.5%
0.7%
26.67
80.0
133.3
186.7
Type 25/38
10.5
31.6
52.6
73.7
Type 50/63 PP 55
6.35 76.3
19.0 —
31.7 —
44.4 —
ST 50/50
—
—
40
—
V S f = -----f ⋅ P f = Af 5 1 3 × --- πd + 7 × --- πd 2.29V 6 2 V f ⋅ ------------------------------------------------------------------------------------ = ---------------f d 3 2 1 2 1 2 10 × --- πd + 9 ------- d – 3 × ------ πd 4 24 4
For the various fibers investigated, the specific surface areas are given in Table A3. ACI Materials Journal/July-August 2000
Table A4—Aspect ratios
ACKNOWLEDGMENTS
Type 19/15
Type 25/15
Type 25/38
Type 50/63
ST 50/50
127
167
65.8
79.3
100
The authors are indebted to Wee Yee Lee of the National University of Singapore for his assistance in the laboratory. The continued financial support of the Natural Sciences and Engineering Research Council of Canada is also thankfully acknowledged.
REFERENCES Table A5—Number of fibers per unit area N,
cm2
N Fiber volume fraction Fiber type Type 19/15 Type 25/15 Type 25/38
0.1%
0.3%
0.5%
0.7%
2.83
8.49
14.15
19.81
0.44
1.32
2.20
3.09
Type 50/63
0.16
0.48
0.80
1.12
ST 50/50
—
—
1.27
—
Aspect ratio The aspect ratios for the five fiber types are given in Table A4. The aspect ratio of the fibrillated polypropylene (PP 55) fiber is not clearly defined. Number of fibers per unit area N The number of fibers are given by N = [K(Vf /Af)] where K is a constant depending upon the fiber distribution (K = 1 for 1-D; K = 2/π for 2-D and K = 1/2 for a 3-D distribution). In Table A5, the numbers of fibers are given assuming a 3-D distribution.
ACI Materials Journal/July-August 2000
1. Rossi, P., “Steel Fiber-Reinforced Concrete (SFRC): An Example of French Research,” ACI Materials Journal, V. 91, No. 3, May-June 1994, pp. 273-279. 2. Banthia, N.; Azzabi, M.; and Pigeon, M., “Restrained Shrinkage Cracking in Fiber-Reinforced Cementitious Composites,” Materials and Structures, RILEM (Paris), V. 26, No. 161, 1993, pp. 405-413. 3. Grzybowski, M., and Shah, S. P., “Shrinkage Cracking of Fiber-Reinforced Concrete,” ACI Materials Journal, V. 87, No. 2, Mar.-Apr. 1990, pp. 138-148. 4. Bloom, R., and Bentur, A., “Free and Restrained Shrinkage of Normal and High-Strength Concrete,” ACI Materials Journal, V. 92, No. 2, Mar.Apr. 1995, pp. 211-217. 5. Khajuria, A., and Balaguru, P., “Plastic Shrinkage Characteristics of Fiber-Reinforced Cement Composites,” Fiber-Reinforced Cement and Concrete, R. N. Swamy, ed., E&FN Spon, London, 1992, pp. 82-90. 6. Banthia, N.; Yan, C.; and Mindess, S., “Restrained Shrinkage Cracking in Fiber-Reinforced Concrete: A Novel Test Technique,” Cement and Concrete Research, V. 26, No. 1, 1996, pp. 9-14. 7. Banthia, N., and Campbell, K., “Restrained Shrinkage Cracking in Bonded Fiber-Reinforced Shotcrete,” RILEM—Proceedings 35, The Interfacial Transition Zone in Cementitious Composites, Katz, Bentur, Alexander, and Arligui, eds., E&FN Spon, London 1998, pp. 216-223. 8. Zollo, R. F., “Synthetic Fiber Reinforced Concrete: Some Background and Definitions,” seminar on Polymeric Fibers: Their Effect on Concrete and Concrete Cracking, World of Concrete ‘89, Atlanta, Ga., Feb. 1989. 9. Paillere, A. M.; Buli, M.; and Serrano, J. J., “Effect of Fiber Addition on the Autogenous Shrinkage of Silica Fume Concrete,” ACI Materials Journal, V. 86, No. 2, Mar.-Apr. 1989, pp. 139-144.
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