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Effect of polypropylene fiber reinforcement on the properties of fresh and hardened concrete in the Arabian Gulf environment
CEMENT and CONCRETE RESEARCH. Vol. 18, pp. 561-570, 1988. Printed in the USA 0008-8846/88 $3.00+00. Copyright (c) 1988 Pergamon Press plc
EFFECT OF POLYPROPYLENE
FIBER REINFORCEMENT
FRESH AND HARDENED CONCRETE
ON THE PROPERTIES
OF
IN THE ARABIAN GULF ENVIRONMENT
A.J. Ai-Tayyib, M.M. Ai-Zahrani, Rasheeduzzafar and G.J. AI-Sulaimani King Fahd University of Petroleum and Minerals, KFUPM Box 1491, Dhahran 31261, Saudi Arabia.
(Communicated by C.D. Pomeroy) (Received Nov. 16, 1987) ABSTRACT
The effect of polypropylene fiber reinforcement (0.2 % by volume of concrete) on the workability, plastic shrinkage, drying shrinkage and strength of concrete has been studied in a detailed test program. Concrete specimens of different mix designs with and without polypropylene fibers have been prepared, cured in laboratory conditions and hot weather environment of the Arabian Gulf, and then tested at different ages for the determination of various properties. The results indicate that the inclusion of polypropylene fibers eliminates the plastic shrinkage cracking of concrete, and reduces the drying shrinkage to some extent. It also slightly improves the tensile and flexural strength of concrete.
Introduction Early cracking of fresh concrete is common in the Arabian Gulf environment where the evaporation of water from the fresh concrete surface is very high due to the prevailing high air temperature and dry winds. These early cracks may not be structurally harmful but they initiate and accelerate some severe deterioration processes such as reinforcement corrosion by facilitating the ingress of moisture, oxygen and chlorides to the steel surface. In normal climatic conditions, these type of cracks may be minimized by following good practices of proportioning, mixing, placing and curing of eonGrete. However, in the Arabian Gulf environment some additional measures are needed to control the early concrete cracking to acceptable level. The use of polypropylene fibers in concrete is such an additional measure. In the past few years, the use of polypropylene fibers in concrete has been considered as a practical alternative to the use of welded wire fabric in controlling the shrinkage and temperature cracks (],2). Although, there are 561
562
Vo[. A.J. Ai-Tavyib,
18, >;o. i
e[ al.
some laboratory investigations 3-9) as well as examples of commercial applications of polypropylene fiber reinforced concrete (2,!0), there is a lack of conclusive experimental data. The data obtained by different investigators on the effect of po[ypropylene fibers on the v~rious properties of concrete is at times conflicting. The differences in the results may be due to different mix proportions, specimen sizes, exposure conditions and testing procedures. Moreover, most of these studf~s ar~ based on a limited number of specimens.
This paper presents the results of an elaborat~ tpst program in which the effect of polypropylene fiber reinforcement on various properties of fresh and hardened concrete has been studied. Workability, plastic shrinkage, drying shrinkage and strength characteristics of polypropylene fiber reinforced concrete (PFRC) have been eva]uated and compared ~ith t h o ~ nf plain concrete (PC).
Experimental Program Materials
Commercially available polypropylene fibers in 20 mm (0.8 in) fibrillated bundles were used in this study. These fibers separate into individual strands during mixing and disperse evenly throughout th~ concrete mix. Some of the properties of the polypropylene fibers are shown ir~ Table [. The other materials used were ASTM Type I portland cement, limestone coarse aggregate, dune sand, 12 mm (No. 4) deformed bars and potable water.
Specimen Preparation and Testing Three mixes with different water-cement (W/C) ratios of 0.45, 0.55 and 0.65 were prepared in this study. A cement content of 400 kg/m 3 (674 Ib/yd 3) and coarse to total aggregate ratio of 0.6 was used in all the mixes. A TABLE I Some Properties of polypropylene fibers
Melting point Ignition point Tensile strength Young's modulus
160-1700C 5900C 550-760 MPa 3.5 GPa
Thermal conductivity
]ow
Electrical conductivity
low
Acid and salt resistance
high
Vol. 18, No. 4
563 FIBER REINFORCEMENT,
POLYPROPYLENE
TABLE II Summary of the experimental
Tests
Type of specimen
Curing condition
program
Testing age (days)
No. of specimens
PFRC
PC
Plastic shrinkage
750 x 375 x 75 mm slabs
field, high wind
first day
18
18
Drying shrinkage
50 x 50 x 250 mm prisms
control
7,14,21,28, 42,56,70,84
72
72
Compressive strength
75 x 150 mm cylinders
control, field
28,60, 120,240
72
72
Tensile strength
75 x 150 mm cylinders
control, field
28,60, 120,240
72
72
Flexural strength
50 x 50 x 250 mm prisms
control, field
28,60, 120,240
72
72
polypropylene fiber content, 0.2% by volume of concrete, was used in the PFRC mixes. From each W/C ratio mix, specimens were made with and without polypropylene fibers. The type of specimens, curing conditions and tests used for the determination of various properties are described below and summarized in Table II:
Plastic Shrfnkago:
Slabs having dimensions of 750 x 375 x 75 mm (30 x 15 x 3 in) were prepared for plastic shrinkage crack monitoring. Both PFRC and PC slabs were subjected to two curing conditions, field curing, and high wind curing. In the field curing condition, the slabs were placed on an exposure site in the natural hot weather environment immediately after casting. Figure 1 shows the climatic conditions of Dhahran (a city located on the Arabian Gulf coast) in a typical summer day when the slabs were exposed. While in the high wind curing condition, the slabs were subjected to uniform wind blowing with a velocity range of 16-24 km/hr (9.6-14.4 mi/hr). The wind was generated by electric fans. The slabs were subjected to this type of curing in the laboratory immediately after casting. Plastic shrinkage crack monitoring was conducted during the first 24 hours of casting.
Drying Shrinkage:
Drying shrinkage measurements were carried out on 50 x 50 x 250 mm (2 x 2 x i0 in) prisms for 3 months after casting according to ASTM C-157. The specimens were cured by complete immersion in water for 7 days 8nd then stored in a moisture conditioning oven at a temperature of 230C (73.5 F) and a relative humidity of 60 ± 5% till the time of testing.
564
Vol.
!~, N o .
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--
L,5
70
~0
60
35
-
--
50 '~ -r-
t~ U-ell
I---
3O
25
I
I
I
I
I
4
8
12
16
20
30 24
Time (hours) FIG. 1 Climatic conditions of Dhahran (located on the Arabian Gulf coast) in a typical summer day
Compressive,
T e n s i l e and F l e x u r ~ l S t r e n g t h : For compressive strength and split tensile strength, 75 x 150 mm (3 x 6 in) cylinders were tested according to ASTM C-39 and ASTM C-496 respectively. While for the flexural strength, 50 x 50 x 250 mm (2 x 2 x i0 in) prisms were tested according to ASTM C-78. The PFRC and PC specimens were cured in the field and ~n normal laboratory conditions. In the field curing, the specimens were demolded 24 hours after casting, covered with wet burlap for 7 days on the exposure site, and then left on the exposure site till the time of testing. While in control curing, the specimens were demolded 24 hour after casting, completely immersed in water for 28 days, and then left in the laboratory til] the time of testing. The strengths were determined at the ages of 28, 60, 120 and 240 days.
All the tests were carried out on a set of at least 3 specimens. Test Results and Discussion Effect of Polypropylene Fibers on the Workabi]ity
Figure 2 shows the variation of slump with W/C ratio for both PFRC and PC. The slump of all the PFRC mixes is lower than that of the corresponding PC mixes. However, during the mixing and placing of concrete it was noticed that the reduction in the fluidity and mobility of the PFRC mixes is not that much as it is indicated by the reduction in the slump. A slight reduction in the fluidity and mobility of PFRC mixes was observed as compared to similar mixes of PC. These results indicate that the conventional slump test is not adequate for measuring the actual workability of PFRC. Similar results have been reported by Barr and Liu (8) where it has been indicated that the addition of polypropylene fibers, 0.5% by volume, reduces the shlmp of a 0.5
Vol.
18, No. 4
565 FIBER REINFORCEMENT,
POLYPROPYLENE
• PoLypropyleneFiber ReinforcedConcrete 20 _ • PtainConcrete 16
/ l -
/
/x
8
E
0.~5
0.55
065
Water - Cement Ra~io FIG. 2 Slump of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC) W/C ratio mix from 88 mm (3.5 in) to 12 mm (0.5 in), although the mix flows satisfactorily and responds well to the vibration. ACI Committee 544 (II) is now considering an inverted slump cone test to replace the conventional slump test for fiber reinforced concrete. This test primarily measures the mobility of the mix and takes into account the effects of size, shape and gradation of the aggregate, air content of the mix, and surface friction of the fibers.
TABLE
III
Results of plastic shrinkage crack monitoring
Mix
designation
PC-0.45 PFRC-0,45 PC-0.55 PFRC-O. 55 PC-O. 65 PFRC-O. 65
Crack width (mm)
Field curing
High wind curing
++ ++ 0. i-0.4 ++ 0.2-1.0 ++
++ ++ 0. i ++ 0.i-0.2 ++
* PC-0.45 :Plain concrete with W/C ratio of 0.45 P F R C - 0 . 4 5 : P o l y p r o p y l e n e fiber reinforced concrete with W/C ratio of 0.45 ++ No cracks
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Effect of Polypropylene Fibers on the Plastic Shrinkage
The PFRC slabs showed no plastic shrinkage cracks, while cracks were noticed in the PC slabs. The width of the cracks in the PC slabs varied from 0.I to i mm (0.004 to 0.04 in) (Table IIl). Figures 3 and 4 show the surfaces of the uncracked PFRC and cracked PC slabs, respectively, having a W/C ratio of 0.55 and cured in the field. The cracking of the PC slabs oceured during the first 6 to 8 hours and there was no significant change either in the number of cracks or the width of the cracks after 24 hours of monitoring. These results
FIG. 3 Uncracked surface of field cured polypropylene fiber reinforced concrete (PFRC) slab with W/C ratio of 0.55
"
t
FIG. 4 Cracked surface of field cured plain concrete (PC) slab with W/C ratio of 0.55
Vol, 18, No. 4
567 FIBER REINFORCEMENT,
POLYPROPYLENE
indicate that polypropylene fibers have the potential to control the plastic shrinkage cracking of concrete exposed to severe environmental conditions like the one existing in the Arabian Gulf countries. In the two test conditions used in this study, the slabs were subjected to temperatures as high as 40-460C (I04-I150F) and wind with a velocity of 16-24 km/hr (9.6-14.4 mi/hr). The results obtained by Dahl (3) and Kraai (6) are supportive of the results obtained in this study, although different exposure conditions have been used in their studies. Effect of Polypropylene
Fibers on the Drying Shrinkage
Figure 5 shows the variation of drying shrinkage with time of PFRC and PC mixes with different W/C ratios. The drying shrinkage of PFRC mixes with W/C ratios of 0.45, 0.55 and 0.65 is 2%, 5% and 11% respectively, less than that of the corresponding PC mixes at an age of 70 days. An analysis of the results obtained by various investigators indicates that the drying shrinkage of PFRC depends upon various factors and the most important ones are the W/C ratio of concrete, age of concrete, fiber content, and fiber length to maximum aggregate size ratio. Taking into consideration these factors, the results obtained in this study contradict the findings of some researchers like Zollo (9) which indicate a drastic reduction in the drying shrinkage of concrete due to the addition of the polypropylene fibers. A 75% reduction in the drying shrinkage of PFRC as compared to PC has been reported by Zollo (9). The results in the present study indicate that there ~s reduction in the drying shrinkage of PFRC as compared to PC, but it is not much. Litvin (5) has also reported that the drying shrinkage of PFRC having a compressive strength of 2 0 - 3 1 M P a (3000-4500 psi) is 5 to 7% less than that of PC.
I 700
8 Po[ypropy{eneFiberReinforcedConcrete Plain Concrete ~ W/C:0.65
•
/~='~~~---~-- W/C:0.55
600 x
g
m
400
W/C:0.4S
300
i" c~
200 t
100 I
I
28
J
I
i
56
I
!
84
Time (Days)
FIG. 5 Drying shrinkage of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
568
Vol. 18, No° 4 A.J. Ai-Tayyib,
et al.
Effect of Polypropylene Fibers on the Strength
The compressive, split tensile and flexural strengths of PFRC and PC mixes with different W/C ratios and curing conditions are shown in Tables IV, V and VI. It is noticed that there is no improvement in the compressive strength of concrete by the inclusion of polypropylene fibers (Table IV), even some of the PFRC mixes show a slight drop in compressive strength. The increase or decrease in the compressive strength of PFRC mixes as compared to PC mixes is in the range of 1-6%. Little improvement has been achieved in the tensile and flexural strengths of PFRC. The tensile strength of PFRC mixes is 2-8% higher than that of the PC mixes (Table V). The improvement in the flexural strength of PFRC is of the order of 1-4% (Table VI). In general, these results are in agreement with those obtained by Dahl (3), Zollo et al. (4) and Litvin (5).
The adverse effect of curing in hot weather conditions on the compressive, tensile and flexural strengths of concrete is clearly noticed from the results obtained in this study. The inclusion of polypropylene fibers does not reduce the strength loss of concrete due to field curing. The loss in compressive strength, tensile strength and flexural strength due to field curing is of the order of 10-35%, 20-50% and 20-40% respectively, for PFRC as well as PC. Conclusions On the basis of the results conclusions can be drawn:
obtained
in the
present
study,
the
following
TABLE IV Compressive strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-0.55-Field PC-0.65-Control PC-0.65-Field PFRC-O.65-Control PFRC-0.65-Field
* PC-0.45-Control:
Compressive strength (MPa)
28 days
60 days
120 days
240 days
42.47 37 94 42 03 37 57 35 38 32 60 35 27 32 75 28 40 22 Ii 28 22 21.45
43.13 38.89 42.84 37.57 37.39 32.71 37.83 32.71 27.30 22.59 28.98 23.21
45.03 39.77 44.19 36.95 38.01 32.49 38.16 31.98 30.37 22.30 30.88 22.33
46 24 39 43 45 61 37 72 38 16 32 27 38 41 31.61 31.47 22.70 30.48 23.06
Plain concrete with W/C ratio of 0.45 and control curing
Vol. 18, No. 4
569 FIBER REINFORCEMENT,
POLYPROPYLENE
TABLE V Tensile strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-0.55-Field PC-0.65-Control PC-0.65-Field PFRC-0.65-Control PFRC-0.65-Field
* PFRC-0.55-Field:
Tensile strength (MPa)
28 days
60 days
120 days
240 days
5.43 4.42 5.52 4.63 4.39 3.61 4.57 3.77 3.47 2.70 3.63 2.83
6 29 4 40 6 30 4 65 4 62 3 50 471 3 83 3 64 2 72 3 98 2 85
6 4 6 4 4 3 4 3 4 2 4 2
6.33 4.35 6.63 4.45 4.95 3.76 5.27 3.90 4.37 3.08 4.29 2.99
02 47 40 66 74 64 86 82 08 77 24 81
Polypropylene fiber reinforced concrete with W/C ratio of 0.55 and field curing
TABLE VI Flexural strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
Flexural strength (MPa)
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-O.55-Field PC-0.65-Contro1 PC-0.65-Field PFRC-0.65-Control PFRC-0.65-Field
28 days
60 days
120 days
240 days
6.63 6.30 6.67 6.36 5.62 4.65 5.70 4.71 4.90 3.82 5.00 4.00
7.05 5.89 7.09 6.00 5.89 4.57 5.99 4.42 5.19 3.66 5.27 3.84
7.11 5.86 7.15 5.94 6.08 4.39 6.16 4.43 5.37 3.79 5.54 3.92
7.41 5.83 7.54 5.97 6.20 4.86 6.14 5.00 5.43 3.83 5.46 3.81
570
Vol. A.J. Ai-Tayyib,
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et al.
I.
The inclusion of polypropylene the concrete.
fibers slightly
2.
The inclusion of polypropylene fibers eliminates the plastic shrinkage cracking in slabs subjected to temperatures as high as 40-460C (104-115DF) and wind with a velocity of 16-24 km/hr (9.6-14.4 mi/hr). On the other hand, PC slabs with similar mix designs have plastic cracks of widths 0.1 to 1 mm (0.004 to 0.04 in).
3.
The influence of polypropylene fibers on the drying shrinkage of concrete is not significant. The drying shrinkage of PFRC mixes is 2 to 11% less than that of the corresponding PC mixes at an age of 70 days.
4.
The inclusion of polypropylene strength of concrete.
5.
The tensile and flexural strengths of concrete are slightly improved with the inclusion of polypropylene fibers. The tensile strength of PFRC mixes is 2-8% and flexural strength is i-4% higher than that of the corresponding PC mixes.
6.
The polypropylene fibers do not help in reducing the strength concrete that occurs due to curing in hot weather conditions.
fibers
does
reduces the workability
not
improve
the
of
compressive
loss
of
Acknowledgements The authors greatly acknowledge the support of the Department of Civil Engineering at King Fahd University of Petroleum and Minerals (KFUPM) for this research. References i. 2. 3.
4.
5.
6. 7.
8. 9. i0. ii.
W.R. Malisch, Concrete Construction 31 (4), 363-368 (1986). W.R. Malisch, Concrete Construction 31 (4), 371-377 (1986). P.A. Dahl, "Plastic Shrinkage and Cracking Tendency of Mortar and Concrete Containing Fibermesh", FCB Cement and Concrete Research Institute, Norway (1985). R.F. Zollo, J.A. Ilter, and G.B. Bouchacourt, "Plastic and Drying Shrinkage in Concrete Containing Collated Fibrillated Polypropylene Fibers", RILEM Symposium on Developments in FRC Composites, Shefield, England, 13-17 July (1986). A. Litvin, "Report to Wire Reinforcement Institute on Properties of Concrete Containing Polypropylene Fibers", Construction Technology Laboratories, Portland Cement Association (1985). P.P. Kraal, "Crack Control Methods: Welded Wire Fabric vs. CFP Fibers", for Fibermesh Company (1985). A.L. Landau, "A System for Inhibiting Steel Corrosion in Concrete", Conference on Improving Performance of Concrete in Marine Environments, ShangriLa, Hong Kong, 3-5 June (1987). B. Barr and K. Liu, Concrete 16 (4), 33-35 (1985). R.F. Zollo, "Collated Fibrillated Polypropylene Fibers in FRC", SP-81, American Concrete Institute, Detroit, Michigan (1984). H. Krenchel and S. Shah, Concrete International: Design and Construction ! (3), 32-34 (1985). ACI Committee 544, "State of the Art Report on Fiber Reinforced Concrete" (ACI.544.1R.82), American Concrete Institute, Detroit, Michigan (1984).
EFFECT OF POLYPROPYLENE
FIBER REINFORCEMENT
FRESH AND HARDENED CONCRETE
ON THE PROPERTIES
OF
IN THE ARABIAN GULF ENVIRONMENT
A.J. Ai-Tayyib, M.M. Ai-Zahrani, Rasheeduzzafar and G.J. AI-Sulaimani King Fahd University of Petroleum and Minerals, KFUPM Box 1491, Dhahran 31261, Saudi Arabia.
(Communicated by C.D. Pomeroy) (Received Nov. 16, 1987) ABSTRACT
The effect of polypropylene fiber reinforcement (0.2 % by volume of concrete) on the workability, plastic shrinkage, drying shrinkage and strength of concrete has been studied in a detailed test program. Concrete specimens of different mix designs with and without polypropylene fibers have been prepared, cured in laboratory conditions and hot weather environment of the Arabian Gulf, and then tested at different ages for the determination of various properties. The results indicate that the inclusion of polypropylene fibers eliminates the plastic shrinkage cracking of concrete, and reduces the drying shrinkage to some extent. It also slightly improves the tensile and flexural strength of concrete.
Introduction Early cracking of fresh concrete is common in the Arabian Gulf environment where the evaporation of water from the fresh concrete surface is very high due to the prevailing high air temperature and dry winds. These early cracks may not be structurally harmful but they initiate and accelerate some severe deterioration processes such as reinforcement corrosion by facilitating the ingress of moisture, oxygen and chlorides to the steel surface. In normal climatic conditions, these type of cracks may be minimized by following good practices of proportioning, mixing, placing and curing of eonGrete. However, in the Arabian Gulf environment some additional measures are needed to control the early concrete cracking to acceptable level. The use of polypropylene fibers in concrete is such an additional measure. In the past few years, the use of polypropylene fibers in concrete has been considered as a practical alternative to the use of welded wire fabric in controlling the shrinkage and temperature cracks (],2). Although, there are 561
562
Vo[. A.J. Ai-Tavyib,
18, >;o. i
e[ al.
some laboratory investigations 3-9) as well as examples of commercial applications of polypropylene fiber reinforced concrete (2,!0), there is a lack of conclusive experimental data. The data obtained by different investigators on the effect of po[ypropylene fibers on the v~rious properties of concrete is at times conflicting. The differences in the results may be due to different mix proportions, specimen sizes, exposure conditions and testing procedures. Moreover, most of these studf~s ar~ based on a limited number of specimens.
This paper presents the results of an elaborat~ tpst program in which the effect of polypropylene fiber reinforcement on various properties of fresh and hardened concrete has been studied. Workability, plastic shrinkage, drying shrinkage and strength characteristics of polypropylene fiber reinforced concrete (PFRC) have been eva]uated and compared ~ith t h o ~ nf plain concrete (PC).
Experimental Program Materials
Commercially available polypropylene fibers in 20 mm (0.8 in) fibrillated bundles were used in this study. These fibers separate into individual strands during mixing and disperse evenly throughout th~ concrete mix. Some of the properties of the polypropylene fibers are shown ir~ Table [. The other materials used were ASTM Type I portland cement, limestone coarse aggregate, dune sand, 12 mm (No. 4) deformed bars and potable water.
Specimen Preparation and Testing Three mixes with different water-cement (W/C) ratios of 0.45, 0.55 and 0.65 were prepared in this study. A cement content of 400 kg/m 3 (674 Ib/yd 3) and coarse to total aggregate ratio of 0.6 was used in all the mixes. A TABLE I Some Properties of polypropylene fibers
Melting point Ignition point Tensile strength Young's modulus
160-1700C 5900C 550-760 MPa 3.5 GPa
Thermal conductivity
]ow
Electrical conductivity
low
Acid and salt resistance
high
Vol. 18, No. 4
563 FIBER REINFORCEMENT,
POLYPROPYLENE
TABLE II Summary of the experimental
Tests
Type of specimen
Curing condition
program
Testing age (days)
No. of specimens
PFRC
PC
Plastic shrinkage
750 x 375 x 75 mm slabs
field, high wind
first day
18
18
Drying shrinkage
50 x 50 x 250 mm prisms
control
7,14,21,28, 42,56,70,84
72
72
Compressive strength
75 x 150 mm cylinders
control, field
28,60, 120,240
72
72
Tensile strength
75 x 150 mm cylinders
control, field
28,60, 120,240
72
72
Flexural strength
50 x 50 x 250 mm prisms
control, field
28,60, 120,240
72
72
polypropylene fiber content, 0.2% by volume of concrete, was used in the PFRC mixes. From each W/C ratio mix, specimens were made with and without polypropylene fibers. The type of specimens, curing conditions and tests used for the determination of various properties are described below and summarized in Table II:
Plastic Shrfnkago:
Slabs having dimensions of 750 x 375 x 75 mm (30 x 15 x 3 in) were prepared for plastic shrinkage crack monitoring. Both PFRC and PC slabs were subjected to two curing conditions, field curing, and high wind curing. In the field curing condition, the slabs were placed on an exposure site in the natural hot weather environment immediately after casting. Figure 1 shows the climatic conditions of Dhahran (a city located on the Arabian Gulf coast) in a typical summer day when the slabs were exposed. While in the high wind curing condition, the slabs were subjected to uniform wind blowing with a velocity range of 16-24 km/hr (9.6-14.4 mi/hr). The wind was generated by electric fans. The slabs were subjected to this type of curing in the laboratory immediately after casting. Plastic shrinkage crack monitoring was conducted during the first 24 hours of casting.
Drying Shrinkage:
Drying shrinkage measurements were carried out on 50 x 50 x 250 mm (2 x 2 x i0 in) prisms for 3 months after casting according to ASTM C-157. The specimens were cured by complete immersion in water for 7 days 8nd then stored in a moisture conditioning oven at a temperature of 230C (73.5 F) and a relative humidity of 60 ± 5% till the time of testing.
564
Vol.
!~, N o .
A.J. Al-Yayyib, et al.
--
L,5
70
~0
60
35
-
--
50 '~ -r-
t~ U-ell
I---
3O
25
I
I
I
I
I
4
8
12
16
20
30 24
Time (hours) FIG. 1 Climatic conditions of Dhahran (located on the Arabian Gulf coast) in a typical summer day
Compressive,
T e n s i l e and F l e x u r ~ l S t r e n g t h : For compressive strength and split tensile strength, 75 x 150 mm (3 x 6 in) cylinders were tested according to ASTM C-39 and ASTM C-496 respectively. While for the flexural strength, 50 x 50 x 250 mm (2 x 2 x i0 in) prisms were tested according to ASTM C-78. The PFRC and PC specimens were cured in the field and ~n normal laboratory conditions. In the field curing, the specimens were demolded 24 hours after casting, covered with wet burlap for 7 days on the exposure site, and then left on the exposure site till the time of testing. While in control curing, the specimens were demolded 24 hour after casting, completely immersed in water for 28 days, and then left in the laboratory til] the time of testing. The strengths were determined at the ages of 28, 60, 120 and 240 days.
All the tests were carried out on a set of at least 3 specimens. Test Results and Discussion Effect of Polypropylene Fibers on the Workabi]ity
Figure 2 shows the variation of slump with W/C ratio for both PFRC and PC. The slump of all the PFRC mixes is lower than that of the corresponding PC mixes. However, during the mixing and placing of concrete it was noticed that the reduction in the fluidity and mobility of the PFRC mixes is not that much as it is indicated by the reduction in the slump. A slight reduction in the fluidity and mobility of PFRC mixes was observed as compared to similar mixes of PC. These results indicate that the conventional slump test is not adequate for measuring the actual workability of PFRC. Similar results have been reported by Barr and Liu (8) where it has been indicated that the addition of polypropylene fibers, 0.5% by volume, reduces the shlmp of a 0.5
Vol.
18, No. 4
565 FIBER REINFORCEMENT,
POLYPROPYLENE
• PoLypropyleneFiber ReinforcedConcrete 20 _ • PtainConcrete 16
/ l -
/
/x
8
E
0.~5
0.55
065
Water - Cement Ra~io FIG. 2 Slump of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC) W/C ratio mix from 88 mm (3.5 in) to 12 mm (0.5 in), although the mix flows satisfactorily and responds well to the vibration. ACI Committee 544 (II) is now considering an inverted slump cone test to replace the conventional slump test for fiber reinforced concrete. This test primarily measures the mobility of the mix and takes into account the effects of size, shape and gradation of the aggregate, air content of the mix, and surface friction of the fibers.
TABLE
III
Results of plastic shrinkage crack monitoring
Mix
designation
PC-0.45 PFRC-0,45 PC-0.55 PFRC-O. 55 PC-O. 65 PFRC-O. 65
Crack width (mm)
Field curing
High wind curing
++ ++ 0. i-0.4 ++ 0.2-1.0 ++
++ ++ 0. i ++ 0.i-0.2 ++
* PC-0.45 :Plain concrete with W/C ratio of 0.45 P F R C - 0 . 4 5 : P o l y p r o p y l e n e fiber reinforced concrete with W/C ratio of 0.45 ++ No cracks
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=~ al
Effect of Polypropylene Fibers on the Plastic Shrinkage
The PFRC slabs showed no plastic shrinkage cracks, while cracks were noticed in the PC slabs. The width of the cracks in the PC slabs varied from 0.I to i mm (0.004 to 0.04 in) (Table IIl). Figures 3 and 4 show the surfaces of the uncracked PFRC and cracked PC slabs, respectively, having a W/C ratio of 0.55 and cured in the field. The cracking of the PC slabs oceured during the first 6 to 8 hours and there was no significant change either in the number of cracks or the width of the cracks after 24 hours of monitoring. These results
FIG. 3 Uncracked surface of field cured polypropylene fiber reinforced concrete (PFRC) slab with W/C ratio of 0.55
"
t
FIG. 4 Cracked surface of field cured plain concrete (PC) slab with W/C ratio of 0.55
Vol, 18, No. 4
567 FIBER REINFORCEMENT,
POLYPROPYLENE
indicate that polypropylene fibers have the potential to control the plastic shrinkage cracking of concrete exposed to severe environmental conditions like the one existing in the Arabian Gulf countries. In the two test conditions used in this study, the slabs were subjected to temperatures as high as 40-460C (I04-I150F) and wind with a velocity of 16-24 km/hr (9.6-14.4 mi/hr). The results obtained by Dahl (3) and Kraai (6) are supportive of the results obtained in this study, although different exposure conditions have been used in their studies. Effect of Polypropylene
Fibers on the Drying Shrinkage
Figure 5 shows the variation of drying shrinkage with time of PFRC and PC mixes with different W/C ratios. The drying shrinkage of PFRC mixes with W/C ratios of 0.45, 0.55 and 0.65 is 2%, 5% and 11% respectively, less than that of the corresponding PC mixes at an age of 70 days. An analysis of the results obtained by various investigators indicates that the drying shrinkage of PFRC depends upon various factors and the most important ones are the W/C ratio of concrete, age of concrete, fiber content, and fiber length to maximum aggregate size ratio. Taking into consideration these factors, the results obtained in this study contradict the findings of some researchers like Zollo (9) which indicate a drastic reduction in the drying shrinkage of concrete due to the addition of the polypropylene fibers. A 75% reduction in the drying shrinkage of PFRC as compared to PC has been reported by Zollo (9). The results in the present study indicate that there ~s reduction in the drying shrinkage of PFRC as compared to PC, but it is not much. Litvin (5) has also reported that the drying shrinkage of PFRC having a compressive strength of 2 0 - 3 1 M P a (3000-4500 psi) is 5 to 7% less than that of PC.
I 700
8 Po[ypropy{eneFiberReinforcedConcrete Plain Concrete ~ W/C:0.65
•
/~='~~~---~-- W/C:0.55
600 x
g
m
400
W/C:0.4S
300
i" c~
200 t
100 I
I
28
J
I
i
56
I
!
84
Time (Days)
FIG. 5 Drying shrinkage of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
568
Vol. 18, No° 4 A.J. Ai-Tayyib,
et al.
Effect of Polypropylene Fibers on the Strength
The compressive, split tensile and flexural strengths of PFRC and PC mixes with different W/C ratios and curing conditions are shown in Tables IV, V and VI. It is noticed that there is no improvement in the compressive strength of concrete by the inclusion of polypropylene fibers (Table IV), even some of the PFRC mixes show a slight drop in compressive strength. The increase or decrease in the compressive strength of PFRC mixes as compared to PC mixes is in the range of 1-6%. Little improvement has been achieved in the tensile and flexural strengths of PFRC. The tensile strength of PFRC mixes is 2-8% higher than that of the PC mixes (Table V). The improvement in the flexural strength of PFRC is of the order of 1-4% (Table VI). In general, these results are in agreement with those obtained by Dahl (3), Zollo et al. (4) and Litvin (5).
The adverse effect of curing in hot weather conditions on the compressive, tensile and flexural strengths of concrete is clearly noticed from the results obtained in this study. The inclusion of polypropylene fibers does not reduce the strength loss of concrete due to field curing. The loss in compressive strength, tensile strength and flexural strength due to field curing is of the order of 10-35%, 20-50% and 20-40% respectively, for PFRC as well as PC. Conclusions On the basis of the results conclusions can be drawn:
obtained
in the
present
study,
the
following
TABLE IV Compressive strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-0.55-Field PC-0.65-Control PC-0.65-Field PFRC-O.65-Control PFRC-0.65-Field
* PC-0.45-Control:
Compressive strength (MPa)
28 days
60 days
120 days
240 days
42.47 37 94 42 03 37 57 35 38 32 60 35 27 32 75 28 40 22 Ii 28 22 21.45
43.13 38.89 42.84 37.57 37.39 32.71 37.83 32.71 27.30 22.59 28.98 23.21
45.03 39.77 44.19 36.95 38.01 32.49 38.16 31.98 30.37 22.30 30.88 22.33
46 24 39 43 45 61 37 72 38 16 32 27 38 41 31.61 31.47 22.70 30.48 23.06
Plain concrete with W/C ratio of 0.45 and control curing
Vol. 18, No. 4
569 FIBER REINFORCEMENT,
POLYPROPYLENE
TABLE V Tensile strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-0.55-Field PC-0.65-Control PC-0.65-Field PFRC-0.65-Control PFRC-0.65-Field
* PFRC-0.55-Field:
Tensile strength (MPa)
28 days
60 days
120 days
240 days
5.43 4.42 5.52 4.63 4.39 3.61 4.57 3.77 3.47 2.70 3.63 2.83
6 29 4 40 6 30 4 65 4 62 3 50 471 3 83 3 64 2 72 3 98 2 85
6 4 6 4 4 3 4 3 4 2 4 2
6.33 4.35 6.63 4.45 4.95 3.76 5.27 3.90 4.37 3.08 4.29 2.99
02 47 40 66 74 64 86 82 08 77 24 81
Polypropylene fiber reinforced concrete with W/C ratio of 0.55 and field curing
TABLE VI Flexural strength of polypropylene fiber reinforced concrete (PFRC) and plain concrete (PC)
Mix
Flexural strength (MPa)
designation
PC-0.45-Control PC-0.45-Field PFRC-0.45-Control PFRC-0.45-Field PC-0.55-Control PC-0.55-Field PFRC-0.55-Control PFRC-O.55-Field PC-0.65-Contro1 PC-0.65-Field PFRC-0.65-Control PFRC-0.65-Field
28 days
60 days
120 days
240 days
6.63 6.30 6.67 6.36 5.62 4.65 5.70 4.71 4.90 3.82 5.00 4.00
7.05 5.89 7.09 6.00 5.89 4.57 5.99 4.42 5.19 3.66 5.27 3.84
7.11 5.86 7.15 5.94 6.08 4.39 6.16 4.43 5.37 3.79 5.54 3.92
7.41 5.83 7.54 5.97 6.20 4.86 6.14 5.00 5.43 3.83 5.46 3.81
570
Vol. A.J. Ai-Tayyib,
18, No. 4
et al.
I.
The inclusion of polypropylene the concrete.
fibers slightly
2.
The inclusion of polypropylene fibers eliminates the plastic shrinkage cracking in slabs subjected to temperatures as high as 40-460C (104-115DF) and wind with a velocity of 16-24 km/hr (9.6-14.4 mi/hr). On the other hand, PC slabs with similar mix designs have plastic cracks of widths 0.1 to 1 mm (0.004 to 0.04 in).
3.
The influence of polypropylene fibers on the drying shrinkage of concrete is not significant. The drying shrinkage of PFRC mixes is 2 to 11% less than that of the corresponding PC mixes at an age of 70 days.
4.
The inclusion of polypropylene strength of concrete.
5.
The tensile and flexural strengths of concrete are slightly improved with the inclusion of polypropylene fibers. The tensile strength of PFRC mixes is 2-8% and flexural strength is i-4% higher than that of the corresponding PC mixes.
6.
The polypropylene fibers do not help in reducing the strength concrete that occurs due to curing in hot weather conditions.
fibers
does
reduces the workability
not
improve
the
of
compressive
loss
of
Acknowledgements The authors greatly acknowledge the support of the Department of Civil Engineering at King Fahd University of Petroleum and Minerals (KFUPM) for this research. References i. 2. 3.
4.
5.
6. 7.
8. 9. i0. ii.
W.R. Malisch, Concrete Construction 31 (4), 363-368 (1986). W.R. Malisch, Concrete Construction 31 (4), 371-377 (1986). P.A. Dahl, "Plastic Shrinkage and Cracking Tendency of Mortar and Concrete Containing Fibermesh", FCB Cement and Concrete Research Institute, Norway (1985). R.F. Zollo, J.A. Ilter, and G.B. Bouchacourt, "Plastic and Drying Shrinkage in Concrete Containing Collated Fibrillated Polypropylene Fibers", RILEM Symposium on Developments in FRC Composites, Shefield, England, 13-17 July (1986). A. Litvin, "Report to Wire Reinforcement Institute on Properties of Concrete Containing Polypropylene Fibers", Construction Technology Laboratories, Portland Cement Association (1985). P.P. Kraal, "Crack Control Methods: Welded Wire Fabric vs. CFP Fibers", for Fibermesh Company (1985). A.L. Landau, "A System for Inhibiting Steel Corrosion in Concrete", Conference on Improving Performance of Concrete in Marine Environments, ShangriLa, Hong Kong, 3-5 June (1987). B. Barr and K. Liu, Concrete 16 (4), 33-35 (1985). R.F. Zollo, "Collated Fibrillated Polypropylene Fibers in FRC", SP-81, American Concrete Institute, Detroit, Michigan (1984). H. Krenchel and S. Shah, Concrete International: Design and Construction ! (3), 32-34 (1985). ACI Committee 544, "State of the Art Report on Fiber Reinforced Concrete" (ACI.544.1R.82), American Concrete Institute, Detroit, Michigan (1984).