Highway IDEA Program
Performance Evaluation of 3-D Basalt Fiber Reinforced Concrete & Basalt Rod
Reinforced Concrete
Final Report for Highway IDEA Project 45
Prepared by:
V Ramakrishnan and Neeraj S. Tolmare, South Dakota School of Mines & Technology, Rapid City, SD Vladimir B. Brik, Research & Technology, Inc., Madison, WI
November 1998
IDEA PROGRAM F'INAL REPORT Contract No. NCHRP-45
IDEA Program
Transportation Resea¡ch Board National Research Council
November 1998
PERFORMANCE EVALUATION OF 3.I) BASALT ITIBER REINFORCED CONCRETE
& BASALT R:D RETNFORCED CONCRETE
V. Rarnalaishnan Neeraj S. Tolmare
South Dakota School of Mines & Technolory, Rapid City, SD Principal Investigator: Madimir B. Brik
Research & Technology, lnc., Madison, WI
INNOVATIONS DESERVING EXPLORATORY ANALYSIS (IDEA) PROGRAMS
MANAGED BY THE TRANSPORTATION RESEARCH BOARD (TRB) This NCHRP-IDEA investigation was completed as part of the National Cooperative Highway Research Program (NCHRP). The NCHRP-IDEA program is one of the four IDEA programs managed by the Transportation Research Board (TRB) to foster innovations in highway and intermodal surface transportation systems. The other three IDEA program areas are Transit-IDEA, which focuses on products and results for transit practice, in support of the Transit Cooperative Research Program (TCRP), Safety-IDEA, which focuses on motor carrier safety practice, in support of the Federal Motor Carrier Safety Administration and Federal Railroad Administration, and High Speed Rail-IDEA (HSR), which focuses on products and results for high speed rail practice, in support of the Federal Railroad Administration. The four IDEA program areas are integrated to promote the development and testing of nontraditional and innovative concepts, methods, and technologies for surface transportation systems.
For information on the IDEA Program contact IDEA Program, Transportation Research Board, 500 5th Street, N.W., Washington, D.C. 20001 (phone: 202/334-1461, fax:
202/334-3471, http://www.nationalacademies.org/trb/idea)
The project that is the subject of this contractor-authored report was a part of the Innovations Deserving
Exploratory Analysis (IDEA) Programs, which are managed by the Transportation Research Board (TRB) with the approval of the Governing Board of the National Research Council. The members of the oversight committee that monitored the project and reviewed the report were chosen for their special competencies and with regard for appropriate balance. The views expressed in this report are those of the contractor who conducted the investigation documented in this report and do not necessarily reflect those of the Transportation Research Board, the National Research Council, or the sponsors of the IDEA Programs. This document has not been edited by TRB.
The Transportation Research Board of the National Academies, the National Research Council, and the
organizations that sponsor the IDEA Programs do not endorse products or manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to the object of the investigation.
PERFORMANCE EVALUATION OF 3-D BASALT FIBER REINFORCED CONCRETE
& BASALT ROD REINFORCED CONCRETE TABLE OF CONTENTS
2.1. LTTERATUREREVTEW... ...1
2.2. CURRENTLY USED FIBERS ...,...2
2.2.4. Synthetic Fibers (Polypropylene Fibers)... ...2
2.2.6. Basalt Rod Reinforced Concrete... ...3
3. PERFORMANCE EVALUATION OF 3-D BASALT FIBER REINFORCED CONCRETE...4
3.1. PROBLEM STATEMENT... ...,...4
3.2. RESEARCH OBJECTIVES... ...4
3.5. MIXTNGPROCEDURE ...4
3.7. TESTS FOR FRESH CONCRETE (pOLypROpyLENE FTBERS)... ...5
3.8. TESTS FORHARDENED CONCRETE .,... ...,..,....5
3.8.1. Static Modulus And Comprehensive Strength... ...5
3.8.2. Static Flexure Test... ...5
3.9. FRESH CONCRETE PROPERTIES ... ...,...6
3.lo.IIARDENED CONCRETE PROPERTIES ... ...6
3.10.1. Compressive Strength And Static Modulus Tes1... ...6
3.10.2. Static Flexural Strength... ...7
3.10.3. Load Deflection Behavior. .,...2
3.10.4. Flexural Toughness... ...7
3.10.5. Japanese Standard Method Of Calculating Flexural Toughness Factor And Equivalent Flexural 3.I2.RECOMMENDATIONS ... ...,...9
4. TENSION TEST FOR BASALT ROD... ...21
4.1.1. Tension Test On Basalt Bar With 14.25mm Diameter... ...21
4.1.2. Tension Test On Basalt Bar And Basalt Cable Of 6mm Diameter ...21
5. CONCRETE REINFORCED WITH BASALT FIBER COMPOSITE REBARS ...28
5.1. PROBLEM STATEMENT... ....:...28
5.3. RESEARCH PROGRAM... ...28
5.4. BASALT BAR REINFORCED CONCRETE BEAMS (DESIGNED AND CAST rN LAB) ...,...,...28
5.4.1. Design Of Beams ...29
5.4.2. Calculation Of Ultimate And Cracking Moments For The 8eam... ...29
5.4.2.1. Ultimate Moment... ...29
5.4.2.2. Cracking Moment ...,.30
5.5. TESTING OF TIIE BASALT REINFORCED CONCRETE BEAMS... ...30
5.6. MEASIIREMENT OF SLIP OF REINFORCEMENT...,... ...,...30
5.7. BASALT BAR REINFORCED CONCRETE BEAMS (SUppLrED By MANUFACTURER) ...31
5.9. RECOMMENDATIONS ... ...32
APPENDIX A: LOAD-DEFLECTION CURVES FOR BASALT FIBER REINFORCED APPENDIX B: LOAD.DEFLECTION CURVES FORBASALT FIBER COMPOSITE REBAR REINFORNCED CONCRETE BEAMS (DESIGNED & CAST rN LAB)... ...,70
APPENDIX C: LOAD-DEFLECTION CURVES FOR BASALT FIBER COMPOSITE REBAR REINFORCED CONCRETE BEAMS (SUPPLIED BY MANUFACTURER) ...,....75
Photo l: Photo 2: Photo 3: Photo 4: Photo 5: Photo 6: Photo 7: Photo 8: Photo 9: Photol0: Photol 1: Photol2: Photol3: Photol4: Photol5: Photol6: PhotolT: Photol8: PhotolSa: Photol9: Photo20: Photo20a: Photo20b: Photo2l: Photo2la: Photo2lb: Photo22: Photo23: Photo23a: LIST OF PHOTOGRAPHS Test Set-Up ...22
Specimen at Failure in the Machine... ...23
Failed Specimen with Anchorages. ...24
Failed Specimen... ...25
Failed Specimen... ...25
Casting of Basalt Reinforced Concrete Beams ...33
Test Set-Up for Basalt Rod Reinforced Beam ...,.33
Test Set-Up for the Basalt Reinforced Concrete 8eam...,. ...,...34
Deflection Measurement fo¡ Basalt Rod Reinforced Concrete Beam... ...34
Close-up of Cracked Beam Still Carrying the Maximum Load... ...35
Close-up of BeamAfterFailure... ...36
Close-up Showing Maximum Width of Crack... ...37
Close-up of Slip ofReinforcement ...3g Slip of Reinforcement at Left End of Beam ...3g Slip of Reinforcement at Right End of Beam ...39
Another View of Slip of Reinforcement After Testing... .."...,..39
Close-up of Test Set-up with True Deflection Measuring Frame & Electrical Stain Gauge Lead Wire ,...40
General View of Test Set-up... ...40
Close-up of Test Set-up... ...41
Typical Flexural Failure (Close-up)... ...41
Test Set-up without Deflection Gauge (Typical Flexural Failure)... ...42
Comparison of Failure of Plain & Basalt Rod Reinforced Beams ...42
Close-up of Typical Flexural Failure ...43
Close-up of Failure in Shear Mode... ...44
General View of Tested Beams... ...44
Close-up of Secondary End Splitting Failure...,... ...45
Close-up of Secondary Shear Failure ...45
Comparison of Cylinder Failure in Compression...,... ...,.46
General View of Both Large & Small and Plain & Reinforced Cylinders... ...,...,46
LIST OF SKETCHES
LIST OF TABLES
Table I : Mix Proportions for Basalt Fiber Reinforced Concrete... ...,... l0
Table 2: Properties of Fresh Concrete ...10
Iuþ1"11' Compressive Strengrh and Elasric Modulus - 7 Days... ...,...,...1I Table3b: Compressive Strength and Elastic Modulus - 2g Dâys.... ...1 I Table4a: Fi¡st Crack Shength and Maximum Flexural Strength - 7 Days... ,...l2 Table4b: Fi¡st Crack Strength and Maximum Flexural Strength - 2g Dãys... ...12
Table5a: ASTM Toughness Indices - 7 Days ... t3 Table5b: ASTM Toughness Indices - 28 Days ...13
1aþfe6a: Japanese Toughness Indices and Equivalent Flexural Strength - 7 Days ...,...14
Table6b: Japanese Toughness Indices and Equivalent Flexural Strengttr - 28 Däys ...14
Table 8: Tension Test for Basalt Rod... ...26
l"þ1" 9.. Mix Proportions for Concrete Reinforced with Basalt Rebars ....,...53
TablelO: Details of Beams Designed & Cast in Lab ... ...53
Tablel0a: Comparison of Actual & Calculated Moments (Beams BRC_A to F)... ...54
Table I I : Details of Basalt Rods Used fo¡ Reinforcing the Concrete ... ...54
!aþþtZ: Toughness Indices (BRC-I to 5)... .,.,...55
Tablel3: Comparison of Calculated & Actual Moments (Beams BRC_l to 5)... ...,55
Appendix B Table B1: Deflection Measurements for Beams BRC_A to C... ...j1 Table B2: Deflection Measurements for Beams BRC-D to F... ...,...71
Appendix C Table Cl: Deflection Measurements for Beams BRC-I to 5 ... ...75
Fig. l: Fig.2; Fig.3A: Fig.3B: Fig,3C: Fig.3D: Fig.3E: Fig.3F: Fig.4: Fig.5: Fig.6: Fig.7: Fig. 8: Fig.9: Fig. 10: Fig. l1: LIST OF FIGURES Comparison of Compressive Strength. ... ¡ó Comparison of Flexural Shength... ... I ó Comparison of Toughness Indices -7 Day ... ...,...1?
Comparison of Toughness Indices -28 Day... ...17
Comparison of Residual Shength Index - R(5, t 0)... ... I E Comparison of Residual Stength Index - R(10,20)... ..,...,...1E Comparison of Toughness Index Ratio - Il0/I5... ...19
Comparison of Toughness Index Ratio -I2O/110... ...19
Comparison of Japanese Toughness Index... ...20
Comparison of Impact Test Results. ...20
Tension Test for Basalt Rod - I (Stess Vs Stain) ...21
Tension Test for Basalt Rod - 2 (Stess Vs Stain) ...27
Comparison of Tougbness Indices for BRC-I to 5... ,...56
Comparison of Toughness Ratios for BRC-I to 5... ...56
Comparison of Residual Strength tndices for BRC-I to 5 ... ...57
comparison of Japanese Toughness & Equivalent Flexural strength for BRC ...57
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Appendix A
Fig. A1: Load-Deflection Curve for BFRC2-l ...61
Fig. A2: Load-Deflection Curve for BFRC2-2 ...61
Fig. A3: Load-Deflection Curve for BFRC2-3 ,...62
Fig. A4: Load-Deflection Curve for BFRC2-4 ...62
Fig. A5: Load-Deflection Curve for BFRC2-5 ...63
Fig. A6: Load-Deflection Curve for BFRC2-6 ...63
Fig. A7: Load-Deflection Curve for BFRC3-I ...64
Fig. A8: Load-Deflection Curve for BFRC3-2 ...64
Fig. A9: Load-Deflection Curve for BFRC3-3 ...65
Fig. 410: Load-Deflection Curve for BFRC3-4 ...65
Fig. Al l: Load-Deflection Curve for BFRC3-5 ...,...,..66
Fig. Al2: Load-Deflection Curve for BFRC3-6 ...66
Fig. Al3: Load-Deflection Curve for BFRC4-l ...67
Fig. Al4: Load-Deflection Curve for BFRC4-2 ...67
Fig. Al5: Load-Deflection Curve for BFRC4-3 ...68
Fig. 416: Load-Deflection Curve for BFRCS-I ...68
Fig. Al7: Load-Deflection Curve for BFRC5-2 ...,....69
Fig. 418: Load-Deflection Curve for BFRC5-3 ...,69
Appendix B Fig. Bl: Load-Deflection Curve for BRC-A ...72
Fig. 82: Load-Deflection Curve for BRC-B ...72
Fig. 83: Load-Deflection Curve for BRC-C ...73
Fig. B4: Load-Deflection Curve for BRC-D ...73
Fig. B5: Load-Deflection Curve for BRC-E... ...74
Fig. B6: Load-Deflection Curve for BRC-F... ...74
Appendix C Fig. Cl: Load-Deflection Curve for BRC-I ...77
Fig, C2: Load-Deflection Curve for BRC-2 ...7'7 Fig. C3: Load-Deflection Curve for BRC-3 ...78
Fig. C4: Load-Deflection Curve for BRC-4 ...78
Fig. C5: Load-Deflection Curve for BRC-5 ...79
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GLOSSARY
The followrng is a glossary of terms for fiber reinforced concrete (FRC) used in this report.
0.1 GENERAL TERMS
Aspect Ratio - The ratio of length to diameter of the fiber. Diameter may be equivalent diameter.
Balling - When fibers entangle into large clumps or balls in a concrete mixture.
Collated - Fiber bundled together either by cross-linking or by chemical or mechanical means.
Equivalent Diameter - Diameter of a circle with an area equal to the cross-sectional area of the fiber.
Fiber content - The weight of fibers in a unit volume of concrete.
Fibrillated - A fiber with branching fibrils.
First Crack - The point on the flexural load-deflection or tensile load-extension curve at which the form of
the curve fust becomes nonlinear.
Hairline Crack - Cracks with widths less than 0.I mm (0.0039 inches) are termed as hairline cracks.
First Crack Deflection - The deflection value on the load deflection curve at the first crack.
First Crack Strength - The stress obtained when the load conesponding to fust crack is inserted in the formula for modulus of rupture given in ASTM Test Method C 78.
First Crack Toughness - The energy equivalent to the area of the load deflection curve up to the first crack.
Flexural Toughness - The area under the flexural load-deflection curye obtained from a static test of a specimen up to a specified deflection. It is an indication of the energy absorption capability of a material.
Toughness Indices - The numbers obtained by dividing the area under the load-deflection curye up to a
specified deflection by the area under the load-deflection curve up to "First Crack" as given in ASTM C 1018.
Toughness Index, Ir - The number obtained by dividing the area up to 3.0 times the fust crack deflection by the area up to the fust crack of the load deflection curye, as given in ASTM C 1018.
Toughness fndex, Iro - The number obtained by dividing the area up to 5.5 times the first crack deflection by the area up to the fust crack of the load deflection curve, as given in ASTM C l0l8
Toughness fndex, Iro - The number obtained by dividing the area up to 10.5 times the flust crack deflection by the area up to the first crack of the load deflection curye, as given in ASTM C 1018
Residual Strength Factor &'0 - The number obtained by calculating the value of 20(I,o-Ir), as
giveninASTMC 1018.
Residual Strength Factor &o,ro - The number obtained by calculating the value of 10(Iro-I,o), as
giveninASTMC 1018.
Flexural Toughness Factor (JCI) - The energy required to deflect the fiber reinforced concrete beam to a
mid point deflection of l/150 of its span.
Equivalent Flexural Strength (JCI) - It is defined by
F" : Tbxs/ô,oxbxd2
where
F" : equivalent flexural strength, psi
Tr = flexural toughness, inchlb
span, inches
ôtu : deflection of l/150 of the span, inches
b : breadth at the failed cross-section, inches
d : depth at the failed cross-section, inches
Impact Strength - The total energy required to break a standard test specimen of a specified size under specified impact conditions, as given by ACI Committee 544.
Monofilament - Single filament fiber.
StatÍc Modulus - The value of Young's modulus of elasticity obtained from measuring stess-strain relationships derived from other than dynamic loading.
High Performânce Concrete - In this report, High Performance Concrete is defined as a concrete with highly enhanced (or improved) desirable properties for the specific purpose and function for which it is
used. It need not necessarily be high-strength concrete. High performance concrete may have one or more
of the following properties enhanced: ductility, fatigue strength, durability, impact resistance, toughness, impermeability and wear resistance.
Whitetopping - Whitetopping is concrete placed over asphalt where the concrete thickness is 101 ( 4 inch ) or more mm thick.
Ultra-Thin Whitetopping - Ulha-Thin Whitetopping is concrete placed over asphalt where the concrete is less than 101 mm ( 4 inch ) thick.
0.2 ACRONYMS USED
ACI - American Concrete Institute
CFP - Collated Fibrillated Poþropylene
F-RC - Fiber Reinforced Concrete
LS - Low Slump
I\MFRC - Non-Metallic Fiber Reinforced Concrete. This acronym refers only to Polyolefin Fiber Reinforced Concrete. These fibers were manufactured and purchased from 3M for the purpose ofthis study.
NMFRS - Non-Metallic Fiber Reinforced Shotcrete
PFRC - Polypropylene Fiber Reinforced Concrete
PCC - Portland Cement Concrete
SFRC - Steel Fiber Reinforced Concrete.
SNFRC - Synthetic Fiber Reinforced Concrete
SIFCON - Slurry Infiltrated Fiber Concrete
SIMCON - Slurry Infiltated Mat Concrete
0.3 ASTM SPECIFICATIONS
A 820 - Specification for Steel Fibers for Fiber Reinforced Concrete
C 31 - Practices for Making and Curing Concrete Test Specimens in the Field C 39 - Test Method for Compressive Stength of Cylindrical Concrete Specimens C 78 - Test Method for Flexural Strength of Concrete (Using Simple Beam with Thfud-
point Loading)
C 94 - Specification for Ready-Mixed Concrete
C138 - Test for Unit rü/eight, Yield and Air Content (gravimetric) of concrete
C 143 - Test Method for Slump of Portland Cement Concrete
C 172 - Method of Sampling Freshly Mixed Concrete
C 173 - Test Method of Air Content of Freshly Mixed Concrete by the Volumetric Method
C 231 - Test Method for Ai¡ Content of Freshly Mixed Concrete by the Pressure Method
C 469 - Test Method for Static Modulus of Elasticity and Poisson's Ratio of Concrete in Compression
C 995 - Test Method for Time of Flow of Fiber Reinforced Concrete Through Inverted Slump cone
C1018 - Test Method for Flexural Toughness and Fi¡st Crack Stength of Fiber
- Reinforced Concrete (Using beam with Third-point Loading) C 1116 - Specification for Fiber Reinforced Concrete and Shotcrete
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0.4 INTERNATIONAL STANDARDS
A - American Concrete Institute Committee 544 Fiber Reinforced Concrete . ACI544,2R.89 Flexural Fatigue Endurance
Impact Resistanee Toughness
B - British Standards Institute
BS188l: Part2, Methods of Testing Concrete-Vebe Test
C - Japanese Society of Civil Engineers
JSCE Standard III-1, Specification of Steel Fibers fo¡ Concrete, Concrete Library, No. 50, March 1983.
- JSCE-SF4 Standard for Flexural Stength and Flexural Toughness, ..Method of
Tests for Steel Fiber Reinforced Concrete," Concrete Library of JSCE, No.3,
June 1984, Japan Concrete Institute (JCI), pp. 58-66,
- "standard rest Method fo¡ Flexural stength and Flexural Toughness of Fiber Reinforced Concrete, (Standard SF4),"JCI Standards for Test Methods of Fíber
Reinforced Concrete, Japan Concrete Institute, I 983, pp. 45-5 l.
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EXECUTIVE'SUMMARY
This report presents the experimental investigation carried out to evaluate the performance characteristics ofbasalt fiber reinforced concrete and basalt bar reinforced concrete. The fibers and bars were supplied by Research & Technology Inc., Madison, WL
All the experiments were conducted following the ASTM standards. The test program was conducted for fresh and hardened concrete properfies. The fresh concrete properties consisted of the following tests:
slump, Vebe slump, Vebe time, concrete temperature, air content and unit weight. The hardened concrete properties determined ìvere compressive shength, static modulus, flexural stength, load-deflection behavior, comparison ofload-deflection curves, ASTM toughness indices, first crack toughness, post crack behavior, Japanese standard method for toughness indices and equivalent flexural strength.
The test results show that the basalt fiber can be easily mixed in the concrete without any balling, bridging or segregation. There was a noticeable increase in the post crack energy absorption capacity and ductility due to the addition of basalt fibers. The impact resistance increased as the fiber ðontent increased.
Tests were also conducted on beams reinforced with basalt bars, which were supplied by the manufacturer. The load deflection curves were plotted and the toughness indices and first ciack toughness were calculated in a similar way as the ASTM standards for the beams reinforced with basalt fibers.
Toughness indices according to the Japanese Standard method and the equivalent flexural strength were also calculated.
Tests were conducted on beams reinforced with basalt bars, which were designed and cast in the lab.
Strain across the depth of the beams was measured, using electrical resistance strain gauges. Deflection was measured, using a magnetic dial gauge. The tests indicated that there was insufficient bond strength and the bars slþed gradually before the ultimate load was released. Beams with increased development lengths failed suddenly with breaking of rods. It was a sudden and brittle failure. The ultimate-load and .ru.kirrg- load moments and the deflections of the basalt Íod reinforced concrete beams were compared.
The tension tests were conducted on the basalt bars, and a cable, having two basalt bars twisted together.
Graphs of stress vs. stain were plotted and the modulus of elasticity of the basalt bars was calculated.
ix
l.INTRODUCTION
Plain concrete has two major deficiencies; a low tensile strength and a low shain at fracture. The tensile strength of concrete is very low because plain concrete normally contains numerous microcracks. It is the rapid propagation of these microcracks under applied stress that is responsible for the low tensile strength of the material. These deficiencies have led to considerable research aimed at developing new approaches to modifying the brittle properlies of concrete.
Current research has developed a new concept to increase the concrete ductility and its energy absorption capacity, as well as to improve overall durability. This new generation technology utilizes fibers, which if raudomly dispersed throughout the concrete matrix, provides better distribution of both internal and external stesses by using a three dimensional reinforcing network. (1,2,3)
The primary role of the fibers in hardened concrete is to modify the cracking mechanism. By modifying the cracking mechanism, the macro-cracking becomes micro-cracking. The cracks are smaller in width;
thus reducing the permeabilþ of concrete and the ultimate cracking strain of the concrete is enhanced. The fibers are capable of carrying a load across the crack. A major advantage of using fiber reinforced concrete (FRC) besides reducing permeability and increasing fatigue shength is that fiber addition improves the toughness or residual load carrying abilþ after the fust crack. Additionally, a number of studies have shown that the impact resistance of concrete can also improve dramatically with the addition of fibeis.
Combining the technical benefits and in place costs, FRC has been found to meet the prerequisites of
value engineering for use, particularly in airport and highway pavements, in bridge deck overlays, curtain walls, sewer pipes and precast concrete products (2). Fibers have also been used in shotcrete for rockfill stabilization, tunnel linings and dome structures. FRC had been used extensively in overlays and repairs of airport pavements and bridge decks.
FRC composites are almost ideal materials for repair, rehabilitation, retrofit and renovation of the world's deteriorating infrastructure. Concrete fiber composites technology has grown over the last th¡ee decades into a mahue industry. The purpose of this paper is to review the current research on basalt fiber reinforced concretes.
2. LITERATURE
2.l LITERATTJRE REVIEW
The addition of fibers in concrete matix has many important effects. Most noticeable among the improved mechanical properties of fiber reinforced concrete are its superior fracture resist¿nce and resistance to impact and impulsive or dynamic loads. Secondly they impart additional strength under all modes of loading which include, direct tensiou, shear, flexural and torsion loading. The degrees of improvement of
the mechanical characteristics of fiber reinforced concrete (FRC), are influenced by specimen size, loading configuration, size and type of fibers (4,5,6).
Even though reinforcing a brittle matrix with discrete fibeis is an age old concept, modern-day use of fibers in concr€te started iu the early 1960s. In the beginning, only straight steel fibers were used. The major improvement occurred in the areas of ductility aud fracture toughness, even though some flexural strength increases were observed. The law of mixtures was applied to analyze the fiber contributions.
Fibers have been produced from steel, plastic, glass and natural materials in various shapes and sizes.
Fibers for commercial application of FRC have been of great variety including (7);
. Diffèring materials: steel, glass, polypropylene, polyolefin and cellulose are representative of the
metallic, mineral, synthetic and natural fiber types.
o Differing amounts: rangrng from relatively high frber additions by volume to low-volume concentrations, with high, being conside¡ed in the range of 3 to 12 percent, intermediate in the range of I to 3 percent, and low in the range of0.1 to I percent, based on the total volume ofconcrete produced.
¡ Differing fiber geometry: prismatic, round or flat, with deformations throughout or at the ends, irregular cross sections, fibrillated monofilament or bundles of fibers, fine (small effective diameter fibers of relatively high aspect (length to diameter) ratios, or coarse fibers of lower aspect ratios).
2.2 CI]RRENTLY USED FIBERS
2.2.1 Steel Fibers
The majority of the applications with steel fibers in the United States, has been with mixes using normal weight concrete. For sfiaight steel fibers, the primary factors that controlled the properties of the composite were fiber volume fraction and aspect ratio of the fibers. The amount of fiber used ranged from 89 to 119 kg/m3 (tSO to 200 lb/cu. yard) of concrete. The major problems encountered in the earþ stages were
difficulty in mixing and workability. At higher volume fractions, fibers were found to ball up during the mixing process. This process called balling was found to occur frequently for longer fibers. The size of the coarse aggregate was normally restricted to facilitate the use of short fibers and to avoid balling.
Additionally, the mortar fraction of concrete was increased to combat the balling problem. There was always a reduction in workability with the addition of fibers. This tends to affect the quality of concrete in place, especially for higher fiber volume fractions (8 to l3).
2.2.2 Carbon Fibers
Until mid 1980's the high cost of carbon fibers limited tlei¡ use in portland cement composites. More recently, low cost carbon fibers have been manufactured with petroleum and coal pitch. Carbon fibers are very light with a specific gravity of about 1.9 and inert to most of the chemicals. Even though their cost is higher than polymeric fibers, carbon fibers have potential for special applications that require high tensile and flexural strength. Carbon fibers are available in stands (tows) that can contain up to 12,000 individual filaments and have elastic modulus as high as steel and are two to three times stronger than steel.(2 ,14,15)
2.2.3 Glass Fibers
Experiments using glass fibers have been conducted in the United States since the early 1950's as well as
in the United Kingdom and in Russia (15,16). Applications of fiber reinforced concrete investigated since the mid 1960's have included road and floor slabs, topping layers, refractory materials, and some precast concrete products. Glass fibers are primarily used for glass fiber reinforced cement (GFRC) sheets (16 to
2t).
2.2.4 Synthetic Fibers
Synthetic fibers are most commonly added to concrete for slab-on-grade construction to reduce early plastic shrinkage cracking and increase impact and abrasion resistance and toughness. The fibers also can be added to precast concrete to improve resistance to handling stresses, to pumped concrete to improve cohesiveness, and to shotcrete, to reduce rebound and material waste.
