Fiber-Reinforced Concrete for Bridge Decks

Fiber-Reinforced Concrete for Bridge Decks final reportFiber-Reinforced Concrete for Bridge Decks Final Report December 2021
Sponsored by Iowa Highway Research Board (IHRB Project TR-767) Iowa Department of Transportation (InTrans Project 19-679)
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Technical Report Documentation Page
1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No.
IHRB Project TR-767
6. Performing Organization Code
Behrouz Shafei (orcid.org/0000-0001-5677-6324), Peter Taylor
(orcid.org/0000-0002-4030-1727), Brent Phares (orcid.org/0000-0001-5894-
InTrans Project 19-679
9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)
Bridge Engineering Center
Iowa State University
Ames, IA 50010-8664
11. Contract or Grant No.
12. Sponsoring Organization Name and Address 13. Type of Report and Period Covered
Iowa Highway Research Board
Iowa Department of Transportation
Visit https://bec.iastate.edu/ for color pdfs of this and other research reports.
16. Abstract
Concrete is made of multiple ingredients that begin in a plastic phase and become solid over time. Additionally, it is well
established that concrete is exposed to various stressors from the initial hours of pouring, making it prone to cracking. The
multiphase nature of concrete along with these stressors require the consideration of several factors, especially for the design of
concrete bridge decks that are exposed to aggressive environmental and mechanical stressors simultaneously. Due to the low
early-age strength of concrete, even small-scale tensions can result in cracking and consequently decrease the longevity of the
concrete structure.
In order to address these issues, a three-stage framework was designed for this project. In Stage 1, multiple binder compositions
were investigated for their performance in terms of early-age plastic shrinkage by recording capillary pressure development,
monitoring crack width, and determining strain development by means of digital image correlation. After binder modification, in
Stage 2 different dosages of microfibers were added to concrete mixtures to compensate for the concrete’s low tensile strength
and control cracking during the life of the concrete. To measure the efficiency of the microfibers, drying shrinkage, compressive
and splitting tensile strength, and rapid chloride migration tests were carried out to determine the cracking potential and
mechanical and durability properties of fiber-reinforced concrete (FRC). In Stage 3, three types of macrofibers (i.e.,
polypropylene [PP], alkali-resistant [AR] glass, and polyvinyl alcohol [PVA]) were incorporated at multiple dosages into FRC
that already contained microfibers to enhance the post-peak strength of the concrete. The compressive, splitting tensile, and
flexural strengths of the concretes were recorded as the pre-peak mechanical properties, and the toughness and residual flexural
strength were recorded as the post-peak mechanical properties.
The results show that Class F fly ash, as opposed to silica fume and Type K (expansive) cement, contributes most to the early-age
cracking resistance of concrete. Furthermore, increasing PP microfiber content significantly reduced the cracking potential and
enhanced the mechanical properties and chloride resistance of concrete. In the case of hybrid FRC (FRC containing both
microfibers and macrofibers), AR glass macrofibers introduced superior performance compared to PP and PVA macrofibers, in
terms of pre- and post-peak mechanical properties.
17. Key Words 18. Distribution Statement
bridge deck concrete—concrete toughness—concrete shrinkage—fiber-
reinforced concrete—hybrid fibers—mechanical properties
No restrictions.
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Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
Co-Principal Investigators
Bridge Engineering Center, Iowa State University
Peter Taylor, Director
Research Assistant
Maziar Kazemian
Sponsored by
Iowa Department of Transportation
through funds provided by the Iowa Department of Transportation
through its Research Management Agreement with the
Institute for Transportation
(InTrans Project 19-679)
A report from
Bridge Engineering Center
Institute for Transportation
Iowa State University
Ames, IA 50010-8664
Types of Shrinkage in Concrete ...........................................................................................5 Factors Affecting Plastic Shrinkage.....................................................................................6 Methods to Mitigate Plastic Shrinkage Cracking ................................................................8
MATERIALS .....................................................................................................................12
Mechanism of Crack Mitigation ........................................................................................27
CHAPTER 5. EXPERIMENTAL PROGRAM .............................................................................59
Stage 1. Binder Investigation .............................................................................................86 Stage 2. Microfiber Investigation ......................................................................................99 Stage 3. Hybrid Fiber Investigation .................................................................................107
Key Findings ....................................................................................................................115
Figure 1. Spalling of concrete due to steel rebar corrosion .............................................................3
Figure 2. Fibers used in the study: (a) from top to bottom, PVA, PP, and AR glass
macrofibers and (b) PP microfibers ..............................................................................60 Figure 3. Restrained slab instrumented with CPSS sensors ..........................................................64 Figure 4. Concrete ring undergoing restrained shrinkage ..............................................................71 Figure 5. Concrete cylinder and bearing device loaded into the testing machine .........................75
Figure 6. RMT setup: (1) cathode in NaCl solution, (2) anode in NaOH solution, (3)
specimen in rubber sleeve, (4) power supply ................................................................80 Figure 7. Three-point bending test setup .......................................................................................84 Figure 8. Capillary pressure development rates for Specimens 0-0 (Control), 7.5K-0, and
15K-0 ............................................................................................................................86
Figure 9. Capillary pressure development rates for Specimens 0-15FA, 7.5K-15FA, and
15K-15FA .....................................................................................................................87
15K-7.5SF .....................................................................................................................87
Figure 11. Onset and rate of plastic shrinkage cracking for Specimens 0-0, 7.5K-0, and
15K-0 ............................................................................................................................89 Figure 12. Onset and rate of plastic shrinkage cracking for Specimens 0-15FA, 7.5K-
15FA, and 15K-15FA ...................................................................................................89 Figure 13. Onset and rate of plastic shrinkage cracking for Specimens 0-7.5SF, 7.5K-
7.5SF, and 15K-7.5SF ...................................................................................................90 Figure 14. Final tensile strain chart for Specimen 0K ...................................................................94 Figure 15. Final tensile strain map for Specimen 0K ....................................................................94
Figure 16. Final tensile strain chart for Specimen 7.5K ................................................................95
Figure 17. Final tensile strain map for Specimen 7.5K .................................................................95 Figure 18. Final tensile strain chart for Specimen 15K .................................................................96 Figure 19. Final tensile strain map for Specimen 15K ..................................................................96
Figure 20. Final tensile strain chart for Specimen 22.5K ..............................................................97 Figure 21. Final tensile strain map for Specimen 22.5K ...............................................................97 Figure 22. Compressive strain development for Mix 1 (0.0% fiber) .............................................99
Figure 23. Compressive strain development for Mix 2 (0.25% fiber) .........................................100 Figure 24. Compressive strain development for Mix 3 (0.50% fiber) .........................................100 Figure 25. Compressive strain development for Mix 4 (1.0% fiber) ...........................................101 Figure 26. Mean compressive strain development for all FRC mixes .........................................101 Figure 27. Tensile strength of FRC mixes at 7, 14, and 28 days .................................................103
Figure 28. Cracking potential over time for all FRC mixes ........................................................104
Figure 29. 28-day compressive strength of FRC mixes ...............................................................106
Figure 30. Rapid chloride migration test results for (a) mean chloride penetration depth and
(b) migration coefficient .............................................................................................107 Figure 31. Water reducer demand of hybrid FRC mixes .............................................................108 Figure 32. Compressive strength of hybrid FRC with PP macrofibers .......................................109 Figure 33. Compressive strength of hybrid FRC with AR glass macrofibers .............................109 Figure 34. Compressive strength of hybrid FRC with PVA macrofibers ....................................110
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Figure 35. Splitting tensile strengths of hybrid FRC with PP, AR glass, and PVA
macrofibers ..................................................................................................................111
Figure 36. Flexural strengths of hybrid FRC with PP, AR glass, and PVA macrofibers ............113
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Table 2. Chemical composition of the binders used in the study (% weight) ...............................59 Table 3. Physical and mechanical properties of the fibers used in the study.................................60 Table 4. Test matrix for plastic shrinkage cracking in restrained concrete slabs ..........................61 Table 5. Test matrix for strain measurement by digital image correlation ....................................66 Table 6. Test matrix for cracking age of concrete under restrained shrinkage ..............................69
Table 7. Test matrix for splitting tensile strength ..........................................................................73 Table 8. Test matrix for compressive strength of concrete ............................................................76 Table 9. Test matrix for chloride penetration by rapid migration..................................................78 Table 10. Test matrix for flexural strength ....................................................................................82 Table 11. Plastic shrinkage cracking images for Type K concrete slabs .......................................91
Table 12. Plastic shrinkage cracking images for fly ash concrete slabs ........................................91 Table 13. Plastic shrinkage cracking images for silica fume concrete slabs and control
sample ...........................................................................................................................92 Table 14. Toughness of FRC specimens......................................................................................114
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ACKNOWLEDGMENTS
This work was sponsored by the Iowa Highway Research Board and the Iowa Department of
Transportation. The authors would like to thank the technical advisory committee on this project:
Michael Nop, Curtis Carter, Todd Hanson, Jesse Peterson, Steve Seivert, and Joseph Stanisz.
The research team would also like to acknowledge the staff of the Portland Cement Concrete
Pavement Materials and Research Laboratory and Structural Engineering Research Laboratory at
Iowa State University for their support in performing the experimental work.
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EXECUTIVE SUMMARY
Bridge deck concretes are exposed to multiple stressors that endanger the performance and life
cycle of the bridge. If special attention is not dedicated to the material design and construction of
the reinforced concrete structures, cracks would be generated and would propagate into the
concrete, providing channels for corrosive agents (such as chloride ions and carbon dioxide) to
penetrate into the concrete at a pace much faster than that of uncracked concrete. The penetration
of these destructive agents results in corrosion of the rebars and a decrease in the structural
performance of the reinforced concrete. At this point, the structure needs to be either repaired,
which hinders the operation of the structure, or demolished and built again. Either of these
options imposes significant economic and operational expenses on the structure. Therefore, a
comprehensive study should be carried out to investigate the multiple factors threatening the
longevity of concrete structures.
Due to its large surface area, bridge deck concrete is significantly prone to shrinkage-induced
cracking caused by water evaporation, either through plastic shrinkage when the concrete is in a
semiplastic phase or through drying shrinkage over longer periods. The low tensile strength of
ordinary portland cement concrete is responsible for shrinkage-induced cracking, which can be
reduced by modifying the binder composition or by adding fibers to the concrete to provide
additional tensile strength capacity.
Since ancient times, people have been putting fibers, such as straw and hair, into mortars and
bricks to improve their tensile properties. These ancient and simple methods of concrete
reinforcement have now been transformed into advanced methods that involve using
discontinuous fibers distributed randomly throughout the concrete matrix. Moreover,
conventional concrete is a brittle material by nature. To compensate for this characteristic and
avoid the sudden brittle failure of concrete structures, reinforcement materials are embedded into
the concrete.
To address the concerns noted above, this project aimed to investigate multiple crack mitigation
scenarios under shrinkage-induced cracking conditions, which are the most important crack-
inducing parameters in bridge deck concrete. Furthermore, the post-peak performance of
concrete was emphasized, with the workability of the concrete being considered as a restrictive
factor.
To pursue the aforementioned objectives, a comprehensive study was conducted that consisted of
three stages:
• Stage 1. Binder Investigation. This stage was designed to investigate the performance of
multiple binder compositions in terms of mitigating early-age cracking. To do so, 7.5% and
15% of portland cement was substituted with expansive (Type K) cement, Class F fly ash,
and silica fume. Plastic shrinkage tests were conducted on slab specimens, and the capillary
pressure development and crack widths of the slabs were recorded for six hours.
Additionally, the digital image correlation (DIC) technique was employed on the mixtures
made with Type K cement to record the strain development. The results of this stage led to
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the determination of the binder composition of the concrete on which tests were performed in
the subsequent stages of this research.
• Stage 2. Microfiber Investigation. In this stage, microfibers were added to the concrete to
further enhance the performance of the concrete against tensile stresses. In particular,
polypropylene (PP) microfibers at dosages of 0.25%, 0.50%, and 1.0% of the concrete
volume were investigated during this stage. Drying shrinkage tests and splitting tensile
strength tests were conducted to measure the cracking potential of the fiber-reinforced
concrete (FRC). Furthermore, compressive strength tests and rapid chloride migration tests
were carried out to determine the mechanical and durability properties of the FRC.
• Stage 3. Hybrid Fiber Investigation. This stage was dedicated to identifying the pre-peak and
post-peak properties of hybrid FRC (FRC containing both microfibers and macrofibers). It is
well known that microfibers are most effective in controlling the low tensile stresses (such as
shrinkage tension) and macrofibers contribute to the post-peak strength of the concrete when
macrocracks are generated. In this stage, three macrofiber types, i.e., PP, alkali-resistant
(AR) glass, and polyvinyl alcohol (PVA), at various dosages were added to concrete
containing two amounts of PP microfibers. The compressive, splitting tensile, and flexural
strengths of the mixtures were recorded and reported as the pre-peak mechanical properties
of the hybrid FRC. Additionally, the toughness and residual flexural strength of the mixtures
were measured and recorded as the post-peak behavior of the hybrid FRC.
The results of this three-stage study can be used to determine a suitable mix design for the
application of fiber-reinforced concrete for bridge decks.
Key Findings of the Research
Stage 1:
• In general, increasing the proportion of Type K expansive cement resulted in an increase in
the rate of capillary pressure development for all types and percentages of supplementary
cementitious materials (SCMs) investigated in this project. Silica fume was found to have a
negative effect on the rate of capillary pressure development, while Class F fly ash decreased
the rate of capillary pressure. Therefore, Class F fly ash was incorporated into the mix design
of the FRC in subsequent stages of this research.
• For each type of concrete investigated, an increase in Type K expansive cement led to a
reduction in plastic shrinkage crack widths at six hours after casting. For concrete containing
Class F fly ash or silica fume, increasing the dosage of Type K cement resulted in a reduction
in the rate of plastic shrinkage crack propagation, provided that adequate workability was
achieved through the use of superplasticizer.
• The DIC results suggest that after the six-hour testing period, the specimens experienced
reduced plastic shrinkage-induced tensile strain at the location of cracking with increasing
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proportions of Type K expansive cement up to 22.5%. Doses of Type K cement up to 22.5%
showed a substantial relative reduction in plastic shrinkage-induced tensile strain.
Stage 2:
• For PP microfiber percentages from 0.25% up to 1.0% by volume, an increase in fiber
proportion did not significantly affect the rate of drying-induced strain development or the
final magnitude of strain in a concrete ring.
• Tensile strength increased with both age and PP microfiber percentage among all ages and
mixes of FRC for fiber doses up to 1.0% by volume. The largest relative increase in tensile
strength occurred at lower PP microfiber doses in the range of 0.25%. The ability of PP
microfibers to improve the tensile strength of concrete decreased in efficiency at fiber
volumes of 1.0% or higher.
• Cracking potential, defined as the ratio of the maximum shrinkage-induced stress
experienced by an FRC mix to the tensile strength of the same FRC mix, decreased with an
increase in PP microfiber percentage for fiber doses up to 1.0%. At volumes of 1.0% and
higher, the relative reduction in cracking potential significantly decreased compared to the
relative reduction in cracking potential at lower doses.
• In general, the data show that the 28-day compressive strength of FRC increases with PP
microfiber proportion for fiber doses up to 1.0% by volume. At PP microfiber volumes of
1.0% and higher, the relative increase in compressive strength provided by the fibers
significantly decreased compared to the relative increase in compressive strength at lower
doses.
• An increase in PP microfiber proportion up to 1.0% by volume corresponded to a decrease in
the rate and magnitude of chloride ion penetration into FRC after 24 hours. Increasing the
fiber dosage to 1.0% appeared to result in less efficient mitigation of chloride penetration
compared to the mitigation provided at lower fiber proportions.
Stage 3:
• PVA macrofibers reduced the workability of FRC more significantly than AR glass and PP
macrofibers due to the water absorption of the PVA fibers.
• PP and PVA macrofibers reduced the compressive strength of concrete, while AR glass
macrofibers provided a compressive strength similar to that of the control sample. In the case
of hybrid FRC, the addition of AR glass macrofibers resulted in superior compressive
strength, which was augmented by increasing the macrofiber dosage.
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• FRC with PP macrofibers showed weaker performance under tensile loads compared to FRC
with AR glass or PVA macrofibers.
• The mechanical test results suggest that AR glass macrofibers show a promising synergy
with PP microfibers, which makes AR glass macrofibers an appropriate choice for hybrid
FRC.
• Regardless of the fiber combination and dosage, the FRC samples studied in Stage 3
exhibited a flexural strength similar to or higher than that of the control sample. FRC with PP
macrofibers showed superior performance in terms of flexural strength when no microfibers
were added to the mixture. However, when microfibers were introduced into the mixture,
FRC with PP macrofibers lost its superiority. Furthermore, in hybrid FRC with low
macrofiber dosages (i.e., 0.125% and 0.1875%), AR glass FRC had the highest flexural
strength. However, at a macrofiber dosage of 0.25%, PVA FRC outperformed the FRCs with
other macrofibers.
• The addition of AR glass macrofibers to concrete, even at a dosage of 0.125%, provided FRC
with some level of post-peak residual strength and toughness. However, PP and PVA
macrofibers provided FRC with post-peak flexural strength and toughness at dosages of
0.25% and 0.1875%, respectively. Moreover, at a macrofiber dosage of 0.5%, AR glass FRC,
in contrast to PP or PVA FRC, showed a well-formed residual flexural strength stretching
beyond 1/150 of the span length.
Summary of the Findings and Recommendations
Based on the research conducted and the literature reviewed for this study, it can be concluded
that replacing a portion of portland cement with Class F fly ash has a positive effect on the
resistance of concrete to plastic shrinkage as well as on the workability and long-term durability
of concrete. Although the addition of Type K cement showed promise in restricting crack width,
it increased…