Concrete international december 2014 35

A New Fiber for Enhanced Crack ControlPolypropylene fiber with chemical bond to concrete

by Emmanuel K. Attiogbe, Steve Schaef, Charles O. Kerobo, Dan Vojtko, and Charles K. Nmai

Early-age cracking of concrete due to shrinkage and temperature effects is typically controlled by the use of welded wire reinforcement (WWR) or small-diameter

steel bars. However, steel reinforcement is susceptible to corrosion, which significantly reduces the life expectancy of reinforced concrete structures. In addition, the placement of WWR or bars can be highly labor-intensive, may not be accurate, and can add significant cost.

As an alternative to steel reinforcement, discrete or short fibers (polymeric, steel, natural, or glass) can be added to concrete mixtures as crack control reinforcement in selected applications. ASTM C1116/C1116M-10a, Standard Specification for Fiber-Reinforced Concrete, recognizes polypropylene, polyethylene, nylon, and carbon fibers as synthetic fibers known for their proven long-term durability. Largely because of their lower cost, the most commonly used synthetic fibers comprise polypropylene. Macrosynthetic polypropylene fibers offer a range of benefits, including reduced labor for concrete placement, reduced crack widths, and extended joint spacing in slab-on-ground applications, high ductility in shotcrete applications, and no corrosion. For slab-on-ground construction, the three-dimensional (3-D) distribution of macrofibers in the concrete matrix ensures that the reinforcement will be where it’s needed.

Interfacial BondThe transfer of forces between fibers and the concrete

matrix is achieved through interfacial bond. Because the chemical structure of polypropylene is such that it has no chemical affinity with the concrete matrix, interfacial bond in commonly available polypropylene fibers is typically achieved through friction and mechanical anchorage.1

Frictional bond depends on the lateral surface of contact between the fiber and the concrete matrix. Fiber intrinsic efficiency ratio (FIER), which is the ratio of the lateral surface area of a fiber to its cross-sectional area, can be used to quantitatively compare the frictional bond for fibers with different geometries.1 A tape-like monofilament macrosynthetic fiber, for example, has a higher FIER than a circular fiber of the same cross-sectional area, so interfacial bond of circular fibers is typically enhanced by embossing or crimping the fibers.

Chemical bond could enhance the performance of polypropylene macrofibers. However, because the composition of basic polypropylene does not include functional groups that can chemically interact with the concrete matrix, special techniques need to be employed to alter the surface of the fiber.

Development of Chemical BondingBASF has recently developed a polypropylene fiber

with chemical bond, achieved by incorporating chemical compounds into the base resin. The reference fiber for evaluation of chemical bond development is a monofila-ment polypropylene macrofiber marketed under the Master Builders Solutions brand for various applications, including slab-on-ground, pavements, bridge decks, precast concrete, and shotcrete. This is a high-performance fiber that has been successfully used in many projects. The fiber is approximately circular in cross section with an equivalent diameter of 0.032 in. (0.81 mm) and a length of 2.1 in. (54 mm). The bond of this fiber to the concrete matrix is enhanced through mechanical means by the embossment process.

36 december 2014 Concrete international

The new, chemical-bond fiber was developed by incorporating specific chemical compounds into the base polypropylene resin before extrusion. Except for the added compounds, the chemical-bond fiber has the same material components and geometry as the reference fiber. The added compounds were designed to improve the performance of the fiber in the cementitious matrix through participa-tion in the cement hydration reactions.

Scanning electron microscope (SEM) micrographs of fractured surfaces of concrete specimens with reference and chemical-bond fibers are

(a)

(b)

f

f

f

f

cm

cm

cm

cm

Fig. 1: SEM micrographs for: (a) reference fiber; and (b) chemical-bond fiber (Note: f is fiber; cm is cementitious matrix)

presented in Fig. 1. For each fiber type, the micrograph shown on the right (4000× magnification) is an enlarged view of the region circled in the first micrograph (45× magnification). The micrographs for the chemical-bond fiber (Fig. 1(b)) show a hydration phase formed on the surface of the fiber, whereas the surface of the reference fiber (Fig. 1(a)) is completely clean. The observed hydration phase on the surface of the chemical-bond fiber would encourage the fiber to develop a stronger bond with the cementitious matrix than in the case of the reference fiber, as is demonstrated

through testing of concrete specimens and discussed subsequently.

Impact of Bond Enhancement on Fiber PerformancePerformance tests

To evaluate the improvement in fiber performance achieved with chemical bonding, standard tests for fiber-reinforced concrete were carried out. Beam specimens were tested per ASTM C1399/C1399M, Standard Test Method for Obtaining Average Residual-Strength of Fiber-Reinforced Concrete, and ASTM C1609/C1609M, Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading). Both are typically used to evaluate the performance of specific fibers for slab-on-ground applications. Round panel specimens were tested per ASTM C1550, Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel). This test is used to evaluate fiber-reinforced concrete for shotcrete applications in underground construction. Figure 2 shows the setups for the three standard tests.

In addition, a single fiber pullout test was performed. The specimen for the test was a 2 in. (50 mm) mortar cube into which a single fiber with a 2.1 in. (54 mm) length was embedded. The fiber embedment depth was 0.6 in. (15 mm). A picture of the specimen under testing is shown in Fig. 3. The pullout load is the maximum load

(a) (b) (c)

Fig. 2: Test setup for: (a) ASTM C1399/C1399M; (b) ASTM C1609/C1609M; and (c) ASTM C1550

Concrete international december 2014 37

attained during the test, with a higher pullout load indicating a stronger bond between the fiber and the cementitious matrix.

Concrete mixtures and curingA fiber dosage of 5 lb/yd3 (3 kg/m3) was used for both the

reference and the chemical-bond fibers for the ASTM C1399/C1399M and C1609/C1609M tests and 9 lb/yd3 (5.3 kg/m3) for the ASTM C1550 test. The concrete mixture proportions and concrete properties are listed in Table 1. All test specimens were moist-cured at 70°F (21°C) and 100% relative humidity for 7 days and tested immediately upon removal from the moist room.

Test resultsFor the ASTM C1399/C1399M and C1609/C1609M tests,

it is known that the level of friction in the beam support rollers affects the magnitude of the measures for fiber performance, with the use of high-friction support rollers

Fig. 3: Single fiber pullout testing of polypropylene macrofiber embedded in a mortar cube

Table 1: Concrete mixture proportions with plastic and hardened concrete properties

Concrete mixture composition and properties

Specimens tested per ASTM C1399/C1399M and C1609/C1609M

Specimen tested per ASTM C1550

Type I/II portland cement, lb/yd3 (kg/m3) 450 (267) 752 (446)

class F fly ash, lb/yd3 (kg/m3) 193 (115) —

Silica fume, lb/yd3 (kg/m3) — 60 (36)

coarse aggregate, lb/yd3 (kg/m3) 1618 (960) 811 (481)

Fine aggregate, lb/yd3 (kg/m3) 1395 (828) 2002 (1188)

Fiber, lb/yd3 (kg/m3) 5.0 (3.0) 9.0 (5.3)

Water, lb/yd3 (kg/m3) 302 (179) 325 (193)

High-range water-reducing admixture,

fl oz/cwt (mL/100 kg) cement3.0 (195) —

Slump, in. (mm) 7.5 (190) 8 (200)

Air content, % 3.1 2.0

7-day compressive strength, psi (mPa) 3900 (26.9) 6000 (41.4)

yielding greater values.2-4 Hence, it is important to compare the performance of different fibers using the same test setup. For the data reported in this article, the coefficient of friction in the roller supports was 0.10, which is considered to be a “low-friction” system.4 Load-deflection curves for the ASTM tests are shown in Fig. 4 through 6.

In each case, the chemical-bond fiber outperformed the reference fiber as shown by the higher post-crack load-carrying capacity of the concrete, indicating that the chemical-bond fiber has a stronger bond to the cementitious matrix and holds cracks tighter. This means that the chemical-bond fiber can be used at a lower dosage relative to the reference fiber, while achieving the same post-crack flexural performance levels. This reduced dosage will result in improved handling properties of the fiber-reinforced concrete in the fresh state, as well as improved finishability. Figure 7 shows test results for concrete specimens with the chemical-bond fiber at a dosage rate of 4 lb/yd3 (2.4 kg/m3) and the reference fiber at 5 lb/yd3 (3 kg/m3). Even though the chemical bond fiber dosage is 20% less than the dosage of the reference fiber, the performance of the specimen with the chemical-bond fiber is comparable to the perfor-mance of the specimen with the reference fiber.

Table 2 provides the percent of improvement in the performance of the chemical-bond fiber relative to the reference fiber for all tests. The improvement ranges from 32 to 45%, indicating that chemical modification further enhances the already excellent performance of this commercially available reference polypropylene fiber.

Concluding RemarksA polypropylene macrofiber with chemical bond to the

concrete matrix has been developed. The novel approach to achieving the chemical bond properties on the inherent inert surface of polypropylene fibers involves incorporating specific chemical compounds into the base polypropylene

38 december 2014 Concrete international

Fig. 7: Load-deflection curves for ASTM C1609/C1609M testing of chemical-bond fiber at 4 lb/yd3 (2.4 kg/m3) dosage rate and reference fiber at 5 lb/yd3 (3 kg/m3) dosage rate (Note: 1 kN = 225 lbf; 1 mm = 0.04 in.)

Table 2: Performance improvement of chemical-bond macrofiber relative to a reference fiber

Polypropylene fiber

ASTM C1399/C1399M average residual

strength, psi (MPa)

ASTM C1609/C1609M equivalent flexural

strength ratio, %ASTM C1550 energy absorption, in.-lbf (J)

Pullout load, lbf (N)

dosage, lb/yd3 (kg/m3) 5 (3) 5 (3) 9 (5.3) Single fiber

reference 176 (1.2) 27 3248 (367) 14.4 (64)

chemical-bond 240 (1.7) 36 4283 (484) 20.9 (93)

Performance improvement, % 37 33 32 45

Fig. 4: Load-deflection curves for ASTM C1399/C1399M testing of fibers at 5 lb/yd3 (3 kg/m3) dosage rate (Note: 1 kN = 225 lbf; 1 mm = 0.04 in.)

Fig. 5: Load-deflection curves for ASTM C1609/C1609M testing of fibers at 5 lb/yd3 (3 kg/m3) dosage rate (Note: 1 kN = 225 lbf; 1 mm = 0.04 in.)

Fig. 6: Load-deflection curves for ASTM C1550 testing of fibers at 9 lb/yd3 (5.3 kg/m3) dosage rate (Note: 1 kN = 225 lbf; 1 mm = 0.04 in.)

resin prior to extrusion of the fiber. The chemical bond of the fiber to the cementitious matrix through chemical interaction of its surface with the matrix is in addition to the bond developed through friction and mechanical anchorage.

0

2

4

6

8

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Load

, kN

Deflection, mm

Chemical-bond fiber

Reference fiber

Chemical-bond fiber

0

5

10

15

20

25

30

35

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Load

, kN

Deflection, mm

Reference fiber

Chemical-bond fiber

Reference fiber05

10152025303540

0 10 20 30 40

Load

, kN

Deflection, mm

Chemical-bond fiberat 4 lb/yd3 (2.4 kg/m3)

Reference fiberat 5 lb/yd3 (3 kg/m3)

05

101520253035

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Load

, kN

Deflection, mm

With these three aspects of interfacial bonding, the new fiber is able to hold cracks tighter, as evidenced by its enhanced performance in various tests, and can be used at a lower dosage relative to existing commercial macrofibers, while achieving

Concrete international december 2014 39

Emmanuel K. Attiogbe, FAcI, is Head of Innovation cluster Technologies, bASF construction chemicals, Global r&d, beachwood, OH. He received his Phd in civil engineering from the University of Kansas, Lawrence, KS, specializing in structural engineer-ing and concrete materials. He was awarded the AcI Wason medal for

materials research in 1995. He is a member of several AcI committees, a past member of the AcI board of direction, and a past chair of the Technology Transfer Advisory Group of AcI’s Strategic development council.

AcI member Steve Schaef manages the engineering Group, supporting research and development for bASF construction chemicals, beach-wood, OH. He has been part of the r&d community within bASF for over 8 years. Schaef received his mS in civil engineering from Purdue Univer-sity, West Lafayette, IN. His research

interests include concrete shrinkage mechanisms, durability test method development, fiber-reinforced concrete, and alkali-silica reactivity.

Charles O. Kerobo is Technical development Leader, bASF care chemicals, Formulation Technolo-gies, Wyandotte, mI. He received his Phd in chemical engineering from New Jersey Institute of Technology, Newark, NJ, and his mbA from michi-gan State University, east Lansing, mI. Kerobo has been with bASF corpo-

ration for 26 years. His activities include product innova-tions for applications in concrete technology.

AcI member Dan Vojtko is a devel-opment engineer at The Goodyear Tire & rubber company in Akron, OH. At the time the research re-ported in this paper was conducted, he was a Senior research engineer with bASF construction chemicals, beachwood, OH. Vojtko received his mS in civil engineering from Purdue University.

Charles K. Nmai, FAcI and past member of the AcI board of direc-tion, is engineering Associate/man-ager, engineering Services, with bASF construction chemicals, beach-wood, OH. He is currently chair of ASTm Subcommittee c09.23, chemical Admixtures. In 2013, Nmai received the AcI Arthur r. Anderson medal for outstanding contributions

to the advancement of knowledge of concrete as a con-struction material in the areas of corrosion and chemical admixtures. He received his Phd in civil engineering from Purdue University.

the same post-crack flexural performance levels. This reduced dosage will result in improved handling properties of the fiber-reinforced concrete in the fresh state, as well as improved finishability. The chemical-bond polypropylene macrofiber is a first for use in reinforcing concrete in applications such as slab-on-ground, pavements, bridge decks, precast, and shotcrete for underground construction.

AcknowledgmentsThe authors gratefully acknowledge the contributions of their colleague R. Reinicker in the development of the chemically modified polypropylene fiber and the support provided by Fabpro Oriented Polymers, LLC, in the production of the fibers. Acknowl-edgments are also due to the Engineering Research and Application group at the BASF R&D Center in Beachwood, OH, for performing the fiber-reinforced concrete testing, and to J. Atienza for preparing the micrographs on the nature of the interface between the fiber and the cementitious matrix.

References 1. Naaman, A.E., “New Fiber Technology,” Concrete International,

V. 20, No. 7, July 1998, pp. 57-62.2. Wille, K., and Parra-Montesinos, G.J., “Effect of Beam Size,

Casting Method, and Support Conditions on Flexural Behavior of Ultra-High-Performance Fiber-Reinforced Concrete,” ACI Materials Journal, V. 109, No. 3, May-June 2012, pp. 379-388.

3. Zollo, R.F., “Analysis of Support Apparatus for Flexural Load-Deflection Testing: Minimizing Bias Caused by Arching Forces,” Journal of Testing and Evaluation, V. 41, No. 1, 2013, pp. 1-6.

4. Bernard, E.S., “Influence of Friction in Supporting Rollers on the Apparent Flexural Performance of Third-Point Loaded Fiber Reinforced Concrete Beams,” Advances in Civil Engineering Materials, V. 3, No. 1, 2014, pp. 158-176.

Note: Additional information on the ASTM standards discussed in this article can be found at www.astm.org.

Selected for reader interest by the editors.