discrete fiber reinforced polymer systems...

DISCRETE FIBER REINFORCED POLYMER SYSTEMS FOR REPAIR OF CONCRETE STRUCTURES: POLYUREA-FIBER CHARACTERIZATION

RESULTS

N. L. Carey and J. J. Myers

Synopsis: This research investigated the development and characterization of different discrete fiber-reinforced polyurea systems for infrastructure applications. The behavior of various systems consisting of several polyureas with different fiber configurations was evaluated. Polyurea coating systems were previously evaluated for blast mitigation and impact resistance, and showed to be adequate in containing debris scatter from blast and impact. The purpose of further testing was an effort to develop a polyurea system for multi-hazard and/or repair-retrofit applications. The addition of fiber to a polymer coating provides improved stiffness and strength to the composite system while the polyurea base material provides ductility. Coupon tensile testing was conducted to determine the material mechanical properties in this study. The two parameters that were varied throughout testing were fiber volume fraction and fiber length. E-Glass fiber was used during specimen fabrication. Several optimal composite configurations of polyurea and fiber resulted from this coupon testing. Keywords: composite material, discrete fiber-reinforced polyurea system, polyurea

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N. L. Carey is currently working on her doctorate under the direction of Dr. John J. Myers. N. Carey graduated with bachelor’s degrees in civil and architectural engineering in 2007 from Missouri University of Science and Technology (Missouri S&T, formerly University of Missouri – Rolla). In 2009, she received her master’s degree from Missouri S&T in civil engineering with an emphasis in structural engineering. J. J. Myers is an Associate Professor in the Department of Civil, Architectural, and Environmental Engineering at Missouri University of Science and Technology in Rolla, MO. He is the current chair of ACI Committee 363 and a member of ACI Committees 201, 342, 423, 440, EAC, S801, S802, S803, S804, SA02, and SOYM.

INTRODUCTION

Research initiatives have been advanced to investigate new materials that can be used for blast mitigation, seismic, and general repair-retrofit applications. Much work has been investigated using fiber reinforced concrete (FRC) (ACI Committee 544 1996). This research investigates the mechanical properties and performance of different discrete fiber-reinforced polyurea systems under development at Missouri University of Science and Technology (Missouri S&T) in Rolla, Missouri. Elastomeric polyurea coating possesses several advantageous characteristics, including elasticity, ductility, and energy absorption. Additionally, polyurea is capable of containing spalling and reducing fragmentation during a blast event (Carey and Myers 2010). The tensile properties of the plain polyurea and composite polymer matrix were determined by conducting coupon testing. In addition, sample ignition loss testing was conducted to determine the fiber reinforcement content. Polyurea material is a two-component 100% solid reactive cure. Polyurea is a low-viscosity liquid, so it bonds very quickly and evenly during application. Several key characteristics of the polyurea coating include chemical and water resistance, excellent elongation, and quick curing. The polyurea coating is capable of withstanding regular thermal or dynamic movement as well. Despite requiring special equipment and experienced operators for mixing, application is easy and the material cures rapidly. Prior to application, the surface should be thoroughly prepared in order to achieve an adequate and strong bond. Also, the material is not very sensitive to temperature and humidity, which eliminates the need for additional arrangements for application procedures and curing environments. Polyurea also aids in the confinement of post-blast materials in compression loaded structures, which produces a residual load-bearing capability.

RESEARCH SIGNIFICANCE This research investigated the development and characterization of different discrete fiber-reinforced polyurea systems for various infrastructural applications. Polyurea coating was selected for further investigation due to its excellent performance during previously conducted blast and impact testing at Missouri S&T. Previously, the authors and others (Carey and Myers 2009; Coughlin 2008; Hrynyk and Myers 2007; Tinsley and Myers 2007; Viswanath 2007) have investigated the use of polyurea systems

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without discrete fiber for hazard mitigation and general strengthening. This discrete fiber-reinforced polyurea work by the authors is novel. The purpose of further testing was an effort to develop a polyurea system for multi-hazard and/or repair-retrofit applications. The addition of fiber to a polymer coating provides improved stiffness and strength to the composite system while the polyurea base material provides ductility.

EXPERIMENTAL STUDY Material description

During this experimental program, five various polyureas from two manufactures were investigated and tested under tension. Coupon specimens were fabricated using each elastomeric polyurea and E-Glass fiber by varying fiber content and fiber length. Tables 1 and 2 list the mechanical properties for the tested materials as specified on product data sheets. Stress vs. strain behavior for each tested polyurea is provided in Figure 1. Polyurea E demonstrated the highest modulus of elasticity and superior ductility compared to the other coatings.

Table 1 — Mechanical properties of elastomeric polyureas

Material Tensile Strength, MPa

(psi) ASTM D 412-06a

Elongation (%) ASTM D 412-06a

Gel Time (sec)

Tack Free Time (sec)

Polyurea A 17 (2466) 480 6 30 Polyurea B 14.8 (2147) 91 3 - 6 6 - 9 Polyurea C 8.3 - 9.0 (1200 - 1300) 400 - 440 20 - 25 120 - 150 Polyurea D 9.0 - 10.3 (1300 - 1500) 135 - 150 6 - 9 9 - 12 Polyurea E 19.3 - 20.7 (2800 - 3000) 430 - 445 11 - 13 78 - 85

Table 2 — Mechanical properties of E-Glass fiber

Mechanical Properties Dry Range Tensile Strength [ASTM D 638-10], MPa (ksi) 59 (8.49) – 98 (14.18) Tensile Modulus [ASTM D 638-10], MPa (ksi) 7542 (1094) – 14893 (2160) Flexural Strength [ASTM D 790-10], MPa (ksi) 166 (24.05) – 307 (44.51) Flexural Modulus [ASTM D 790-10], MPa (ksi) 6939 (1006) – 12065 (1750)

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Unit conversion: 1 MPa = 0.14504 ksi, 1 mm/mm = 1 in/in

Figure 1 — Stress-strain behavior of tested plain polyurea coatings. Test matrix description

Various coupon samples were fabricated by varying matrix material, fiber content, and fiber length. Fiber volume was adjusted during coupon fabrication by increasing the speed of fiber chopping and integration into the polyurea matrix. During specimen fabrication, a mechanical chopper/spray gun was used to combine and simultaneously distribute a two-component polyurea mixture, chop the E-Glass fiber roving, and disperse the glass fibers into the coating system creating a uniformly discretely integrated fiber-reinforced system. The E-Glass fiber roving used during specimen fabrication was specifically designed for chopping and spray-up applications. The chopper assembly on the spray gun contains a cutter head with blades. The cut lengths of the fiber roving vary depending upon the number of blades in the cutting head. Any number of blades can be omitted to achieve the desired fiber length. Examined fiber lengths were dictated by the chop blade configuration. Based upon knowledge of other fiber-reinforced systems, such as FRC, the researchers expected shorter fiber lengths to yield desirable performance. Fiber chopping speed was adjusted directly on the spray gun by turning a dial. Three fiber lengths were investigated for each fiber volume using polyurea A. Fiber length of 6 mm (0.25 in) was kept constant during polyurea B thru E composite coupon testing. Fiber length was easily adjusted and maintained consistent throughout testing. Great care was taken to maintain the fiber content consistency for each fiber length. Table 3 provides test matrix description.

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Stre

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Strain (mm/mm)

Polyurea EPolyurea D

Polyurea A

Polyurea B

Polyurea C

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Table 3 — Test matrix

Material Fiber length, mm (in) Fiber chopping speed (# of dial turns)

Polyurea A 6 (0.25) 3, 3.5, 3.75, 4, 5 Polyurea A 13 (0.5) 3, 3.5, 3.75, 4 Polyurea A 38 (1.5) 3, 3.5, 3.75 Polyurea B 6 (0.25) 3, 4, 5, 6, 7, 9 Polyurea C 6 (0.25) 3, 4, 5, 6, 7, 8 Polyurea D 6 (0.25) 3, 4, 5, 6, 7, 8 Polyurea E 6 (0.25) 3, 4, 5, 6, 7, 8

Coupon sample fabrication

Coupon specimen fabrication was conducted according to ASTM D 3039-08 (2008). In order to obtain coupon samples, the material was sprayed on a steel oiled plate and then peeled off, as shown in Figure 2. Coupon samples were cut out of the peeled sections. Because sample surfaces were slightly uneven, several width and thickness measurements were taken with calipers to obtain average coupon dimensions. Several coupon samples were made from the plain polyurea material and the fiber-reinforced polyurea composite material for benchmarking. Chopped E-Glass fibers were discretely integrated in with the polyurea to develop a fiber-reinforced system. General purpose gun roving fibers were chopped and mixed instantaneously during the application process.

Figure 2 — Polyurea application process.

The specimen dimensions and modified testing procedure were adopted from the

previous study by Carey and Myers (2009). All test specimens had the same gage length of 127 mm (5 in), but varied in cross-sectional geometry. The coating thickness varied from 2.5 to 4 mm (0.1 to 0.16 in) with an average specimen width of 40 mm (1.57 in). Figure 3 provides the layout of the polyurea coupon sample used during plain and composite polyurea A specimen testing. Thin aluminum sheets 51 x 51 mm (2 x 2 in) were glued with epoxy to each end of the specimen in order to ensure a tighter grip for

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the machine and effective load transfer to the sample. Several coupon test samples are displayed in Figure 4. Several coupon specimens were tested without aluminum tabs demonstrating similar results as obtained previously during polyurea A specimen testing. Aluminum tabs, therefore, were not used during further testing of plain and composite specimens in order to promote tearing in the gage length.

Unit conversion: 1 mm = 0.0394 in

Figure 3 — Coupon test specimen layout.

Figure 4 — Plain polyurea (left) and discrete fiber-reinforced polyurea (right) coupon test

samples. Coupon tensile testing

Tensile testing was conducted at Missouri S&T. Coupon specimens were tested until failure using an Instron 4485 tensile testing machine, as shown in Figure 5. The stress-strain behavior of the material was determined and evaluated. The testing procedure specified by ASTM D 3039-08 (2008) was followed. A loading rate of 12.7 mm/min (0.5 in/min) was used, and specimens were tested without end tabs. To record the strain data a 25.4 mm (1 in) extensometer was used, attached at the midpoint of the gage length. To prevent damage to the testing machine and extensometer, the extensometer was removed when a 100% strain value was achieved. Minimum of four

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samples were tested for each material configuration. Some coupon samples exhibited tearing within the tabs which eliminated them from the data analysis.

Figure 5 — Tensile testing equipment (left) and composite coupon sample undergoing

tension (right). Ignition loss testing

Ignition loss testing was conducted according to the following procedure. Initially, a small test sample was weighed, then placed on a glass substrate and reweighed. The samples on glass substrates were heated in the muffle furnace at 600°C (1112°F) until all polyurea resin had disappeared (see Figure 6). The samples were than cooled and weighed again including the substrate. Fiber content was calculated to check and compare to the estimated value. Samples prior and after ignition loss testing are demonstrated in Figure 7.

Figure 6 — Samples in the muffle furnace prior to heating.

Figure 7 — Samples prior (left) and after (right) ignition loss test.

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RESULTS AND CONCLUSIONS Results and discussion

Data acquired during coupon tensile testing was graphed and analyzed. For each fiber-reinforced polyurea system minimum of four samples were tested. If during testing, the tearing in the sample occurred within the tabs, the sample was eliminated. The most conservative result from each tested combination was graphed. Stress vs. strain behavior for 6 mm (0.25 in), 13 mm (0.5 in), and 38 mm (1.5 in) fiber lengths combined with polyurea A are presented in Figures 8, 9, and 10. A few samples with the highest fiber content plotted lower than expected due to delamination. Some samples with long fibers or high fiber volume content exhibited delamination and high amount of voids leading to lower ultimate strength. It was noted that as the fiber content increased, material strength increased, but ductility decreased. Fiber-reinforced polyurea systems with longer fiber length exhibited decreased ductility. As the fiber content and the fiber length increased, the modulus of elasticity increased as well. Samples with shorter fibers exhibited higher ductility due to the weaker bond between the fiber and the matrix; therefore 6 mm (0.25 in) fiber length was used for the fiber-reinforced polyurea B thru F specimens.


Note: The value above each curve indicates fiber volume percentage. Figure 8 — Stress-strain behavior of 6 mm (0.25) fiber-reinforced polyurea A systems.

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12%

Polyurea A

3%

10%

6.5%

8.5%

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Figure 9 — Stress-strain behavior of 13 mm (0.5 in) fiber-reinforced polyurea A systems.


Figure 10 — Stress-strain behavior of 38 mm (1.5 in) fiber-reinforced polyurea A systems.

Stress vs. strain behavior for plain and fiber-reinforced polyurea systems

consisting of polyurea B and 6 mm (0.25 in) E-Glass fiber are presented in Figure 11. Polyurea B is a fast setting material with a 90% elongation capability, which is considerably lower compared to the other evaluated polyureas. It was observed that as the

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0.00 0.02 0.04 0.06 0.08 0.10 0.12

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6%

3.5%

11.5%

14%

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0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Stre

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Strain (mm/mm)

10%

7%

5%

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fiber volume increased, the strength and modulus of elasticity increased compared to the plain polyurea, as noted previously.


Figure 11 — Stress-strain behavior of 6 mm (0.25 in) fiber-reinforced polyurea B systems.

Stress vs. strain behavior for plain and composite fiber-reinforced polyurea

systems incorporating polyurea C and 6 mm (0.25 in) E-Glass fiber are illustrated in Figure 12. Polyurea C is a slow set material with about 420% elongation capability. It was noted that as the fiber content increased the strength and modulus of elasticity increased, but ductility decrease compared to the plain polyurea material. Polyurea C demonstrated the highest elongation capability, but the lowest tensile strength compared to the other plain polyureas and composite polyurea systems at similar strength level. As the fiber content increased, the strength of the composite samples increased for all polyureas, but ductility decreased. In case of the polyurea C, the addition of the fiber increased the specimen strength, but not as significantly as for polyureas A and B. Additional fiber has to be added to the polyurea C to achieve a comparable strength level similar to polyurea A and B.

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0.00 0.10 0.20 0.30 0.40 0.50

Stre

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Strain (mm/mm)

2%

4%4%

5.5%4.7%

Polyurea B

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Figure 12 — Stress-strain behavior of 6 mm (0.25 in) fiber-reinforced polyurea C systems.

Stress vs. strain behavior for plain and fiber-reinforced polyurea systems

consisting of polyurea D and 6 mm (0.25 in) E-Glass fiber are presented in Figure 13. Polyurea D is a fast set material with a 135 to 150% elongation capability. Stress vs. strain behavior for plain and composite systems incorporating polyurea E and 6 mm (0.25 in) E-Glass fiber are presented in Figure 14. Polyurea E is a slow set material with 430 to 445% elongation similar to polyurea C, but has a higher tensile strength. It was noted that as the fiber volume increased, the strength and modulus of elasticity increased, but ductility decreased compared to the plain material for polyurea D and E, similar to polyurea A, B, and C.

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Strain (mm/mm)

Polyurea C2.7%

3.3%

6.2%

5.1%

3.9%5.3%

6.5%

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Figure 13 — Stress-strain behavior of 6 mm (0.25 in) fiber-reinforced polyurea D systems.


Figure 14 — Stress-strain behavior of 6 mm (0.25 in) fiber-reinforced polyurea E systems.

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Polyurea D2.9%

3.8%

5.7%

8.4%

6.1%7.5%

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Polyurea E

0.8%

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3.5%

2.8%

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Conclusions Coupon tensile testing was conducted to determine mechanical properties and

performance of different E-Glass fiber-reinforced polyurea systems. Five various polyureas were tested by varying fiber volume fraction and fiber length. Sample ignition loss testing was conducted to determine the fiber volume fraction. Several conclusions resulted from this study:

1. As the fiber content increased for all polyureas fiber-reinforced systems, material strength and the modulus of elasticity increased, but ductility decreased.

2. As the fiber content and the fiber length increased for polyurea A composite systems, the modulus of elasticity increased. In addition, fiber-reinforced polyurea systems with longer fiber length exhibited decreased ductility.

3. Polyurea B should be further investigated. By increasing the fiber content to 5%, the strength increased significantly and some minor ductility was gained.

4. Polyurea E should be further investigated as well, due to high ductility and strength. Further testing is currently in progress, simultaneously examining the repair effectiveness on reinforced concrete members for flexure and shear.

ACKNOWLEDGMENTS

The research presented in this paper was supported by the Awareness and Localization of Explosives-Related Threats (ALERT) Center of Excellence located at Northeastern University and funded through the United States Department of Homeland Security. Their financial support is gratefully appreciated. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied of the U.S. Department of Homeland Security.

REFERENCES Referenced standards and reports ASTM D 412-06a Standard Test Methods for Vulcanized Rubber and Thermoplastic

Elastomers—Tension D 638-10 Standard Test Method for Tensile Properties of Plastics D 790-10 Standard Test Methods for Flexural Properties of Unreinforced and

Reinforced Plastics and Electrical Insulating Materials D 3039-08 Standard Test Method for Tensile Properties of Polymer Matrix

Composite Materials These publications may be obtained from this organization: ASTM International 100 Barr Harbor Drive West Conshohocken, PA 19428 www.astm.org

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Cited references

ACI Committee 544, 1996, “Report on Fiber Reinforced Concrete (ACI 544.1R-96),” American Concrete Institute, Farmington Hills, MI, 64 pp.

Carey, N.L., and Myers, J.J., 2009, “Impact Testing of Polyurea Coated Reinforced Concrete and Hybrid Panels,” 9th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS-9), Sydney, Australia, 4 pp.

Carey, N.L., and Myers, J.J., 2010, “Full scale blast testing of hybrid barrier systems,” American Concrete Institute (ACI) Special Publication Journal.

Coughlin, A., 2008, “Contact Charge Blast Performance of Fiber Reinforced and Polyurea Coated Concrete Vehicle Barriers,” Master’s thesis, Pennsylvania State University, University Park, PA, 123 pp.

Hrynyk, T.D., and Myers, J.J., 2007, “Static Evaluation of the Out-of-Plane Behavior of URM Infill Walls Utilizing Modern Blast Retrofit Systems,” Master’s thesis, Missouri University of Science and Technology, Rolla, MO, 201 pp.

Tinsley, M.E., and Myers, J.J., 2007, “Investigation of a High-Volume Fly Ash-Wood Fiber Material Subjected to Low-Velocity Impact and Blast Loads,” Master’s thesis, Missouri University of Science and Technology, Rolla, MO, 178 pp.

Viswanath, T., 2007, “Experimental Study on the Impact Resistance of Polyurea Coated Concrete,” Master’s thesis, Pennsylvania State University, University Park, PA, 69 pp.

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