Center for By-Products Utilization

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1 Center for By-Products Utilization Testing and Evaluation of Concrete Using High- Carbon Fly Ash and Carbon Fibers By Rudolph N. Kraus and Tarun R. Naik Report No. CBU REP-615 June 2006 Report Submitted to We Energies, Milwaukee, WI Department of Civil Engineering and Mechanics College of Engineering and Applied Science THE UNIVERSITY OF WISCONSIN - MILWAUKEE

2 FINAL PROJECT REPORT Testing and Evaluation of Concrete Using High-Carbon Fly Ash and Carbon Fibers By Rudolph N. Kraus Researcher and Assistant Director UWM Center for By-Products Utilization University of Wisconsin-Milwaukee And Tarun R. Naik Professor, Department of Civil Engineering and Mechanics and Director UWM Center for By-Products Utilization University of Wisconsin-Milwaukee ABSTRACT This project consisted of performance testing and evaluation of concrete containing high-carbon fly ash and carbon fibers. One of the primary aims of this project was to further determine the effect of using high-carbon fly ash and carbon fibers on the electrical properties of concrete. This is a part of the continuing work performed initially by the UWM Center for By-Products Utilization (UWM-CBU) in 2000 and The intended uses of these conductive construction materials are in electrical grounding, or similar applications where conductive construction materials are needed. In electrical grounding applications, lowering the electrical resistance of bedding materials for grounding electrodes should result in reduced length, or entirely eliminate, the need for the grounding electrodes currently in use for protection of electrical equipment from lightning strikes. Many other potential applications exist for this type of conductive concrete. Fourteen different mixtures were specified by We Energies for testing of electrical properties. These mixtures consisted of: -i-

3 - Seven high-volume fly ash concrete mixtures without high-carbon fly ash and containing 0, 1, 3, 5, 7.5, 10, and 20 lb of conductive carbon fibers per cubic yard of concrete. The nominal mixture proportions specified by We Energies was 350 lb Class C fly ash, 350 lb cement, 315 lb water, 1375 lb SSD fine aggregate, and 1375 lb coarse aggregate, per cubic yard of concrete. Actual mixture proportions slightly differed from those specified by We Energies due to differences in the yield of the mixtures produced at the UWM-CBU laboratory. - Seven high-volume fly ash concrete mixtures containing high-carbon fly ash and containing 0, 1, 3, 5, 7.5, 10, and 20 lb of conductive carbon fibers per cubic yard of concrete. The nominal mixture proportions specified by We Energies was 350 lb Class C fly ash, 300 lb high-carbon fly ash, 350 lb cement, 450 lb water, 1075 lb SSD fine aggregate, and 1375 lb coarse aggregate per cubic yard of concrete. Actual mixture proportions differed from those specified by We Energies due to differences in the yield of the mixtures produced in the laboratory. Due to electrical resistance measurements, mixture proportions, and other corrections made in the laboratory, an additional eight concrete mixtures were produced in the UWM-CBU laboratory for this project. However, only the results of the final mixtures are given in this report to avoid confusion between the final test results test results and the test results that were eliminated due to testing and equipment problems. Electrical properties and compressive strength measurements for the concrete mixtures show the following general trends: 1. There was a significant decrease in electrical resistance of the hardened concrete as the amount of carbon fibers was increased in the mixtures. This trend was apparent in both series of mixtures, with and without the use of high-carbon fly ash. 2. The electrical resistance of the concrete decreased for similar concrete mixtures when highcarbon fly ash was used. This is an indication that the use of high-carbon fly ash will decrease the electrical resistance of the concrete. -ii-

4 3. The most significant drop in the electrical resistance occurred when a minimum of 2.6 lb/yd 3 of carbon fibers were used in the concrete that contained high-carbon fly ash. A similar drop in electrical resistance was obtained in the concrete mixtures without the highcarbon fly ash, but at a much higher carbon fiber content, approximately 10 lb/yd 3. This makes the concrete mixtures without high-carbon fly ash much more expensive than similar mixtures using high-carbon fly ash due to the required amount of expensive carbon fibers needed to produce a low electrical resistance. 4. Compressive strength of mixtures containing high-carbon fly ash was lower than similar mixtures without this ash. This is attributed to the higher water to cementitious materials necessary in mixtures containing high-carbon fly ash. The additional water was needed to maintain the desired workability of the fresh concrete. The compressive strength of these mixtures may be increased through the proper use of water reducing admixtures, and revising mixture proportions. Concrete mixtures containing the high-carbon fly ash in combination with a small amount of carbon fibers (approximately 2.5 lb per cubic yard of concrete) should be further evaluated using larger batches of concrete for further grounding enhancement studies. The compressive strength and durability of these mixtures should be optimized for the intended use though the use of chemical admixtures. Other by-products in lieu of the carbon fibers should also be investigated in the future. -iii-

5 ACKNOWLEDGMENT The authors express deep sense of gratitude to the We Energies, Milwaukee, Wisc., for the carbon fibers and fly ash used for this project. Special appreciation is expressed to Mr. Bruce W. Ramme for his continued interest in this project and monitoring project progress and achievements. Thanks are also due to the UWM Center for By-Products Utilization laboratory staff, for their contributions in gathering test data for this project. The Center was established in 1988 with a generous grant from the Dairyland Power Cooperative, La Crosse; Madison Gas and Electric Company, Madison; National Minerals Corporation, St. Paul, Minn.; Northern States Power Company, Eau Claire; We Energies, Milwaukee; Wisconsin Power and Light Company, Madison; and, Wisconsin Public Service Corporation, Green Bay. Their financial support, and additional support from Manitowoc Public Utilities, Manitowoc, is gratefully acknowledged. -iv-

6 TABLE OF CONTENTS Section Page ABSTRACT... i ACKNOWLEDGEMENT... iv LIST OF TABLES... vi LIST OF FIGURES... vii INTRODUCTION AND BACKGROUND...1 SCOPE OF WORK...2 MATERIALS AND TEST PROCEDURES...2 Materials...2 Fine Aggregate...2 Coarse Aggregate...3 Cement...3 Carbon Fibers...3 Fly Ash...3 Manufacturing of Concrete Mixtures...4 Concrete Specimen Preparation and Testing...5 Concrete Compressive Strength Testing...5 Electrical Resistance Measurements and Resistivity Calculations...5 MIXTURE PROPORTIONS...6 Concrete Mixture Proportions...6 DISCUSSION OF RESULTS...7 Mechanical Properties...7 Concrete Mixtures...7 Concrete Compressive Strength...8 Electrical Properties...9 Electrical Resistance of Concrete...9 Electrical Resistivity of Concrete...10 RECOMMENDATIONS...11 Electrical Properties...12 Mechanical Properties v-

7 LIST OF TABLES Table No./Title Page Table 1 - Gradation of Aggregates...15 Table 2 - Physical Properties of Aggregates...15 Table 3 - Chemical Composition of Portland Cement...16 Table 4 - Physical Properties of Portland Cement...16 Table 5 - Chemical Analysis of Fly Ash...17 Table 6 - Physical Properties of Fly Ash...18 Table 7 - Mixture Proportions of Concrete Without High-Carbon Fly Ash...19 Table 8 - Mixture Proportions of Concrete Containing High-Carbon Fly Ash...20 Table 9 - Compressive Strength of Mixtures Without High-Carbon Fly Ash...21 Table 10 - Compressive Strength of Mixtures Containing High-Carbon Fly Ash...21 Table 11 - Electrical Resistance of Concrete Without High-Carbon Fly Ash...22 Table 12 - Electrical Resistance of Concrete Containing High-Carbon Fly Ash...23 Table 13 - Electrical Resistivity of Concrete Mixtures Without High-Carbon Fly Ash...24 Table 14 - Electrical Resistivity of Concrete Mixtures Containing High-Carbon Fly Ash vi-

8 LIST OF FIGURES Figure No./Title Page Fig. 1 - Fine Aggregate Used in Concrete Mixtures...25 Fig. 2 - Coarse Aggregate Used in Concrete Mixtures...25 Fig. 3 - Carbon Fibers Used in Concrete Mixtures...26 Fig. 4 - ASTM Class C Fly Ash and High-Carbon Fly Ash...26 Fig. 5 - Storage of Test Cylinders in the Laboratory...27 Fig. 6 - Application of Conductive Gel to Concrete Cylinder...27 Fig. 7 - Positioning Cylinder for Electrical Resistance Test...28 Fig. 8 - Electrical Resistance Test...28 Fig. 9 - Compressive Strength at 28-Day Age Concrete Without High-Carbon Fly Ash...29 Fig Compressive Strength at 28-Day Age Concrete Containing High-Carbon Fly Ash...29 Fig Electrical Resistance of Concrete Cylinders Without High-Carbon Fly Ash...30 Fig Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash...30 Fig Electrical Resistance of Concrete Cylinders Containing High-Carbon Fly Ash...31 Fig Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash...31 Fig Electrical Resistance of Concrete Mixtures Without Carbon Fibers...32 Fig Electrical Resistance of WE-1 and WE-1CA Concrete...32 Fig Electrical Resistance of WE-3 and WE-3C Concrete...33 Fig Electrical Resistance of WE-5 and WE-5C Concrete...33 Fig Electrical Resistance of WE-7.5B2 and WE-7.5C Concrete...34 Fig Electrical Resistance of WE-10 and WE-10CA Concrete...34 Fig Electrical Resistance of WE-20 and WE-20CA Concrete vii-

9 INTRODUCTION AND BACKGROUND Conductive concrete is made by using electrically conductive materials in close contact with each other throughout the concrete matrix. Some of the conventional materials that have been used to produce electrically conductive concrete include carbon fibers, steel fibers, steel shavings, carbon black, coke breeze, and other similar types of materials. Prior to the start of the testing work conducted by UWM-CBU for We Energies, high-carbon fly ash had not been tested in applications for conductive concrete or for other related applications such as Controlled Low Strength Materials (CLSM). Based upon the current state-of-knowledge and test results from previous testing projects conducted at UWM-CBU for We Energies in 2000 and 2003, this current project was proposed as an additional testing phase to evaluate We Energies high-carbon fly ash and carbon fibers in concrete. The aim of the first phase of this project (2000) was to determine the feasibility of incorporating high-carbon fly ash in CLSM and concrete to lower the electrical resistance of these materials. This first phase consisted of an initial laboratory evaluation of both CLSM and concrete containing high-carbon fly ash without carbon fibers. Results of the initial testing showed that incorporating high-carbon fly ash in the CLSM and concrete reduced the electrical resistivity. A second phase of testing on concrete (2003) evaluated high-carbon fly ash, carbon fibers, and Class C fly ash in concrete. The work summarized in this report builds upon the testing work on concrete performed by the UWM-CBU in 2000 and The lowered electrical resistance of concrete has the potential to reduce the required length, or entirely replace, the grounding electrodes currently in use for protection of electrical equipment from lightning strikes as well as numerous other applications. -1-

10 SCOPE OF WORK As authorized by We Energies, the following work was completed for this project: - Laboratory production of concrete mixtures; - Measurement of compressive strength; - Measurement of density; and - Measurement of electrical resistance and calculation of electrical resistivity. MATERIALS AND TEST PROCEDURES MATERIALS Materials utilized in this project consisted of two sources of fly ash, cement, clean concrete sand, rounded pea gravel used as coarse aggregate, and carbon fibers. Material characterization was not part of the scope of this project. Material characteristics reported for this project were taken from manufacturer s literature, We Energies data, and previous UWM-CBU projects. Fine Aggregate One source of clean concrete sand was used in this investigation as fine aggregate for all concrete mixtures (Fig. 1). Absorption (ASTM C 128) of the sand was determined per ASTM C 33 requirements in order to determine the water to cementitious materials ratio of the mixtures. The gradation and physical properties of the fine aggregate are shown in Tables 1 and 2. These test results were obtained from the work associated with other previous UWM-CBU projects. Coarse Aggregate One source of coarse aggregate was used for all concrete mixtures. The aggregate used for the project was a semi-crushed river gravel with a maximum size of 3/8 inch (Fig. 2). The gradation -2-

11 and physical properties are reported in Table 1 and 2. Absorption (ASTM C 128) of the coarse aggregate was determined per ASTM C 33 requirements in order to determine the water to cementitious materials ratio of the mixtures. The aggregate test results were obtained from the other previous UWM-CBU projects. Cement Type I/II cement (Type I/II cement Lafarge North America, Alpena, Michigan) was used throughout this investigation. The chemical analysis and physical properties of the cement with a comparison to ASTM C 150 requirements for Type I cement are reported in Table 3 and 4. The characterization information was obtained from Lafarge North America. Carbon Fibers Conductive carbon fibers were used for this project. The fibers used for this project were Panex 33 chopped carbon fibers manufactured by the Zoltek Corporation, St. Louis, MO. The carbon fibers were pan-type fibers ½-inch long and approximately mils (7.2 microns) in diameter. The density of the fibers reported by the manufacturer was lb/in 3. The carbon fibers also had a water-soluble sizing applied. The as-received carbon fibers were in the form of small groups of fibers, as well as individual fibers. The carbon fibers are shown in Fig. 3. These groups of fibers, during the process of mixing of the concrete, typically separated into individual fibers and were effectively dispersed in the concrete. Fly Ash Two sources of fly ash were used for this project, a typical ASTM Class C fly ash obtained from the We Energies Pleasant Prairie Power Plant (P4), and one source of a high-carbon fly ash from the We Energies, Valley Plant. These selections were made to represent the typical fly ashes available from We Energies. As shown in Fig. 4, the two sources of fly ash had a distinct color difference, the We Energies P4 fly ash was beige in color, and the Valley high-carbon fly ash -3-

12 was dark gray in color. Chemical and physical properties of the fly ashes reported by We Energies are given in Table 5 and 6. As reported in Table 5, the carbon content, as indicated by the Loss on Ignition, was over 20% for the high-carbon fly ash. Manufacturing of Concrete Mixtures The test concrete was produced in the concrete laboratory of the UWM Center for By-Products Utilization. Concrete mixture proportions were produced for obtaining a large number of mixtures and test specimens with the least amount of laboratory work. Since the volume of concrete to be produced was very small (approximately 0.04 cubic feet), the mixing was done by hand in a small bucket. Prior to mixing, the moisture content of the fine and coarse aggregate was determined to account for absorption of the aggregate and the net water to cementitious materials ratio. All concrete ingredients were weighed and added to the mixture following the sequence specified in ASTM C 192. The carbon fibers were added and mixed with the aggregates to allow the fibers to disperse uniformly in the mixture. Admixtures were not specified to maintain the desirable water content, so water was added as needed (and recorded) to maintain the necessary workability of the mixture. The workability was based on a visual evaluation since there was insufficient material for the standard slump test. A modified test procedure was used for determination of unit weight (ASTM C 138). The unit weight was determined from the weight of the concrete in the cylinder molds. A preliminary unit weight of the fresh concrete was determined after all materials were added and mixed in order that corrected mixture proportions could be determined based on the yield of the mixture. If the corrected mixture proportions had a cementitious materials content that was below the design value specified by We Energies, additional cement, fly ash and water was added to the mixture, and the unit weight of the concrete was again determined. The resulting mixture was then placed in the cylinder molds and consolidated. Concrete Specimen Preparation and Curing Cylindrical test specimens, 3-inch dia. x 6-inch length, were prepared from each mixture for electrical resistance, compressive strength (ASTM C 39), and density tests. A total of five -4-

13 cylinders were cast from each concrete mixture. All test specimens were cast in accordance with ASTM C 192. Concrete specimens were cured for one day in their molds in the UWM-CBU laboratory at about 70 ± 5 F. These specimens were then demolded and placed in a standard moist-curing room maintained at 100% R. H. and 73 ± 3 F temperature until the age of seven days. After seven days, the concrete cylinders were stored in the laboratory at approximately 30 to 40% R.H., and 70 ± 5 F on a plastic grid to allow for increased air exposure on the supported end of the cylinder. The storage of the concrete test specimens in the laboratory is shown in Fig. 5. Concrete Compressive Strength Testing Cylinders, 3-inch dia. x 6-inch length were used for testing of compressive strength. Two cylinders were tested at the age of 28 days. Testing of compressive strength was modified from the standard ASTM C 39. Thin neoprene pads similar to those used for testing Controlled Low- Strength Materials (CLSM) were used in lieu of the standard sulfur caps or standard unbonded capping system used for testing cylinders in compression since the standard 4-inch or 6-inch diameter cylinders were not used (due to the small amount of concrete made for each mixture). The compressive strength of the concrete measured for this project was meant as preliminary values to be used for determining if significant changes to concrete mixture proportions are needed for future work. Electrical Resistance Measurements and Resistivity Calculations In order to test the effect of the moisture on the electrical resistance of the material and the reliability of the measurement, five identical cylinders were made from each concrete mixture. Two cylinders from each mixture were randomly selected for resistance measurements. Resistance measurements were taken using a Leader LCR multimeter at one predetermined location on two cylinders for each mixture up to the age of 28 days. A conductive gel was applied to the ends of the test cylinder (Fig. 6), then the cylinder was positioned for testing (Fig. 7), and then copper plates were used to measure the resistance across the ends of the cylinder (Fig. 8). At the age of 28 days, all five cylinders were tested for resistance, and at the -5-

14 56 and 91 day age, three cylinders were tested. Based on the results of the previous project, We Energies determined that the electrical resistance of the cylinders would be measured in one location: across the length (Fig. 8). This location corresponds to Location 1 of the UWM- CBU 2000 report (CBU ). An average was determined for each type and age of test cylinder. The average result for each test specimen is presented in the plots of electrical resistance in this report. The electrical resistivity is calculated from the resistance measurements obtained from each test cylinder. Electrical resistivity is related to resistance by the following expression: where: R A L = Electrical Resistivity (ohm-cm); R = Electrical Resistance (ohms); A = Cross Sectional Area (cm 2 ); and L = Length of Electrical Path (cm). MIXTURE PROPORTIONS Concrete Mixture Proportions The materials used for producing concrete for this project included a standard clean concrete sand and 3/8" coarse aggregate meeting ASTM C33 requirements, Type I cement, ASTM Class C fly ash, high-carbon fly ash, and carbon fibers. Per the instruction of We Energies, two series of concrete mixtures were produced. Seven high-volume fly ash concrete mixtures were produced without high-carbon fly ash and containing 0, 1, 3, 5, 7.5, 10, and 20 lb of conductive carbon fibers per cubic yard of concrete. The nominal mixture proportions specified by We Energies were 350 lb Class C fly ash, 350 lb cement, 315 lb water, 1375 lb SSD fine aggregate, and 1375 lb coarse aggregate, per cubic yard of concrete. Seven high-volume fly ash concrete mixtures were produced containing high-carbon fly ash and containing 0, 1, 3, 5, 7.5, 10, and 20 lb of conductive carbon fibers per -6-

15 cubic yard of concrete. The nominal mixture proportions specified by We Energies were 350 lb Class C fly ash, 300 lb high-carbon fly ash, 350 lb cement, 450 lb water, 1075 lb SSD fine aggregate, and 1375 lb coarse aggregate per cubic yard of concrete. Due to resistance measurements, mixture proportions, and other corrections made in the laboratory, an additional eight concrete mixtures were produced in the laboratory for this project. However, only the results of the final mixtures are given in this report to avoid confusion between the final test results and the test results that were eliminated due to testing and equipment problems. Mixture proportions of the final concrete mixtures are given in Table 7 and 8. The mixture proportions for the final mixtures differ from the quantities specified by We Energies due to differences in the yield from those estimated by We Energies. DISCUSSION OF RESULTS MECHANICAL PROPERTIES Concrete Mixtures For the first series of concrete mixtures without the high-carbon fly ash (Table 7), the amount of the carbon fibers in the concrete was very close to the amount specified by We Energies. The density of the fresh concrete was reduced as the amount of carbon fibers was increased in the mixture. The density of the fresh concrete ranged from lb/ft 3 for the mixture without carbon fibers, to approximately lb/ft 3 for mixtures containing 10 to 20 lb/yd 3 of carbon fibers. The total cementitious materials content, and the aggregate content of the mixtures also reduced as the amount of carbon fibers increased. The cementitious materials content reduced from 746 lb/yd 3 to 716 lb/yd 3, while the total aggregate content reduced from 2,930 lb/yd 3 to 2,810 lb/yd 3. The water to cementitious materials (W/Cm) ratio was 0.45 for all of the concrete mixtures in this series. The differences in mixture proportions may be attributed to the reduction in unit weight of the concrete mixtures due to the presence of the carbon fibers. -7-

16 The second series of concrete mixtures contained the high-carbon fly ash (Table 8). The amount of the carbon fibers in the concrete was approximately 10 to 20% less than mixtures in the first series of mixtures without high-carbon fly ash. The density of the fresh concrete also reduced as the amount of carbon fibers was increased in the mixture. The density of the fresh concrete was much lower than the comparable mixtures without the high carbon fly ash and varied from lb/ft 3 for the mixture without carbon fibers, to lb/ft 3 for the mixture containing 17 lb/yd 3 of carbon fibers. This lower unit weight also lowered the amount of carbon fibers in the mixtures. The total cementitious materials content, and the aggregate content of the mixtures also reduced as the amount of carbon fibers increased. The total cementitious materials content reduced from 969 lb/yd 3 to 955 lb/yd 3, while the total aggregate content reduced from 2,185 lb/yd 3 to 2,075 lb/yd 3. The amount of high-carbon fly ash in the mixtures remained approximately the same, about 290 lb/yd 3. The water to cementitious materials (W/Cm) ratio was 0.53 for all of the concrete mixtures in this series with the exception of Mixture WE-7.5C that had a W/Cm ratio of The higher W/Cm ratio for this series of mixtures indicates that the water demand of the concrete will increase when high-carbon fly ash is used. Concrete Compressive Strength The compressive strength test results are given in Tables 9 and 10, and Figs. 9 and 10. The values of compressive strength given in the Tables are the average strength of two cylinders tested at the age of 28 days. The compressive strength of the mixtures containing high-carbon fly ash typically were lower than the strength obtained for mixtures without high-carbon fly ash. The overall average of compressive strength for concrete mixtures without high-carbon fly ash is 3,355 psi, while the average strength for mixtures containing high-carbon fly ash is 2,540 psi. The lower compressive strength was expected since the W/Cm ratio of the mixtures containing the high-carbon fly ash was 0.49 to 0.53, while the mixtures without the high-carbon fly ash was Compressive strength of concrete decreases when increasing the W/Cm ratio. Test results of the mixtures containing high-carbon fly ash would be expected to improve if the water in the mixtures were reduced by judiciously using chemical admixtures such as water reducing admixtures. -8-

17 ELECTRICAL PROPERTIES Electrical Resistance of Concrete The electrical resistance measurements obtained for the concrete mixtures without high-carbon fly ash is given in Table 11 and Fig. 11. Two distinct trends are observed from the data. One is that the electrical resistance of the concrete generally increases with increasing age (Fig. 11), and the second trend is that as the amount of carbon fibers increases in the concrete, the resistance generally decreases (Fig. 12). This second trend is also evident after the concrete test cylinders were removed from the moist curing environment and placed in the laboratory environment (after seven days). As the concrete cylinders dried, the electrical resistance generally quickly increased, particularly for mixtures containing low amounts of carbon fibers (less than 7.9 lb per cubic yard of concrete). However, this drying effect was not apparent in mixtures containing high amounts of carbon fibers, such as Mixtures WE-7.5B2, WE-10 and WE-20. Overall, it appears that the lowest amount of fibers where electrical resistance is significantly reduced starts with Mixture WE-7.5B2, which contains 7.9 lb of fibers per cubic yard of concrete. This is further improved when higher quantities of fibers are incorporated into the concrete. The electrical resistance of concrete Mixtures WE-10 (10.3 lb carbon fibers) and WE-20 (20.5 lb carbon fibers) were rated as Very Low (less than 100 ohms for any test age), Mixture WE- 7.5B2 (7.9 lb carbon fibers) was rated as Low (100 to 500 ohms), Mixture WE-5 (2.6 lb carbon fibers) was rated as Medium (500 to 5000 ohms), Mixtures WE-1 (1.1 lb carbon fibers) and WE-3 (3.1 lb carbon fibers) were rated as High (5,000 to 20,000 ohms), and Mixture WE- 0-A (0 lb carbon fibers) was rated as Very High (greater than 20,000 ohms) Test results for electrical resistance of concrete mixtures containing high-carbon fly ash is given in Table 12 and Fig. 13. Similar trends in electrical resistance test results were observed for these mixtures as those observed for concrete mixtures without high-carbon fly ash. The electrical resistance of the concrete generally increased as the concrete ages (Fig. 13), and as the quantity of carbon fibers increased in the concrete, the resistance decreased (Fig. 14). However, the effect of the drying of the concrete on the electrical resistance was not apparent in concrete mixtures with a much lower carbon fiber content, starting at 2.6 lb per cubic yard (Mixture WE- -9-

18 3C) than similar mixtures without the high-carbon fly ash where a minimum of 7.9 lb was needed. Overall, the electrical resistance of concrete Mixtures WE-5C (4.3 lb carbon fibers), WE-7.5C (6.6 lb carbon fibers), WE-10CA (8.5 lb carbon fibers), and WE-20C-A (16.9 lb carbon fibers) were rated as Very Low (less than 100 ohms). The electrical resistance of concrete Mixture WE-3C (2.6 lb carbon fibers) was rated a Low (100 to 500 ohms), while resistance of Mixture WE-1CA (0.9 lb carbon fibers) was rated as High (5,000 to 20,000 ohms), and resistance of Mixture WE-0C-A (0 lb carbon fibers) was rated as Very High (greater than 20,000 ohms). A comparison between the two series of electrical resistance measurements were made between similar mixtures with and without high-carbon fly ash to better visualize the effect of the highcarbon fly ash (Figs ). Although the level of carbon fibers and the aggregate contents were typically lower for the mixture series containing the high-carbon fly ash, the resistance of the concrete mixtures with high-carbon fly ash was typically lower than comparable mixtures without the high-carbon fly ash. This indicates that the high-carbon fly ash does have some effect on lowering the electrical resistance of the concrete. The reduction in electrical resistance in concrete containing high-carbon fly ash makes the use of carbon fibers more effective, allowing lower carbon fiber contents to be used to achieve a similar level of electrical resistance. Electrical Resistivity of Concrete The electrical resistivity of the concrete was calculated for all of the electrical resistance test data. The electrical resistivity calculation takes into account the diameter and length of the test specimen, so if this work is repeated using different sizes of test specimens, comparisons may be made. Comparisons between test data presented in this report and test results from the previous UWM-CBU tests conducted in 2000 and 2003 may also be made using electrical resistivity. Electrical resistivity for concrete mixtures with and without high-carbon fly ash are shown in Table 13 and 14. Only the average values of electrical resistivity for each test age are presented in these Tables, however, individual electrical resistivity data is available for each test. Since the electrical resistivity is directly related to the resistance of the concrete, the same trends in the -10-

19 values of resistivity are observed as those for the values of electrical resistance. The resistivity of the concrete containing the high-carbon fly ash was typically lower than the resistivity of the concrete without the high-carbon fly ash. Electrical resistivity ranged from 173 to over 60,000 ohm-cm for concrete mixtures without high-carbon fly ash, and from 131 to over 50,000 ohm-cm for mixtures with high-carbon fly ash. Mixtures containing the high-carbon fly ash had an overall lower resistivity (less than 350 ohm-cm) starting at dosages of carbon fibers at 2.6 lb per cubic yard of concrete. Concrete mixtures without the high-carbon fly ash achieved this level of resistivity only when 10.3 lb or higher were incorporated into a cubic yard of concrete, nearly four times the carbon fiber content in the high-carbon fly ash mixtures. This indicates that conductive concrete mixtures containing the high-carbon fly ash would be much more economical than mixtures without such high-carbon ash. The amount of carbon fibers is the overwhelming factor when determining the economy of the mixtures since the cost of the fibers are very expensive, over $10 per pound. RECOMMENDATIONS Only limited tests were conducted as a part of this study on small batch sizes of concrete. Therefore, results obtained here should be carefully used in drawing general conclusions and recommendations. Adequate batch sizes for standard testing of test specimens should be conducted to verify and validate the test data collected in this project. Electrical Properties The results of the electrical property testing conducted for this project suggest that using a combination of high-carbon fly ash and carbon fibers will provide enhanced electrical properties (lower resistance and resistivity) useful for applications such as in electrical grounding, conductive concrete for melting of ice and snow, self-monitoring concrete for cracks and stresses, and other similar applications. The high-carbon fly ash used for this project is not pure carbon but it has been determined that using the high-carbon fly ash will reduce the electrical resistance and resistivity of concrete, particularly when used in combination with carbon fibers. -11-

20 The optimum dosage of carbon fibers when used with the aggregate sources selected for this project is approximately 2.5 lb per cubic yard. This compares with over 10 lb of carbon fibers per cubic yard of concrete for concrete mixtures without high-carbon fly ash. The cost of carbon fibers used for this project was more than $10 per pound. Therefore, to achieve a Very Low electrical resistance, a cubic yard of concrete that does not use the high carbon fly ash would cost $75 more than a cubic yard of comparable concrete using the high-carbon fly ash, and $100 per cubic yard more than typical ready-mixed concrete, approximately twice the cost of such concretes. Based on the cost of the carbon fibers, conductive concrete using high-carbon fly ash in the concrete would cost approximately $25 per cubic yard more than typical concrete. Based on the electrical performance of the concretes tested in this project, the focus of the next step for this process would be to further optimize and test the combination of high-carbon fly ash and carbon fibers using judiciously selected batch weights. If a specific range of electrical resistance is desired for electrical grounding or an other application, a concrete mixture could be developed to achieve this desired resistance value, by a judicious blend of high-carbon fly ash and carbon fibers. Mechanical Properties The compressive strength of the concrete with high-carbon fly ash was found to be lower than comparable concrete without the high-carbon fly ash. This reduction was in part due to the higher water demand observed in concrete mixtures using the high carbon fly ash, and the corresponding increase in the water to cementitious materials ratio. The next phase of testing should reduce the amount of water used in the concrete mixtures with the use of chemical admixtures such as water reducing admixtures. This may increase the compressive strength to the desired level. Another aspect that should be addressed is the durability of the concrete mixtures that use the high-carbon fly ash. In northern climates, such as in Wisconsin, concrete that is subjected to moisture and freezing and thawing cycles requires air entrainment for durability. Prior experience with high-carbon fly ash has shown that higher dosages of airentraining admixtures, and/or specialized air entraining admixtures may be required to produce and maintain the desired air content, and therefore, desired level of durability for such concretes. -12-

21 UWM-CBU has experience with other types of materials (e.g., pulp and paper mill residuals) that may be used in lieu of air entraining admixtures to produce concrete resistant to damage from freezing and thawing cycles. The effectiveness of such by-products in concrete using highcarbon fly ash should also be investigated in future project work. -13-

22 TEST RESULTS -14-

23 Sieve Size Table 1 - Gradation of Aggregate (ASTM C 136) % Passing ASTM C 33 % Passing Coarse Fine for Coarse Aggregate Aggregate Aggregate ASTM C 33 % Passing for Fine Aggregate 3/8" (9.5 mm) #4 (4.75 mm) #8 (2.36 mm) #16 (1.18 mm) #30 (600 µm) #50 (300 µm) #100 (150 µm) Table 2 - Physical Properties of Aggregates Bulk Density (lb/ft 3 ) Bulk Specific Gravity SSD Bulk Specific Gravity Apparent Specific Gravity SSD Absorption (%) ASTM Test Designation C 29 C 127/C 128 Fine Aggregate Coarse Aggregate Void Content (%) Fineness Modulus Material Finer than #200 Sieve (75 μm) (%) Clay Lumps and Friable Particles (%) Organic Impurity ASTM Test Designation C 29 C 136 C 117 C 142 C 40 Fine Aggregate Passes Coarse Aggregate

24 Table 3 - Chemical Composition of Portland Cement Item Lafarge Standard requirement of ASTM C (% by mass) 150 for Type I cement Silicon dioxide, SiO Aluminum oxide, Al 2 O Ferric oxide, Fe 2 O Calcium oxide, CaO Magnesium oxide, MgO maximum Sulfur trioxide, SO maximum, when C 3 A 8% 3.5 maximum, when C 3 A > 8% Loss on ignition maximum Insoluble residue maximum Free lime Tricalcium silicate, C 3 S Dicalcium silicate, C 2 S N.A. -- Tricalcium aluminate, C 3 A 8 -- Tetracalcium aluminoferrite, C 4 AF N.A. -- Equivalent alkalies, Na 2 O K 2 O N.A.: Result not available. Table 4 - Physical Properties of Portland Cement ASTM Item Lafarge Standard requirement of ASTM C 150 for Type I cement C 185 Air content of mortar (volume %) 6 12 maximum C 204 Fineness (specific surface) by Blaine minimum air-permeability apparatus (m 2 /kg) C 151 Autoclave expansion (%) maximum Compressive strength of cement mortar (psi): C day days minimum 7 days minimum 28 days C 191 Initial time of setting by Vicat needle 105 Between 45 to 375 (minutes) C 188 Density (g/cm 3 )

25 Table 5 - Chemical Analysis of Fly Ash Analysis Parameter Material, % ASTM C 618 Requirements, % Valley Pleasant (highcarbon (Class C Prairie Class N Class C Class F fly ash) fly ash) Silicon Dioxide, SiO 2 Aluminum Oxide, Al 2 O Iron Oxide, Fe 2 O SiO 2 + Al 2 O 3 + Fe 2 O 3 Calcium Oxide, CaO Magnesium Oxide, MgO Titanium Oxide, TiO 2 Potassium Oxide, K 2 O Sodium Oxide, Na 2 O Min Min Min Sulfate, SO Max. 5.0 Max. 5.0 Max. Loss on Ignition, LOI (@ 750 C) Max.* 6.0 Max.* 6.0 Max.* Moisture Content Max. 3.0 Max. 3.0 Max. Available Alkali, Na 2 O Equivalent (ASTM C-311) 0.5 N.A. 1.5 Max.** 1.5 Max.** N.A. Result not available * Under certain circumstances, up to 12.0% max. LOI may be allowed. ** Optional. Required for ASR Minimization. 1.5 Max.** -17-

26 Table 6 - Physical Properties of Fly Ash Analysis Parameter Material ASTM C 618 Requirements Valley Pleasant (highcarbon (Class C Prairie Class N Class C Class F fly ash) fly ash) Retained on No. 325 Sieve (%) max. 34 max. Strength Activity Index with Cement (% of Control) 7-day 28-day min. 75 min. 75 min. 75 min. 75 min. 75 min. Water Requirement, (% of Control) max. 105 max. 105 max. Autoclave Expansion, (%) max. 0.8 max. 0.8 max. Density

27 Table 7 - Mixture Proportions of Concrete Without High-Carbon Fly Ash Mixture Number Mixture WE-0-A Mixture WE-1 Mixture WE-3 Mixture WE-5 Mixture WE- 7.5B2 Mixture WE-10 Mixture WE-20 Fly Ash [A1/(C+A1)] (%) 50% 50% 50% 50% 50% 50% 50% Class C Fly Ash (lb/yd 3 ), A High-Carbon Fly Ash (lb/yd 3 ), A Cement (lb/yd 3 ), C Water (lb/yd 3 ), W [W/(C+A)] SSD Fine Aggregate (lb/yd 3 ) SSD 3/8" Aggregate (lb/yd 3 ) Carbon Fibers (lb/yd 3 ) Fresh Concrete Density (lb/ft 3 )

28 Table 8 - Mixture Proportions of Concrete Containing High-Carbon Fly Ash Mixture Number Mixture WE-OC- A Mixture WE-1C-A Mixture WE-3C Mixture WE-5C Mixture WE-7.5C Mixture WE-10C- A Mixture WE-20C- A Fly Ash [A1/(C+A1)] (%) 65% 65% 65% 65% 65% 65% 65% Class C Fly Ash (lb/yd 3 ), A High-Carbon Fly Ash (lb/yd 3 ), A Cement (lb/yd 3 ), C Water (lb/yd 3 ), W [W/(C+A)] SSD Fine Aggregate (lb/yd 3 ) SSD 3/8" Aggregate (lb/yd 3 ) Carbon Fibers (lb/yd 3 ) Fresh Concrete Density (lb/ft3)

29 Table 9 - Compressive Strength of Mixtures Without High-Carbon Fly Ash Mixture No. Compressive Strength (psi) (Average of two tests) Test Age, Days 28 WE-0-A 2430 WE WE WE WE-7.5-B WE WE Table 10 - Compressive Strength of Concrete Mixtures Containing High-Carbon Fly Ash Mixture No. Compressive Strength (psi) (Average of two tests) Test Age, Days 28 WE-0C-A 2215 WE-1C-A 5110 WE-3C 2195 WE-5C 1825 WE-7.5C 2320 WE-10C-A 2080 WE-20C-A

30 Table 11 Electrical Resistance of Concrete Without High-Carbon Fly Ash Test Age, days RESISTANCE, ohms Mixture Number / Carbon Fiber Content (lb per cubic yard of concrete) WE-0-A / 0 WE-1 / 1.1 WE-3 / 3.1 WE-5 / 5.3 WE-7.5-B2 / 7.9 WE-10 / 10.3 WE-20 / 20.5 Actual Ave. Actual Ave. Actual Ave. Actual Ave. Actual Ave. Actual Ave. Actual Ave

31 Table 12 Electrical Resistance of Concrete Containing High-Carbon Fly Ash Test Age, days RESISTANCE, ohms Mixture Number / Carbon Fiber Content (lb per cubic yard of concrete) WE-OC-A / 0 WE-1C-A / 0.9 WE-3C / 2.6 WE-5C / 4.3 WE-7.5C / 6.6 WE-10C-A / WE-20C-A / Actual Ave. Actual Ave. Actual Ave Actual Ave. Actual Ave. Actual Ave. Actual Ave * * * *Test results at the age of 29 days. -23-

32 Table 13 - Electrical Resistivity of Concrete Mixtures Without High-Carbon Fly Ash Mixture No. Resistivity of Concrete, (ohm-cm) Test Age, Days WE-0-A WE WE WE WE-7.5B WE WE Table 14 - Electrical Resistivity of Concrete Mixtures Containing High-Carbon Fly Ash Mixture No. Resistivity of Concrete, (ohm-cm) Test Age, Days WE-0C-A WE-1C-A * WE-3C WE-5C WE-7.5C WE-10C-A WE-20C-A * Test results at the age of 29 days. -24-

33 Fig. 1 - Fine Aggregate Used in Concrete Mixtures Fig. 2 - Coarse Aggregate Used in Concrete Mixtures -25-

34 Fig. 3 - Carbon Fibers Used in Concrete Mixtures Fig. 4 - ASTM Class C Fly Ash (left) and High-Carbon Fly Ash (right) -26-

35 Fig. 5 - Storage of Test Cylinders in the Laboratory Fig. 6 - Application of Conductive Gel to Concrete Cylinder -27-

36 Fig. 7 - Positioning Cylinder for Electrical Resistance Test Fig. 8 - Resistance Test -28-

37 Compressive Strength (psi) Compressive Strength (psi) WE-0A WE-1 WE-3 WE-5 WE- 7.5B2 WE-10 WE-20 Mixture Number Fig. 9 - Compressive Strength at 28-Day Age Concrete Without High-Carbon Fly Ash WE-0C- A WE-1C- A WE-3C WE-5C WE- 7.5C WE- 10C-A WE- 20C-A Mixture Number Fig Compressive Strength at 28-Day Age Concrete Containing High-Carbon Fly Ash -29-

38 Electrical Resistance, ohms Resistance, Ohms WE-0A WE-1 WE-3 WE-5 WE-7.5B2 WE-10 WE Test Age, Days Fig Electrical Resistance of Concrete Cylinders Without High-Carbon Fly Ash Test Age, 1-day Test Age, 3-days Test Age, 7-days Test Age, 14-days Test Age, 28-days Test Age, 56-days Test Age, 91-days Carbon Fiber Content (lb per cubic yard of concrete) Fig Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash -30-

39 Electrical Resistance, ohms Resistance, Ohms WE-0CA WE-1CA WE-3C WE-5C WE-7.5C WE-10CA WE-20CA Test Age, Days Fig Electrical Resistance of Concrete Cylinders Containing High-Carbon Fly Ash Test Age, 1-day Test Age, 3-days Test Age, 7-days Test Age, 14-days Test Age, 28-days Test Age, 56-days Test Age, 91-days Carbon Fiber Content (lb per cubic yard of concrete) Fig Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Containing High-Carbon Fly Ash -31-

40 Resistance, ohms Resistance, ohms Mixture WE-0A Mixture WE-0CA Test Age, days Fig Electrical Resistance of Concrete Mixtures Without Carbon Fibers Mixture WE-1 Mixture WE-1CA Test Age, days Fig Electrical Resistance of WE-1 and WE-1CA Concrete -32-

41 Resistance, ohms Resistance, ohms Mixture WE-3 Mixture WE-3C Test Age, days Fig. 17 -Electrical Resistance of WE-3 and WE-3C Concrete Mixture WE-5 Mixture WE-5C Test Age, days Fig Electrical Resistance of WE-5 and WE-5C Concrete -33-

42 Resistance, ohms Resistance, ohms Mxiture WE-7.5B2 Mixture WE-7.5C Test Age, days Fig Electrical Resistance of WE-7.5B2 and WE-7.5C Concrete Mixture WE-10 Mixture WE-10CA Test Age, days Fig Electrical Resistance of WE-10 and WE-10CA Concrete -34-

43 Resistance, ohms Mixture WE-20 Mixture WE-20CA Test Age, days Fig Electrical Resistance of WE-20 and WE-20CA Concrete -35-