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-2006-16 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
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 2003. 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-
- 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-
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-
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-
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...12 -v-
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...24 -vi-
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. 10 - Compressive Strength at 28-Day Age Concrete Containing High-Carbon Fly Ash...29 Fig. 11 - Electrical Resistance of Concrete Cylinders Without High-Carbon Fly Ash...30 Fig. 12 - Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash...30 Fig. 13 - Electrical Resistance of Concrete Cylinders Containing High-Carbon Fly Ash...31 Fig. 14 - Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash...31 Fig. 15 - Electrical Resistance of Concrete Mixtures Without Carbon Fibers...32 Fig. 16 - Electrical Resistance of WE-1 and WE-1CA Concrete...32 Fig. 17 - Electrical Resistance of WE-3 and WE-3C Concrete...33 Fig. 18 - Electrical Resistance of WE-5 and WE-5C Concrete...33 Fig. 19 - Electrical Resistance of WE-7.5B2 and WE-7.5C Concrete...34 Fig. 20 - Electrical Resistance of WE-10 and WE-10CA Concrete...34 Fig. 21 - Electrical Resistance of WE-20 and WE-20CA Concrete...35 -vii-
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 2003. 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-
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-
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 0.283 mils (7.2 microns) in diameter. The density of the fibers reported by the manufacturer was 0.065 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-
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-
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-475-01 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-
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-2000-18). 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-
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 148.5 lb/ft 3 for the mixture without carbon fibers, to approximately 143.5 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-
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 135.9 lb/ft 3 for the mixture without carbon fibers, to 131.7 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 0.49. 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 0.45. 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-
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-
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. 15-21). 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-
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-
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-
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-
TEST RESULTS -14-
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) 100 100 90-100 100 #4 (4.75 mm) 63 99 20-55 95-100 #8 (2.36 mm) 7 86 5-30 80-100 #16 (1.18 mm) 4 72 0-10 50-85 #30 (600 µm) 3 51 -- 25-60 #50 (300 µm) 2 16 0-5 5-30 #100 (150 µm) 1 4 -- 0-10 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 107 2.69 2.73 2.79 1.3 Coarse Aggregate 106 2.62 2.68 2.79 2.3 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 36 2.7 1.4 0.0 Passes Coarse Aggregate 35 5.2 -- 0.0 -- -15-
Table 3 - Chemical Composition of Portland Cement Item Lafarge Standard requirement of ASTM C (% by mass) 150 for Type I cement Silicon dioxide, SiO 2 20.2 -- Aluminum oxide, Al 2 O 3 4.5 -- Ferric oxide, Fe 2 O 3 2.6 -- Calcium oxide, CaO 64.2 -- Magnesium oxide, MgO 2.5 6.0 maximum Sulfur trioxide, SO 3 2.4 3.0 maximum, when C 3 A 8% 3.5 maximum, when C 3 A > 8% Loss on ignition 1.4 3.0 maximum Insoluble residue 0.4 0.75 maximum Free lime 1.5 -- Tricalcium silicate, C 3 S 67 -- 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 + 0.53 -- 0.658K 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 364 280 minimum air-permeability apparatus (m 2 /kg) C 151 Autoclave expansion (%) 0.07 0.80 maximum Compressive strength of cement mortar (psi): C 109 1 day 2080 -- 3 days 3590 1740 minimum 7 days 4400 2760 minimum 28 days 5620 -- C 191 Initial time of setting by Vicat needle 105 Between 45 to 375 (minutes) C 188 Density (g/cm 3 ) 3.15 -- -16-
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 3 44.8 37.7 -- -- -- 18.4 19.4 -- -- -- Iron Oxide, Fe 2 O 3 6.8 5.6 -- -- -- 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 70.0 62.7 70.0 Min. 50.0 Min. 70.0 Min. 3.9 22.4 -- -- -- 0.1 4.2 -- -- -- 0.6 -- -- -- -- 1.25 0.5 -- -- -- 0.7 1.9 -- -- -- Sulfate, SO 3 0.3 2.0 4.0 Max. 5.0 Max. 5.0 Max. Loss on Ignition, LOI (@ 750 C) 21.2 0.6 10.0 Max.* 6.0 Max.* 6.0 Max.* Moisture Content 0.3 0.1 3.0 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-
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 (%) 68.6 17.7 -- 34 max. 34 max. Strength Activity Index with Cement (% of Control) 7-day 28-day 54.0 53.4 101 99 75 min. 75 min. 75 min. 75 min. 75 min. 75 min. Water Requirement, (% of Control) 111.6 94 115 max. 105 max. 105 max. Autoclave Expansion, (%) 0.01 0.07 0.8 max. 0.8 max. 0.8 max. Density 2.04 2.57 -- -- -- -18-
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 ), A1 373 370 369 367 355 358 358 High-Carbon Fly Ash (lb/yd 3 ), A2 0 0 0 0 0 0 0 Cement (lb/yd 3 ), C 373 370 369 367 355 358 358 Water (lb/yd 3 ), W 336 333 332 331 320 323 323 [W/(C+A)] 0.45 0.45 0.45 0.45 0.45 0.45 0.45 SSD Fine Aggregate (lb/yd 3 ) 1465 1455 1450 1445 1455 1405 1405 SSD 3/8" Aggregate (lb/yd 3 ) 1465 1455 1450 1445 1455 1405 1405 Carbon Fibers (lb/yd 3 ) 0 1.1 3.1 5.3 7.9 10.3 20.5 Fresh Concrete Density (lb/ft 3 ) 148.5 147.7 146.9 146.5 146.2 143.1 143.5-19-
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 ), A1 339 337 337 334 335 332 334 High-Carbon Fly Ash (lb/yd 3 ), A2 291 288 289 287 287 285 287 Cement (lb/yd 3 ), C 339 337 337 334 335 332 334 Water (lb/yd 3 ), W 515 511 512 507 469 504 508 [W/(C+A)] 0.53 0.53 0.53 0.53 0.49 0.53 0.53 SSD Fine Aggregate (lb/yd 3 ) 960 935 925 920 945 915 910 SSD 3/8" Aggregate (lb/yd 3 ) 1225 1200 1185 1180 1210 1170 1165 Carbon Fibers (lb/yd 3 ) 0 0.9 2.6 4.3 6.6 8.5 16.9 Fresh Concrete Density (lb/ft3) 135.9 133.6 132.8 132.1 132.8 131.3 131.7-20-
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-1 3735 WE-3 3300 WE-5 3415 WE-7.5-B2 4680 WE-10 2935 WE-20 2990 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 2030-21-
Table 11 Electrical Resistance of Concrete Without High-Carbon Fly Ash Test Age, days 1 3 7 14 28 56 91 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. 408 199 219 187 98.3 53 42.6 411 192 207 186 95 57 413 185 196 184 91.5 61.2 48.2 45 590 272 283 250 137.2 54.6 36.9 559 268 247 225 134 61 527 264 211 199 130.8 68.3 43.2 40 724 309 316 288 179.8 66.7 47.2 697 308 275 260 175 74 670 306 234 231 170.0 80.4 48.9 48 5230 1035 1080 1030 190.1 70.3 45.9 5325 1010 886 820 190 82 5420 985 692 609 190.3 93.8 52.7 49 19600 2500 2230 1390 200 70 46.7 20600 1930 2630 2000 180 81.2 47.8 22400 19680 2270 2045 1220 2004 1130 1437 235 214 41.4 64 32.7 42 20100 2290 2130 985 238 57.9 40.1 15700 1235 1810 1682 215 70.2 41.9 17860 4080 4000 1303 252 87.6 63.4 24900 22087 3610 3250 3160 3253 1122 1598 247 253 56.1 75 45.0 54 23500 2060 2600 2370 260 81.6 53.2 21100 10210 9430 2130 251 93.1 67.9 21600 19900 7630 6980 7820 7380 2060 2697 238 240 64.2 81 49.0 58 16980 3100 4890 3720 229 84.5 56.2-22-
Table 12 Electrical Resistance of Concrete Containing High-Carbon Fly Ash Test Age, days 1 3 7 14 28 56 91 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 / 8.5 16.9 Actual Ave. Actual Ave. Actual Ave Actual Ave. Actual Ave. Actual Ave. Actual Ave. 159.1 126 34 31.1 32.8 17.1 15 156 126 36 30 32 18 152.2 126 37.3 28 30.3 18 15 15 293 189.2 38.6 34.2 31.5 25 29.2 296 182 42 32 34 29 299 174.5 44.5 29.8 36.6 33.9 28.4 29 356 183 45 40.1 47.3 27.1 33.6 353 191 48 38 45 32 349 199 51.3 35.2 41.6 37 31.5 33 1100 440 91.2 67.5 62.8 35.3 43 1042 427 100 62 57 42 984 413 108.7 56 51.6 47.7 38.6 41 11700 4950* 106.3 74.6 69.6 46.2 48 12300 4130* 127.3 54.8 57.7 59.2 43.9 11090 12930 4423 90.9 102 55 57 54.5 64 44.7 51 35.9 43 13230 4190* 85.6 42.1 64.1 54.7 40.5 16330 101.6 59 74.1 49.3 44 12670 16540 110.3 62.5 71.2 49.9 43 17360 16710 16820 15930 105 110 64 65 61.2 72 53.4 51 45.9 45 20100 14430 115.2 67.1 82.7 49.6 45.7 14800 19800 115.2 67.3 69.4 43.9 42.8 10200 12640 13900 15967 109.5 115 65.3 68 61.4 72 49.2 47 42.9 44 12920 14200 119.4 72.1 85.3 48.1 45.9 *Test results at the age of 29 days. -23-
Table 13 - Electrical Resistivity of Concrete Mixtures Without High-Carbon Fly Ash Mixture No. Resistivity of Concrete, (ohm-cm) Test Age, Days 1 3 7 14 28 56 91 WE-0-A 1229 1672 2086 15938 58902 66105 59540 WE-1 575 802 920 3022 6121 9727 20891 WE-3 584 733 825 2605 5999 9737 22088 WE-5 569 660 760 2401 4302 4784 8071 WE-7.5B2 284 401 524 569 639 757 716 WE-10 171 184 220 246 171 225 241 WE-20 136 120 144 148 125 161 173 Table 14 - Electrical Resistivity of Concrete Mixtures Containing High-Carbon Fly Ash Mixture No. Resistivity of Concrete, (ohm-cm) Test Age, Days 1 3 7 14 28 56 91 WE-0C-A 466 886 1055 3118 38699 50012 37831 WE-1C-A 377 544 572 1277 13238* 47678 47787 WE-3C 107 124 144 299 306 330 343 WE-5C 88 96 113 185 171 194 204 WE-7.5C 94 102 133 171 192 215 216 WE-10C-A 53 88 96 124 152 153 141 WE-20C-A 45 86 97 122 127 134 131 * Test results at the age of 29 days. -24-
Fig. 1 - Fine Aggregate Used in Concrete Mixtures Fig. 2 - Coarse Aggregate Used in Concrete Mixtures -25-
Fig. 3 - Carbon Fibers Used in Concrete Mixtures Fig. 4 - ASTM Class C Fly Ash (left) and High-Carbon Fly Ash (right) -26-
Fig. 5 - Storage of Test Cylinders in the Laboratory Fig. 6 - Application of Conductive Gel to Concrete Cylinder -27-
Fig. 7 - Positioning Cylinder for Electrical Resistance Test Fig. 8 - Resistance Test -28-
Compressive Strength (psi) Compressive Strength (psi) 6000 5000 4000 3000 2000 1000 0 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 6000 5000 4000 3000 2000 1000 0 WE-0C- A WE-1C- A WE-3C WE-5C WE- 7.5C WE- 10C-A WE- 20C-A Mixture Number Fig. 10 - Compressive Strength at 28-Day Age Concrete Containing High-Carbon Fly Ash -29-
Electrical Resistance, ohms Resistance, Ohms 100000 10000 1000 100 10 1 WE-0A WE-1 WE-3 WE-5 WE-7.5B2 WE-10 WE-20 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, Days Fig. 11 - Electrical Resistance of Concrete Cylinders Without High-Carbon Fly Ash 100000 10000 1000 100 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 10 0 5 10 15 20 25 Carbon Fiber Content (lb per cubic yard of concrete) Fig. 12 - Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Without High- Carbon Fly Ash -30-
Electrical Resistance, ohms Resistance, Ohms 100000 WE-0CA WE-1CA WE-3C WE-5C WE-7.5C WE-10CA WE-20CA 10000 1000 100 10 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, Days Fig. 13 - Electrical Resistance of Concrete Cylinders Containing High-Carbon Fly Ash 100000 10000 1000 100 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 10 0 5 10 15 20 Carbon Fiber Content (lb per cubic yard of concrete) Fig. 14 - Electrical Resistance Vs. Carbon Fiber Content for Concrete Mixtures Containing High-Carbon Fly Ash -31-
Resistance, ohms Resistance, ohms 25000 20000 15000 10000 5000 0 Mixture WE-0A Mixture WE-0CA 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 15 - Electrical Resistance of Concrete Mixtures Without Carbon Fibers 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 Mixture WE-1 Mixture WE-1CA 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 16 - Electrical Resistance of WE-1 and WE-1CA Concrete -32-
Resistance, ohms Resistance, ohms 8000 7000 6000 5000 4000 3000 2000 1000 0 Mixture WE-3 Mixture WE-3C 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 17 -Electrical Resistance of WE-3 and WE-3C Concrete 3000 2500 2000 1500 1000 500 0 Mixture WE-5 Mixture WE-5C 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 18 - Electrical Resistance of WE-5 and WE-5C Concrete -33-
Resistance, ohms Resistance, ohms 300 250 200 150 100 50 0 Mxiture WE-7.5B2 Mixture WE-7.5C 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 19 - Electrical Resistance of WE-7.5B2 and WE-7.5C Concrete 90 80 70 60 50 40 30 20 10 0 Mixture WE-10 Mixture WE-10CA 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 20 - Electrical Resistance of WE-10 and WE-10CA Concrete -34-
Resistance, ohms 70 60 50 40 30 20 10 0 Mixture WE-20 Mixture WE-20CA 0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 Test Age, days Fig. 21 - Electrical Resistance of WE-20 and WE-20CA Concrete -35-