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High Early Strength Fly Ash Concrete for Precast/Prestressed Products

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High Early Strength Fly Ash Concrete for Precast/Prestressed Products High Early Strength Fly Ash Concrete for Precast/Prestressed Products Presents field applied research to report the advantages of using a high quality ASTM C-618 Class C fly ash on water de­ mand, workability and compressive strength of concrete. The research was performed at two precast/prestressed concrete plants to identify optimum mixture proportions for production of high early strength concrete with high fly ash contents. Tests were carried out on nominal 5000 psi (34 MPa) concrete utiliz­ ing fly ash produced at Wisconsin Electric Power Company's Pleasant Prairie Power Plant. Fly ash replacement improved workability, decreased water demand, and increased strength Tarun R. Naik while maintaining the high early strength requirements of pre­ Associate Professor of Civil Engineering cast/prestressed concrete operations. Director, Center for By-Products Utilization Department of Civil Engineering and Mechanics Milwaukee, Wisconsin he purpose of this project was to pavement construction and other proj­ develop mixture proportioning ects have successfully used structural Tinformation for the production of grade concrete with up to 70 percent ce­ high early strength concrete with high ment replacement. fly ash content for precast prestressed The objective of this paper is to re­ concrete products. The fly ash used in port that high early strength concrete this project was produced by Wisconsin can be produced with high replacement Electric Power Company (WE) at the of cement by fly ash for precast/pre­ Pleasant Prairie Power Plant located in stressed concrete operations. Effects of Kenosha County, Wisconsin. This fly fly ash content on water demand and ash meets the Class C requirements of workability are also reported. ASTM C-618. Tests were carried out on nominal Test data from mixture proportioning 5000 psi (34 MPa) 28-day compressive 1 5 reported in earlier publications · strength concrete, where fly ash was clearly established that this source of substituted for cement at levels up to 30 Bruce W. Ram me fly ash can be used for structural grade percent replacement on a 1.25 to 1.00 Senior Project Manager concrete in quantities.of up to 60 per­ fly ash replacement for cement basis. A Engineering and Construction Department cent replacement of cement. Demon­ literature search was also conducted to Wisconsin Electric Power Company stration projects are also reported in further study the water demand, work­ Milwaukee, Wisconsin these publications which show that ability and strength characteristics for 72 PCI JOURNAL fly ash concrete. Rather than compiling Table 1. Chemical and physical properties test data. an exhaustive annotated bibliography Pleasant Prairie Power Plant (Class C fly ash). of the available literature, some impor­ tant publications were reviewed and are Number Range Avg. ASTM 1 24 Chemical composition of samples (percent) (percent) C-618 listed in the references. - Silicon oxide (Si02) 9 32.90- 35.60 34.39 - PLEASANT PRAIRIE Aluminum oxide (AI20 3) 9 17.10-18.20 17.74 - CLASS C FLY ASH Iron oxide (Fe20 3) 9 15.10- 6.30 5.93 - Total (Si02+Al203) 9 55.70-59.30 58.07 50.0min. The Pleasant Prairie Power Plant Sulfur trioxide (S03) 9 2.68- 3.42 3.08 5.0max. Class C fly ash is a by-product of West­ Calcium oxide (CaO) 9 26.60- 28.60 27.52 - em United States sub-bituminous coal Moisture content 9 0.04- 0.44 0.12 3.0max. combustion. The fly ash is captured by Loss on ignition 9 0.20- 0.63 0.39 6.0max. electrostatic precipitators from flue gas Magnesium oxide (MgO) 7 4.10- 4.80 4.56 5.0max. prior to discharge through exhaust Available alkalies as Na20 7 0.87- !.'55 1.19 1.5 max. chimneys, and meets the requirements of ASTM C-618 for Class C fly ash Number ASTM (see Table 1). Until about 10 years ago, Physical Test of samples Range Avg. C-618 most of the fly ash available from coal Fineness, percent retained on burning power plants in the United #325 wet sieve 9 11.03- 13.34 18.83 34.0max. States was of the Class F (low calcium) Pozzolanic activity index: variety. However, the introduction of With cement, 28 days, percent 9 91- 133 Ill 75.0min. low sulphur sub-bituminous coal in the Pozzolanic activity index: 1970s made Class C (high calcium) fly With lime, 7 days, psi 9 810-1486 1029 800min. ash more readily available. Water requirement, percent of Class C fly ash has a higher lime con­ the control 9 88-92 89 105 max. tent than Class F fly ash and possesses Soundness: some cementitious properties of its Autoclave expansion (percent) 9 0.07-0.21 0.13 0.8 max. own. Therefore, this Class C fly ash can Specific gravity 9 2.55-2.71 2.66 - be used in higher proportions than the 15 to 20 percent range typically used for the Class F fly ash for structural SPECIMEN PREPARATION 2, were cured with the prestressed quality concrete. AND TESTS member where the test concrete was used for casting it. The remaining cyl­ Each batch of concrete produced was MIX PROPORTIONING inders were cured in a protected area tested for acceptability before concrete away from the prestressing bed and Mix proportions were developed for tests were undertaken. Fresh concrete traffic. Each cylinder was covered with producing concrete on a 1.25 to 1.00 fly tests including slump and air content a plastic bag which had a rubber band ash to cement weight substitution basis. were performed and mix proportions around it. A Type I cement was used and the re­ were recorded (see Tables 2 and 3). At­ Cylinders from both plants were placement quantities were 0, 10, 15, 20, tention was also paid to maintaining taken to the laboratory the next day. 25 and 30 percent. Twelve different constant workability, which is not en­ They were then stripped, marked and mixture proportions were developed tirely evident from the slump values be­ stored in a lime-saturated water bath based upon a nominal 5000 psi (34 cause of the use of a superplasticizer in until the time of their tests. Cylinders MPa) control mixture that contained no the concrete. From each concrete mix­ for the very early age strength test were fly ash. Mixture proportions and test ture, standard specimens were prepared tested after stripping them. data for the 12 mixes are given in Ta­ for compressive strength tests. bles 2 and 3. TEST RESULTS CONCRETE MIXING CURING AND DISCUSSION Concrete was produced at two differ­ Cylinders were cured following the Compressive Strength ent precast/prestressed concrete plants actual practice of the individual pre­ The compressive strength results are 3 in 2 cu yd (1.5 m ) test batches. Based cast/prestressing plant. For Plant No. 1, shown in Tables 4 and 5. Figs. I and 2 on the preliminary mixture proportions all cylinders were cured under a plastic show the compressive strength vs. age developed, the final mixture propor­ sheet after the tops of these cylinders comparison for the 5000 psi (34 MPa) tions were completed after consultation were sprayed with a liquid curing com­ concrete mixtures produced at the two with the concrete producers. Standard pound. They were cured in the open air different prestressing plants. The re­ hatching and mixing procedures for on top of a prestressing bed which was sults represent the testing of two cylin­ ready mixed concrete were followed, in not being used at the time. ders at each test age as opposed to the accordance with ASTM C-94. Early age test cylinders, at Plant No. more common testing of three cylinders November-December 1990 73 Table 2. Concrete mix proportions and test data (5000 psi specified strength). were compared to the non-fly ash con­ Concrete supplier: Prestressed Concrete Plant No. 1. crete mixes from the same concrete plant). Mix No.2, which has 10 percent 2 3 4 5 6 Mix No. 1 fly ash replacement, showed strength Specified design increases of roughly 12 percent when strength, psi 5000 5000 5000 5000 5000 5000 compared to the strength results for the Cement, lbs 628 572 554 528 491 459 concrete without fly ash (Mix No. 1) at Fly ash, lbs 0 77 119 160 198 238 the various test ages (see Table 4). When the amount of fly ash replace­ Water, lbs* 283 263 253 248 237 227 ment was increased, the strength gain at Sand@ SSD, lbs 1278 1294 1328 1343 1332 1370 early age was more pronounced. For I in. aggregates @ example, Mix No.4, which has 20 per­ SSD, lbs 1807 1830 1877 1899 1884 1887 cent fly ash replacement, showed strength increases of around 50 percent W/(C +FA) 0.45 0.41 0.38 0.36 0.34 0.33 for the 19-hour, 22-hour, 3-day and 7- Slump, in. 2%t 6\12 6% 4% 7 4\14 day ages, when compared to the con­ Air content, percent 5.4 4.5 2.4 2.0 2.1 1.6 crete without fly ash (Table 4 ). Mix No. 6, which has the highest fly ash replace­ Air temperature, °F 70 70 70 70 70 70 ment, at 30 percent, had an even higher Concrete temperature, °F 69 66 70 69 69 69 strength gain at the 7 -day age, at 65 per­ Concrete density, pcf 148.0 149.5 153.0 154.7 153.4 154.9 cent. These results clearly indicate that * 90 fluid oz. of a nominal 42 percent solid sodium napthalene condensate ASTM C-494 Type F admixture Class C fly ash usage increased the (superplasticizer) was added to all mixes.
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