SUPERPLASTICIZED HIGH-VOLUME FLY ASH STRUCTURAL CONCRETE *

Transcription

1 SUPERPLASTICIZED HIGH-VOLUME FLY ASH STRUCTURAL CONCRETE * By Tarun R. Naik 1 and Shiw S. Singh 2 ABSTRACT This research was carried out to develop structural grade concrete containing high-volumes of ASTM Class C fly ash. A Portland cement concrete, proportioned to have 28-day compressive strength of 6000 psi (41 MPa) was used in this study. Concrete mixes were also proportioned to have various levels of cement replacement by fly ash ranging from 40-70% by weight. Properties of concrete, namely, compressive strength, splitting tensile strength, and modulus of elasticity were measured as a function of fly ash amounts and age. Analysis of results showed that ASTM Class C fly ash could be substituted for cement replacement up to 70% for high strength structural grade concretes without sacrificing its performance significantly. INTRODUCTION Fly ash has been utilized in mass concrete for more than 50 years in the United States. Most early studies were primarily concerned with the use of fly ash in construction of dams and highways. More recently, attempts have been made towards the use of fly ash in high-strength structural grade (greater than 5000 psi) concretes [1,2]. This work was undertaken to determine performance of ASTM Class C fly ash in concrete. EXPERIMENTAL PROGRAM A Portland cement concrete, designed to have 28-day strength of 6000 psi (41 MPa) was used as a reference concrete. In addition, concrete mixes were also 1 Director, Center for By-Products Utilization, Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, P.O. Box 784, Milwaukee, WI Post-Doctoral Fellow, Center for By-Products Utilization. proportioned to incorporate fly ash at various percentages of cement replacements ranging between 40-70%.

2 Portland cement (ASTM Type I) obtained from one source was used in this investigation. High-calcium fly ash (ASTM Type C) was secured from Pleasant Prairie Power Plant located in Kenosha, Wisconsin. Chemical composition and physical properties of the fly ash were determined using appropriate ASTM techniques, see Table 1. The fine aggregate was natural sand, which was obtained from a local ready-mix concrete producer. Natural gravel was used as a coarse aggregate. The coarse aggregate was also obtained from the same local concrete producer which had maximum size of 3/4 in (19 mm). A Melamine based superplasticizer was utilized to control workability of concrete. The mix proportions and their properties for the Concrete Containing various amounts of fly ash in the range of 0-70% used in this study are shown in Table 2. Cylindrical specimens of 6x12 in. (152 x 305 mm) were manufactured and tested for compressive strength, tensile strength, and secant modulus of elasticity in accordance with the ASTM test methods. Three specimens were tested for each experimental condition. RESULTS AND DISCUSSION Relationship between compressive strength and fly ash percentage at various ages is shown in Figure 1. The results showed increased strength with increasing age at all the levels of fly ash tested. The compressive strength decreased with the increase in fly ash in the concrete at one day age. However, at the 7-day age, no significant difference was observed in the performance of the concrete containing fly ash up to 50% cement replacements compared to the concrete with no fly ash. The fly ash concrete outperformed the reference Portland concrete at all the levels of cement replacements (up to 70%) at the 28-day age. The relation between tensile strength and fly ash percentage at various ages is plotted in Figure 2. The fly ash concrete showed the tensile strength results to be essential, identical to that of the reference concrete up to cement replacement of 50%. Even at 60% cement replacement, the fly ash concrete maintained its tensile strength in excess of 85% of the strength achieved by the Portland cement concrete. In this investigation, the 28-day secant modulus of elasticity of fly ash concrete was computed using ACI Building Code 318 equation 1.5 E c = W c x 33 f 1/2 c, where E c = the static modulus of elasticity, psi ; W c = unit weight, lb/ft 3 ; and f c = the 28-day compressive strength of standard cylinder. The secant modulus of elasticity data is displayed in Figure 3. At 7 days, the secant modulus of elasticity decreased somewhat with increasing fly ash addition for all the test

3 conditions. It was 20% lower at 70% fly ash replacements compared to no fly ash concrete. The data presented in Figure 3 indicates that the secant modulus values for the concretes tested were sufficient for structural applications. Based on the results reported above, it can be concluded that superplasticized concrete can be proportioned to have 70% cement replacement by fly ash to obtain desired workability and strength for structural applications. REFERENCES (1)Naik, T.R. and Ramme, B.W., "High Strength Concrete Containing Large Quantities of Fly Ash", ACI Materials Journal, V. 86, No. 2, March-April 1989, pp (2) Papayianni, J., "Strength and Bond Data for Greek High-Lime Fly Ash Concrete", in "Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete", V.M. Malhotra, Ed., Proceedings of the Second International Conference, Vol. I, Madrid, Spain, l986, pp REP-57

4 Table 1:Chemical and Physical Test Data for the Class C Fly Ash from Pleasant Prairie Power Plant Chemical Composition Average, percent ASTM-C-618 Silicon Oxide, SiO Aluminum Oxide, Al 2 O Iron Oxide, Fe 2 O Total, SiO 2 +Al 2 O 3 +Fe 2 O Max. Sulfur Trioxide, SO Max. Calcium Oxide, CaO Magnesium Oxide, MgO Max. Moisture Content Max. Loss on Ignition Max. Physical Tests ASTM C-618 Fineness, % Retained on #325 Sieve Max. Pozzolanic Activity Index with Portland Cement, 28 days, % with lime, 7 days, psi Min. Water Requirement, % of Control Soundness, Autoclave Expansion, % Max Max. Specific Gravity Note: 1 psi = MPa

5 Table 2:Concrete Mix Using Pleasant Prairie Power Plant Class C Fly Ash psi (41 MPa) Specified Strength Mix No. 0-A P4-F P4-I P4-G P4-J Specified Design Strength, psi Cement, lbs./cu. yd Fly Ash, lbs./cu. yd Water, lbs./cu.yd Water to Cementitious Ratio Sand, SSD, lbs./cu. yd /4" aggregates, SSD, lbs./cu. yd Slump, inches 2-1/ /2 2 2 Air Content, % Air Temperature, degrees F Concrete Temperature, degrees F Concrete Density, pcf Superplasticizer liters/cu. yd Note: 1 psi = MPa; 1 inch = 25.4 mm 1 degree C = (degree F - 32)/1.8 1 lb/cu yd = kg/m 3 ; 1 pcf = kg/m 3 1 liter = x 10 3 oz.

6 Table 3:Concrete Strength Data* Using Pleasant Prairie Power Plant Class C Fly Ash psi (41 MPa) Specified Strength Mix No. 0-A P4-F P4-I P4-G P4-J Specified Strength, psi Percent Fly Ash Test Age, Days Compressive Strength, psi Test Age, Days Splitting Tensile Strength, psi Test Age, Days Modulus of Elasticity, psi x ** *Average of three test speciments **Not measured, computed by using the ACI Building Code 318. Note: 1 psi = MPa