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Class C Fly Ash As a Full Or Partial Replacement for Portland Cement Or Lime

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Class C Fly Ash As a Full Or Partial Replacement for Portland Cement Or Lime 68 TRANSPORTATION R ESEARCH RECORD 1219 Class C Fly Ash as a Full or Partial Replacement for Portland Cement or Lime KENNETH L. McMANrs AND ARA ARMAN A study was undertaken to evaluate the stabilization or mod­ Designation C 207, Type N) and a Type I portland cement ification of sands and clays using ASTM Class C fly ash as a (AASHTO Designation M85) were used in the study. full or partial replacement for hydraulic cement or hydrated A base or subbase material is usually evaluated with respect lime. Strength and durability tests demonstrated that the Class to its strength and durability. Louisiana Department of Trans­ C fly ashes of the study could be substituted for cement in portation and Development (DOTD) criteria (TR 432-82) some sands. Improvement of the sands was provided by the were used to evaluate the performance of the sand specimens matrix formed with fly ash acting as a filler and as a cementing (i.e., sand plus fly ash alone, sand plus fly ash and lime, sand agent. The test results indicate the importance of the gradation plus fly ash and cement, and sand plus cement alone). This characteristics of the materials and the effects on matrix quality due to the presence of fines in the natural sands. Also, improve­ specification required the minimum cement content to cor­ ments in the plastic properties and gains in soil support with respond to a strength of 250 psi with a 7-day curing period at the addition of fly ash and/or lime were evaluated for two clays. a temperature of 73° ± 3° F. The strength and durability of However, test results and analyses demonstrated that the Class the specimens were tested using ASTM C 593 procedures, C fly ash does not effectively compete as a substitute for lime except that the curing temperature was modified to agree with in the treatment of clays. DOTD TR 432-82. Class C fly ash used as a clay modifier was also studied. Electrical power plants in Louisiana use western coals and Both clays were evaluated with respect to changes in their are producing ASTM Class C fly ash. In addition to being plastic properties, strength index, and curing time. Atterberg pozzolanic, many Class C fly ashes have been shown to exhibit limit tests were conducted on the clays to evaluate the effects cementitious properties similar to portland cement. In recent of the additives on the soil plasticity. R-value tests were per­ years, an increasing number of stabilization projects has been formed to measure resistance to deformation and change in performed using Class C fly ash alone (1). Class C fly ash soil support values. with calcium oxide contents of 20 percent or more has report­ The standard Proctor (ASTM D 698) procedure was used edly adequately stabilized fine-grained plastic soils, as well as in molding the mixtures into sets of three specimens. In com­ coarse-grained soils, without the use of lime (1-4). The objec­ pacting the specimens, moisture contents were not allowed tives of this research were (a) to evaluate ASTM Class C fly to vary beyond ± 1 percent of optimum; and dry densities ash as a lone or partial replacement for portland cement in were maintained to within ± 3 lb of the theoretical dry weight sands and lime in clays and (b) to identify the manner in density. Those specimens not meeting these criteria were which improvements to the soils occur and the important remolded. characteristics of the materials. The molded specimens were extruded and cured for 7, 28, and 56 days at a temperature of 73° ± 3° F and at a relative humidity of 90 percent or greater. At the end of each curing TEST METHODOLOGY period, the mixtures were tested for compressive strength and durability. The average compressive strength of the three The testing program included two sands commonly used in specimen groups was used for comparison purposes. The coef­ base construction and two clays: a nonplastic A-3-0 sand, a ficient of variance of the individual group test values was slightly plastic A-2-4 silty sand (liquid limit = 21, plastic index computed and compared as a measure of the relative test = 7), an A-6(9) silty clay, and an A-7-6(20) clay. Ta­ dispersion. The coefficient of variance ranged from 5 to 14 ble 1 gives the engineering properties of the natural soils. percent with an overall value of 9 percent. Three fly ashes produced in Louisiana's Big Cajun, Nelson, DOTD TR 433-81 criteria for lime treatment of soils used and Rademacher power plants were included in the testing in Louisiana highways were used to determine the minimum program. The physical and chemical properties were analyzed amount of lime required. This requires that the liquid limit according to ASTM C 311 (Table 2). All the fly ashes were (LL) and plastic index (PI) after lime treatment cannot exceed ASTM Class C. The calcium oxide content ranged from 21.5 40 and 10, respectively, for soils used as a base course and percent to 27.2 percent. A hydrated, high-calcium lime (ASTM 40 and 15, respectively, when soils are used as a subbase. These criteria were also applied when fly ash was evaluated K. L. McManis, Civil Engineering Department, University of New Orleans, New Orleans, La. 70148. A. Annan, Louisiana Transpor­ as a replacement for lime with the clays. Both the A-3 sand tation Research Center, Louisiana State University, 4101 Gourrier, and the A-2-4 silty sand met this requirement without Baton Rouge, La. 70804-9245. treatment. McManis and Arman 69 TABLE 1 SOIL PROPERTIES Soil Variable A-3 A-2-4 A-7-6(20) A-6(9) Coarse sand (Ret #40) (%) 40 3 0 1 Fine sand (Ret #200) (%) 52 62 14 3 Clay and colloids (%) 3 17 55 34 Liquid limit(%) NP 21 60 31 Plastic index (%) NP 7 40 13 Max. dry wt. den . (pcf) 109.7 119.7 98.8 104.9 Optimum moisture(%) 13.0 12.l 23.1 19 .0 R-value < 5 30 NOTE: NP = nonplastic. TABLE 2 CHEMICAL AND PHYSICAL ANALYSES OF FLY ASHES Fly Ash Source Variable Cajun Rademacher Nelson Retained #325 (%) 9.2-7.6 12.0-11.0 13 .9-18.3 Loss on ignition (%) 1.3-0.5 0.5-1.9 0.7-1.0 Total oxides• (%) 65.8-66.5 62.3-64.7 51.5-62.9 Calcium oxide (%) 21.5-24.5 27.20-24.0 25.2-25.8 Magnesium oxide (%) 4.4-4.7 4.9-4.5 4.9-2.9 Sulfur trioxide ( % ) 2.8-2.8 2.7-2.9 3.1-3.3 Alkalies(%) 1.34-0.66 1.49-1.06 1.45-1.74 NOTE: Materials were tested according to ASTM Designation C 311. 0 Si02 + Al 20 3 + Fe,0 3 • As a measure of performance of the two treated clays in sands are shown in Figure 1. All three fly ashes gave similar an embankment or subbase, the R-value test (resistance value­ results. The addition of fly ash to the A-3 sand significantly ASTM D 2844) was used. A few specimen sets were subjected increased the maximum dry density and decreased the opti­ to an R-value retest with an additional 7 days to observe any mum moisture content throughout the range of fly ash per­ increase that may have occurred. centages tested. The reduction in optimum moisture content is attributed to the spherical shape of the fly ash particles in the sand voids, which lubricates the mix and aids in the den­ sification efforts. EVALUATION OF TEST RESULTS WITH The addition of the fly ash to the A-2-4 silty sand produced SANDS a small increase in dry density between 3.3 and 6.9 pcf with the different fly ashes. The maximum density occurred at fly To satisfy the stated acceptance criteria (250 psi compressive ash percentages of 15 to 20 percent for all three fly ashes. strength with a 7-day cure), the following minimum propor­ Additional fly ash beyond 20 percent produced a decrease in tions of stabilizing agents were required with the sands: density. The fly ash percentage corresponding to the maxi­ Stabilizing Agent A-3 Sand(%) A-2-4 Silty Sand mum density of the A-2-4 silty sand also seemed to coincide Fly ash 20-25 None meeting with the minimum value of the optimum moisture content. criteria Other mix combinations of cement plus fly ash and lime Lime + fly ash 4-6 lime+ None meeting plus fly ash with the A-3 sand also produced gains in density. 15 fly ash criteria Both lime and cement increased the density at different per­ All acceptable Cement + fly ash All centage levels. The resulting increase proportions or decrease varied among acceptable the different fly ashes. This was not the case for the A-2-4 except 4 silty sand. A decrease in density, below that of the raw soil, cement + 5 occurred with the addition of lime and was attributed to the fly ash effects of the fines present. Cement 8 All acceptable The aggregate gradation had a significant effect on the den­ The test results were evaluated in an attempt to identify or sity, strength, and durability of the mix (2,6). Using Talbot's explain the behavior of the fly ash mixtures. relationship for computing the ideal gradation necessary for maximum density, a grain size analysis was made for each sand. Moisture Density p = (dfD)O 5 (1) where The density of the mixture has a major effect on the strength and durability of cement- and lime-fly ash stabilized materials P = percent finer by weight for grain size, (5,6).
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