Compaction Characteristics and Bearing Ratio of

Compaction Characteristics and Bearing Ratio of Pond Ash Stabilized with Lime and Phosphogypsum Ambarish Ghosh1 Abstract: Recycling of waste material is one of the effective solutions of its disposal problem. Fly ash produced by coal-based thermal power plants and phosphogypsum 共PG兲 produced by fertilizer plants producing phosphoric acid as constituent of fertilizers, take huge disposal area and creates environmental problems. Stabilization/solidification of fly ash improves the engineering properties and reduces the environmental problem like leaching and dusting. This paper presents the laboratory test results of a Class F pond ash alone and stabilized with varying percentages of lime 共4, 6, and 10%兲 and PG 共0.5, and 1.0兲, to study the suitability of stabilized pond ash for road base and subbase construction. Standard and modified Proctor compaction tests have been conducted to reveal the compaction characteristics of the stabilized pond ash. Bearing ratio tests have been conducted on specimens, compacted at maximum dry density and optimum moisture content obtained from standard Proctor compaction tests, cured for 7, 28, and 45 days. Both unsoaked and soaked bearing ratio tests have been conducted. This paper highlights the influence of lime content, PG content, and curing period on the bearing ratio of stabilized pond ash. The empirical model has been developed to estimate the bearing ratio for the stabilized mixes through multiple regression analysis. Linear empirical relationship has been presented herein to estimate soaked bearing ratio from unsoaked bearing ratio of stabilized pond ash. The experimental results indicate that pond ash-lime-PG mixes have potential for applications as road base and subbase materials. DOI: 10.1061/共ASCE兲MT.1943-5533.0000028 CE Database subject headings: Compaction; Lime, Ashes; Stabilization; Recycling. Author keywords: Bearing ratio; Compaction; Lime; Phosphogypsum; Pond ash; Stabilization.

Introduction Fly ash produced by thermal power plants takes huge disposal area and creates environmental problems like leaching and dusting. Actually, there are three types of ash produced by thermal power plants, viz. 共1兲 fly ash; 共2兲 bottom ash; and 共3兲 pond ash 共Bera et al. 2007兲. Fly ash is collected by mechanical or electrostatic precipitators from the flue gases of power plant; whereas, bottom ash is collected from the bottom of the boilers. When these two types of ash, mixed together, are transported in the form of slurry and stored in the lagoons, the deposit is called pond ash. The volume of pond ash produced by thermal power plants is very large compared to that of the other two ash, viz. fly ash and bottom ash. Presently, in India, extensive road network is under construction. In some of the road projects, attempt has been made to use pond ash as a construction material, solution to the scarcity of conventional construction material, and disposal of fly ash. The major problems the world is facing today are the scarcity of conventional construction material on one hand while on the other hand, large amount of unutilized industrial wastes causing serious environmental problems and ecological imbalance. Utilization of 1 Professor, Dept. of Civil Engineering, Bengal Engineering and Science Univ., Shibpur, Howrah-711 103, India. E-mail: ambarish@ civil.becs.ac.in Note. This manuscript was submitted on September 8, 2008; approved on June 6, 2009; published online on June 10, 2009. Discussion period open until September 1, 2010; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, Vol. 22, No. 4, April 1, 2010. ©ASCE, ISSN 0899-1561/ 2010/4-343–351/$25.00.

fly ash in construction, such as embankments and structural fills and dykes, is the most promising solution to the problem of the disposal of fly ash and also to reduce the construction cost of the projects. Previous researchers studied different uses of fly ash such as bulk fill material 共Raymond 1958; DiGioia and Nuzzo 1972; Gray and Lin 1972; Joshi et al. 1975兲, soil stabilization 共Chu et al. 1955; Goecker et al. 1956; Viskochil et al. 1957; Vasquez and Alonso 1981兲, and land reclamation 共Kim and Chun 1994兲. Potential application of fly ash alone or soil stabilized with fly ash or fly ash and admixtures for road construction has been reported by a number of researchers 共Ghosh et al. 1973; Manjesh et al. 2003; Satyanarayana Reddy and Rama Moorthy 2004; Ghosh and Subbarao 2006兲. Jute-geotextile reinforcing fly ash was found to be a promising technique to improve the bearing capacity of the foundation medium 共Ghosh et al. 2005兲. Fly ash has found potential application in the construction field because of its self-hardening characteristics which depends on the availability of lime. According to the ASTM classification 共ASTM C618-03兲 fly ashes fall in two types; Class C and Class F. Class C fly ash with high in calcium content undergoes high reactivity with water even without addition of lime 共Parsa et al. 1996兲. Class F fly ash contains lower percentages of lime. To improve the engineering properties of Class F fly ash, attempt has been made to stabilize fly ash with lime or cement 共Ghosh 1996; Ghosh and Subbarao 2007兲. Gypsum has also been used to stabilize fly ash 共Pandian 2004; Ghosh and Subbarao 2007兲. The suitability of Class F fly ash stabilized with lime and analytical quality anhydrous gypsum for road construction material was studied through tensile strength, bearing ratio, and slake durability 共Ghosh and Subbarao 2006兲.

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Table 1. Properties of Pond Ash Chemical composition Constituents SiO2 Al2O3 Fe2O3 CaO MgO LOI Others

Percentages

Properties

Property values

60.80 15.20 8.60 5.50 4.40 4.60 0.90

Specific gravity Uniformity coefficient Coefficient of curvature Maximum dry density Optimum moisture content Cohesion Angle of internal friction

2.186 2.077 1.256 11.50 kN/ m3 31.14% 30 kPa 37°

Phosphogypsum 共PG兲 is another kind of waste calcium sulfate, produced by fertilizer plants during production of phosphoric acid, a major constituent of many fertilizers. The PG contains a variety of impurities which may contaminate the ground soil and ground water. This material has not been used in the housing industry for the presence of impurities resulting in huge stockpiles around the world. Researchers and professionals have come forward to utilize PG as a construction material for various field applications 共Ong et al. 1993; Parreira et al. 2003兲. The potential use of PG stabilized with Class C fly ash and lime or cement for marine applications has been explored 共Rusch et al. 2002; Deshpande 2003, Guo et al. 2003兲. The suitability of PG for construction of road bases and subbases has also been demonstrated by earlier researchers 共Gregory et al. 1984; Chang et al. 1989兲. This paper presents the compaction characteristics and bearing ratio of a Class F pond ash stabilized with 共4–10%兲 lime alone or in combination with PG 共0.5 and 1.0%兲 to study the suitability of pond ash as road construction material through recycling. For the bearing ratio test, specimens were cured up to 45 days. Both unsoaked and soaked bearing ratio tests were conducted on specimens compacted at maximum dry density 共MDD兲 and optimum moisture content 共OMC兲 of the respective mixes, obtained from standard Proctor compaction tests. The influence of lime content, PG content, and curing period 共CP兲 on the bearing ratio of the stabilized pond ash is discussed in this paper. Based on the experimental findings and analysis of the test results, the following aspects of the stabilized pond ash are highlighted in this paper: • Compaction characteristics of pond ash stabilized with lime and PG; • Influence of lime content, PG content, and CP on the bearing ratio of the stabilized pond ash; • Development of empirical relationship to estimate bearing ratio of stabilized pond ash; and • Empirical relationship between soaked bearing ratio and unsoaked bearing ratio of stabilized pond ash.

Fig. 1. Grain size distribution curve of pond ash

pond ash. Lime Hydrated lime was used to stabilize the pond ash. The chemical compositions of the lime on dry weight basis are as follows: SiO2 = 4.50%; Fe2O3 = 1.50%; Al2O3 = 3.50%; CaO= 67.00%; MgO= 0.80%; loss on ignition= 22.50%; and others 0.20%. Phosphogypsum PG used in this study was collected from Haldia Fertilized Plant, West Bengal, India. The chemical compositions of the PG on dry weight basis are as follows: SiO2 = 15.50%; Al2O3 = 2.40%; CaO = 35.20%; MgO= 8.40%; SO4 = 23.50%; loss on ignition 14.50%; and others 0.50%.

Experimental Program and Test Procedures To study the compaction characteristics and the influence of the governing parameters on the bearing ratio of compacted pond ash stabilized with lime and PG, the experimental program was scheduled in the following phases: 共a兲 characterization of pond ash sample, lime, and PG; 共b兲 Standard Proctor and Modified Proctor compaction tests of pond ash alone and pond ash modified with lime and PG; and 共c兲 bearing ratio tests of pond ash mixes compacted with standard Proctor Energy for both unsoaked and soaked conditions. The percentages of lime added to pond ash on dry weight basis are 4.0, 6.0 and 10.0 and the percentages of PG added to lime modified pond ash are 0.5 and 1.0%. Total of ten mixes are used in this study. The details of the mixes are given in Table 2.

Materials The following section presents the materials used in the present investigation. Pond Ash The pond ash used in this investigation has been collected from Kolaghat Thermal Power Plant, West Bengal, India. Physical and compositional properties of the pond ash are presented in Table 1. In accordance with ASTM C618-03, the pond ash belongs to Class F. Fig. 1 presents the grain size distribution curve of the

Moisture Density Relationships of Stabilized Pond Ash The quality control tests of a fill material are designed to check the required engineering properties for the specific project. In most of the cases, the specification for quality control is prescribed in terms of dry density and molding moisture content, depending on the requirements higher energy is being used for compaction in the field. The current Ministry of Surface Transport 共Road Wing兲共2000兲, specification for road and bridge works recommend that subgrade shall be compacted to 97% of dry density achieved with heavy compaction as per IS 2720 共Part 8兲. This

344 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2010


Table 2. Detail of Mixes Sl. number

Mix designation

1 2 3 4 5 6 7 8 9 10

PA+ 0L + 0PG PA+ 4L + 0PG PA+ 6L + 0PG PA+ 10L + 0PG PA+ 4L + 0.5PG PA+ 6L + 0.5PG PA+ 10L + 0.5PG PA+ 4L + 1.0PG PA+ 6L + 1.0PG PA+ 10L + 1.0PG

Description Pond Pond Pond Pond Pond Pond Pond Pond Pond Pond

density requirement is recommended for subgrade compaction for Expressways, National Highways, Major District Roads, and other heavily trafficked roads. In other cases the subgrade should be compacted to at least 97% of the standard density conforming to IS 2720 共Part 7兲. In accordance with this recommendation in the present study, both standard and modified Proctor compaction tests have been carried out to study the influence of energy on compaction of stabilized pond ash. The MDD and OMC of all the 10 mixes were obtained through Standard Proctor compaction tests 共ASTM D698-92兲 and Modified Proctor compaction tests 共ASTM D1557-92兲. Table 3 presents the values of MDD and OMC for both standard and modified Proctor compaction tests. Bearing Ratio Test Bearing ratio is one of the vital parameters, used in the evaluation of soil subgrades for both rigid and flexible pavements design. It is also an integral part of several pavement thickness design methods. To assess the suitability of pond ash stabilized with lime and PG as base and/or subbase material both unsoaked and soaked, bearing ratio tests have been conducted in accordance with ASTM D1883-05. The specimens were soaked in water for 96 h to determine soaked bearing ratio. Annular weight of 5.00 kg surcharge was placed on the top of the specimen both at the time of soaking and testing. The bearing ratio mold is a rigid metal cylinder with an inside diameter of 152 mm and a height of

ash ash ash ash ash ash ash ash ash ash

+0% lime +0% phosphogypsum +4% lime +0% phosphogypsum +6% lime +0% phosphogypsum +10% lime +0% phosphogypsum +4% lime +0.5% phosphogypsum +6% lime +0.5% phosphogypsum +10% lime +0.5% phosphogypsum +4% lime +1.0% phosphogypsum +6% lime +1.0% phosphogypsum +10% lime +1.0% phosphogypsum

178 mm. The specimens were prepared in accordance with the procedures given in ASTM D1883-05, using standard compaction effort with OMC and MDD obtained from standard Proctor compaction test. The specimens of pond ash mixes were left in the mold after compaction and was sealed using plastic wrap for curing. The specimens for bearing ratio tests were cured in humidity control chamber at a temperature of 30⫾ 1 ° C and a relative humidity ⬎95%. The specimens were cured for 7, 28, and 45 days to evaluate the effect of curing. After respective CPs, bearing ratio tests were conducted in a mechanical loading machine equipped with a movable base that moves at a uniform rate of 1.2 mm/min and a calibrated proving ring is used to record the load. The proving ring is attached with a piston, which penetrates into the compacted specimen. The diameter of the piston is 50 mm. The loads are carefully recorded as a function of penetration up to a penetration of 12.5 mm. The specimens were cured up to 45 days to consider the effect of pozzolanic reaction of lime on the bearing ratio of stabilized pond ash.

Results and Discussions The experimental results of compaction tests and bearing ratio tests are presented in this section. Compaction Characteristics of Stabilized Pond Ash

Table 3. Maximum Dry Density and Optimum Moisture Content of Pond Ash

Lime content 共%兲

PG content 共%兲

Standard Proctor compaction ␥d max 共kN/ m3兲

OMC 共%兲

Modified Proctor compaction ␥d max 共kN/ m3兲

OMC 共%兲

0 0.0 11.50 31.14 12.30 27.50 4 0.0 11.63 30.67 12.44 26.45 6 0.0 11.74 30.26 12.73 23.79 10 0.0 11.43 32.65 12.74 22.70 4 0.5 11.45 33.23 12.74 23.00 6 0.5 11.62 30.96 12.95 22.51 10 0.5 11.83 29.52 12.82 22.81 4 1.0 11.65 30.09 12.84 22.93 6 1.0 11.61 31.27 12.82 23.74 10 1.0 11.65 29.07 13.00 22.65 Note: PG⫽phosphogypsum; ␥d max⫽maximum dry density; and OMC ⫽optimum moisture content.

Moisture content and dry density obtained from standard Proctor compaction tests for the pond ash stabilized with lime 共4.0, 6.0, and 10.0%兲, and lime along with 1.0% PG 共PG兲 are presented in Figs. 2 and 3, respectively. Figs. 4 and 5 illustrate the typical moisture content and dry density relationships obtained from modified Proctor compaction tests. The compaction curves of the pond ash depict the similar nature illustrated by earlier researchers 共Bera et al. 2007兲. The change in dry density with molding water content is not abrupt; this nature is useful for field application. Compaction of stabilized pond ash does not show any appreciable change in the nature of the compaction curve compared to that of unstabilized pond ash. This is may be due to addition of lower percentages of lime and PG compare to the percentages of pond ash and no time was also allowed for the pozzolanic reaction to take place during compaction tests. The compaction test results are summarized in Table 3. The values of MDD and OMC of the stabilized pond ash vary from 11.43 to 11.83 kN/ m3 and 33.23 to 29.07%, respectively, for standard compaction. For modified compaction tests the values vary from 12.44 to 13.00 kN/ m3 and 26.45 to 22.51%, respectively. With the in-



Fig. 2. Standard Proctor compaction curve of pond ash with varying percentages of lime content

crease in compaction energy from standard to modified, the nature of compaction curve changes which is similar to that of pond ash reported by earlier researchers 共Bera et al. 2007兲. The increase in MDD varies from 7.0 to 12.0%, depending on mixed proportions

Fig. 5. Modified Proctor compaction curve of pond ash with varying percentages of lime content along with 1.0% PG

for increase in compaction energy from standard Proctor compaction to Modified Proctor compaction. The decrease in OMC varies from 12.0 to 22.0% for the mixes of this study. Bearing Ratio of Stabilized Pond Ash Bearing ratio test results of lime stabilized pond ash mixes are illustrated in Figs. 6共a and b兲 for unsoaked and soaked specimens, respectively. Bearing ratio test results of pond ash stabilized with lime alone or in combination with PG are presented in Figs. 7–9

Fig. 3. Standard Proctor compaction curve of pond ash with varying percentages of lime content along with 1.0% PG

Fig. 4. Modified Proctor compaction curve of pond ash with varying percentages of lime content

Fig. 6. Bearing ratio of lime stabilized pond ash at different CPs 共a兲 unsoaked test; 共b兲 soaked test



Fig. 7. Bearing ratio of stabilized pond ash with varying lime and gypsum contents, 7 days curing: 共a兲 unsoaked test; 共b兲 soaked test


for 7, 28, and 45 days CP, respectively. The effects of various parameters such as lime content, PG content, and CP on bearing ratio of stabilized pond ash are highlighted item wise in the following sections. Discussion on the development of empirical model by regression technique has also been made in a separate section.

lime 共Table 4兲. The soaked bearing ratio of unstabilized pond ash is 5.07 at 7 days curing. This is may be due to softening of the unstabilized specimen during soaking. Addition of lime may restrict the softening by forming compact matrix of the stabilized specimen. Influence of Phosphogypsum

Influence of Lime Content To investigate the influence of lime content on bearing ratio, varying percentages of lime 共i.e., 4, 6, and 10%兲 were added to the Class F pond ash. Fig. 6共a兲 illustrates the increasing trend of unsoaked bearing ratio of the Class F pond ash up to 10% addition of lime. This increase in the bearing ratio may be due to the availability of lime for pozzolanic reaction. The unsoaked bearing ratio increases from 34.03 for unstabilized pond ash to 69.80, 98.50, and 111.93 for addition of 4, 6, and 10% lime to pond ash, respectively, at 7 days curing. Similar trend of increase in bearing ratio is observed for 28 and 45 days CP also. Ghosh and Subbarao 共2006兲, reported such increase of bearing ratio of Class F fly ash due to addition of lime. To highlight the influence of lime on the bearing ratio of stabilized pond ash, the percentage increase in the bearing ratio, due to addition of lime to pond ash, is presented in Table 4. The rate of increase of bearing ratio is more for addition of lime from 0 to 4% and 4 to 6% compared to the addition of lime from 6 to 10%. It is revealed from Fig. 6共b兲 that the effect of lime in case of soaked bearing ratio of stabilized pond ash is more prominent. The percentage increase in the soaked bearing ratio is more compared to the unsoaked bearing ratio due to addition of

Varying percentages of PG 共i.e., 0.5, and 1.0%兲 were added to lime stabilized Class F pond ash to study the influence of PG on the bearing ratio of stabilized pond ash. Figs. 7–9 revealed that addition of small percentage 共0.5 or 1.0%兲 of PG to lime-pond ash mixes enhances the bearing ratio of the stabilized pond ash. The unsoaked bearing ratio of pond ash stabilized with 10% lime increased from 111.93 to 137.90 for addition of 1.0% PG at 7 days curing. Similar improvements are noticed for other mixes for lime content at different CPs 共Figs. 7–9兲. Table 5 presents the percentage increase in bearing ratio of stabilized pond ash due to addition of PG to lime-pond ash mixes. The contribution of PG is found to be more prominent for the mixes with lower percentage of lime 共4%兲 in soaked condition. It implies that PG plays an important role for formation of compact matrix for mixes with lower percentage of lime. Ghosh and Subbarao 共2001兲 illustrated the formation of compact matrix of Class F fly ash stabilized with lime and analytical quality gypsum through microstructural analysis. Mixes with less lime addition is likely to be softened while soaking. Ghosh and Subbarao 共2006兲 have reported that addition of analytical quality gypsum to lime modified fly ash is effective in enhancing the bearing ratio of stabilized fly ash. Pandian 共2004兲



Table 5. Percentage Increase in Bearing Ratio due to Addition of Phosphogypsum to Lime Stabilized Class F Pond Ash Curing period 共days兲 7 Mix

US

28 S

US

45 S

PA+ 4L + 0.5PG 45 136 13 48 PA+ 6L + 0.5PG 24 19 16 3 PA+ 10L + 0.5PG 11 14 16 3 PA+ 4L + 1PG 61 177 40 104 PA+ 6L + 1PG 34 43 42 26 PA+ 10L + 1PG 23 29 39 36 Note: US⫽unsoaked test; S⫽soaked test; PA⫽pond ash; of lime; and PG⫽percentage of phosphogypsum.


stated that the addition of gypsum alone has no effect on strength of the stabilized matrix, even if the ash contains both free lime and reactive silica.

US

S

7 89 19 9 18 29 48 168 44 52 37 49 L⫽percentage

formation of pozolanic reaction products, i.e., active participation of lime and PG added to pond ash. At higher CPs 共45 days兲, the influence of curing is prominent for soaked bearing ratio of stabilized pond ash 关Figs. 9共a and b兲兴. In India, the specifications for the base and subbase of roads are followed in accordance with IRC 37 共2001兲. From the bearing ratio point of view, the subbase material should have a minimum bearing ratio of 20% for cumulative traffic up to 2 msa and 30% for traffic exceeding 2 msa. It is recommended that normally with bearing ratio value less than 100% should not be used in base construction. The pond ash stabilized with lime 共6% and above兲 and PG cured for 28 days and above, meets the requirements of bearing ratio for base course material. The leaching aspect of this stabilized material may be studied as a part of suitability study of this stabilized material as road base and subbase material. Ghosh and Subbarao 共1998兲 reported the reduction of leaching from fly ash due to stabilization with lime alone or in combination with analytical quality gypsum.

Nondimensional Parameter Study Influence of Curing Period To study the influence of the CP on the bearing ratio of stabilized pond ash, the specimens were cured for 7, 28, and 45 days. Fig. 6 shows that the values of bearing ratio of lime stabilized pond ash increases with increase in CP for each lime content. The increasing trend of the bearing ratio of pond ash stabilized with lime and PG is also revealed in the Figs. 7–9 for increasing CP up to 45 days. The percentages increase in the bearing ratio of pond ash stabilized with 10.0% lime and 1.0% PG are 35 and 69% at 28 and 45 days CP, respectively, with respect to 7 days cured specimens. This enhancement of bearing ratio is may be due to the

Stabilized pond ash may find potential application in road construction with the high values of bearing ratio compared to that of unstabilized pond ash with small amount of lime present in it for pozzolanic reaction. Large numbers of experiments are to be conducted in the laboratory to obtain the design mix of stabilized pond ash to be used in the field to achieve the target bearing ratio values. Empirical relationships are generally sought for preliminary estimation of the mix design. In this present study an attempt has been made to study the influence of the governing parameters 共lime content, PG content, and CP兲 on unsoaked bearing ratio of stabilized pond ash through a nondimensional parameter BRgain, defined as follows:

Table 4. Percentage Increase in Bearing Ratio due to Addition of Lime to Class F Pond Ash Curing period 共days兲 7 Mix

US

28 S

US

45 S

PA+ 4L + 0PG 105 617 129 512 PA+ 6L + 0PG 189 1,578 217 1,091 PA+ 10L + 0PG 229 1,866 231 1,150 Note: US⫽unsoaked test; S⫽soaked test; PA⫽pond ash; of lime; and PG⫽percentage of phosphogypsum.

US

S

110 165 142 428 168 455 L⫽percentage

BRgain =

BRstabilized BRunstabilized

共1兲

where BRstabilized = unsoaked bearing ratio of pond ash stabilized with lime 共4, 6, and 10%兲 alone or in combination with PG 共0.0, 0.5, and 1.0%兲, cured for 7, 28, and 45 days; and BRunstabilized = unsoaked bearing ratio of pond ash without stabilization at 7 days curing 共34.03%兲. The values of BRgain are presented in Figs. 10共a–c兲 for 7, 28, and 45 days cured specimens. The values of the nondimensional parameter ranges between 2.05–4.75, 2.98–5.60, and 3.31–6.52 for pond as stabilized with lime only, lime +0.5% PG, and lime



+1.0% PG, respectively, considering the variation of the parameters of this present study. Edil et al. 共2006兲 presented the enhancement of California bearing ratio 共CBR兲 of three types of soft fine-grained soils due to stabilization with fly ash in terms of the normalized term CBR gain; it was reported that with addition of 10% fly ash to the soft soils, the CBR increased on average by a factor of 4. Multiple Regression Analysis of the Nondimensional Parameter, BRgain The bearing ratio of the stabilized pond ash of this study depends on a number of governing parameters like type of pond ash, lime content, PG content, CP, dry density, and molding water content. In this study, an empirical model for the nondimensional parameter BRgain as a function of the governing parameters like lime content 共L兲, PG content, and CP, has been developed through multiple regression analysis. Analyzing the scatter plot matrix obtained from the experimental data the following model has been chosen for the proposed nondimensional parameter.

BRgain =

BRstabilized = a0 + a1共L兲 + a2共PG兲 + a3共CP兲 BRunstabilized

共2兲

where a0, a1, a2, and a3 = multiple regression coefficients. Based on the values of the nondimensional parameter obtained from the experimental results of this study, the values of the multiple regression coefficients of the above model 关Eq. 共2兲兴 are calculated. The calculated values of multiple coefficient of determination 共R2兲, adjusted multiple coefficient of determination 共R2adj兲, and standard error 共Es兲 of the above model 关Eq. 共2兲兴 are 0.908, 0.897, and 0.441, respectively. Significance of the Model The significance of the model 共Draper and Smith 1998兲 for BRgain has been studied through the following statistical tests. Significance of the Multiple Regression Coefficients as a Whole „F Test…

Fig. 10. Unsoaked bearing ratio gain as function of lime and PG contents, cured at 共a兲 7 days; 共b兲 28 days: and 共c兲 45 days

Significance of the multiple regression coefficients as a whole of the model presented in Eq. 共2兲 is tested using the F test. The calculated value of “F” is 69.12. From the table of F distribution with ␣ = 0.05, F共0.95, 3 , 23兲 = 3.03. Therefore, Fcal = 69.12 is greater then the tabulated Fcritical = 3.03, which rejects the null hypothesis, i.e., at least one of the explanatory variables of Eq. 共2兲 helps explain the variation of the nondimensional parameter 共BRgain兲.

Table 6. Values of t Statistics for Different Parameters of the Model 关Eq. 共2兲兴 Parameters

Coefficients

Standard error

t statistics

tcritical = t共0.975,23兲

Intercept Lime content 共L兲 Phosphogypsum content 共PG兲 Curing period 共CP兲

a0 = 1.059651 a1 = 0.180988 a2 = 0.176238 a3 = 0.04676

0.255258 0.028844 0.176238 0.004629

4.15129 6.274779 8.121526 10.1009

2.069



Summary and Conclusions

Fig. 11. Relationship between unsoaked and soaked bearing ratios of pond ash stabilized with lime and PG

Significance of the Partial Multiple Regression Coefficients The significance of the partial multiple regression coefficients of Eq. 共2兲 has been studied through the t statistics. Table 6 presents the summary of the t statistics of the coefficients; the decision rule of rejection of null hypothesis is satisfied, i.e., all the explanatory variables help explain the variation of the nondimensional parameter.

Relationship between Unsoaked and Soaked Bearing Ratios In this study, an attempt has been made to develop empirical model to estimate soaked bearing ratio of stabilized pond ash from unsoaked bearing ratio values. The bearing ratio of the stabilized pond ash depends on a number of parameters such as type of pond ash, lime content, PG content, CP, dry density, and molding water content. However, the bearing ratio values of unsoaked specimens as the reference variable in estimating the soaked bearing ratio values may represent the combined effect of the parameters mentioned above. This relationship will help the design engineers for quick estimation of soaked bearing ratio of stabilized pond ash for pavement design. Fig. 11 illustrates the variation of soaked bearing ratio versus unsoaked bearing ratio of stabilized pond ash. Analyzing this scatter plot, the following linear relationship between soaked and unsoaked bearing ratio has been developed: BRsoaked = 0.889BRunsoaked, R2 = 0.985

共3兲

where BRsoaked = bearing ratio of soaked stabilized specimens and BRunsoaked = bearing ratio of unsoaked stabilized specimens. This empirical relationship 关Eq. 共3兲兴 presented above in simple form is selected based on physical significance. This relationship is developed based on limited test results. Due to paucity of data in literature for pond ash stabilized with lime alone or in combination with PG, the proposed relationship could not be verified with previous studies. The proposed model should be used with good judgment and engineering experience to provide a quick method to estimate soaked bearing ratio.

The compaction and bearing ratio characteristics of pond ash stabilized with lime 共4–10%兲 alone or in combination with PG 共0.5 and 1.0%兲 were studied through laboratory experiments. The compaction tests were conducted for both standard and modified Proctor compaction energy. The specimens for bearing ratio tests were cured for 7, 28, and 45 days. Both unsoaked and soaked bearing ratio tests were conducted. Empirical model for a nondimensional parameter has been developed to find out unsoaked bearing ratio of stabilized pond ash as function of bearing ratio of unstabilized pond ash at 7 days curing, lime content, PG content, and CP. Linear relationship between unsoaked bearing ratio and soaked bearing ratio of the stabilized pond ash has also been developed. The following conclusions may be drawn from the analysis of experimental results presented in this paper: • The variation of dry density with moisture content of pond ash stabilized with lime and PG is similar to that of unstabilized pond ash in this study; • With increase in compaction energy from standard to modified Proctor compaction energy, the MDD of stabilized pond ash increases and OMC decreases; • Bearing ratio of stabilized pond ash increases with increase in lime content up to 10%, however the contribution of lime is more at the lower percentage 共4%兲 of lime addition to pond ash compare to that of 10% of lime addition; • The contribution of lime is more prominent to enhance soaked bearing ratio of lime stabilized pond ash compare to that of unstabilized pond ash; • Addition of small percentages of PG 共0.5 and 1.0%兲 to lime stabilized pond ash increases both soaked and unsoaked bearing ratio; • Linear relationship exists between soaked and unsoaked bearing ratio of pond ash stabilized with lime and PG; and • Pond ash stabilized with lime and PG may find potential application in road construction.

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