Experimental Research on Foamed Mixture Lightweight Soil Mixed with Fly-Ash and Quicklime as Backfill Material behind Abutments of Expressway Bridge

1Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Jiangsu Research Center for Geotechnical Engineering Technology, Institute of Tunnel and Underground Engineering, Hohai University, Nanjing, China 2Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Jiangsu Research Center for Geotechnical Engineering Technology, Geotechnical Research Institute, Hohai University, Nanjing, China


Introduction
Foamed Mixture Lightweight Soil (FMLS) as a new kind of geotechnical material, because of its light weight, independence, self-tightness, regulatory of bulk density and intensity, construction convenience, and thermal insulation properties, and so forth, has been applied successfully in the underground pipeline and cavity filling, heat preservation and heat insulation, and so on, especially in highway embankment filling [1,2].Because of its advantages, FMLS was put forward by Japanese scholars in the early 1980s, and a series of indoor and outdoor experimental research were carried out to show that FMLS is a good material in the field of civil engineering (e.g., [3][4][5][6]).Experimental research is mainly focused on compressive strength, stress-strain characteristics, pore structure, fiber improvement, and durability currently [7].
Although a lot of progress has been made, new improved materials for FMLS remain to be studied.
In recent years, fly-ash was gradually used in road engineering, ocean engineering, and other types of engineering.Predecessors (e.g., [8][9][10][11][12]) had done research on the utilization of fly-ash, and some beneficial conclusions were obtained.In general, previous scholars' research showed that fly-ash utilization research usually focuses on using fly-ash as the solely material for embankment filling, while research on the combination of FMLS and fly-ash for embankment filling is less.Using fly-ash as solely material for embankment filling may cause the following problems.Firstly, the dry density of fly-ash after roller compaction is 10.7-11.0kN/m 3 , which is a third to fifth lighter than soil, but heavier than widely used FMLS to more than 50%.Therefore, there is a big disadvantage if considering embankment weight.Secondly, fly-ash is the powder material in the process of filling, and the compaction effect is not very ideal [13]; especially when its moisture content is higher, the degree of compaction is hard to meet the requirements.In addition, the permeability of flyash is high and easily induces fly-ash being washed by rainwater, so it needs to do package-edge processing with cohesive soil before construction, which will impact the integrity, strength, and stability of embankment [14].Some scholars (e.g., [15][16][17][18]) tried mixing fly-ash with soil or cement, even lightweight aggregate, and proved that it is not only a good way to promote the utilization of fly-ash, but also a good way to solve the above-mentioned problems.Despite recent progress, the research on mixing fly-ash with FMLS is less, so the main point of this work is to explore the best utilization program of fly-ash mixed in FMLS.
The differential settlement between the rigid abutment and the flexible embankment has resulted in huge maintenance costs for the bridge highway, which prompts some experts to find a lightweight and low-cost backfill material for the abutment to effectively solve the problem of strength and deformation.Alizadeh et al. [19,20] used fly-ash to produce a suitable controlled low strength material as a backfill for bridge abutment.The application of argil fly-ash concrete to highway-bridge abutment backfill was introduced by Zhesheng et al. [21] and proved that its soil pressure and settlement are smaller than that of other materials markedly.Li and Zhang [22] and Jamnongpipatkul et al. [23] discussed the actual application state of FMLS in handling of road foundation and showed that FMLS is a lightweight, environmentally friendly embankment backfill material and it can reduce settlement and maintain stability.However, there is little research and application on FMLS mixed with fly-ash and quicklime (FMLSF) as abutment backfill.This paper will study the application of FMLSF in abutment backfill.This paper starts from the experimental study on wet density and unconfined compressive strength of FMLS mixed with fly-ash and quicklime (FMLSF) based on the orthogonal experiment method [24,25].Then, changing laws of wet density and unconfined compressive strength are revealed, with different content of cement, fly-ash, quicklime, and foam.After that, the microscopic mechanism of fly-ash improvement in FMLSF and primary and secondary influence order of mixing content are obtained later, as well as the optimal mixture combination.Finally, selected sections of abutments of expressway bridge are filled with FMLSF, and the corresponding construction techniques and key points of quality control are discussed.

Mixture Proportion Experiment Program
2.1.Dosage of Water and Foaming Solution.Define   as the total volume of slurry composed of cement, fly-ash, quicklime, and water in 1 m 3 mixture of FMLSF.It can be showed as follows: where   ,  fa ,   , and   are the mass of cement, fly-ash, quicklime, and water, respectively, and   ,  fa ,   , and   are the density of cement, fly-ash, quicklime, and water, respectively, which are 3100 kg/m 3 , 2600 kg/m 3 , 1200 kg/m 3 , and 1000 kg/m 3 , respectively.The dosage of water can be expressed using following equation: where  is water-binder ratio.The designing density of slurry composed of cement, fly-ash, quicklime, and water is around 1650 kg/m 3 , so the water-binder ratio  could be calculated using following equation: where   is the density of slurry composed of cement, fly-ash, quicklime, and water and   is the mass of slurry composed of cement, fly-ash, quicklime, and water.The residual volume for   in 1 m 3 mixture of FMLSF would be filled by foam, so the dosage of foaming solution   would be calculated using the following equation: where  is extracoefficient, usually taking 1.1∼1.3. = 1.1 in this experiment.We firstly did some preexperiments to decide the suitable dosage of foaming solution based on above calculation.Because of lots of losses of foam when added to the mixture of FMLSF, we should add 2∼3 times the amount of theoretical foam to get the ideal porosity content or specific density.

Orthogonal Experiment Method.
The composition of FMLSF can be divided into two classes: one is the main components as cement, water, and foam, and the other is auxiliary components as fly-ash, quicklime, and others.For the wet density and unconfined compressive strength of FMLSF, the main affecting factors are cement, fly-ash, quicklime, and foam.Each affecting factor is set four levels further in Table 1, using  16 (4 4 ) orthogonal table to arrange four factors and four levels of tests in Table 2.

Experimental Materials and Instruments. A Composite
Polymer Foaming Agent (CPFA) is used to produce FMLSF in this research, which is a colorless liquid, and PH value is 7.5-9.0.It has many advantages, such as high foaming rate, low absorption rate, high strength, and persistence.In addition, CPFA is pollution-free and green environmental.There are two methods to form air bubbles: mechanical mixing method and prefoaming method.To control the amount and stability of air bubbles, the prefoaming method is selected.With the prefoaming method, the air bubbles were first foamed via a foaming machine and then mixed with cement slurry.CPFA is mixed with water according to the scale of 1 : 40.Generated bubbles are shown in Figure 1.The fly-ash used in this work is collected from Yulian thermal power plant in Henan, China.According to the test result, the 45 m sieve residues of fly-ash are 12%, which meet the fineness requirement of ASTM C618 [26].The hardening agent is ordinary Portland cement whose grade is 42.5.Table 3 shows the chemical and mineralogical compositions of cement and fly-ash used in this investigation, and Table 4 shows the physical properties of cement which meet the requirements of ASTM C311 [27].conducted according to the Chinese Standard of Technical Specification for Foamed Mixture Lightweight Soil Filling Engineering [28].Firstly, the cement content, quicklime content, and fly-ash content are mixed according to the orthogonal experimental design of 16 different mix proportion groups.Stirring time lasts approximately 5 min and then stirring with water, until seriflux's wet density is around 1650 kg/m 3 .This process is very important, because cement and fly-ash must be fully mixed into slurry state until the seriflux no longer contains larger particles.Secondly, the foaming agent is diluted with water in a ratio of 1 : 40, and then frothing through foaming machine.Thirdly, the generated bubbles are added to fully stirred seriflux, then loading the foamed mixture into cube mould whose size is 10 cm × 10 cm × 10 cm after mixing for 5 minutes.Because foamed mixture has high liquidity, it can fully dense as long as mould vibration gently.Finally, put samples covered with plastic film in an incubator chamber at 20 ∘ C, demould after conserving 24 hours, then number them, and conserve to designing age.Before the foamed mixture loaded into cube mould, it was measured to obtain its wet density.Three specimens were tested and the average wet density was computed.The wet density is measured by a container and an electronic weigher; the range of them, respectively, is 600 mL and 2000 g.Three specimens were tested at the age of 28 d to determine their unconfined compressive strength and the average strength was computed.The unconfined compressive strength is measured by electronic universal testing machine which is made in Changchun Machinery Factory (CSS-44050) and the loading rate is 2 kN/s.The specimen preparation process is shown in Figure 2. Samples before demoulding are shown in Figure 3. Different mixture proportions of FMLSF which have been numbered are shown in Figure 4.

Analysis of Test Results
Unconfined compressive strength test results of FMLSF are shown in Table 2. To compare the effect of various factors on the strength and wet density, we calculated the mean value of strength and wet density at different levels of each affecting factor; the results are shown in Figures 5-9.To investigate the microscopic mechanism of fly-ash improvement, the microstructure of FMLSF at different cement content was  observed by Scanning Electron Microscope (SEM), and the representative images are shown in Figure 6.
As can be seen from the Figure 5, the wet density and unconfined compressive strength of FMLSF increase with the increase of cement content.It is well known that cement is a cementitious material.For FMLSF, cement is the primary cementitious material and the primary source of strength.We all know that hydration occurs when cement encounters water.Hydration is the precondition of cement hardening and hardening is the reason for the structural strength.The hydration products of cement are mainly amorphous calcium silicate hydrates (C-S-H), cubic plate of calcium hydroxide crystals, and needle-like ettringite (see Figure 6(b)).These hydrated products grow interchangeably, making the structure become compacted and dense.
As can be seen from the Figure 7, the addition of fly-ash makes the wet density and unconfined compressive strength of FMLSF increase firstly and then decrease.Fly-ash particles are smooth spherical beads, which are similar to the lubrication effect of ball bearing that can reduce the friction between particles.It is well known that the flowability of spherical particles in the slurry is much greater than that of any other shaped particles under the identical conditions.Furthermore, the 45 m sieve residues are used as fly-ash fineness requirements according to the ASTM C 618.That is to say, most of the particle size of fly-ash is less than 45 m.Because of the small size of fly-ash particles and the shape of smooth spherical beads, as is shown in Figure 6(a), the fly-ash particles can be evenly distributed in the cement slurry, which makes the cement particles evenly disperse to reduce the accumulation of cement.As a result, space of hydration and generated hydration products is expanded and the porosity after hardening is reduced.The description of the preceding paragraph is called ball effect or microaggregate effect of fly-ash.With the ball effect (e.g., [29][30][31][32][33]), the structure between spherical particles of fly-ash and cement particles can be closer, thus improving the wet density and unconfined compressive strength of FMLSF to a certain extent.However, when the fly-ash content is small, the ball effect is not obvious.Meanwhile, excessive fly-ash content will lead to less hydration products and as a result reduce wet density and strength.So too much fly-ash has little good effect on wet density or strength; there is an optimal value.
As can be seen from the Figure 8, with the mixing content of quicklime increase, the wet density and unconfined compressive strength of FMLSF increase firstly within a narrow range and then decrease.It follows that quicklime has little effect on wet density and strength, so it should not be used as the additive agent for FMLSF.As is reported by Cong and Bing [34], the water existing in the surface of the foam can be assimilated by quicklime, which results in the breaking of foam.The great instability in foam is also the reason that quicklime cannot be applied in the FMLSF.
As can be seen from the Figure 9, the increase of foam content makes the wet density and unconfined compressive strength of FMLSF decrease.It is because with the increase of foam content, the amount of fine closed foams in the slurry increases.Thus, the slurry volume will increase, which leads to decreasing of wet density and strength.Based on Kikuchi' microscopic observation [35], the air voids in the FMLS will be connected when the air fraction is more than 30% by volume; it is disadvantageous for the strength of FMLS.Foam reduces the wet density but also reduces the strength, so the foam content should not be too high.

Range Analyses
The factors which influence wet density and strength of FMLSF include cement content, fly-ash content, quicklime content, and foam content.It can found through experiments that four factors have different level of effects on wet density and strength.In order to analyze magnitude of the effect, the range analysis method is selected, and the results are shown in Table 5.It can be seen from it that the main factor which influences wet density and compressive strength is the cement content, which follows the foam content.Therefore, the cement content and foam content should be strictly controlled to achieve the purpose of controlling construction quality.The calculation steps of range analysis are as follows: (1) Calculate   value, where   is the sum of corresponding test results for affecting factor in different level;  is serial number of levels of affecting factor, as 1, 2, 3, and 4;  is serial number of affecting factors, as 1, 2, 3, and 4.
(4) According the value of   , judge the influence order of affecting factors.
The greater the value, the larger the influence on test results and the more important the affecting factor.

The Optimum Mixture Proportion
From the view of engineering application, in general, the mixture proportion of FMLSF when wet density is low (<1000 kg/m 3 ) and strength is high (>1 MPa) is the optimum mix proportion, therefore taking the data of Table 3 for further analysis.
(1) Cement.The corresponding compressive strengths of 400 kg/m 3 , 550 kg/m 3 , and 700 kg/m 3 are all greater than 1 MPa.However, with the increase of mixing amount, the corresponding cost increases also.As a result, the recommended optimal mixing proportion of cement is 400 kg/m 3 , and the corresponding wet density also meets the requirement.
From the point of improving the lightweight, 750 L/m 3 is a better choice.
(3) Fly-Ash.Because all the compressive strength of different addition amounts can meet the requirements, it is suitable for analysis through wet density.It is not difficult to find that 150 kg/m 3 and 200 kg/m 3 are more reasonable.From the view of promoting the use of fly-ash, 200 kg/m 3 is a better choice.
(4) Quicklime.Both the wet density and compressive strength changed very little, so it is better not to mix quicklime.and it is a separate abutment.Behind the abutments of Lianjing Bridge, FMLSF or FMLS are filled as backfill material, so as to reduce the load and basic stress, reduce settlement, and increase the dense of fillings [36,37].In order to verify the optimum mixture proportion of FMLSF based on laboratory experimental data in this article, two abutments are selected to fill with FMLSF and FMLS, respectively.The height behind the abutment is 8 m, width is 12 m, and length is 17.7 m.Along the longitudinal of the road, four steps with spacing 2 m are set at the joint of the FMLSF and subgrade.Each step laid steel mesh as  8@200 to reinforce the integrity and stability of FMLSF and subgrade.Filling diagram of FMLSF or FMLS as backfill material behind abutments is shown in Figure 10.

Construction Technology.
The construction technology of FMLSF and FMLS is almost the same, and the key points of quality control are briefly analyzed as follows.According to the previous studies on FMLS, because its fluidity is high, the pouring method can be selected, and there is a wealth of construction data of FMLS [38].Single layer thickness of the pouring for FMLSF is 0.6 m and FMLS is 0.8 m.Single layer time of the pouring should be controlled in the concrete initial setting time.A layer of pouring should be after the final set of the under layer, so the time interval is at least 7 hours.Avoid the rain, in the process of construction, and curing at moisture-retention with plastic film or nonwoven fabric.Before filling construction, testing preparation of four practical engineering mixture proportions of FMLS is conducted, and the results are shown in Table 6.Testing preparation of mixture proportion of FMLSF is the same as laboratory experiments.Because foam density is far less than the cement density, this paper uses the same wet density cement slurry by adding different amount of foam content to pouring specimens.Material composition of cement slurry is as follows: cement 1109.2 kg/m 3 , water 650.9 kg/m 3 , and wet density 1760.1 kg/m 3 .

Fieldwork Experiment Results
. See Figure 11 for the results of unconfined compressive strength test of FMLS.Through the stress-strain curves with different rate of foam, it can be seen that the unconfined compressive strength of FMLS decreases with the rate of foam increase.In addition, main failure pattern which is brittle failure is gradually transformed into ductile failure.When compressive strength meets the design requirements, wet density of FMLS should be as small as possible, so the first (wet density 970 kg/m 3 ; unconfined compressive strength 1.4 MPa) or second (wet density 800 kg/m 3 ; unconfined compressive strength 1.2 MPa) group is more appropriate.Furthermore, according to the optimum mixture proportion of FMLSF (cement 400 kg/m 3 , fly-ash 200 kg/m 3 , and foam 750 L/m 3 ) to prepare sample and do unconfined compressive strength test, experimental results show that wet density of FMLSF is 720 kg/m 3 , and compressive strength is 1.4 MPa.
Based on the above test results, two mixture proportions are used to manufacture FMLS and FMLSF as backfill material, respectively.The recent monitor report shows that the settlement and stability of the two abutments all meet the requirements.Because of the low price of fly-ash and because utilization of it can reduce the pollution of environment, from this point of view, FMLSF is better than FMLS.

Conclusions
Foamed Mixture Lightweight Soil mixed with fly-ash and quicklime as backfill material behind abutments of expressway bridge is studied in this paper.Based on two abutments of Guang-Fo-Zhao Expressway Bridge in China, the construction techniques and key points of quality control filled with FMLSF behind abutment are proposed as follows: (1) The wet density and unconfined compressive strength of FMLSF increase with the increase of cement content, while decreasing with the increase of foam  content.With the mixing content of fly-ash increase, the wet density and unconfined compressive strength of FMLSF increase firstly and then decrease.With the mixing content of quicklime increase, the wet density and unconfined compressive strength of FMLSF increase firstly within a narrow range and then decrease.
(2) The microscopic mechanism of fly-ash improvement in FMLSF is investigated through images observed by Scanning Electron Microscope (SEM).Ball effect or microaggregate effect of fly-ash improves the wet density and unconfined compressive strength of FMLSF.(3) The range analysis shows that the content of cement has a significant effect on the wet density and unconfined compressive strength and then in turn air content, fly-ash content, and quicklime content.The optimum mixture proportion is cement 400 kg/m 3 , fly-ash 200 kg/m 3 , and foam 750 L/m 3 .(4) The settlement and stability of the two abutments filled with FMLSF and FMLS all meet the requirements.Because of the low price of fly-ash and because utilization of it can reduce the pollution of environment, from this point of view, FMLSF is better than FMLS.

Figure 4 :
Figure 4: Different mixture proportion of FMLSF which have been numbered.

Figure 10 :
Figure 10: Filling diagram of FMLSF or FMLS as backfill material behind abutments.

Figure 11 :
Figure 11: The stress-strain curves of FMLS with different rate of foam.

Table 1 :
Four levels of each affecting factor.

Table 3 :
Chemical compositions of cement and fly-ash.

Table 4 :
Physical properties of cement.
Materials and samples preparation, as well as measurement of wet density and unconfined compressive strength, are

Table 5 :
The range analysis results.

Table 6 :
Mixture proportion in the practical engineering of FMLS.