Use of Residual Rice Husk ash as Structural and...

International Journal of Engineering & Technology IJET-IJENS Vol:16 No:06 11

165606-7474-IJET-IJENS © December 2016 IJENS I J E N S

Use of Residual Rice Husk ash as Structural and

Sustainable Conditioner of Clayey Soil Decio Lopes Cardoso

1, a, Lidiane Regina Braun

2,b, Simone Minuzzo

2,c and Camila Daiane Cancelier

2,d

11701 8 C, Pio XII Street, Neva, Cascavel, PR, Brazil, Postal Code 85802-170

22069, Universitaria Street, Jardim Universitario, Cascavel, PR, Brazil, Postal Code 85819-110

[email protected],

[email protected],

[email protected],

[email protected]

Cardoso, Décio Lopes. Doctor in Civil Engineering, Geotechnical Engineering, [email protected]

Abstract-- Clayey soils are chemically reactive towards certain

compounds, allowing manipulate their engineering properties,

particularly mechanical strength, which opens up the possibility

of strengthening them mixing with proper reinforcements. This

research treated the soil adding rice husk ash at doses 0; 2.5; 5;

7.5 and 10%. The residue were chosen because it constitute

major environmental liabilities in places where is produced and

had already demonstrated potential as fine soils structural

modifier. The specimens were compacted and sheared in triaxial

compression tests, with confining pressures of 25; 50 and 100

kPa, presenting failure deviator stress 718.2, 722.1 and 731.1%

higher than reference, respectively. Also highlights the change in

material rheological behavior which changed from ductile to

brittle, with significant increase in elasticity modulus, implying

stiffness gain. The most striking effect was observed in the

cohesion parameter while the effect on the internal friction angle

was smaller. It was suggested that reinforcement promoted

formation of new product which more than fill the voids of the

soil changed the soil for a substance with nature of continuum.

The improvement in strength indicated that the waste has ability

to decrease environmental liability and improve the natural

clayey soil to a material with sustainable engineering properties.

Index Term-- Composite, RHA, deviator stress, shear strength,

mechanical properties.

1. INTRODUCTION

Rice Husk Ash (RHA) characteristics: Rice is the food base for

more three billion of people and shows the title of second

more cultivated cereal in the world, with 29%, only behind

corn with 33 % [1]. Brazil is among the major global

producers with 12.432 million tons in the 2013/14 crop [2].

Post harvest, according to [3], in the processing phase, various

wastes are generated, highlighting the RHA, because of the

large generated volume. [4] Studies showed an amount of

2,525,640 tons of rice husk in the country in the 2011/2012

harvest. Despite the existence of federal regulations instituting

restrictive policies at the disposal to them much waste is still

disposed improperly in valley, rivers and crops. Postulate [5]

that an ethical, ecological and cost-effective alternative is the

generation of energy through the burning of rice husk, which

generates ash as its residue main. This ash is considered an

agriculture industry residue widely found in the grain

producing regions, and due its high level of silica, [6]

proposed to use the waste as a potential structural soil

conditioner. About 20% of the weight of rice production is its

husk which is peeled in the rice processing industries [7]. A

large quantity of that husk is utilized in the processes of drying

and parboiling the beans, replacing the wood in the generation

of heat and steam. The use of RHA has been studied by many

researchers, replacing part of the cement in the concrete gray,

with satisfactory results. This effect has been attributed to the

silica constituent in the RHA. It improved the quality of

concrete due to physical and chemical effects and the particle

size correction [8] and [9]. The high calorific value of rice

husk and virtually no cost is making it a substitute for wood as

an energy source in the processing industries. Research's [10]

evaluated the potential of RHA as corrective of acidity,

fertilizer and agricultural soil conditioner applying doses from

10 to 140 ton ha-¹, with significant results regarding the

structural improvement of the soil. [11] Used residual RHA in

structural concrete and decided that was feasible the partial

replacement of Portland cement by RHA in natural and ground

conditions in 15 and 25% doses. Studies [12] discussed RHA

production methods for use in high performance concrete and

considered the RHA as a superpozzolan, for its peculiar

properties of engineering, as well as ASTM Designation C

989 [13] which classifies the RHA produced under special

conditions as high reactivity pozzolan, along with the active

silica. According the studies of [14] replacement of Portland

cement by 5% and 10% increased compressive strength from

50 MPa to 54 MPa and 60 MPa, respectively. It was examined

the gain of compressive strength of a Uruguay's sandy soil by

adding RHA and lime [15], which mixtures showed

convergence to a stress-strain behavior stiffer than the natural

soil. The mechanical performance assessment of concrete mix

with RHA made by [16], verified according to rules

regulations NBR 5739 [17] along with the economic viability

of the technical results concluded that the addition of 3% ash

rice hull showed the best performance among the tested

mixtures of 3, 5 and 7% for all curing periods. Research's [18]

showed strength gains of an organic silt when mixed with



Portland cement and pozzolan in the form of RHA and

microsilica. The highlight in this study was the use of organic

soil, seen as problematic geotechnical standpoint and therefore

always avoided. It was concluded that the use of pozzolans

such as RHA provides the cement consumption reduction

without loss of strength. From an environmental perspective it

is noteworthy the [19] research work, whose relevant results

showed that the addition of 35%RHA reduced cement

consumption by 25% and allowed storage, without emission to

the atmosphere, 1.9 tonnes of CO2 per tonne of cement

consumed, thus contributing for CO2 capture, which can

stimulate rural constructions under the ecological point of

view.

Based on the above and from the physical and chemical

characteristics, in particular the high specific surface area and

cation exchange capacity of RHA, the possibility of its use as

a structural soil conditioner arises. In the chemical

composition of the RHA are among other chemicals SiO2,

Al2O3, CaO and Fe2O3 [5]. In contact with the fine particles of

the clay soil of the region, which have electrically negative

surfaces, and alkaline and aqueous environment, electrostatic

bonds between the cations present in the RHA and surface

charges occur, The increase in the number of contacts between

the soil particles and the strengthening of preexisting contacts

are two important effects of RHA on properties of

compounds, thus providing more stiffness at the composite

structure. Modifying the structure of the material changes its

engineering properties and affect its mechanical performance

and for this purpose the research aimed to increase the soil

shear strength adding residual RHA, assessed through failure

deviator stress measured in triaxial compression tests.

2. MATERIALS AND METHODS

The soil was from the Experimental Center of Agricultural

Engineering, located in Cascavel city, Parana state, Brazil, a

typical occurrence in the central and southern Parana Plateau

Third, being classified pedologically as Oxisol Red [20], and

geotechnically as A7 by AASHTO and CH by USCS systems

[21]. It is a residual soil from strong weathering of the basalt,

clayey predominantly, porous structure, and typically have

high levels of iron oxides, about 20%, in the Fe2O3 form of

hematite. The RHA was derived from rice processing industry

Peruchi Food Industry and Trade S.A., Venice city, Santa

Catarina state, Brazil. The experiments were performed at the

Geotechnical Laboratory of the State University of Parana

Western. The specimens were cast in the dimensions 50 mm in

diameter and 100 mm in high in the mini-Moisture Condition

Value (mini-MCV) device at doses 2.5%; 5.0%; 7.5%; 10%

RHA in dry soil mass, and the reference dose. The mix were

compacted in the Proctor curve optimum content, using

normal energy, and adjusting the amount of soil to be

compacted in the mold to achieve the desired void ratio and

density as recommended by [22]. The shear strength of the

composite was measured in consolidated undrained triaxial

compression tests [23], in confining pressures of 25; 50 and

100 kPa, applied by compressor of 5 HP power and reversion

panel with pressure regulator valves. The volumetric variation

of the specimens were read in the reversal panel burette, and

the drain valve was closed when the reading was stabilized.

After consolidation, the specimens were sheared with strain

rate 0.34 mm min-1

, recording the vertical displacement by

Linear Vertical Displacement Transducer (LVDT) and the

axial load by the test press dynamometer ring. Data post-

processing were drawn on Sigma-Tau diagrams, as well as

Mohr's circles of stresses and Coulomb rupture envelopes,

whose linear and angular coefficients provided, respectively,

the parameters cohesion and internal friction angle of the

materials.

3. RESULTS AND DISCUSSION

3.1. Characterization: The soil textural classification taken

from the particles distribution curve is clay 60%, silt 30% and

sand 10%. Specific gravity is 29.46 kN m-³. The optimum

moisture content is 31.20% and maximum specific weight is

14.34 kN m-³. These values from compaction tests were

reproduced in all the specimens to permit comparision

between the several mixtures.

3.2. Mohr circles to soil-RHA composite: In Figs. 1, 2, 3, 4

and 5 are shown, respectively, the Mohr circles to dosages 0%;

2.5%; 5%; 7.5% and10%RHA.



Fig. 1. Mohr circles to RHA 0%



Fig. 2. Mohr circles to RHA 2.5%



Fig. 4. Mohr circles to RHA 7.5%




Observe itself both a gradual increase and approximation of

the Mohr circles with dosages and with confining pressures.

For all treatments the failure envelopment was well defined,

validating the application of the Mohr-Coulomb rupture

criterion. There was substantial increase of the largest

principal stress with RHA increments. The essential aspects

of these qualitative patterns are discussed better after

presentation of the numerical values of failure deviator stress.

3.3. Strength increase to soil-RHA composite: Table 1 shows

the effect of RHA reinforcement and confinement on failure

deviator stress, which can be best seen in the curves of Fig. 6.

Table I

Failure deviator stress as function of reinforcement and confinement

RHA

Confining

pressure

[%]

[kPa]

25 50 100

0 56.21 64.63 77.61

2.5 169.44 253.53 419.41

5 241.94 347.54 508.93

7.5 346.13 428.29 558.86

10 454.90 531.33 644.99



Fig. 6. Failure deviator stress behavior in the soil-RHA composite

The results presented in Table 1 are plotted in Figure 6 show

that at the lowest level confinement (σ3 = 25 kPa), the

relationship between the failure stress deviator (q = σ1-σ3) and

RHA treatments was highly linear, according Eq. 1.

The high determination coefficient r² testifies the very strong

cause-effect relationship between the variables. For maximum

applied dosage of 10% the value of 454.90 kPa corresponding

to a gain of 718.2% compared to 56.21 kPa obtained in the

control specimen.

Increasing the level of containment to σ3 in 50 kPa the

relationship remains highly linear, according Eq. 3.

𝑞 [𝐾𝑃𝑎] = 39.363𝑅𝐻𝐴 [%] + 57.91 (1)

𝑟² = 0.9953

𝑞 [𝐾𝑃𝑎] = 4.326𝑅𝐻𝐴 [%] + 108.43 (2)

𝑟² = 0.9696



The shear resistance obtained in the maximum dosage 10%,

531.33 kPa, was 722.1% higher than the shear stress in the

reference, 64.63 kPa. With maximum confinement level used

in the research, σ3 = 100 kPa, was obtained in rupture a

deviator stress of 644.99 kPa, 731.1% higher than the one

presented by reference specimen 77.61 kPa. The best

statistical adjustment curve stopped being linear and passed to

obey power law, as Eq. 3.

The displayed patterns suggest that:

i) in relation to the treatment effects by adding RHA in

resistance to soil shear appears to be a large initial impact

which can be noticed by higher slopes of the three curves in

Fig. 6 in the section between the dosages 0%RHA and

2.5%RHA. After dosing 2.5%RHA the increase in the

resistance due to the treatment is consistent but constant.

These results are very consistent with those obtained by [16]

who added RHA to concrete in the dosages 3%, 5%, 7% in the

cement replacement and evaluated the performance according

to [17], concluding that the concrete made with 3%RHA

showed the best performance in compressive strength, with

values respectively of 20.10 MPa, 8.93% above the reference

(second best result with three days of curing); 26.46 MPa,

4.23% above the trace of 7% (second best result after 7 days

of curing); and 32.09 MPa which represent 3.17% over the

control line (second best result after 28 days of curing). Also

[14] reported resistance initial gain in concrete with the

addition of 5% and 10%RHA, passing from 50 MPa in the

control to 54 MPa and 60 MPa respectively, highlighting the

benefits of using RHA in reducing environmental pollution,

improvement of the properties of engineering and

sustainability of concrete.

ii) The second pattern that should be highlighted of these

results regarding of confinement impacts associated with the

treatment in resistance to shear of the composite. First of all

the larger effect of confining stress was observed for the first

treatment, 2.5%RHA, wherein the higher confining pressure

greater the slope of the q - RHA curve. For doses above

2.5%RHA curves show trends of convergence, albeit a slight

one, suggesting that for any higher level confinement of the

same no longer take effect. This pattern is typical of a material

which is becoming progressively more rigid, since a rather

rigid structure does not suffer confinement effect.

Studies with compression tests on a soil stabilized with RHA

and lime in dosages 15% and 5%, 20% and 5%, respectively,

observed that the mixtures showed brittle fracture and stress-

strain behavior stiffer than natural soil [15]. Also evaluates the

California Bearing Ratio, CBR, in order to apply for road

paving getting an average increase of 7% in the control to 25%

in soil mix with 20%RHA. The secant deformability modulus

was also another parameter strongly impacted by the addition

of RHA and lime, with mixtures of 20%RHA presenting a

gain 18 times the natural after 20 days of curing.

The (i) and (ii) effects can be better understood from the

standpoint of curves shown in Fig. 7. Clearly, the increase in

longitudinal elasticity modulus of the composite is too weak

for maximum confining pressure used in the experiment, σ3 =

100 kPa, and rather sharp in the minimum confinement level,

σ3 = 25 kPa. Again, the trend of convergence to higher doses

is noted, supporting the argument presented to Fig. 6 with

respect to the progressive stiffening.

In the Fig. 7 the values of longitudinal elasticity modulus or

Young's modulus as a function of treatment and confining

pressures are shown. The elasticity moduli were determined

from the line connecting the second point of the stress-strain

curve with the point that finished the elastic phase and

anticipated the yield phase. The first point was discarded due

to the inertial effects of the machine, which can be

compromised the accuracy of the first reading.

𝑞 [𝑘𝑃𝑎] = 411.55𝑅𝐻𝐴 [%] 0.146 (3)

𝑟² = 0.9912



Fig. 7. Young's modulus behavior of the soil-RHA composite

The lower content of RHA applied caused a slight reduction in

the value of E and from there it is noted an increasing stiffness

of the composite. Taking, for example, natural soil and

treatment of 10%, it was obtained for confining pressure of 25

kPa gain of 50.15%, for the confining pressure of 50 kPa gain

dropped to 35.64%; and at the pressure of 100 kPa the

corresponding gain was only 10.88%. This fact indicates again

to be taking place a progressive structural stiffening of system.

The results shown in Fig. 8 are emblematic. The almost

constant slope of the three curves related to higher dosages of

RHA suggests that a same and single phenomenology is

involved. The key aspect that should be emphasized here is the

impact of treatment on cohesion parameter. It seems to be

undoubted, because the correlation is straight line and very

strong. The statistical correlation was quantified with the data

in Table 2 and presented for treatments up 2.5% with behavior

essentially linear whose angular coefficients of the straight

sections were 12.07, 13.23 and 13.58; with determination

coefficients r² equals to 0.9994, 0.9989 and 0.9989, for

confining pressures of 25, 50 and 100 kPa, respectively. The

high determination coefficients along with the almost same

values of slope suggest a cause-effect relationship very strong,

indicating that the RHA triggers chemical reactions in the

system that promote fundamentally cohesion and giving it a

shear resistance intrinsically linked to cementitious effects.

In Table 2 the values of cohesion in the treatments are shown.

There is a reduction of cohesion with the dosage 2.5%

compared to the natural soil cohesion, and thereafter takes

place a systematic increase in the parameter. The gains were

of 333.24%, 224.54% and 92.25% for confining pressures of

25, 50 and 100 kPa, respectively. The parallelism between

cohesion curves in up 2.5%RHA doses indicates that the

confinement was not effective.



Table II

Cohesion parameter as function of reinforcement and confinement

RHA Coehsion

[%] [kPa]

25 50 100

0 20.52 32.31 59.86

2.5 21.60 23.07 24.56

5 28.53 38.70 47.12

7.5 59.98 69.87 79.11

10 88.90 104.86 115.02

Fig. 8. Change in the cohesion parameter of the soil-RHA

Extensive research [11] used residual RHA in structural

concrete and concluded that there was greater efficiency of the

mixture when incorporated RHA, natural or ground. Such

cementitious effect is attributable to the cohesion. Obtained

higher unconfined compressive resistance (RCS) in all curing

time (7, 28 and 91 days) for mixtures containing natural and



ground 15%RHA. The average values of RCS in the mixtures

with natural and ground 15%RHA were respectively 20.7%

and 10.7% higher than those RCS of the reference mixture for

all water/cement ratio investigated. Another relevant

discovery of the research was that ground admixtures with

15% presented values in average 8% higher that of mixture

with 25%, and with a natural RHA mix the difference in

average was more pronounced yet. The importance of this

discovery is that a RHA that has not undergone any processing

can be used at low levels, which represents a significant

advance for the productive sector, by enabling that add more

value to those produced concrete, insofar as they provide

smaller energy consumption.

PAULA [18] concluded in study that the use of pozzolan as

RHA provides reduction in cement consumption without loss

of strength. The research used mix with stabilizer contents of

100 and 200 kg m-³ of soil, which is typically in the range of

10% to 20%. There was no loss of strength because the

pozzolans added provides the action cementitious that the

Portland cement would give at the mix, thus seems reasonable

to attribute to the RHA and other pozzolan added, microsilica,

the role of cementitious agent, which is eventually the

parameter cohesion in the perspective of the Mohr Coulomb

failure criterion.

The friction angle effectiveness should not be assessed on the

values of the angle itself, but on the called Terzaghi's load

capacity factors, given by the expressions:

𝑁𝑞 = 𝑒𝜋 tan ∅ 𝑡𝑎𝑛2 𝜋

4+

∅

2 (4)

𝑁𝑐 = 𝑁𝑞 − 1 𝑡𝑎𝑛−1 ∅ (5)

𝑁𝛾 = 2 𝑁𝑞 + 1 tan∅ (6)

The curves in Fig. 9 were plotted with the values of the Table

3 and are the response of the internal friction angle of the

composite to the treatment with RHA and confinement. First

of all the effect of containment is undoubted, but the

treatments lead to a peak with 5%RHA dose which then

recedes to lower values of composites's friction angle. The

mirror pattern compared to the cohesion plot is striking,

confirming once again the correctness of the application of

Mohr-Coulomb failure criterion to the behavior of the soil-

RHA composite. For section where the cohesion rapidly

decreases the friction angle is growing too quickly, while that

in the section wherein the cohesion parameter persistently

increases the friction angle decreases also systematically.

Apart from all this, at the dosage of 2.5%RHA both

parameters of Hvorslev [24] converge to the same and unique

value.

Table III

Internal friction angle parameter as function of reinforcement and confinement

RHA Internal fricition angle

[%] [graus]

25 50 100

0 21.8 28.2 36.0

2.5 37.5 37.7 37.9

5 38.0 40.0 42.8

7.5 34.9 36.4 39.5

10 31.9 33.6 37.5



Fig. 9. Change in the internal friction angle of the soil-RHA composite

With respect to the Terzaghi factors is discussed herein the

treatment effect in Nq, due to being the most impacted by a

variation in friction angle. The 5%RHA dose raised the Nq

factor to peak values 48.93, 64.20 and 96.09, which compared

with values 7.66, 5.15 and 37.75 in the reference samples

represent increases of 538.8%, 326.6% and 154.5%,

respectively, to the confining pressures 25, 50 and 100 kPa.

However as stated above higher dosages of RHA dropped Nq

to 22.91, 28.04 and 45.81, representing modest gains of 199.1,

86.3 and 21.4 relative to the reference, at the three respective

confinement levels. As was the case for the other properties

already discussed is noticeable once again the effect

progressively smaller of confinement.

The cohesion and internal friction angle behaviors have been

shown inversely correlated, suggesting that the Mohr-

Coulomb criterion can be successfully applied to study the

behavior of composite formed by soil-RHA. In short, the

effect of RHA on composite cohesion is substantial, but the

increase in the internal friction angle is smaller.

The Mohr-Coulomb equation is by far most widely used for

strength [25]. It states that

𝜏𝑓𝑓 = 𝑐 + 𝑓𝑓 𝑡𝑎𝑛 ∅ (7)

where τff is shear stress at failure on the failure plane, c is the

cohesion intercept, ff is the normal stress on the failure plane

and is the friction angle. Analysis of equation 7 shows that

the confinement, represented by the normal stress in the

rupture plane, affects the parcel of shear resistance due to

friction between the material particles but does not affect the

part due to the cohesion. This explains why larger confinement

had no overall effect on the system as a whole.

3.4. Strain modes and failure planes in the soil-RHA

composite: In Fig. 10 is shown photography of the specimens

after fracture, illustrating the progressive reduction in axial



and radial deformations with increasing RHA reinforcement.

In the soil in natura the rupture occurred in several closely

spaced planes and is evidenced by a blistering of the sample,

characteristic of plastic and ductile flow of materials breaking.

The stress-strain curves are well behaved presenting linear

initial stretch followed by viscoplastic flow that eventually

takes on a plateau where large deformations are sustained until

the failure occurs with large compressive deformation of 6.2,

6.3 and 11.8 mm mm-1

in confining pressures 25, 50 and 100

kPa, respectively. For the treatment of 10%RHA the stress-

strain curves show the initial linear elastic section, which pass

to a very short section of yield and immediately reach the

break in well defined peaks. They are typical of materials with

brittle fracture, which occurs in much lower strain of 2.8, 3.1

and 4.6 mm mm-1

in confining pressures 25, 50 and 100 kPa,

respectively. Research investigating artificially cemented soil

[26] found that at low confining pressures the shear strength is

function of the cohesion provided by the cementing agent, and

under higher confining pressures the behavior is frictional. In

the same containment relationships [27] also found transition

from ductile-compressive to brittle-dilatant behavior in

artificially cemented soils. Venson [28] postulates that

strongly cemented soils are fragile behavior in any confining

pressure levels, and moderate or weakly cemented soils

present the ductile-brittle transition. All the above assumptions

corroborate the results of this research regarding the effects of

a cementing agent such as RHA modifying the clayey soil

properties from ductile to fragile.

Fig. 10. Photograph of broken test bodies for increasing RHA levels

4. CONCLUSION This paper presented an experimental investigation of the

nature of composite shear resistance formed by soil with

RHA. The results indicated as main effects (i) increase in

soil shear strength due a substantial increase on cohesion

parameter; (ii) smaller increase on the internal friction

angle, and due to this fact, the confinement caused relativity

small increase in shear strength; (iii) increase in the Young's

modulus indicating an increase in the composite structural

rigidity. Coupled analysis of the effects listed above

suggests the formation of new products that fill the

composite porous space changing the clayey soil in a

material of a continuum nature, as the traditional

engineering materials made from steel, concrete and wood.

It was found strict obedience to the Mohr-Coulomb rupture

theory used in modeling the soils mechanical behavior. The

survey also demonstrated the ability to reuse waste

considered environmental liabilities as structural conditioner

of clayey soils lending it a sustainable character.

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