Mechanical characteristics of cement stabilised

Road Materials and Pavement Design

ISSN: 1468-0629 (Print) 2164-7402 (Online) Journal homepage: http://www.tandfonline.com/loi/trmp20

Mechanical characteristics of cement stabilised granular lateritic soils for use as structural layer of pavement Dipti Ranjan Biswal, Umesh Chandra Sahoo & Suresh Ranjan Dash To cite this article: Dipti Ranjan Biswal, Umesh Chandra Sahoo & Suresh Ranjan Dash (2018): Mechanical characteristics of cement stabilised granular lateritic soils for use as structural layer of pavement, Road Materials and Pavement Design To link to this article: https://doi.org/10.1080/14680629.2018.1545687

Published online: 22 Nov 2018.

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Road Materials and Pavement Design, 2018 https://doi.org/10.1080/14680629.2018.1545687

Mechanical characteristics of cement stabilised granular lateritic soils for use as structural layer of pavement Dipti Ranjan Biswala∗ , Umesh Chandra Sahoob and Suresh Ranjan Dashb a School of Civil Engineering, KIIT Deemed to be University, Bhubaneswar, Odisha, 751024, India; b School of Infrastructure, Indian Institute of Technology, Bhubaneswar, Odisha, 752050, India

(Received 2 May 2018; accepted 31 October 2018 )

Granular lateritic soils are available in many parts of the world including India and usually this does not meet the specifications for aggregates used in unbound granular layers of pavements. But, with depletion of sources for good quality aggregates, it has become necessary to use such marginal materials in pavement structural layers through proper stabilisation. Material characterisation is very essential in mechanistic design of pavements and therefore an attempt has been made in this study to evaluate the mechanical properties of cement stabilised granular lateritic soils (CLS), which would be useful during mechanistic design of pavements. The characterisation covers determination of unconfined compressive strength (UCS) in soaked and un-soaked state, flexural strength (FS) or modulus of rupture, indirect tensile strength (IDT) and flexural modulus (FM ) through a detailed laboratory investigation process. For carrying out this study, soils were collected from three different sources located in the eastern part of India. Relationships have been established between different mechanical properties, which will help the designers to predict suitable stiffness/modulus values for pavement analysis. Effectiveness of the strength ratio (soaked/un-soaked strength) in evaluating the suitability of CLS as a bound material for pavements has also been discussed in this paper. Keywords: granular lateritic soil; cement stabilised base; unconfined compressive strength; flexural strength; flexural modulus

Introduction Source of good quality aggregates are getting depleted day by day in many parts of the world. Strict environmental regulations has also restricted further expansion of existing stone quarries in developed countries, which has necessitated the use of low quality aggregates or marginal materials in pavement structural layers through suitable stabilisation techniques (Biswal, Sahoo, & Dash, 2016; Disfani, Arulrajah, Haghighi, Mohammadinia, & Horpibulsuk, 2014; Liebenberg & Visser, 2003). In recent years, the fast pace of road construction in India has also necessitated considerable interest among the road administrators to think about the use of sub-standard locally available gravel materials or soils in road construction. Chemical stabilisers such as cement, lime, lime-flyash have been successfully used to stabilise different types of soils and marginal materials that resulted in enhancement of strength, stiffness, durability and workability, thereby improving quality of pavement structure (Liebenberg & Visser, 2003; Xuan, Houben, Molenaar, & Shui, 2012 and Paul, Theivakularatnam, & Gnanendran, 2015). Cement-treated bases (CTB) are in use for many years in the developed countries like USA, China, Australia, Brazil and South Africa etc. (Jitsangiam, Nusit, Chummuneerat, Chindaprasirt, & Pichayapan, 2016; Ma, Gu, & Li, 2015; *Corresponding author. Email: [email protected]

© 2018 Informa UK Limited, trading as Taylor & Francis Group

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Paige-Green, 1998; Paul & Gnanendran, 2016 and Portelinha, Lima, Fontes, & Carvalho, 2012), but very limited applications were reported in India. Granular lateritic soils (locally known as moorum) are abundantly available in many parts of India and are normally used as granular sub-base material in its untreated state subjected to meeting the specifications laid down by Indian Roads Congress (IRC). However, in many cases, it has been observed that these soils fail to meet the specifications in terms of liquid limit, plasticity index and other strength parameters and need to be treated with suitable hydraulic binders for use in pavement structural layers. Presently, there is a sway from empirical to mechanistic empirical (M-E) pavement design approach, where modulus is one of the important input parameters. Considering the necessity of characterisation of CLS, a detailed investigation has been carried out on mechanical characterisation of such materials in this study. It is also important to study the durability characteristics of stabilised materials to be used as structural layers of pavements. Wet-dry durability test or freeze–thaw durability test are common tests to relatively assess the durability of stabilised materials. The mass loss in 12 wet-dry or freeze–thaw cycles is compared to the limiting mass loss stipulated by codes or standards. However, the durability properties of stabilised granular lateritic soils are not within the scope of this paper.

Literature review In twentieth century, some studies have been conducted on characterisation of lateritic soils found in different African countries, Brazil and Australia (Charman, 1988; Fall, Tisot, & Cisse, 1997; Fall, Sawangsuriya, Benson, Edil, & Bosscher, 2008; Gidigasu, 1976; Iyer & Williams, 1997). Geotechnical properties of lateritic soils mostly depend on the parent rock, degree of laterisation, and the nature of cementing or sesquioxide minerals (Bayewu, Olountola, Mosuro, & Adeniyi, 2012; Iyer & Williams, 1997; Lyons, 1971; Fall et al., 1997). Many studies are available on effectiveness of lateritic soil stabilised with cement, lime, flyash, geopolymer or a combination of any two binder in last one decade (Caro et al., 2018; Fall et al., 2008; Joel & Agbede, 2011; Mengue, Mroueh, Lancelot, & Eko, 2017; Portelinha et al., 2012; Ravi, Suresha, & Kashinath, 2008). Important literatures of recent past on stabilised lateritic soil are presented in the following paragraphs. Fall et al. (2008) studied the effect of soil compacity and cement percentage on resilient modulus of treated gravel lateritic soils collected from different locations of Senegal by conducting repeated load triaxial tests. They observed that the percentage of cement is the significant parameter which changes the soil rigidity. The resilient modulus of treated gravel lateritic soils was found to vary between 300 to 1800 kPa for 1% cement, 500 to 2500 kPa for 2% cement and 400– 5600 kPa for 3% cement. The material constants of resilient modulus models, such as Andrei (Andrei, 1999) model and Uzan-Witczak (Witchzak & Uzan, 1988) model, significantly depends on the percentage of cement. Recently, Mengue et al. (2017) studied improvement of mechanical properties of fine-grained lateritic soil of Cameroon and concluded that 3% and 6% cement is sufficient to obtain acceptable mechanical performance of stabilised lateritic soil to be used as sub-base and base layer respectively. Joel and Agbede (2011) showed that use of sand in addition to cement results in economic stabilisation of a poorly graded lateritic soil of Ikpayongo, Nigeria to make it suitable for base layer. A mixture of 45% sand and 6% cement with lateritic soil results in an increase of 7-day UCS by 270% and the mixture satisfy the provision of Nigerian general Specification in terms of CBR (i.e. 180%). Portelinha et al. (2012) demonstrated the effectiveness of low dosage of lime and cement in enhancing workability of a fine-grained lateritic soil of Vicosa, Brazil. They found that highest modification in terms of PI occurs at 3% cement. XRD analysis showed


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Table 1. Criteria in terms of UCS for suitability of stabilised sub-base and base layers in pavements. UCS (MPa) References

Medium to high volume roads

Low volume roads

NCHRP (2004)

1.72 for sub-base 5.17 for base 2 1.5–3 for sub-base 4.5 for base

1.72 for sub-base 5.17 for base 1–2

Austroads (2012) IRC (2012) IRC (2015)

3 for base 1.7 for sub-base

that the addition of cement and lime results in the formation of calcium cilicate hydrate mineral rankinite. Similarly, a study on Indian lateritic soil by Ravi et al. (2008) reported that 4% cement with Pondash resulted in 7-day UCS of 1 MPa, which is 440% improvement over untreated lateritic soil. An experimental work done by Caro et al. (2018) showed that water content affects significantly the resilient modulus of untreated and treated poorly graded sandy lateritic soil of Colombia. Caro et al. (2018) suggested a minimum cement dosage of 6% for better field performance. In addition to the compressive strength, evaluation of tensile or flexural strength of bound materials is also necessary as the failure of the CTB layer is largely governed by its flexural strength. As flexural strength and tensile strength of any material is closely related, indirect tensile tests and flexural beam tests can be used to determine the tensile strength of the stabilised materials (Paul et al., 2015). However, four-point flexural beam test is one of the most preferred test method as the test conditions simulate the stress or strain gradients developed in a bound layer of a pavement (Wen et al., 2014; Freeme, Maree, & Viljoen, 1982; Austroads, 2012; Yeo, Jitsangiam, & Nikraz, 2011 and Paul et al., 2015). The default value of MOR of soil cement and cement-treated aggregate is taken as 0.69 MPa corresponding to minimum 28 days UCS value of 5.17 MPa (NCHRP, 2004). However, due to the simplicity of the test procedure, unconfined compressive strength is widely used as a criterion for examining the suitability of bound material in pavements (Chakrabarti & Kodikara, 2003; Paige-Green, 1998 and Yeo et al., 2011). Some of these UCS criteria specified by various codes of practices are provided in Table 1. Strength parameters in terms of UCS, FS and IDT are usually determined under static loading, whereas the determination of modulus is done under cyclic or dynamic loading from flexural beam tests. In this paper, the term ‘modulus’ refers to the flexural modulus of CLS. Very few studies are available on flexural properties of cement stabilised lateritic soils. Many studies have tried to establish relationships between UCS and FM for cemented materials, some of which are presented in Table 2. Paul and Gnanendran (2016) conducted flexural tests on lightly stabilised materials to determine the static modulus, dynamic modulus and modulus of rupture. They observed that dynamic modulus is approximately twice of the static modulus and approximately 1000 times the modulus of rupture. They determined the dynamic modulus with respect to the 50% stress ratio unlike Yeo (2008) and Paul et al. (2015), who have proposed to determine the modulus at 30% stress ratio. Piratheepan, Gnanendran, and Lo (2010) carried out investigations on slag-lime stabilised granular materials by conducting compression testing, monotonic IDT and cyclic IDT testing with internal displacement measures. Reliable correlations were developed in their study to predict IDT, static stiffness modulus and dynamic stiffness modulus from UCS values. Liebenberg and

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Table 2. Established relationships between modulus and UCS for cemented materials as per literature. References

Relationships

NCHRP (2004)

E = 1200 × UCS

Austroads (2012)

Eflex = k × UCS

Wen et al. (2014)

Ef = 131.08 × UCS + 62382

Description E = Modulus of elasticity: UCS = 28-day Unconfined compressive strength in MPa as per ASTM 1633 Eflex = Flexural modulus: K = 1000–1250, UCS = Unconfined compressive strength in MPa of field beams at 28-day moist curing Ef = Flexural modulus in psi: UCS = Unconfined compressive strength in psi

Visser (2003) studied cement stabilisation of marginal ferricrete, milled from the sub-base layer of an existing road and determined elastic modulus by conducting dynamic triaxial tests. The average elastic modulus of cement-treated ferricrete was found to be 2100 MPa as compared to the elastic modulus of 600 MPa of untreated frerricrete. Significant improvement in the strength of cement-treated ferricrete led them to conclude that these materials can be used for base and sub-base layers of pavement for low to medium volume of traffic. It can be observed that most of the studies on cement stabilised materials have been conducted with good quality aggregates or crushed stones. The mechanical properties reported in these literatures are more appropriate for similar materials and should not be adopted for other stabilised materials using sub-standard or marginal aggregates like granular lateritic soils. Very few studies were found on characterisation of CLS and also, geotechnical properties of lateritic soils largely depend on the geography of formation. Therefore, there is a need to study the strength and stiffness properties of stabilised granular lateritic soils available in the Indian region for subsequent use in mechanistic design of pavements.

Material and methods Experimental programme adopted in this study for mechanical characterisation of CLS are described in the following paragraphs that includes materials, sample preparation and testing. Materials Lateritic soil Granular lateritic soils were collected from three different sources (denoted as A, Band C) located in the eastern part of India as shown in the Figure 1. Sufficient care was taken to take the representative samples from a reasonable depth of the quarries. The relevant physical and engineering properties of untreated granular lateritic soils are presented in Table 3 and chemical compositions are summarised in Table 4. Gradation curve of all three soils is presented in Figure 2. According to the Indian soil classification system, the soils A and B are classified as silty sand (SM) and soil C is classified as silty clay (SC). As per AASHTO (2011), soils A and B can be classified as A-2-7 and soil C as A-2-6. Cement Ordinary Portland cement of grade 43 was used for stabilisation of granular lateritic soil, whose chemical compositions are given in Table 4.


Figure 1.

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Map showing location of collected granular lateritic soil samples.

Table 3. Physical and engineering properties of the lateritic soils. Lateritic soil Property Gravel Sand Silt and clay Specific gravity Liquid limit (LL) plastic limit (PL) Plasticity index (PI) Natural moisture content Optimum moisture content (OMC) Maximum dry density (MDD) Classification (as per USCS)

Unit

A

B

C

Standard methods

% % % – % % % % % kN/m3 –

30.15 47.49 22.36 2.71 48.25 31.58 16.67 2.2% 12.5 19.4 SM

37.65 43.35 19 2.78 41.25 28.4 12.85 2.5% 10.7 22.1 SM

45.48 39 15.52 2.83 36.5 20.61 15.89 1.8% 9.6 21.5 SC

ASTM D2487–11 (2011) ASTM D854–14 (2014) ASTM 4318-17 (2007)

ASTM D1557 (2012)

In order to maintain consistency among the prepared samples, the collected lateritic soils were initially separated based on their particle size gradation and again mixed in a particular mix proportion which is very close to the gradation of the parent material. The gradation of the reconstituted samples remained same throughout the testing regime to have uniform samples for all associated tests. Figure 2 presents the gradations of all three samples. The gradation and plasticity characteristics of these soils suggest that both cement and lime can be used for stabilisation (IRC, 2010). However, cement has been chosen in this study for stabilisation of granular lateritic

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D.R. Biswal et al. Table 4. cement.

Chemical compositions of granular lateritic soils and Lateritic soil

Figure 2.

Constituents

Soil A

Soil B

Soil C

Cement

Fe2 O3 Al2 O3 SiO2 MgO K2 O CaO TiO2 CuO SO2

21.6 18.59 50.32 0.95 2.1

32.49 14.26 44.05 1.02 1.34 1.03 2.97

8.47 12.51 64.06 0.76 1.69 < 1% 1.36

1.67 4.41 16.33 5.49 1.09 67.99

2.49

< 1% < 1%

Gradation curve of collected granular lateritic soil.

soils because of its wide availability and the lime of desired purity was not available within an economical distance. Sample preparation Modified Proctor tests were conducted on the untreated soils and also for soils mixed with cement in varying dosage (ranging from 2% to 10%) to determine the compaction parameters as per ASTMD 1557 (2012). It was observed that cement content has a little positive effect on optimum moisture content (OMC) whereas the effect on maximum dry density (MDD) was inconclusive which can be observed from Figure 3. OMC determined from modified Proctor test against various cement content were used to develop a relationship and the OMC determined from this relationship for each cement content were later used for sample preparation throughout the testing regime. The lateritic soils were stabilised by mixing cement in the proportions of 2, 3, 4, 6, 8 and 10% by dry mass of the soil. Initially, predetermined amount of air-dried soil and cement were mixed thoroughly to ensure homogeneity of mix and then, a calculated amount of water was added to the mix prior to sample preparation for any test. For unconfined compressive strength test, cylindrical samples of size 100 mm diameter and 115 mm height were prepared using a split mould


Figure 3.

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Variation of OMC and MDD with cement content of soil A.

to ensure that the samples were not disturbed during extraction. Samples were compacted in 5 layers with 25 blows to each layer similar to that of modified compaction test, to achieve the dry density equal to maximum dry density. A small part of the sample (soil + cement + water) was kept for determination of its moulding moisture content. The prepared specimens were wrapped with polythene sheets in order to avoid moisture evaporation after extraction and were kept for curing for specified periods. Twelve cylindrical specimens were prepared at each cement content for UCS testing after 7-day and 28-day curing periods. Similarly, cylindrical specimens of size 100 mm diameter and 60 mm height were prepared and cured for a period of 28 days for indirect tensile strength test. Superpave gyratory compactor was used for the preparation of IDT specimen instead of compaction by rammer. Flexural beam specimens of size 75 mm × 75 mm × 285 mm were prepared according to ASTM D 1632-07 (2007) and cured for 28-days for the flexure test. The detail of test procedures, specimen shape, size, curing periods and number of tests have been presented in Table 5.

Tests Unconfined compressive strength test In most of the specifications for road works, for stabilised base or sub-base, 7-day UCS values are normally used as guiding criteria for use in flexible or semi-rigid pavements (Yeo et al., 2011). However, as per Australian standard (AS 5101.4, 2008), UCS testing is conducted after 28 days of curing followed by 4 hours of soaking in water. Therefore, in this study, the UCS tests were conducted on CLS samples after 7 days and 28 days of curing period for both soaked and un-soaked specimens. Monotonic UCS tests (Figure 4(a) shows the test setup) were conducted using a universal testing machine (UTM).

Indirect tensile strength test Many researchers (Foley and Group, 2001 and Piratheepan et al., 2010) have used indirect tensile tests to determine the modulus and fatigue life of stabilised granular materials in addition to the determination of indirect tensile strength. However, in this study, only monotonic IDT tests were conducted to determine the IDT values of stabilised materials, as suggested by Yeo (2008) with certain modifications. The specimens were tested (Figure 4(b) shows the test setup) after 28 days of curing for un-soaked specimen.

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Table 5.

Test procedure selected for evaluation.

Properties

Tests to be conducted

Specimen shape

Specimen size

Unconfined compressive strength

UCS test

Cylinder

diameter-100 mm, height-115 mm

Flexural strength

Four-point flexural beam test IDT test

Beam sample

75 × 75 × 300

Cylinder

diameter-100 mm, height-60 mm 75 × 75 × 300

Indirect tensile strength Flexural modulus

Cyclic flexural beam test

Beam sample

Curing periods

Cement content

Number of test

2, 3, 4, 6, 8, 10

3 × 4 × 6 = 72

3, 4, 6, 8, 10

3 × 5 = 15

28 days

3, 4, 6, 8, 10

3 × 5 = 15

28 days

3, 4, 6, 8, 10

3 × 5 = 15

7 days, 7 days + 4 hours soaking, 28 days, 28 days + 4 hours soaking 28 days


Figure 4.

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(a) Unconfined compressive strength test (b) indirect ensile strength test.

Flexural strength test Flexural beam testing was conducted in two stages: (1) monotonic flexure test to determine flexural strength and (2) cyclic flexure test to determine the flexural modulus. Flexural strength or modulus of rupture is the maximum tensile stress before failure, measured at the bottom fibre of the beam sample which was determined using the following equation: FS =

Pl , bd2

(1)

where FS is flexural strength (MPa); P is applied load at failure in kN; l is centre to centre length between supports; b is width and d is height of specimen. Support span of samples in monotonic flexure test was kept 225 mm (l) and the loading span was 75 mm (l/3). The flexure test was conducted in stress controlled mode with a constant stress of 690 kPa/min as specified in ASTM D 1635 (2012). Flexural modulus test Determination of flexural strength is a prerequisite for conducting cyclic flexural test at a particular stress ratio. Stress ratio (SR) is defined as the ratio of applied bending stress to the flexural strength of the material. Thus cyclic flexure test (Haversine loading pulse at a frequency of 1 Hz) was conducted in stress controlled mode using a servo hydraulic loading machine. A total of 100 loading cycles were applied to the specimen, out of which first 50 cycles were used for conditioning of the specimen and the last 50 cycles were used to determine the flexural modulus. Two linear variable differential transformers (LVDTs) were used to measure the central deflection of the beam. Though the cyclic flexure test was conducted at various stress ratio of 0.3, 0.4 and 0.5, modulus was determined corresponding to 30% stress ratio (Wen et al., 2014; Mandal, Tinjum, Gokce, & Edil, 2015 and Yeo, 2008) using the following equation: Ef =

23Fl3 , 108bd3 δd

(2)

where Ef = flexural modulus in MPa; F = applied cyclic load; δ d = vertical deformation at midpoint.

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Figure 5.

Four-point flexure test arrangement.

The modulus is usually determined at a low range of stress ratio, so that the specimen is not damaged. Arrangement for flexural modulus test is shown in Figure 5.

Results Mineralogical characterisation of cemented lateritic soil X-ray diffractograms of untreated and cemented granular lateritic soil samples are shown in Figure 6(A), (B) and (C). Analysis of this diffractograms indicates that major minerals before cement treatment (Figure 6(A)), are kaolite, quartz, hematite and goethitite. However, cement addition has resulted in the development of hydration products which is evident from Figure 6(B) and (C). Tricalcium silicate and dicalcium silicate present in portland cement reacts with water to form calcium silicate hydrate (CSH) gel and calcium hydroxide [Ca(OH)2 ]. Ca(OH)2 present in soil water reacts with pozzolana to form additional cementing material containing CSH. The XRD plots of cement stabilised powdered samples (Figure 6(B) and (C)) indicates the presence of minerals like tobermorite, calcite, portlandite and rankite which are formed due to hydration of cement and pozzolanic reactions. Portlandite and calcilte enhances the basic properties of soil (Mengue et al., 2017). CSH minerals like tobermorite results in the enhancement of strength of cemented granular lateritic soil. The microstructure of untreated and cemented (8% cement) lateritic soil is shown in Figure 7. Figure 7(A) shows the quartz particles coated with fine particles of kaolinite. Figure 7(B) shows the presence of rod-like crystalline structure such as ettringite which is formed as a result of cement hydration reactions. Honeycombing matrix-like structure can be observed on a higher zoom SEM image of cemented lateritic soil as shown in Figure 7(C). This matrix structure


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Figure 6. X-ray diffraction of untreated and cement treated lateritic soil: (a) untreated; (B) 6% cement (C) 8% cement.

provides the mechanical strength against sliding of the particles subjected to shear force. The strength and stiffness of cemented granular lateiritc soil are presented in the following sections. Strength and stiffness properties of CLS Unconfined compressive strength of CLS Figures 8–10 represent UCS values after 7 days and 28 days of curing with varying cement content for soils A, B and C respectively. Strength ratios (i.e. ratio of soaked to un-soaked UCS values denoted by Rucs ) for all three soils after7 days and 28 days curing with varying cement content are presented in Figures 11 and 12 respectively. It may be observed that strength ratios increase with increase in cement content. The values increased from 0.15 to 0.4 or 0.5, with the increase in cement content from 2% to 6% for all three soils and no remarkable change was noticed after 6% of cement dosage. Therefore, strength ratio can be used as a better indicator of the bound nature of stabilised samples. This indicates that cement stabilised lateritic soils becomes a stable stabilised material (i.e. adverse effect of moisture do not increase) beyond 6% cement. The average 7-day strength ratios at 6% cement content were found to be 0.4, 0.42 and 0.5 for soils A, B and C, respectively. From Figures 11 and 12, it may be observed that there is no remarkable difference in the strength ratio of 7-day cured and 28-day cured samples with cement content in the range of 2%

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Figure 7. Scanning electron microscopy of untreated and cemented granular lateritic soils: (A) untreated; (B) 8% cement; (C) 6% cement (honeycomb structure).


Figure 8.

UCS after 7-day and 28-day curing with varying cement content for soil A.

Figure 9.

UCS after 7-day and 28-day curing with varying cement content for soil B.

Figure 10.

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UCS after 7-day and 28-day curing with varying cement content for soil C.

to 6%. However, for cement content in the range of 6% to 10%, the 28-day strength ratio was found to be more than the 7-day strength ratio. For example, 28-day strength ratios (Rucs28d ) at 8% cement are 0.6, 0.65 and 0.7 as compared to 7-day strength ratio (Rucs7d ) of 0.4, 0.41 and

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Figure 11.

Seven-day UCS strength ratio with varying cement content.

Figure 12.

Twenty-eight-day UCS strength ratio with varying cement content.

Figure 13.

Relationship between 7-day UCS and 7-day strength ratio.

0.55 at the same cement content for the soils A, B and C, respectively. Figure 13 presents 7-day strength ratio vs. 7-day UCS for all three soil samples. It may be observed that a 7-day minimum UCS value of 3.25 MPa ensures suitable stabilised material that possess sufficient strength in the presence of water.


Figure 14.

Relationship between 7-day and 28-day UCS values.

Figure 15.

Flexural strength of the CLS samples at varying cement content.

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From the present study, it was found that the emersion of stabilised lateritic soil samples in water for 4 hours results in decrease of strength by more than 40% to 50%, which confirms that the strength of cement stabilised granular lateritic soils is very sensitive to moisture level even after 7-day or 28-day curing although moluded at OMC, unlike the cement concrete. Therefore, the authors feel that the suitability of stabilised granular soils should be based on soaked strengths rather than un-soaked values to be conservative in design, because the strength of stabilised granular lateritic soil decreases remarkably after immersion in water. It can be observed from Figure 13, that the strength ratio corresponding to 7-day unsoaked UCS of 4.5 MPa is 0.4. Hence, instead of using un-soaked UCS in design, 7-day soaked UCS of 1.8 MPa is recommended (corresponding to un-soaked UCS of 4.5 MPa as per IRC (2012) with strength ratio of 0.4), as criteria to define the suitability of a CLS for base layer. However, as per NCHRP (2004), using the same principle, 7-day soaked UCS of 2.06 MPa may be specified as the requirement for CLS as base layer. Using the results of this study, an attempt has also been made to establish a correlation between 28-day UCS and 7-day UCS (Figure 14). This relationship, given by Equation (3) has a good coefficient of determination (R2 ) of 0.90. This equation is valid for the 7-day UCS value ranging between 1 to 5.5 MPa. As the 28-day UCS is more often used to predict the modulus of stabilised materials for mechanistic design of pavements, this relationship will be useful to determine the

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D.R. Biswal et al.

Figure 16.

Indirect tensile strength of the CLS samples at varying cement content.

Figure 17.

Relationship between UCS and FS for CLS.

28-day strength from 7-day UCS results. UCS28d = 0.94 × UCS7d + 1.73.

(3)

Flexural strength and indirect tensile strength of CLS Though, UCS is the major parameter for assessing the suitability of stabilised materials, other strength parameters such as flexural strength (FS) and indirect tensile strength (IDT) are also equally important for evaluating the performance of stabilised materials. FS and IDT of the CLS at various cement contents are presented in Figures 15 and 16 respectively. From the figures, it may be observed that FS varies from 0.4 to 1.37 MPa for soil A, 0.7 to 1.52 MPa for soil B and 0.33 to 1.05 MPa for soil C with the cement content in the range of 3% to 8%. A strong correlation (with an R2 value of 0.81) given by Equation (4) was established between FS and UCS from the characterisation of all three CLS samples (see Figure 17). FS = 0.235 × UCS − 0.265

(4)

The FS values predicted using the correlation developed under this study [Equation (4)] are very close to the values estimated using other similar relationships proposed by Wen et al. (2014)


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Figure 18.

Comparison of relationships between UCS and FS.

Figure 19.

Relationship between indirect tensile strength and unconfined compressive strength for CLS.

and Mandal, Edil, and Tinjum (2017), which can be observed from Figure 18. However, these values are less than that obtained by PCA model (1992), which was primarily developed for cement concrete. A good correlation represented by Equation (5) was also observed between IDT and UCS and the relationship between these two parameters is plotted in Figure 19. Another good correlation (with R2 = 0.89) was also established between IDT and FS as given by Equation (6). The relationship is plotted in Figure 20. IDT = 0.123 × UCS − .168 R2 = 0.90

(5)

IDT = 0.486 × FS R2 = 0.89

(6)

In addition to the flexural strength, flexural braking strain is also used in some fatigue models (e.g. Freeme et al., 1982). Therefore, an attempt has also been made to study the relationship between flexural strength and flexural braking strain ( f ). Due to the uncertainty in the measurement of braking strain at the point of failure, the bending strain corresponding to 95% of the failure load (which was obtained from four-point static flexure test), was considered as the flexural braking strain in this study (Litwinowicz & Brandon, 1994). The braking strain of soil samples A and B varied in the range of 4000–8000 microstrains, whereas for soil C, it was found

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Figure 20.

Relationship between indirect tensile strength and flexural strength for CLS.

Figure 21.

Relationship between flexural braking strain and flexural strength for CLS.

to be in the range of 700–3500 microstrains. When the flexural strain data of all three soils were aggregated for regression analysis, a weak correlation was observed between flexural strength and flexural braking strain. The relationship between flexural braking strain and flexural strength for each soil is shown separately in Figure 21. Breaking strain increase with an increase in flexural strength upto a certain value and decreased thereafter. However, further studies are required to conclude any relationship between these flexural properties and the effect of other soil parameters on flexural braking strain.

Flexural modulus of CLS Flexural modulus (FM ) values were determined from cyclic flexure tests conducted at stress ratios of 0.3, 0.4 and 0.5 for all the three soil samples. It was observed that flexural modulus increases with an increase in stress ratio (Figure 22), which is in conformation with the study by Yeo (2008) and Biswal, Sahoo, and Dash (2017). It was suggested by him that the modulus corresponding to a stress ratio of 0.3 should be considered as flexural modulus of any sample and the same has been adopted here. FM of all the soils at various cement contents is presented in Figure 23. It may be observed that flexural modulus increases with an increase in cement content. The FM values for CLS were found to be in the range of 400 MPa to 2275 MPa with cement content in the range of 3% to 10%.


Figure 22.

Flexural modulus of cement-treated soil B at different stress ratios (0.3, 0.4 and 0.5).

Figure 23.

Flexural moduli of CLS at varying cement content.

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The relationship between FM and UCS [given by Equation (7)] is presented in Figure 24 and the relationship between FM and FS [given by Equation (8)] is presented in Figure 25. FM = 367.7 × UCS − 467.9,

(7)

FM = 1488 × FS.

(8)

The determination of flexural modulus is a tedious process that requires cyclic loading machine and flexural test arrangements. Therefore the relationships established under this study will be helpful in predicting the modulus and other strength parameters from basic soil test. Figure 26 presents a comparison of predicted flexural modulus determined from unconfined compressive strength using model developed in the present study (Equation (7)) with those models cited in the literature for cement stabilised materials. It may be observed from Figure 26 that, the modulus predicted from the model developed in the present study is very close to the values predicted by Wen et al. (2014) who studied the effect of stabilisation with cement and lime on clay, silt, sand and gravels. However, the values predicted by NCHRP (2004) and Austroad (2012) are much higher than those obtained from this study. It may be due to the fact that most of the other models were developed using the results of cement-treated good quality graded aggregates. As per IRC (2012), minimum 7 day UCS

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D.R. Biswal et al.

Figure 24.

Relationship between UCS and flexural modulus for CLS.

Figure 25.

Relationship between flexural strength and flexural modulus for CLS.

Figure 26.

Relationship between flexural modulus and unconfined compressive strength for CLS.

required for stabilised base is 4.5 MPa. Using Equation (3), 7-day UCS of 4.5 MPa corresponds to 28-day UCS of 6 MPa. Thus, using the minimum 28-day UCS and Equation (7), an average flexural modulus of 1800MPa is recommended for use of pavement design having stabilised


21

granular lateritic soil. Therefore, it is very important to use appropriate models for the type of material being used for construction of stabilised bases.

Conclusions A detailed laboratory investigation was carried out to study the mechanical properties of cement stabilised granular lateritic soils available in the eastern part of India. The findings are summarised below: • Mineralogical alterations in the cement-treated lateritic soil samples as compared to the untreated lateritic samples in terms of formation of CSH mineral tobermorite results in enhancement of strength. • Unlike cement concrete, the strength of cement stabilised granular lateritic soils are very sensitive to the moisture content after a curing period of 7 days to 28 days, even though moluded at OMC. • The criteria for judging the suitability of CLS should be based on soaked UCS values rather than un-soaked values. A minimum 7-day un-soaked UCS value of 3.25 MPa ensures a more stable stabilised material in the presence of water. • Minimum 7-day soaked UCS of 1.8 and 2.06 MPa are recommended as criteria to find the suitability of stabilised granular lateritic soil to be used as a base layer as per IRC (2012) and NCHRP (2004) respectively. • From the correlations established in this study, a very good relationship was established between 28-day UCS and 7-day UCS for CLS samples. • UCS strength ratio after 7 days of curing increases with cement content upto 6% cement (approx.) and then the rate of increase diminishes. • Strong correlations were established between UCS-FS, UCS-IDT and IDT-FS for cement stabilised granular lateritic soils. No conclusive relationship was observed between flexural strain and flexural strength. • For cement stabilised granular lateritic soils, a flexural modulus of 1800 MPa is recommended as obtained from the present study. This is less than the presumptive modulus values predicted from the models specified by NCHRP(2004) and Austroad (2012) corresponding to the specified UCS values for base layer. This may be attributed to the type of soil/aggregates used for stabilisation. Acknowledgements The authors would like to acknowledge the SERB, Department of Science and Technology, Govt. of India for providing the financial support for carrying out the research work.

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