Soil and Aggregate Stabilization for Sustainable Pavement

Satander Kumar, Ex-Scientist CRRI, Consultant, New Delhi Anukul Saxena, Senior Engineer, Alchemist Touchnology. Ltd., New Delhi

In Pavement Design and Construction, there is an urgent need of economical and innovative Technology that reduces energy consumption and Green House Gas (GHG) emission apart from providing sufficient strength and durability to pavement. Soil and aggregate Stabilization is one of the solutions for obtaining sustainable pavement and economically mitigating the critical issues such as energy consumption and GHG emissions. Stabilization with RBI Grade 81 is one such solution. It is a natural inorganic material that is used in aggregate and soil as pavement material and soil Stabilizer. Soil or aggregate can be stabilized with RBI grade 81 and then can be used in Subgrade and also as Sub base and base Layers.

This paper presents and analyses various lab test results of RBI Grade 81 with five different type of soils, mixture of Black cotton and stone dust and a mixture of 10mm down Aggregate and stone dust to prove its suitability as a soil Stabilizer and Pavement material in Sub base and Base layers.

Introduction

‘Sustainability’ is becoming a central concern for all. It is a concern that has grown out of wider recognition that rising population and economic development are threatening a progressive degradation of the earth’s resources and polluting environment.

In Pavement Design and Construction, there is an urgent need of economical and innovative Technology that provides sufficient strength and durability to pavement and also reduces energy consumption and Green House Gas (GHG) emission. Presently, there is lot of energy consumption and GHG emission in the manufacture of pavement material as well pavement construction. Soil and aggregate Stabilization is a solution for obtaining sustainable pavement and economically mitigating the critical issues such as energy consumption and GHG emissions.

Stabilization with RBI Grade 81 is one such solution. It is a natural inorganic material that is used with aggregate and soil as pavement material and soil Stabilizer. Soil or aggregate can be stabilized with RBI grade 81 and then can be used in Subgrade and also as Sub base and base Layers. The whole Pavement can be constructed by using RBI grade 81 thus reducing energy consumed and GHG emissions in manufacturing and placing of unbound granular material (WBM/WMM) and bituminous bound granular material (DBM/BM) without compromising on Strength and durability.

This paper presents and analyses various lab test results of RBI Grade 81 with five different soils, mixture of Black cotton and stone dust and a mixture of 10mm down Aggregate and stone dust to prove its suitability as a pavement material and soil stabilizer. Firstly CBR and PI lab test are conducted with different low CBR soils to prove its suitability as Subgrade Stabilization and then PI, CBR and UCS lab test are conducted with Black Cotton soil and Black Cotton soil mixed with different percentage of Stone dust to verify its suitability in Sub base Layer. Compressive Strength and Modulus of Elasticity test are conducted with aggregate mix to verify its suitability in Base Layer. Lab Test Results verifies that RBI grade 81 can be used in Stabilizing Subgrade as well as Sub Base and Base.

Layers in Pavement. RBI Grade 81 suitability to stabilize Sub Base and base layer are proved by comparing the strength requirement in Design specifications of different countries.

Objective

The Objective of this paper is to prove by lab test results that:

1. RBI Grade 81 can be used for Subgrade Stabilization.
2. RBI Grade 81 stabilized mixtures of very weak soil like Black cotton soil and Stone dust is suitable for Sub base Layer.
3. RBI grade 81 Stabilized mixtures of 10mm down aggregate and Stone dust is Suitable for base Layer.

Subgrade Stabilization with RBI Grade 81

Four different types of soils are tested with RBI Grade 81 to check its suitability in Subgrade Stabilization. As the Subgrade stabilization is done in most of the cases when the Subgrade CBR strength is low hence the four soil samples are selected having CBR in the range of 2% to 4%.

The details of the soil samples are as follow:

• Soil A: This soil is taken from Dadri, Uttar Pradesh at Ansal API site. The gradation of the Soil is 0% Gravel, 17% Sand and 83% Silt and Clay.
• Soil B: This soil is taken from Mohali, Punjab at Ansal site. The gradation of the Soil is 10% Gravel, 38% Sand and 52% Silt and Clay.
• Soil C: This soil is taken from Noida, Uttar Pradesh at 3C site. The gradation of the Soil is 14% Gravel, 34 Sand and 52% Silt and Clay.
• Soil D: This soil is taken from Jajjhar Bye Pass Haryana, near the site of State Highway. The gradation of the Soil is 1% Gravel, 55% Sand and 44% Silt and Clay.

The samples for CBR are first cured for 7 days and then soaked for 4 days. The cured samples are tested for CBR as per IS 2720, part-XIV. Lab Test Results are shown in Table 1.


In Soil A PI is reduced to 5.7 from 10.5 and in Soil B PI is reduced to 4.8 from 6.4 with 2% RBI Grade 81, hence in both the cases PI

In Soil A CBR is increased 17.5 times with 2% dosage, 27.3 times with 3% dosage and 37 times with 4% dosage.

In Soil B CBR is increased 4 times with 2% dosage, 10.5 times with 3% dosage and 17.5 times with 4% dosage.

In Soil C CBR is increased 8.5 times with 2% dosage, 9.5 times with 3% dosage and 17 times with 4% dosage.

In Soil D CBR is increased 11.6 times with 2% dosage, 27.6 times with 3% dosage and 31 times with 4% dosage.

In Soil A the increase of the CBR is the highest. Soil B and Soil C shows nearly same rate of increase of CBR for 3% and 4% dosage of RBI Grade 81.On an Average considering these four soil samples the increase in CBR with dosage of 2% RBI grade 81 is 10 times, with dosage 3% 19 times and with 4% 26 times.

The trends of reduction in PI and increase in CBR for all the soil samples are shown in Fig 1 and Fig 2 respectively.

RBI Grade 81 Suitability in Subbase Layer

For checking the suitability of RBI Grade 81

Stabilized materials in a sub base layer, a soil sample of low CBR (2%), Black cotton soil is chosen for the lab test. Black cotton soil sample is taken from jajjhar district of Haryana for testing. The gradation and the properties of the soil are given in Table 2.

CBR test was conducted as per IS 2720, part-XIV and UCS test are conducted as per IS 2720, part–X. The Samples are cured for 7 days and soaked for 4 days and UCS samples are cured for 7 days.

The Results of PI, CBR and UCS for Black Cotton Soil stabilized are sown in Table 3.

In order to further reduce the PI and increase CBR and UCS a sample of Black Cotton soil mixed with some percentage of stone dust is stabilized with RBI Grade 81 to make the material more suitable to act as a Sub Base layer. Two different mixtures are chosen for testing. The mixtures are 80% Black Cotton Soil + 20% Stone dust and 70% Black Cotton Soil + 30% Stone dust

The Results of PI, CBR and UCS for both the mixtures are shown in Table 4 and Table 5 respectively.


CBR and UCS values are increased considerably and PI values reduced with the addition of different dosages of RBI Grade 81 in all the three different samples. With 6% dosage PI is reduced from 27.3 to 7.5 in Black Cotton soil, reduced to 6.5% in 80% Black Cotton Soil + 20 % Stone dust and reduced to 4.6 % in 70% Black Cotton Soil + 30 % Stone dust.

The graphs of behavior of each of the parameter values i.e. PI, CBR and UCS with different dosage of RBI grade 81 for all the three test sample are shown below in Fig 3, Fig 4 and Fig 5.


In all the three tested samples, the minimum value of CBR is 40 for Black Cotton soil with 3% dosage and minimum value of UCS is 2.1 MPa for 80% Black Cotton soil + 20% Stone dust with 3% dosage. IRC code strength criteria are verified against these minimum values.

As per IRC: 51-1992 “Guidelines for the use of Soil-Lime Mixes in Road Construction”, Minimum CBR value for the lime stabilized sub base should be 15% for low trafficked rural roads, 20% for cumulative traffic up to 2 Msa and 30% for traffic exceeding 2 Msa.

This strength criterion of CBR is satisfied by the stabilized samples as the minimum CBR is 40%. For the UCS, the lime stabilized soil used for sub base should have a strength 700 KN/sq m (0.7 MPa), this strength criterion of UCS is satisfied by the stabilized samples as the minimum UCS is 2.1 MPa. Hence RBI grade 81 Stabilized soil layer can be used as Sub Base layer in the pavement.


By analyzing the figure 4, 5 and 6 it can be concluded that 70% Black cotton Soil + 30% Stone dust is the best sample mixture to be used as Sub Base layer. PI is reduced to less than 6 in this sample and the average increase of CBR and UCS are maximum.RBI Grade 81 dosage of 4% and 5% is best as it is economical apart from high Strength. The results of this composition are compared with Design specifications of different Countries.

As per USA practice (AASHTO 1993), it is expected to have a 7 day strength of at least 2 MPa for Stabilized layer. The selected sample satisfies this criterion with values of 3.3 MPa and 3.6 MPa at 4 % and 5% of RBI Grade 81 respectively.

As per South Africa Manual TRH 14, 1985 (Guidelines for Road Construction Materials), there are four classes of Stabilized material C1-C4, where C1 is the strongest. The Specifications limits become less strict as the material is used further below the road surface. C1 materials are seldom used because of their tendency to form wide shrinkage cracks. Material class C2 is used for high Quality Sub base. The lower strength materials C3 and C4 are used for lower layers or for bases on low volume roads. The table 6 shows the UCS Values suggested for Stabilized materials

According to the above table C2 material classification UCS Strength requirement is satisfied by the selected sample. Hence according to the South African manual RBI grade 81 stabilized layer is considered as high quality sub base and also the other RBI grade 81 tested samples satisfies Strength requirement of C3.

As per Philippines Standard specifications for Public Works and Highways, Volume 2 (DPWH, 1995), for fine textured soils, UCS test seven day compressive strength should be minimum of 2.1 MPa. This Strength requirement is also satisfied by the selected sample and also other samples tested with RBI Grade 81.

As per UK specification TRL ORN31 design guide, the Stabilized materials recommended are road base (CB1 and CB2) and Sub Base (CS), with unconfined compressive strength (UCS) values as shown in Table 7

The Strength requirement of CB1 is satisfied by the selected sample and also the other RBI grade 81 tested samples satisfies Road base Strength requirement of CB2. Moreover, according to this design guide for stabilized sub Base material, the CBR test can be used as an alternative to the UCS requirement. A minimum value of CBR 70% after 7 days moist curing is recommended. This Strength requirement is also satisfied by the selected sample.

Hence RBI Grade 81 stabilized layer can be used as sub base layer in a pavement.

RBI Grade 81 Suitability in Base Layer

For checking the suitability of RBI Grade 81 stabilized materials as a base layer, compressive Strength test are performed on a sample of mixture of 50% 10mm down aggregate + 50% Stone dust. Dosage of 4%, 6% and 8% of RBI grade 81 by weight of the sample are used in this testing.

Reason to select 50% 10mm down aggregate and 50% Stone dust composition is to have a dense and strong mixture with lot of fines for the reaction to take place with RBI Grade 81 .The other reason is to also take into account the effect RBI Grade 81 with marginal aggregate as the size of aggregates selected in sample mixture are 10mm down along with quarry dust or stone dust.

Three samples are tested each after 7 days, 14 days and 28 days for each dosage. The Test Results are shown in table 8.

The trend of increase of compressive strength is shown in Fig 6

Compressive Strength of sample increases with increase in number of days of curing and dosage of RBI Grade 81.

Elastic Modulus test are conducted in order to check whether RBI grade 81 Stabilized Aggregate mixture layer act as flexible or Rigid, so that if the Modulus of Elasticity is high the pavement consisting of RBI Grade 81 Aggregate Stabilized layer and Bituminous layer will be considered as Semi-Rigid and then the suitability of RBI grade 81 Stabilized layer as a base layer will be compared with respect to semi rigid pavement. Moreover, for Pavement Design the value of Elasticity Modulus of stabilized layer is needed.For this purpose Four Point Loading beam test are conducted. The equipment is a computerized cyclic beam loading set up.

The Testing Equipment for Elastic Modulus is shown in Photo 1. Three Beam samples (500mmX100mmX100mm) of each 4% and 6% dosage with Aggregate mixture is prepared at Maximum dry density and OMC. The prepared beams were kept for curing for 28 days. After Curing the Beam samples is put on the four point Cyclic Loading beam set up and load was applied at 0.01 KN per second. Deflections and applied load were measured by using LVDT (linear variable displacement transducers) and load cell respectively. From the test measurements, elastic modulus values were calculated and are given in Table 9.

As the Modulus of Elasticity values are high above 1500 MPa in both the case, Pavement consisting of RBI grade 81 stabilized Aggregate layers and Bituminous layer is considered as semi rigid pavement and thus the strength criteria of RBI grade 81 stabilized layers as base layer is considered based on semi rigid pavement.

Therefore, as per IRC 74-1979 “Tentative Guidelines for Lean-Cement concrete and Lean Cement-Fly Ash Concrete as a Pavement Base or Sub Base”, to act as a semi rigid pavement layer, lean cement concrete/ lean cement fly ash concrete should be designed to give a minimum compressive strength of 40 -60 Kg/cm² (4-6MPa) at 28 days in the field and having regard to process variations in the field, the laboratory mix should be designed for 1.25 times the required 28 days field strength.

Hence in laboratory the minimum compressive strength of 28days is 50-75 kg/cm² (5-7.5 MPa). The 28 days Laboratory compressive strength of RBI grade 81 stabilized Aggregate mixture sample with 6% dosage is 5.3 MPa and with 8% dosage is 5.8 MPa which are greater than 5 MPa. Hence the RBI grade 81 stabilized Aggregate Layer is suitable as a base layer.

As the pavement consisting of Aggregate stabilized layer and bituminous layer is semi rigid, Danish Road institute Report on Semi Rigid pavement is also taken into consideration.

As per “Mechanistic Design of Semi-Rigid Pavements” by Danish Road Institute Report 138, 2004, there are three different types of mix design for the treated material: A high rigidity mix (lean concrete) with a 7-day compressive strength greater then 10MPa and Elasticity modulus in the range of 15,000-30,000; a low strength soil cement or sand cement type mix with a 7- day compressive strength less than 2 MPa and Elasticity modulus in the range of 1000-2000 MPa or an intermediate mix type with 7 day compressive strength of 2.5 – 4.5 MPa and Elasticity modulus of 7000-10000 MPa. As per the lab test result of 7 days compressive strength and Elasticity modulus of Aggregate mix with 4% and 6% the values of low strength mix is fulfilled and hence the suitability of Stabilized Aggregate as base layer material is verified.

Observation and Conclusion

1. Plasticity index is reduced and CBR is considerably increased with the addition of RBI Grade 81 stabilizer in all the four soil samples of low CBR. PI is reduced to less than 6 with 2% or 3% dosage in all the four soil samples and considering all the soil samples the average increase in CBR with 2% dosage is 10 times as compared to conventional, with 3% dosage 19 times and with 4% dosage 26 times. Hence RBI grade is suitable to stabilize Subgrade.
2. With Black cotton soil PI is reduced from 27 to 15 with 3% dosage, 4% dosage PI is reduced to 12, 5% dosage PI is reduced to 10 and 6% dosage PI is reduced to 7.5. CBR is increased from 2 to 40 with 3% dosage, 4% dosage CBR is increased to 67, 5% dosage CBR is increased to 79 and 6% dosage CBR is increased to 107. UCS is 2.3 MPa with 3% dosage, 4% dosage UCS is increased to 2.7 MPa, 5% dosage UCS is increased to 3.4 MPa and 6% dosage UCS is increased to 3.8 MPa.
3. With the mixture of 80% Black cotton soil and 20% Stone dust PI is reduced from 27 to 12 with 3% dosage, 4% dosage PI is reduced to 10, 5% dosage PI is reduced to 8 and 6% dosage PI is reduced to 6.5. CBR is increased from 2 to 95 with 3% dosage, 4% dosage CBR is increased to 100, 5% dosage CBR is increased to 109 and 6% dosage CBR is increased to 141. UCS is 2.1 MPa with 3% dosage, 4% dosage UCS is increased to 2.5 MPa, 5% dosage UCS is increased to 3 MPa and 6% dos- age UCS is increased to 4.4 MPa.
4. With the mixture of 70% Black cotton soil and 30% Stone dust PI is reduced from 27 to 7 with 3% dosage, 4% dosage PI is reduced to 5.2, 5% dosage PI is reduced to 5 and 6% dosage PI is reduced to 4.6. CBR is increased from 2 to 82 with 3% dosage, 4% dosage CBR is increased to 121, 5% dosage CBR is increased to 143 and 6% dosage CBR is increased to 153. UCS is 2.9 MPa with 3% dosage, 4% dosage UCS is increased to 3.3 MPa, 5% dosage UCS is increased to 3.6 MPa and 6% dosage UCS is increased to 3.8 MPa.
5. The mixture of 70% Black cotton soil + 30% Stone Dust gives the best results with RBI Grade 81 in all respect. PI is reduced to less than 6 in this sample and the average increase of CBR and UCS are maximum.
6. As per IRC:51-1992, AASHTO 1993, South African Manual TRH 14, Philippines Standard specifications for Public Works and Highways, Volume 2 (DPWH, 1995) and UK Specification TRL ORN31 the Strength requirement for sub base layer is satisfied by the tested samples. Hence RBI grade 81 stabilized layers are suitable as Sub Base Layer.
7. The 28 days Compressive Strength of mixture of 50% 10mm down aggregate and 50% Stone dust stabilized with 4% RBI grade 81 is 3.8 MPa, 6% dosage 5.3 MPa and 8% dosage 5.8 MPa. Elastic Modulus of Aggregate mixture with 4% dosage is 1518 MPa and with 6% dosage is 1829 MPa. As Elastic modulus is high the pavement consisting of RBI grade 81 Stabilized Aggregate layer and bituminous layer is semi rigid.
8. As per IRC 74-2979, the minimum 28 days lab compressive strength should be in the range of 5-7.5 MPa which is satisfied by the Aggregate mixture stabilized with RBI grade 81 dosages of 6% and 8%. As per “Mechanistic Design of Semi-Rigid Pavements” by Danish Road Institute Report 138, 2004, there are three different types of mix design for the treated material and criteria of low strength mix is fulfilled by RBI grade 81 stabilized Aggregate Mix. Hence RBI Grade 81 Aggregate Stabilized layer is suitable as a base layer.

Future Research

1. Similar more Lab tests have to be conducted on various soils and aggregate mixtures with different dosage of RBI grade 81 to have more concrete and precise results for further analysis. For Aggregate mixture lab test has to be performed on different percentage composition of 10mm down aggregate and stone dust and also on different gradation of aggregate.
2. In order to check the durability of RBI Grader 81 stabilized layers, durability lab test has to be conducted on various soils, mixture of soil with stone dust and mixture of 10mm down aggregate and stone dust.
3. Cyclic beam load test or triaxial cyclic loading test has to be conducted on various soils and aggregate mixture with different dosage of RBI grade 81 to develop the Fatigue equation.
4. Once the fatigue equation is developed, thickness of RBI Stabilized layers can be estimated by using the Mechanistic Empirical methodology. Pavement Design of RBI grade 81 layers can be done by using available Elastic layered software e.g. EVERSERIES and KENLAYER.
5. Trial Patch has to be constructed and the trial thickness of RBI Grade 81 Stabilized layer used in trial patch is taken from the pavement design done as described in previous point. Field Evaluation has to be performed on trial patch over a period of time say one to two year and deterioration model of RBI Grade 81 Stabilized pavement has to be developed.

Refrences

• DFID (Department for International Development), UK Project Report “Stabilised Sub-Bases for Heavily Trafficked Roads”
• Bujang B.K.Huat, Asmidar Alias and Azlan Abdul Aziz, “Evaluation, Selection and Assessment of Guidelines for Chemical Stabilization of Tropical Residual Soils”, American Journal of Environmental Sciences 4(4):303-309, 2008
• Abu Siddique and Bipradas Rajbongshi, “An Analytical Study on Design and Analysis of Stabilized Rural Roads,” Proceedings of the Eastern Asia Society for Transportation Studies, Vol. 5 pp. 813-828,2005
• Dr.Animesh Das and Dr.B.B.Pandey, “Trial Design of Bituminous pavement with cemented base/Sub-Base”
• Final Contract Report VTRC (Virginia Transportation Research Council) 09-CR2, “Composite Pavement Systems: Synthesis of Design and Const. Practices”
• IRC :50-1973, “Recommended Design criteria for the use of Cement – Modified Soil in Road Construction”
• IRC:51-1992,”Guidelines for the use of Soil-Lime mixes in Road Construction”
• IRC:88-1984 “Recommended Practice for Lime Flyash Stabilised Soil Base/Sub Base in Pavement Construction”
• IRC:74-1979, “Tentative Guidelines for Lean-Cement Concrete and Lean Cement-Fly Ash Concrete as a Pavement Base or Sub Base”