CEMENT STABILIZATION OF RDA ROAD BASES

V. Mohan MSc, CEng, FCIHT, FIE, IntPEng

Deputy Project Director, ADB Project Road Development Authority, Ministry of Highways,

Introduction

Since ancient times, mankind has used the natural earth surface for transportation. The soil surface roads have been compacted by human and animal traffic and the tracks were developed as a natural result of the movement of traffic. These road surfaces were widened and covered with rock and gravel, and thus made suitable to tackle the traffic better. However, these road surfaces were transformed in to masses of mud by the spring and rain, where as in the summer season the carts created clouds of dust. Several attempts were made to take care of these problems. The outcome of these techniques subsequently results in various pavement design concepts and guidelines.

Most of the pavement design guidelines are based on the assumption that aggregates are important ingredients of pavement structure. However, availability of good quality aggregates may be a constraint in some locations. To transport good quality aggregates from long distance may not be economically feasible. Due to the excessive investment and maintenance cost, new methods of design had to be sought and new building materials are introduced. Some researchers tried with soil, which is available everywhere. The engineering properties of soil were modified using certain treatment. At the same time various waste products are created by several industrial plants. These waste products could be used in the road construction projects after following certain treatment procedure. By treating natural soil or fly-ash, or by addition of certain materials to it, new road construction materials can be developed. Thus use of stabilized soil/waste product as a base/sub-base material of pavement leads not only economic solution, but also offers a potential use of the industrial/domestic waste materials. Thus cemented bases and sub-bases can be designed to economize the design where, locally available inferior quality materials are stabilized using cemented material.

Some soils are characterized by high swelling on wetting and excessive shrinkage on drying. When swelling is restricted, it results in development of swell pressure. On account of these peculiar properties, this soil poses serious problems in the construction of roads. Even at places where condition for development of swell pressure do not exist, the road still fails because of poor supporting power of the sub grade in wet condition. Therefore there is need of a technique for an effective and economic stabilization of such type of soil.

Stabilization of low quality aggregate or soil can be done by mechanical, chemical, electrical or thermal means. Chemical stabilization involves addition of cement, lime, fly-ash, bitumen, chemical compounds etc. In this research work, it is proposed to investigate the best combination of soil-cement-

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water and fly-ash-lime-water-sodium chloride-fiber-bitumen for the purpose of deciding its performance as pavement base/sub-base material. Fly-ash is stabilized by adding water, lime, sodium chloride, fibers, and bitumen. Combination of soil-cement in the presence of water is considered as another material for pavement base or sub-base.

What is Soil Stabilization?

Soil is one of nature’s most abundant construction materials. Almost all construction is built with or upon soil. When unsuitable construction conditions are encountered, a contractor has four options;

1. Find a new construction site

2. Redesign the structure so it can be constructed on the poor soil.

3. Remove the poor soil and replace it with good soil.

4. Improve the engineering properties of the site soils.

In general option 1 and 2 tends to be impracticable today. While in the past, option 3 has been mostly commonly used method. However, due to improvement in technology coupled with increased transportation costs, option 4 is being used more often today and is expected to dramatically increase in the future.

Improving an on-site (in situ) soil’s engineering properties is referred to as either “soil modification” or “soil stabilization.” The term “modification” implies a minor change in the properties of a soil, while stabilization means that the engineering properties of the soil have been changed enough to allow field construction to take place.

There are two primary methods of soil stabilization used today:

Mechanical

Chemical or additive

Nearly every road construction project will utilize one or both of these stabilization techniques. The most common form of “mechanical” soil stabilization is compaction of the soil, while the addition of cement, lime, bituminous, or other agents is referred to as a “chemical” or “additive” method of soil stabilization. There are two basic types of additives used during chemical soil stabilization: mechanical additives and chemical additives. Mechanical additives, such as soil cement, mechanically alter the soil by adding a quantity of a material that has the engineering characteristics to upgrade the load-bearing capacity of the existing soil. Chemical additives, such as lime, chemically alter the soil itself, thereby improving the load-bearing capacity of the soil.

Why and when is it used?

Traditionally, stable sub-grades, sub-bases and /or bases have been constructed by using selected, well-graded aggregates, making it fairly easy to predict the load-bearing capacity of the constructed layers. By using select material, the engineer knows that the foundation will be able to support the design loading.

Gradation is an important soil characteristic to understand. A soil is considered either “ well-graded” or “uniformly-graded” (also referred to as “poorly-graded”).

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This is a reference to the sizes of the particles in the materials. Uniformly-graded materials are made up of individual particles of roughly the same size. Well-graded materials are made up of an optimal range of different sized particles.

It is desirable from an engineering standpoint to build upon a foundation of ideal and consistent density. Thus, the goal of soil stabilization is to provide a solid, stable foundation. “Density” is the measure of weight by volume of a material, and is one of the relied-upon measures of the suitability of a material, and is one of the relied-upon measures of the suitability of a material for construction purposes. The more density a material possesses, the fewer voids are present.

Voids are the enemy of road construction; voids provide a place for moisture to go, and make the material less stable by allowing it to shift under changing pressure, temperature and moisture conditions.

Uniformly-graded materials, because of their uniform size, are much less dense than well-graded materials. The high proportion of voids per volume of uniformly-graded material makes it unsuitable for construction purposes. In well-graded materials, smaller particles pack in to the voids between the larger particles, enabling the material to achieve high degrees of density. Therefore, well-graded materials offer higher stability, and are in high demand for construction. With the increased global demand for energy and increasing local demand for aggregates, it has become expensive from a material cost and energy use

Standpoint to remove inferior soils and replace them with choice, well-graded aggregates, ”One way to reduce the amount of select material needed for base construction is to improve the existing soil enough to provide strength and conform to engineering standards. This is where soil stabilization has become a cost-effective alternative essentially; soil stabilization allows engineers to distribute a larger load with less material over a longer life cycle.

Advantages to soil stabilization:

Stabilized soil functions as a working platform for the project

Stabilization waterproofs the soil

Stabilization improves soil strength

Stabilization helps reduce soil volume change due to temperature or moisture

Stabilization improves soil workability

Stabilization reduces dust in work environment

Stabilization upgrades marginal materials

Stabilization improves durability

Stabilization dries wet soils

Stabilization conserves aggregate materials

Stabilization reduces cost

Stabilization conserves energy

Need of Stabilization

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The aggregates are important ingredients of pavement structure. Good quality aggregates may not always be available nearby road construction site. Transporting the aggregates from long distance may not be economically feasible. Under such circumstances, locally available inferior quality material like soil or industrial waste material like fly-ash may be proposed to be used in pavement as base/sub-base material. These materials are of inferior quality, and hence may not satisfy the requirement as pavement material. Therefore engineering properties of these materials are modified by means of a process, known as stabilization. This is not only economic solution, but also offers a potential use of the industrial/domestic waste materials. Advantages of stabilization are summarized in the following.

1. Improved stiffness and tensile strength of the material.

2. Reduction in pavement thickness.

3. Improved durability and resistance to the effect of water.

4. Reduction is swelling potential

Mechanism of stabilization can be broadly divided into two categories, mechanical and chemical stabilization. Mechanical stabilization includes compaction, blending of aggregates to improve gradation, and addition of asphalt. Asphalt, as a stabilizer, does not generally react chemically with the materials being stabilized, but coats the particles and imparts adhesion and helps waterproofing. Chemical stabilization includes addition of materials such as lime, cement or fly-ash in combination or alone. These materials either react chemically with materials being stabilized (for example, lime reacts with clays) or react on their own to form cementing compounds (for example, Portland cement).

Basic ingredients for cemented base/sub-base in pavement

Portland cement and lime are most commonly used cementing materials for construction of cemented base with soil, sand and aggregates for pavements of roads and runways. Low grade or marginal aggregates with suitable proportioning of coarse and fine fractions, lime, clay, lime-late rite-soil, lime-fly-ash, lime-granulated blast-furnace-slag-soil mixture, etc. can be used as a cemented base or sub-base. Various types of materials are considered by several researchers to study the suitability of those materials as pavement base and sub-base layer. Natural soil, fly-ash, sand, stone dust, river bed materials, reclaimed asphalt pavement, low quality aggregates etc. are considered as ingredients of pavements with cemented base/sub-base. Cemented materials may be classified in to three categories:

1. Traditional stabilizers: hydrated lime, Portland cement, and fly-ash

2. By-product stabilizers: cement kiln dust, lime kiln dust and other forms of byproduct lime and

3. Non-traditional stabilizers: sulfonated soils, potassium compounds, ammonium chloride, enzymes, polymers

Environmental Benefits of Soil Stabilization

The purpose of stabilization is to enhance the strength of the soil. This increased strength is then taken into account in the pavement design process Soil stabilization used to improve the properties of soils to transform otherwise unusable materials

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into suitable fills and/or substitute material for expensive imported granular aggregates. It is an extremely cost-effective and environmentally-sound method of converting areas of weak soil into construction materials. Our technological advances and innovative techniques have paved the way for the growing use of soil stabilization in infrastructure construction. By using natural materials on site, construction carried out more cheaply, quickly and effectively. Key cost benefits include significant savings, from landfill to transportation, and reduced programme in time.

The environmental advantages are also extensive:

Life of landfill is extended, as materials are kept on site

Natural resources are extended, by eliminating use of stone

Emissions associated with transport are radically reduced

Transport miles are eliminated, reducing wear and tear on roads

Design Procedures for Soil Modification or Stabilization Methodology and Stabilization Mechanisms

Stabilization requires more thorough design methodology during construction than modification. The methods of stabilization include physical processes such as soil densification, blends with granular material, use of reinforcements (Geogrids), undercutting and replacement, and chemical processes such as mixing with cement, fly ash, lime, lime byproducts, and blends of any one of these materials. Soil properties such as strength, compressibility, hydraulic conductivity, workability, swelling potential, and volume change tendencies may be altered by various soil modification or stabilization methods..

Strength requirements for stabilization and modification

The reaction of a soil with quick lime, or cement is important for stabilization or modification and design methodology. The methodology shall be based on an increase in the unconfined compression strength test data. In the case of soil modification, enhanced sub-grade support is not accounted for in pavement design. However, an approved chemical , cement, and fly ash class C) or a combination of the chemicals shall attain an increase in strength of natural soils when specimens are prepared and tested in the same manner as stabilization.

Laboratory Test Requirements

Soil Sampling and Suitability: An approved Geotechnical Engineer should visit the project during the construction and collect a bag sample of each type of soil in sufficient quantity for performing the specified tests. The geotechnical engineer should review the project geotechnical report and other pertinent documents such as soil maps, etc., prior to the field visit. The geotechnical consultant shall submit the test results and recommendations, along with the current material safety data sheet or mineralogy to the engineer for approval.

When the geotechnical engineer determines the necessity of chemical-soil stabilization

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During the design phase, they should design a sub-grade treatment utilizing the chemical for the stabilization in the geotechnical report in accordance with guidelines.

Following tests should be performed and the soils properties should be checked prior

To any modification or stabilization.

a. CBR at 98% MDD (AASHTO T-193)

b. Atterberg limits LL, PL PI (AASHTO T-89 & 90),

c. Max. Dry Density, OMC (AASHTO T-180)

d. Particle Size Distribution (TRRL RD-31)

e. Loss of ignition (LOI ) not more than 3% by dry weight of soil in accordance with AASHTO T 267,

f. Carbonates not more than 3 % by dry weight of the soils, if required,

g. As received moisture content in accordance with AASHTO T 265.2

Cement Stabilization

Stabilization with cement is a common treatment technology for the safe management, reuse and treatment for disposal of contaminated waste. Portland cement is composed of calcium-silicates and calcium-aluminates that, when combined with water, hydrate to form the cementing compounds of calcium-silicate hydrate and calcium-aluminates-hydrate, as well as excess calcium hydroxide. Because of the cementations material, as well as the calcium hydroxide (lime) formed, Portland cement may be successful in stabilizing both granular and fine-grained soils, as well as aggregates and miscellaneous materials. A pozzolanic reaction between the calcium hydroxide released during hydration and soil alumina and soil silica occurs in fine-grained clay soils and is an important aspect of the stabilization of these soils. The permeability of cement stabilized material is greatly reduced. The result is a moisture-resistant material that is highly durable and resistant to leaching over the long term. Cement-soil Stabilization is another technique, Where Portland cement can be used to stabilize and strengthen certain type of soils. CSB constructed in accordance of the requirements of the RDA standard specifications for construction of roads & bridges sub sections 1708.3 for soils and 1703 for cement. In this regard granular soils are very effective for cement stabilization. The required quantity of Portland cement is mixed with soil, which is to be stabilized, followed by mixing with water uniformly, preferably with a pulverize-type machine like pug mill, and transported and deposited at the site and spread to the specified depth, followed by fine grading compaction. The principal materials used for the cementations stabilization of highway pavement materials with Portland cement. Stabilization projects are almost always site-specific, requiring the application of standard test methods, along with fundamental analysis and design procedures, to develop an acceptable solution. As with any such process, adherence to strict environmental constraints is vital to project success. The use of cementations materials makes a positive contribution to economic and resource sustainability because it allows enhancement of both standard and substandard in situ soils to levels consistent with the requirements of a given application.

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What is Stabilization with cement?

Stabilization protects human health and the environment by immobilizing hazardous contaminants within the treated material. The technology involves mixing Portland cement into contaminated material. The cement chemically reacts with water in the material being treated, creating changes in its physical and chemical properties. Physical changes include making the treated material more solid, thereby reducing its permeability and encapsulating contaminants. Chemical changes to the contaminants themselves stabilize these hazardous constituents within the treated material. These physical and chemical changes prevent movement of hazardous contaminants into the environment.

Advantages of Cement Stabilization

While several reagents can be used for CSB, Portland cement has advantages that make it more economical and easy to use than others:

Cement is manufactured under strict ASTM standards, ensuring uniformity of quality and performance.

Cement’s success in CSB is supported by more than 50 years of use on a variety of projects.

Cement has a long-term performance record.

Using cement can minimize volume increase compared with other reagents.

Cement is a non-proprietary manufactured product, readily available across the country in bag or bulk quantities.

Strength requirements for stabilization

The reaction of soil cement is important for stabilization and design methodology. The methodology shall be based on an increase in the unconfined compression strength test data. To determine the reactivity of the soils for cement stabilization with 2- 4% cement by dry weight of the soils and tested as described above.

The criteria for cement percentage required for stabilization shall be as follows. The following methodology shall be used for quality control and soil-cement stabilization.

1. Perform the mechanical and physical property tests of the soils.

2. Select the Cement Content based on the following:

AASHTO Classification Usual Cement Ranges for Stabilization (% by dry weight of soil)

A-1-a 3 – 5

A-1-b 5 – 8

A-2 5 – 9

A-3 7 – 10

Suggested Cement Contents

3. Perform the Standard Proctor on soil-cement mixtures for the change in maximum dry unit weight in accordance with AASTO T -180.

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4. Perform the unconfined compression and CBR tests on the pair of specimens molded at 98% of the standard Proctor in case of stabilization. A gain of 100 psi of cement stabilization is adequate enough for stabilization and % cement shall be adjusted. Although, there is no test requirement for the optimum cement content when using cement to modify the sub-grade.

Mix design

Mix design is the process of choosing the optimum stabilized content of various ingredients of the pavement. The general principle of mix design is that the mixture should provide satisfactory performance when constructed in the desired position in the pavement structure of sub-grade. Design proportions of the ingredients are generally based on an analysis of the effect of various proportions on selected engineering properties of the mix. Engineering properties which are considered, depending on the objective are-

1. Attenberg limits

2. California Bearing Ratio (CBR),

3. Swell potential, unconfined compression strength (UCS) of cured or uncured mixtures, Freeze-thaw and wet-dry test etc.

4. The mix design procedure includes the testing for strength and for durability.

`Mix Design Considerations

Mix design requirements vary depending on the objective. Soil-cement bases generally have more stringent requirements than cement-modified soil sub-grades. For soil-cement bases, two types of testing have typically been used—durability tests and strength tests. The Portland Cement Association has developed requirements for AASHTO soils A-1 to A-7 that make it possible to determine the durability of cement on the basis of maximum weight losses under wet-dry (ASTM D559) and freeze-thaw (ASTM D560) tests. This requirement is often based on many years of experience with soil-cement. The advantage of using these strength tests is that they can be conducted more rapidly than the durability tests in 7 days and require less laboratory equipment and technician training. However, achievement of a specified strength does not always ensure durability. For cement-modified soils, the engineer selects an objective and defines the cement requirements accordingly. Objectives may include one or more of the following: reducing the plasticity index (Atterberg limits, ASTM D4318); increasing the shrinkage limit; reducing the volume change of the soil (AASHTO T116); reducing clay/silt-sized particles (hydrometer analysis); meeting strength values/indexes such as the California Bearing Ratio (ASTM D1883) CBR> 80 for bases.

Surface Preparation

The shoulder is constructed on sub base surface before the Cement Stabilized Base (CSB) constructed since shoulder materials are different from CSB and after completing the shoulder, sub base surface will be prepared to the levels conforming to the required.

Shoulder is under Construction to Facilitate CSB

After the levels are confirmed by the Surveyor, the laboratory is requested to carry on the test of compaction on sub base which is expected to be above 98 %. Once the surfaces satisfied the requirements are accepted to continue the CSB

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construction. The surface is confined and protected from traffic and other damage till the CSB is complete.

Material Handling and Transport

The soil for CSB is stock piled at the yard near the Pug Mill Plant (The Plant which produces CSB material) after screening for over sizes and other impurities. Regular testing of raw materials stock piled is carried out and the Pug Mill Plant is under observation of Consultancy staff regularly. The Pug Mill plant is producing CSB materials continuously by adding all raw materials (Soil, Cement & Water) to the required proportions. The plant is calibrated to the required proportions after doing plant trials and site trials we use 2.5% CSB. Materials transported to the site by Dump trucks covered with tarpaulin and unloaded to the location.

CSB Materials are loaded to Dump Trucks at Pug Mill Plant

Method of Construction of CSB

The approved confined surface of sub base is sprinkled with water just before the CSB is laid to have a proper bonding with previous layer. The unloaded CSB materials is leveled at the site by using a JCB and followed with a Motor Grader for fine leveling. The laying levels are predetermined after doing trials to achieve required compaction factor. A 20T Double Drum Vibratory Roller is used to compact the leveled surface to achieve the required compaction of 98% or more. After the compaction the levels and test for compaction is carried out. The surface is applied with water and covered with sand or Geo-textile for curing purposes. Curing of surface with application of water is continued for 7 days

Cement Stabilization process

The soil stabilization process is carried out in layers and consists of:

Excavation and spreading of material to the required layer thickness for stabilizing

Lime or cement spreading, with regular checks to control dosage

Mixing, to a depth depending on the soil and on the design requirements

Sealing the material, preventing carbonization of the lime while it reacts with the moisture in the soil. This involves trimming of the treated layer using bulldozers and passing over by a smooth roller

Allowing (or maturation) period - to allow time for the exothermic chemical reaction to take place between the lime and clay

Compacting the treated layer with a roller until required compaction is achieved

7 days curing

Assessment and Testing

The soils of the site are thoroughly tested to determine the existing conditions. Based on analysis of existing conditions, additives are selected and specified. Generally, a target chemical percentage by weight and a design mix depth are defined for the sub-base constructed. The selected additives are subsequently

mixed with soil samples and allowed to cure. The cured sample is then tested to ensure that the additives will produce the desired results.

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Curing

Sufficient curing will allow the additive to fully achieve its engineering potential. For cement, lime, and fly ash stabilization, weather and moisture are critical factors, as the curing can have a direct bearing on the strength of the stabilized base. Bituminous-stabilized based often require a final membrane of medium-curing cutback asphalt or slow-curing emulsified asphalt as a moisture seal. Generally, a minimum of seven days are required to ensure proper curing. During the curing period, samples taken from the stabilized base will reveal when the moisture content is appropriate for surfacing.

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