Methods of Soil Stabilization for Pavements and their Quality Control
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Methods of soil stabilization for pavement construction with materials such as cement, lime, lime-fly ash, asphalt and its quality control are discussed.
Quality control is essential to ensure that the final product will be adequate for its intended use. It must also ensure that the contractor has performed in accordance with the plans and specification, as this is a basis for payment.
Those factors which are most important for a quality control standpoint in cement stabilization are: pulverization, cement content, moisture content, uniformity of mixing, time sequence of operations, compaction, and curing. These are described in detail below.
a. Pulverization.Pulverization is generally not a problem in cement construction unless clayey or silty soils are being stabilized. A sieve analysis is performed on the soil during the pulverization
process with the No. 4 sieve used as a control. The percent pulverization can then be determined by calculation. Proper moisture control is also essential in achieving the required pulverization.
b. Cement content.Cement content is normally expressed on a volume or dry weight basis. Field personnel should be aware of quantities of cement required per linear foot or per square yard of
pavement. Spot check can be used to assure that the proper quantity of cement is being applied, by using a canvas of known area or, as an overall check, the area over which a known tonnage has been spread.
c. Moisture content.The optimum moisture content determined in the laboratory is used as an initial guide when construction begins. Allowance must be made for the in situ moisture content of the soil when construction starts. The optimum moisture content and maximum density can then be established for field control purposes. Mixing water requirements can be determined on the raw soil or on the soil-cement mix before addition of the mixing water. Nuclear methods can be used to determine moisture content at the time construction starts and during processing.
d. Uniformity of mixing.A visual inspection is made to assure the uniformity of the mixture
throughout the treated depth. Uniformity must be checked across the width of the pavement and to the desired depth of treatment. Trenches can be dug and then visually inspected. A satisfactory mix will exhibit a uniform color throughout; whereas, a streaked appearance indicates a no uniform mix. Special attention should be given to the edges of the pavement.
e. Compaction.Equipment used for compaction is the same that would be used if no cement were
present in the soil, and is therefore dependent upon soil type. Several methods can be used to
determine compacted density: sand-cone, balloon, oil, and nuclear method. It is important to determine the depth of compaction and special attention should be given to compaction at the edges.
f. Curing.To assure proper curing a bituminous membrane is frequently applied over large areas. The surface of the soil cement should be free of dry loose material and in a moist condition. It is
important that the soil-cement mixture be kept continuously moist until the membrane is applied. The recommended application rate is 0.15 to 0.30 gallons per square yard.
2. Lime Stabilization of Soil for Pavements
The most important factors to control during soil-lime construction are pulverization and scarification, lime content, uniformity of mixing, time sequence of operations, compaction and curing.
a. Pulverization and scarification.Before application of lime, the soil is scarified and pulverized.
To assure the adequacy of this phase of construction, a sieve analysis is performed. Most specifications are based upon a designated amount of material passing the 1 inch and No. 4 sieves. The depth of scarification or pulverization is also of importance as it relates to the specified depth of lime treatment.
For heavy clays, adequate pulverization can best be achieved by pretreatment with lime, but if this method is used, agglomerated soil-lime fractions may appear. These fractions can be easily broken down with a simple kneading action and are not necessarily indicative of improper pulverization.
b. Lime content.When lime is applied to the pulverized soil, the rate at which it is being spread can be determined by placing a canvas of known area on the ground and, after the lime has been spread, weighing the lime on the canvas. Charts can be made available to field personnel to determine if this rate of application is satisfactory for the lime content specified.
To accurately determine if this rate of application is satisfactory for the lime content specified. To accurately determine the quantity of lime slurry required to provide the desired amount of lime solids, it is necessary to the quantity of lime slurry required to provide the desired amount of lime solids, it is necessary to know the slurry composition. This can be done by checking the specific gravity of the slurry, either by a hydrometer or volumetric-weight procedure.
c. Uniformity of mixing.The major concern is to obtain a uniform lime content throughout the depth of treated soil. This presents one of the most difficult factors to control in the field. It has been reported that mixed soil and lime has more or less the same outward appearance as mixed soil without lime.
The use of phenolphthalein indicator solution for control in the field has been recommended. This method, while not sophisticated enough to provide an exact measure of lime content for depth of treatment, will give an indication of the presence of the minimum lime content required for soil treatment. The soil will turn a reddish pink color when sprayed with the indicator solution, indicating that free lime is available in the soil (pH = 12.4).
d. Compaction.Primarily important is the proper control of moisture-density. Conventional Procedures such as sand cone, rubber balloon, and nuclear methods have been used for determining the density of compacted soil lime mixtures.
Moisture content can be determined by either oven-dry methods or nuclear methods. The influence of time between mixing and compacting has been demonstrated to have a pronounced effect on the properties of treated soil. Compaction should begin as soon as possible after final mixing has been completed.
The National Lime Association recommends an absolute maximum delay of one week. The use of phenolphthalein indicator solution has also been recommended for lime content control testing. The solution can be used to distinguish between areas that have been properly treated and those that have received only a slight surface dusting by the action of wind. This will aid in identifying areas where density test samples should be taken.
e. Curing.Curing is essential to assure that the soil lime mixture will achieve the final properties desired. Curing is accomplished by one of two methods: moist curing, involving a light sprinkling of water and rolling; or membrane curing, which involves sealing the compacted layer with a bituminous seal coat.
Regardless of the method used, the entire compacted layer must be properly protected to assure that the lime will not become non reactive through carbonation. Inadequate sprinkling which allows the stabilized soil surface to dry will promote carbonation.
3. Lime-Fly Ash (LF) and Lime-Cement-Fly Ash (LCF)
The nature of lime-fly ash and lime cement-fly ash stabilization is similar to that forlime only. Consequently, the same factors involvedfor quality control are suggested.
4. Bituminous Stabilization of Soil
The factors that seem most important to control during construction with bituminous stabilization are surface moisture content, viscosity of the asphalt, asphalt content, uniformity of mixing, aeration, compaction, and curing.
a. Surface moisture content.The surface moisture of the soil to be stabilized is of concern. Surface moisture can be determined by conventional methods, such as oven-drying, or by nuclear methods. The Asphalt Institute recommends surface moisture of up to three percent or more for
use with emulsified asphalt and a moisture content of less than three percent for cutback asphalt. The gradation of the aggregate has proved to be of significance as regards moisture content. With densely graded mixes, more water is needed for mixing than compaction.
Generally, a surface moisture content that is too high will delay compaction of the mixture. Higher plasticity index soils require higher moisture contents.
b. Viscosity of the asphalt.The Asphalt Institute recommends that cold-mix construction should not be performed at temperatures below 50 degrees F. The asphalt will rapidly reach the temperature of the aggregate to which it is applied and at lower temperature difficulty in mixing will be encountered.
On occasion, some heating is necessary with cutback asphalts to assure that the soil aggregate particles are thoroughly coated.
c. Asphalt content.Information can be provided to field personnel which will enable them to
Determine a satisfactory application rate. The asphalt content should be maintained at optimum or slightly below for the specified mix. Excessive quantities of asphalt may cause difficulty in compaction and result in plastic deformation in service during hot weather.
d. Uniformity of mixing.Visual inspection can be used to determine the uniformity of the mixture. With emulsified asphalts, a color change from brown to black indicates that the emulsion has broken.
The Asphalt Institute recommends control of three variables to assure uniformity for mixed-in-place construction: travel speed of application equipment; volume of aggregate being treated; and flow rate (volume per unit time) of emulsified asphalt being applied. In many cases, an asphalt content above design is necessary to assure uniform mixing.
e. Aeration.Prior to compaction, the diluents that facilitated the cold-mix operation must be allowed to evaporate. If the mix is not sufficiently aerated, it cannot be compacted to acceptable limits
The Asphalt Institute has determined that the mixture has sufficiently aerated when it becomes tacky and appears to “crawl.” Most aerating occurs during the mixing and spreading stage, but occasionally additional working on the roadbed is necessary.
The Asphalt Institute has reported that over mixing in central plant mixes can cause emulsified asphalts to break early, resulting in a mix that is difficult to work in the field.
f. Compaction.Compaction should begin when the aeration of the mix is completed. The Asphalt
Institute recommends that rolling begin when an emulsified asphalt mixture begins to break (color change from brown to black). Early compaction can cause undue rutting or shoving of the mixture due to overstressing under the roller. The density of emulsion stabilized bases has often been found to be higher than that obtained on unstabilized bases for the same compaction effort.
g. Curing.Curing presents the greatest problem in asphalt soil stabilization. The Asphalt Institute has determined that the rate of curing is dependent upon many variables: quantity of asphalt applied, prevailing humidity and wind, the amount of rain, and the ambient temperature.
Initial curing must be allowed in order to support compaction equipment. This initial curing, the evaporation of diluents, occurs during the aeration stage. If compaction is started too early, the pavement will be sealed, delaying dehydration, which lengthens the time before design strength is reached.
The heat of the day may cause the mixture to soften, which prohibits equipment from placing successive lifts until the following day. This emphasizes the need to allow sufficient curing time when lift construction is employed.
The Asphalt Institute recommends a 2- to 5-day curing period under good conditions when emulsified bases are being constructed. Cement has been used to accelerate curing.
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