Geotech FAQs

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Geotech FAQs

Because there is no noticeable color change, how can I tell how deep my roadway materials were treated with cement?

Basically, when cement is added to the soil material, the resultant mixture experiences an increase in pH—usually into the moderate to strongly alkaline range from 8.0 to 9.0. This is a measurable difference from the untreated soil which commonly has a lower pH. Therefore, knowing the soil pH provides various clues about soil properties (such as the presence of calcium-rich materials like portland cement) and is easily determined. A hole dug through the completed cement-stabilized work (post-hole diggers work very well for this) can be tested at various depths to determine at what point the pH increase ceases— thereby indicating the depth of treatment. The most accurate method of determining soil pH is by a pH meter.

A second method, which is simple and easy but less accurate then using a pH meter, consists of using certain indicators or dyes. Many dyes change color with an increase or decrease of pH making it possible to estimate soil pH. In making a pH determination on soil, the sample is saturated with the dye for a few minutes and the color observed. This method is accurate enough for most purposes. Both pH meters and pH kits containing the necessary chemicals and color charts are available from garden supply stores. Here are some sites that you might find useful:

http://www.gemplers.com/soil-test-kits

http://www.biconet.com/testing/st-t.html

More sophisticated (and usually more expensive) testing devices are also available through laboratory supply companies. Some helpful sites include:

http://www.omega.com/pptst/ITK_RITK.html

A commonly used chemical (although strictly controlled in some areas of the country) to determine the presence of calcium in cement-stabilized materials is phenolphthalein. While very effective for the intended use, it has been reasonably considered by many to be a human carcinogen. It is, however, still available for purchase at the following site:

http://www.sciencecompany.com/Search.aspx?k=phenolphthalein

Another method that can be helpful in determining the presence of portland cement in stabilized soils is through the use of "natural indicators." Nature provides us with pH indicators in the form of plant pigments known as antocyanines, which change color over different pH ranges, depending on the source. The juice from a red cabbage will function over a wide pH range, from as low as 1.0 up to 12.0, while the juice from red beets will change from red to yellow somewhere between 11.0 and 12.0. The following preparation information comes from the website referenced below:

"The indicator solutions are prepared by chopping up the red beets or red cabbage into small pieces, preferably by running them through a blender. Then the juice is strained off from the resulting mush, and the mush can also be extracted with water to yield even more of the pigment. The solutions are filtered to remove any remaining plant matter, and can then be used as is, or diluted if the color is too intense."
http://www.rhodium.ws/chemistry/equipment/ph-indicator.htm
For base material, is there a cost advantage in using a cement-treated base over an unbound graded aggregate base?

Obviously, the difference in cost between a cement-treated base (CTB) and an unbound graded aggregate base (GAB) will vary depending upon location, availability and cost of materials, and costs associated with placing the materials. However, in many cases, the cost ofcement-treated base is less than graded aggregate base. As an example, in the southeastern United States during the past two years, the cost of a 6-inch CTB section has been between 20 and 30 percent less than a 6-inch GAB section (including all associated costs such as materials, hauling, spreading, compacting, finishing, etc.).

But simply looking at the percent savings for a comparable thickness of pavement does not tell the whole story. One of the major engineering benefits of cement-treated base is that because it is a bound material it requires less thickness to provide the same structural capacity as graded aggregate base. By using the AASHTO layer coefficients for pavement materials, a commonly accepted “rule of thumb” is that six inches of CTB is equal to eight inches of GAB. This results in even greater cost savings!

Another item to consider is that the extra structural capacity provided by having a cement-stabilized pavement layer can be factored into an overall pavement evaluation. By increasing the cement-treated base thickness and reducing the surface course material (normally hot-mix asphalt) even further savings can be realized, especially considering the spiraling price increase of petroleum products today. For further information on the applications, benefits, design, construction, testing, and performance of CTB, please refer to the PCA publication Guide to Cement-Treated Base (CTB), EB236.
How does cement slurry compare with lime slurry?

Although the application of cement and lime are relatively similar, there are some distinct differences on how each works. In most cases, slurry products are applied through the use of a modified water truck. A rearmounted distributer bar applies the slurry by gravity onto the desired surface. Also, application by means of a drum-mounted sprayer bar on pulverization equipment is available.

The differences are noticed in several ways. The water to cement ratio in cement slurry averages 1.5:1. This is not the case with lime slurry. A large tank with an internally mounted mixing paddle is used in the “slaking” (hydration) of the dry lime and at the very least 2.5 times the amount of water to dry lime is needed for proper hydration. This means 40 percent more water is needed in lime slurry production. The exothermic reaction of hydrating lime can produce temperatures as high as 212 degrees Fahrenheit whereas temperatures of cement slurries can range from 70 to 120 degrees Fahrenheit. Heat burns can occur to skin quickly at temperatures exceeding 120 degrees Fahrenheit.

The engineering benefits of cement slurry are superior also. While both portland cement and lime are capable of providing calcium, the primary ingredient necessary for stabilizing a clay soil, they differ in their chemical nature, mode of reaction in the presence of water, and resulting reaction products. Although both portland cement and lime produce the pozzolanic reaction to form calcium-silicate-hydrate (C-S-H) and calcium-aluminate-hydrate (C-A-H), portland cement has the capability to form C-S-H and C-A-H immediately similar to portland cement concrete. This characteristic allows cement treated soils to produce higher strengths and work more effectively with a broader range of soils. The figure below shows higher unconfined compressive strengths are produced by cement slurry when compared to lime slurry in sample clay materials.

When considering the use of a slurry product for modification, stabilization or reclamation, cement slurry has distinct advantages over lime slurry in economic, health and engineering aspects.
How exactly does a pugmill work to blend materials together for CTB?

Pugmills provide excellent mixing for cement-treated materials because of the violent action of the mixing process. Typically a pugmill will have two horizontal shafts, each with dozens of paddles. The shafts turn in opposite directions, causing one set of paddles to turn clockwise, and the other set to turn counter-clockwise. This motion hurls the particles toward each other in a violent action that allows for uniform mixing.

The pugmill operates on a continuous basis, with an aggregate feed belt, water spray, and cement vane that are all computer controlled to provide accurate metering of the constituents. Pugmills can be set up with more than one silo, so cementitious products (such as portland cement and fly ash) can be blended at the plant.

The mixed material is taken by belt to a surge hopper (or gob hopper) where it will be dumped into a truck to be transported to the job site. The trucks will have tarps to cover the freshly mixed material so that moisture is not lost on the way to the site.

Pugmills on paving projects are typically portable, and can be set up and running in less that a day. Production for pugmills on these projects are normally in the range of 300 – 500 tons per hour.

Click here for a diagram of a pugmill layout
How soon can a newly constructed cement-stabilized roadway base—whether a cement-treated base (CTB) or a full-depth reclamation (FDR) base—be opened to traffic?

A properly constructed cement-stabilized roadway base is one in which the roadway materials are homogenously blended with the designed amount of portland cement and water, and then compacted, graded, finished, and cured through the application of a bituminous compound, a sealing membrane, or kept continuously moist for a period of seven days.

Completed portions of these cement-stabilized bases can be opened immediately to low-speed local traffic and to construction equipment, provided the curing material or moist curing operations are not impaired, and provided the cement-stabilized base is sufficiently stable to withstand marring or permanent deformation. The section can be opened up to all traffic after the cement-stabilized base has received the curing compound or subsequent bituminous or concrete surfacing and is sufficiently stable to withstand marring or permanent deformation. If continuous moist curing is employed in lieu of a curing compound or subsequent surfacing within seven days, the cement-stabilized base can be opened to all traffic after the 7-day moist curing period, provided the cement-stabilized base has hardened sufficiently to prevent marring or permanent deformation.
Is it less expensive to rehabilitate an entire roadway through FDR using cement rather than removing and replacing the roadway?

Full-depth reclamation (FDR) recycles the materials from deteriorated asphalt pavement, and, with the addition of cement, creates a new stabilized base. Another alternative, the complete removal and replacement of the existing base and asphalt surface, can be very expensive and wasteful of virgin aggregates. The recycled base will be stronger, more uniform, and more moisture resistant than the original base, resulting in a long, low-maintenance life. And most important, recycling costs are normally 25 - 50 percent less than removal and replacement of the old pavement.

Full-depth reclamation with cement makes the reconstruction of old roads a largely self-sustaining process. The original “investment” in virgin road materials becomes a one-time cost, which is reclaimed through cement stabilization and the addition of a new, thin surface course.
Is there a general "rule of thumb" about how much water to use in soil-cement?

Designing the proper amount of water for soil-cement materials is not only important to obtain a good final product, but also provides important information for quality control during construction. Research has shown that cement-stabilized materials have better strength and performance when they are fully compacted, so determining final compaction density is fundamental to the design procedure.

Compaction density is determined through the Standard Test Method for Moisture-Density Relations of Soil-Cement Mixtures (ASTM D558). This test method is a common (as well as inexpensive) procedure for most construction testing labs. The test determines the maximum dry density (unit weight) for the soil-cement mixture and its corresponding optimum moisture content. If the mixture is too dry, there is not enough moisture available to lubricate the particles into a denser formation. If the mixture is too wet, the excess moisture pushes the particles apart.

Obviously, the nature of the materials being mixed with cement, the quantity of cement incorporated, and environmental conditions will all have an influence on the water requirement for soil-cement mixtures, and is the reason that a design "rule-of-thumb" is not recommended. However, there does exist a construction "rule-of-thumb" used by many contractors in the field to determine if a soil-cement moisture content is near the optimum moisture content. The procedure goes roughly as follows:

You can tell if you have the right amount of water in a soil-cement mixture if you can squeeze a sample of the mixture using both hands into a ball. The ball should hold together without excessively wetting the hands. Hold the ball using the fingers of each hand. The ball should be capable of being pulled apart into two distinct halves without crumbling. If the ball crumbles during the splitting process, then the mixture is too dry. If the ball is sticky or difficult to pull apart, then the mixture is too wet. A small film of moisture and soil should remain on your hands after breaking the ball.

It is important to remember that the above mentioned procedure - while frequently used and with reported success - is only valid for estimating field moisture control. Proper field or laboratory testing to determine the actual water content of a soil-cement mixture is always preferred and recommended.
Is there a test procedure to determine how much portland cement to use for a given FDR pavement project?
METHOD OF TEST FOR LABORATORY DESIGN OF SOIL-CEMENT MATERIAL

A. SCOPE

This method of test covers the procedures for making and testing hardened soil-cement samples prepared with portland cement. The laboratory maximum dry density and optimum moisture content are determined for samples at three different percentages of cement. Samples are then compacted in a mold before cement hydration to the established laboratory maximum dry density and optimum moisture content and tested for unconfined compressive strength.

B. MATERIALS

The materials shall consist of the following:
C. APPARATUS

The apparatus shall consist of the following:
D. SAMPLE PREPARATION

Sieve a sufficient quantity of the soil material through the ¾-inch (19 mm) sieve to determine maximum dry density and optimum moisture content and to provide at least nine, 4.0-inch (100 mm) diameter by 4.6-inch (115 mm) high compacted specimens having a total volume of 1/30-cubic foot (940 cc). (Usually about 100 pounds (45 kg) of dry soil is required.)

E. PROCEDURE
F. TESTING

Three samples for each cement content are tested uncapped for unconfined compressive strength after 7 days curing in the moist room. These samples are vertically loaded in a testing machine at a loading rate of .05 in/min (1.25 mm/min) until failure.

G. CALCULATIONS

Compressive Strength (PSI) = Load (lbs) at failure area of sample (in²)

Compressive Strength (kg/m²) = Load (kg) at failure area of sample (m²)

NOTE: No consideration is given to the length-diameter ratio (K-factor).
I've heard that improperly constructed soil-cement bases have a tendency to crack due to shrinkage. Is there a process that can be used to help minimize this problem?

To learn more see Field Investigation: Pre-Cracking of Soil-Cement Bases to Reduce Reflection Cracking, by Tom Scullion, PE, Research Engineer, Texas Transportation Institute, Texas A&M University, College Station, Texas.
Should the acceptance criteria of completed soil-cement be based on compressive or flexural strength test results of in-place cores or beams?
The Portland Cement Association (PCA) does not recommend the use of strength test results from in-place cores or beams as acceptance criteria for completed soil-cement including cement treated base (CTB) and full-depth reclamation (FDR). In addition, acceptance criteria for unconfined compressive or flexural strength test results based on molded specimens in the field are also not recommended.

The problem is the difficulty in obtaining representative samples of the in-place material as well as the time delay in addressing a possible problem that may exist. It is extremely rare for state or local transportation agencies to base their acceptance criteria on the in-place unconfined compressive strength of completed soil-cement bases due to the many difficulties involved and questionable results. Occasionally cores, beams, or field molded cylinders are obtained and tested, but these results are used for record purposes, and not used as acceptance criteria.

As an alternative to taking cores or beams, PCA promotes the adherence to quality control during design and construction. The normal steps to ensure that a quality project meeting the required specifications is met are as follows:
1. In the laboratory, determine the proper cement content that will be necessary to adequately stabilize the project materials as well as the optimum moisture content and maximum dry density for these materials.
2. In the field, apply the amounts of cement and water determined in the laboratory to the in-place materials.
3. In the field, thoroughly mix the in-place materials, cement, and water until a homogenous blend is obtained.
4. In the field, thoroughly compact and then shape the soil-cement material to project requirements.
5. In the field, properly cure the completed soil-cement until a final pavement surfacing is applied.
The purpose of field inspection and control of soil-cement during construction— cement content, moisture content, mixing, compaction, and curing—is to ensure that the results set out in the plans and specifications are obtained and if problems do arise they can be handled immediately.

Further information on the proper construction and quality control of cement-stabilized pavement materials are available in PCA publications Soil-Cement Construction Handbook and Soil-Cement Inspector’s Manual.
What are the different types of soil-cement materials?
Soil-cement is an engineered material designed and constructed for various pavement applications or material characteristics. The best soil-cement product is the one best suited to the specific application. Each of these cement-based pavement materials are engineered specifically for their intended purpose as follows:
The four major variables that control the properties and characteristics of soil-cement are (1) the nature of the soil material; (2) the proportion of cement in the mix; (3) moisture conditions; and (4) the degree of compaction. It is possible, simply by varying the cement content, to produce mixes ranging from those which result in only modification of the compacted soil to those which result in hard soil-cement that will meet durability and strength requirements.
What is micro-cracking (or pre-cracking) of cement-treated bases?

Microcracking is a construction process used to reduce the potential for reflective cracks in pavements that have cement-stabilized layers such as in cement-treated base (CTB) or full-depth reclamation (FDR). Sometimes in the United States this process is also referred to as pre-cracking, but care must be used with this term because in Europe pre-cracking refers to the construction of control joints in CTB. Microcracking was first tried in Austria, and has since been evaluated extensively in the United States by agencies such as the Texas Transportation Institute.

Any cement-treated material will shrink slightly as it cures and gains strength. Thin cracks in a cement-treated pavement layer occur naturally every 20 to 40 feet as the result of this shrinkage. The objective of microcracking is to induce hundreds of tiny cracks to accommodate the shrinkage, rather than larger individual cracks that have the potential to reflect up into the surface layer.

Microcracking is accomplished by loading the CTB/FDR layer with a vibrating roller approximately two days after construction. About four passes of the roller will complete the process. This action does not permanently damage the CTB/FDR layer since it is still “green” and will regain any strength lost due to the microcracking procedure.

The use of microcracking has moved from research to reality with many agencies now incorporating the process in their cement-stabilized pavement specifications. Results have shown that it will reduce the amount and severity of cracks, although it may not always eliminate reflection cracking. In addition, there does not appear to be a “down-side” to the process, since no negative effects of microcracking have been observed.
Which product is better—portland cement or hydrated lime—in stabilizing clay soils?

In 2003, an investigation was completed that evaluated the performance of portland cement and hydrated lime in stabilization and in improving engineering properties of several soils. Three soils were examined that had Plasticity Index (PI) values of 25, 37, and 42. The following general conclusions can be drawn based upon the investigation performed:

1. For the soil with a PI of 25, portland cement performed better than lime even at dosages as low as 3 percent. For the soils with a PI of 37 and 42, the performance increased significantly when the cement dosage was increased to 6 percent or more.

2. The maximum dry density of all three soils stabilized with portland cement attained higher maximum dry density than the lime-stabilized soils.

3. The unconfined compressive strength (UCS) of the soil with a PI of 25 stabilized with portland cement was almost always higher than that of lime-stabilized soil.

4. Strength of cement-stabilized soil is generally higher than lime-stabilized soil at all ages.

5. California Bearing Ratios (CBR) measured for all three soils stabilized with 6 percent and 9 percent portland cement or lime, clearly indicate that cement-stabilized soils have superior load bearing capacity.

6. For all three soils, stabilized with 6 percent and 9 percent portland cement, there was a significantly higher strength than lime-stabilized soils after vacuum saturation tests.

7. The physiochemical nature of cement-stabilized soils results in better wet-dry durability.

8. Hydraulic conductivities of the soil with a PI of 25 stabilized with portland cement were significantly lower than lime-stabilized ones at all dosage levels.

9. The concentration of calcium ions in leachates from lime-stabilized soils is generally higher than those from cement-stabilized soils.

For the complete findings contained in this investigation, please see PCA publication RD125 – Comparative Performance of Portland Cement and Lime Stabilization of Moderate to High Plasticity Clay Soils.