UFC 3-260-02
30 June 2001
lift. Generally, stabilized layers used in subbase and base courses of military airfields should be
compacted to 100 percent of the laboratory modified compaction-energy density. TM 5-822-14/AFJMAN
32-1019 provides more comprehensive guidance on requirements for laboratory compaction and testing
procedures to be used with different stabilized materials. Addition of the stabilizer changes the
laboratory compaction characteristics of the soil or aggregates, and the trends are not always
predictable. For example, increasing the percent of portland cement used to stabilize a soil may either
shift the laboratory compaction curve up and to the left (i.e., increase maximum density and decrease
optimum moisture content) or down and to the right (i.e., decrease maximum density and increase
optimum moisture content). On the other hand, increasing lime contents decrease the laboratory
maximum density and increase the optimum moisture content for compaction. If field stabilizer contents
are increased for in situ mixing as noted in the previous paragraph, this may affect the laboratory
maximum density value that the contractor is required to meet in the field, and assessment of the
contractor's field compaction must take this into account. For instance, if the lime content is increased in
the field over that used in the laboratory, the contractor may encounter problems achieving the specified
density because the actual laboratory target density was decreased by the additional lime. When these
complex soil-stabilizer interactions are combined with field variation from distribution and mixing of the
stabilizer, fairly assessing the contractor's compaction efforts may become difficult. In circumstances
where stabilizer contents are being increased in the field, supplemental one-point compaction tests of
the in situ stabilized materials may prove helpful for assessing compaction compliance. HQUSACE
(CEMP-ET), appropriate Air Force MAJCOM pavements engineer, or the Naval Facilities Engineering
Service Center may be consulted for assistance with difficult cases.
h. Curing. In the subsequent sections, curing requirements are identified for many stabilizers. It is
crucial that this curing take place adequately for the stabilizer to achieve the desired results. Generally,
this means that temperatures must be high enough for the desired chemical reactions to occur, and
moisture must be maintained within the material and evaporation stopped or at least severely retarded.
Inadequate curing can negate the benefits of stabilization.
i. Testing. Tight financial restraints on military construction today often discourage adequate
testing. However, when working with stabilized materials, it is important to verify in the laboratory that
the proposed stabilization scheme will achieve the desired results. For instance, it is not sufficient to
simply select a suggested lime content for stabilizing a clay because the soils/clay mineralogy or the
presence of organic or some iron compounds in the soil may totally change or inhibit the chemical
reactions that occur. It is always prudent to perform sufficient laboratory work to verify that the
percentages of stabilizer, stabilizer type, and actual soil or aggregate will achieve the desired results
when they are mixed, compacted, and cured.
j. Lime Stabilization. Hydrated lime (Ca (OH)2), quick lime (CaO), or the dolomitic variants of these
limes are suitable for lime stabilization of soils. Requirements for the limes for soil stabilization are
contained in ASTM C 977. Calcium carbonate (CaCO3) is often sold under names such as agricultural
lime and is not suitable for soil stabilization.
(1) Mechanisms. Several things happen when lime is added to a soil. As the lime hydrates, it
dries the soil. Anhydrous quicklime is particularly effective for this. Some fine clay-sized soil particles
agglomerate when lime is added to the soil which results in a decrease in the measured number of clay-
sized soil particles. Essentially, a clayey soil fabric becomes siltier, and the soil is easier to work, dry,
etc. Also, cation exchange occurs, and the calcium from the lime replaces sodium and potassium in clay
minerals. This results in a reduction in plasticity of the soil. The above reactions (drying, particle
agglomeration, and cation exchange) occur rapidly after the lime is added to the soil. With time, some,
but not all, clays may undergo a further pozzolanic reaction with the lime and develop additional strength
9-3
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