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Soil-Cement Frequently Asked Questions
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Sandy Soils
Q: What are “Poorly Reacting Sandy Soils”
and how could they be detected prior to use in soil-cement applications?
In soil-cement construction, certain types of sandy soils are found
that cannot be treated successfully with normal amounts of portland
cement. These types of soils will experience lower compressive strengths
then would be expected based on the gradation of the soil. Although
a black or dark brown colored soil is a good indicator for the potential
for a poorly reacting sandy soil, appearance alone should not be
relied upon as the only indicator. Poorly reacting sandy soils,
which are typically acidic and/or have high organic contents, have
been encountered mainly in glaciated areas in the northern United
States and in isolated areas in the Eastern and Southeastern coastal
areas. It was determined that organic content and low pH do not
in themselves constitute a definite indication of poorly reacting
sand. However, sandy soil with an organic content greater than 2%
or having a pH lower than 5.3 will generally not react normally
with cement1.
Laboratory tests must be performed to detect the presence of these
types of sandy soils. The standard ASTM-AASHTO wetting and drying2
and, freezing and thawing3 test methods may be used; however, these
tests take more than one month to complete. The PCA’s “short-cut”
test procedure for sandy soils4 may also be used, but this procedure
requires a 7-day compressive strength to complete. These procedures
were developed in the 1950’s and have been widely used, even
today, to determine adequate cement contents for durability soil-cement.
A quicker test method to detect poorly reacting sandy soils was
developed by Robbins and Mueller5 in late 1950’s. The test
takes about one hour and involves determining the ability of the
soil to immobilize or absorb calcium. High absorption indicates
the cement may not react normally with the soil. To measure the
absorption of calcium by a soil, calcium ions must be placed in
contact with the soil grains. A standard saturated and carbonate-free
solution of calcium hydroxide is used as the agent. When a standard
solution of calcium hydroxide is mixed with a predetermined amount
of soil, the amount of calcium actually absorbed by the soil can
be determined by titrating the calcium remaining in solution. This
amount of calcium, when compared with that available in the standard
solution, provides a direct measure of the calcium absorption ability
of the soil. A complete description of the testing procedure is
given in reference 5.
Ways to mitigate the problem of poorly reacting sandy soils include
the use of calcium chloride and other chemicals to effectively neutralize
this adverse reaction6,7. The use of a small percentage of calcium
chloride (CaCl2) on the order of 0.6 to 1.0 percent by weight of
dry soil or blending normal reacting aggregate into the mixture
have also been effective in minimizing the problem. Research also
shows that the addition of sodium chloride (NaCl) may also be effective.
Typically poorly reacting sandy soils are located in the top few
feet of the surface. These soils should be stripped and (1) used
on the job in applications other than soil-cement, (2) diluted with
normally reacting soil or treated with admixtures as mentioned above,
or (3) wasted. The underlying normally reacting soils can be excavated,
stockpiled and used in soil-cement.
References:
- M. D. Catton, “Research on Physical Relations of Soil
and Soil-Cement Mixtures.” Highway Research Board Proceedings,
1940
- Methods of Wetting and Drying Test of Compacted Soil-Cement Mixtures,
ASTM Designation: D 559; AASHTO Designation: T 135
- Methods of Freezing and Thawing Test of Compacted Soil-Cement
Mixtures, ASTM Designation: D 560; AASHTO Designation: T 136v
- Portland Cement Association Publication EB052.07S, 1992, Soil-Cement
Laboratory Handbook, Chapter 6
- E. G. Robbins and P. E. Mueller, “Development of a Test
for Identifying Poorly Reacting Sandy Soils Encountered in Soil-Cement
Applications.” Highway Research Board, Bulletin 267, 1960
- M. D. Catton and E. J. Felt, “Effect of Soil and Calcium
Chloride Admixtures on Soil-Cement Mixtures.” Highway Research
Board Proceedings, 1943
- T. W. Lambe, A. S. Michael, Za-Chieh Moh, “Improvement
of Soil-Cement with Alkali Metal Compounds.” Highway Research
Board, Bulletin 241, 1960
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