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Bound Bases (Subbases)

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UFC 3-260-02
30 June 2001
of 152 by 152 millimeters (6 by 6 inches) with a length long enough to permit testing over a span of
457 millimeters (18 inches). The recommended procedures are widely accepted and extensively used
for determining the properties of PCC. The test procedure for determining flexural strength and modulus
of elasticity will be determined in accordance with ASTM C 78 and CRD-C 21, respectively. When
aggregate larger than the 51-millimeter (2-inch) nominal size is used in the concrete, the mix will be wet-
screened over a 51-millimeter (2-inch) square mesh sieve before it is used for casting the beam
specimen.
(3) Mix proportioning and control. Proportioning of the concrete mix and control of the concrete
for pavement construction will be in accordance with TM 5-822-7/AFM 88-6, Chapter 8. Normally, a
design flexural strength at 90-days will be used for pavement thickness determination. Should it be
necessary to use the pavements at an earlier age, consideration should be given to the use of a design
flexural strength at the earlier age or to the use of high early-strength cement, whichever is more
economical.
b. Bound Bases (Subbases).
(1) General. Chemically stabilized materials (portland cement, lime, fly ash, etc.) and
bituminous-stabilized materials need to be discussed separately, even though the conclusions regarding
inclusion of effects of repeated loading are the same. Due to the viscous and temperature-dependent
behavior of the bituminous binder, bituminous-stabilized materials are affected by temperature and rate
of loading to a much greater extent than any other component in a pavement structure.
(2) Requirements. Bituminous base materials are designed in accordance with TM 5-822-8/
AFM 88-6, Chapter 9. The design for frost consideration will be in accordance with Chapter 20, herein.
Chemically stabilized materials should meet requirements set forth in TM 5-822-14/AFM 32-1019.
Among these are requirements for durability and the requirement that strength increase with age. These
requirements are intended to ensure that the materials continue to function with age and that no adverse
chemical reactions occur. However, in terms of ensuring that the material functions as a bound material
(sustains flexural loading), it is required that the material attain an unconfined compressive strength of
1.7 MPa (250 psi) at 28 days. This requirement should be used in lieu of strength requirements in the
above references. Chemically treated soils in which no substantial increase in strength is considered
are modified soils and should be characterized using the methods presented herein for unbound base,
subbase, and subgrade materials. Chemically treated soils having unconfined compressive strengths
greater than 1.7 MPa (250 psi) should be tested in accordance with the methods specified for stabilized
materials. Pavement designs that result in a nonstabilized (pervious) layer sandwiched between a
stabilized or modified soil (impervious) layer and the pavement present the danger of entrapped water
with subsequent instability in the nonstabilized layer. These designs will not be used unless the
nonstabilized layer is positively drained, and its use on Air Force bases will require the approval of the
appropriate Air Force Major Command.
(3) Modulus and Poisson's ratio. The modulus of elasticity Ef of bound base material will be
determined from cyclic flexural tests of beams. The recommended test procedures have not been
standardized but are described in Appendix J. There are differences in the procedures for chemically
stabilized materials and those stabilized with bituminous binders. These differences are necessary
because of the sensitivity of bituminous-stabilized bases to rates of loading and temperature.
(a) A simply supported unconfined beam loaded at the third point with essentially point
loads will be used for bound bases (subbases). For chemically stabilized bound bases, the ultimate load
is first determined. Loads of 0.4, 0.6, and 0.8 times the ultimate load are applied repetitively, and the
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