TM 5-822-7/AFM 88-6, Chap. 8
outweigh its cost and long-term strength loss.
comparable to the project under investigation.
c. Recycled concrete as aggregate. Existing
d. There are also some expansive cements,
designated as Types K, M, and S in ASTM C 845,
concrete may be taken up, crushed to suitable
that may find some application where concrete
gradation, and re-used as concrete aggregate. It
shrinkage needs to be minimized or avoided.
must meet all the requirements for gradation and
However, there is little experience with these
quality that conventional aggregates must meet.
cements, and they will require careful investigation
All reinforcing steel should be removed. During
crushing, hammermill secondary crushers tend to
before use in pavement construction.
produce excess fines and should not be used. The
e. At least one United States manufacturer is
recycled concrete aggregates will not normally
actively promoting a slag cement made by grinding
require washing unless they have been contaminated
iron blast furnace slag. This may be substituted for
with base or subgrade material. If the original pave-
up to 50 percent of the portland cement in a pavement
ment that is being recycled is D-cracked, then the
mixture if tests show the required final properties
concrete should be crushed to a maximum size of
are achieved in the hardened concrete. Preblended
inch. When crushed concrete is being used as fine
mixes of portland-pozzolan (Type IP), and portland-
aggregate, the inclusion of some natural sand may
slag cements (Type IS) are described in ASTM C 595
be required to improve the new concrete mixture's
and are acceptable for use in pavements.
workability.
d. Alkali-aggregate reactions. Minerals in some
aggregates can chemically react with the cement
5. Aggregates.
a. Approval of aggregates. The contractor will use
alkalis, resulting in an increase in volume that
aggregates from approved sources. If the contractor
places the concrete under expansive stresses which
proposes to use aggregates from an unapproved
may result in map cracking, popouts, strength loss,
source, the Government will be responsible for
and concrete expansion. Once started, the reaction
conducting tests to determine whether the proposed
and the resulting progressive deterioration of the
aggregates will meet the requirements of the project.
concrete cannot be stopped. These reactions are
generally associated with aggregates containing
Sampling and aggregate delivery costs will be borne
by the contractor. The contract specifications will
poorly ordered forms of silica, such as opal,
chalcedony, chert, or flint (alkali-silica reaction),
state aggregate sample size, delivery location, and
required evaluation time for the proposed aggre-
complex layer-lattice, and silicate minerals such as
gates. For small jobs requiring 1,600 cubic yards or
graywackes, phyllites, siltstones, or argillites (some-
less of concrete, all tests for aggregates from an
times differentiated as alkali-silicate reaction), or
unapproved source will be done by the contractor.
argillaceous, dolomitic limestone (alakali-carbonate
reaction). A petrographic investigation in conjunction
b. Quality. Aggregates must generally meet the
requirements of ASTM C 33, as modified in the
with results of tests in accordance with ASTM C 227
following paragraphs. These requirements can be
for alkali-silica expansion and ASTM C 586 for
alkali-carbonate expansion is the most promising
adjusted as necessary to reflect local experience
with specific aggregates to insure economical use of
approach for identifying these problems. If possible,
aggregates to meet project requirements. The
reactive aggregates should be avoided. If they cannot
magnesium or sulfate soundness test (ASTM C 88)
be avoided, low-alkali cement should be specified. If
required in ASTM C 33 has not been consistently
low-alkali cement is not available or is overly expen-
successful in identifying frost-resistant aggregate.
sive, some pozzolans and slag cements have been
Consequently, if an otherwise suitable aggregate
successful in countering alkali-silica reactions, or
fails this test, it should be further investigated
reactive aggregate may be blended with non-reactive
aggregate to lessen the effect of the reactions.
using freezing and thawing tests as described in
e. Coarse aggregate.
ASTM C 666 or ASTM C 682 before it is finally
rejected. Similarly, if an otherwise suitable aggre-
natural gravel, crushed gravel, crushed stone, crushed
gate fails the Los Angeles (LA) abrasion test
recycled concrete, or iron blast furnace slag. The
(ASTM C 131 or C 535), it can be accepted if it has a
crushing of gravel tends to improve the quality and
history of local use showing that it can be processed
the bond characteristics and generally results in a
without unacceptable degradation and that it is
higher flexural strength of concrete than if uncrushed
durable under weathering and traffic conditions
4
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