TM 5-822-7/AFM 88-6, Chap. 8
g. Aggregate for calibration hardstands. To avoid
affect several qualities of the concrete, and some
inclusion in the pavement concrete of iron oxides or
desirable properties may be adversely affected.
Consequently, the decision to use an admixture
other iron-rich materials having magnetic properties
must be based on experience and tests with the
that will interfere with the operation of the facility,
the concrete aggregate proposed for paving calibration
cement, aggregates, and admixtures representative
of those to be used on the project. It must be demon-
hardstands will be subjected to detailed petrographic
strated that the final concrete proposed pavement
analyses in accordance with ASTM C 295 prior to
quality is not adversely affected by the use of any
acceptance. Special attention will be given to the
proposed admixture. More extensive guidance on
existence of magnetite in granites, high-iron
minerals in traprock, pyrite in limestone, and free
admixtures is provided in ACI 212.lR and ACI
iron or iron oxide in slag aggregate. When a list of
212.2R.
aggregate sources is included in the contract specifi-
cations, the sources not approved for concrete used
7. Pozzolans.
a. Pozzolans are defined in ASTM C 618 as
in calibration hardstands will be indicated.
"siliceous or siliceous and aluminous materials
h. Aggregate for power check pads. Concrete that
which in themselves possess little or no cementitious
will be exposed to extended jet engine blast during
value, but will, in finely divided form and in the
maintenance can use igneous rocks with a low silica
presence of moisture, chemically react with calcium
content and a high content of ferromagnesian
hydroxide at ordinary temperatures to form
minerals or slag as aggregate to improve its perfor-
compounds possessing cementitious properties."
mance under high temperature. Aggregates rich in
This ASTM specification recognizes three classes of
silica such as quartzite or chert should be avoided.
pozzolan and provides detailed requirements for
Aggregate with a high silica content such as sand-
each class. Class N pozzolans are raw or calcined
stone or granite, especially if coarse grained, can
natural pozzolans such as some diatomaceous earths,
cause problems. Calcareous aggregates such as lime-
opaline cherts, tuffs, volcanic ashes, clays, and
stone or dolomite are acceptable.
shales. Fly ash is divided into two classes. Class F is
6. Admixtures.
fly ash produced by burning anthracite or bituminous
a. Admixtures are used to modify properties of
coal, and Class C fly ash is produced by burning lignite
fresh or hardened concrete. Admixtures may be used
or subbituminous coal. Environmental Protection
to improve workability, control initial set, reduce
Agency (EPA) guidelines published in 1983 (40
heat generation, accelerate strength gain, increase
CFR, Part 249) require that the use of fly ash must
be allowed on all federal projects unless it can be
strength, and improve durability. Seven chemical
proven technically unsuitable.
admixtures are listed for use in portland cement
concrete in ASTM C 494, as follows: Type A--water-
b. Pozzolans may be substituted for a maximum
reducing admixture; Type B --retarding admixture;
of 25 percent by volume of portland cement in concrete
Type Caccelerating admixture; Type Dwater-
for pavements. If substitution of pozzolan above
reducing and retarding admixture; Type E--water-
this limit is contemplated, a thorough test program
reducing and accelerating admixture; Type F--
must show that the proposed substitution limit will
not adversely affect workability, admixture perfor-
water-reducing, high-range admixture; and Type
mance strength, and durability of the final concrete.
Gwater-reducing, high-range and retarding
admixture.
Pozzolans typically have specific gravities appreciably
lower than portland cement, and pozzolan limits are
b. Air-entraining admixtures provide resistance
to freezing and thawing and should meet the
expressed as a percent of the volume of cementitious
requirements of ASTM C 260, and will normally be
materials. In general, concrete containing pozzolans
required in all concrete for pavements. For small
has improved workability, lower heat of hydration,
jobs under 1,600 cubic yards, air-entraining
and a slower strength gain. When pozzolans are to
cements may be considered. The use of any other
be used, the required time to gain the design strength
may be increased beyond the standard 28 days for
admixture will depend on the economics of its use
roads and streets and 90 days for airfield pavements
and site conditions. The effect of an admixture will
vary, depending on conditions such as cement type,
to the extent that the concrete is not to be exposed
specific mixture proportions, ambient temperature,
to traffic.
or water quality. Furthermore, an admixture may
c. Class N pozzolans will not generally be
11
Previous Page
