Civil Engineering Reference
In-Depth Information
Fire Resistance and Thermal Properties
Table 5-6. Harmful Materials in Aggregates
Effect on
Test
The fire resistance and thermal properties of concrete—
conductivity, diffusivity, and coefficient of thermal expan-
sion—depend to some extent on the mineral constituents
of the aggregates used. Manufactured and some naturally
occurring lightweight aggregates are more fire resistant
than normal-weight aggregates due to their insulating
properties and high-temperature stability. Concrete
containing a calcareous coarse aggregate performs better
under fire exposure than a concrete containing quartz or
siliceous aggregate such as granite or quartzite. At about
590°C (1060°F), quartz expands 0.85% causing disruptive
expansion (
ACI 216
and
ACI 221
). The coefficient of ther-
mal expansion of aggregates ranges from 0.55 x 10
-6
per °C
to 5 x 10
-6
per °C (1 x 10
-6
per °F to 9 x 10
-6
per °F). For
more information refer to Chapter 15 for temperature-
induced volume changes and to Chapter 18 for thermal
conductivity and mass concrete considerations.
Substances
concrete
designation
Organic
Affects setting
ASTM C 40
(AASHTO T 21)
impurities
and hardening,
ASTM C 87
(AASHTO T 71)
may cause
deterioration
Materials finer
Affects bond,
ASTM C 117
(AASHTO T 11)
than the 75-
µ
m
increases water
(No. 200) sieve
requirement
Coal, lignite,
Affects durability,
ASTM C 123
(AASHTO T 113)
or other
may cause
lightweight
stains and
materials
popouts
Soft particles
Affects durability
ASTM C 235
Clay lumps
Affects work-
ASTM C 142
(AASHTO T 112)
and friable
ability and
particles
durability, may
cause popouts
Chert of less
Affects
ASTM C 123
(AASHTO T 113)
than 2.40
durability, may
ASTM C 295
relative density
cause popouts
Alkali-reactive
Causes
ASTM C 227
POTENTIALLY HARMFUL MATERIALS
aggregates
abnormal
ASTM C 289
expansion,
ASTM C 295
Harmful substances that may be present in aggregates
include organic impurities, silt, clay, shale, iron oxide, coal,
lignite, and certain lightweight and soft particles (Table
5-6). In addition, rocks and minerals such as some cherts,
strained quartz (
Buck and Mather 1984
), and certain dolo-
mitic limestones are alkali reactive (see Table 5-7). Gypsum
and anhydrite may cause sulfate attack. Certain aggre-
gates, such as some shales, will cause popouts by swelling
(simply by absorbing water) or by freezing of absorbed
water (Fig. 5-18). Most specifications limit the permissible
amounts of these substances. The performance history of
an aggregate should be a determining factor in setting the
limits for harmful substances. Test methods for detecting
harmful substances qualitatively or quantitatively are
listed in Table 5-6.
Aggregates are potentially harmful if they contain
compounds known to react chemically with portland
cement concrete and produce any of the following: (1)
significant volume changes of the paste, aggregates, or
both; (2) interference with the normal hydration of
cement; and (3) otherwise harmful byproducts.
Organic impurities may delay setting and hardening
of concrete, may reduce strength gain, and in unusual
cases may cause deterioration. Organic impurities such as
peat, humus, and organic loam may not be as detrimental
but should be avoided.
Materials finer than the 75-µm (No. 200) sieve, espe-
cially silt and clay, may be present as loose dust and may
form a coating on the aggregate particles. Even thin coat-
ings of silt or clay on gravel particles can be harmful
because they may weaken the bond between the cement
paste and aggregate. If certain types of silt or clay are pres-
ent in excessive amounts, water requirements may
increase significantly.
map cracking,
ASTM C 342
and popouts
ASTM C 586
ASTM C 1260 (AASHTO T 303)
ASTM C 1293
Table 5-7. Some Potentially Harmful Reactive
Minerals, Rock, and Synthetic Materials
Alkali-carbonate
Alkali-silica reactive substances*
reactive substances**
Andesites
Opal
Calcitic dolomites
Argillites
Opaline shales
Dolomitic limestones
Certain siliceous
Phylites
Fine-grained dolomites
limestones
Quartzites
and dolomites
Quartzoses
Chalcedonic cherts
Cherts
Chalcedony
Rhyolites
Cristobalite
Schists
Dacites
Siliceous shales
Glassy or
Strained quartz
cryptocrystalline
and certain
volcanics
other forms
Granite gneiss
of quartz
Graywackes
Synthetic and
Metagraywackes
natural silicious
glass
Tridymite
* Several of the rocks listed (granite gneiss and certain quartz for-
mations for example) react very slowly and may not show evi-
dence of any harmful degree of reactivity until the concrete is over
20 years old.
** Only certain sources of these materials have shown reactivity.
