Page 91 - ICJ Jan 2023

POINT OF VIEW COLLECTOR’S EDITION

MPa) specified strength and exposed to deicing MINERAL ADMIXTURES
chemicals or seawater, is presented below. Mineral admixtures, such as ground granulated

blast-furnace slag and ASTM Class F flyash, are
CEMENT CONTENT highly effective in reducing the heat of hydration,

According to the code, a maximum 0.40 w/cm and strength, and elastic modulus of concrete at early
2
a minimum 5000 lb/in (35 MPa) concrete mixture age. This is why properly cured concrete mixtures
containing high volumes of slag or flyash (50
shall be specified. Generally, the average concrete
percent or more by mass of the cementitious
2
strength will be from 700 to 1400 lb/in (5 to 10 MPa)
material) are generally less crack-prone and
higher than the specified strength, depending on
therefore less permeable in service, which is an
whether or not field strength test data are available important factor in controlling the deterioration
to establish a standard deviation. For a 1 in (25 of concrete from causes such as reinforcement

mm) maximum-size aggregate and 4 in (100 mm) corrosion, alkali-aggregate expansion, and sulfate
slump, the ACI 211 tables for non air-entrained attack. The construction codes should incorporate
3
3
concrete recommend 325 lb/yd (195 kg/m ) guidelines on the use of a high volume of mineral
water content. A normal water—reducing admixtures in concrete structures designed for
durability.
admixture, by reducing the water—requirement
7 to 8 percent, will bring down the water content
CRACK WIDTH AND DURABILITY
3
3
to 300 lb/yd (180 kg/m ). Thus, at the maximum
There are no clear guidelines in the ACI Manual of
3
permitted 0.40 w-cm, one would need 750 lb/yd
Construction Practice on the relationship between
3
(450 kg/m ) cement content. If the water reduction
crack width and durability of reinforced concrete
is doubled by the use of a high-range water-
structures exposed to different environmental
reducing admixture, one would still need 690 lb/ conditions. Although ACI 224R-98 suggests 0.15
3
3
yd (410 kg/m ) cement. Theoretical considerations and 0.18 mm as maximum tolerable crack widths
as well as field experience shows that these cement at the tensile face of reinforced concrete structures
contents are too high to obtain crack-free, durable exposed to deicing chemicals or seawater,
structures. respectively, the report also contains a disclaimer
that the crack-width values are not a reliable
WATER CONTENT indicator of the expected reinforcement corrosion
and concrete deterioration. For a designer to
ACI 318 controls the water content by specifying
exercise engineering judgement on the extent of
a maximum limit on w-cm. As shown above, this
needed crack control, at least some understanding
approach is unsatisfactory when the cementitious of the effect of cracks and microcracks (less than 0.1
material happens to be exclusively or mostly mm) on the permeability of concrete is necessary.
portland cement. From standpoint of durability, it A brief summary is presented herein. Generally, at
is apparent that a direct control on the maximum the interfacial transition zone between the cement
allowable water content in the concrete mixture is mortar and coarse aggregate or reinforcing steel,
essential. a higher than average w-cm exists, which results

92 The Indian Concrete Journal | November 2018