Page 93 - ICJ Jan 2023

POINT OF VIEW COLLECTOR’S EDITION

be the responsibility of ACI Committee 224 • To serve the goal of materials conservation, a
which does not want to deal with durability. paradigm shift is needed from prescriptive to
The root causes of many durability problems performance-based standard specifications
can be traced to this kind of reductionistic for materials. For example, ASTM C 1157-
approach. By ignoring the cracking-durability 98a, Standard Performance Specification for
relationship and by overemphasising the Hydraulic Cement, describes a general-use
relation between strength and durability, ACI cement (Type GU), which has maximum
318 is not helping the cause of constructing limits of 2900 lb/in (20 MPa) and 4350 lb/in
2
2
durable and environmentally sustainable
concrete structures. A paradigm shift to (30 MPa) on the 3- and 7-day compressive
a holistic approach to control cracking in strength, respectively. This specification
concrete structures is necessary to create a also covers a moderate-heat cement (Type
2
much closer working relationship between MH) with 2175 and 2900 lb/in (15 and 20
the structural designer, materials engineer, MPa) maximum strength at 3 days and 7
and construction personnel than exists today. days, respectively. There are no restrictions
on the composition and fineness of the
• The belief that the durability of concrete can
be controlled by controlling the w-cm is not cements; however, to satisfy the maximum
3
correct because it is not the w-cm but the on strength, the fineness and the C S content
water content that is more important for the of modern portland cement will have to be
control of cracking. A reduction in the water controlled. This can be achieved by making a
content will bring about a corresponding coarse-ground, low-C S portland cement or
3
reduction in the cement content at a given by blending normal portland cement with a
value of strength, which in turn, will reduce high volume of fly ash or slag. Compared to
thermal contraction, autogenous shrinkage, the Type I/II cements conforming to ASTM
and drying shrinkage of concrete. Therefore, C 150, the Type GU and Type MH cements
to achieve durability, the standard practice produced according to ASTM C 1157-98a are
for selecting concrete-mixture proportions expected to be less crack-prone.
will have to undergo a fundamental change.
Note that a change in emphasis from the CONCLUSIONS
w-cm strength relation to the water content In the 20th century, the concrete construction
durability relation will provide the needed industry, driven primarily by the economics
incentive for a much closer control of the of higher and higher speeds of construction,
aggregate grading than is customary in the increasingly used cements and concrete mixtures
current construction practice. A substantial possessing high-early strength. Consequently,
reduction in water requirement can be the field experience with many modern concrete
achieved by using a well-graded aggregate. structures shows that they are crack-prone and
Additional reductions in the water content of those exposed to severe environments tend to
concrete mixtures can be realized by the use
of midrange or high-range water-reducers, deteriorate much faster than their anticipated
high-volume fly ash or slag cements, and service life. To build environmentally sustainable
coarse-ground portland cements. concrete structures, it is clear that instead of

94 The Indian Concrete Journal | November 2018