PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 297

procedures, and typical curing practices. These requirements may include
the following enhancements:
ease of placement and compaction
long-term mechanical properties
early-age strength
toughness
volume stability
extended life in severe environments
These enhanced characteristics may be accomplished by altering the ag-
gregate gradation, including special admixtures, and improving mixing and
placement practices. Currently, a compressive strength in the order of 70 to
175 MPa (10,000 to 25,000 psi) can be obtained. As the need for HPC is bet-
ter understood and embraced by the engineering community, there will prob-
ably be a transition in concrete specification from the current prescriptive
method to the performance-based or performance-related specifications. A
Strategic Highway Research Program (SHRP) study (Zia et al. 1991) defined
HPC as
maximum water-cementitious materials ratio of 0.35;
minimum durability factor of 80%, as determined by ASTM C 666,
Procedure A; and
minimum strength criteria of either:
a. 21 MPa (3000 psi) within 4 hours after placement (Very Early
Strength, VES),
b. 34 MPa (5000 psi) within 24 hours (High Early Strength, HES), or
c. 69 MPa (10,000 psi) within 28 days (Very High Strength, VHS).
Thus, high-performance concrete is characterized by special performance,
both short-term and long-term, and uniformity in behavior (Nawy 1996).
Such requirements cannot always be achieved by using only conventional
materials or applying conventional practices. Since concrete is the most
widely used construction material worldwide, new concrete construction
has to utilize the currently available new technology in order to eliminate
costly future rehabilitation.
Revolutionary new construction materials and modifications and im-
provements in the behavior of traditional materials have been developed in
the last three decades. These developments have been considerably facilitat-
ed by increased knowledge of the molecular structure of materials, studies of
long-term failures, development of more powerful instrumentation and mon-
itoring techniques, and the need for stronger and better performing materials,
suitable for larger structures, longer spans, and more ductility.
In spite of the current advances of the concrete technology and the devel-
opment of high-performance concretes, it is expected that the concrete indus-
try will continue improving through the development of new components
and admixtures, microstructural studies, blended cement compositions, better
material selection and proportioning techniques, and more efficient placement
techniques.