Page 89 - ICJ Jan 2023

POINT OF VIEW COLLECTOR’S EDITION

with concrete bridge decks probably increased was finished, Fig 3. This cracking was due to
in the mid-1970s after AASHTO mandated the very high thermal contraction and autogenous
use of high-strength concrete mixtures, and the shrinkage resulting from the use of a high
problem was not resolved in the 1980s when cement content (w/c = 0.31), and a fast-hydrating
high-performance concrete with even higher Type II cement. The fineness was 391 m /kg
2
early strength was incorporated into the highway and the C A-plus-C S content was 72 percent and
construction practice. 3 3
was the highest of approximately 200 Type II North
American cements produced in 1994. The cracking
According to Krauss and Rogalla: tendency of this concrete mixture was further
“When high cement content HRWR admixtures exacerbated by the addition of silica fume, which
(superplasticiser) and silica fume are used, one- is known to increase the autogenous shrinkage
day moist-cured compressive strengths of 27.6 to of concrete. In conventional concrete, the
2
55 MPa (4000 to 8000 lb/in ) have been achieved. autogenous shrinkage of less than 50 millionths
These concretes would have 1- day modulus of can be ignored, but a high-strength concrete may
6
elasticity of 28.8 to 35.8 GPa (3.6 to 5.2 x 10 lb/ have an autogenous shrinkage of several hundred
2
in ), - values 3 to 7 times those of a nominal 20.7 millionths, which is as high as the drying shrinkage.
2
MPa (3000 lb/in ) concrete used before 1974. These
very high strength concretes also have significantly
reduced creep potential. The brittleness relates
to dramatically reduced creep potential and the
observed early cracking or other unusual cracking
that is not consistent with engineer’s experience
12
with more conventional concrete .

Field experience with bridge decks in Virginia,
Kansas, Texas, and Colorado cited by Burrows
7
confirms Krauss and Rogalla’s conclusions . In
1974, bridge deck cracking in Virginia reportedly
increased when the strength requirement was
2
raised from 3000 to 4000 lb/in . Similarly, a 1995
report on the condition of 29 bridges in Kansas
stated that there was twice as much cracking with
2
2
6400 lb/in (44 MPa) concrete than with 4500 lb/in
(31 MPa) concrete. In 1997, the high—performance
concrete deck in the Louetta Overpass—a
showcase bridge in Texas— cracked more than
the conventional concrete deck in the adjoining Fig 3 Early-age cracking in the high-performance
lane. In Denver, the high-strength concrete in the concrete used in the construction of the new 23rd Street
Viaduct in Denver, Colo.
23rd Street Viaduct cracked before construction

90 The Indian Concrete Journal | November 2018