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POINT OF VIEW COLLECTOR’S EDITION

Type II cements, Fig 2 shows that until 1953, at corrosive environments. One unfortunate result of
least 50 percent of the cements had less than the AASHTO reduction of the w/c from 0.53 to 0.445
2
3000 lb/in strength at 7 days, whereas in was that some people thought that if reducing the
1994, none had such a low strength. Moreover, w-c from 0.53 to 0.445 was a good idea, it would be
approximately 50 percent of the Type II cements an even better idea to reduce it further to values
2
had 7-day strength in the 4500 to 5400 lb/in (31 like 0.3 because this is now possible with the high-
to 38 MPa) range. Now, commercially available range water-reducing admixtures. As discussed
portland cement easily meets the ASTM 28-day next, cases of severe cracking have been reported
minimum strength requirement in 3 to 7 days. Well in many structures built with very low w-c concrete
suited for the fast schedules of the construction mixtures.
industry, the demand for today’s portland cements
have virtually driven the slower-hardening and 1980 TO PRESENT
more durable portland cements of the past out of Since the early 1980s, increasing use of high-
the market place. range water reducing admixtures and highly
reactive pozzolans like silica fume has made it
Krauss and Rogalla proposed another reason possible to make concrete mixtures possessing
why the cracking and deterioration of concrete in high workability at very low water-cementitious
bridge decks have increased substantially since materials ratio (w-cm). Called high-performance
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the mid-1970s . They pointed out the coincidence concrete*, the product is normally characterised
2
between an upsurge in deterioration problems by 50 to 80 MPa (7,500 to 12,000 lb/in ) compressive
and a major change in the AASHTO specification strength at 28 days and a very low permeability in
laboratory specimens. Due to the high strength
in 1974. For over 40 years, from 1931 to 1973, the and high elastic modulus at relatively early ages,
AASHTO specification for bridge deck concrete the product quickly found its way into fast-track
2
required 3000 lb/in (20.7 MPa) as the minimum projects such as structural members for tall
28-day compressive strength. This concrete buildings. The use of high performance concrete
is characterized by a low elastic modulus and where impermeability and durability are prime
high creep at early ages and is therefore less considerations has generated considerable
prone to cracking from thermal and drying- controversy, as explained below.
shrinkage stresses. In response to increasing
cases of reinforcement corrosion resulting from The 1996 report by Krauss and Rogalla contains the
the widespread use of deicing salts on roads results of a survey of 200,000 newly-constructed
and bridges, AASHTO decided that something bridges in the U.S. and Canada . The report
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had to be done to reduce the permeability of showed that more than 100,000 concrete bridge
concrete. Consequently, in 1974, AASHTO made a decks had developed transverse cracks soon after
change in the concrete specification requiring a construction. This was attributed mainly to thermal
3
maximum 0.445 w-c, a minimum 362 kg/m (610 contraction by the authors. Usually, the cracks
2
3
lb/yd ) cement content, and a minimum 4500 lb/in were full depth and spaced 1 to 3 m (3.3 to 10 ft)
(30 MPa) compressive strength at 28 days. Krauss apart along the length of the bridge. The authors
and Rogalla believe that, due to the high thermal concluded that, under adverse environmental
conditions, the crack growth reduced the
and drying shrinkage, low creep, and high elastic permeability of concrete and accelerated the rate
modulus at early ages, these concrete mixtures of corrosion of reinforcing steel and deterioration
were crack-prone and therefore less durable in of concrete. It seems that deterioration problems

The Indian Concrete Journal | November 2018 89