Building Design and Construction Handbook - Merritt - Ventech!

4.26 SECTION FOUR
TABLE 4.6 High-Performance Concrete Strength Characteristics
Characteristic Grade 1 Grade 2 Grade 3 Grade 4
Compressive strength,
ksi
Modulus of elasticity,
103 ksi
Shrinkage, microstrain,
in/in
Creep, microstrain,
in/in
6 � X � 8 8 � X � 10 10 � X � 14 X � 14
4 � X � 6 6 � X � 7.5 X � 7.5
600 � X � 800 400 � X � 600 X � 400
0.41 � X � 0.52 0.31 � X � 0.41 0.21 � X � 0.31 X � 0.21
naphthalene-based superplasticizers are used, whereas lignosulfonate-based water
reducers may actually increase air content. Larger amounts of air-entraining agent
may be needed for high-strength concretes, especially for low-slump mixes with
high cement content and mixes with large amounts of some types of fly ash. Furthermore,
some types of superplasticizers and air-entraining admixtures may not be
compatible with each other.
(‘‘State-of-the-Art Report on High-Strength Concrete,’’ ACI 363.)
4.17.5 Nonstructural or Foamed Cellular Concretes
These are formed by the use of admixtures that generate or liberate gas bubbles in
concrete in the plastic stage. Aluminum powder, which reacts with the alkalies in
cement to release hydrogen, is generally used for this purpose, although hydrogen
peroxide, which generates oxygen, or activated carbon, which liberates absorbed
air, can be used. These foaming agents create stable, uniformly dispersed air spaces
within the concrete when it sets. Perlite and vermiculite are most frequently used
as aggregates. The resulting concrete may weigh 50 lb/ft 3 or less and have a compressive
strength up to 2500 psi. Applications of such lightweight concretes include
topping and soundproofing barriers over structural concrete slabs.
The effectiveness of the admixture is controlled by the duration of mixing, handling,
and placing of the mix relative to the gas-generation rate. The amount of
unpolished aluminum powder to be added to a mix may range from 0.005 to 0.02%
by weight of cement under normal conditions. Larger quantities, however, may be
used to produce lower-strength concretes. More aluminum may be needed at low
temperatures to achieve the same amount of concrete expansion, for example, twice
as much as 40�F asat70�F. Furthermore, at low temperatures, to speed up gas
generation, it may be necessary to add to the mix alkalies such as sodium hydroxide,
hydrated lime, or trisodium phosphate. Also, to prevent the powder from floating
on the surface of mixing water, the aluminum may be premixed with sand or
combined with other admixtures.
Curing is very important. If good curing practices and jointing are not followed,
extensive drying shrinkage may result.
4.18 POLYMER CONCRETES
Plastics with long-chain molecules, called polymers, are used in several ways to
enhance concrete properties: replacement of portland cement, incorporation in a
mix as an admixture, and impregnating hardened concrete.