Building Design and Construction Handbook - Merritt - Ventech!

4.16 SECTION FOUR
with less cement at constant water-cement ratio. And they may be used to produce
self-compacting, self-leveling flowing concretes, for such applications as longdistance
pumping of concrete from mixer to formwork or placing concrete in forms
congested with reinforcing steel. For these concretes, the cement content or watercement
ratio is not reduced, but the slump is increased substantially without causing
segregation. For example, an initial slump of 3 to 4 in for an ordinary concrete mix
may be increased to 7 to 8 in without addition of water and decrease in strength.
Superplasticizers may be classified as sulfonated melamine-formaldehyde condensates,
sulfonated naphthaline-formaldehyde condensates, modified lignosulfonates,
or synthetic polymers.
4.13.2 Air-Entraining Admixtures
These create numerous microscopic air spaces within concrete to protect it from
degradation due to repeated freezing and thawing or exposure to aggressive chemicals.
For concrete exposed to repeated cycles of freezing and thawing, the air gaps
provide room for expansion of external and internal water, which otherwise would
damage the concrete.
Since air-entrained concrete bleeds to a lesser extent than non-air-entrained, there
are fewer capillaries extending from the concrete matrix to the surface. Therefore,
there are fewer avenues available for ingress of aggressive chemicals into the concrete.
The ‘‘Standard Specification for Air-Entraining Admixtures for Concrete,’’
ASTM C260, covers materials for use of air-entraining admixtures to be added to
concrete in the field. Air entrainment may also be achieved by use of Types IIA
and IIIA portland cements (Art. 4.2.2).
4.13.3 Set-Accelerating Admixtures
These are used to decrease the time from the start of addition of water to cement
to initial set and to increase the rate of strength gain of concrete. The most commonly
used set-accelerating admixture is calcium chloride. Its use, however, is
controversial in cases where reinforcing or prestressing steel is present. The reason
is that there is a possibility that the accelerator will introduce free chloride ions
into the concrete, thus contributing to corrosion of the steel. An alternative is use
of one of many admixtures not containing chloride that are available.
4.13.4 Retarding Admixtures
To some extent, all normal water-reducing admixtures retard the initial set of concrete.
A Type B or D admixture will allow transport of concrete for a longer time
before initial set occurs. Final set also is delayed. Hence, precautions should be
taken if retarded concrete is to be used in walls.
Depending on the dosage and type of base chemicals in the admixture, initial
set can be retarded for several hours to several days. A beneficial side effect of
retardation of initial and final sets is an increase in the compressive strength of the
concrete. A commonly used Type D admixture provides higher 7- and 28-day
strengths than a Type A when used in the same mix design.