PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 225

Retarders can reduce the strength of concrete at early ages (e.g., one to
three days). In addition, some retarders entrain air and improve workability.
Other retarders increase the time required for the initial set but reduce the
time between the initial and final set. The properties of retarders vary with
the materials used in the mix and with job conditions. Thus, the use and ef-
fect of retarders must be evaluated experimentally during the mix design
process.

■

Hydration-Control Admixtures

6.11.4

These admixtures have the ability to stop and reactivate the hydration
process of concrete. They consist of two parts: a stabilizer and an activator.
Adding the stabilizer completely stops the hydration of the cementing ma-
terials for up to 72 hours, while adding the activator to the stabilized con-
crete reestablishes normal hydration and setting. These admixtures are very
useful in extending the use of ready-mixed concrete when the work at the
jobsite is stopped for various reasons. They are also useful when concrete is
being hauled for a long time.

■ 6.11.5 Accelerators

Accelerators are used to develop early strength of concrete at a faster rate
than that developed in normal concrete. The ultimate strength, however, of
high early strength concrete is about the same as that of normal concrete. Ac-
celerators are used to

1. reduce the amount of time before finishing operations begin
2. reduce curing time
3. increase rate of strength gain
4. plug leaks under hydraulic pressure efficiently

The first three reasons are particularly applicable to concrete work placed
during cold temperatures. The increased strength gained helps to protect the
concrete from freezing and the rapid rate of hydration generates heat that
can reduce the risk of freezing.
Calcium chloride, is the most widely used accelerator (ASTM
D98). Both initial and final set times are reduced with calcium chloride.
The initial set time of 3 hours for a typical concrete can be reduced to 1.5
hours by adding an amount of calcium chloride equal to 1% of the ce-
ment weight; 2% reduces the initial set time to 1 hour. Typical final set
times are 6 hours, 3 hours, and 2 hours for 0%, 1%, and 2% calcium chlo-
ride. Figure 6.10 shows that strength development is also affected by
for plain portland cement concrete (PCC) and portland cement concrete
with 2% calcium chloride. Concrete with develops higher early
strength compared with plain concrete cured at the same temperature
(Hewlett 1978).

CaCl 2 ,

CaCl 2

CaCl 2