Civil Engineering Reference
In-Depth Information
admixture dosage, fineness of the cement, mix proportions, and the time of
addition of admixture. According to the standards, the minimum initial set
time should be 1 hour earlier with a maximum of 1½ hours later, with
respect to the control. The compressive strength at 3, 7, and 28 days should
be at least 110% of the corresponding control mixes.
The water-reducing-retarding admixtures are used for avoiding
cold joints and facilitating large pours. They are used particularly in hot
weathering operations. These admixtures should increase the initial set
time by at least one hour, with a maximum by 3½ hours. The compressive
strength should be at least 110% of the control at 3, 7, and 28 days.
The water-reducing-accelerating admixtures may be used in win-
ter concreting as they permit early form removal and enable concrete to be
available for service earlier. The addition of these admixtures should result
in an initial set one hour earlier, with a maximum at 3½ hours earlier. The
compressive strength requirement is 125% of control mix at 3 days and
110% at 28 days.
Set retarders are used to offset the effects of high temperatures,
which decrease the set time. They are also applicable when unnecessary
delays occur between mixing and placement. In mass concrete, retarders
prolong the plasticity of fresh concrete, and that has advantages. Elimina-
tion of cold joints is possible with the use of retarders. With retarders, the
minimum initial set time should be 1 hour later, with a maximum of 3½
hours. The compressive strength should be a minimum of 90% of the
control at 3, 7, and 28 days.
Composition.
The materials that are used for water reducers and
retarders are generally composed of the following compounds:
1. Lignosulfonic acids and their salts.
2. Modifications and derivatives of lignosulfonic acids and
their salts.
3. Hydroxycarboxylic acids and their salts.
4. Modifications and derivatives of hydroxylated carboxy-
lic acids and their salts.
5. Miscellaneous formulations such as inorganic salts in-
cluding zinc salts, borates, phosphates, chlorides, amines
(their derivatives), carbohydrates, polysaccharides, poly-
meric materials, and sugar acids.
