Page 167 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Accelerators 151

2.4 Effect of Calcium Chloride on Concrete

Calcium chloride reduces significantly both the initial and final
setting times of concrete. This is useful for concreting operations at low or
moderate temperatures. It permits quicker finishing and earlier use of slabs.
The setting times are decreased as the amount of CaCl is increased.
2
Excessive amounts, e.g., 4–5%, may cause rapid set and should be avoided.
Even using the same type of cement, obtained from different sources, shows
differences in setting characteristics. There is no direct correlation between
the acceleration of setting and subsequent strength development in the
hardened state.
It is recognized that CaCl increases the early strength of cement
2
paste, mortar, and concrete. The actual values depend on the amount of
CaCl added, the mixing sequence, temperature, curing conditions, w/c
2
ratios, and the type of cement. Calcium chloride increases the strengths of
all types of cements (Types I to V ASTM designation). The time required
for concretes to attain a strength of 13.8 MPa using 2% CaCl is indicated
2
in Table 2. [36] The optimum dosage suggested varies between 1 and
4%. [37][40] Most practitioners, however, recommend a dosage not exceeding
2% flake CaCl or 1.5% anhydrous CaCl .
2
2
The acceleration of strength development is also achieved at
temperatures lower than the ambient temperature. Figure 3 shows the
relative strengths developed in concrete cured for periods 1, 3, 7, and 28
days and cured at temperatures of -4.0, -4.5, 13, and 23°C. [1][36][40] Although
the addition of CaCl results in greater strengths at ambient temperatures of
2
curing, the percentage increase in strength is particularly high at lower
temperatures of curing.

Table 2. Time Required for Concrete to Attain a Compressive Strength of
13.8 MPa Using 2% CaCl 2

Time (days)
Types of Cement Plain Concrete Concrete Containing
Calcium Chloride
I 4 1.5
II 5 2.0
III 1 0.6
IV 10 4.0
V 11 5.5