Use of Admixture and Its Effects on Setting Time

Initial set, as defined by ACI 116R, is a degree of stiffening of a mixture of cement less than final set, generally stated as an empirical value indicating the time in hours and minutes required for the cement paste to stiffen sufficiently to resist to an established degree, the penetration of a weighted test needle. The concrete setting time at various temperatures is given at table below:

Table 1 Setting Time of Concrete at Various Temperature
Temperature Approximate Setting Time (hours)
100oF (38oC) 1-2/3
90oF (32oC) 2-2/3
80oF (27oC) 4
70oF (21oC) 6
60oF (16oC) 8
50oF (10oC) 11
40oF (4oC) 14
30oF (-1oC) 19
20oF (-7oC) Set will not occur
The retardation of initial setting time by the use of admixture is affected by three factors, that is, the ambient temperature, the dosage used, and the time of adding to the batch.


Temperature Effect on Retardation of Initial Setting Time

Temperature can have a detrimental effect to concrete strength development. However, proper cold weather concrete curing will enhance concrete strength development. Hot weather is defined as any combination is high ambient temperature, high concrete temperature, low relative humidity, and wind velocity. Cold weather period, as defined by ACI Committee 306, is when one of the following conditions occur for three consecutive days:


The effect of concrete temperature and retardation of setting time is given by PCA in the chart below. It is concluded from the chart that the retardation effect is more pronounced when a higher concrete temperature is used.

Figure 1 Effect of Concrete Temperature and Retarder on Setting Time


Retardation of setting time is influenced by the type of admixtures used. The chart below illustrates the effect of various lignosufonates (1 and 2) and carboxylic (3 and 4) admixtures on setting time.


Figure 2 Effect of Various Admixtures on Concrete Setting Time


The time of adding the admixture to the batch here is significant and may affect final results. More retardation may take place if the admixture is added as the last ingredient and the cement is wet.

Dosage Effect on Retardation of Initial Setting Time

Higher dosage may be used up to a certain level only prior to when any rapid stiffening and slump loss occurs. This admixture is sensitive to ambient temperature when introduced to the batch. The lower the ambient temperatures, the longer time of setting for the concrete will be. The following figure is used to estimate initial setting time according to the dosage of retarder and concrete ambient temperature.

Figure 3 Increasing Initial Setting Time with Retarder Content