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Effect of Mixing Time on Properties of Concrete

The various components of a mix are proportioned so that the resulting concrete has adequate strength, proper workability for placing and cost effective. To achieve such properties the mixing should such that it produce an intimate mixture of cement, water, fine and coarse aggregate and suitable admixture of uniform consistency throughout each batch. The average strength of concrete increases with an increase in mixing time as it improve uniformity of mix.


Conventional Practice


In a construction site , it is common practice to mix concrete as rapidly as possible. But this time varies with type of mixture and basically it is not a function of time but of number of revolutions of the mixing. Generally, about 20 revolution are sufficient. As the manufacturer recommended certain speed of rotation, the number of revolutions and mixing time are independent.


Standard Value of Mixing


For a particular mixer, a relation between mixing time and uniformity is provided. Mixing less than 1 to 1.25 minutes result a significantly variable concrete but prolonging the time of mixing beyond these values render no significant improvement in uniformity. The length of mixing time also depends on quality of blending materials during charging of mixture.


The exact value of mixing time is also a function of size of mixture. A minimum mixing time of 1 minute for mixer size of 1 cum. yd (3/4 cum. m) and 15 additional seconds for each addition cubic yard giving satisfactory uniformity of mixing.

ASTM C 94-94


Mixing time is counted from the time when all the solid materials have been put in the mixer, and it is also required that all the water has to be added not later than after mixing time.



ACI 304R-89


Mixing time should count from the time when all the ingredient have been discharged into the mixer.



Exceptions


1) Many modern large mixer performs satisfactorily with a mixing time of 1 to 1.5 minutes.

2) In high speed pan mixers, the mixing time can be as short as 35 seconds.

3) When lightweight aggregate is used, the mixing time, less than 5 minutes, may hamper developments of strength for better result. Sometimes mixing of aggregate with water for 2 minutes is done first followed by 3 minutes mixing after cement is added.


Effect of Pro-longed Mixing Time

Generally, evaporation of water from the mix takes place resulting decrease in workability and increase in strength. The another effect is grinding of the aggregates, particularly if soft. The grinding makes aggregate more finer resulting lower workability. The friction effect also produces an increase in the temperature of the mix.

In case of air-entrained concrete, prolonged mixing reduces their content by about 1/6 per hour (depending on the type of air-entraining agent), while a delay in placing without continuous mixing causes a drop in air content by only about 1/10 per hour. On the other hand, a decrease in mixing time below 2 or 3 minutes may lead to inadequate entrainment of air.

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