After several trials 1% of admixture is fixed and in such case can we put 0.6% admixtures in starting time and remaining 0.4% after sometime(maybe 2hrs later) such that workability of concrete remains good for longer time. Actually i want to clear can we put admixtures two times in a concrete?
The use of additives is allowed only with the approval of the monitoring device.
Adding admixture in a given cement is based on compatibility of that admixture with a particular brand and batch of cement. So this must be tested for adding admixture at two stages and only then this must be used. Admixture are added to charge the cement particles so that water molecules are repelled and these are available for workability of concrete and time is important factor in maintaining workability.
First, this depends on the type of admixtures and the mix design of concrete. If you use the plasticizer type of admixture in mix design (to reduce the water cement ratio and increase the workability), you can use it like this in some urgent situations only, like the site preparations getting delayed and the batching already completed or some unexpected delay in pouring. As a case, if you dose additional plasticizer type admixture before the initial setting started, it will extend the concrete workability more and if you use it after the initial setting time started, it will extend the workability but for a short period. (But the maximum dosing percentage will be differed for each admixture)
But as per your question, if you use partial admixture (plasticizer type) initially, the concrete will take it as the design mix admixture volume and the later you add will be considered as the additional, and this will not give a good workability, since the mix design is done for the optimum quantities of materials to achieve the required strength and other parameters of concrete, but using the less amount of admixture you will not get the required worability of that mix design.
Further, the retarder type admixture is not used twice in general, because its chemical action is mainly related to time, and it may affect the strength of concrete.
Mohammad Lak first of all fluidizig admixture is not an "additive". "Admixtures" and "additives" are different things/components in concrete mix.
This will be a difficult task from a technological point of view. The plasticizer should be added to the mix together with the mixing water so that it is evenly distributed throughout the mix. There is a risk that if you have already prepared the mix (cement with water and the first portion of the admixture), when you add the second portion of the admixture to the mix, it will not be distributed in the same way throughout the entire volume and then it may turn out that the hardened composite microstructure is not homogeneous. Especially with large volumes of the concrete mix. In a small mixer in the laboratory it will probably be mixed well, but what about a large mixer on a construction site?
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