Super-plasticizer (SP) admixture in concrete mixture usually guarantees the initial workability. However, the time dependent workability of mixtures principally depends on the chemical structure and type of SP and its compatibility with cement. Polycarboxylate-based superplasticizers (PC-based SPs) have chemical structures, which have the potential to be modified to improve their performances. This study investigates the different available PC-based SPs in order to achieve the optimal type and amount of SP in production of precast concrete tunnel segments required for the line 3 of Mashhad urban railway project. To this aim, 19 mixture designs incorporating various SPs obtained from 11 companies were constructed and the appropriate dosage of each SP to obtain the desired initial (0 min) and final (15 min) slump values was achieved. Cubic specimens were also constructed and subjected to the compressive test at the ages of 6 hours, 7 and 28 days.
The results indicate that the use of the optimal amount of each type of SP leads to obtain the minimum allowable compressive strength at all 19 mixture designs. Moreover, the mixtures with an appropriate appearance were accompanied by an appropriate slump value; while, reducing the appearance characteristics of each mixture resulted in an inproper slump condition. Finally, the mixture design with 0.54% SP admixture and the least cost was selected as the optimal mixture design.