The effects of plasticizing admixtures on the properties of fresh and hardened ready mixed concrete were studied. Eight different concrete mixtures were designed by two different polycarboxylate based plasticizing admixtures, CEM III/42,5 N type Portland cement, ground granulated blast furnace slag, natural sand, crushed limestone, and water. Flow ability of the freshly mixed concretes were measured and the compressive strength tests were carried out on 150 mm cubic specimens at the ages of 7 and 28 days. It was observed that flow values increased when the amount of admixture used in the mixtures increased. The composition of the admixture affected the flow values of the mixtures significantly. Compressive strength test results revealed that the increase in amount of slag in the amount of total binding material resulted in lower 7-days strength values. Type of chemical admixtures significantly influenced the 7-days strength values. This was attributed to the different secondary effects of different plasticizing chemical admixtures. Type of chemical admixture influenced both 7-days strength and 28-days strength even the mixtures had equal amounts of ingredients and also equal water/binder ratios. This result showed that plasticizing chemical admixtures can affect both early and later age strengths of ready mixed concrete even for equal water/binder ratios.