Retempering (the addition of water to concrete held in truck mixers for long periods to maintain a given slump) is generally prohibited. In view of the lack of knowledge regarding the effects of retempering the restriction has probable been necessary. Today though considerable research is underway and the tentative results of which are already showing that the effects of retempering may not be so serious and that a more lenient interpretation of the total prohibition rule may be possible. The effect of prolonged mixing on compressive strength under field conditions goes down the first hour but then starts returning to its originally strength which it maintains till hour number 3 when it starts going down again. These results were obtained by the Bureau of Reclamation with a regular 1 and one half inch gravel mix with saturated surface dry proportions of 1: 2.65: 4.15, using 39 percent sand and 5.3 sacks of Type II low alkali cement per cubic yard. The batches were controlled to a uniform 3 and one half inch to 4 inch slump. Remixing for 3 minutes was done each hour, during which sufficient water was added to bring the mix back to the original slump. The compressive strength probably does not follow the usual water/cement ratio curve, particularly over the first 4 hours. This can probably be attributed to the reduction in air caused by the extra mixing. Results in respect to durability were similarly good.