the effect of re-tempering with superplasticizer on consistency and compressive strength
The effect of re-tempering with super plasticizer on concrete can be different depending on some factors. Temperature, time of concrete in the mixer, original dosage of the plasticizer, cement and water content, amount of superplasticizer used for retempering, etc. If the concrete has been long in the mixer and you re-temper it with super plasticizer maybe you wont get the expected workability but a delay in compressive strength (depending also the chemical compound on the superplasticizer). If the concrete has already the super plasticizer in the original recepie, both effects could be bigger. If the concrete has not been long in the mixer, you will see an increase in workability, and, depending on the amount of superplasticizer used, a good beheavior in strength development. The good news are that at the end, the final strength usually will not be affected when retempering with superplasticizer, because if you do so with water, the strength will be affected dramatically.
The superplasticizer should be added after the mix of the concrete materials and water. The engineer should check the workability of the concrete before the addition of the super plasticizer. This is suggested due to some reasons that can not be controlled and numerically estimated at the work site, like the moisture of the sand and the aggregates. Also the super plasticizer should be divided in two parts and added in two stages. The effect of the super plasticizer is the increase of the workability of the concrete in case of use of low water/cement ratios. Additionally the porosity of the concrete is reduced. Due to both aforementioned reasons the compressive strength and the consistency of the concrete may be increased, but the engineer can not be based on this and is suggested the use of the design strength that results according the composition of concrete. Additionally a concrete mixture with low water/cement ratio, without the use of super plasticizer is very possible to have lower strength (even lower than the design strength) than the strength of the same concrete where super plasticizer was used.
Fresh concrete loses workability due to one of the reasons. As mentioned in many literatures , the mixing water being absorbed by the aggregate if it is not in SSD state, or evaporation of the mix water,or early hydration. The loss of slump increases in higher temperatures and in higher initial slump ,or in higher cement contents and in high alkali and low sulfate content of the cement.A significant effect on strength if only that water which would be added to retempet the concrete leads to increase w/c ration in more than 5%. During my experiences in concreting in hot weather concreting in UAE, the retempering of concrete by using admixture type F , ASTM C494 can improve the workability of concrete and improve other concrete properties in addition to clearly improve the pumpability ,placeability and finishability of cast concrete.So the retempering of concrete by superplasticizer which to increase the workability of the concrete to the specified limits determine in specifications will reflect positively on properties of both fresh and hardend concrete.