If the concrete water reducer is too much be added, it will affect the setting and hardening of cement, thus affecting the concrete strength performance.
Adding water reducing agent will make the slump retention rate of concrete increase, adding too much will cause concrete water secretion, and even segregation.
Theoretically, adding a small amount of water reducing agent will only improve the slump and will not affect the w/c ratio, and the concrete strength will not be affected. However, if not handled properly, it will definitely affect the conrete final strength.
The impact on the strength can be considered from the following aspects.
1) Test block production. Water reducing agent is to increase the concrete slump and concrete is easy to cause segregation and water secretion, so the requirements for the test block production is very high, if the test block production is not standardized, then the strength will be reduced.
2) Concrete slump. Prepared concrete if has the slump below 200mm, adding less admixture will has little effect; if the slump is relatively large, say above 220mm, then the impact of the second addition of admixture will be greater.
3) Adjustment of concrete. If the concrete slump by transport to the site is small, then it is necessary to add admixture for the second time. It is stronger than adding water. However, it should be noted that it should be poured out quickly, otherwise there will be problems such as blocking the pump.