Excessive superplasticizer dosage is essentially a waste of resources and can severely affect concrete quality and construction performance. By mastering the following three practical site-based checks, overdosing can be identified more accurately and avoided.
1.bserve the placing condition
If, after placement, a distinct surface water-rich layer forms with a thickness of approximately 5 mm or more, and clear vertical stratification is visible, which cannot be eliminated even after proper vibration, there is a high likelihood of superplasticizer overdosing or imbalance in the water–binder system.
2. Assess the handling feel of the fresh concrete
When lifting the concrete with a shovel, if the mixture appears loose, with mortar failing to adhere to the coarse aggregates and the aggregates becoming exposed, resulting in very poor cohesion and almost no coating ability, this generally indicates a significantly excessive superplasticizer dosage.
3. Check the setting behavior
Under normal ambient conditions, if no obvious initial setting is observed after more than 4 hours, and finger pressure leaves an indentation of approximately 1 cm or deeper with slow recovery, the concrete should be considered to have severe superplasticizer overdosing or incompatibility with the cementitious system.
Note:
These three items serve as practical on-site reference criteria. When they occur simultaneously, the superplasticizer dosage, water content, and raw material variations should be immediately reviewed to prevent quality defects and unnecessary cost losses.
Also, by observing the fresh concrete's mixing condition, it is possible to preliminarily judge whether the admixture dosage is excessive or insufficient, and whether the water content is appropriate.
When the water content is excessive, the concrete generally does not exhibit excessive slumping. The coarse aggregates show a certain degree of mortar coating and are not 'bare' or shiny. Workability is acceptable, the slump is relatively high, and a certain level of bleeding may occur after vibration.
When the admixture dosage is excessive, the concrete tends to slump and segregate. The coarse aggregates have poor coating, appear relatively shiny, and the mixture feels sticky and heavy during handling. Under pumping conditions, it shows high resistance, and pipeline blockage may easily occur after pump stoppage.
In cases where the water content is sufficient but the admixture dosage is insufficient, the concrete does not adhere to the shovel, but overall flowability is poor and workability is inadequate. The mixture appears unnatural and mainly relies on gravity for movement rather than proper flow.
If the admixture dosage is generally appropriate but the water content is insufficient, the concrete surface appears glossy, but the flow rate is slow and the coating ability is slightly reduced. The mixture also feels sticky and heavy during handling.
In summary, only when a proper water content is well matched with a suitable admixture dosage can concrete with good workability and reliable construction performance be produced.
