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Concrete water-reducing agent technology is increasingly adopted in engineering construction and plays an important role in concrete quality control. However, excessive use of water-reducing agents in commercial concrete mixing plants has become common; this article examines the impact of excessive water-reducing agent dosage on concrete performance and proposes treatment methods.
If the water-reducing agent dosage increases while water consumption remains unchanged, flowability may exceed the expected range, and concrete workability may deteriorate. If the concrete is in poor condition, it may lead to increased bleeding, which negatively affects pumping construction. Moreover, increased bleeding may reduce the surface strength and appearance quality of concrete. If the water-reducing agent is over-mixed but the water consumption is correspondingly reduced, the concrete’s workability is good. In this case, because the water-cement ratio decreases, the concrete strength increases correspondingly, with no adverse effect on other properties.
For water-reducing agents with retarding effects, excessive mixing may prolong the setting time and reduce early strength compared with normal concrete dosage. Generally, it sets within 48 hours and usually has no adverse effect on the later strength. However, early curing is required to prevent excessive water loss in the plastic state on site, which may cause surface cracking and affect the concrete’s strength and structural durability.
For water reducers with air-entraining components, air content increases after overmixing, and the early and later strength of concrete may decrease. Generally, air content below 4.5% has little effect on the strength of concrete, and an appropriate increase in air content can have a positive and beneficial effect on anti-freezing and anti-harmful medium erosion; If the air content exceeds 5%, it will cause a serious decrease in the strength of concrete at all ages, which may lead to quality accidents.
Overall, the water-reducing agent dosage is within 2 times the normal dosage, and the concrete is in good condition with an air content of less than 4.5%. After pouring, implement appropriate surface curing to avoid adverse effects on concrete performance.
Determine the effect of water-reducing agent dosages several times the normal dosage on concrete performance based on specific circumstances.
In the first case, in ultra-high strength concrete, because the water-cement ratio is ≤ 0.3 or even as low as 0.2, the concrete is usually insensitive to the amount of water-reducing agent added. To achieve the ideal flowability state, the amount of water-reducing agent added is usually 5-8 times the normal amount; that is, the amount of polycarboxylate acid added needs to reach 5%- 8%. For concrete below C50, such a high dosage is unimaginable.
However, experimental results show that strength development at different ages is good at this dosage, and concrete with a 28-day strength greater than 100 MPa is prepared based on this. This is because water-reducing agents only physically adsorb to disperse cement. Water-reducing agent molecules are adsorbed on the surface of cement particles. Through steric hindrance and electrostatic repulsion, the flocculent structure of cement particles is disintegrated, releasing encapsulated free water and increasing concrete fluidity. In addition, polycarboxylate water-reducing agents, due to their comb-like structure, can prevent cement particles from coalescing again for a certain period, thus providing good slump retention performance.
After a certain period, cement hydration products completely encapsulate the water-reducing agent molecules adsorbed on the surface of cement particles. Once shielded, the water-reducing agent molecules lose their dispersion effect and no longer affect the concrete. Cement hydrates normally, and the concrete strength develops normally.
Of course, because of the high dosage of water-reducing agents, the concentration of water-reducing agent molecules in the concrete is high. After cement hydration products cover some molecules, new molecules adsorb on the surface of the hydration products, preventing cement particles from quickly overlapping and forming a network, thereby extending the setting time to a certain extent. However, generally, cement setting does not exceed 24 hours.
In the second scenario, if the water-reducing agent itself has air-entraining and retarding properties, then overmixing or repeated over-mixing may significantly adversely affect concrete performance. Generally, the dosage of the retarding component is determined by the ambient temperature, engineering requirements, and the normal dosage of the water-reducing agent.
If the retarding component is added in excess of several times, it will affect the normal hydration of the cementitious material due to the large adsorption of the retarding component on the surface of the cementitious material particles, which can significantly prolong the setting time or cause the concrete not to set for several days or permanently. Generally, concrete that sets for 2 days or longer may experience a permanent decrease in strength due to delayed hydration, changing the types and quantities of hydration products.
Subway interlocking piles (usually requiring 72-90 hours of initial setting) and large-volume concrete such as pile foundations, abutments, and dams also require a long setting time. Generally, the mix design increases the strength grade to ensure the 28-day strength meets the design requirements.
Air-entraining water-reducing agents are overmixed several times. Under normal mixing conditions, when the air content of concrete is appropriate, overmixing several times greatly increases it. The concrete slurry is exceptionally rich, and when shoveled up, it is light and floats. In severe cases, the concrete is loose and porous like bread, and its strength is severely reduced.
In the third scenario, even if the water-reducing agent itself has no air entraining and slow setting properties, if the water consumption is not adjusted promptly after being excessively mixed, it may seriously deteriorate the workability of the freshly mixed concrete, resulting in serious phenomena such as bleeding, segregation, bottom grabbing, and compaction. Moreover, the stability of the poured concrete is poor, leading to internal layering and an increased water-cement ratio around the steel bars, reduced strength, and a serious decrease in the grip strength of the steel bars.
Severe overmixing can also cause heavy bleeding on the concrete surface and on parts in contact with the formwork, reducing their strength. When removing the formwork, defects such as cracks, honeycombs, and rough surfaces are likely, greatly reducing the concrete’s resistance to external erosion and seriously affecting its durability.
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