Water-reducing agents are commonly used admixtures in concrete. However, if added in excessive amounts, they can have multiple negative impacts on the performance of concrete, the construction process, and the final quality. The specific drawbacks are as follows:
Damage to the workability of concrete
1. Abnormally increased fluidity: Excessive water reducer will significantly reduce the slump loss of concrete, causing it to be overly soft during transportation and pouring. This may lead to segregation (separation of aggregates and mortar) and bleeding (excessive water on the surface) during the process. For example, pumped concrete may be blocked by segregation, affecting the construction efficiency.
2. Excessive setting time: It will significantly prolong the initial and final setting times of concrete. In low-temperature environments, it may cause the final setting time of concrete to exceed 24 hours, which not only affects the normal progress of subsequent processes (such as removal of formwork and curing), but also increases the risks of freezing and being washed by rain due to prolonged exposure to harsh conditions.
Weakening of concrete mechanical properties
1. Significant reduction in strength: Excessive addition of water-reducing agents will increase the water-cement ratio in the concrete (with a relatively excessive amount of water used), resulting in an increase in the internal porosity of the hardened concrete and a decrease in structural density. Generally, if the amount of water-reducing agent is over 10%-20%, the 28-day compressive strength of the concrete may decrease by 10%-30%, seriously affecting the structural carrying capacity.
2. Reduced durability: The increase in internal pores will reduce the impermeability, freeze resistance, and carbonation resistance of the concrete. For example, in rainy areas, water is prone to seep into the concrete interior, and when the temperature changes, water freezing and expansion may cause the concrete to crack; in industrial environments, harmful gases (such as carbon dioxide, chloride ions) are also more likely to invade, causing problems such as steel corrosion.
Impact on construction costs and quality control
1. Increase in additional costs: Due to unqualified concrete performance, rework and re-construction may be necessary, which not only wastes raw materials (such as cement, sand, admixtures, etc.), but also increases labor, machinery rental, etc. fees. In addition, to compensate for the impact of excessive water reducer, additional cement or other adjusting agents may be needed, further increasing costs.
2. Difficult control of quality risks： The concrete performance problems caused by excessive water reducer are concealed. Some defects (such as internal pores, insufficient strength) may not be revealed until a certain period after the construction is completed, laying long-term risks for structural safety. For example, if important structures such as bridges and high-rise buildings have deformation or collapse due to insufficient strength, the consequences would be unimaginable.
Impact on the Volume Stability of Concrete
1. Increased risk of cracking: Excessive addition of water reducer will cause the concrete to undergo greater drying shrinkage and plastic shrinkage during the hardening process. Drying shrinkage refers to the volume reduction caused by water evaporation after the concrete hardens, while plastic shrinkage occurs during the initial setting period after pouring, when the surface moisture evaporates rapidly, leading to volume contraction. The combined effect of these two shrinkages can easily cause a large number of cracks on the concrete surface, affecting the overall integrity and appearance of the structure.
2. Excessive addition of water reducer will have adverse effects on concrete from multiple aspects such as workability, mechanical properties, durability, and construction cost. Therefore, during construction, the dosage of water reducer must be strictly controlled according to the mix ratio to ensure that the concrete quality meets the design requirements.
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