Definition of water reduction rate
Water reduction rate refers to the percentage of water consumption reduced by concrete with water reducer compared with benchmark concrete under the condition of achieving the same concrete working performance (usually based on slump). This parameter intuitively reflects the ability of water reducer to disperse cement particles and release free water. Water reduction rate is the core indicator for evaluating the performance of water reducer, which directly affects the workability, strength and economic benefits of concrete.
Calculation Method of Water Reduction Rate
Water reduction rate (%) = (W0-W1)/W1 X 100%
W0: Water consumption of benchmark concrete (without water reducer) (kg/m³).
W1: Water consumption of concrete with water reducer to achieve the same slump (kg/m³).
Key points:
Slump consistency. The slump of the benchmark and water reducer concrete must be strictly guaranteed to be the same, otherwise the result will be distorted. If the slump increases significantly after adding water reducer, the sand ratio or thickener needs to be adjusted.
Material stability. The sources of cement and aggregates need to be fixed, and different batches of materials may cause fluctuations in water consumption. The amount of water reducer needs to be accurately weighed (error≤0.1%).
Environmental factors. Temperature and humidity affect slump loss, and the test should be carried out under standard conditions (20±2℃, relative humidity ≥50%).
Repeatability verification. Carry out at least 3 parallel tests and take the arithmetic mean (the single difference should not exceed 1%).
Influence Mechanism of water reduction rate on concrete performance
>> Influence mechanism on workability
Dispersion effect. The higher the water reduction rate, the stronger the dispersion effect and the better the fluidity of the slurry. Water reducer molecules are adsorbed on the surface of cement particles to form a double electric layer or steric hindrance effect, destroy the cement flocculation structure, release the encapsulated water, and thus improve fluidity.
Lubrication effect. At high water reduction rate, the lubrication effect is more significant and the slump loss is smaller. Water reducer reduces the surface tension of the water-cement interface, making it easier for water molecules to form a lubricating film and reduce friction between particles.
>> Influence mechanism on compressive strength
Reduction of water-cement ratio (W/B). Increasing the water reduction rate can significantly reduce the amount of water used, reduce the capillary pores and porosity in the cement paste, and make the structure denser. This is the main reason for the strength improvement.
Optimization of cement hydration. At a low water-cement ratio, the distance between cement particles is reduced, the distribution of hydration products is more uniform, and the strength of the interface transition zone is improved.
>> Influence mechanism on durability
Pore structure refinement. High water reduction rate reduces the proportion of harmful pores, reduces the chloride ion diffusion coefficient and permeability, and improves frost resistance and carbonization resistance.
Crack control. Dense structure reduces shrinkage stress concentration and reduces the risk of shrinkage and plastic cracking. However, it should be noted that excessive water reducer may cause negative effects due to air entrainment.
>> Effect on setting time and hydration heat
Retarding effect. Some water reducers will delay the initial hydration of cement and prolong the setting time. High water reduction rate may aggravate this phenomenon, which needs to be controlled by adjusting the retarding component.
Hydration heat release rate. Under low water-binder ratio, the total hydration heat may increase, but the heat release peak is delayed, which is beneficial to the temperature control of large-volume concrete.
What Are Key factors affecting water reduction rate test
Raw material factors
(1) Cement properties
Mineral composition. When the C3A content is high, it will quickly absorb water reducer molecules, reduce the effective water reduction rate, and increase the water reducer dosage. Excessive alkali content will cause the dispersibility of cement particles to deteriorate and the water reduction rate to decrease.
Fineness and specific surface area. The finer the cement, the greater the water demand, and the water reduction rate may decrease at the same water reducer dosage. For every 100 m²/kg increase in specific surface area, the water consumption increases by about 1~2%.
(2) Aggregate properties
Mud content. When the mud content of aggregate is greater than 1%, clay particles will absorb water reducers and significantly reduce the water reduction rate. If the mud content increases from 0.5% to 2%, the water reduction rate may decrease by 5% to 10%.
Grading and particle shape. When the grading is poor or there are many needle-like particles, the water demand increases, causing the measured water reduction rate to deviate from the theoretical value.
(3) Influence of admixtures
Fly ash, mineral powder. High-quality fly ash can reduce water demand and increase water reduction rate. However, high-calcium ash or fly ash with a loss on ignition greater than 5% may absorb water reducers and reversely reduce the water reduction rate.
Test conditions and factors
(1) Ambient temperature and humidity
Temperature. When the temperature is greater than 25℃, cement hydration is accelerated, the adsorption rate of water reducers is accelerated, and the slump loss increases, which may lead to a low water reduction rate test value and the test time needs to be shortened.
Humidity. When the humidity is less than 50%, the water in the concrete evaporates quickly and the slump is difficult to control stably.
(2) Mixing process
Feeding sequence. The water reduction rate of water reducer in direct contact with cement (pre-mixing method) may be 5%~8% lower than that of post-mixing method.
Mixing time. Insufficient mixing time (<2min) leads to uneven dispersion. Excessive mixing (>5min) may lead to excessive air entrainment, affecting the judgment of water consumption.
(3) Timing of slump measurement
If the test is not completed within 5min after mixing, the water consumption may be misjudged due to slump loss, especially for naphthalene-based water reducers.
Operation specification factors
(1) Water consumption adjustment accuracy
The water consumption adjustment step should be ≤0.5 kg/m³. Too large a step will lead to slump matching deviation and increase the error in water reduction rate calculation.
(2) Slump test consistency
Operational differences such as slump cone lifting speed and bottom plate wetness can cause a measurement error of ±10mm, which directly affects the determination of water consumption.
(3) Water reducer dosage control
When the weighing error of powder water reducer is >0.1% or the volume measurement of liquid water reducer is used, the actual dosage deviation will cause water reduction rate fluctuation.
Mix design factors
(1) Benchmark concrete fluidity
When the slump of benchmark concrete exceeds the standard range (80±10mm), the linear relationship between water consumption and water reduction rate may be destroyed.
(2) Sand ratio selection
A deviation of ±2% in sand ratio will cause changes in the slurry encapsulation, and the adjustment of water demand may mask the actual water reduction effect.