Regarding the control of water usage in concrete, there are some things you need to know!

The amount of water used directly affects the water-cement ratio, which in turn affects the strength of the concrete. Using too much water can cause problems such as cracks, honeycombing, and laitance, which can reduce the durability of concrete. It is well known that reducing the amount of water can increase the strength and durability of concrete while saving some costs, so it should be strictly controlled during the production process.

?

Factors affecting the amount of water used:

The effect of cementitious materials on the amount of water used in concrete:

The factors that affect the amount of water used in concrete by cementitious materials mainly include the standard consistency of cement, the water demand of fly ash, and the water demand of slag.

(1) The amount of water used for cement standard consistency:

The amount of water used for cement standard consistency reflects how much water is needed when the cement slurry reaches the standard consistency. Generally, the amount of water used for cement standard consistency is related to the type of cement, fineness, and raw materials.

?

When other conditions are constant, the amount of water used in concrete increases with the increase of the amount of water used for cement standard consistency. If the amount of water used for cement standard consistency increases by 1%, the amount of water used in concrete to achieve the same slump requirement will increase by at least 3-5 kg/m3. To reduce the amount of water used in concrete, reduce the water-cement ratio, and reduce the amount of cement used per cubic meter of concrete. When preparing concrete, it is advisable to choose cement with the same strength but lower standard consistency water demand to improve the strength and durability of the concrete.

?

(2) Water demand of fly ash:

Fly ash can be divided into three grades: class I, class II, and class III. The water demand ratio of fly ash refers to the ratio of the water demand for a mixture of 30% fly ash and sand to the water demand for a reference cement mortar. The water demand of fly ash directly affects the amount of water used in concrete, which in turn affects the strength and durability of concrete. As the water demand ratio of fly ash increases, the mixing water also increases when the concrete mix needs to achieve the same slump. In practice, it has been found that adding fly ash with low water demand to concrete will not increase the amount of water used, and may even reduce it. However, it has also been found that fly ash with a high carbon content (high loss on ignition) has a high-water demand and will significantly increase the amount of water used in concrete. When preparing concrete, it is advisable to use fly ash with a smaller water demand ratio (less than 100%) to reduce the amount of water used in concrete, improve the workability of the concrete mix, enhance its pump-ability, and reduce creep, hydration heat, and increase the impermeability and durability of concrete.

?

(3) The flow-ability ratio of slag:

The flow-ability ratio of slag also has a certain impact on the amount of water used in concrete. The larger the flow-ability ratio of slag, the lower the amount of water used in concrete when preparing a concrete mix with a certain slump, and vice versa. The flow-ability ratio of slag is related to its production process, particle size distribution, specific surface area, and other factors. When the particle size distribution of slag is reasonable and spherical particles are more abundant, the flow-ability ratio is larger. When the specific surface area is larger, the flow-ability ratio is lower. The specific surface area has an important effect on the activity of slag. When the specific surface area is less than 400 m2/kg

II. Influence of Aggregate on Water Consumption of Concrete

Variety of Aggregates

Natural aggregates have smooth surfaces, while artificial sand and gravel have rough surfaces and more edges and corners, resulting in a larger surface area and higher water consumption. Generally, under the same slump of the concrete mix, the water consumption of natural aggregates is 5-10 kg/m3 lower than that of artificial sand and gravel.

Particle Size of Aggregates

The smaller the particle size of the aggregate, the larger the surface area, and the higher the water consumption when mixing concrete. Generally, for coarse aggregates, a change in the maximum particle size by one grade will cause a change in water consumption of about 5 kg. For example, when the maximum particle size of crushed stone changes from 5-31.5 mm to 5-20 mm, the water consumption will increase by about 5 kg/m3 to achieve the same slump of?the concrete. Similarly, changes in the fineness modulus of sand will cause changes in the particle size of fine aggregates. If the fineness modulus of the sand changes by one grade, the water consumption of concrete will differ by 10-15 kg. For example, when using medium sand with a fineness modulus of about 2.7 to mix concrete, the water consumption will be 10-15 kg higher than that of coarse sand and 10-15 kg lower than that of fine sand.

Clay Content and Fine Powder Content

Under the condition of constant water consumption of concrete, as the amount of fine powder increases, the slump of the concrete mix gradually decreases. To maintain the same slump, the water consumption or the amount of admixture should be correspondingly increased. If the amount of fine aggregates used in one cubic meter of concrete is 800 kg and the content of fine powder increases by 1%, the amount of fine powder will increase by 8 kg/m3. Therefore, if the content of fine powder is too high, it will increase the viscosity of the concrete and reduce its workability. If the fine particles contained in the aggregates are mainly clay, they will have a greater impact on the water consumption of concrete.

Influence of Water Absorption of Aggregates on Water Consumption of Concrete

The water absorption rate of aggregates refers to the moisture content of the aggregate in the saturated state. The rougher the surface of the aggregate, the higher its water absorption rate. Generally, the surface of natural aggregates is smoother than that of artificial aggregates, and the water absorption rate of artificial aggregates is higher than that of natural sand. Aggregates with larger capillary pore rates have higher water absorption rates than those with smaller capillary pore rates. For example, the water absorption rate of granite is higher than that of bluestone. Aggregates with rough surfaces have poor flow-ability in concrete mixes and require more water to achieve the same flow-ability. Aggregates with high water absorption rates can absorb free water in the concrete mix, resulting in poor flow-ability, greater slump loss over time, and decreased workability and pump-ability of the concrete. The water absorption rate of aggregates is an important physical index that has a significant impact on the water consumption of concrete. Therefore, the water absorption rate of aggregates should be controlled below 1% and should not exceed 2%.

III. The influence of admixture water-reducing rate on concrete water consumption

The water-reducing rate of admixtures has an important impact on the water consumption of concrete. The higher the water-reducing rate of admixtures, the lower the water consumption of concrete can be achieved while still maintaining satisfactory workability. However, a higher water-reducing rate is not always better. Although it can help reduce the water consumption of concrete and improve its workability, an excessively high water-reducing rate can cause poor cohesiveness, segregation, and bleeding of the concrete mixture, which can also make it difficult to control production quality. Therefore, an appropriate water-reducing rate should be selected to achieve the desired workability of the concrete mixture. Generally, the water-reducing rate of admixtures increases with the increase of their dosage, and when the dosage of admixtures increases to a certain value, the increase in water-reducing rate with additional dosage is no longer significant, which is the maximum water-reducing rate at the saturation dosage. Under the same dosage conditions, the better the compatibility of admixtures with other concrete raw materials, the higher the water-reducing rate.

IV. The influence of measurement and machinery on water usage.

(1) The weighing of materials in the production plant and the testing equipment in the laboratory should be regularly self-calibrated and accurately calibrated by relevant testing departments. As the saying goes, "losing a hair will ruin a whole head."

(2) It is also necessary to regularly check whether the position of the sensor has changed, whether there is dust accumulation on the weighing scale, whether the dust collector bag will cause over-weighing or under-weighing, whether the metering of admixtures and water has drips, and whether the automatic detection of moisture content is accurate, etc.

(3) In addition, the order of adding admixtures can also have an impact. Generally, the method of adding admixtures later has a higher utilization rate of admixtures, and the machine can be appropriately modified.

(4) The uniformity of mixing is different for different machines, and regular maintenance should be carried out to ensure that the mixer blades and bearings are not excessively worn, and whether there is material spilled from the belt. Sometimes, there may be a significant difference in water usage for the same material and mix ratio among different mixers.

(5) The use of waste materials under the belt should be prohibited or only used after being processed and verified. Prevent the site cleaning workers from using the cleaned garbage in the concrete. Some small stations may think that a small amount of garbage has no effect, which may increase water usage or even result in the concrete not setting for several days.

Fifth, personnel factors:

The technical level, experience, and sense of responsibility of the personnel directly affect the accuracy of the water consumption. The operator can directly judge whether it is normal during the feeding process based on basic control standards such as slump and aggregate usage, which can be preliminarily judged based on the size of the ammeter and the intuitive judgment of the display. The production control personnel then conduct sampling and testing confirmation and maintain a certain frequency.

VI. The influence of weather and temperature

During the summer, the high temperatures can cause the temperature of raw materials such as sand and gravel to increase, leading to complete dehydration and reduced workability. The slump loss during the summer is also faster than other seasons, so it is necessary to adjust the slump retention and retarder components of the admixture appropriately.

During the dry and windy autumn season, it is important to ensure adequate curing of concrete without increasing the water content to improve workability. It is also important to control transportation and avoid long waiting times for mixing trucks on the construction site, plan driving routes to avoid traffic, and prevent drivers and construction personnel from adding water to the concrete at will in pursuit of construction speed. In the cold winter season, reducing water usage is necessary to prevent excess free water from causing freezing of the concrete.

Methods to achieve low water usage:

(1) Strictly monitor the performance of each material and control the above factors to reduce water usage.

(2) Increase the dosage of admixtures or use high-efficiency and high-water-reducing admixtures, and choose admixtures and cement varieties with better adaptability.?

(3) Improve the sand and gravel gradation to find the optimal gradation for each mix design to improve workability and reduce water usage.

(4) Use high-dosage cementitious materials to improve workability.

(5) Even if the material storage tank is covered, it is necessary to check the moisture content of sand and gravel multiple times, strengthen responsibility and ensure that the slump of each batch of concrete is within the control range.

(6) Communication with the construction team is important. Gain the understanding of construction personnel and cooperate with the technical personnel of the construction team to avoid excessive slump. Correct understanding: a higher slump does not mean easier pumping; workability and aggregate content should be adjusted.

(7) Actual water usage during production often differs significantly from that in the mix design. Therefore, strictly select materials that are superior or close to the mix design, and control water-cement ratio more strictly than in the mix design to ensure sufficient excess strength and ensure that the final product delivered to the construction site is qualified. The principle is to maintain production superiority over the mix design.

Methods to determine water usage:

(1) Analyze a large amount of test data to accurately determine water usage.

(2) Take samples on site and conduct small-scale mix design tests (1-2L) before production to confirm water usage.

(3) Take multiple samples and conduct frequent slump tests to confirm adjustments.

(4) The first batch and first mix must be sampled and tested before starting work.

(5) The first batch must be accompanied to the construction site to confirm slump and workability, and provide timely feedback to production control personnel through phone calls, WeChat videos, etc. Maintain timely communication with construction personnel to ensure quality stability.

(6) All information must be prepared in advance. Generally, night shift personnel should gather all material, production, and construction site information before starting work in the morning and discuss it in a group meeting. Confirm the adjustment of water usage and mix design, and avoid many details issues.

Closing Remarks

Strictly control the amount of water used in the process to avoid errors and mistakes. It is necessary to establish rigorous work processes, strictly inspect every detail and key node, and enhance everyone's sense of responsibility. Science is rigorous and does not tolerate any carelessness. In the work, we cannot ignore quality control due to momentary greed and irresponsibility.