Precast concrete can easily cause cracks, honeycombs, pockmarks and other problems due to excessive water consumption, which will reduce the durability of concrete. Reducing water consumption can improve the strength and durability of concrete while saving some costs, so it should be strictly controlled in the production process. Of course, don't make the mistake of thinking that you can reduce water consumption indefinitely!
1. Factors affecting water consumption：
1.1 Influence of raw materials
(1) The increase in the amount of cement and admixtures, the storage temperature (sometimes the storage temperature is too high will cause the collapse too fast, and the water consumption will be greatly increased), and the larger the specific surface area, will cause the concrete water consumption to increase. The size of the water consumption of the standard consistency of cement directly leads to the size of the water consumption. The poor adaptability of cement and admixtures will lead to changes in the proportion of water consumption.
(2) The higher the mud content and powder content of sand and stone, the worse the continuity of particle gradation and the greater the water consumption. The accuracy of moisture content detection directly affects the amount of water consumption. The moisture content of the newly stored sand and gravel from the top to the bottom of the pile is often very different. It should be measured several times during use, otherwise the slump will be greatly affected during production.
(3) The type of admixture, the water-reducing rate of the admixture, the amount of air-entraining agent, and the difference in adaptability to cement.
(4) The properties of fly ash and mineral powder, such as loss on ignition, fineness, water demand ratio, etc., and the amount of cement replaced.
(5) The use of other materials, expansion agents, fibers, synergists, etc., has greatly increased or reduced water consumption.
(6) Different levels of sewage have different effects on the performance of concrete, which should be verified and then determined. The amount and performance of each material may directly affect the amount of water consumption. It should be tested one by one when selecting materials or before actual production. The verification and opening identification will finally determine a reasonable amount of water.
1.2 The impact of metering and machines on water consumption
(1) The scales of materials used in the production of mixed buildings and the testing equipment in the laboratory should be calibrated regularly by self-calibration and by regularly entrusting relevant testing departments to conduct accurate calibrations. The so-called "missing a tiny bit is a thousand miles away".
(2) It is also necessary to regularly check whether the position of the sensor has changed, whether there is dust accumulation on the scale, whether the dust-returning cloth bag will cause overweight or underweight, whether there is dripping of additives and water scales, and automatic detection of moisture content. Is it accurate and so on.
(3) In addition, the order of blending first and then blending the admixtures also has an impact. Generally, the post-blending method has a higher utilization rate of the admixtures, and the machine can be modified appropriately.
(4) The degree of mixing uniformity of different machines is different. Regular maintenance should be made to ensure that the blades and bearings of the mixer are not worn too much, and whether the belt has thrown materials. Sometimes there is a big difference between the uniform proportion and the water consumption of different mixers of the same material.
(5) The use of waste under the belt should be avoided or it can be used after processing verification. Prevent site cleaners from using cleaned up waste into concrete. In order to save trouble, some small stations believe that a small amount of garbage has no effect, and sometimes leads to an increase in water consumption or even the phenomenon that the concrete does not set for a few days.
1.3 Slump requirements
Different construction sites have different requirements for slump. When high-rise concrete, underwater cast-in-place piles, bored piles, and steel bars are denser, the slump requirements are generally larger, and the workability is also required to be better. However, the requirements for caps, floors, foundations, and post-casting belts should not be too large. Therefore, the proportion of water consumption should be adjusted in advance according to different construction sites.
1.4 Personnel
The technical level, experience and responsibility of personnel directly affect the accuracy of water consumption. The operator's basic control standards such as slump and the amount of crushed stone can directly judge whether it is normal during the discharging process, and can make a preliminary judgment according to the size of the ammeter and the display can judge intuitively. Production control personnel will then conduct sampling and testing to confirm, and maintain a certain frequency.
1.5 Influence of weather and temperature
Due to the high temperature in summer, the temperature of raw materials such as sand and stone will rise, and even the water will be completely lost in the air to produce water absorption. The slump loss in summer is correspondingly faster than in other seasons, so adjust the slump-preserving and retarding components of admixtures appropriately. In the high wind season in autumn, due to the dry weather, it is necessary to pay attention to the moisture retention of the concrete, instead of increasing the water consumption of the concrete to increase the slump. Control all transportation links, avoid the mixer truck waiting too long at the construction site, plan the driving route to avoid traffic jams on the road, and prevent drivers and construction personnel from randomly adding water to the concrete in pursuit of construction speed. In the low temperature season in winter, the water consumption should be reduced to prevent the concrete from freezing due to excessive free water.
2. How to achieve low water consumption：
(1) Strictly test the performance of each material and control the above factors to reduce water consumption.
(2) Increase the amount of admixtures or use admixtures with high efficiency and high water-reducing rate, and select admixtures and cement varieties with better adaptability.
(3) Improve the gradation of sand and gravel, find out the best gradation of sand and gravel for each mix ratio to improve workability, thereby reducing water consumption.
(4) Workability is improved by using high content of cementitious material.
(5) Even if the silo is covered, the moisture content of the sand and gravel should be checked many times, and the sense of responsibility should be strengthened to ensure that the slump of each plate of concrete is within the control range.
(6) It is very important to communicate with the construction party of the construction site. To obtain the understanding of the construction personnel, it is necessary to cooperate with the technical personnel of the construction party to avoid excessive slump. Correct understanding: It is not that the larger the slump, the easier the pumping will be, but the workability and the amount of crushed stone should be adjusted.
(7) Usually the actual production water consumption and the trial water consumption will be very different. Therefore, it is necessary to strictly select materials that are superior or close to the test content, and strictly control the water-to-binder ratio compared with the test to ensure that there is enough surplus strength, and then to ensure that the final entry into the construction site is also qualified. Maintain the principle of production over trial fit.
3. Methods of determining water consumption
(1) Accurate judgment by accumulating a large amount of test data through the analysis of a large number of test data.
(2) The water consumption is confirmed by taking materials on site and making a small-capacity trial mix (1~2L) before pre-production.
(3) Carry out more sampling and more opening appraisal to confirm the adjustment.
(4) To achieve the first car before the start of construction, the first plate of material must be sampled for testing.
(5) The first car goes to the construction site to confirm the slump and workability, etc., and timely feedback the information to the production control personnel through telephone, WeChat video, etc. Maintain timely communication with construction personnel on site to ensure fluctuations in quality.
(6) All information must be prepared in advance. Generally, the evening shift personnel will summarize all materials, production, and construction site information in advance before starting work in the morning, and conduct unified discussions in the form of group meetings. Confirm the adjustment of water consumption and mix ratio, and avoid many detailed problems.
