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1What is concrete slump
Concrete slump degree mainly refers to the plasticization properties and pumpable properties of concrete, and the factors affecting the slump of concrete are mainly gradation changes, moisture content, weighing deviation of the scale, the amount of admixture, and the temperature of cement that is easy to be ignored. Slump refers to the harmony and ease of concrete, specifically to ensure the normal progress of construction, including the water retention, fluidity and adhesion of concrete.
And ease refers to whether the concrete is easy to operate and uniform compact performance, is a very comprehensive performance which contains fluidity, adhesion and water retention. The impact and susceptibility mainly include water consumption, water ash ratio, sand rate and several aspects, including cement varieties, aggregate conditions, time and temperature, admixtures and so on.
2 The importance of slump
Slump is an important indicator of concrete working, the so-called working, simple image, refers to whether the newly mixed concrete is convenient for construction. Why control slump during construction?
First, convenient and concise, simple operation, easy to promote the use of construction enterprises, no other instrument than the slump test is simpler to determine the working performance of concrete.
Second, the degree of collapse from another perspective is also to control the quality of the concrete. Concrete construction must have a mix ratio, in the mix ratio design, there will also be a chip value of the mix ratio design, then the construction should refer to the collapse value of the matching design to control.
3 Supervision measurement of concrete slump qualified unqualified
With an upper mouth 100mm, the lower mouth 200mm, high 300mm horn-like slumber barrel, poured into the concrete after pounding, and then pull up the barrel, the concrete due to its own weight to produce a collapse phenomenon, with the barrel height (300mm) minus the height of the highest point of the concrete after the collapse, called the degree of collapse. If the difference is 10mm, the slump is 10mm.
According to the different slump degrees, the concrete mixture can be divided into large flow concrete (slump degree >150mm), fluid concrete (slump degree 100~150mm), plastic concrete (slump degree 50~90mm), and low plastic concrete (slump degree 10~40mm).
4 Factors affecting the slump of concrete
(1) Aggregate gradation of concrete. Since the parcel rate of coarse aggregate and fine aggregate of equal volume of water and cement is very different, the slump degree of fine aggregate concrete is much smaller than that of coarse aggregate concrete under the same water content.
(2) The moisture content of the concrete. The effect of changes in concrete moisture content on concrete slump is obvious. Due to the large change in the water content in the sand, if the change in the water content in the sand is not taken into account when the concrete is mixed, the slump of the concrete will be affected. (3) The temperature of the cement. The impact of cement temperature on concrete slump is often overlooked by construction workers. The high temperature of cement will not only increase the temperature of the concrete, but also the degree of collapse will become smaller due to the high temperature of the cement and the larger water absorption. (4) The error of the measuring scale. The weighing deviation of the water scale and the cement scale has a great influence on the slump of the concrete, and if the weighing deviation of the scale and the cement scale is unstable, the slump is not easy to control. (5) The amount of admixture. The amount of admixture used has a direct effect on the slump of the concrete. In the production process, the dosage of the admixture should be relatively stable to play a better role. (6) Dehydration of gypsum in cement. During the grinding process of cement, due to the increase in temperature, it is easy to cause the dehydration of the dihydrous gypsum in the cement into semi-aqueous gypsum. After the semi-aqueous gypsum is added to the cement concrete, it quickly reacts with the water to re-form the dihydral gypsum, which reduces the fluidity of the concrete and affects the slump of the concrete. (7) Adaptability of admixture and cement. There is an adaptability problem between the type of concrete admixture and the cement variety, if the adaptability of the concrete admixture and cement is not good, it will seriously affect the fluidity of the concrete, resulting in the loss of the slump of the concrete. (8) The fineness of the grinding of cement. The fineness of the cement affects the standard consistency water requirement of the cement. In general, the larger the specific surface area of cement, the greater the water demand. In particular, cement mixed with volcanic ash is often very large in specific surface area, and the standard consistency of cement requires high water, which will reduce the slump of concrete under the same conditions of concrete water-ash ratio. If the fineness of the cement fluctuates greatly, it will cause fluctuations in the slump of the concrete. (9) The cement condensation time is abnormal. In the process of cement clinker calcination, for some reasons, it often causes changes in the content of certain fast-setting minerals in cement clinker, so that the condensation time of cement is abnormal, or fluctuates greatly (fast and slow), resulting in a large change in the slump of concrete
5 Causes of concrete slump loss
1. Concrete material
Cement grinding is too fine or the particle size distribution range is too narrow, the amount of gypsum-aluminum does not match or the gypsum form is not correct and the mixture is not conducive to the normal hydration of the cement water reducer system, etc.; the fly ash carbon content is too high and abnormally adsorbed; the sand and gravel particle size gradation is unreasonable or the mud content, the water absorption rate is abnormal, etc. The concrete work performance cannot meet the requirements.
2. The free water inside the concrete evaporates and dissipates
Under certain conditions, the greater the amount of water used, the greater the degree of slump, so to a certain extent, the change of free water inside the concrete affects the change of the harmony of the concrete, the evaporation and loss of free water inside the concrete, the temperature, humidity, wind speed and the area of the concrete water loss will affect the divergence rate of its water; sand and stone If the concrete self-water mixing begins to enter the pump and enter the mold, the sand and gravel water absorption will naturally affect the change of the free water in the concrete.
3. Concrete mixing and comparison of the selection of mixed water
Concrete mix ratio of its own parameters, such as water-to-glue ratio, unilateral water consumption, the proportion of admixture retardation components, the initial slump of concrete and other factors will have an impact on the change of concrete slump degree; at present, many site technicians are limited to the water-glue ratio - strength, ignoring the impact of unilateral water consumption on the volume of concrete slurry and concrete viscosity and yield value, too low water consumption not only causes too low amount of rubber and makes the concrete viscosity too large, Construction difficulties also exacerbate the loss of free water to mixed water ratio naturally increased concrete collapse;
4. The unreasonable amount of water reducer causes abnormal concrete working performance
The unreasonable amount of doping or improper proportioning in the concrete production process will cause the slump degree (working) to be small, the normal cement hydration consumption, the physical adsorption (activated carbon, long-empty substances and other harmful substances in the system) and the cement hydrate will produce cement entanglement loss, etc., resulting in the effective content of the remaining water reducer in the system is difficult to maintain, the concrete slump can not meet the construction requirements, or the excessive amount of mixing causes the concrete to segregate the water and other undesirable use effects.
5. The unreasonable formula of the water reducer causes the abnormal working performance of the concrete
The water reducer is mixed in a reasonable amount, but the improper combination of water reduction- slump retention - retardation - gas inlet will also cause the slump of the concrete to fail to meet the performance requirements of each stage.
6 Control of concrete slump
1. Control the concrete material
For a variety of materials for parameter inspection and comparison, unqualified raw materials or materials that affect the main parameters of concrete performance deviations are too large as possible, or in the design of concrete mix ratio, the parameters are focused on the selection of parameters, such as the water demand of rubber is relatively large, the sand fine mud content is high, consider the theoretical water reduction rate to take a smaller value, unilateral water consumption is more, and at the same time in the proportion of rubber materials, the proportion of high-quality materials is more significant and other reasonable means are applied.
2. Aimed at the factor of free water loss
High temperature, windy weather construction pay attention to all aspects of cooling and moisturizing, such as sand and gravel materials can be drizzled and cooled; the cement storage warehouse is marked as far as possible with cement that has been aged and cooled first, to avoid the use of high-temperature cement; reasonable scheduling to avoid production, transportation, pumping, pouring disconnected communication caused by broken material or concrete pressure vehicle overtime; transportation (transmission) process on the mixer truck, pump pipe cooling measures; construction operation surface to be windproof maintenance, while strict production, construction related operating procedures for construction organization, A principle is to make the production of concrete poured into the mold as soon as possible to avoid excessive loss of free water.
3. Attention to the use of water reducers
(1) Ensure that the amount of water reducer is mixed in full, improve the water reduction rate, to enlarge the slump degree of the newly mixed concrete, and the concrete state is actually based on the pump, the mold is still relatively scientific, so sometimes for some tendency, you can make some concessions in the workability of the concrete out of the machine, that is, there can be slight water leakage or even separation. (2) Appropriately improve the water retention (culvert) performance of concrete by improving the suction, and the air content is lost over time, of which the bubble culvert water capacity is reduced and part of the water storage can be replenished. The actual workingness of the concrete with large slump degree is more convincing than the slump degree according to past experience, and the general expansion degree will reach more than 500mm to have a better retention effect. (3) Appropriately extend the condensation time of concrete, the cement with large collapse is basically shorter in its own condensation time, so a larger proportion of retardant can be used in this case; (4) increase the proportion of the mother liquor of the sustained-release water reducer, the use of this type of mother liquor in the early stage of the water reduction rate is low, and then with the cement hydration generated by the Ca(OH)2 environmental stimulation gradually release the water reduction effect to make up for the loss of slump. (5) The time-consuming water absorption rate of sand and gravel material is too large, which will also aggravate the consumption of the internal water reducer solution of concrete, and the high temperature production of concrete in summer can consider pouring water on the aggregate 3-5 hours in advance to pre-saturate the water, and also has the effect of reducing the temperature of sand and gravel, considering the weight factor of sand and gravel to reduce the temperature of the concrete itself.
4. Other factors
For example, the inaccuracy of the slump test requires us to operate carefully and responsibly, and control the slump of the concrete in all aspects.