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The main influence factors of concrete slump are analyzed

Release time: August 31, 2020 popularity: nine

one introduction

At present, commercial concrete is widely used in China, which is characterized by centralized mixing and commercial supply. This will separate the concrete from the traditional construction site, which is an innovation of engineering construction technology and an inevitable trend of concrete development. With the wide application of commercial concrete, it requires that the concrete can still maintain a relatively high slump after a long time of transportation and parking. In the construction process, the loss of slump is easy to cause pump blockage and construction difficulties, as well as honeycomb and pitted surface of concrete after formwork removal, and even cause engineering quality problems. These have seriously affected the pumping distance and pumping height of commercial concrete and the supply radius of commercial concrete mixing station.

two Mechanism of concrete slump loss in time

It is generally believed that the slump loss mechanism lies in the following aspects:

(1) Due to the occurrence of cement hydration reaction, at the same time, some free water is adsorbed on the surface of hydration products, and some free water is continuously evaporated, resulting in the gradual reduction of free water in concrete mixture. In addition, molecular force and external force promote the condensation of hydration products.

(2) For the concrete with superplasticizer, with the extension of time, the water reducing effect of superplasticizer decreases, which also causes the loss of concrete slump. Because the superplasticizer is adsorbed on the surface of hydration products, some water reducing agents are wrapped by hydration products, and some water reducing agents are consumed with the occurrence of hydration reaction. Therefore, the repulsion force between cement particles is reduced, and the cement particles are flocculated, which makes the slump of concrete smaller.

(3) Due to the hydration of cement, a large amount of cement will be produced in the hydration process Ca(OH) two as well as C-S-H This will increase the viscosity of the system and increase the slump loss of concrete. In practical engineering, the wide application of admixtures such as water reducer will enhance the dispersion of cement and increase the reaction area of cement particles. Therefore, the slump loss of concrete with concrete admixtures, especially water reducer, will be greater over time. At the same time, a large number of polar groups in superplasticizer form complex with some metal ions, resulting in the decrease of ion concentration in the liquid phase, accelerating the speed of cement hydration at the initial stage, increasing the viscosity of the whole concrete system, and leading to the loss of concrete slump over time.

three Analysis of factors affecting concrete slump loss in time

3.1 cementitious materials

3.1.1 cement fineness

The process of cement hydration is the reaction process of cement clinker and water. In this process, cement clinker and water continuously react to generate hydrate, which reduces the liquid phase. The results show that the hydration process of cement at 30 μ M m The results show that the strength of the clinker particles increases with the increase of cement strength sixty μ m However, the strength is not affected by the particle size, which is less than 0 ten μ m The results show that the grain size mainly plays an early strengthening role, three μ m The particles below play the role of early strength only. less than ten μ m The water requirement of the granules is large. Cement with good rheology ten μ m The following particles should be less than 10% The finer the particles, the more fine particles, the greater the water demand and the higher the early strength, which will aggravate the collapse loss. At the same time, due to the hydration reaction, the solid phase increases and the solid particles connect with each other, which leads to the slump loss of concrete. Therefore, the fluidity of concrete has an important relationship with the hydration process of cement: the faster the hydration speed of cement is, the greater the slump loss of concrete is. Under the same conditions, the finer the cement particles are, the greater the water demand for mixing concrete is, and the more intense the hydration reaction is, which inevitably leads to the slump loss of fresh concrete. On the other hand, the smaller the distance between cement particles, the smaller the water loss ratio of cement particles.

In the process of cement clinker grinding, we try to make the clinker particles form reasonable particle gradation with different sizes. The particles are filled with each other to form the lowest porosity, so as to optimize the structural performance of cement paste. Good rheological properties can be obtained if the coarse and fine particles are properly graded. The concrete prepared with this cement has good fluidity, low water demand and small slump loss.

3.1.2 mineral composition of cement clinker

The adsorption capacity of aluminate minerals to water reducer is greater than that of silicate minerals, and the larger the adsorption capacity, the worse the adaptability. With tricalcium aluminate( C three A )The slump loss of concrete increases with the increase of content. Because of the high content of aluminates, especially C three A High content cement, C three A It can adsorb polycarboxylate superplasticizer and affect the silicate mineral tricalcium silicate( C three S )And dicalcium silicate( C two S )At the same time, Liu houfen et al, C three A It is easy to cause slump loss and low water reduction rate due to high hydration heat and fast hydration reaction rate.

3.2 mineral admixtures

At present, mineral admixtures such as fly ash will be added in the preparation of high-performance concrete, which has the advantages of reducing the cost and improving the workability, strength and durability of concrete.

The experiments of Huang Yubin and others show that the trend of concrete slump loss can be effectively controlled by adding fly ash and slag. The reasons are as follows:

1) Because the surface of mineral admixtures is smooth and dense, they are dispersed among cement particles when mixing concrete. These dense particles play the role of dispersant, and the physical dispersion effect will not be weakened with the extension of time;

2) The addition of mineral admixtures slows down the hydration reaction and slows down the reaction rate of the whole system;

3) In the system with large amount of mineral admixtures, due to the low saturated adsorption capacity of superplasticizer, the saturated adsorption capacity of superplasticizer in the whole system is low, and the concentration of residual superplasticizer in the liquid phase of concrete mixture is high;

4) Admixtures, especially fly ash, have the function of water retention and reduce the water overflow;

5) The temperature rise of the cement mixture is greatly reduced by adding the admixture.

3.3 ambient temperature

Through the experiment, Zhang Dengxiang concluded that the early slump loss is directly proportional to the temperature of concrete, and when the temperature is higher, the slump loss is larger. At the same time, the slump loss is directly proportional to the initial slump. The greater the initial slump, the greater the slump loss. Therefore, it is not appropriate to use the method of increasing the initial slump in design to offset the expected slump loss, as shown in Fig. 1 and FIG two As shown in Fig.

 Concrete slump

Xu will also get the result that the higher the ambient temperature, the greater the loss of concrete slump, as shown in Table 1. The reason is that the higher the ambient temperature is, the faster the hydration speed of cement is, and the greater the slump loss of concrete is. At the same time, at higher temperature, water evaporation is faster, which also affects the slump loss of concrete.

 Concrete slump

3.4 flooring

When the content of sand and clay in concrete increases, the most obvious effect is the content of sand and clay 1% The slump of concrete decreases 10mm And the degree of slump decrease will increase with the increase of concrete strength grade.

In the practical engineering application, the sand and stone with less mud content should be selected as far as possible. If the sand and stone with less mud content can not be obtained due to environmental constraints, the amount of water reducing agent should be appropriately increased to reduce the slump loss of concrete.

3.5 concrete mix proportion

5

The size of water cement ratio is determined by the concrete strength and can not be changed at will. When the amount of cement slurry is fixed, the larger the water cement ratio is, the thinner the cement slurry is, and the worse the cohesion and water retention of concrete mixture are, resulting in the phenomenon of slurry flow and segregation, and the slump of concrete will decrease. The reason is that the coarse and fine aggregates are not wrapped by enough cement paste, the lubrication is seriously insufficient, the friction between aggregates increases, and the fluidity of concrete mixture decreases. The smaller the water cement ratio is, the more dry and thick the cement slurry is, the worse the fluidity of concrete mixture is and the smaller the slump is. In order to increase the fluidity of concrete, the method of adding water reducing agent properly or increasing the amount of water and cement at the same time can be adopted, and the method of simply increasing water consumption can be avoided as far as possible. Because the water cement ratio is too high, the cohesiveness and water retention of concrete will be poor, and the produced concrete is easy to segregate, thus affecting the pumping.

3.5.2 sand rate

Yang Zhi obtained through the experiment that there should be a reasonable sand ratio value among different sand rates of concrete, as shown in Table 2 39% The slump loss of concrete is relatively small. The reason is that under the condition of a certain amount of cement slurry. When the sand ratio is too high, the total surface area of aggregate increases. At this time, the thickness of cement slurry wrapped on the aggregate becomes thinner, the lubrication effect decreases, and the slump of concrete mixture decreases. When the sand ratio is too small, although the total surface area of the aggregate decreases, the amount of mortar also decreases. The coarse aggregate does not have enough mortar to wrap and lubricate. At this time, the concrete appears coarse and astringent, the workability is poor, and the slump of concrete mixture is small.

 Concrete slump

four Conclusion and suggestion

From the above analysis, the following factors should be considered in the control of concrete slump:

one )Fully realize the characteristics of cementitious materials. Retarder can be added to delay C three A In order to solve the influence of cement fineness on concrete slump loss, the hydration reaction of aluminate or the addition of slump retaining agent can solve the problem.

2) Reasonable and appropriate amount of mineral admixture should be added. The proper addition of fly ash and other admixtures can save cement, reduce cost, reduce hydration heat of concrete and improve its later strength.

3) The influence of ambient temperature should be fully considered. Generally, the higher the ambient temperature, the greater the slump loss of concrete. Therefore, in the summer operation, the concrete can be mixed at a lower temperature in the morning and evening.

4) Sand and stone with less mud content should be selected as far as possible. In practical engineering, if the sand with less mud content can not be obtained due to environmental constraints, the amount of water reducing agent should be appropriately increased to reduce the slump loss of concrete.

5) Reasonable design of concrete mix proportion. In the design of concrete mix proportion, the factors affecting concrete slump loss, such as environmental temperature, transportation distance and admixture variety, should be fully considered. After the design of mix proportion, laboratory adaptation should be carried out to ensure that it meets the requirements of strength, workability and durability. Problems should be adjusted in time.

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