As fresh concrete, more emphasis is placed on its workability. The workability of fresh concrete includes fluidity, cohesion and water retention.
1. Factors affecting the fluidity of fresh concrete
There are many methods for evaluating the fluidity of fresh concrete, the most common being the slump test method. The slump test method is mainly applicable to plastic concrete with medium fluidity.
The expansion test method is suitable for concrete with high fluidity, and the test process is the same as that of the slump method. After the slump is lifted, the concrete expands around under its own weight. The average diameter of the expanded concrete was measured as an indicator for judging the fluidity of the concrete.
(1) Water consumption
The water consumption of concrete is one of the main factors affecting the fluidity. Within a certain range, the fluidity of concrete increases with the increase of water consumption.
In the case of using a certain aggregate, if the water consumption is constant, within the actual application range, even if the cement consumption changes, the slump remains generally unchanged. This law is called the fixed water addition rule, or the water demand rule. . Although the fixed water addition rule is not strict, this rule is quite convenient for the design of concrete mix ratio, that is, by fixing the water consumption and changing the water-cement ratio, it is possible to obtain the workability requirements of fresh concrete. It is also designed to meet the requirements of concrete strength.
(2) ashes ratio,
If the water-cement ratio remains unchanged, reducing the bone-cement ratio will increase the fluidity of the concrete.
(3) Sand rate
The particles of sand are much smaller than those of coarse aggregate, so within a certain range, it can be filled in the voids of coarse aggregate, so that the bulk density of aggregate increases and the porosity of aggregate decreases. As the rate increases, the fluidity of the concrete increases. When the amount of sand exceeds a certain level, the voids of the coarse aggregate have been filled, and the entry of sand can no longer be accommodated. In this case, continue to increase the content of sand, the porosity of the aggregate begins to rise, and the fluidity of the fresh concrete decreases. . So there is an optimum sand rate problem for concrete.
The optimum sand ratio is related to the accumulation state of coarse aggregate, the particle gradation of sand and the amount of cement. The greater the porosity of the coarse aggregate, the greater the amount of sand that can be accommodated in the voids, and thus the greater the optimum sand ratio. The optimal sand ratio of coarse sand is large, and the optimal sand ratio of fine sand is small. In low-strength grade concrete, a larger sand ratio should be used due to the less cementitious material used. In high-strength grade concrete, due to the large amount of cementitious materials, a smaller sand rate should be used accordingly.
(4) Aggregate gradation
The graded aggregate has less voids, and better fluidity can be obtained under the same amount of cement slurry.
(5) Mineral admixtures
The water demand of high-quality fly ash is relatively small, which can significantly improve the fluidity of concrete, or greatly reduce the water consumption while keeping the fluidity of concrete unchanged. The finely ground slag powder has little effect on the slump of concrete.
(6) Admixtures
The addition of water reducing agent can significantly improve the fluidity of fresh concrete, or reduce the water consumption of concrete by a large margin while maintaining the same fluidity. The influence of superplasticizer on the fluidity of fresh concrete has a great relationship with the type and dosage of superplasticizer. Generally speaking, there is an optimal dosage of water reducing agents, but the optimal dosage of different water reducing agents is different. There is a mutual adaptation problem between the water reducing agent and the cementitious material. If the water reducing agent is not compatible with the cementitious material, its effect will also be greatly affected.
2. Slump loss of fresh concrete
The fluidity of fresh concrete decreases with time, which is an inevitable process of concrete hydration and hardening.
The causes of concrete slump loss are as follows:
(1) Hydration of the cementitious material. In concrete, the cementitious material reacts with water to form a hydration product, and the formation of the hydration product makes the cement paste transfer from a dispersed state to a cohesive structure. This transfer process will inevitably cause slump loss of the concrete.
(2) Decreased effect of superplasticizer.
(3) Aggregate absorbs water.
(4) Water evaporates.
(5) Bubble escape.
Reasons for large concrete slump loss:
(1) The admixture is not suitable for cement. The use of water reducing agent can not only significantly increase the slump of fresh concrete, but also may lead to the loss of slump of fresh concrete. Water reducing agent and cement have a problem of mutual adaptation and mutual matching. If they are not adapted to each other, it will cause a large slump loss.
(2) The hydration of the cementitious material is too fast. The slump loss of fresh concrete is closely related to the hydration process of cement. The faster the hydration rate of the cement, the greater the slump loss of the fresh concrete. my country's cement generally develops in the direction of refinement and high C3S. Therefore, the hydration rate of cement is generally accelerated, which is also one of the reasons for the accelerated slump loss of concrete. When the gypsum content in the cement is not suitable and the hemihydrate gypsum is too much, the slump loss of the fresh concrete will be too fast.
(3) The water absorption rate of aggregate and mineral admixture is large.
(4) High ambient temperature and low humidity.
Main technical measures to reduce concrete slump loss:
(1) Select a suitable cementitious material-admixture system.
(2) Delay the hydration of the gelling material. There are usually two ways to delay the hydration of cementitious materials: one is to adjust the cementitious material system and increase the amount of mineral admixtures; the other is to use retarders.
(3) The aggregate is pre-absorbed before use.
3. Properties of fresh concrete - workability (segregation, laitance and bleeding)
Segregation refers to the phenomenon that the components of the concrete mixture are separated from each other, resulting in uneven internal composition and structure, usually manifested as the separation of coarse aggregate and mortar or the phenomenon of laitance, and laitance refers to the separation of cement slurry and aggregate particles. Segregation is a manifestation of poor cohesion of concrete. Bleeding refers to the phenomenon in which water migrates from the interior of the mixture to the surface after the mixture is poured and compacted, but before setting and hardening. Hazards of Segregation and Bleeding on Hardened Concrete Properties: The separation of different levels of fresh concrete leads to uneven distribution of concrete, which will lead to uneven concrete strength and due to surface concrete and underlying concrete grout. The inconsistency of the content may cause cracking of the concrete surface.
Technical measures to prevent concrete bleeding and segregation:
(1) There are two ways to solve bleeding: one is to reduce the particle size of the cementitious material particles, and the other is to reduce the density of the cementitious material particles. Improving the grinding fineness of cement can effectively reduce bleeding, which is essentially a technical approach to reduce particle size. The addition of high-quality fly ash, silica fume and other mineral admixtures can also effectively reduce bleeding. From the point of view of reducing bleeding, fly ash is more effective than grinding fine mineral powder.
(2) To prevent various segregation of concrete, the key is that there can be a good gradation between various particles in concrete, and the movement of particles can be hindered at different levels, so as to maintain good uniformity of concrete. There are two technical ways to solve the laitance: one is to reduce the particle size of the fine aggregate particles, and the other is to increase the viscosity of the cement slurry. Considering the former technical approach, the use of finer sand helps to reduce laitance, and from the latter technical approach, it can be achieved by reducing the water-binder ratio and increasing the volume content of the cement slurry. If the coarse aggregate is separated, it indicates that the viscosity of the mortar is too small or the particle size of the coarse aggregate is too large, and the sand rate can be appropriately increased and the particle size of the coarse aggregate can be reduced.
4. Properties of fresh concrete - workability (setting and hardening)
Initial setting: The fresh concrete basically loses its fluidity, and the fresh concrete reaches the initial setting at this time.
Final setting: The hydration reaction continues, and the hydration products continue to strengthen the reticulated coagulation structure, so that the concrete has mechanical strength, and the fresh concrete reaches the final setting at this time.
(1) The influence of cement setting time. The faster the cement sets, the shorter the setting time of the fresh concrete.
(2) The influence of chemical admixtures. Chemical admixtures that have a significant effect on concrete setting time are some setting modifiers.
(3) The addition of mineral admixtures will affect the hydration reaction rate of the cementitious material, thus also affecting the setting time of the concrete. Under the same conditions, the larger the amount of mineral admixture, the longer the setting time of concrete. When adding mineral admixtures, the amount of retarder should be reduced accordingly. Especially when the dosage is large, the retarder may not even be used.
(4) Under normal circumstances, if the cement is the same, the greater the water-cement ratio, the longer the setting time of the fresh concrete.
(5) With the increase of ambient temperature, the setting time of fresh concrete is shortened.
(6) The level of ambient humidity affects the evaporation rate of water in the concrete. In a dry environment, the water in the fresh concrete evaporates faster, and the concrete setting time is shorter. This kind of coagulation is not coagulation in the true sense. This kind of coagulation is based on the loss of moisture, so the cement paste will be accompanied by large drying shrinkage, which will lead to more serious cracking. This condensation process is a process of crack formation and development. Normal coagulation is achieved by the formation of a sufficient number of hydration products, with no loss of water, only the transfer of water. That is to say, the conversion of some moisture from the natural state to a part of the hydration product does not necessarily produce shrinkage, and even if it does, it is much smaller than the former. Therefore, there are generally no obvious cracks.
*Note: In a dry environment, especially in a windy environment, measures must be taken to prevent the loss of moisture inside the concrete.
5. Performance of fresh concrete - workability (air content)
(1) As the air content increases, the bleeding rate of fresh concrete decreases.
(2) Under the condition that other raw materials remain unchanged and the mixing ratio is also the same, the air content increases and the bulk density of concrete decreases.
(3) With the increase of air content, the strength of concrete decreases, and the strength of high-strength grade concrete decreases faster.
(4) Appropriate air content helps to improve the frost resistance of concrete.
(5) Appropriate air content does not affect the impermeability of concrete.
