With the continuous development of materials science, the application fields of concrete are becoming wider and wider. No matter which field the concrete is used in, the key to its mix proportion design is to target the workability, mechanical properties, and durability of concrete. Select appropriate raw materials and optimize proportion parameters to meet design requirements. This article starts from the inherent requirements of mix ratio design and explores the direction of mix ratio adjustment under the influence of different factors, which has practical guiding significance for the design and production of concrete.
Technical requirements for concrete mix design
When designing the mix proportion of concrete, the main considerations are to meet the technical requirements of workability, mechanical properties, and durability. In the production and construction stage, the workability of the concrete mixture must be met, and its properties mainly include fluidity, cohesion, and water retention; in the concrete forming, curing, and later use stages, the requirements for mechanical properties must be met, which mainly refers to the strength of the concrete. Cubic compressive strength, flexural strength, and the ability to resist concrete deformation (mainly including chemical shrinkage, carbonation shrinkage, dry and wet deformation, temperature deformation, creep, etc.); in the later use stage, durability requirements must also be met, and its performance Mainly includes anti-permeability, anti-freeze, carbonization, steel corrosion, corrosion resistance, etc.
02 Analysis and adjustment of factors affecting concrete mix ratio
The composition and selection of concrete raw materials, the environment, structural parts, production and construction techniques, etc. all have a great impact on its technical requirements, which in turn affects the mix design of concrete.
2.1 Raw materials
The raw materials for preparing concrete directly determine the various properties of concrete. For example, the strength of cement, the main raw material, plays a decisive role in the strength of concrete; the water-cement ratio, sand rate, porosity of stones, admixtures, and admixtures all play a decisive role in the strength of concrete. The workability of concrete has a great influence.
2.1.1 The influence of raw materials on the workability of concrete
Water-cement ratio: The size of the water-cement ratio affects the fluidity of the concrete mixture: the larger the water-cement ratio, the better the fluidity and the greater the slump. However, if the water-cement ratio is too large, it is easy to cause the mixture to stratify and segregate. ; The smaller the water-cement ratio, the worse the fluidity. If the water-cement ratio is too small, the concrete will be difficult to vibrate and compact. When designing the mix ratio, the water-cement ratio should be reduced as much as possible to reduce cement consumption and save costs. To achieve the required slump, water reducing agent can be added.
Sand rate: If the sand rate is too high, the fluidity of the concrete will be reduced and the slump will become smaller; if the sand rate is too small, the cohesion and water retention of the concrete will become poor, and bleeding, segregation, and slurry will easily occur. When designing the mix ratio, a sand rate that can fill the gaps between the stones and have a certain margin should be selected.
Aggregate: The selection of aggregates has a greater impact on the workability of concrete. When the total amount of aggregates remains unchanged, concrete mixed with pebbles and river sand has better workability, while gravel and machine-made sand are used. The workability of the mixed concrete is relatively poor; the fineness modulus of the selected aggregates is too large or too small, which will lead to poor fluidity of the concrete mixture. When designing the mix ratio, if a large fineness modulus results in poor fluidity, the sand rate can be appropriately increased, or the cement dosage can be increased to improve workability; if the fineness modulus is too small, the sand rate can be reduced and water consumption can be increased for adjustment.
Admixtures: Admixtures such as fly ash and mineral powder can replace part of the cement. On the one hand, they can reduce the amount of cement and reduce costs. On the other hand, they can improve the workability of concrete. For large-volume concrete and high-temperature construction concrete, admixtures such as fly ash and mineral powder can be added to adjust the workability of the concrete.
Admixtures: Admixtures can improve the workability of concrete. For example, water-reducing agents can significantly improve the fluidity of the mixture without reducing the amount of mixing water. The more water-reducing agents are added, the greater the fluidity of the concrete; Pumping agents can be used to improve concrete pumping performance.
2.1.2 Effect of raw materials on mechanical properties of concrete
Strength of cement: The higher the strength level of cement, the higher the strength of the prepared concrete; for concrete prepared with the same cement strength level, the greater the cement dosage, the higher the strength.
Water-cement ratio: If the water-cement ratio is too large, there will be few cement particles in the concrete mixture and large distances, resulting in gaps between the particles, which will lead to low strength of the concrete and large deformation in the later period; the water-cement ratio will become smaller, the distance between cement particles will be small, and the particles will If the gaps are densely filled, the strength of the concrete will be relatively high.
Sand rate: The compressive strength of concrete changes slightly with the increase of sand rate.
Aggregate: Aggregate mainly plays a role in transmitting loads in concrete structures. The selection of aggregates can improve the strength and elastic modulus of concrete, and reduce deformation caused by load, thereby enhancing the anti-deformation ability of concrete and improving durability. sex.
Admixtures: Adding fly ash, mineral powder, and other admixtures will slow down the hydration rate of concrete and reduce the early strength of concrete. However, the active ingredients in the admixture can promote the later strength of concrete. Conducive to later strength development and durability.
Admixtures: The use of admixtures can adjust the strength of concrete. For example, adding early strength additives to concrete can accelerate the hardening of concrete and increase the early strength rapidly, so it can be used in emergency repair projects. Adding a retarder to concrete can delay the hydration rate of concrete and avoid the phenomenon of low strength and cracks in the later stage of concrete caused by the large temperature difference between the inside and outside. It is mainly used for concrete in high-temperature seasons, large-volume concrete, pumping, and long-distance transportation. Concrete. Water-reducing admixtures can increase the strength of concrete by reducing water without reducing the amount of cement.
2.1.3 Effect of raw materials on the durability of concrete
Water-cement ratio: The greater the water-cement ratio, the smaller the cement dosage, the less Ca(OH)2 produced during the hydration process of concrete, the smaller the diffusion resistance, and the faster the carbonization speed. At the same time, The total volume and pore diameter of the internal pores of the concrete is getting larger and larger, the ice expansion pressure and seepage pressure generated during the freezing and thawing process become larger, and the impermeability and frost resistance become worse; conversely, the water-cement ratio decreases and the concrete becomes denser. The strength is enhanced, the porosity is reduced, the carbonization speed is slower, and the frost resistance and impermeability are better.
Sand rate: An appropriate sand rate facilitates vibration and compaction. It not only improves the compactness and impermeability of concrete, but also improves resistance to the destructive effect of external corrosive media, reduces the degree of erosion, and delays the rate of erosion.
Aggregate: During the setting and hardening process of concrete, the shrinkage value of coarse aggregate is much smaller than that of cement slurry, which can limit the shrinkage of concrete to a certain extent. The coarse aggregate has a good surface structure and is easy to bond with the mortar matrix, which can effectively improve the density of the interface between aggregate and slurry and improve durability.
Admixture: The incorporation of fly ash can cause the concrete to undergo a secondary hydration reaction to produce hydrated calcium silicate, fill the gaps inside the concrete, and improve the impermeability of the concrete.
Admixtures: Adding water-reducing admixtures to concrete can significantly improve the durability of concrete. Incorporating air-entraining agents can evenly introduce a large number of stable and closed micro-bubbles, which can improve the frost resistance and impermeability of concrete. At the same time, it increases the air content and improves the crack resistance of concrete.
2.2 The environment in which concrete is located
The environment in which concrete is located mainly refers to the influence of temperature (high temperature, high cold, freezing and thawing, etc.), the influence of humidity, and salt-alkali corrosion. These environmental factors directly affect the strength of concrete, as well as durability indicators such as carbonization, crack resistance, and steel bar corrosion.
2.2.1 Effect of environmental concrete on workability
Temperature and relative humidity have an impact on the workability of concrete mixtures. When the temperature is high, the water in the concrete evaporates quickly during the mixing process, which will lead to reduced fluidity, reduced slump, and poor workability of the concrete. The water-cement ratio should be increased accordingly.
2.2.2 Effect of environment on mechanical properties of concrete
The higher the temperature and the greater the wind speed, the greater the evaporation rate of water in the concrete, which can accelerate the early strength of the concrete, thus leading to a reduction in the later strength. At the same time, it will cause the concrete to shrink, deform, and crack. When designing the mix ratio, the water consumption per unit volume can be appropriately increased. In high cold and freeze-thaw environments, the temperature is low and the hydration rate is slow, which directly affects the early strength and later frost resistance and crack resistance of concrete. To ensure the mechanical properties of concrete, the concrete is required to have a lower water-cement ratio. An appropriate amount of fly ash admixtures and water-reducing agents should be added to the concrete to limit the heat of hydration from the early hardening of the concrete so that the later hardening can be reduced. Shrinkage and deformation.
2.2.3 Impact of environment on concrete durability
High temperatures will cause the free water in the concrete to evaporate quickly and the concrete to solidify quickly, resulting in large plastic shrinkage of the concrete and cracks, especially in reinforced concrete structures, which will further cause corrosion of steel bars. When designing the concrete mix proportion, based on ensuring strength, the water-cement ratio can be appropriately increased and a retarder can be added to slow down the hydration rate. In high-cold areas, especially in freezing environments with water contact, reducing the water-cement ratio can make the capillary pores and bubbles inside the concrete smaller, greatly improving the frost resistance of the concrete.
Moisture and saline-alkali corrosive environments will cause concrete to crack. When designing the mix ratio, adding mineral admixtures can improve the pore structure of the concrete, significantly improve the concrete's resistance to sulfate corrosion, and thereby improve the durability of the concrete. At the same time, rust inhibitors can be added appropriately to prevent steel bars from rusting in the later stages of the concrete structure.
2.3 Structural parts
Different structural parts of concrete projects have different characteristics, which have different requirements for the workability, strength, and durability of concrete. When designing the mix ratio, adjustments should be made according to the characteristics of different structural parts.
2.3.1 Requirements for the workability of structural parts
Concrete with larger structural parts has higher workability requirements. During the construction process, the design requirements for the mix proportion of large-volume concrete should be fully considered. The use of large amounts of active admixtures and retarders can effectively reduce the early stage of cement Heat of hydration.
For parts with higher design elevations that require pumping construction, the concrete is required to have better pumpability. When designing the mix ratio, pumping agents must be added to adjust the fluidity of the concrete.
2.3.2 Requirements for mechanical properties of concrete in structural parts
The strength requirements of cushions, beams, slabs, columns, and foundation concrete in construction projects are different. In construction projects, the strength of columns is higher than the strength of beams and slabs of the same layer; the concrete strength of different parts of the bridge such as piers, abutments, and steel beams The requirements are also different. When designing the concrete mix proportion, the strength of concrete can be improved by selecting a cement strength grade that is 1.5 to 2 times the concrete strength grade, reducing the water-cement ratio, and selecting well-graded medium sand and continuous-grained gravel.
2.3.3 Requirements for concrete durability in structural parts
Different structural parts bear different stresses, which will have an impact on durability. For example, the cable towers of bridges require concrete to have good crack resistance. When designing the mix ratio, consider adding active admixtures such as fly ash and retarder. Water agents can reduce the heat of hydration of cementitious materials as much as possible, reduce the chemical shrinkage and dry shrinkage of concrete, and improve its anti-shrinkage performance; improve the anti-deformation and anti-cracking properties of concrete itself.
2.4 Other factors
Production, transportation, and construction techniques such as concrete transportation methods and pouring methods also have a great impact on the workability of concrete, which in turn affects the mix design of concrete. For example, for concrete transported over long distances, the setting time must meet the requirements of transportation and construction. Pumping concrete requires good fluidity, etc. The corresponding mix ratio design should be adjusted. For example, for pumped concrete, the sand should be increased accordingly. rate, add an appropriate amount of pumping agent to increase fluidity and reduce resistance during concrete pouring.
03Conclusion
Factors such as structural parts, environment, raw materials, and processes are not independent but influence each other. For example, the mix ratio of the same structural part will be different due to different environments; the mix ratio of different structural parts in the same environment also needs to be adjusted appropriately; and the selection of raw materials also needs to consider the characteristics of the structural part and the environment. When designing the concrete mix ratio, it is necessary to determine the working performance, structural strength, and durability index of the long-term environmental impact of the concrete based on the structural parts of the project, the environment in which it is located, the selection of raw materials, and the technology, and then the water-cement ratio is determined. , sand rate, aggregate gradation, admixtures, and additive dosage, and finally determine the unique mix ratio design for each part of the project.
