Influence of Water Cement Ratio on Concrete Performance

What is water cement ratio？

Water cement ratio refers to the weight ratio of the amount of water used in the concrete mixture to the amount of cement. When the constituent materials are determined, the water-cement ratio is the main parameter that determines the strength, durability and other physical and mechanical properties of the concrete.

1.The influence of water cement ratio on concrete strength

The water cement ratio directly affects the strength of concrete. When the water-cement ratio is large, the cement particles in the concrete mixture are relatively small, and the distance between the particles is relatively large. The colloid produced by hydration is not enough to fill the gaps between the particles. In addition, Too much water evaporates and leaves more water voids, which reduces the strength of concrete. On the contrary, when the water ash is relatively small, the distance between cement particles is small, the colloid produced by cement hydration is easy to fill the gaps between the particles, the water and air left after evaporation are also low, and the concrete strength is high. However, a too low water-cement ratio will cause too little water and difficulty in hydration of the cement. Part of the cement cannot be fully hydrated, which is not conducive to the improvement of strength.

2.The influence of water cement ratio on the workability of concrete

The water-cement ratio decreases, the consistency of the slurry increases, the fluidity of the concrete mixture decreases, the mixture becomes astringent, and it is difficult to vibrate and compact. At this time, more additives are needed to improve workability and improve the construction performance of concrete. The water-cement ratio becomes larger, the consistency of the slurry becomes thinner, although the fluidity increases, the cohesion and water retention become worse, the sinking speed of the aggregate becomes faster, and the concrete mixture is prone to delamination, segregation and bleeding This phenomenon seriously affects the strength and durability of concrete.

3.The influence of water cement ratio on concrete durability

Concrete durability is the ability of concrete to resist various physical and chemical damages under the use environment, which directly affects the safety and performance of the structure, including impermeability, frost resistance, chemical erosion and alkali-aggregate reaction, etc. The ash ratio plays a key role in the durability of concrete.

(1) Impact on concrete carbonization

Concrete carbonation refers to the process by which CO2 in the air diffuses into the concrete. Considering the factors of the concrete itself, the carbonation rate is affected by the amount of cement or the content of Ca(OH)2 in the cement stone and the compactness of the concrete. On the one hand, the smaller the amount of cement in concrete, the smaller the amount of Ca(OH)2 produced by hydration, the smaller the diffusion resistance, and the faster the carbonization rate. On the other hand, when the water-cement ratio increases, the porosity of concrete increases, the density decreases, and the carbonization rate increases. On the contrary, the water-cement ratio decreases, the concrete density increases, the porosity decreases, and the carbonization speed is slower.

(2) Impact on the diffusion of chloride ions

The water-cement ratio has an important influence on the ability of concrete to resist the diffusion of chloride ions. The greater the water-cement ratio, the greater the chloride ion binding capacity of the concrete.

(3) Impact of anti-seepage

Impermeability refers to the ability of concrete to resist the penetration of water into the concrete pores. Generally speaking, the smaller the water-cement ratio of concrete, the higher the density and the better the impermeability. The larger the water-cement ratio, the more interconnected and irregular capillary pores inside the concrete. The porosity of the cement stone increases and the water permeability is strong. When the water-cement ratio is greater than 0.6, the impermeability of the concrete increases sharply. However, a too small water-cement ratio is not conducive to the full hydration of the cement, the density will also be reduced, and the water permeability will increase. The water-cement ratio during construction should be strictly controlled in water level fluctuation areas such as hydraulic building foundations, water-retaining and flow-through buildings, and wing walls, generally between 0.38 and 0.4.

(4) The impact of antifreeze

The frost resistance of concrete refers to the performance that the concrete will not be damaged and the strength will not be significantly reduced after being subjected to multiple freeze-thaw cycles under the conditions of use. The influence of water-cement ratio on the frost resistance of concrete has the following points: First, the water-cement ratio is too large, and the sinking rate of aggregates with different particle diameters is different during the vibrating process, resulting in delamination of paste and aggregate. The cement slurry floats on the surface and has poor abrasion resistance. When the concrete is damaged by freezing and thawing, it is easy to form surface erosion. Therefore, in high-cold areas, especially in frozen environments with water contact, the water-cement ratio of concrete should be appropriately reduced to improve the frost resistance. Secondly, the size of concrete bubbles and the distance between bubbles decrease as the water-cement ratio decreases, and increase as the water-cement ratio increases. When the air-entraining amount of concrete is similar, the larger the water-cement ratio, the larger the spacing of the bubbles, and the worse the frost resistance of concrete is. Finally, as the water-cement ratio increases, the total volume and pore size of the internal pores in the concrete become larger and larger, and the icing pressure and penetration pressure generated during the freezing and thawing process will be greater, and the frost resistance of the concrete will inevitably decrease.

4.The influence of water-cement ratio on concrete cracks

The larger the water-cement ratio, on the one hand, the more water in the concrete mixture, the more water evaporates, and the greater the plastic shrinkage. On the other hand, the increase of the water-cement ratio will relatively prolong the setting time of the concrete mixture, prolong the generation time of the force of the concrete to resist plastic shrinkage, weaken the force of resisting plastic shrinkage, and the concrete is prone to cracks. When the water-cement ratio is low, the homogeneity and cohesion of the concrete become better, the plastic settlement produced is smaller, and the width and total area of plastic shrinkage cracks are smaller. Therefore, under the conditions of meeting the construction requirements, the water-cement ratio should be reduced as much as possible.

5.The influence of water-cement ratio on concrete shrinkage

The shrinkage of concrete is composed of the volume shrinkage of the cement gel itself (the so-called condensation) and the volume shrinkage caused by the loss of water (the so-called dry shrinkage). The water-cement ratio has a great influence on the drying shrinkage of concrete. The larger the water-cement ratio, the greater the drying shrinkage. The shrinkage value of concrete with a water-cement ratio of 0.6 is about 40% higher than that of 0.4. The greater the water consumption of the concrete mixture, the greater the drying shrinkage. It is very necessary to use additives to control the water-cement ratio and workability. The lower the water-cement ratio, the higher the temperature of concrete due to hydration, and the greater its early temperature deformation value. The autogenous shrinkage of concrete and its rate increase with the smaller water-cement ratio. Concrete with low water-cement ratio hardens. There will be a lot of self-shrinkage early on.

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