The water-binder ratio is the ratio of the amount of water used in concrete to the amount of cementitious materials. The water-binder ratio is an important parameter of the concrete ratio. Many properties of concrete are directly related to the water-binder ratio, such as workability, strength, durability, etc.
I will discuss the effect of the water-binder ratio on concrete strength, workability, etc. through this article.
The relationship between water-binder ratio and concrete strength
Under the condition that the variety, quality, and quantity of cementitious materials are determined, the size of the water-binder ratio directly determines the concrete strength. The strength of concrete becomes larger as the water-binder ratio decreases, and the strength decreases as the water-binder ratio increases. The change in the water-binder ratio and the change of strength is not a simple linear relationship. In different water-binder ratio ranges, the influence of water-binder ratio change by 0.01 on the strength is different. The smaller the water-binder ratio, the greater the effect of the same change on the strength.
In the past, only one cementitious material was used. Once the variety and quality of cement are determined, the size of the water-binder ratio directly affects the concrete strength. Today, cementitious material is no longer a single cement, but also includes mineral admixtures. The relationship between cementitious ratio and strength has become relatively complex. With the same water-binder ratio, the strength is not necessarily the same, and sometimes there is even a big difference.
For example, when the cement and fly ash species and quality remain unchanged, with the same water-binder ratio of 0.5, the 28d strength of concrete prepared with 30% fly ash and 50% fly ash obviously has a great difference.
For example, with the same water-binder ratio of 0.5, the 28d strength of concrete prepared by 30% fly ash and 30% mineral powder is also different.
For example, with the same water-binder ratio of 0.5, the 28d strength of concrete prepared with 30% of Class I fly ash and Class II fly ash is not the same.
All of the above shows that the impact of concrete water-binder ratio and strength is not a single relationship. The relationship between the two is very complex. It is affected by many factors such as mineral admixture species, quality, fineness, activity, and admixture amount. Once the raw material and the amount of admixture are determined, it is still consistent with the inverse relationship between the water-binder ratio and strength, but more than a linear relationship.
The effect of water-binder ratio on workability
The size of the water-binder ratio has a direct effect on the consistency of the concrete slurry. The larger the water-binder ratio, the lower the consistency of the slurry, and the smaller the buoyancy of the slurry to inhibit the sinking of aggregates, and the easier it is for the concrete to delaminate. Conversely, the greater the consistency of the slurry, the stronger the ability of concrete to resist segregation. The large water-binder ratio of low-strength grade concrete has low slurry concentration, poor concrete cohesion, and insufficient water retention. Concrete is easy to secrete, segregation.
It is recommended to use a low admixture amount and appropriately increase the sand ratio to improve water retention. The low water-binder ratio of high-strength concrete has high slurry concentration, better concrete cohesion, and good water retention, but large viscosity, and poor workability. Without increasing the amount of water, a higher admixture should be used to improve the workability of concrete.
The relationship between water-binder ratio and mineral admixture admixture
In the case of a constant water-binder ratio, the early strength of concrete decreases with the increase of mineral admixture, because the activity of mineral admixture is lower than the activity of cement. In order to obtain satisfactory early strength, while increasing the amount of mineral admixture, the water-binder ratio is appropriately reduced to improve the early strength of concrete to meet the needs of construction. Reducing the amount of water-binder ratio required for mineral admixture increased has a great relationship with the concrete water-binder ratio.
For example, when the concrete water-binder ratio is about 0.6, fly ash admixture increased by 10%, and the water-binder ratio should be reduced by about 0.04 to ensure that the concrete 28-day strength is not significantly reduced. When the water-binder ratio is about 0.4, fly ash admixture increased by 10%, the water-binder ratio reduced by 0.01 can ensure that the concrete 28-day strength is not significantly reduced.
In addition, as the amount of fly ash increases, the magnitude of the water-binder ratio reduction gradually increases. The use of additives can achieve a low water-binder ratio of the concrete formulation. But the concrete water-binder ratio is not the lower the better. The too-low water-binder ratio increases the amount of admixtures, and water sensitivity increases, which brings difficulties to control concrete quality.
Therefore, we should not pursue a large amount of mineral admixture and a low water-binder ratio. It is important to choose the appropriate amount of mineral admixture and suitable water-binder ratio according to the needs of engineering practice.
The influence of fine particles in the aggregate on the water-binder ratio
Aggregates are inevitably mixed with fine particles less than 0.075mm in size. These particles are sometimes not active though, and only play a filling role in the concrete. In the calculation of the water-binder ratio, do we need to consider these particles? That is, will the increase in these particles affect the size of the actual water-binder ratio? Should the cementitious material be taken into account to control the effective water-binder ratio?
The composition of the fine particles mixed into the aggregate is very complex. Part of it is stone powder or sand powder, which can play a filling role. But there is also a part of the fine mud powder particles, these mud powder particles adsorb water and admixtures, hinder the bond between cement and aggregate, and reduce the strength of concrete. To be part of the cementitious material or not, should be treated differently.