01Understanding the consistency and workability of concrete mixtures

Workability is a crucial and mysterious concept in the field of concrete construction. It is not just a simple term, but a comprehensive reflection of the multidimensional properties of concrete mixtures during construction operations. So, what exactly is the workability of concrete mixtures? What are the key elements involved?

Workability, in short, is the ability of a concrete mixture to cope easily with a series of construction processes such as mixing, transportation, pouring and compaction, and ultimately to achieve excellent performance characteristics such as a dense and uniform structure, no segregation and no bleeding. This performance cannot be achieved without the three key elements covered by workability: fluidity, cohesiveness and water retention.

02The “main factors” affecting concrete workability

(1) Water consumption per unit volume:

This key parameter plays a decisive role in concrete preparation, directly determining the amount of cementitious paste produced and its consistency. More importantly, the water content per unit volume is a key factor influencing the workability of concrete. Within a certain range of water content, by adjusting this parameter, we can significantly change the flowability of concrete mixtures based on different coarse aggregates. Specifically, there is a positive proportional relationship between the water content per unit volume and the flowability of the mixture: as the water content is moderately increased, the flowability of the concrete will also be correspondingly improved, making it easier to construct.

However, this adjustment is not without limits. When the amount of water per unit volume is too high, it will trigger a series of adverse chain reactions. First, the cohesiveness of the mixture will be significantly weakened, which means that the binding force between the components of the concrete is reduced, which can easily lead to structural instability. What’s more serious is that excessive water consumption may also induce serious problems such as segregation, delamination and bleeding, which not only destroy the evenness and integrity of the concrete, but also significantly reduce its strength and durability after hardening.

(2) Sand rate:

The sand rate, as one of the key parameters in the concrete mix ratio, has a significant impact on the aggregate total surface area and void ratio, which in turn has a profound effect on the workability of the concrete mixture. Within an appropriate sand rate range, as the sand rate gradually increases, the fluidity of the concrete is effectively improved, which helps to improve the plasticity and workability of the concrete during construction. However, when the sand content increases to a certain critical value, the fluidity of the concrete begins to decrease, accompanied by a decrease in the strength of the concrete. This phenomenon can be attributed to the fact that an excessively high sand content leads to too many voids between the aggregates, reducing the effective filling of the paste and weakening the internal structure of the concrete.

On the other hand, an excessively low sand content can also cause a series of problems. It can significantly reduce the cohesiveness and water retention of the concrete mixture, increasing the risk of segregation and bleeding. This is because a low sand ratio reduces the wrapping and filling effect of the fine aggregate on the coarse aggregate, making the internal structure of the concrete loose and unstable.

The choice of sand ratio is affected by many factors, including but not limited to the shape of the stone, the particle size distribution, the particle gradation, and the specific construction method. These factors together determine the comprehensive impact of the sand ratio on the properties of the concrete.

(3) Water-cement ratio and amount of cementitious material:

The water-cement ratio and the amount of cementitious material are two crucial parameters in the concrete preparation process, and they have a significant impact on the properties of the concrete mixture. When the amount of cementitious material is kept constant, a change in the water-cement ratio will directly lead to a change in the fluidity of the mixture. Specifically, when the water-cement ratio increases, the fluidity of the mixture will increase accordingly, and vice versa. This is because the change in the water-cement ratio affects the consistency of the slurry, which in turn changes the workability of the mixture.

However, the water-cement ratio should not be adjusted arbitrarily, but reasonably selected according to the design requirements for the strength and durability of the concrete. A too-low water-cement ratio will result in a slurry that is too viscous, making the mix less fluid and not conducive to construction operations. A too-high water-cement ratio will result in a mix with poor cohesiveness and water retention, seriously affecting the strength and durability of the concrete. Therefore, in practice, the water-cement ratio must be accurately controlled according to specific design requirements and construction conditions.

On the other hand, while keeping the water-cement ratio constant, the amount of cementitious material per unit volume is also an important factor affecting the properties of the concrete mixture. When the amount of cementitious material is increased, the slurry layer surrounding the aggregate particles becomes thicker, providing better lubrication. This increases the fluidity of the concrete mixture, improves its cohesiveness and water retention, and is conducive to pumping and construction. However, there is no limit to the increase in the amount of cementitious material. Excessive amounts of admixture are not only uneconomical, but also increase the drying shrinkage of the hardened concrete, which adversely affects the long-term performance of the concrete. Conversely, if the amount of admixture is too low, the necessary fluidity of the mixture cannot be ensured, and the cohesiveness will deteriorate, which is not conducive to pumping.

(4) Properties of constituent materials:

1. Cement:

As the core component of concrete, the type, mineral composition and mixing materials of cement all significantly affect the workability of the concrete mixture. Specifically:

①The choice of cement type is crucial. Cement has a significant effect on the workability of the concrete mix, especially its ease of mixing. This effect is mainly reflected in the standard water content of the cement consistency. Due to the differences in mineral composition and the use of mixed materials between different cement types, their water requirements will also be different, which directly affects the consistency and construction performance of the concrete mix.

②The C3A content of cement is an indicator that requires particular attention. Among the many mineral components of cement clinker, C3A is known for its rapid setting and hardening characteristics and high heat of hydration. When the C3A content of cement clinker exceeds the threshold of 8%, it not only accelerates the hydration process of cement, but also may lead to compatibility issues between cement and admixtures, which in turn affects the overall performance of concrete.

③The temperature of cement is also a factor that should not be ignored. Experimental studies have shown that changes in the temperature of cement can affect its water demand. Specifically, when the temperature of cement increases from 50°C to 90°C, its water demand will increase by about 5%. In addition, taking 50°C as a reference point, for every 10°C increase in the temperature of cement, the loss of concrete slump over time will also increase by about 15%. This change will not only affect the construction performance of concrete, but may also adversely affect its mechanical properties and durability after hardening.

2. Aggregates:

As an important component of concrete, the aggregate has a profound effect on the workability of the concrete mixture due to its particle grading, shape, clay content and water absorption rate. Concrete made with high-quality aggregates that meet the standards exhibits excellent fluidity, cohesiveness and water retention during the mixing process, while also experiencing relatively little loss of slump. Such concrete is easier to work with and has stable structural properties after hardening.

On the other hand, concrete made with aggregates with poor particle grading, a rough surface, a high content of needle- and flake-shaped particles, a high clay content and a high content of clay lumps will have significantly reduced flow properties, and its cohesiveness and water retention will also be poor. In addition, the loss of slump of this type of concrete will accelerate significantly, which will not only make construction difficult, but will also adversely affect the strength and durability of the final hardened concrete.

3. Active mineral admixtures:

The effect of reactive mineral admixtures in concrete varies depending on the type and quality. High-quality fly ash, for example, is a highly effective reactive mineral admixture that can significantly slow down the hydration rate of the cementitious material. It has excellent water retention capacity and can enhance the viscosity of the slurry thanks to its hard surface properties and non-persistent adsorption of water. This is why high-quality fly ash shows excellent results in reducing the loss of concrete slump and improving the pumpability of concrete.

4. Concrete admixtures:

Concrete admixtures play a vital role in regulating the workability of the mixture and improving economic efficiency. By adding admixtures with a significant water-reducing effect, such as ordinary water reducers, high-efficiency water reducers, and pumping agents, to the concrete, the fluidity of the mixture can be greatly enhanced, its cohesiveness effectively improved, and bleeding significantly reduced. The introduction of these admixtures not only optimizes the workability of the concrete, but also further improves its mechanical strength and durability by reducing the amount of water used without reducing the amount of cementitious material while maintaining the fluidity of the mixture.