What is workability of concrete and why is it important?
The workability of concrete refers to the ease of construction operation (mixing, transportation, pouring, tamping) of the concrete mixture and the performance of small molding, uniform quality, no segregation, and no bleeding. The workability of concrete generally includes three aspects: rheological properties, cohesion, and water retention. Rheological properties refer to the performance of concrete mixture that can flow under the action of gravity or mechanical vibration and fill the formwork uniformly and densely. The size of rheological properties will directly affect the difficulty of conveying, pouring, and vibrating construction and the quality of concrete; cohesiveness means that there is a certain cohesion between the various components in the concrete mixture, and it will not cause damage during transportation and pouring. Delamination and segregation occur so that the internal structure of the concrete maintains consistent performance. Water retention is the performance of concrete mixtures that have an individual water retention capacity and will not cause severe bleeding during construction. It is an essential indicator of the stability of concrete mixtures.
How do you measure the workability of concrete?
The workability of concrete is a comprehensive concept, and it isn't effortless to express it comprehensively and appropriately with a simple evaluation method. Usually, the rheological properties of concrete mixtures are determined by slump experiments, supplemented by visual experience to evaluate cohesion and water retention. To this day, there is no simple measurement method that can fully reflect the workability of concrete mixes. Generally, the fluidity is determined experimentally, and the degree of cohesion and water retention are evaluated visually and empirically. The rheological properties of concrete are expressed by consistency. There are two methods to determine the rheological properties of the slump, slump extension method, and weber consistency method.
Leading factors affecting the workability of concrete
(1) Water consumption per unit volume
Water consumption per unit volume determines the quantity and consistency of the cementitious material slurry (after this referred to as slurry), which is the most critical factor affecting the workability of concrete. Within a specific range of water consumption per unit volume, the fluidity of the concrete prepared with different coarse aggregates is proportional to the unit water consumption. That is, as the unit water consumption increases, the fluidity also increases. However, when it is too large, it will lead to poor cohesion of the mixture, and even cause severe segregation, delamination, bleeding, and severely reduce the strength and durability of the concrete.
(2) Sand rate
The change of sand rate will cause a significant difference in the total surface area and porosity of the aggregate, so it has a more substantial impact on the workability of concrete. Within a specific range of sand rate, as the sand rate increases, particular rheological properties can be effectively improved; when the sand rate increases to a certain degree, the specific rheological properties become worse as the sand rate increases and affect the strength of the concrete. Besides, an excessively low sand rate will deteriorate the cohesiveness and water retention of the concrete mixture, and it is prone to segregation and bleeding. The main factors affecting the sand rate are the shape of the stones, particle size, particle gradation, and construction methods. Therefore, when designing the concrete mix ratio, a reasonable sand rate should be selected through experiments.
(3) Water-gel ratio and quantity of gelling material
With the same amount of cementitious material in the concrete, the water-binder ratio increases, and the fluidity of the mixture increases, and vice versa. The hydrogel ratio is too small, the slurry is thick, and the rheological properties of the mixture are low. If the water-to-binder rate is too large, it will cause weak cohesiveness and water retention of the mix, and it will seriously affect the strength and durability of the concrete. Therefore, the water-binder ratio should be reasonably selected according to the concrete strength and durability design requirements.
With the same water-binder ratio, the amount of glue in a unit volume is large. The thicker the slurry layer wrapped on the surface of the aggregate particles is, the better the lubrication effect is. The rheological properties of the concrete mixture are extensive and cohesive. It also has excellent features and water retention, which is suitable for pumping. If the amount of glue is too large, it will not only be uneconomical, but also increase the shrinkage deformation of the concrete after hardening; if the amount of adhesive is too small, the necessary fluidity cannot be guaranteed, and the cohesiveness will be weak, which is not conducive to pumping. It can be known that the amount of glue in concrete should not be too much or too small. The amount of glue should be determined according to the actual needs of the construction.
(4) Influence of the properties of constituent materials
The effect of cement on the workability of concrete is mainly the water consumption of standard consistency of cement. Different cement types, mineral compositions, and mixed materials all have a more significant impact on the water consumption of the natural texture of cement.
C3A content in cement
In the mineral composition of cement clinker, C3A has the fastest setting and hardening speed and the most significant heat of hydration. If the content of C3A in the cement clinker exceeds 8%, it not only causes the cement to hydrate too quickly but also has poor adaptability to additives.
The test shows that the water demand at 90℃ about 5% higher than the water demand at 50℃ The cement temperature is based on 50℃, and the slump loss over time increases by 15% for every 10℃ increase.
Active mineral admixture
Different mineral admixtures and various quality mineral admixtures have different effects. High-quality fly ash can effectively reduce the hydration speed of the gelling material, has excellent water retention performance, can also improve the viscosity of the slurry, and the surface is hard. There is no continuous adsorption of water. Therefore, it has a better effect on reducing concrete slump loss and improving practical pump ability; grinding slag powder has a worse impact on reducing tangible slump loss than fly ash, especially in high temperature and dry environments, grinding slag The effect of the dust is significantly reduced.
(4) Concrete additives
Concrete additives have a significant impact on the workability of tangible and economic benefits. For example, adding ordinary water-reducing agents, superplasticizer, pumping agents, and other admixtures with water-reducing results in concrete can significantly improve the fluidity of the mixture, improve cohesion, and reduce water bleeding. While maintaining rheological properties, reducing the amount of water used without reducing the cementitious material can enhance the strength and durability of concrete.
The adaptability of concrete additives to cementitious materials, sand, and stone mud content have a significant effect on the workability of concrete. When the versatility is excellent, the amount of concrete additives not only decreases, but also the workability of concrete is excellent. Conversely, the number of concrete additives will not only increase significantly, but also the workability of concrete will deteriorate, especially the slump loss will be substantial. When it is found that the workability of concrete is reduced due to the adaptability of concrete additives to the raw materials used, it is necessary to replace the elements or cooperate with the manufacturer of concrete additives to adjust the additive component.
4.How to improve the workability of concrete
Workability of concrete can be improved by
(1) When the slump of the concrete mixture is too small, the water-cement ratio can be maintained, and the amount of cement slurry should be appropriately increased; when the recession is too massive, or the sand rate is maintained, adjust the amount of sand and gravel;
(2) Through experiments, use a reasonable sand rate;
(3) Improve the gradation of sand and gravel, and generally use continuous grading as much as possible;
(4) Adding admixtures, using Superplasticizer, foaming agent, retarder can effectively improve the workability of concrete;
(5) According to the specific environmental conditions, shorten the transport time of the fresh concrete as much as possible. If it is not allowed, a retarder can be added to reduce the slump loss.