Analysis and research prospects on the problem of concrete segregation and bleeding

Concrete is one of the most widely used building materials in the world. Due to the emergence and widespread application of high-performance water-reducing agents, modern concrete materials have undergone revolutionary changes. The raw materials have changed from the traditional four-component to six-component. Components, high-performance water-reducing admixtures and mineral admixtures have become essential components of modern high-performance concrete. Modern construction technology and actual engineering requirements have promoted the development of concrete from the original dry-hard and plastic concrete to the current large fluid concrete. The problem of concrete segregation and bleeding has seriously restricted the healthy development of the concrete industry. Therefore, research on the problem of concrete bleeding has also attracted great attention from concrete scholars and practitioners around the world.

The factors that lead to concrete bleeding are very complex and have a lot to do with every aspect of concrete production, including raw material quality, mix ratio design, mixing process, construction technology, types and amounts of admixtures etc.[2]. The cement paste in the concrete system is an unstable system, and studying its stability is one of the most effective ways to understand the bleeding mechanism of concrete. In the case of relatively scarce resources, it is more feasible to apply cement concrete admixture technology to solve the problem of concrete bleeding. This is also the need for the continuous transformation of the concrete industry from a labor-intensive to a technology-based industry [3].

Concrete bleeding has become a practical problem that must be faced in concrete material science. The current research including the mechanism, model, evaluation and measures of concrete bleeding is not in-depth enough. Therefore, from the raw materials of concrete Only by taking targeted solutions to concrete bleeding in aspects such as , ratio design, and construction, and organically combining the technical principles of admixtures, can we develop new functional admixtures that can inhibit concrete bleeding and are well compatible with high-efficiency water-reducing admixtures. This article compares This paper conducts an in-depth and systematic discussion of the research on concrete bleeding problems at home and abroad, and proposes some solutions.
1. Bleeding model of cement slurry

The stability of fresh concrete includes two aspects: the settlement of coarse aggregate in the slurry and the bleeding of cement slurry. In the comparison of water glue In large cases, differences in movement or fluidity between cement particles may lead to inconsistent settlement and bleeding of the cement slurry itself. In view of the self-settlement and bleeding process of cement slurry, a suspension control agent is needed to inhibit the settlement of powder particles.

1.1 Instability of cement slurry

1.1.1 Cement slurry and suspension

The stability of cement slurry can be described as its retention over time Unstable phenomena such as uniformity, precipitation and bleeding can occur in various situations depending on the material composition. First of all, cement slurry is a multi-phase material that contains active cement particles in a wide range of particle sizes and has a high solid content. Secondly, due to the hydration of cement materials in fresh slurry, the viscosity and yield stress of the slurry must change with time. Usually the compounding of concrete admixtures ignores the cohesion and stability issues of concrete. In other words, the compounding of admixtures generally only uses water-reducing admixture components and various functional components, and lacks attention to the stability of concrete. The compounding of concrete functional admixtures is based on concrete slurry stabilizer components plus other functional components. Concrete functional admixtures are a supplement to current concrete admixtures. Their greatest role is to flexibly solve the common problems in the current application of concrete admixtures. Insufficient performance problems, improve concrete strength, collapse resistance, cohesion and reduce cracks in hardened concrete. Some people use viscosity modifiers, which have been proven to improve slurry properties and have a certain impact on particle settlement and bleeding rates, but their functions are far from achieving the effect of suspension stabilizers. Regarding the problem of segregation and bleeding of cement slurry, many scholars also use some classic theoretical models to explain it, including Stokes theory, Richardson–Zaki formula and the Kynch theory [5-7]. However, these models are used to explain the cement slurry in concrete theoretically. The sedimentation and bleeding process is relatively reluctant.

1.1.2 Cement slurry and concrete

There are many studies on the correlation between cement slurry and concrete. A large amount of experimental data proves that the rheology of concrete and the rheology of cement slurry related. Park et al. [8] studied the relative properties of fresh cement slurry, cement mortar and concrete by assuming that cement slurry, cement mortar and concrete have the same water cement composition, and found that there is a correlation between their properties. Ferraris et al. [9] experimentally concluded that the properties of fresh cement slurry can be used to predict the rheological properties of the same concrete. The research results of Peng, Y et al. [10] also show that the main influencing factors of the precipitation process include the viscosity and yield stress of the fluid, particle size, density difference between particles and fluid, solid volume fraction and the distance between particles. Saaket et al. explained that the settlement and segregation of aggregates are controlled by the yield stress, viscosity and density of the cement paste.

1.2 Cement slurry bleeding model

Kynch proposed a particle settlement theory based on the consolidation of particles during the settlement process. This theory assumes that a cylindrical suspension anywhere in the The settling rate is the only function of concentration at that point, and the dispersion does not consolidate or flocculate. Tiller[13] in K� Low; adding air-entraining agents has been proven to be very effective in reducing concrete bleeding, which not only affects the strength, impermeability and frost resistance of concrete, but also causes excessive bubbles in the concrete surface layer; in modern concrete production During the process, functional admixtures aimed at improving the stability of concrete have become popular in the market. They are characterized by suspension stabilizing components plus functional adjustment components.

(2) According to the practical experience of using additives in other related industries, additives can be mainly divided into two different series: dispersants and stabilizers, and the main method of compound additives is dispersant + stabilization agent + functional additives. At present, in the cement concrete industry, pumping agents for commercial concrete are mainly composed of water reducing agent components, retarders, air entraining agents, etc., and basically lack concrete stabilizer components. Retarders and thickeners cannot replace stabilizers. As a technical route for compounding concrete pumping agents, concrete stabilizers, which are dispersion-maintaining components, must be included. However, the actual situation is that most products cannot cope with concrete segregation and bleeding. To solve the conflicting problem of slump loss, some methods of increasing the amount of water reducing agent and adding thickener components cannot solve the problem at all. In view of the problem of concrete bleeding, the research on admixtures to improve the cohesion stability of concrete is a general trend in the research on functional concrete admixtures.

(3) At present, my country's construction projects are developing rapidly, concrete raw material resources are in short supply, and there are common problems that are difficult to ensure high quality, such as poor aggregate gradation, high mud and stone powder content, and excess concrete production capacity. Compared with the situation abroad, there are fewer construction projects abroad and stable sources of aggregates, which can ensure stable quality. Since the regional differences in concrete materials are very large and directly affect the adaptability of concrete admixtures and cement-based materials, when conducting compounding or modification research on admixtures, it is necessary to pay attention to the compatibility of admixtures with cement and sand and gravel raw materials. Regarding the capacitance problem, in view of the quality differences of raw materials in my country, especially the characteristics of high clay content, it is very necessary to conduct research on special functional admixtures.

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