The appropriate mix design, strength, hardening process, workability, and durability of concrete directly impact the final precast products’ quality. A proper understanding of concrete technology is key to consistent quality, effective production, and low costs.
The concrete mix design and professionally managed production process determine the structural quality (load-bearing capacity) and visual quality of precast elements. The concrete mix and curing technology also influence the speed of the production cycle.
Looking at the process from the other end, the desired properties of the precast product, the quality of raw materials, production process, and structural requirements determine the concrete’s mix design
Article: ABC's of concrete production
Concrete is the most used building material in the world. Concrete is a material formed by mixing cement, fine and coarse aggregate, and water with or without admixtures and additions, which develops its properties by hydration of cement.
Concrete is a primary raw material in precast and directly impacts the quality, production effectiveness and costs of precast products and structures. Concrete is also the main cost factor in many product groups. The share of concrete in different precast products is 20-40% in wall panels while in hollow core slabs its share rises to 40-50%.
Depending on the mix design, different types of concrete can be produced: dry-mix concrete for hollow core slabs, wet-cast concrete for walls, TT-slabs, beams, columns and stairs, self-compacting concrete, high strength concrete, facade concrete.
The primary raw materials needed for concrete are typically natural and local. The local supply chains for precast mean shorter delivery distances and minimized fuel use during transportation. Precast concrete is a sustainable choice for reducing the carbon footprint of the construction industry.
Sand and gravel or aggregates are the main components of concrete (over 70% by volume). The aggregates form the base of the concrete, and cement acts as a glue between aggregate particles.
The aggregate is a natural material, and variations in quality and properties are more extensive than with cement and other components. Therefore, the focus of quality control in a factory laboratory should be on aggregates.
The aggregates’ type and quality are vitally important for the concrete properties, both fresh and hardened. The aggregates’ quality also affects the durability and condition of the machines used in the casting process. For example, washing can improve the quality of the aggregates.
Cement is a hydraulic binder that hardens when combined with water. The use of different binder combinations can reduce costs and improve quality. Cement properties depend on cement type, and they have a direct impact on concrete properties. The cement types used in the precast industry are mostly CEM I (Portland cement), CEM II (Portland-composite cement), and CEM III (blast furnace slag cement).
It is essential in the concrete mix design to select and interactively proportion the raw materials so that the right properties of concrete paste and hardened concrete are reached. In the concrete mix, we should maximize the volume of aggregates and minimize cement and water volume. The goal is to use as little water as possible but still reach a suitable level of concrete workability.
The production of concrete for precast products includes storing of raw materials, batching, and mixing. Storage should provide conditions that minimize possible changes and preserve the raw material’s standard properties. Particular attention should be paid to humidity control.
The batching equipment must be built appropriately and fulfil the accuracy requirements set by local standards. The mixers must achieve a uniform distribution of the raw materials and the concrete’s uniform consistency within the available mixing time and capacity.
Batching and mixing plants
Most countries control the properties of concrete by applicable standards. These standards set the requirements, test, and quality procedures for both raw materials and final products. Standards may differ in different countries due to various climatic conditions, raw material availability and local traditions. Local standards usually provide the maximum amount of recycled materials that can be used in the mix design.
Modern concrete plants often have concrete wastewater and concrete recycling systems. Recycling different waste materials like water, fresh and hardened concrete and reinforcement decreases the environmental impact and improves the overall economy of precast production.
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