Precast concrete is a form of concrete that is prepared, cast and cured off-site, usually in a controlled factory environment, using reusable moulds. Precast concrete elements can be joined to other elements to form a complete structure. It is typically used for structural components such as; wall panels, beams, columns, slabs, staircases, pipes, tunnels, and so on. It is a high performance material that integrates easily with other systems and inherently provides the versatility, efficiency, and resiliency needed to meet the multi-hazard requirements and long-term demands of high performance structures. Choosing precast concrete during the design build process can provide a multitude of architectural, construction, and design benefits.
Precasting is good at producing large numbers of identical components. For example, building an affordable housing project with identical apartments could use precasting to produce wall slabs and floor slabs for all the apartments, and then lift them into place and connect them. Today precast cast concrete plays a significant role in construction of high rise residential buildings, affordable housing, warehouses, commercial buildings, bridges & flyovers and other infrastructural structures. Construction with precast concrete components is faster and cost efficient.
Manufacturers have developed precast concrete plants for mass production of high quality precast concrete components. One of the important processing units of the production line is the casting Mould. Moulds play a critical role in giving form and affecting the look of the precast concrete components. The variation in component size, shape and surface finish depends on their architectural function and effect. High-quality moulds can help achieve these while ensuring that the production runs smoothly and reliably.
Mould quality matters because any flaws in the mould will also be visible in the final product. Precast concrete components today come with many types of surface finish – from raw grey to paint, dye, patterns, exposed aggregate or tile coverage. Any pits or unevenness in the mould can result in wavy product surfaces that undermine the desired architectural effect. Using weak or low quality moulds, for example, in casting wall panels may result in propeller-like twists that are costly or next to impossible to fix afterwards at the construction site.
A well-made mould helps to improve not only product quality and consistency but also the efficiency of the production process itself. Moulds need to be rigid, dimensionally accurate and uniformly over the whole surface area to produce even product. They must be as flexible and easy as possible to use to enable cost-efficient production.
During designing of the moulds for various elements, special importance should be given to easy de-moulding and assembly of the various parts. At the same time rigidity and strength and water tightness of the mould are also important taking into consideration forces due to pouring of green concrete and vibration. Mould design in terms of strength is carried out under consideration of static loads that include the weight of element, mould and mould equipment. In addition, the mould must carry dynamic loads, mainly due to the vibration generated during the compaction of a concrete mix.
In flow production, in which elements move through the subsequent production stations, the moulds are usually light and non-separable and have small dimensions. Precast concrete elements that are prepared on the stationary production lines require massive and separable moulds.
In order to obtain a proper shape and surface of the precast elements it is required to use not only durable and rigid molds but also easily adjustable solutions according to different shapes and dimensions of products. Intensive development of concrete technology in recent years, including broadening of Self Compacting Concrete applications, has contributed to the progress in mould planning and manufacturing.
Concrete technology affects leak-tightness of the moulds. Self-compacting concrete (SCC) can be poured into the moulds of a high level of leak-tightness. In addition, the lack of vibrations influences a higher durability of moulds. Concrete reinforcement technology also influences the massiveness of moulds. Pre-stressed elements due to the need of transferring large tensioning forces are designed with larger cross-section of steel with high mechanical strengths. Casting of several elements at the same time requires a usage of battery moulds, which can be non-separable, separable and partially separable.
Precast concrete components used in buildings and infrastructural projects today come in many sizes, shapes and with diverse surface finishes. Consistent quality product can be made by using moulds that are built with precision and skill from the best available materials. Moulds for mass production of structural precast concrete elements such as columns, beams, slabs, walls and staircases are usually made from steel due to several repetitions. Steel moulds are used in a production of reinforced and unreinforced precast concrete elements. Design and shape of the moulds depend mainly on the production technology in the plant, type of concrete, weight and shape of the element and number of elements in one casting.
One can create any kind of mould based on the precast element designed. The mould designer uses steel plates ranging from 4 mm, C-channels ranging from 50 to 200 mm, Angles ranging from 50 to 150 mm and flat bar thickness from 100 mm to 30 mm. Usually, the supporting steels such as angles/ channels/ flats used depend upon the plate thickness chosen based on the span of the mould designed. Hydraulic jacks for pre-stressing or tilting the table mould used range from 2 tons to 50 tons capacity. The common types of precast concrete moulds are Column Moulds, Beam Moulds, Wall Moulds, Slab Moulds and Staircase Moulds.
