Precast concrete elements
Author: Mirjana Terzić | Allplan Teamleader | NS Drafter
What is prefabrication?
Prefabrication or precast presents a different point of view compared to on-site construction since it brings many benefits of the manufacturing.
Starting with a controlled environment, it induces the elimination of weather factors and weather delays. Prefabrication enhances the quality of used materials, and the factory-controlled process generates minimal waste. There is also a reduction in construction time, as a result of providing of parallelization of works. Additionally, construction time can significantly minimize financing costs.
Disadvantages come when prefabricated items don’t fit, or are difficult to transport from the fabrication location. The main problem is that they are not suitable for high seismic zones since joints in prefabricated structures cannot handle the stress as well as cast-in-situ concrete.
As manufacturing technology continues to improve, the benefits and advantages of this construction method are expected to rise in the future.
Prefabricated wood components can help us solve many design and engineering challenges, including many benefits to the construction industry, such as aesthetics, environmental performance, lighter weight (compared to concrete), and energy efficiency. On the contrary, structural steel frames can provide an alternative for prefabricated structural components while, precast concrete can be more economical and, in some cases, more practical. Today, many buildings include a mixture of construction techniques, such as incorporating structural steelwork, in-situ concrete, and precast concrete elements.
Nowadays, the building industry embraces new digital tools such as 3D modeling, building information modeling (BIM), and computer numeric control (CNC) machines, making prefabrication and information flow much easier.
This text will further deal with the basic principles of working with precast concrete elements, for which we produce details – Shop drawings.
Precast concrete elements
These types of elements are usually joined to other elements in order to form a complete structure. They are used for structural components such as wall panels, beams, columns, slabs, staircases, pipes, tunnels, and so on.
Connection technology plays a fundamental role. Mostly, they should provide a good load-bearing connection along with connecting two precast elements. Therefore, an assembly of metal components and anchors is used. Joints around each element are cast in place with a suitable mortar.
On the other hand, the stability of elements must be provided, whether in transport or during their installation. In addition to this, a wide range of easy-to-install fixing elements is placed during concreting.
Another technology that needs to be analyzed is lifting technology, which sometimes can be various for transport and mounting. Depending on the weight and type of element, a proper type and number of lifting anchors according to manufacturer instructions are used and positioned.
All of these additional elements are installed during the concreting process, as well as electrical and plumbing rough-in components. In order to fit them into the rest of the structure, high precision is required.
Different elements can be concreted in different positions. It is relevant to know from which side of the element the pouring of concrete will take place and on which sides the formwork will be. This can be determined by the architect if it’s due to the esthetics, or it can depend on lifting technology in order to hide the holes for anchors. For some complicated elements, the factory can have its requirements too. If the “pouring side” mark is not well positioned on the shop drawing, the reinforcement itself will be incorrectly positioned, which leads to more errors when it comes to this.
One element represents a single unit during its construction but observing the whole model has its conveniences. It contains a lot of useful information, such as the way of their connecting to other units, as well as the location and type of those connections. Also, it is easier to spot errors in it, for example. It can be provided if parts of vertical electrical installations do not continue on each other.
Concrete reinforcement
Reinforcement in solid construction elements gets from one to another element being able to transfer loads. Unlike them, most of the loads in precast elements are transferred via added components, and for that reason, in those cross-sections, additional reinforcement is needed. Additional reinforcement is also required for the lifting system, depending on the load cases and boundary conditions.
Another difference with precast concrete elements is that most factories prefer to use a reinforcement mesh whenever it is possible. This is the result of much easier placement and securing of welded meshes than reinforcement bars, where each bar must be individually placed and tied. It is also simpler to provide concrete cover resulting in expected precision. One more reason for the greater use of reinforcement meshes in prefabrication is that precast plants have the necessary equipment and space for bending them.
In addition to this, while doing edge reinforcement for walls, slabs, and stair landings, we use open bent up meshes instead of open stirrups for the larger number of our projects.
What is the most common problem we face while drawing reinforcement for precast elements?
The manufacturing process itself allows them to be produced with finished surfaces, which reduces the amount of work once the element is set in place on-site. They are typically constructed on steel tilting tables, vibrating tables, or specially designed steel moldings.
Due to the way of production and the specified finished surface appears a problem – sometimes it’s unavoidable to interrupt the concreting within one element.
For example, we have a wall panel that has to be produced on a steel table in a horizontal position with a cantilever beam on the formwork side. A wall panel after being finished and cared separately will be turned to the other side so the beam can be subsequently concreted.
In this case, it is necessary to ensure their good connection and continuity of reinforcement, which can be achieved by installing specially designed and manufactured rebend and joint connections.
The casing unit consists of a hollow galvanized steel body which should provide an effective concrete bond. It is placed inside the formwork with the reinforcement belonging to the wall. The pre-bended bars for the beam are hidden in the casing, and the guard is fixed with a plasticized steel cover. The casing is fixed to the formwork with nails or secured to the reinforcement with the wire. When the wall is finished and turned around, the cover can be removed, and the bars can be moved and straightened to the main reinforcement in the beam. Only then the cantilever beam can be concreted and cured, and after that, the entire wall is considered finished.
Using many combinations of rebar, profile widths, various connections, and joints can be achieved for different cases.
Another problem appearing with the development of construction joints appears in the case of connecting a precast concrete element with the element concreted on the construction site. For example, a precast concrete wall in its bottom zone is connected with a slab that is made on the construction site. It is necessary to create a monolithic structure without any weaknesses that could compromise the structure’s durability and to achieve the continuity of reinforcement. Therefore, the reinforcement should pass through the precast wall, together with a rough surface of concrete on the entire length of the joint, in order to assist the bond with the new concrete. The rough surface has special rules for its development, and it is very important to mark it in the drawings.
What do we know so far?
Besides many benefits provided by precast concrete elements in constructions, there are certain observations and work in practice, which implies that they should be made according to the above-mentioned rules while following main engineering regulations and structural characteristics of elements. Also, the main design, shop drawings, structural analysis, and recommendations by engineers provide the essential starting point for the selection and development of prefabricated concrete elements in projects.
What are your thoughts on this subject?
