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In this article, we will discuss concrete placing.
Concrete placing may be defined as the process of pouring concrete into the formwork to have a defined shape and size.
Concrete placement is an essential process in construction that defines the success of the structure and its life. Technical and environmental needs are carried into tough care while setting the concrete.
A. Concrete Placing
Here are 2 things you need to take care of while concrete placing listed below:
1. Avoiding Segregation
The considerable practices for ignoring segregation at the time of placing concrete, in any element, are:
a. Concrete must be positioned vertically and closely as much as feasible to its final position.
b. It should not be completed to flow into position. Where concrete must be transported it must be shoveled into the final position.
Other techniques for neglecting segregation at the time of placing rely on the type of element being created and on the type of distribution equipment being employed.
For concrete placing of flat work and slabs incorporating ribs and beams (i.e. shallow forms), the techniques shown below should be considered.
For concrete placing of walls and columns (i.e. deep, narrow forms), problems arise when the concrete is poured from a height and bounces off the reinforcement and forms faces, outcoming in segregation.
The means of evading this vary with the type of distribution equipment being employed as shown in the figure below.
2. Aiding Compaction
To assist in subsequent compaction of the concrete, care should be carried to place concrete in layers that are of a suitable depth for the compaction gear.
Layers that are too deep make it virtually unfeasible to sufficiently compact the concrete, leaving entrapped air that will make voids and blow holes in the surface of the concrete, and prevent it from achieving its potential durability and strength.
The two main types of compaction equipment are immersion (poker) vibrators and vibrating-beam screeds. The effective radius of action of an immersion vibrator relies on its frequency and amplitude.
The standard sizes available in normal concrete construction work include a radius of action between 200 and 350 mm. This means, in practice, concrete should be positioned in uniform layers varying from 250 to 400 mm, relying on the vibrator employed.
To ensure each layer is correctly melded together, the vibrator should penetrate about 150 mm into the lower layer as shown in the figure below.
B. Precautions of Concrete Placing
The precautions that need to be carried out during concrete placing are as follows:
1. During placing, it should be observed that all edges and corners of the concrete surface stay unbroken, sharp, and straight in line.
2. As far as possible, the concrete should be set in a single thickness. In the case of deep sections, the concrete should be placed in successive horizontal layers and proper care should be carried out to develop sufficient bonds between successive layers.
3. It is hoped to prepare concrete as near as practicable to its conclusive position.
4. Large quantities of concrete should not be deposited at a time. Otherwise, the concrete will start to flow along the formwork and consequently, the resulting concrete will not have uniform composition.
5. The concrete must be poured vertically from a proper height. For vertical laying of concrete, care must be carried to employ a stiff mix.
Otherwise, the bleeding of concrete through cracks in forms will occur. The term bleeding is employed to convey the diffusion or running of concrete through formwork.
6. The placing of concrete should be carried out uninterrupted between predetermined construction joints.
7. The concrete should be deposited in horizontal layers of about 150 mm in height. For mass concrete, the layers may be 400 mm to 500 mm in height.
The collection of surplus water in upper layers is known as the laitance and it should be controlled by employing shallow layers with a stiff mix or by placing dry batches of concrete to soak the surplus water.
8. The concrete should be simply worked around the reinforcement and tapped in such a way that no honeycombed surface occurs on the disposal of the formwork. The term honeycomb is employed to indicate a comb or mass of waxy cells initiated by bees in which they hold their honey.
Hence, if this precaution is not considered, the concrete surface so formed would have a honeycomb-like surface.
9. The concrete should be positioned on the formwork as soon as possible. But in no case, it should be positioned after 30 minutes of its preparation.
10. The formwork or the surface which is to obtain fresh concrete should be adequately cleaned, prepared, and well watered.
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