As you all know, concrete is the primary material required for most construction work and is known for its durability, strength and versatility. For concrete to perform well, it is very vital to ensure proper placement of concrete. This article will guide you and provide you with all the techniques, considerations, and practises involved in the placement of concrete, ensuring the material’s best performance in various construction projects.
What is Concrete Placement?
Concrete placement is the delivery of the freshly prepared concrete mix to the specific location in a form that ensures it meets the design specifications. To prevent defects, proper placement of concrete is important as it provides uniformity and achieves the desired structural integrity. The concrete placing process involves transportation, pouring, spreading, compacting and finishing of the concrete.
Concrete Placement in Various Settings
Concrete placement is an important part of construction, and its methods depend on the project requirements, site conditions, and type of structure. Below are different methods of concrete placement, along with their detailed processes, which help in meeting the project’s structural and aesthetic requirements.
1. Process Before Concrete Placement
Before any concrete is placed, careful planning and preparation are essential. This includes selecting the right equipment, laying out on the project site, and choosing appropriate tools to ensure efficient and safe concrete placement. The steps are:
(i) Formwork Preparation: The formwork, which acts as a mould for the concrete, must be set up and inspected for strength and stability. It should be free of leaks and positioned to ensure the smooth flow of concrete without segregation of materials.
(ii) Setting Dimensions: The area and volume of concrete to be poured must be calculated and marked. This ensures that the formwork will hold the exact quantity of concrete required for the structure.
(iii) Placement Timing: Once the concrete mix is prepared, it must be placed within 30 minutes of leaving the mixer to prevent it from hardening early on. Concrete should be poured as close to its final position as possible to avoid the risk of weakening the structure or causing displacement of the reinforcement.
2. Placement of Concrete in Layers
Concrete is often placed in layers for large structures like dams, piers, and abutments. This method is necessary to secure uniformity and prevent issues like cold joints or weak points within the structure. The important features of this technique are:
(i) Layer Thickness: Concrete should be placed in horizontal layers of uniform thickness. For reinforced concrete, the layer thickness is between 15 to 30 cm, while for plain concrete, it ranges from 35 to 45 cm.
(ii) Bonding Layers: The first layer, also known as the jointing layer, provides a rough surface for bonding with the next layer. This is followed by a smoother finishing layer to prepare the surface for further construction.
(iii) Avoiding Cold Joints: It is vital to ensure that each layer is properly compacted and stiffened before the next is applied. The placement process should be quick enough to avoid cold joints, which can form when there is a delay between pouring layers, leading to weak points in the structure.
3. Concrete Placement in Special Conditions
Special situations need specific concrete placement methods to ensure the material reaches its final position without segregation or weakening. These conditions can include confined spaces, challenging site layouts, or proximity to existing structures. Some methods are:
(i) Vertical Placement: When concrete must be placed in confined forms, tools like chutes, buckets, or elephant trunks are used to deposit the material vertically. This prevents the segregation of the aggregate and ensures even distribution within the formwork.
(ii) Temperature Regulation: In hot or cold weather conditions, the temperature of the concrete must be monitored to ensure that it cures properly. Measures may include cooling the mix with chilled water or using insulation to maintain the ideal temperature range.
(iii) Transitioning Between Old and New Concrete: In scenarios like extending a sidewalk, it is important to create a smooth transition between old and new concrete by using proper fill material and ensuring that the surfaces bond effectively.
4. Framework for Concrete Placement
The framework or formwork is a temporary structure that holds the concrete in place until it hardens. It must be sturdy, durable, and well-designed to withstand the pressure of the wet concrete. Important factors for formwork include:
(i) Structural Integrity: The formwork should be strong enough to maintain its shape and prevent deformation during the pouring process. Materials like steel, wood, or plastic are commonly used, depending on the type and size of the structure being built.
(ii) Watertightness: Formwork should be sealed tightly to stop the concrete from leaking out, which could lead to defects in the finished structures.
(iii) Release Agents: To ensure easy removal of the formwork once the concrete is cured, release agents like oil or grease are applied to the inner surfaces. This prevents the concrete from sticking to the formwork and allows for clean removal without damaging the final structure.
Key Considerations in Concrete Placement
Following are the vital considerations you must know during the placement of concrete.
1. Workability
It affects the ease of mixing, transportation, placing, and finishing concrete. Workability influences the consolidation and surface quality of the placed concrete. Factors like water-cement ratio, aggregate size and shape, admixtures, and suitable temperature all affect workability.
2. Segregation and Bleeding
The separation of aggregate from cement paste can lead to non-uniformity and weak zones within the concrete. Segregation is caused by excessive handling or improper mix proportions. The moving of water to the surface after placement can weaken the surface layer and cause durability issues.
3. Compaction
Proper compaction eliminates air voids, ensuring that concrete surrounds the reinforcement and fills the framework, improving strength and durability. For pavements or large slabs, compaction methods like internal or external vibrators, roller compaction and manual methods like rods are used.
4. Temperature Control
While cold weather concrete placement slows down curing and strength development, hot weather concrete placement increases hydration, which speeds up the setting and may cause thermal cracking. To accelerate curing methods of concrete placement in cold weather, use heaters, insulating blankets, chilled water, and shade.
Conclusion
In construction projects, the quality and performance of a structure are mostly affected by the placement of concrete. Using the right methods and taking important factors like workability and compaction into account is important for successful concrete placement. These measures enhance the structural integrity and durability of construction and contribute to the overall safety and longevity of the built environment. If you want your project to be done perfectly, consult the engineers and experts at Brick & Bolt, who can help you achieve optimal results in concrete placement, leading to structures that are strong, durable and meet your design expectations.