Concrete is one of the most widely used construction materials, valued for its durability, strength, and versatility. However, these desirable properties depend not only on the mix design but also critically on how concrete is handled, transported, placed, compacted, and cured. Improper procedures during any of these stages can compromise the quality and longevity of the structure. This article explores the essential steps and best practices to ensure concrete performs as intended, following the specifications laid down by the Engineer-in-charge.
Steps Involved in the Concreting Process
The process of concreting involves several key steps:
- Selecting quantities of materials according to the chosen mix proportion
- Mixing the ingredients thoroughly
- Checking the workability of the concrete
- Transporting the concrete to the site
- Pouring the concrete into formwork
- Vibrating the concrete for proper compaction
- Removing the formwork after a suitable curing period
- Curing the concrete with appropriate methods to ensure strength development
Each step plays a vital role in achieving the desired properties of concrete, and neglecting any can reduce its durability and strength.
Placing of Concrete
Before starting concreting, a thorough inspection of centering, shuttering, and reinforcement is mandatory to ensure everything is clean, properly aligned, and free from foreign materials like sawdust or shavings.
For slabs and beams, wooden planks or catwalks (such as chequered MS plates supported on wooden blocks) should be used to convey concrete without disturbing the reinforcement. Workers must avoid walking directly on reinforcement to prevent displacements.
When placing concrete in columns and walls, it is preferable to avoid construction joints. The vertical progress of concreting should be limited to about one meter per hour to ensure proper bonding between layers.
Concrete must be placed carefully to avoid segregation. In deep trenches and footings, chutes can be used to place concrete accurately. For columns and walls, the vertical drop of concrete should not exceed 1.5 meters at a time to prevent separation of coarse and fine aggregates.
Temperature plays an important role in placing concrete. Concreting should not be done when temperatures fall below 4.5°C, and protection against frost damage is essential, as frost-damaged concrete must be removed and replaced. In hot weather, the temperature of wet concrete should not exceed 38°C to avoid rapid setting and loss of workability.
The time between mixing and placing concrete should not exceed 30 minutes, or appropriate retarders should be used to maintain workability during longer periods.
Compaction of Concrete
Immediately after placing, concrete should be compacted to eliminate voids and air pockets, creating a dense, uniform mass. Mechanical vibrators designed for continuous operation are the preferred tool for compaction.
For areas around reinforcement, embedded fixtures, or corners, hand compaction using tamping rods is effective to ensure thorough consolidation. Layers of concrete should be placed so that the bottom layer has not set before the next layer is added.
Vibration must continue until the concrete is fully compacted and the mortar begins to rise to the surface, indicating the filling of spaces between aggregates. At this point, the vibrator should be stopped and withdrawn carefully to avoid leaving air pockets.
When both internal and external vibrators are used, the internal vibrator is removed first, followed by the external vibrator, to prevent loose pockets or gaps. It is important to avoid shaking reinforcement during compaction to maintain its position.
All compaction must be completed before the initial setting of concrete, which typically occurs within 30 minutes after mixing water into the dry mix.
Curing of Concrete
Curing is critical to the strength and durability of concrete. After the concrete begins to harden—about 1 to 2 hours after placement—it must be protected from rapid drying. Coverings such as moist gunny bags, sand, or hessian cloth are used to retain moisture.
After 24 hours, curing by ponding (keeping the surface covered with water) is recommended for a minimum of seven days. This ensures adequate hydration of cement, which is essential for strength development.
Rectification of Defects after Concreting
Once the concrete sets, inspection is necessary to identify any defects. For roof slabs, the top surface should be finished smooth before setting.
Concrete with significant sagging or honeycombing that threatens structural integrity must be rejected and replaced. Minor surface defects can be repaired as follows:
- Surface Defects After Form Removal
Bulges from form movement, ridges at form joints, honeycombing, and damage from form stripping can be rectified by careful chipping and tooling. Edges of honeycombed areas should be cut cleanly or undercut for better bonding. - Patching Shallow Defects
These are treated with a grout coat (1 part cement to 1 part fine sand) followed by layers of mortar similar to the original concrete. Each layer, up to 10 mm thick, is given a scratch finish before applying the next to ensure bonding. The final layer is finished to match the surrounding surface by floating or rubbing. - Repairing Large and Deep Defects
Such defects are reinforced, dowelled to the existing concrete, and filled with concrete held in place by forms. - Filling Bolt Holes
Holes left by bolts are packed with dry-mix mortar tightly compacted into place. - Pressure Filling Deep Holes
For deep or tiered holes, mortar can be injected with a pressure gun. - Color Matching Patches
Patches may appear darker due to less cement laitance. To correct this, 10-20% white Portland cement can be added to the patch mortar to achieve uniform color. - Curing of Repairs
Patches must be cured with the same care as the main structure to prevent early drying and cracking.
Conclusion
Proper handling, placing, compaction, and curing of concrete are indispensable steps for ensuring the structural integrity and longevity of concrete works. Following the outlined best practices not only helps achieve the desired strength and durability but also reduces the risk of defects and costly repairs. Always adhere to engineer specifications and environmental considerations for the best outcomes in concreting projects.