Plain cement concrete (PCC) plays a crucial role in building construction. It acts as a foundational layer that prevents direct contact between reinforced concrete and soil or water, protecting the structure from moisture and chemical attacks. This article explores the materials involved, the step-by-step procedure for laying PCC, important dos and don’ts, and the benefits PCC brings to construction projects.
Materials Used in Plain Cement Concrete
Selecting the right materials is essential to ensure durable and effective PCC.
- Coarse Aggregate: The coarse aggregate must be composed of hard, broken stones like granite, free from dust, dirt, and foreign matter. It should be sized at 20 mm or smaller and pass through a 5 mm square mesh. Proper grading is necessary to limit voids to no more than 42%.
- Fine Aggregate: Portland Pozzolana Cement (PPC) is generally the preferred cement type for PCC, offering required tensile and compressive strengths along with adequate fineness.
- Cement: The cement should conform to standard specifications and match the demands for strength and durability, with PPC typically used due to its superior properties.
- Water: Water used in PCC must be clean and free from harmful substances such as oils, acids, alkalis, salts, or organic matter. Ideally, potable water with a pH of 6 or above is used, complying with IS 456:2000 standards.
Proportioning of Plain Cement Concrete
The concrete mix ratio is fundamental for the performance of PCC. Commonly, mixes of 1:2:4 or 1:3:6 (cement:sand:aggregate) are used depending on structural requirements. Proportioning can be done by weight or volume, with volume batching using measuring boxes equivalent to one cement bag being common on-site.
Mixing of Plain Cement Concrete
PCC can be mixed by hand or machine, depending on the scale of the project:
- Hand Mixing: Suitable for small jobs, hand mixing involves creating a clean, watertight surface. Cement and sand are mixed uniformly before adding coarse aggregates and water. The aim is to achieve a consistent color and texture.
- Machine Mixing: For larger projects, mechanical mixers ensure uniformity. Dry materials are loaded into separate hoppers, then water is gradually added while mixing to achieve the correct water-cement ratio.
Laying of Plain Cement Concrete
PCC is laid in layers not exceeding 150 mm thickness to ensure proper compaction. Mechanical vibrators are typically employed for thorough compaction, though hand compaction is sometimes necessary, especially near edges or corners. All compaction must be completed within 30 minutes of water addition to avoid premature setting.
Curing of Plain Cement Concrete
Once laid, the PCC surface should be protected from rain and rapid drying by covering it with moist materials like wet burlap or plastic sheets. Proper curing maintains moisture for at least 14 days, essential for strength development and durability.
Dos and Don’ts of PCC Works
To ensure quality PCC, follow these guidelines:
Dos:
- Use shuttering that matches the exact size and thickness required.
- Mix water and cement according to specified water-cement ratios.
- Use chutes or extra labor for pouring concrete in deep areas.
- Remove loose soil from pit sides to prevent collapse during concreting.
- Implement dewatering methods if the water table is high.
Don’ts:
- Avoid mixing materials directly on bare ground.
- Do not proceed without formwork in place.
- Never pour concrete without proper leveling and compaction.
- Avoid dropping concrete from heights greater than 1.5 meters into the pit.
- Prevent excess cement mortar buildup on top of PCC for a smooth finish.
Advantages of Laying Plain Cement Concrete
Using PCC in construction provides several benefits:
- It ensures the necessary cover to bottom reinforcement by providing a firm base for cover blocks.
- Helps achieve the effective depth of reinforced concrete members, improving dimensional accuracy.
- Prevents direct contact between reinforcement steel and chemically active soil, reducing corrosion risk.
- Simplifies the placement of steel cages, improving productivity.
- Acts as a barrier to soil, promoting better bonding with overlaying structural concrete.
Conclusion
Plain cement concrete is a foundational element in construction that contributes significantly to the structural stability and durability of buildings. Proper selection of materials, accurate proportioning, careful mixing, correct laying, and effective curing are all vital to ensure the quality and longevity of PCC. Following best practices and guidelines ensures that the PCC layer performs its protective and structural role efficiently, safeguarding the reinforced concrete structure above it.