ch 22 lecture p 2

Chapter Overview

Discusses structural applications of concrete and reinforced steel. Reinforced concrete construction is versatile for various structures.

Advantages of Reinforced Concrete Construction

• Availability of Materials:Materials and labor are locally sourced, minimizing transportation costs and contributing to local economies.

• Type 1A Construction:Fire resistive properties allow for taller structures, making it suitable for high-rise buildings where safety standards are paramount.

• Flexibility in Design:Structural engineers can vary the amount of concrete and steel in structural elements, allowing for creative architectural designs and meeting specific load requirements.

• Custom Finishes:Almost any finish can be applied to reinforced concrete, including polished surfaces, textured coatings, and integrative colors which enhance aesthetic appeal.

Concrete Formwork Cost Breakdown

• 25-30%: Placing and finishing concrete.

• 20-25%: Cost of rebar.

• 25%: Cost of concrete itself.

• 50%: Cost of formwork, often underestimated but critical for the shaping and integrity of structures.

Types of Formwork

• Wooden Formwork:Typically used for shallow footings; affordable and easy to shape but not as durable as metal.

• Metal Formwork:Reusable for complex shapes and provides enhanced structural strength; slightly higher upfront costs, but savings in labor and repeat use.

• Glass Fiber Reinforced Plastic (GFRP):Allows for intricate designs and is lightweight yet strong, suitable for decorative projects and where complex geometries are required.

Releasing Agent

• Prevents bonding between formwork and concrete for easy removal; essential for maintaining the integrity of formwork for multiple uses.

Formwork Design and Considerations

• Formwork is temporary and often designed by the installing contractor based on specifications.

• Standardizing shapes and dimensions enhances reusability and reduces onsite adjustments.

• Bracing Needed:Must support lateral pressure from wet concrete to prevent deformation or collapse during curing.

Elevated Slabs and Shoring

• Overhead pours require robust temporary supports (shoring).

• Shoring systems include vertical shores and plywood sheathing, which create a stable platform for pouring.

• Reshores:Used when concrete isn't fully cured; provide support during this critical phase to avoid structural failure.

Concrete Beams

Reinforcement Types:

• Tension Reinforcement:Steel bars added at the bottom to resist tension during bending, essential for structural stability.

• Stirrups:Hooped reinforcements provide resistance to shear forces, particularly near supports where shear stress is greatest.

• Continuous Reinforcement:Both top and bottom reinforcement is used in beams of complex structures to enhance load-bearing capabilities.

Cover Requirements for Reinforcement

• Adequate cover is necessary to prevent corrosion and buckling; variations in cover thickness are based on exposure conditions and application (e.g., 3" for slabs on grade).

Reinforced Concrete Columns

• Ties are used to hold reinforcement bars together, maintaining spacing and preventing buckling under load.

• Lap Splicing:A method to join longer rebar pieces together; couplers can reduce congestion and increase effective load distribution.

Reinforced Concrete Walls

• Columns are tied into foundations with vertical reinforcement, ensuring overall stability and load transfer.

• Proper formwork and bracing are required to withstand the weight during curing, ensuring the structural integrity post-pour.

Concrete Slabs

Types of Slabs:

• Isolated Slabs:Independent sections placed on stable soil, allowing for movement relative to the building structure without cracking.

• Stiffened Slabs:Supported by thickened edges and additional beams for increased strength and load distribution, often used in industrial buildings.

• Composite Slabs:Incorporating steel profiled decking which creates composite action with concrete, enhancing structural efficiency.

Vapor Retarder Use

• Essential to prevent moisture-related issues in concrete; improper application may lead to differential curing and cracking.

Joints in Concrete

Control Joints:

• Shallow cuts made to prevent random cracking by allowing controlled separation as the concrete shrinks during curing.

Isolation Joints:

• Full-depth joints that allow independent movement between sections of concrete to prevent stresses due to thermal expansion or contraction.

Construction Joints:

• Used when multiple pours are done; need careful management to maintain integrity and alignment, often requiring dowels for load transfer.

Prestressed Concrete

Definition:

• Uses tensioned steel strands to enhance concrete's strength and resistance to cracking, allowing it to bear larger loads.

Types:

• Pre-tensioning:Steel strands are stressed before the concrete is poured, creating an initial compressive force.

• Post-tensioning:Stressing occurs after the concrete hardens, allowing for adjustments based on site conditions and requirements.

Benefits:

• Enables smaller, stronger structural elements capable of supporting heavier loads; reduces material usage while enhancing performance.

Conclusion

Concrete is a fundamental element in construction, with significant applications in formwork, beams, columns, walls, and slabs. Understanding its properties yields cost savings and enhances structural efficiency.

Next Steps in the Course

Further exploration of reinforced concrete slabs and advanced construction techniques.