Construction Technology (CE-402) - Unit 2 Short Notes

UNIT 2: FORMWORK AND CONCRETE CONSTRUCTION

This unit covers the temporary structures (formwork) used to shape and support concrete until it gains sufficient strength, along with the planning and execution of construction joints.

1. Formwork Materials

Formwork materials must be strong, stiff, durable, and produce a smooth concrete finish.

[!TIP] Exam Focus: Be prepared to compare steel vs. timber formwork (Jun 2023) and discuss plastic formwork applications (Nov 2023).

2. Formwork Types

A. Stationary Formwork (Conventional)

• Definition: Formwork assembled, concreted, and stripped in place for each section.

• Applications: Building foundations, walls, columns, beams for typical projects.

• Features: Flexible for complex shapes, slower cycle time, higher labor cost.

B. Slip Formwork (Continuous)

• Definition: Formwork continuously raised vertically or horizontally as concrete is placed, without stopping.

• Applications: Vertical: Towers, silos, bridge piers. Horizontal: Pavements, canals, slabs.

• Key Features:

  • Continuous operation: No construction joints in the vertical direction.

  • High speed & quality: Uniform finish, reduced joints, faster construction.

  • Requires: High initial setup, skilled supervision, consistent concrete supply & mix design.

  • Components: Form panels, working platforms, hydraulic jacks, guiding system.

[!TIP] Common Pitfall: Slip form is for continuous, repetitive elements. It is not used for isolated, complex structural elements.

3. Formwork Design & Construction

Design Considerations:

1. Strength & Stability: Must withstand fresh concrete pressure (lateral pressure formula: $$\displaystyle P = \rho h $$, where $\rho$ = density, $h$ = depth) plus live loads (workers, equipment).

2. Stiffness: Limit deflection to avoid excessive deformation or bulging.

3. Surface Finish: Material and joints must provide specified finish.

4. Ease of Stripping: Design for safe, undamaged removal.

5. Safety: Guardrails, access platforms, secure ties.

Stripping & Removal (Jun 2024):

1. Determine Stripping Time: Based on concrete strength (typically 70-100% of design strength for supports), often 7-28 days. Use test cubes.

2. Remove Non-Load-Bearing Parts First: Wall forms, soffit forms.

3. Systematic Removal: Start from top (for walls) or edges (for slabs). Use proper tools to avoid impact damage.

4. Safety: Inspect for stability, use crowbars carefully, clear debris promptly.

5. Cleaning & Maintenance: Clean immediately, repair damages, apply release agent, store properly.

Safety & Efficiency:

• Safety: Design for load safety factors, provide access, secure against wind, inspect regularly.

• Efficiency: Optimize panel sizes for reuse, standardize components, plan for rapid assembly/dismantling.

4. Formwork Comparisons (Advantages/Disadvantages)

Steel vs. Timber Formwork:

Plastic Formwork:

• Advantages: Corrosion-proof, lightweight, easy to clean, non-adhesive surface, good for chemical environments.

• Disadvantages: Lower strength limits span, UV degradation, limited reuse compared to steel, higher cost than timber.

5. Construction Joints

Need (Jun 2023):

• When concrete placement must be stopped (end of day, equipment breakdown).

• To divide large structures for controlled curing and stress relief.

• To accommodate construction sequencing.

• Without joints: Creates a cold joint (weak plane) with poor bonding and potential leakage.

Types of Construction Joints (with Sketches - Describe for Diagram):

A. Based on Orientation:

1. Vertical Joints: Between adjacent lifts/wall sections.

   • Keyed Joint: A protruding key (tongue) in the first pour fits into a groove in the next. Provides shear key.

   • Butt Joint: Simple straight edge. Least effective, requires careful cleaning and grouting.

   • DiagramSEARCH: "construction joint keyed butt vertical concrete"

2. Horizontal Joints: Between successive layers (e.g., slab-on-ground, beam bottoms).

   • Often includes a shear key or dovetail.

   • Must be roughened and cleaned before next pour.

   • DiagramSEARCH: "horizontal construction joint slab beam key"

B. Based on Water Tightness:

• Waterstop: A flexible PVC or rubber strip embedded across the joint to prevent water passage (critical in water tanks, basements).

• DiagramSEARCH: "PVC waterstop installation construction joint"

Construction Procedure for Joints:

1. Stop concrete at predetermined location.

2. Terminate properly: Slope the end, avoid leaving loose aggregate.

3. Prepare for next pour: After initial set, remove laitance, roughen surface (hacking), clean thoroughly.

4. Apply bonding agent (cement slurry, epoxy) before placing next concrete.

5. Place and vibrate new concrete carefully against the prepared face.

[!TIP] Exam Key: Always link the type of joint to its function (shear transfer, water tightness). Cold joint is a defect; construction joint is a planned, prepared feature.

Quick Reference for Exams

• Formwork Design Core: Strength (pressure), Stiffness (deflection), Finish, Safety.

• Slip Form: Continuous, no vertical joints, for towers/pavements.

• Stripping Rule: Based on concrete strength, not just time.

• Construction Joint Must-Do: Roughen, Clean, Bond.

• Steel Formwork: High reuse, high cost, for repetition.

• Timber Formwork: Low cost, low reuse, for flexibility.

\boxed{\text{Formwork is a temporary mold; Construction joints are planned, prepared interfaces to ensure structural integrity.}}