Precast & Modular Construction (CE-603 (B)) - Unit 3 Short Notes

UNIT 3: PRECAST & MODULAR CONSTRUCTION

1.0 FUNDAMENTALS OF PREFABRICATION & MODULAR CONSTRUCTION

1.1 Need, Necessity, and Driving Factors

• Need: Addresses shortage of skilled labor, ensures quality, speeds up construction, reduces site disturbance, and improves safety.

• Driving Factors: Urbanization, demand for rapid construction, labor cost escalation, need for precision and quality control, sustainability goals (less waste), and technological advancements.

1.2 Aims and Basic Philosophy

• Aims: To shift construction from site to factory, achieve industrial production, ensure consistent quality, reduce time and cost, and minimize on-site waste.

• Philosophy: "Manufacture in controlled environment, assemble on site." Emphasizes standardization, repetition, and integration of design with production.

1.3 Systems of Prefabrication

1.4 Prefabrication vs. Conventional Cast-in-Place

1.5 Merits and Demerits of Large Panel Construction

• Merits: Very fast erection, excellent surface finish, reduced on-site labor, good thermal insulation (if sandwich panels), suitable for repetitive structures (apartments, hotels).

• Demerits: Requires heavy lifting equipment, complex connections (structural & services), high transportation cost/logistics, less flexible for last-minute design changes, requires precise foundation and alignment.

2.0 MODULAR COORDINATION & STANDARDIZATION

2.1 Concept of Modular Coordination (MC)

• Definition: Adoption of a basic module (M) as the fundamental unit of length for coordinating dimensions of building components and spaces.

• Preferred Modular Sizes: \boxed{100 \text{ mm}} (basic module) or \boxed{300 \text{ mm}} (multiples for planning grids). Ensures dimensional harmony and reduces cutting/waste.

2.2 Significance in Prefabricated Structures

• Enables interchangeability of components from different manufacturers.

• Simplifies design, detailing, and production planning.

• Reduces inventory of different component sizes.

• Facilitates future expansion or modification.

• Minimizes on-site fitting errors and adjustments.

2.3 Standardization

• 2.3.1 Need: Achieves economy of scale, simplifies production, ensures quality, reduces design time, and supports modular coordination.

• 2.3.2 Levels:

  • Component Level: Standard sizes of bricks, blocks, tiles, windows.

  • Element Level: Standardized wall panels, floor units, door sets.

  • System Level: Standardized structural systems (e.g., standardized frame spacing).

2.4 Modular Planning for Buildings

• 2.4.1 Principles:

  • Design based on modular grid (e.g., 3M or 6M = 300mm or 600mm).

  • Room dimensions and component sizes are multiples of the module.

  • Structural grid aligns with modular grid for easy integration of floors/walls.

• 2.4.2 Integration of Services & Finishes:

  • Service ducts, conduits, and finishes (plaster, tiles) must be designed within the module thickness.

  • Requires early coordination between structural, architectural, and services engineers.

3.0 STRUCTURAL SYSTEMS & COMPONENTS

3.1 Classification of Structural Systems

3.2 Wall Panels

• 3.2.1 Types (by function):

  • Load-Bearing: Carry vertical dead/live loads.

  • Non-Load Bearing/Partition: Only provide separation.

  • Cladding/Facade: Weatherproofing, aesthetic.

  • Insulated Sandwich: Two concrete wythes with insulation core.

• 3.2.2 Classification (by connection):

  • Panel-to-Panel: Vertical/horizontal joints between wall panels.

  • Panel-to-Floor/Column: Connections at intersections.

3.3 Shear Walls

• 3.3.1 Role: Primary lateral load-resisting system (wind, earthquake). Provides stiffness and ductility.

• 3.3.2 Types of Precast Shear Walls:

  • Solid Walls: Simple, heavy.

  • Couple Walls: Two walls connected by beams/spandrels.

  • Walls with Openings: Strategically placed openings.

  • Sandwich Walls: Insulated, with connectors for composite action.

• 3.3.3 Shear Wall vs. Brick Masonry Wall:

  | Parameter | Precast Shear Wall | Load-Bearing Brick Wall | | :--- | :--- | :--- | | Strength & Stiffness | High, predictable | Low, variable | | Ductility | Good (with proper reinforcement) | Poor (brittle) | | Construction Speed | Fast | Slow | | Quality Control | Excellent (factory) | Poor (site) | | Lateral Load Resistance | Designed for | Not adequate for high seismic zones |

3.4 Column Structures

• 3.4.1 Types:

  • Single Columns: Most common.

  • Double/Twin Columns: For heavy loads or large spans.

  • Embedded Columns: Cast into foundation or wall panels.

• 3.4.2 Design Considerations:

  • Length-to-thickness ratio for stability during handling/erection.

  • Reinforcement detailing at ends for connection zones.

  • Lifting devices (inserts, anchors) must be considered in design.

3.5 Floor and Roof Slabs

• 3.5.1 Types:

  • One-Way Slabs: Span in one direction (supported on two sides). Common for strip loading.

  • Two-Way Slabs: Span in two directions (supported on all four sides). More efficient for square panels.

• 3.5.2 Common Precast Slab Types:

  • Hollow Core Slabs: Continuous voids reduce weight, increase stiffness. Very common for floors.

  • Double-Tee Slabs: Wide flange, used for long spans (parking, roofs).

  • Ribbed/Waffle Slabs: For large spans with heavy loads.

• 3.5.3 Manufacturing Process Flow Chart:

Mould Preparation (cleaning, oiling)

    ↓

Reinforcement Cage Placement (including lifting inserts)

    ↓

Concrete Placement (vibration, finishing)

    ↓

Curing (Steam curing for early strength)

    ↓

Stripping & Finishing (repair, edge treatment)

    ↓

Storage & Dispatch (stacking, marking)

#### **3.6 Special Systems**

*   **3.6.1 Box Prefabricates (Volumetric Construction):**

  *   Complete 3D units (rooms with finishes, fixtures, services).

  *   **Features:** Maximum factory completion (up to 90%), crane-lifted like containers.

  *   **Advantages:** Fastest construction, highest quality, minimal site waste.

  *   **Applications:** Hotels, apartments, student housing, hospitals.

*   **3.6.2 Earthen Walls in Prefabrication:** *(Not a major focus in past papers; typically refers to rammed earth or compressed earth blocks made off-site. Less common in modern high-rise precast.)*

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### **4.0 PRODUCTION, TRANSPORTATION & ERECTION PROCESS**

#### **4.1 Production Phase**

*   **Factory Setup:** Requires large, flat yard, overhead cranes, storage areas, and efficient production line layout.

*   **Moulds/Formwork:** **Reusable, precision steel/wood moulds**. Must be rigid, accurate, and allow for easy demoulding. Tolerances are critical (±2-3mm).

*   **Concrete Mix Design:** **High early strength** (for rapid turnover), good workability (for compaction), and durability. Often uses superplasticizers and steam curing.

*   **Curing Methods:** **Steam curing** (most common – accelerates strength gain), membrane curing, or water curing.

*   **Quality Control:** Testing of materials, concrete slump, cube strength, dimensional checks, and visual inspection for cracks/honeycombing.

#### **4.2 Transportation Phase**

*   **Handling & Lifting:** Use **lifting anchors/inserts** designed for the element. Lifting stresses must be checked.

*   **Logistics:** **Route survey** for overhead clearances, load limits, and turning radii. Use **specialized trailers** with supports to prevent bending during transit.

*   **Damage Prevention:** Adequate **supporting points**, protective coverings, gentle loading/unloading, and securing against movement.

#### **4.3 Erection Phase**

*   **Site Preparation:** **Precise foundation alignment** (grouting, shims). Setting out based on grid lines.

*   **Equipment:** **Mobile/Tower cranes** selected based on lift capacity, radius, and site constraints.

*   **Temporary Supports:** **Props, braces, and guy wires** used until connections are fully secured and grouted.

*   **Sequence:** Typically **vertical elements first (columns/walls)**, then **horizontal elements (beams/floors)**, following a **bay-by-bay or floor-by-floor** sequence.

#### **4.4 Disuniting (Demounting/Dismantling)**

*   **4.4.1 General Steps:**

  1.  Assessment & planning (identify connection types).

  2.  Remove non-structural finishes/services.

  3.  Dismantle connections (cut bolts, break mortar in wet joints).

  4.  Sequential removal using appropriate equipment.

  5.  Segregate components for reuse/recycling.

*   **4.4.2 Precautions:** Prevent damage to reusable elements, ensure stability during removal, manage dust/debris, safety of workers.

*   **4.4.3 Reusability & Recycling:** Design for **disassembly** (use dry connections where possible). Concrete can be crushed for aggregate; steel is recyclable.

---

### **5.0 CONNECTIONS AND JOINTS**

#### **5.1 Types of Connections**

*   **5.1.1 Wet Connections:** **Cast-in-place concrete** at site. Provides monolithic action but slower. (e.g., beam-column joints, wall panel splices).

*   **5.1.2 Dry Connections:** **Mechanical** (bolted, welded, clamped). Fast erection. (e.g., bolted steel plates, welded studs, keyed joints).

*   **5.1.3 Wall Panel Connections:**

  *   **Vertical:** Butt joints with grout, keyways, or welded plates.

  *   **Horizontal:** Often use **tongue-and-groove** or **grouted splices**.

*   **5.1.4 Other Connections:** Floor-to-wall (support angles, corbels), wall-to-column (embedded plates), column-to-foundation (base plates, anchor bolts).

#### **5.2 Joints in Precast Concrete**

*   **5.2.1 Expansion Joints:**

  *   **Purpose:** Accommodate **thermal expansion/contraction**, moisture movement, and creep/shrinkage. Prevent uncontrolled cracking.

  *   **Location:** Typically at **building corners, changes in height/length, and between different structural systems**.

  *   **IS Code Recommendations (IS 3414):** Joint width based on temperature range and coefficient of thermal expansion. Minimum width often 10-20mm.

  *   **Materials:**

      *   **Joint Fillers:** Pre-molded bituminous, foam, or cork – compressible, resilient.

      *   **Sealing Compounds:** Polysulfide, silicone, acrylic – applied over filler for weatherproofing.

  *   **Merits:** Protects structure from induced stresses.

  *   **Demerits:** Potential for **leakage, maintenance, tripping hazard**, and thermal bridging if not detailed properly.

*   **5.2.2 Contraction/Control Joints:** Pre-planned weakened planes to control cracking due to **shrinkage**.

*   **5.2.3 Construction/Weak Plane Joints:** Intentionally created planes of weakness to control crack location.

*   **5.2.4 Flexibility Joints:** Joints designed to **accommodate differential movement** between adjacent structural elements (e.g., between precast and cast-in-place sections).

#### **5.3 Tie Bars in Concrete Pavement/Wall Panels**

*   **5.3.1 Purpose:**

  *   **Hold-Down:** Resist uplift due to friction (e.g., in pavements under braking).

  *   **Load Transfer:** Transfer shear across longitudinal joints in pavements.

*   **5.3.2 Design of Tie Bars (for Longitudinal Joint in Pavement - Example):**

  *   **For Friction Resistance:** $$\displaystyle A_{st} = \frac{\mu \cdot W}{f_{st}} $$

      *   $$\displaystyle A_{st} $$ = Area of one tie bar

      *   $\mu$ = Coefficient of friction (1.5 for concrete on concrete)

      *   $W$ = Weight of slab per unit length = $\gamma \times t \times L$ ($\gamma$=unit wt, $t$=thickness, $L$=width of slab)

      *   $$\displaystyle f_{st} $$ = Allowable tensile stress in steel.

  *   **For Bond Stress:** Check $$\displaystyle L_d \geq \text{required length} $$ where $$\displaystyle L_d = \frac{\phi \cdot f_{st}}{4 \cdot \tau_{bd}} $$ ($$\displaystyle \tau_{bd} $$ = allowable bond stress).

  *   **Spacing:** Determined from total force required per unit length.

*   **5.3.3 Difficulties during Installation:**

  *   Misalignment, difficulty in achieving correct embedment length, congestion of reinforcement, damage to bar ends during handling.

#### **5.4 Problems Arising from Joint Flexibility**

*   **5.4.1 Analysis of Issues:**

  *   **Excessive Movement:** Leads to serviceability issues (e.g., door/window jamb distortion).

  *   **Stress Concentration:** At connection points due to unintended rotations.

  *   **Leakage & Waterproofing Failure:** Through moving joints.

  *   **Noise & Vibration:** Rattling in flexible connections.

  *   **Differential Settlement:** Amplified effects.

*   **5.4.2 Design Strategies to Mitigate:**

  *   Use **semi-rigid connections** where possible.

  *   Provide **adequate stiffness** in connection elements (thicker plates, larger welds).

  *   Incorporate **flexible sealants** with good movement capacity.

  *   **Isolate** movement to designated flexible joints only.

  *   Perform **detailed analysis** (e.g., finite element) for complex connections.

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### **6.0 DESIGN ASPECTS & CONSIDERATIONS**

#### **6.1 Design of Precast Wall Panels**

*   **6.1.1 Out-of-Plane (Cantilever):** Designed as vertical cantilever slabs for wind pressure. Check for bending, shear at fixed base.

*   **6.1.2 In-Plane:** Check for axial load (compression/tension) and shear (from lateral loads or diaphragm action).

*   **6.1.3 Numerical Problems:** Typically involve checking **combined stresses** (bending + axial + shear) as per IS 456. Use interaction diagrams or limit state equations.

#### **6.2 Design of Cross-Sections based on Material Efficiency**

*   **6.2.1 Principles:** Shape should place material **away from neutral axis** to maximize moment of inertia (I) for given area. Efficient for bending.

*   **6.2.2 Optimization for Precast:**

  *   **I-Section:** Most efficient for beams/long spans.

  *   **T-Section:** Efficient for slabs with flange in compression.

  *   **Box Section:** Excellent for torsion and high bending in both directions (e.g., columns, bridge girders).

  *   **Double-Tee:** Efficient for wide, shallow floor units.

#### **6.3 Factors Influencing Response of Precast Components**

*   **6.3.1 Connection Stiffness:** **Rigid connections** distribute moments; **pinned** do not. Drastically changes analysis model.

*   **6.3.2 Support Conditions:** Fixed, pinned, roller – define boundary conditions.

*   **6.3.3 Load Application Sequence:** **Erection sequence** and **time of casting** (creep/shrinkage) affect long-term stresses.

*   **6.3.4 Material Properties & Time-Dependent Effects:** **Creep and shrinkage** of concrete cause redistribution of stresses, especially in continuous systems. Must be considered in long-term serviceability.

---

### **7.0 PERFORMANCE, SAFETY & CODE PROVISIONS**

#### **7.1 Abnormal Effects and IS Code Provisions (IS 1893, IS 13920)**

*   **7.1.1 Definition:** Loads/events beyond normal service conditions: **Earthquake, Impact, Explosion, Fire**.

*   **7.1.2 Codal Requirements:**

  *   Design for **appropriate load combinations** (e.g., 1.2(DL+LL) ± 1.0(EQ) for seismic).

  *   Ensure **ductility** through proper detailing (confinement reinforcement, strong column-weak beam concept).

  *   Provide **redundancy** and **robustness** to prevent progressive collapse.

#### **7.2 Earthquake Loading**

*   **7.2.1 Intensity vs. Magnitude:**

  *   **Magnitude:** Energy released at source (Richter scale).

  *   **Intensity:** Effects at a location (MSK/MMI scale).

*   **7.2.2 Equivalent Static Load Method (IS 1893):**

  *   Base shear $$\displaystyle V_b = A_h \times W $$

      *   $$\displaystyle A_h = \frac{Z \cdot I \cdot S_a}{R \cdot g} $$ (horizontal seismic coefficient)

      *   $Z$ = Zone factor, $I$ = Importance factor, $$\displaystyle S_a $$ = Spectral acceleration, $R$ = Response reduction factor.

  *   Distribute $$\displaystyle V_b $$ to floors based on mass and height.

*   **7.2.3 Calculation of Equivalent Design Load:** Seismic force at each level $$\displaystyle F_i = \frac{W_i \cdot h_i}{\sum W_j \cdot h_j} \times V_b $$.

#### **7.3 Damping in Precast Structures**

*   **7.3.1 Concept:** Mechanism that **dissipates vibrational energy** (reduces amplitude).

*   **7.3.2 Sources:** Material damping (concrete, steel), friction at **connections**, non-structural elements (partitions, facades).

*   **7.3.3 Importance:** Higher damping → lower seismic forces & displacements. Precast structures may have **lower inherent damping** than monolithic due to joint slip, but can be designed for adequate damping.

#### **7.4 Degree of Progressivity**

*   **7.4.1 Definition:** Measure of the **extent to which a structure can develop plastic hinges and redistribute moments** without collapse. High degree = ductile, robust.

*   **7.4.2 Implications for Design & Construction:**

  *   Requires **strong connections** (stronger than connected elements).

  *   **Sequencing** of construction affects progressivity (e.g., ensuring stability at each stage).

  *   Influences **redundancy** and **load path** continuity.

---

### **8.0 COMPARATIVE STUDIES & SPECIAL TOPICS**

#### **8.1 Comparative Analysis: Shear Wall vs. Conventional Load-Bearing Brick Masonry Wall**

| **Aspect** | **Precast Shear Wall** | **Load-Bearing Brick Wall** |
| :--- | :--- | :--- |
| **Lateral Strength** | **High**, designed for seismic/wind. | **Very Low**, inadequate for high seismic zones. |
| **Ductility** | **Good** (with proper reinforcement). | **Poor** (brittle failure). |
| **Construction Speed** | **Fast** (panel erection). | **Slow** (masonry work). |
| **Quality Control** | **Excellent** (factory). | **Poor** (site, mortar quality, workmanship). |
| **Weight** | Can be lightweight (sandwich). | **Heavy**. |
| **Space Efficiency** | Thinner walls possible. | Thicker walls (space loss). |
| **Cost** | Higher initial, lower lifetime. | Lower initial, higher maintenance. |

#### **8.2 Special Prefabrication Types**

*   **8.2.1 Box Prefabricates (Volumetric Units):**

  *   **Features:** Complete 3D modules with **all finishes, fixtures, and services** installed in factory.

  *   **Advantages:** **Maximizes off-site work** (80-90%), fastest on-site assembly, highest quality control, minimal site disruption.

  *   **Disadvantages:** **High transport/logistics cost**, requires large crane, less design flexibility, complex MEP coordination.

  *   **Applications:** High-rise residential, hotels, student accommodations, hospitals.

*   **8.2.2 Large Panel Systems:**

  *   **Merits:** Very fast erection, good thermal performance (sandwich), excellent surface finish, reduced on-site labor.

  *   **Demerits:** Heavy elements require **heavy cranes**, complex **connections** (structural & services), high **transportation cost**, requires **precise foundation**, less adaptable to design changes.

#### **8.3 General Recommendations for Design of Expansion Joints (Summary)**

1.  **Location:** At points of stress concentration (corners, changes in height/length, between different structural systems).

2.  **Width:** Calculate based on **temperature range** and **coefficient of thermal expansion** (per IS 3414). Provide minimum 10-20mm.

3.  **Material:** Use **compressible, resilient fillers** (bituminous, foam) and **elastic sealants** (polysulfide, silicone) on exterior.

4.  **Detail:** Ensure **water tightness**, accommodate **movement in all 3 directions**, avoid **thermal bridging**.

5.  **Maintenance:** Design for **accessibility** for inspection and repair.