UNIT 4: PRESTRESSED CONCRETE LABORATORY
4.1 Laboratory Fundamentals & Safety Protocols
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4.1.1 Introduction: Lab focuses on fabrication, testing, and evaluation of prestressed concrete elements to understand real-world behavior vs. theory.
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4.1.2 Safety Procedures & Hazard Identification - CRITICAL
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High-Stress Systems: Hydraulic jacks (over-pressurization risk), anchors (sudden release), and tensioning strands (snap-back hazard). Always verify jack calibration and secure anchorage before stressing.
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Materials: Cement dust (respiratory), grout chemicals (skin/eye), and tendon handling (sharp edges). Use gloves, masks, and eye protection.
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Testing Machines: Ensure proper specimen alignment on UTM/beam supports to prevent eccentric loading or sudden failure.
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PPE & Emergency: Mandatory safety shoes, helmets, and gloves. Know location of emergency stop switches and first-aid.
[!TIP] Exam Focus: Common viva question: "What are the top 3 safety hazards during post-tensioning?" Answer: 1) Strand snap-back, 2) Jack/anchorage failure, 3) Grout chemical exposure.
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4.1.3 Calibration: All instruments (pressure gauges, load cells, LVDTs) must have valid calibration certificates. Record calibration factors for data correction.
4.2 Material Characterization for Prestressed Concrete
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4.2.1 High-Strength Concrete
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Target compressive strength $$\displaystyle f'_{c} > 40-50 $$ MPa. Low water-cement ratio, silica fume, superplasticizers.
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Experiment: Compressive Strength Test on 150mm cubes/cylinders (cured 28 days). Load applied axially at $0.25$ N/mm²/sec. Stress-strain curve obtained.
\boxed{f_c = \frac{P}{A}} where $P$ = max load, $A$ = cross-sectional area.
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4.2.2 Prestressing Tendons & Strands
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Properties: High-tensile steel (wires, strands, bars). Key curve: Linear up to yield strength $$\displaystyle f_{py} $$, then strain hardening to ultimate strength $$\displaystyle f_{pu} $$. Modulus $$\displaystyle E_p \approx 195 $$ GPa.
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Experiment 1: Tensile Test
- Determine $$\displaystyle f_{pu} $$, $$\displaystyle f_{py} $$ (0.2% offset), $$\displaystyle E_p $$ (slope of initial linear portion), and elongation % at failure.
\boxed{E_p = \frac{\Delta f}{\Delta \varepsilon}} (from initial linear part of load-elongation curve).
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Experiment 2: Relaxation Test
- Strand held at constant length (initial stress ~70-80% of $$\displaystyle f_{pu} $$). Measure loss of stress (%) over time (typically 1000 hours). Governs time-dependent losses.
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4.2.3 Anchorage Systems & Couplers
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Types: Wedge-type (common for pre-tensioning), screw-type, grouted couplers (for post-tensioning).
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Experiment: Grip Efficiency Test. Apply load to strand through anchorage. Measure slip at specified load (e.g., 0.8 $$\displaystyle f_{pu} $$). Slip must be within code limits (e.g., < 1mm).
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4.3 Prestressing Operations & Loss Measurement
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4.3.1 Methods:
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Pre-tensioning: Tendons tensioned before concrete casting. Transferred via bond after concrete hardens.
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Post-tensioning: Tendons placed in ducts, tensioned after concrete hardens. Ducts later grouted (bonded) or left unbonded.
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4.3.2 Jacking Procedures & Force Measurement
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Hydraulic Jack: Force $$\displaystyle P = \frac{\pi}{4} D^2 \times p \times \eta $$. $D$ = jack piston diameter, $p$ = pressure (gauge reading), $\eta$ = efficiency (from calibration).
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Experiment: Jack Calibration using a proving ring or load cell. Apply known loads, record gauge pressure, plot $P$ vs. $p$ to find $\eta$.
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4.3.3 Measurement of Prestress Losses
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Immediate Losses: Occur during/just after stressing.
- Elastic Shortening ($$\displaystyle \Delta f_{pES} $$): Concrete shortens under prestress, reducing tendon stress. Measured by comparing strand elongation before/after transfer (pre-tensioned).
\boxed{\Delta f_{pES} = \frac{E_p}{E_c} \cdot \Delta f_{cp}} where $$\displaystyle \Delta f_{cp} $$ = concrete stress at c.g.s. of tendons.
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Friction ($$\displaystyle \Delta f_{pF} $$): In post-tensioning, due to duct curvature and wobble. Measured by "jacking-end vs. far-end pressure" during stressing.
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Wedge Draw-in ($$\displaystyle \Delta f_{pA} $$): Anchor slip. Measured by "set-back" in strand length after anchor seating.
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Time-Dependent Losses: Measured by long-term monitoring of pressure at anchor or strain on tendon.
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Creep & Shrinkage: Concrete volume change under sustained load.
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Relaxation: Tendon stress decay under constant strain.
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Experiment: Monitoring Force Over Time. Install pressure gauge at live end of post-tensioned beam. Record pressure at intervals (1h, 24h, 7d, 28d). Calculate force loss.
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4.4 Structural Testing of Prestressed Concrete Members
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4.4.1 Flexural Testing (Beam Tests) - CORE EXPERIMENT
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Specimen Fabrication:
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Pre-tensioned: Tendons stressed on abutments, concrete cast, cured, strands cut (transfer).
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Post-tensioned: Concrete cast with ducts, cured, tendons stressed via jacks, ducts grouted.
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Test Setup: Simply supported beam, two-point loading (to create constant moment region). Supports allow rotation.
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Instrumentation:
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Strain Gauges: On concrete top (compression) and bottom (tension) at mid-span. On tendons (if accessible).
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Deflection: LVDTs at mid-span (main) and supports (to correct for support settlement).
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Loading & Data Recording: Incremental loading (e.g., 20% of predicted ultimate load). Record load, mid-span deflection, mid-span strains at each stage. Note cracking load (first visible crack).
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Analysis & Graphs:
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Load vs. Deflection Curve: Initial linear (uncracked), post-cracking (stiffness drops), yield plateau (if non-prestressed steel yields), peak (ultimate), post-peak drop.
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Moment vs. Curvature: Curvature $$\displaystyle \phi = \frac{\varepsilon_c + \varepsilon_t}{h} $$ (approx). Moment $$\displaystyle M = P \times a $$ (for two-point load). Slope = flexural rigidity $EI$.
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Key Strengths:
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Cracking Moment ($$\displaystyle M_{cr} $$): From first crack. Compare with $$\displaystyle M_{cr} = \frac{f_r I_g}{y_t} $$ (gross section).
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Ultimate Moment ($$\displaystyle M_u $$): Max load. Compare with code (e.g., ACI 318, IS 1343):
\boxed{M_u = A_{ps} f_{ps} \left( d - \frac{a}{2} \right)} where $$\displaystyle a = \frac{A_{ps} f_{ps}}{0.85 f'_c b} $$.
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Failure Mode: Ductile (tendon yields, large deflection) vs. Brittle (concrete crushes, sudden). Prestressing increases ductility.
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[!TIP] Exam Focus: Be able to sketch the Load-Deflection curve for a prestressed beam, labeling uncracked, cracked, yield, and ultimate phases. Explain why initial stiffness is high.
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4.4.2 Shear/Torsion & 4.4.3 Axial Tests: Follow similar instrumentation principles. Shear tests observe diagonal tension cracks. Axial tests show increased load capacity due to prestress compression.
4.5 Non-Destructive Evaluation (NDE) & Advanced Techniques
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4.5.1 Locating Tendons & Grout Quality
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Pachometer/Covermeter: Electromagnetic tool to locate tendons and measure concrete cover.
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Ultrasonic Pulse Velocity (UPV): Low velocity indicates voids/grout defects in ducts. Test along tendon path.
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4.5.2 Strain & Stress Monitoring
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Vibrating Wire Strain Gauges (VWSG): Embedded in concrete or attached to tendon. Frequency changes with strain. Good for long-term monitoring.
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Fiber Bragg Grating (FBG): Optical sensor. Wavelength shift $\Delta \lambda$ proportional to strain. Allows distributed sensing along a single fiber.
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4.6 Data Analysis, Reporting & Interpretation
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4.6.1 Stress Calculation: From measured strain $\varepsilon$ using modulus.
\boxed{\sigma_c = E_c \cdot \varepsilon_c} (concrete), \boxed{\sigma_p = E_p \cdot \varepsilon_p} (tendon).
- Note: For concrete in compression, use parabolic stress block for ultimate state.
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4.6.2 Graph Interpretation:
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Load-Deflection: Slope = $1/(EI)$. Change in slope at $$\displaystyle M_{cr} $$.
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Moment-Curvature: Initial slope = $$\displaystyle E_c I_g $$. Post-cracking slope decreases.
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4.6.3 Calculating Experimental Losses:
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Elastic Shortening Loss (Pre-tensioned): $$\displaystyle \Delta f_{pES} = f_{pi} - f_{pe} $$ at transfer. $$\displaystyle f_{pi} $$ = initial jacking stress, $$\displaystyle f_{pe} $$ = stress in tendon immediately after transfer (measured by strain gauge or calculated from elongation).
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Total Loss: $$\displaystyle \Delta f_{pT} = f_{pi} - f_{pe} $$ (at service) from long-term monitoring.
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4.6.4 Comparison with Theory: Tabulate experimental $$\displaystyle M_{cr} $$, $$\displaystyle M_u $$ vs. calculated values. Compute % error. Discuss reasons (material property variations, friction not accounted, etc.).
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4.6.5 Lab Report Structure:
- Objective, Theory, Apparatus, Procedure, Data (Tables), Results (Graphs), Discussion (error analysis, behavior), Conclusion.
[!TIP] Common Pitfall: Forgetting to correct deflection for support settlement. Always subtract support LVDT reading from mid-span reading.
4.7 Modern & Special Topics (Time Permitting)
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4.7.1 External Prestressing: Tendons outside concrete section, deviators to change direction. Test setup similar but with anchorages at ends and deviators. Lower bond loss, easier inspection.
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4.7.2 CFRP Tendons: Linear stress-strain up to failure (no yield point). Lower $$\displaystyle E_p $$ (~150 GPa) than steel. Higher ultimate strain (~1.5-2%). Handling: no kinking, avoid sharp bends.
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4.7.3 Partial Prestressing: Combines prestressed and non-prestressed (reinforcing) steel. Allows controlled cracking under service loads. Observe wider, fewer cracks vs. fully prestressed.
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4.7.4 Prestressed Slabs: Test slab strip (one-way) or panel (two-way). Loading may be hydraulic jacks at points. Observe punching shear or flexural failure. Prestress reduces deflections and cracks.