Prestressed Concrete Structures Lab (CE-704) - Unit 5 Short Notes

UNIT 5: Advanced Testing & Performance Evaluation of Prestressed Concrete Systems

5.1 Laboratory Preparation & Safety Protocols for Advanced Testing

• 5.1.1 Safety Hazards: High hydraulic pressures (risk of sudden release/rupture), high-strength material handling (strand snap-back), heavy specimen movement, and electrical systems for instrumentation.

• 5.1.2 Pre-test Planning: Define load schema (e.g., monotonic, cyclic, sustained). Layout instrumentation (strain gauges, LVDTs, dial gauges, load cells). Calibrate Data Acquisition System (DAS) and all sensors. Prepare test procedure and emergency stop protocol.

• 5.1.3 Specimen & Instrumentation: Handle specimens with care to avoid pre-existing damage. Install strain gauges (surface preparation, waterproofing), LVDTs (magnetic bases or brackets), and dial gauges (on independent reference frames). Protect wiring from mechanical damage.

[!TIP] Exam Focus: Always verify jack calibration certificate and DAS channel assignments before testing. A common error is miswired or uncalibrated strain gauges giving erroneous data.

5.2 Material Characterization for Prestressing Elements

• 5.2.1 Prestressing Strands/Wires:

  • Tensile Test (IS 1786 / ASTM A1061): Determine ultimate tensile strength ($$\displaystyle f_{pu} $$), 0.2% offset yield strength ($$\displaystyle f_{py} $$), modulus of elasticity ($$\displaystyle E_p $$), and elongation at failure.

  • Relaxation Test (IS 1786): Measure loss of stress under constant strain over time (typically 1000 hours). Critical for long-term loss prediction.

• 5.2.2 Anchorages & Couplers:

  • Static Load Test: Apply increasing load to failure. Record load vs. slip. Anchorage efficiency = (measured capacity / strand strength).

• 5.2.3 High-Strength Concrete:

  • Compressive Strength (IS 516): Test cylinders/cubes at transfer age and 28 days.

  • Modulus of Elasticity (IS 516): Load-unload cycle on cylinder to determine initial tangent/secant modulus.

  • Splitting Tensile Strength (IS 5816): Indirect tension test on cylinder.

5.3 Measurement and Analysis of Prestress Losses (Experimental Determination)

• 5.3.1 Direct Measurement:

  • Jack Pressure Gauge: $$\displaystyle F_{jack} = P_{gauge} \times A_{jack} $$. Measures force during stressing.

  • Load Cell/Dynamometer: Placed at anchorage for direct force measurement after seating.

• 5.3.2 Indirect Measurement:

  • Strand Strain: Use electrical strain gauges or DEMEC (demountable mechanical) points on exposed strands. $$\displaystyle \Delta f_p = E_p \times \Delta \varepsilon_p $$.

  • Concrete Strain: Strain gauges on concrete surface at critical section (e.g., midspan). Elastic shortening loss: $$\displaystyle \Delta f_{pES} = E_p \times \varepsilon_{c,transfer} $$.

• 5.3.3 Calculation of Total Losses:

  • Effective Prestress: $$\displaystyle f_{pe} = f_{pi} - \Delta f_{total} $$

  • Total Loss Components: $$\displaystyle \Delta f_{total} = \Delta f_{pF} + \Delta f_{pES} + \Delta f_{pCR} + \Delta f_{pR} + \Delta f_{pSR} $$ (Friction, Elastic Shortening, Creep, Relaxation, Shrinkage).

  • Compare initial jacking force ($$\displaystyle F_{pi} $$) to force measured at a later age ($$\displaystyle F_{pe} $$) via anchor load cell or strain recovery.

[!TIP] Critical Lab Skill: Elastic shortening loss is immediate after transfer. Measure concrete strain at the exact time of transfer for accurate $$\displaystyle \Delta f_{pES} $$. Friction loss ($$\displaystyle \Delta f_{pF} $$) is determined from pressure gauge readings during stressing (difference between jack end and far end).

5.4 Flexural Testing of Prestressed Concrete Beams

• 5.4.1 Test Setup: Simply supported beam, four-point bending (two equal point loads) or third-point loading for shear tests. LVDTs at midspan for deflection, dial gauges at supports for settlement.

• 5.4.2 Instrumentation:

  • Strain: Concrete surface gauges (top/bottom at midspan), strand strain gauges (if accessible).

  • Cracks: Visual observation, crack width microscope (e.g., 10x magnification with 0.02 mm scale).

• 5.4.3 Load-Deflection Behavior:

  • Stage 1 (Uncracked): Linear, high stiffness. $$\displaystyle M_{cr} = f_{r} \times Z $$ (modulus of rupture).

  • Stage 2 (Cracked): Stiffness drops after first crack. Load redistributes.

  • Stage 3 (Ultimate): Steel yields or concrete crushes. Ductile failure for bonded tendons.

  • Moment-Curvature: $$\displaystyle \phi = \frac{\varepsilon_c + \varepsilon_p}{d} $$. Plot $M$ vs. $\phi$ to find yield moment $$\displaystyle M_y $$ and ultimate moment $$\displaystyle M_u $$.

• 5.4.4 Crack Pattern & Width:

  • Cracks propagate vertically from bottom. Width $w \propto$ steel stress and concrete cover.

  • Compare max crack width with code limit (e.g., ACI 318: 0.3 mm for moderate exposure).

• 5.4.5 Continuous Beams: Observe negative moment region (top cracks) and positive moment region. Look for hinge formation and moment redistribution before failure.

[!TIP] Exam Formula: For a simply supported beam with two point loads at $L/3$ from each support, shear force at supports $$\displaystyle V = \frac{P}{2} $$, bending moment at load points $$\displaystyle M = \frac{P \cdot L}{6} $$.

5.5 Shear and Torsion Testing of Prestressed Concrete Members

• 5.5.1 Shear Without Stirrups:

  • Failure: Diagonal tension (shear) crack initiating from bottom.

  • Effect of Prestress: Increases shear capacity slightly by reducing principal tensile stress.

• 5.5.2 Shear With Stirrups:

  • Failure Modes:

    1. Shear-Compression: Diagonal crack followed by concrete crushing in compression zone.

    2. Shear-Tension: Stirrups yield, followed by diagonal tension failure.

• 5.5.3 Torsion Test (Hollow Sections Common):

  • Loading: Pure torsion (e.g., via lever arms) or combined bending-torsion.

  • Crack Pattern: Spiral cracks at ~45° on surfaces. For hollow sections, cracks appear on both inner and outer faces.

  • Ultimate Torsion: $$\displaystyle T_u $$ recorded. Compare with theoretical $$\displaystyle T_{u,calc} $$ based on strut-and-tie model.

[!TIP] Common Pitfall: In shear tests, ensure load points are close enough to supports to create a pure shear region (constant shear diagram). For torsion, warping effects can complicate analysis—hollow sections minimize this.

5.6 Testing of Prestressed Concrete Slabs and Grid Systems

• 5.6.1 One-Way Slab: Reinforced/PT in one direction. Test as simply supported strip. Observe bending cracks perpendicular to span.

• 5.6.2 Two-Way Slab: PT in two directions (often banded or distributed). Test on four supports. Observe cracking in both directions, load distribution to all supports.

• 5.6.3 Post-Tensioning Effect: Compared to RC slab, PT slab shows:

  • Higher load capacity.

  • Reduced deflections (camber).

  • Much finer crack pattern (crack-controlled).

[!TIP] Key Observation: In two-way PT slabs, the load distribution is more uniform. Failure often occurs at column supports (punching shear) or along middle strips.

5.7 Performance Under Service Loads & Long-Term Effects

• 5.7.1 Sustained Load Test:

  • Apply constant load (e.g., 0.6-0.8 $$\displaystyle P_u $$) for weeks/months.

  • Measure: Deflection vs. time (creep), strain in concrete/steel.

  • Creep Coefficient: $$\displaystyle \phi(t) = \frac{\varepsilon_{creep}(t)}{\varepsilon_{elastic}} $$.

• 5.7.2 Fatigue Test:

  • Repeated cyclic loading (e.g., 2-5 Hz) between low and high load levels.

  • Monitor: Stiffness degradation (slope of load-deflection), crack growth (length/width per cycle), eventual failure.

• 5.7.3 Camber & Deflection: Record initial camber (after transfer), deflection under service load, and long-term deflection increase.

[!TIP] Lab Report Must Include: Plot of deflection vs. log(time) for sustained load. Fatigue life is number of cycles to failure—often millions for PT members due to low stress range.

5.8 Failure Mode Investigation & Ultimate Capacity Validation

• 5.8.1 Controlled Failure: Increase load monotonically until collapse. Observe and document crack propagation.

• 5.8.2 Failure Modes:

  • Flexural: Steel yields → concrete crushes (ductile).

  • Shear: Diagonal tension or compression failure (brittle).

  • Bond/Anchorage: Strand slip or anchorage pull-out.

• 5.8.3 Theoretical vs. Experimental:

  • Calculate $$\displaystyle M_{u,calc} $$ using strain compatibility method (assume rectangular stress block for concrete, linear strain in steel).

  • Compare: $$\displaystyle M_{u,exp} $$ vs. $$\displaystyle M_{u,calc} $$. Good agreement validates theory.

• 5.8.4 Ductility & Energy Absorption:

  • Ductility Ratio: $$\displaystyle \mu = \frac{\delta_u}{\delta_y} $$ (deflection at ultimate / at yield).

  • Energy: Area under load-deflection curve up to failure.

[!TIP] Code Check: IS 1343 requires $\mu \geq 2.5$ for ductile flexural members. A lower $\mu$ indicates brittle failure (e.g., shear).

5.9 Non-Destructive Evaluation (NDE) Techniques in Lab Context

• 5.9.1 Ultrasonic Pulse Velocity (UPV):

  • Principle: $$\displaystyle V = \frac{L}{t} $$ (path length / transit time).

  • Use: Higher velocity indicates better concrete quality/compaction. Detect voids, cracks.

• 5.9.2 Rebound Hammer:

  • Principle: Impact rebound number correlates with surface hardness and compressive strength.

  • Use: Quick, relative strength estimate. Calibrate on known cubes.

• 5.9.3 Electromagnetic Covermeter:

  • Use: Locate reinforcement and prestressing ducts in existing structures. Measures cover depth.

• 5.9.4 Acoustic Emission (AE):

  • Principle: Listen for high-frequency "pops" from micro-cracking during loading.

  • Use: Real-time monitoring of damage progression before visible cracks.

5.10 Data Analysis, Reporting, and Code Correlation

• 5.10.1 Data Processing: Correct DAS zero drift, apply calibration factors. Smooth noisy data if needed (but preserve peaks).

• 5.10.2 Key Plots:

  • Load vs. Deflection (global behavior).

  • Load vs. Strain (concrete top/bottom, steel).

  • Moment vs. Curvature (section analysis).

  • Crack width vs. Load (serviceability).

• 5.10.3 Parameter Calculation:

  • Effective Prestress: $$\displaystyle f_{pe} = \frac{F_{anchor}}{A_p} $$.

  • Flexural Strength: $$\displaystyle M_u = \frac{P_u \times \text{lever arm}}{} $$.

  • Shear Strength: $$\displaystyle V_u = \frac{P_u}{2} $$ (for midspan point load).

  • Ductility: $$\displaystyle \mu = \frac{\delta_u}{\delta_y} $$.

• 5.10.4 Code Correlation: Compare results with IS 1343 (Indian Standard for Prestressed Concrete) or ACI 318 / Eurocode 2. Note discrepancies.

• 5.10.5 Error Sources:

  • Material variability (concrete strength, strand properties).

  • Friction & seating losses not fully captured.

  • Instrumentation lag or misalignment.

  • Boundary conditions (support settlement, friction).

• 5.10.6 Lab Report Structure: Objective, Theory, Methodology (specimen details, test setup), Results (tables, graphs), Discussion (compare theory/experiment/code, explain errors), Conclusion.

[!TIP] Golden Rule: In discussion, always state whether experimental capacity was higher or lower than theoretical/code prediction, and provide at least two plausible technical reasons (e.g., higher concrete strength than assumed, better bond than model).