1 Health Monitoring of Early Age Concrete Surendra P. Shah Northwestern University, Illinois, USA Keynote CONSEC 04, Seoul, June 30, 2004.
2 Scope of Research Test method for in-situ testing of early age concrete Requirements versatile in application nondestructive in nature measure physical properties access to one side of the structure Life Cycle of Concrete green early age young mature concrete 4-6 hours 5 days 28 days
3 Outline 1. Basics of Wave Propagation 2. Principle of Wave Reflection Method 3. General Application Setting Behavior Compressive Strength Dynamic Shear Modulus Measures of Cement Hydration Microstructural Parameters 4. Field Application 5. Conclusions
4 Wave Propagation
5 Longitudinal Waves (L-Waves) also: Primary (P-) Waves, Compression Waves Excitation Wave Velocity: v L = Direction of Travel E( 1 ν) ( + ν)( 1 2ν) ρ 1 Direction of Particle Motion Governing Parameters Young s Modulus E Poisson s Ratio v Density ρ
6 Transverse Waves (T-Waves) also: Secondary (S-) Waves, Shear Waves Excitation Direction of Travel Wave Velocity: v T = G ρ Direction of Particle Motion Governing Parameters Shear Modulus G Density ρ no propagation in liquids or gases!
7 Wave Reflection at Boundaries if P- or S-wave encounters an interface between two materials the wave is partially reflected partially transmitted material 1 material 2 (steel) (mortar) reflection coefficient r = Z Z mortar mortar + Z Z steel steel acoustic impedance reflected incident ρ s, v s transmitted ρ m, v m Z = ρ v governs the reflection process
8 Principle of WR-Method
9 Principle of Wave Reflection Method shear wave transducer (2.25 MHz) steel plate fresh concrete Case 1: 1: concrete is is liquid no no wave transmission at at interface shear waves: do not propagate in liquids
10 Principle of Wave Reflection Method shear wave transducer (2.25 MHz) steel plate fresh Concrete hardened concrete Case 2: 2: concrete is is hardening transmission losses at at interface
11 Signal Analysis 1 Main Bang 0.8 1. Reflexion 0.6 R 1 2. Reflexion R 0.4 2 0.2 0 0 5 10 15 20-0.2 Zeit (µs) FFT 1 0.8 0.6 0.4 F 1 1. Reflexion F2 2. Reflexion -0.4 0.2-0.6-0.8 Time Domain 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Frequency Domain Reflection Coefficient r F 2 (f) F 1 (f) = L r F 1, F 2. FFT of reflections at 2.25 MHz L. Losses (material, coupling, geometry) Reflection Loss R L (db) = - 20 log (r)
12 Attenuation Development Attenuation Time Phase 1 Phase 1: immediately after casting Concrete is liquid no transmission attenuation is zero
13 Attenuation Development Attenuation Point A Time Phase 1 Phase 2 Phase 2: accelerating hydration Concrete hardens rapidly transmission attenuation increases
14 Attenuation Development Point B Attenuation Point A Time Phase 1 Phase 2 Phase 3 Phase 3: decelerating hydration Concrete hardens constant transmission attenuation levels off
15 Test Equipment t = 12 mm Laptop Computer Transducers Pulser/Receiver Steel Plates Concrete
16 Reflection Loss vs. Setting
17 Influence of Admixtures 0.8 Concrete Reflection Loss (db) 0.6 0.4 0.2 Accelerator Plain Retarder 0 0 5 10 15 20 25 Time (hours) Shifting of Point A
18 Influence of Admixtures Initial setting time (hours) 10 8 6 4 2 0 Silica Accelerator Superplasticizer Plain Retarder 2 3 4 5 6 Time of point A (hours)
19 Influence of w/c-ratio Cement Mortar w/c = 0.35, 0.5, 0.6 0.8 w/c = 0.35 w/c = 0.5 Reflection Loss (db) 0.6 0.4 0.2 R L = 0.12 ± 0.02 w/c = 0.6 0.0 0 3 6 9 12 Time (Hours) initial setting
20 Summary Setting WR-Method can measure influence of admixtures w/c-ratio temperature (not shown) on the setting behavior of cement-based materials
21 Reflection Loss vs. Viscoelastic Properties
22 Background WR-method can be used to measure rheological parameters of fresh cement paste Parameters of interest Shear Modulus G Storage modulus G Energy storage Loss modulus G Energy dissipation Viscosity η Comparative Experiments Rheometer WR-Method Cement Paste w/c = 0.4, 0.5, 0.6 Time range 0 4 hours
23 Rheometric Measurements for comparison Rheometer coaxial cylinder barrel with paste
24 Storage Shear Modulus Normalized G' 1 0.8 0.6 0.4 0.2 Portland Cement Pastes Temperature = 25 C w/c=0.4 w/c=0.5 w/c=0.6 G' from Rheometric method G' from WR method 0 0 1 2 3 4 Hydration Age (hours) normalized values show similar trends
25 Viscosity Viscosity (Pa.s) 0.35 0.3 0.25 0.2 0.15 0.1 0.05 SR-Method WR-Method Portland Cement Paste Temperature=25 C Hydration Age=15mins 0 0.4 0.5 0.6 water/cement-ratio 1.4 Good correlations of the viscosity Viscosity (Pa s) (WR-method) 1.2 1 0.8 0.6 0.4 0.2 Portland Cement Pastes Temperature=25 w/c=0.4 w/c=0.5 w/c=0.6 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Viscosity (Pa s) (SR-method)
26 Summary Visco-elastic Parameters WR-Method can reproduce storage modulus (measure of elasticity) viscosity during the setting of cement-based materials
27 Reflection Loss vs. Strength
28 191 128 64 Strength Test on Extruded Cylinders Ram-Extruder Strength Test Piston Barrel Ram Mortar Die 32 mm 64 mm Cut Cylinder
29 Reflection Loss vs. Strength Compressive Strength (MPa) 30 25 20 15 10 5 0 Extruded Cement Mortar 0 6 12 18 24 30 36 42 48 54 Power Law f(t) = a t b Reflection Loss () t Time (hours) f Hyperbolic Law (Knudson 80, Carino 84) = F u k 1+ Strength ( t t0 ) k( t t ) 0 64 mm 32 mm 3 2.5 2 1.5 1 0.5 0 Reflection Loss (db)
30 2.9 h 6.5 h 7.6 h 8.6 h 10.6 h 15.1 h 19.1 h 12 1.6 Compressive Strength (MPa) 10 8 6 4 2 Strength Reflection Loss 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Reflection Loss (db) 0 0 3 6 9 12 15 18 Time (hours) 0.0
31 R L vs. Strength Compressive Strength (MPa) 50 40 30 20 10 0 Cement Mortars different w/c-ratios R 2 = 0.92 Transition Point between 6 15 hours R 2 = 0.97 w/c = 0.6 w/c = 0.35 w/c = 0.5 0 1 2 3 4 Reflection Loss (db)
32 Summary Compressive Strength Strength and reflection loss follow similar trends Reflection loss is (bi-) linearly related to compressive strength at early ages. Relationship is independent of w/c-ratio (for mortar) of temperature (results not shown)
33 Reflection Loss vs. Shear Modulus
34 Determination of Shear Modulus Reflection Loss local measurement R L = f (Shear Modulus) G r Shear Wave Velocity measurement along wave path v s = f (Shear Modulus) G vs Torsional Resonant Frequency bulk measurement f tor = f (Shear Modulus) G tor
35 Dynamic Shear Modulus Dynamic Shear Modulus (GPa) 12 9 6 3 S-wave w/c = 0.5 (mortar) resonance WR resonance (paste) Dynamic Shear Modulus (GPa) 12 9 6 3 w/c = 0.6 (mortar) resonance S-wave resonance (paste) WR 0 0 12 24 36 48 60 72 Time (h) 0 0 12 24 36 48 60 72 Time (h) Reflection Loss is governed by cement paste properties
36 Summary Shear Modulus Reflection loss is governed by dynamic shear modulus, measures shear modulus of cement paste portion of mortar
37 Reflection Loss vs. Direct Measures of Hydration
38 Reflection Loss (db) 5 4 3 2 1 Reflection Loss vs. Degree of Hydration w/c = 0.35 reflection loss degree of hydration 0.6 0.4 0.2 0 0.0 1 10 100 Time (Hours) 1000 Direct comparison linear relationship Degree of Hydration (-) Reflection Loss (db) 4 3 2 1 Degree of hydration measured by TGA Comparison in time w/c = 0.35, 0.5, 0.6 0.35 0.5 0.6 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Degree of Hydration (-)
39 Reflection Loss vs. Microstructure
40 Reflection Loss vs. Capillary Porosity Reflection Loss (R L, db) 3.5 3 2.5 2 1.5 1 0.5 cement paste w/c=0.35 w/c=0.5 w/c=0.6 R 2 = 0.9888 0 0 0.1 0.2 0.3 0.4 Decrease of Capillary Porosity (Pc 0 -Pc) Reflection loss uniquely related to decrease in porosity
41 Gel-Space Ratio vs. Reflection Loss 4 3.5 R L is closely related to microstructural changes in cement paste Reflection Loss (db) 3 2.5 2 1.5 1 X = 0.68 α 0.32 α + w c 0.5 0 R L = 7.48 X 1.63 R 2 = 0.96 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Gel-Space Ratio (-)
42 Numerical Simulation HYMOSTRUC3D Connectivity of Solid Phase initial stage no contacts hydration step x clusters hydration step y closed path Results: percolation threshold total amount of solids connected solid
43 Solid Phase vs. Reflection Loss 0.8 0.7 total solid connected solid 3 Fraction of Solid Phase (-) 0.6 0.5 0.4 0.3 0.2 0.1 Reflection Loss Cement Mortar w/c = 0.5 2.5 2 1.5 1 0.5 Reflection Loss (db) 0 0 24 48 72 96 0 Percolation Threshold Time (hours)
44 Contact Area Reflection loss uniquely related to contact area connected particles exposed contact area
45 Summary Microstructure Reflection loss is closely related to microstructural changes. Unique relationships to: decrease of capillary porosity gel-space ratio contact area
46 Field Application
47 Test Object Prestressed Box Girder Production Process placement of reinforcement and Styrofoam core Styrofoam core pouring concrete 4 ft ~15 h curing removing girder from steel bed prestress cables 6 ft 130 ft Need for Quality Control When has concrete reached the critical strength for removing girder?
48 Pulser/ Receiver Temperature Logger Main Power Switch Laptop Computer Transducer Central Power Supply
49 On-Site Measurements Steel Plates
50 Result of Field Test 50 Strength (MPa) 40 30 20 strength measured by wave reflection method NDT-method strength determined in plant (cylinder test at 16h) quality control precast plant strength determined in ACBM lab (precalibration) calibration ACBM-lab 10 0 0 12 24 36 48 Equivalent Age ( C h)
51 Options for Vertical Structures e.g. walls, columns non-steel formwork transducer steel plate steel formwork
52 Summary Field Application WR-method can be used for field testing during production process. in-situ strength can be assessed equipment can be made portable advance laboratory testing necessary
53 Final Conclusions Wave Reflection Method can nondestructively monitor: setting behavior viscoelastic properties compressive strength dynamic shear modulus progress of cement hydration microstructural changes in-situ strength of concrete structures
54 Vision Task Expert System Early Age Concrete Properties decision support on construction site