ICRI 2017 Spring Convention March 15-17, 2017 Evaluating Shrinkage Cracking Potential of Concrete Using Ring Test Bruce G. Menu, PhD candidate, Laval University, Canada Marc Jolin, professor, Laval University, Canada Benoît Bissonnette, professor, Laval University, Canada Laurent Molez, associate professor, INSA, Rennes, France
Overview Early-age shrinkage cracking Shotcrete AASHTO PP34-99 Ring Test Restrained shrinkage analysis Stress development and stress rate Creep evaluation Results and discussions Free Ring Test AASHTO PP34-99 Ring Test Summary
Objectives General Objectives Predict the likelihood of restrained shrinkage cracking occurrence in repair materials Specific Objectives 1.Study the influence of curing on shrinkage cracking 2.Understand the relationship between volumetric compatibility and cracking potential (using ring test ) 3. Find a correlation between stress rate and the time-to-cracking 4.Quantify tensile creep behavior of concrete using the ring tests Ultimate goal => crack free repair material
What is shotcrete Shotcrete A technology for placing concrete Mortar or concrete sprayed on a surface under pneumatic pressure There are two ways to do this: a wet mix and a dry mix process «Water line» «Shooting» «Substrate» «Water ring» «Nozzle» «rebond» «In-Place concrete» McCormick Dam and Power Station, QC, CA KING Shotcrete Solution
Basic principles Limit paste content Aggregates usually are volume stable, does NOT shrink Use of supplementary cementitious materials Silica fume Fly Ash (reduced water demand) Proper use of admixtures In search of crack free concrete High range Water reducers (HRWR) Shrinkage reducing admixtures (SRA) direct shrinkage reduction - up to 40 % Expansive admixtures (EA) shrinkage compensation Fibers Reduction of early age cracks No direct influence on the shrinkage itself Use wet curing and protect the surface of the concrete Delays drying Extremely important
The problem with all concretes: Cracking Why does concrete crack?
Time-dependent volume changes Volume change starts soon after cement and water come in contact All possible types of volume change are interrelated (very complex problem) Early Age Volume Change Thermal Shrinkage creep External Influences Heat of hydration Autogenous shrinkage External drying shrinkage Basic creep Drying creep self-desiccation Cement hydration Chemical shrinkage results from change in moisture or loss of water by evaporation
Volumetric behavior of concrete exposed to drying Drying shrinkage Drying shrinkage cracking Evaporation of moisture H 2 O «restraint to volume change concrete» Cracking H.R < 100 % «moisture loss» «drying shrinkage» pore Capillary Force Water evaporates Bond with substrate do not allow repair material to shrink
Why curing is important Prevent early surface desiccation Provide the right conditions for cement hydration Minimize shrinkage and shrinkage cracking Drying shrinkage cracking No Drying = No Drying Shrinkage = No Drying Shrinkage Cracking
Stress Level When will shrinkage cracking occur? Tensile Strength Induced Stress due to Restraint Tensile creep Age at cracking without creep Age at cracking with creep Relaxation of stress due to creep T 0 = start of drying Time of Drying
Quantifying Shrinkage Cracking How do we capture the combined effect of shrinkage and tensile creep simultaneously in a scientifically sound test? Shrinkage Stress rate Tensile creep (relaxation) = actual cracking time Answer : The RING TEST Girard, 2013
Shrinkage cracking characterization Ring test Measures the resistance of concrete to cracking under restrained shrinkage Restrained ring Free ring Strain Gauge Steel Restraining Ring Polystyrene Insert Ring Modulus Tensile Strength Cracking Potential Tensile Creep Shrinkage Standardized Test AASHTO PP-34-99 Non-Standardized Test Alternative for ASTM C 157 Cracking Index: CI σ(t) residual stress tensile strength ' f t
The concept of restrained ring test T=23 ± 4 C @ 50 ± 4% RH Shrinkage cracking characterization
Shrinkage cracking characterization The restrained ring test experimental procedure Crack
Shrinkage cracking characterization Unrestrained ring test (New experimental set-up) Ø381 mm DEMEC points 76 mm Template for positioning of the DEMEC H=152 mm DEmountable MEChanical strain gauge
Shrinkage cracking characterization The restrained and free ring specimen exposed to drying Restrained ring specimen Free ring specimen (Circumferential drying) (Top & Bottom drying)
Stress development in the concrete ring stress distribution in the ring R ic R oc σ c P s + P c σ c σ θθ (r) = 1 r 2 R os 2 r 2 +R oc 2 R oc 2 R os 2 P s
Stress development in the concrete ring Average tensile stress analysis: Equilibrium of forces : σ b force in steel F -F s c force in concrete Stress rate S t = dσ c_avg t dt = К dε s dt Adapted from See et al. [1] f c f s a b f s f c F c = 2*f c F s = 2*f s steel average residual stress residual stress σ s_avg t = 1 2 E b + a s t ε s (t) b σ c_avg t = 1 2 σ c_avg t = K ε s (t) σ ) s _ avg A. s σ c _ avg ( t A. c area of steel area of concrete Steel elastic modulus A s b + a E A s t ε s (t) c b К= 31.55 GPa where dε s Τdt is the net strain rate at time t ε s as function of time is a regression fit given as: ε s t = α t + c The stress rate after drying is given by : S t = К α 2 t α is the slope of the line t is time to cracking
Net steel strain [µ-strain] Shrinkage cracking characterization Example of time-to-cracking versus square root of time 0-24 -48-72 1-96 -120 0 0.8 1.6 2.4 3.2 4 Square root of drying time [days] α is the slope of the line and is known as the strain rate
+ Principle of superposition of strains: Quantifying tensile creep behavior σavg _ c( t) K Concrete strain components: where: εelastic ε s( t) E E c c ε ε total Strain creep - t 0 Elastic strain Elastic + Creep strain ε ε ε elastic steel deformation Creep strain shrinkage ε - ε - ε steel elastic ε creep ε elastic ε total ε total = ε steel ε shrinkage Time free shrinkage creep shrinkage creep coefficient 1 ε Ec. K shrinkage ε steel elastic modulus Tensile strain capacity elastic t) creep εcreep ε steel ε shrinkage 1 ε ε elastic ε ε elastic elastic ( ε 1 ( t) -1-1
Shrinkage cracking characterization Characteristics of the concrete mixture studied (at 28-days) w/c=0.45 w/c=0.60 Compressive strength (MPa) 34.5 33.1 Tensile strength (MPa) 3.22 2.74 Elastic modulus (GPa) 30.0 28.8 Poisson s ratio 0.18 0.13 Mix ID Cement Coarse aggregate Fine aggregate Water w/c (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) S1 445 736 1054 197 0.45 S2 417 689 988 247 0.60
Free shrinkage strain [µm/m] Free shrinkage strain [µm/m] Free ring shrinkage (circumferential drying) Results and discussion 0-160 w/c=0.45 0-160 w/c=0.60-320 -320-480 -480-640 3-days curing (w/c=0.45) 7-days curing (w/c=0.45) -800 0 14 28 42 56 70 Time after initiation of drying [days] -640 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) -800 0 14 28 42 56 70 Time after initiation of drying [days]
Steel ring strain [µ-strain] Before demolding Steel ring strain [µm/m] Before demolding Strain recorded during curing of the ring specimen Results and discussion 15 0 (Curing) 0.45 w/c (Drying) 20 10 0 (Curing) 0.60 w/c (Drying) -15-10 -30-45 (24hrs) C drying T&B drying T=23 o C; RH=50% -60 0 1 2 3 4 5 6 7 Time after placement [days] -20-30 (24hrs) C drying T&B drying T=23 o C; RH=50% -40 0 1 2 3 4 5 6 7 Time after placement [days] w/c = 0.45 w/c = 0.60
Free shrinkage strain [µm/m] Results and discussion Restrained ring shrinkage (circumferential drying) 0-20 -40-60 -80 3-days curing (w/c=0.45) 7-days curing (w/c=0.45) w/c=0.45 Free shrinkage strain [µm/m] 0-20 -40-60 -80 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) w/c=0.60-100 0 3 6 9 12 15 Time after initiation of drying [days] -100 0 3 6 9 12 15 Time after initiation of drying [days]
Stress - strength ratio Results and discussion Stress development and stress-to-strength ration (cracking index) 3 1.0 Average tensile stress [MPa] 2.4 1.8 1.2 3-days curing (w/c=0.45) 0.6 7-days curing (w/c=0.45) 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) 0 0 3 6 9 12 15 Time after initiation of drying [days] 0.8 0.6 0.4 0.2 0.0 0 9 18 27 36 45 Time-to-cracking
Time to cracking [days] Results and discussion Stress rate in the ring specimen 45 R 2 =0.9898 36 y = 1.1107x^-1.11099 27 Low Experimental Power fit 18 9 Moderate-low Moderate-high 0 High 0.00 0.08 0.16 0.24 0.32 0.40 Stress rate [MPa/day]
Tensile Creep Coefficient Results and discussion Evaluation of tensile creep parameters from ring tests (up to time of cracking) 1.2 1.0 0.8 0.6 0.4 0.2 3-days curing (w/c=0.45) 7-days curing (w/c=0.45) 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) 0.0 0 2.4 4.8 7.2 9.6 12 Time after initiation of drying [days]
Results and discussion Evaluation of tensile creep parameters from ring tests (up to time of cracking) Specific Tensile Creep [µ-strain/mpa] 40 32 24 16 3-days curing (w/c=0.45) 8 7-days curing (w/c=0.45) 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) 0 0 2.4 4.8 7.2 9.6 12 Time after initiation of drying [days]
Tensile creep strain/shrinkage strain Stress relaxation [%] Results and discussion Evaluation of tensile creep parameters from ring tests (up to time of cracking) 0.4 40% 0.3 32% 0.2 24% 0.2 16% 3-days curing (w/c=0.45) 0.1 7-days curing (w/c=0.45) 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) 0.0 0 2.4 4.8 7.2 9.6 12 Time afer initiation of drying [days] 8% 0% 3-days curing (w/c=0.45) 7-days curing (w/c=0.45) 3-days curing (w/c=0.60) 7-days curing (w/c=0.60) 0 2.4 4.8 7.2 9.6 12 Time afer initiation of drying [days]
In Summary The restrained ring test facilitates the prediction of cracking resistance and is suitable for evaluating the tensile creep behavior of concrete exposed to drying Cracking resistance of concrete under restrained shrinkage depends on the combined properties of shrinkage (stress rate), tensile creep, and tensile strength The cracking potential of concrete can be evaluated based on either the timeto-cracking or the rate of stress development The lower the stress rate, the longer the cracking time under restrained shrinkage A simple procedure is developed for a quantifying creep behavior using free shrinkage, restrained shrinkage and modulus 1 k E c. ε shrinkage ε steel -1-1
In Summary Restrained stress to strength ratio can be used to define limits for stress development (or cracking index) to minimize shrinkage cracking This limit may be in the range of 50-60%, but further research is needed for a definitive conclusion Prolonged moist curing has the potential of delaying cracking of a concrete under restrained shrinkage Additional evaluations are ongoing to evaluate the influence of other parameters such as drying condition, surface-to-volume ratio and curing on cracking Free ring test ASTM C157
References 1. AASHTO, PP-34-99. 1999. " Standard Practice for Estimating the Cracking Tendency of Concrete, American Association of State Highway and Transportation Officials, Washington, DC 2. See, H. T., Attiogbe, E. K., & Miltenberger, M. A. (2003). Shrinkage cracking characteristics of concrete using ring specimens. ACI Materials Journal, 100(3) 3. Attiogbe, Emmanuel K, WJ Weiss, and Heather T See. 2004. "A look at the stress rate versus time of cracking relationship observed in the restrained ring test. a Rilem International Conference 4. See, Heather T, Emmanuel K Attiogbe, and Matthew A Miltenberger. 2004. "Potential for restrained shrinkage cracking of concrete and mortar." Cement, concrete and aggregates 26 (2):1-8 5. Benoit, B., Le fluage en traction: un aspect important de la problématique des réparations minces en béton, in Thèse de doctorat. 1996, Département de génie civil, Université Laval, Canada. p. 279 pages.
Acknowlegements
Commentaires? Questions?
Steel strain [µm/m] Strain recorded for the first 24 hours after casting the ring specimen Curing 5 0-5 -10-15 T=23 o C; RH=100% -20 0 4 8 12 16 20 24 Time after placement [hours]