Early Age Cracking: Causes, Measurements, and Mitigation Dale P. Bentz (dale.bentz@nist.gov) National Institute of Standards and Technology NIST/ACBM Computer Modeling Workshop August 13, 2009

Early-Age Performance of Concrete Criticality of early-age performance for long term durability Cracking at early ages Thermal considerations Autogenous deformation considerations Measurement technologies Effect of cement fineness Coarse cement – 311 m2/kg Fine cement – about 380 m2/kg Mitigation strategies

Concrete Goals Structural and non-structural members that withstand their physical and environmental loads throughout their intended service life Strength considerations (design-based) Durability Diffusivity, permeability, sorptivity Freeze-thaw performance, corrosion, sulfate attack, ASR Want to assure that HPC stands for “High Performance Concrete” and not “High Probability for Cracking”

Example of Importance of Early-Age Cracking 2003 FHWA Nationwide HPC Survey Results --- Most Common Concrete Distresses 1) Early-age deck cracking (56.6 % of responses were a 4 or 5=often) 2) Corrosion (42.3 % ---- definitely linked to cracking) 3) Cracking of girders, etc. (31.4 %) Others (sulfate attack, ASR, F/T, overload, poor construction quality were all below 25 % level) 2005 NRC/Canada report stated that “over 100,000 bridges in the U.S. have developed transverse cracking of their deck shortly after construction”

Importance of Early Age All of the long term performance properties of a concrete are dictated by its proportioning, placement, and curing If the concrete isn’t designed, prepared, transported, placed, finished, and cured properly, it will likely not perform as intended It is during this early age, that concrete undergoes a remarkable transition from a viscous suspension to a rigid load-bearing durable solid

Cracking during Transition Thermal cracking Heat released during hydration causes temperature rise (50 oC or more) and expansion of concrete - reports of water boiling inside concrete in California Shrinkage will occur during subsequent cooling If heating/cooling is too rapid and/or concrete is restrained (externally or internally), cracking may occur Heat generation Cooling

Cracking during Transition Autogenous deformation Volume occupied by cement hydration reaction products is significantly less (10 % or more) than that of starting materials After setting (in a sealed system), this chemical shrinkage will be directly translated into a measurable autogenous deformation Once again, if concrete is restrained (externally or internally), cracking may occur Analogous to drying shrinkage, but drying is internal Autogenous shrinkage

What about high performance concretes (HPCs)? Thermal cracking HPCs usually contain more cementitious binder and perhaps silica fume, both of which accelerate and increase heat release rate and may exacerbate thermal cracking Autogenous deformation cracking Finer pore structure of HPC and greater self-desiccation both greatly increase autogenous deformation (and cracking)

Measurement Technologies Early age heat generation and reaction rates Isothermal calorimetry Differential Scanning Calorimetry (DSC), TAMAir, Isocal, etc. ASTM C1679-07 Standard Practice for Measuring Hydration Kinetics of Hydraulic Cementitious Mixtures Using Isothermal Calorimetry Semi-adiabatic calorimetry Adiacal, coffee cup, Q-drum, etc. If you have a thermocouple, you can build your own!!! Being standardized in ASTM C09.48/C01.48 Chemical shrinkage ASTM C1608-05 Standard Test Method for Chemical Shrinkage of Hydraulic Cement Paste Loss on ignition Difficult at early ages due to hydration continuing during specimen preparation, resolution issues

Measurement Technologies Isothermal calorimetry assesses heat production at constant temperature conditions Results can be expressed as heat flow or integrated to get total heat release vs. time Generally limited to cement pastes or mortars Isocal for concrete Experiments at three different temperatures (10, 25, and 40 °C) can be used to estimate activation energy (needed for application of maturity method) w/c=0.35 cement pastes

Measurement Technologies Semi-adiabatic calorimetry assesses temperature rise with controlled (or at least measured) heat loss Results expressed as temperature vs. time Can be used on pastes, mortars, or concretes Can be used in both the field and in the laboratory Mixture design Quality control w/c=0.35 cement pastes

Some Limitations on Calorimetry Isothermal calorimetry from NIST Calorimetry is being advocated by some as a means of assessing the setting time of cement-based materials Basic problem with this is that calorimetry assesses the extent of hydration of the material while setting is a physical phenomenon (e.g., different extents of hydration are required to achieve setting depending on mixture proportions, etc.) …let’s consider w/c Setting data from NIST

Measurement Technologies Chemical shrinkage assesses the imbibition of external water into a hydrating cement paste due to the fact that the hydration products occupy less volume than the reactants Standardized in 2005 as ASTM C1608 by subcommittee C01.31 Burrows has advocated that the 12-h chemical shrinkage be less than or equal to 0.0105 mL/g cement for a crack resistant cement (Burrows, et al., Three Simple Tests for Selecting Low-Crack Cement, Cement and Concrete Composites, 26 (5), 509-519, 2004.)

Example of Chemical Shrinkage (CS) Hydration of tricalcium silicate C3S + 5.3 H  C1.7SH4 + 1.3 CH Molar volumes 71.1 + 95.8  107.8 + 43 CS = (150.8 – 166.9) / 166.9 = -0.096 mL/mL or -0.0704 mL/g cement For each lb (g) of tricalcium silicate that reacts completely, we need to supply 0.07 lb (g) of extra curing water to maintain saturated conditions (In 1935, Powers measured a value of 0.053 for 28 d hydration – 75 %)

Measurement Technologies Early age deformations and cracking Corrugated tubes (with digital dilatometers) for mortar and paste autogenous deformation Concrete cylinder or prism molds for concrete autogenous deformation Cracking frames Ring test for restrained shrinkage and early age cracking Acoustic emission measurements

Measurement Technologies Custom-built digital dilatometers (Developed by Prof. O.M. Jensen – Technical University of Denmark) Specimens sealed in corrugated polymeric tubes and stored at constant temperature Method nearly approved by ASTM C09.68 AutoShrink equipment now available from Germann Instruments

Measurement Technologies for Concrete For autogenous deformation of concrete, can use larger corrugated tubes, sealed prisms (next slide) or…. Use plastic concrete cylinder molds with a simple dial gauge ---- (like conventional creep experiments) Feasibility demonstrated by Craeye and De Schutter in Belgium and Brooks et al. in U.K. (references available upon request) NRMCA, Dec. 2007 Temperature control is critical

Measurement Technologies for Concrete Linear measurement of autogenous shrinkage (Japan 1999) Similar setup being used by Virginia Transportation Research Council and by one or more concrete producers in U.S. (specimen wrapped in plastic film) Possibilities for confounding: Temperature variation

Measurement Technologies for Concrete Linear variants Schleibinger Shrinkage Drain 1 m long channel 60 mm by 38 mm (mortar) 60 mm by 100 mm (concrete) Neoprene sheets used to avoid wall friction Double-walled channel with heating and cooling also available Accuracy of about 2 μm Need to seal sample Used by group of Hansen et al. at University of Michigan

Measurement Technologies for Concrete Linear variants Modification to reduce friction Use a V-channel 285 mm by 40 mm by 45 mm One fixed and one moveable end LVDT with 0.15 μm accuracy Sealed with plastic film Newlands, M.D., Paine, K.A., Vemuri, N.A., and Dhir, R.K., “A Linear Test Method for Determining Early-Age Shrinkage of Concrete,” Magazine of Concrete Research, 60 (10), 747-757, 2008.

Measurement Technologies for Concrete Modification to further reduce friction Spin the V-channel Schleibinger Shrinkage Cone Uses a laser to measure movement 0.3 μm resolution Temperature-controlled

Measurement Technologies for Concrete Larger corrugated tubes Concrete dilatometer Presently, neither standardized nor commercially available Place in a temperature- controlled bath to minimize temperature variation? Under evaluation in Denmark, Sweden, Purdue, etc.

Measurement Technologies for Concrete Corrugated PVC tube immersed in temperature-controlled bath at LCPC in France Shape of corrugations and type of tube (impermeable, stable) can be important considerations In a very high performance concrete (VHPC), can still measure a temperature rise of 2 ºC during first 24 h of curing http://www.argenco.ulg.ac.be/etudiants/Multiphysics/StaquetS-Methodes_experimentales_part1.pdf

Measurement Technologies Large scale frame-based machines for monitoring early-age performance Rigid Cracking Frame developed at Technical University of Munich (based on Mangold 1998) Temperature-stress testing machine (Springenschmid 1980, 1985) Being employed in the U.S. by the group of Schlinder et al. at Auburn University

Measurement Technologies Restrained ring shrinkage and cracking test (ASTM C 1581 test method) Any questions: Contact Prof. Jason Weiss at Purdue Univ.

Restrained Ring Measurements

Measurement Technologies Acoustic emission can be used to indicate microcracking and cavitation (self-desiccation) under sealed curing Group at Purdue University (Prof. Weiss again) is also very active in this field

Mitigation Strategies Thermal cracking Use a low heat release cement (ASTM Type IV or invoke optional heat of hydration requirement on ASTM Type II – 290 J/g cement at 7 d) Difficult to find in U.S. currently U.S. cement finenesses constantly increasing since 1950

Mitigation Strategies Thermal cracking Reduce heat release rate by replacing cement with slowly reacting fly ash or slag may increase autogenous shrinkage Chilled aggregates or ice additions as part of mixing water Use cooling pipes in large concrete pours ($$$$) Liquid nitrogen cooling Use phase change materials (PCMs) that “absorb” hydration heat during their phase change from solid to liquid (melting)

Mitigation Strategies Mihashi, H, Nishiyama, N, Kobayashi, T, Hanada, M. Development of a Smart Material to Mitigate Thermal Stress in Early Age Concrete. In: Control of Cracking in Early Age Concrete, 2002. p. 385-392. Use a hydration retarder encapsulated in a (paraffin) wax phase change material During first hydration temperature rise, wax melts (absorbing energy during its phase change) and releases hydration retarder

Mitigation Strategies Paraffin Wax as an example of a phase change material Wax melts at around 50 °C ΔH of about 150 J/g For a typical concrete with 400 kg/m3 of cement Maximum calculated temperature rise of 85 °C With an addition of 350 kg/m3 of wax, maximum calculated temperature rise is reduced to 63 °C Wax can be added in “powder” form to concrete or can be “embedded” in lightweight aggregates (LWA)

Mitigation Strategies Semi-adiabatic results on mortars with paraffin wax additions; the maximum temperature rise is decreased by about 8 °C and shifted to 1 h later; temperature decrease post peak is more gradual; could be extended to a blend of PCMs Bentz, D.P., and Turpin, R., “Potential Applications of Phase Change Materials in Concrete Technology,” Cem Concr Comp, 29 (7), 527-532, 2007.

Mitigation Strategies Autogenous deformation and cracking Magnitude of autogenous stresses are controlled by Kelvin-Laplace equation Two variables that end user can exploit to their advantage are: Decreasing surface tension (γ) of pore solution Increasing the size of pores (rpore) being emptied during the self-desiccation that accompanies the ongoing chemical shrinkage Shrinkage reducing admixtures (SRA) Increased Cement Particle Spacing Internal Curing

Autogenous Deformation - SRAs SRAs can reduce surface tension of pore solution by 50 % Autogenous stresses should also be 50 % of level without SRA Thus, SRAs may reduce autogenous shrinkage as well as drying shrinkage SRAs also modify pore solution viscosity, and influence evaporative drying rates and plastic shrinkage cracking in a positive manner

Autogenous Deformation --- SRA w/cm=0.35 mortar, sealed curing, 30 oC

Drying with and without SRAs No SRA – Faster drying SRA – Slower drying

Some SRA References Bentz, D.P., Geiker, M.R., and Hansen, K.K., “Shrinkage-Reducing Admixtures and Early Age Desiccation in Cement Pastes and Mortars,” Cement and Concrete Research, 31 (7), 1075-1085 2001. Bentz, D.P., “Curing with Shrinkage-Reducing Admixtures: Beyond Drying Shrinkage Reduction,” Concrete International, 27 (10), 55-60, 2005. Bentz, D.P., “Influence of Shrinkage-Reducing Admixtures on Early-Age Properties of Cement Pastes,” Journal of Advanced Concrete Technology, 4 (3), 423-429, 2006. Lura, P., Pease, B. Mazzotta, G. Rajabipour, F., and Weiss, J. “Influence of Shrinkage-Reducing Admixtures on the Development of Plastic Shrinkage Cracks,” ACI Materials Journal, 104 (2), 187-194, 2007.

Concrete Solutions Coarser Cements w/c = 0.35 pastes Blaine fineness: Reduce temperature rise and decrease rate of temperature decrease during semi-adiabatic curing due to reduced reactivity (lower surface area) Reduce autogenous shrinkage due to increased interparticle spacing Strength reduction: 25% at 28 d w/c = 0.35 pastes Blaine fineness: Coarse – 310 m2/kg Fine – 380 m2/kg w/c = 0.35 mortars

Concrete Solutions Increased w/c Pastes Mortars Reduces temperature rise and decrease rate of temperature decrease during semi-adiabatic curing due to reduced cement content and increased heat capacity Reduces autogenous shrinkage due to increased interparticle spacing Strength reduction: 7% at 28 d Pastes Mortars

Concrete Solutions Limestone (coarse) replacements for cement Pastes Reduce temperature rise and decrease rate of temperature decrease during semi-adiabatic curing due to reduced cement content Reduce autogenous shrinkage due to increased interparticle spacing (D50 of 50 to 100 μm) Strength reduction: 7 % at 28 d Pastes Mortars

Some Cement Fineness References Bentz, D.P., and Haecker, C.J., “An Argument for Using Coarse Cements in High Performance Concretes,” Cement and Concrete Research, 29, 615-618, 1999. Bentz, D.P., Garboczi, E.J., Haecker, C.J., Jensen, O.M., “Effects of Cement Particle Size Distribution on Performance Properties of Cement-Based Materials,” Cement and Concrete Research, 29, 1663-1671, 1999. Bentz, D.P., Jensen, O.M., Hansen, K.K., Oleson, J.F., Stang, H., and Haecker, C.J., “Influence of Cement Particle Size Distribution on Early Age Autogenous Strains and Stresses in Cement-Based Materials,” Journal of the American Ceramic Society, 84 (1), 129-135, 2001. Bentz, D.P., Sant, G., and Weiss, W.J., “Early-Age Properties of Cement-Based Materials: I. Influence of Cement Fineness,” ASCE Journal of Materials in Civil Engineering, 20 (7), 502-508, 2008. Bentz, D.P., and Peltz, M.A., “Reducing Thermal and Autogenous Shrinkage Contributions to Early-Age Cracking,” ACI Materials Journal, 105 (4), 414-420, 2008.

Autogenous Deformation - IC In internal curing (IC), internal reservoirs of extra curing water are supplied within the 3-D concrete microstructure Saturated surface-dry (SSD) lightweight aggregates (LWA) Superabsorbent polymers (SAP) Saturated wood fibers Crushed returned concrete aggregate Significantly reduces autogenous shrinkage at early and later ages Reduces plastic and drying shrinkage as well May also enhance hydration and strength in the longer term (7 d and beyond)

Question: Why do we need IC? Answer: Particularly in HPC, it is not easily possible to provide curing water from the top surface (for example) at the rate that is required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities that are often achieved in the concrete as the capillary pores depercolate. Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959).

Cement paste Water reservoir

Autogenous Deformation Results w/cm=0.35 mortar, sealed curing, 30 oC Control

Autogenous Deformation Results CCA = crushed (returned) concrete aggregates Mortars with slag (20 %) blended cement IC added via fine LWA/CCA to increase total “w/c” from 0.30 to 0.38 (0.36) Note – chemical shrinkage of slag hydraulic reactions is ~0.18 g water/g slag or about 2.6 times that of cement

Degree of Hydration and Strength w/cm = 0.35 mortars, sealed curing w/cm = 0.3 HPM with silica fume blended cement

Three-Dimensional X-ray Microtomography X-ray microtomography allows direct observation of the 3-D microstructure of cement-based materials Example: Visible Cement Data Set http://visiblecement.nist.gov In October 2005, experiments were conducted at Pennsylvania State University to monitor three-dimensional water movement during internal curing of a high-performance mortar over the course of two days

After mixing 1 d hydration 2 d hydration All images are 13 mm by 13 mm Aqua indicates drying Red indicates wetting Subtraction: 1 d – after mixing

Three-Dimensional X-ray Microtomography Three-dimensional subtracted image of 1 d hydration – initial microstructure showing water-filled pores that have emptied during internal curing (4.6 mm on a side) 2-D image with water evacuated regions (pores) overlaid on original microstructure (4.6 mm by 4.6 mm)

Three-Dimensional X-ray Microtomography Empty porosity within LWA from analysis of 3-D microtomography data sets scales “exactly” with measured chemical shrinkage of the cement for first 36 h of curing

SEM Observations IC Control w/cm=0.30, 8 % silica fume Courtesy of P. Stutzman w/cm=0.30, 20 % slag

Questions to Consider When Using IC How much water (or LWA) do I need to supply for internal curing? How far can the water travel from the surfaces of the internal reservoirs? How are the internal reservoirs distributed within the 3-D concrete microstructure? Answers May be found at the NIST internal curing web site: http://concrete.nist.gov/lwagg.html

http://concrete.nist.gov/lwagg.html Menu for Internal Curing with Lightweight Aggregates 1)Calculate Lightweight Aggregates Needed for Internal Curing 2)Estimation of Travel Distance of Internal Curing Water 3)Simulate Mixture Proportions to View Water Availability Distribution 4)View Water Availability Distribution Simulation Results 5)Learn more about FLAIR: Fine Lightweight Aggregates as Internal Reservoirs for the autogenous distribution of chemical admixtures 6)View presentation on internal curing made at 2006 Mid-Atlantic Region Quality Assurance Workshop Link to Workshop homepage 7)Internal Curing Bibliography 8)Direct Observation of Water Movement during Internal Curing Using X-ray Microtomography

Question: How much water (or LWA) do I Question: How much water (or LWA) do I need to supply for internal curing? Answer: Equation for mixture proportioning (Menu selection #1) MLWA =mass of (dry) LWA needed per unit volume of concrete Cf =cement factor (content) for concrete mixture CS =(measured via ASTM C 1608-05 or computed) chemical shrinkage of cement αmax =maximum expected degree of hydration of cement, [(w/c)/0.36] or 1 S =degree of saturation of LWA (0 to 1] when added to mixture øLWA = (measured) absorption of lightweight aggregate (use desorption measured at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498–04a) Nomograph in SI or English units downloadable at IC web site

Question: How far can the water travel from the surfaces of the LWA? Answer: Equation balancing water needed (hydration) vs. water available (flow) (Menu selection #2) “Reasonable” estimates --- early hydration ---- 20 mm middle hydration --- 5 mm late hydration --- 1 mm or less “worst case” --- 0.25 mm (250 μm) Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing

Question: How are the internal reservoirs Question: How are the internal reservoirs distributed within the 3-D concrete microstructure? 30 mm by 30 mm Answer: Simulation using NIST Hard Core/Soft Shell (HCSS) Computer Model (Menu selections #3 and #4) Returns a table of “protected paste fraction” as a function of distance from LWA surface Yellow – Saturated LWA Red – Normal weight sand Blues – Pastes within various distances of an LWA

Some Internal Curing References Bentz, D.P., Lura, P., and Roberts, J., “Mixture Proportioning for Internal Curing,” Concrete International, 27 (2), 35-40, 2005. Bentz, D.P., Halleck, P., Grader, A., and Roberts, J.W., “Water Movement during Internal Curing: Direct Observation Using X-ray Microtomography,” Concrete International, 28 (10), 39-45, 2006. Bentz, D.P., “Internal Curing of High Performance Blended Cement Mortars: Autogenous Deformation and Compressive Strength Development,” ACI Materials Journal, 104 (4), 408-414, 2007. Kim, H., and Bentz, D.P., "Internal Curing with Crushed Returned Concrete Aggregates," in NRMCA Technology Forum: Focus on Sustainable Development, 2008. Henkensiefken, R., Castro, J., Kim, H., Bentz, D., and Weiss, J., “Internal Curing Improves Concrete Performance throughout Its Life,” to appear in Concrete Infocus, summer 2009.

What about Modeling Numerous modeling efforts are in progress for early-age predictions for concrete structures HIPERPAV II (Transtec and FHWA) FEMMASSE (The Netherlands) DuCoM (Japan) www.texasconcreteworks.com National Concrete Pavement Technology Center (Iowa State University) VCCTL (some aspects relevant to early-age)

Virtual Heat of Hydration Test Based on CEMYHD3D v3.0 hydration model Use a 12 h chemical shrinkage test to calibrate kinetics factor for a specific cement Predict ASTM C186 7 d and 28 d heats of hydration (entire heat of hydration vs. time curve) Ref: Bentz, D.P., “Verification, Validation, and Variability of Virtual Standards,” Proceedings of the 12th International Congress on the Chemistry of Cement, Montreal, 2007.

Summary Early age performance is controlled by materials, production, and exposure environment which makes it triply hard to ensure Many advances made in measurement (and modeling) technologies Mitigation strategies are available and are actually being employed in practice! Of the mitigation strategies presented here, only the utilization of coarser cement systems (coarser grind cement, increased w/c, or coarse limestone additions) has the potential to reduce both thermal and autogenous deformation contributions to early-age cracking, and with the added benefit of a cost reduction!