1. Early Age Cracking: Causes, Measurements, and Mitigation
Dale P. Bentz
National Institute of Standards and Technology
NIST/ACBM Computer Modeling Workshop
August 13, 2009

2. 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

3. 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”

4. 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”

5. 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

6. 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

7. 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

8. 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)

9. Measurement Technologies
Early age heat generation and reaction rates
Isothermal calorimetry
Differential Scanning Calorimetry (DSC), TAMAir, Isocal, etc.
ASTM C 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 C 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

10. 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

11. 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

12. 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

13. 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 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), , 2004.)

14. Example of Chemical Shrinkage (CS)
Hydration of tricalcium silicate
C3S H  C1.7SH CH
Molar volumes 
CS = (150.8 – 166.9) / = mL/mL or
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 for 28 d hydration – 75 %)

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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), ,
2008.

21. 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

22. 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.

23. 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

24. 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

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

26. Restrained Ring Measurements

27. 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

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

29. 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)

30. 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, p
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

31. 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)

32. 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), , 2007.

33. 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

34. 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

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

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

37. 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),
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), , 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), , 2007.

38. 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

39. 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

40. 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

41. 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, , 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, , 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), , 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), , 2008.
Bentz, D.P., and Peltz, M.A., “Reducing Thermal and Autogenous Shrinkage Contributions to Early-Age Cracking,” ACI Materials Journal, 105 (4), , 2008.

42. 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)

43. 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).

44. Cement
paste
Water reservoir

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

46. 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

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

48. 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
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

49. 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

50. 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)

51. 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

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

53. 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:

54.
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

55. 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 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

57. 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 mm
middle hydration mm
late hydration mm or less
“worst case” mm (250 μm)
Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing

58. 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

59. 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), , 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.

60. 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)
National Concrete Pavement Technology Center (Iowa State University)
VCCTL (some aspects relevant to early-age)

61. 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.

62. 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!