Module 3 Page 1 of 23

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1 Module 3 Page 1 of 23 Internal Curing Module III: and Volume Change Presented by Jason Weiss, Purdue University, March 19 th, 2013 Module 3: and Slide 1 of 66 To understand how shrinkage relates to stress development and how stress development relates to cracking To understand the different types of shrinkage and their causes To understand why low w/cm are prone to cracking To understand how IC can reduce the potential for cracking To review data that compares the cracking behavior of plain and IC concrete Module 3: and Slide 2 of 66 Introduction frequently observed Transverse cracking in 100,000+ bridges 62% of DOT s consider cracking as a problem (NCHRP) Cracks shorten service life, increase maintenance cost, and accelerate corrosion Module 3: and Slide 3 of 66 Photo

2 Module 3 Page 2 of 23 Early Age Stress Development Concrete shrinks (reduces volume) due to moisture loss or temperature reduction Initial Specimen If this volume Effect reduction is not restrained, the Restraint Effect sample will shrink and get smaller If restrained, tensile stresses develop that can result in cracking Module 3: and Slide 4 of 66 Restraint in Concrete Many concrete elements are restrained Slabs on grade restrained by subgrade or items penetrating slabs Slabs on girders or beams restrained by connection to the beams or girders Bonded overlays or topping slabs are restrained by the substrate Patches are restrained by the material into which the patch is placed It is hard to remove restraint in practice Module 3: and Slide 5 of 66 Early Age Stress Development If stress is estimated using Hooke s law the stress can be relatively high (> f t) Module 3: and Slide 6 of 66

3 Module 3 Page 3 of 23 Early Age Stress Development Fortunately, concrete is an aging viscoelastic material, stresses relax and reduce Module 3: and Slide 7 of 66 Will the Concrete Crack? Stress that develops due to restraint can be compared with the age-dependent tensile strength (concern if stress > 70% strength) Module 3: and Slide 8 of 66 Early Age Stress Development The stress (after accounting for relaxation) can be compared with the tensile strength Module 3: and Slide 9 of 66

4 Module 3 Page 4 of 23 What Factors Influence DOR Stress Level Hydration Volume Elastic Change Modulus () Failure Criteria (Strength) Viscoelasticity Elastic Modulus Age Weiss et al Module 3: and Slide 10 of 66 Stress Level Common Attempts to Reduce Many people will suggest to increase the strength of the concrete to make the concrete more resistant to cracking Stress Time of Time of Drying Material Resistance i.e., Strength Weiss et al Module 3: and Slide 11 of 66 Common Attempts to Reduce While its true that the strength could increase, this may not be the entire story Stress Level Material Resistance i.e., Strength Time of Drying Weiss et al Module 3: and Slide 12 of 66

5 Module 3 Page 5 of 23 Stress Level Common Attempts to Reduce While it is true that the strength could be increased, we need to remember that the modulus increase and viscoelastic effects decrease Time of Time of Drying Material Resistance i.e., Strength Weiss et al Module 3: and Slide 13 of 66 Higher Strength? Increasing the strength frequently outweighed by reduced relaxation and higher stiffness Stress Level Time of Material Resistance i.e., Strength Time of Drying Weiss et al Module 3: and Slide 14 of 66 Stress Level What Factors Influence (A Quick Recap) DOR Hydration Volume Elastic Change Modulus () Failure Criteria (Strength) Viscoelasticity Elastic Modulus Age Weiss et al Module 3: and Slide 15 of 66

6 Module 3 Page 6 of 23 of Different Cement Based Materials - volumetric change associated with movement of water is a paste property This means that the volume of water & cement are important Measured Aggregate (Generally Drying Time Paste Concrete After L Hermite, 1956 Module 3: and Slide 16 of 66 Changes in It is commonly thought that a low w/c is the key to reducing shrinkage Module 3: and Slide 17 of 66 Changes in In low w/c it is critical to measure the shrinkage at early ages (especially in HPC) Module 3: and Slide 18 of 66

7 Module 3 Page 7 of 23 Why Does Increase in Lower w/c at Early Ages? This idea that shrinkage (at early age in low w/c concrete) could increase seems to be out of line with conventional wisdom However we can examine why this is the case To do so we will need to define two types of shrinkage: Chemical Module 3: and Slide 19 of 66 Fundamental Volume Change Le Chatelier ( ) Volume of reactants larger than volume of the products Chemical + = + = Module 3: and Slide 20 of 66 Chemical Module 3: and Slide 21 of 66

8 Module 3 Page 8 of 23 Chemical the volume reduction associated with the hydration reactions in cementitious materials Powers conceptual model shown ~ 6.4% reduction (after Powers) Module 3: and Slide 22 of 66 Chemical Test A standard was adopted by ASTM (C1698) Measure the water that is absorbed in the hydrating product Measure volume reduction with hydrating material Chemical Module 3: and Slide 23 of 66 Chemical Over Time Recall, CS is proportional to reacted cement Sant et al Module 3: and Slide 24 of 66

9 Module 3 Page 9 of 23 (Strain) Vapor Module 3: and Slide 25 of 66 Tests Standard shrinkage test for autogenous shrinkage length-to-diameter of approximately 400:30 mm fresh paste permits measurement 30 min after water added Several autogenous tests compared Reasonable agreement Sant et al Module 3: and Slide 26 of 66 Determining the Time of Structure Development Before setting, the system collapses as it shrinks and does not lead to stress development Difference between chemical & autogenous shrinkage signals solidification or setting Sant et al Module 3: and Slide 27 of 66

10 Module 3 Page 10 of 23 and Pore Size (RH) is driven by the menisci, whether external drying or autogenous CS worse in low w/c due to fewer big pores Radlinska et al Module 3: and Slide 28 of 66 Summary Definitions Drying the volume reduction associated with water loss (Change in RH) Chemical - volume reduction associated with hydration reactions in a cementitious material - volume reduction of a closed, isothermal, cementitious material system not subjected to external forces Module 3: and Slide 29 of 66 Internal Curing Module 3: and Slide 30 of 66

11 Module 3 Page 11 of 23 Internal Curing Supplying water throughout a freshly placed cementitious mixture using reservoirs, via prewetted lightweight aggregate, that readily release water as needed for hydration or to replace moisture lost through evaporation or self desiccation ACI 2011 Hiding water in LWA to increase hydration and strength while reducing transport, shrinkage, and cracking Module 3: and Slide 31 of 66 How Does Internal Curing Work? Conventional Concrete Porous lightweight aggregate is prewetted before mixing Water moves from the pores in LWA to the paste when it is needed This movement is due to fact that smaller pores want to remain water filled Conceptual Model of Pores In Concrete Internally Cured Concrete Module 3: and Slide 32 of 66 LWA Radlinska et al Chemical & Vapor Spaces How much vapor space would be created, estimated from chemical shrinkage Vapor space develops after set Henkensiefken et al. (2004) Module 3: and Slide 33 of 66

12 Module 3 Page 12 of 23 Desire for Pores to be Water Filled Sealed - vapor-filled spaces will develop Access to water - If water is available (water curing) water will be drawn in (pulled) Water Sealed Vapor Module 3: and Slide 34 of 66 Comments on Space Happens every day in concrete This is the reabsorption of bleed water that occurs at the time of set Module 3: and Slide 35 of 66 Concept LWA Supplies Water Water stays in LWA until the time that this capillary pressure develops In a sealed system the pressure results in vapor-filled space When there is prewetted LWA, water is drawn out of bigger LWA pores to reduce pressure and keeps paste wet Module 3: and Slide 36 of 66

13 Module 3 Page 13 of 23 Chemical (IC Water Volume) Typically, amount of water used for internal curing is based on replacing chemical shrinkage (can adjust for drying, other, etc.) Henkensiefken et al. (2004) Module 3: and Slide 37 of 66 Details of the LWA for IC Recall Module 2 discusses the determination of the mass of LWA using the water from the previous slide The LWA also needs to be able to desorb ( give water up ) easily The LWA also needs to be well spaced in the mixture Castro (2011) Lura (2003) Module 3: and Slide 38 of 66 Measured autogenous shrinkage for a concrete The plain concrete mixture shrinks however the IC mixture expands and does not shrink Barrett (2013) Module 3: and Slide 39 of 66

14 Module 3 Page 14 of 23 As LWA replacement volume increases, autogenous shrinkage decreases 25.3% accounts for the CS volume Henkensiefken et al. (2008) Module 3: and Slide 40 of 66 Unrestrained in Sealed Conditions As LWA replacement volume increases, autogenous shrinkage decreases 25.3% accounts for the CS volume Henkensiefken et al Module 3: and Slide 41 of 66 RH Measured with Internal Curing Can relate the size of the meniscus to a separate RH measure Henkensiefken et al. (2008) Module 3: and Slide 42 of 66

15 Module 3 Page 15 of 23 Restrained Weiss and Furgeson 2001 Hossain and Weiss, 2002 Module 3: and Slide 43 of 66 Restrained (Sealed Samples) Strain (stress) development in sealed plain systems (blue), cracking at 6 days IC system nearly zero stress development Henkensiefken et al. (2008) Module 3: and Slide 44 of 66 Restrained (Sealed Samples) The age of cracking is plotted as a function of LWA addition As the volume approaches the CS replacement (25%) no cracking is observed No Is Seen For Higher Volumes Henkensiefken et al. (2008) Module 3: and Slide 45 of 66

16 Module 3 Page 16 of 23 A Comment on Sealed vs Unsealed Behavioral Differences Sealed different size menisci will result as the volume (CS) of water loss is similar Drying Eventually the same size menisci (r 3 ) is formed Radlinska et al Module 3: and Slide 46 of 66 Restrained (Unsealed Samples) When the sample is unsealed we have autogenous and drying effects More severe, higher stress, earlier cracking Henkensiefken et al. (2008) Module 3: and Slide 47 of 66 Restrained (Unsealed Samples) As before we see that increasing the LWA volume decreases the potential for cracking Unsealed samples require a higher volume No Is Seen For Higher Volumes Henkensiefken et al. (2008) Module 3: and Slide 48 of 66

17 Module 3 Page 17 of 23 Thermal Effects Dual ring test used to capture expansion and contraction, Invar rings (low COTE) Coil system used to regulate sample temperature from 60 C to -10 C Schlitter et al Module 3: and Slide 49 of 66 Temperature Required to Crack Internally cured systems are more robust Schlitter et al Module 3: and Slide 50 of 66 Robustness at Early Ages Repeat temperature drop at different ages Schlitter et al Module 3: and Slide 51 of 66

18 Module 3 Page 18 of 23 Robustness at Early Ages IC has greater load or temp. capacity Schlitter et al Module 3: and Slide 52 of 66 Tendency of LWC Auburn University Studied 3 LWA (shale, clay, slate) summer (95 o ) fall (73 o ) Control IC, fine LWA Sand LWC: coarse LWA ALWC: all fine and coarse LWA Byard (2010) Module 3: and Slide 53 of 66 Tendency of LWC Auburn University Studied 3 LWA (shale, clay, slate) summer (95 o ), fall (73 o ) Note IC is not designed to fully replace chemical shrinkage water, designed for 135 lb/ft 3 Byard (2010) Module 3: and Slide 54 of 66

19 Module 3 Page 19 of 23 Tendency of LWC Auburn University 73F, Slate Byard (2010) Module 3: and Slide 55 of 66 Tendency of LWC Auburn University 73F, Slate 73F, Clay Plain cracking <2 days IC delays cracking (recall not CS repl.) Coarse LWA better ALWC lower stress 73F, Shale Byard (2010) Module 3: and Slide 56 of 66 Bloomington Indiana Decks At 18 months Plain decks - three large cracks IC decks no cracking observed Module 3: and Slide 57 of 66

20 Module 3 Page 20 of 23 Utah Bridge Decks Summer of 2012 UDOT constructed two bridge decks with two plain Reported at the 2013 TRB annual meeting 23x less crack length with internal curing (cracks at cold joint) Research by Guthrie et al at BYU Module 3: and Slide 58 of 66 Occurs in first hours Causes: Settlement Evaporation Module 3: and Slide 59 of 66 Bleeding and Evaporation Initial Surface Bleeding Evaporation Bleeding Water exuded from plastic concrete as particles settle due to gravity Evaporation Water lost from the surface If bleed rate exceeds evaporation no cracking will occur Module 3: and Slide 60 of 66

21 Module 3 Page 21 of 23 Benefits of Internal Curing Henkensiefken et al Module 3: and Slide 61 of 66 with IC As LWA volume Henkensiefken et al increases the crack width and number of cracks decrease For environmental conditions used, plastic shrinkage was controlled when the IC was at the CS volume however depends drying Water used for plastic shrinkage is not available for other benefits Module 3: and Slide 62 of 66 Review of Main Topics Concrete is susceptible to cracking when it is restrained from shrinking The cracking potential increases with lower w/c higher strength concrete Chemical shrinkage occurs as cement hydrates, can result in autogenous shr. Internal curing is simply using LWA as a reservoir to hide water, reduce self-desc. Internal curing reduces shrinkage and shrinkage cracking Module 3: and Slide 63 of 66

22 Module 3 Page 22 of 23 Conclusions - 1, elastic modulus, visco-elasticity (relaxation), strength (fracture properties), rate, all influence cracking Chemical shrinkage due to chemical reaction shrinkage internal vapor space created (self-desc.) by CS Drying shrinkage water lost to environment As w/c decreases: autogenous shrinkage increases, stiffness increases (more stress), relaxation decreases (more stress) Module 3: and Slide 64 of 66 Conclusions - 2 Internally cured mixtures hide water in the LWA, this water is released after setting to increase the pore saturation If the pores are more full, the meniscus in the pore is larger & resulting shrinkage is smaller IC concrete is less susceptible to restrained shrinkage cracking than plain concrete shrinkage cracking can be reduced but stored water is used and not available Module 3: and Slide 65 of 66 Acknowledgements/Disclaimer These slides were developed as a part of a series for the Expanded Shale, Clay and Slate Institute by Jason Weiss. These materials are provided as general information and do not constitute legal or other professional advice. Any use of this information in the design or selection of materials for practice should be approved by the project owner and engineer-of-record. Module 3: and Slide 66 of 66

23 Module 3 Page 23 of 23 Module 3: and Slide 67 of 66 Formation of Vapor Filled Space Development of vapor filled space occurs rapidly after set Module 3: and Slide 68 of 66 External Water Supply Versus Sealed If there is access to water the water will be drawin in Couch et al Module 3: and Slide 69 of 66