5 cm 4. 6 mm on a side Dale P. Bentz dale. bentz@nist. gov
Question: What is internal curing (IC)? Answer: As defined by ACI in 2010, IC is “supplying water throughout a freshly placed cementitious mixture using reservoirs, via prewetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation. ” For many years, we have cured concrete from the outside in; internal curing is for curing concrete from the inside out. Internal water is generally supplied via internal reservoirs, such as pre -wetted lightweight aggregates (LWA), superabsorbent polymers (SAPs- baby diapers), saturated wood fibers, or pre-wetted crushed (returned) concrete aggregates (CCA).
Question: Why do we need IC? Answer: In practice, IC is being used mainly to reduce early-age cracking by maintaining a high relative humidity within the hydrating cement paste! This can be particularly important in lower w/cm (≤ 0. 4) concretes when capillary pores depercolate within just a few days. If your concrete isn’t cracking at early ages, you may not need internal curing (may help with curling/warping). Capillary pore percolation/depercolation first noted by Powers, Copeland Mann (PCA-1959).
Question: How does IC work? Answer: IC distributes the extra curing water (uniformly) throughout the entire 3 -D concrete microstructure so that it is more readily available to maintain saturation of the cement paste during hydration, avoiding self-desiccation (in the paste) and thereby reducing autogenous shrinkage. Because the generated capillary stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste and should also be well connected. Internal curing is not a substitute for external curing. At a minimum, evaporative moisture loss (after set) must be prevented using conventional external measures (misting, fogging, curing membrane or compound).
Cement paste Water reservoir Larger “sacrificial” pores within the reservoirs to minimize stress/strain
Question: What are the documented benefits that IC can provide? Answers: - Reduced autogenous deformation and less early -age cracking • Early-age deck cracking identified as #1 distress in 2003 FHWA Nationwide High Performance Concrete Survey Results - Maintenance of a higher internal RH, reduced plastic shrinkage (cracking) and settlement, enhanced (long term) hydration and strength development, reductions in creep, improved interfacial transition zone (ITZ) microstructure, reduced transport coefficients, increased sulfate attack resistance
A Brief (57 year) Timeline • 1957 - Paul Klieger writes “Lightweight aggregates absorb considerable water during mixing which apparently can transfer to the paste during hydration. ” in Klieger, P. , Early High Strength Concrete for Prestressing, Proceedings World Conference on Prestressed Concrete, San Francisco, CA, July 1957, A 5 -1 to A 5 -14. • 1991 - Bob Philleo writes “. . a way must be found to get curing water into the interior of high-strength structural members…. A partial replacement of fine aggregate with saturated lightweight fines might offer a promising solution. ” • Mid 1990 s – Research groups such as Weber and Reinhardt in Germany and Bentur et al. in Israel begin to actively investigate internal curing • 1999 – NIST enters the arena with the publication of Bentz, D. P. , and Snyder, K. A. , Protected Paste Volume in Concrete: Extension to Internal Curing Using Saturated Lightweight Fine Aggregate , Cement and Concrete Research, 29 (11), 1863 -1867, 1999. • 2000 – In Denmark, Jensen and Hansen conceive and demonstrate the idea of using superabsorbent polymers (SAPs) as internal curing agents • 2005 – TXI places 238, 000 yd 3 of concrete with internal curing (mid-range LWA) in a commercial paving project (railway transit yard) -- Feb. 2007 issue of Concrete International • 2006 – Continuously reinforced concrete pavement with internal curing placed in Texas • 2007 – Full-day session on internal curing held at Fall ACI convention in Puerto Rico; bridge deck with internal curing placed in Ohio • 2010 – Bridge decks with internal curing placed in New York and Indiana • 2011 – Bentz and Weiss publish NISTIR 7765 - Internal Curing: A 2010 State-of-the-Art Review • 2012 – Special session on internal curing held at TRB Annual Meeting in D. C. ; bridge decks with internal curing placed in Utah; three ACI sessions on internal curing held in Toronto; ASTM issues ASTM C 1761 -12 Standard Specification for Lightweight Aggregate for Internal Curing of Concrete • 2013 – ACI 308/213 publishes R-13 Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate; ACI webinar on Internal Curing delivered in English and Spanish in September
A Brief Dictionary (from RILEM ICC committee) • Chemical shrinkage – An internal volume reduction that is the result of the fact that the absolute volume of the hydration products is less than that of the reactants (cement and water); can be on the order of 10 % by volume; ASTM standard test method C 1608 -12, first approved in 2005 • Self-desiccation (internal drying) – The reduction in the internal relative humidity (RH) of a sealed system when empty pores are generated. • Autogenous shrinkage – The external (macroscopic) dimensional reduction of the cementitious system under isothermal, sealed curing conditions; can be 100 to 1000 microstrain; along with thermal strains can be a significant contributor to earlyage cracking; ASTM standard test method C 1698 -09 for pastes and mortars
Example of Chemical Shrinkage (CS) Hydration of tricalcium silicate(major component of portland cement) C 3 S + 5. 3 H C 1. 7 SH 4 + 1. 3 CH Molar volumes 71. 1 + 95. 8 107. 8 + 43 C=Ca. O S=Si. O 2 H=H 2 O 10 % by volume CS = (150. 8 – 166. 9) / 166. 9 = -0. 096 m. L/m. L or -0. 0704 m. L/g cement For each lb of tricalcium silicate that reacts completely, we need to supply 0. 07 lb of extra curing water to maintain saturated conditions (In 1935, T. C. Powers measured a value of 0. 053 for 28 d of hydration – 75 %) Chemical shrinkage of blended cements with fly ash and/or slag can be significantly higher (2 X to 3 X) than that of ordinary portland cement by itself
From Chemical to Autogenous Shrinkage • CS creates empty pores within hydrating paste • During self-desiccation, internal RH and capillary stresses are both regulated by the size of the empty pores being created; larger empty pores mean lower stresses and higher internal RH • These stresses result in a physical autogenous deformation (shrinkage strain) of the specimen • Analogous to drying shrinkage, but drying is internal • Autogenous shrinkage is a strong function of both w/c and cement fineness; trends towards increasing fineness and lower w/c have both substantially increased autogenous shrinkage in recent years
IC Agent Characterization • Need to assess – Total water (pre-wetted condition) – Available curing water (desorption isotherms) – Particle size distribution (PSD) In final conditions (expanded SAPs, saturated wood fibers) • “Primum non nocere” – in addition to supplying internal curing water, a worthy goal for the IC agent is that it “First, do no harm” to the desirable properties of the control concrete – Physical and chemical stability during mixing, etc. • In 2012, ASTM committee C 09 published ASTM C 1761/C 1761 M-12 (now 13 b) Standard Specification for Lightweight Aggregate for Internal Curing of Concrete – Provides instructions on measuring physical properties and absorption and desorption of LWA for internal curing applications
Sample Desorption Isotherms Saturated salt solutions of K 2 SO 4, KNO 3, and KCl Ref: Greenspan, L. , Journal of Research of the National Bureau of Standards, 81 (1), 89 -96, 1977, see also ASTM C 1498 -04 and ASTM C 1761 -13 b.
Concrete Mixture Proportioning For lightweight aggregate (LWA) demand supply MLWA =mass of (dry) LWA needed per unit volume of concrete Cf =cementitious factor (content) for concrete mixture CS =(measured via ASTM C 1608 -12 or computed) chemical shrinkage of cementitious binder αmax =maximum expected degree of hydration of cementitious materials, OPC = min{[(w/c)/0. 36], 1} S =degree of saturation of LWA (0 to 1] when added to mixture ΦLWA = (measured) sorption of lightweight aggregate [use desorption measured at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498– 04 a; see also ASTM C 1761 -12] for Nomograph available at http: //concrete. nist. gov/ICnomograph. Englishunits. pdf (SI units also) Calculator available at Expanded Shale, Clay, and Slate Institute (ESCSI) web site Question of how uniformly water is distributed throughout the 3 -D concrete microstructure remains ---- we will cover this soon
Performance Evaluation Question: How can the effectiveness of IC be quantified? Answer: By direct and indirect experimental measurements of performance including – internal relative humidity (RH) autogenous deformation (ASTM C 1698) plastic shrinkage cracking and settlement compressive strength development drying shrinkage creep degree of hydration restrained shrinkage or ring tests (ASTM C 1581) sorptivity and diffusion coefficients 3 -D X-ray microtomography Scanning Electron Microscopy (SEM) observations
Autogenous Deformation Results Mortars w/cm = 0. 35, 8 % SF
SEM Observations w/cm=0. 30, 8 % silica fume IC Courtesy of P. Stutzman (NIST) w/cm=0. 30, 20 % slag Control
Web site for more information 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) Internal Curing and Reductions in Settlement and Plastic Shrinkage Cracking 6)Learn more about FLAIR: Fine Lightweight Aggregates as Internal Reservoirs for the autogenous distribution of chemical admixtures 7)View presentation on internal curing made at 2006 Mid-Atlantic Region Quality Assurance Workshop Link to Workshop homepage 8)Internal Curing Bibliography 9)Direct Observation of Water Movement during Internal Curing Using X -ray Microtomography
Question: How are the internal reservoirs distributed within the 3 -D concrete microstructure? 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 30 mm by 30 mm
Future Visions • Blending of LWA with crushed returned concrete fine aggregate (CCA) and other underutilized porous materials to optimize economics and performance • Utilization of pre-wetted LWA to distribute chemical admixtures as well as IC water throughout the concrete – Particularly for those admixtures that boost long term performance but may sometimes negatively impact fresh concrete properties (such as workability and air void stability) – Shrinkage-reducing admixtures and viscosity modifiers • NIST has published extensively on this (VERDi. CT) – Self-healing agents --- Ongoing research in Europe – Lithium admixtures --- Purdue and USBR studies ongoing – Phase change materials – research at NIST
Autogenous Deformation Results (LWA/CCA) (60: 40) Mortars with slag (20 %) blended cement w/cm=0. 3 IC added via fine LWA/CCA to increase total “w/cm” from 0. 30 to 0. 38 (0. 36) Note – chemical shrinkage of slag hydraulic reactions is ~0. 18 lb water/lb slag or about 2. 6 times that of cement
LWA for admixture distribution Improvement in chloride penetration resistance via addition of a viscosity modifier (VERDi. CT) w/c=0. 4 OPC mortars WLW = water in LWA WLT= VERDi. CT admixture solution (50: 50) in LWA Txx = VERDi. CT in mix water (10 % solution) Snyder, K. A. , Bentz, D. P. , and Davis, J. M. , “Using Viscosity Modifiers to Reduce Effective Diffusivity in Mortars, ” ASCE J Mat Civil Eng, 24(8), 1017 -1024, 2012.
Potential Benefit – Resistance to Sulfate Attack ASTM C 1012 Testing of Mortar Bars Measured average expansion vs. exposure time in replenished sulfate solution. Internal curing used pre-wetted fine LWA to replace a portion of the mortar sand. IC-VERDi. CT used a 50: 50 solution of SRA in water to pre-wet the same LWA. In both cases, expansion rates are dramatically decreased. (Bentz et al. , Materials and Structures, 2013). X-ray microfluorescence imaging S map X-ray microtomography imaging Control IC VERDi. CT-IC Control VERDi. CT-IC
Internal Curing - Prospectus • Practices and procedures are in place for utilizing IC in infrastructure concrete • IC slowly being adopted by the U. S. construction industry – Pavements and railway transit yard in Texas – Bridge decks in Ohio, Indiana, New York, Georgia, Virginia, North Carolina, and Utah – Water tanks in Colorado • Natural extensions are – to blend LWA with CCA and perhaps other porous “waste” materials to optimize economics and performance – to use the LWA to distribute chemical admixtures in addition to/instead of water