ACI Webinar Introducing ACI (308-213)R-13: Report ...

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Internal curing – process by which the hydration of cement continues because of .... measured at 93 % RH (potassium nitrate saturated salt solution) via. ASTM C ...
ACI Webinar Introducing ACI (308-213)R-13: Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate  5 cm 

4.6 mm on a side

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

Let’s Begin with a Definition • ACI Concrete Terminology ACI CT-13 • Internal curing – process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water • Previous version (circa 2010): • Curing, internal – supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

Question: Why do we need internal curing (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 a few days. If your concrete isn’t cracking at early ages, you may not need internal curing. Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959).

Examples 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”

Cements Trends and Cracking

Cement fineness has consistently increased from 1954 to the present day Finer cements: - have an increased early-age heat release - increased semi-adiabatic temperature rise (thermal cracking) - increased autogenous shrinkage - greater propensity for early-age cracking

Residual Stress (MPa)

3.0 Fine Cement Coarse Cement

Visible Cracking

2.0

1.0 Tension

0.0 Compression

-1.0 0

12

24

36

48

60 72 Time (h)

84

96

108

120

Restrained ring measurements Dual ring Similar cement chemistry Fine cement - 380 m2/kg Coarse cement – 311 m2/kg (Bentz et al., ASCE J Mat CE, 2008)

Concrete International, Jan. 2011

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

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 reducing autogenous shrinkage (and cracking). During IC, water is drawn from the water reservoirs into the surrounding hydrating cement paste. 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).

A Brief History of IC • The basic premises of internal curing were first recognized in 1957 (Proceedings of the World Conference on Prestressed Concrete) – Paul Klieger wrote “Lightweight aggregates absorb considerable water during mixing which apparently can transfer to the paste during hydration.”

• IC suggested directly in 1991 (Concrete Science and Reality) – Bob Philleo wrote “..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.”

• Following this, IC actively researched in Germany, Israel, Denmark and the U.S. from the mid 1990s onward – Commonly explored reservoirs include (fine) lightweight aggregates, superabsorbent polymers, and wood fibers

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 C1608-12 approved in 2005

• Self-desiccation – 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 microstrains; along with thermal strains can be a significant contributor to early-age cracking. ASTM C1698-09.

Example of Chemical Shrinkage (CS) Hydration of tricalcium silicate(major component of portland cement) C3S + 5.3 H  C1.7SH4 + 1.3 CH

Molar volumes 71.1 + 95.8  107.8 + 43

C=CaO S=SiO2 H=H2O

10 % by volume

CS = (150.8 – 166.9) / 166.9 = -0.096 mL/mL or -0.0704 mL/g cement For each g of tricalcium silicate that reacts completely, we need to supply 0.07 g of extra curing water to maintain saturation. Can think of this as 7 lbs of water per 100 lbs of cement in the concrete mixture Chemical shrinkage of blended cements (fly ash, slag, or silica fume) is generally significantly higher than that of ordinary portland cement by itself

From Chemical Shrinkage to Autogenous Shrinkage • CS creates empty pores within hydrating paste

 cap 

2  ln(RH) RT  rpore Vm

• During self-desiccation, internal RH and capillary stresses are both regulated by the size of the empty pores being created • These stresses result in a physical autogenous deformation (shrinkage strain) of the specimen

  (Scap / 3)[(1 / K )  (1 / Ks)] • 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

Internal Curing via Water Reservoirs Cement paste Water reservoir

Larger “sacrificial” pores within the reservoirs to minimize stress/strain

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

Mixture Proportioning with Internal Curing • Goal is to provide sufficient internal water to avoid self-desiccation and maintain saturation of the hydrating cement paste (promoting hydration and property development) • Simple rule of thumb is to supply 7 lbs of water per 100 lbs of cementitious materials • More detailed calculations can be employed – Equation or chart form

• Important that lightweight aggregates are added to the concrete mixture in pre-wetted conditions – When added dry, they will pull water from the mixture leading to problems with rheology, workability, and yield

Concrete Mixture Proportioning For lightweight aggregate (LWA)

C f  CS   max M LWA  S  ΦLWA

demand supply

MLWA =mass of (dry) LWA needed per unit volume of concrete Cf =cement factor (content) for concrete mixture CS =(measured via ASTM C 1608-12 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 ΦLWA = (measured) absorption of lightweight aggregate [use desorption measured at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498–04a; see also ASTM C1761-12] Nomograph available in ACI (308-213)R-13 report and at: http://ciks.cbt.nist.gov/~bentz/ICnomographEnglishunits.pdf (also in SI)

Question of how uniformly water is distributed throughout the 3-D concrete microstructure remains ---- we will cover this soon

MIXTURE PROPORTIONING WITH INTERNAL CURING 80

80

70

70

60

60

CS=0.07

50

50

CS=0.08

40

40

30

30

20

20

10

10 400

CS=0.05 CS=0.06

w /c>=0.36 w /c=0.33 w /c=0.3 w /c=0.27 w /c=0.24 w /c=0.21 w /c=0.18

0

10

20

30

40

50

60

70

80

10

20

30

40

50

60

70

80 1400

abs= 5 % abs= 10 %

1200 1000

abs= 25 % abs= 30 %

800

abs= 35 % abs= 40 %

600 400 200 0

LWA addition (lb/yd 3)

abs= 15 % abs= 20 %

600

700

800

900

Cement content (lb/yd3)

Water demand (lb/yd3)

0

500

Starting with the cement content in the graph on the upper right, find the chemical shrinkage of the mixture (a good default value is 0.07). Proceed to the value on the y-axis and starting with this same value in the graph on the upper left, find the line for the mixture’s w/c ratio. (Note that there is a single (thick) line for all w/c ratios greater than or equal to 0.36 as for these w/c ratio values, it is assumed that complete hydration of the cement powder can be achieved.) Proceed to the value on the x-axis and starting with this same value in the graph on the lower left, find the line for the absorption (dry mass of aggregate basis) of the lightweight aggregate. Finally, proceed to the value on the y-axis to obtain the recommended level of lightweight aggregate (dry mass basis) to be added to the concrete mixture. This replacement should then be conducted on a volumetric basis, replacing an equal volume of normal weight aggregates with pre-wetted (SSD) lightweight aggregates.

Characterizing Properties of LWA for IC • In 2012, ASTM committee C09 published ASTM C1761/C1761M-12 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

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)

30 mm by 30 mm

Returns a table of “protected paste fraction” as a function of distance from LWA surface

Yellow – Pre-wetted LWA Red – Normal weight sand Blues – Pastes within various distances of an LWA

http://concrete.nist.gov/lwagg.html

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

Potential Additional Benefits • In addition to reductions in autogenous shrinkage and early-age cracking, IC has been documented in the lab and the field to: – Reduce plastic shrinkage cracking (robustness of construction) – Reduce curling – Increase strengths (compressive and flexural), particularly at later ages – Reduce modulus of elasticity – Improve interfacial transition zone (ITZ) microstructure – Reduce transport rates (RCPT and bulk diffusion) – Reduce expansion under sulfate attack testing (accommodation mechanism of lightweight aggregates)

Where has IC been used in practice? • 2005 – Railway transit yard and CRC paving in Texas (w/cm=0.43 for transit yard) • 2008-present – More than a dozen bridge decks in New York (w/cm=0.33 to 0.40) • 2011-present - Bridge decks in Indiana (w/cm=0.39 to 0.42) • 2012 - Bridge decks in Utah (w/cm=0.44) • 2011-12 - Water tank (10 million gallons) in Colorado

Internal Curing Applications (TXI & Texas DoT – Villarreal, Crocker, Reeves, Friggle)

• 2005 - RR intermodal facility constructed • 250,000 yd3 of low slump IC material • 6 months: 1 crack, 5.5 years: miniscule drying or plastic shrinkage cracking • CRC Paving for TxDOT

CRC Pavement

Railway Transit Yard

Indiana Field Trials - Conventional and IC mixtures in Sister Bridges

Courtesy of Prof. Jason Weiss, Purdue University

• • •

Implemented as a change order to existing Monroe County Bridges in 2010 Bridges cast using conventional ready mix concrete and conventional procedures Shows that this is a ‘very off the shelf technology’ – replace some FA with FLWA

Monroe County IN (DiBella et al. 2012) • Simple Change in Mixture Proportions • IN - Plain Slabs Cracked; IC has not cracked to date

• IC has lower transport properties

Field comparison (Monroe Co In) Plain bridge deck (Monroe Co.) 1 year after casting.

Bridge deck with internal curing (Monroe Co.) “Crack free 18+ mos after casting”

Use by NYDOT (Wolfe et al. 2012)

Tonawanda NY

Tonawanda (190/I290) NY Results • Similar RCPT results (DiBella et al. 2011) *note cond. LWA in test

• Similar fresh properties • Similar Strength (Wolfe et al. 2012)

Comp. Str. 7 day Comp. Str. 28 day Comp. Str. 56 day Concrete Density Air Content Slump

Class HP 3,040 psi 4,677 psi 5,343 psi 140.2 pcf 5.5 % 5.0”

Class HP-IC 3,500 psi 4,683 psi 5,417 psi 135.2 pcf 6.0 % 4.5”

Crack Surveys on Utah Bridge Decks at 8 Months Age

Data courtesy of Prof. Spencer Guthrie, BYU

Potential Benefit – Resistance to Sulfate Attack ASTM C1012 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-VERDiCT 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

VERDiCT-IC

Control

VERDiCT-IC

Learning Objectives • Identify why internal curing may be needed • Describe how internal curing works • Describe mixture proportioning procedures to incorporate internal curing into a concrete mixture • Distinguish additional benefits that internal curing may provide beyond crack reduction

Thank you for your time and attention!

QUESTIONS?