Dale Bentz, Phillip Halleck, Abraham Grader, and John Roberts RILEM Conference- Volume Changes of Hardening Concrete:

Testing and MitigationAugust 2006

Outline

• Need for internal curing– Blended cements

• “Undercuring” with internal curing

• Microtomography observations of water movement during internal curing– Quantitative analysis of 3-D images

• Mixture proportioning for internal curing

What is internal curing (IC)?

Answer: As being considered by ACI-308, “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water.”

For many years, we have been curing concrete from the outside in, internal curing is for curing from the inside out. Internal water is generally supplied via internal reservoirs, such as saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers.

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).

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.

Because the autogenous 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 (percolated).

Cement paste

Water reservoir

Blended Cements• Internal curing can be particularly important in

high-performance (low w/cm) blended cement systems– Increased chemical shrinkage of pozzolanic and slag

reactions• Cement: 0.06 to 0.07 mL/g cement• Silica fume: 0.22 mL/g cement• Slag: ~ 0.18 mL/g cement• Fly ash (Type F): ~ 0.12 to 0.16 mL/g cement

– Possible earlier depercolation of capillary pores and reduced permeability limiting water transport distances within the hydrating blended cement paste microstructure

Autogenous Deformation Results

IC added via fine LWA to increase total “w/c” from 0.30 to 0.38 or 0.40

Note – chemical shrinkage of pozzolanic reactionof silica fume with CH is ~0.22 g water/g silica fume or

about 3.2 times that of cement

w/c=0.3 HPM silica fume blended cement

-500

-400

-300

-200

-100

0

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0 7 14 21 28 35 42 49 56

Time (d)

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rain

Control

IC - 8

IC - 10

Autogenous Deformation Results

IC added via fine LWA to increase total “w/c” from 0.30 to 0.38

Note – chemical shrinkage of slag hydraulic reactionsis ~0.18 g water/g slag or about 2.6 times that of

cement

w/c=0.3 HPM slag blended cement

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-400

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0

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0 7 14 21 28 35 42 49 56

Time (d)

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Control

IC

“Undercuring” with Internal Curing

• Hydrating cement paste is a complex and dynamic porous media and as such, internal curing mixture proportions that supply only part of the total needed water (demand) can potentially exhibit some interesting results as illustrated in the schematic on the following slide

Empty and Full PoresSaturated curing Sealed curing

RH = 98 % RH = 93 %

Sufficient Internal curing

IC Reservoir Cement paste

RH = 97 %

Insufficient Internal curing

Cement paste

RH = 90 %

IC Reservoir

Better hydrationOnly pores in reservoirs empty

Some increase in hydrationPores in both reservoirsand paste empty

Cement pasteCement paste

Less hydrationLargest pores in paste empty

Four-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 (time is the 4th dimension)

Mixture ProportionsTable 1. Mixture proportions for the control and IC high-performance mortars. Material Control Mortar Mass IC Mortar Mass Cement 984.6 g (2.17 lb) 953.3 g (2.10 lb) Water 344.6 g (0.759 lb) 333.7 g (0.735 lb)

Sand (total) 1870.8 g (4.12 lb) 1529.4 g (3.37 lb) F95 fine sand 467.7 g (1.03 lb) 452.8 g (0.997 lb)

Graded sand (ASTM C778) 355.4 g (0.783 lb) 344.1 g (0.758 lb) 20-30 sand (ASTM C778) 355.4 g (0.783 lb) 287.8 g (0.634 lb)

S16 coarse sand 692.2 g (1.52 lb) 444.7 g (0.980 lb) LWA (SSD) --- 183.6 g (0.404 lb)

Water in LWA --- 35.2 g (0.0775 lb)

w/c = 0.35Blend of four sands (Ferraris) to improve particle packingLWA added in saturated surface dry (SSD) condition

SSD specific gravity of 1.7

Commercial cement – no particles larger than 30 μm diameterHydration conducted at 30 oC

maintained by circulating fluid from a temperature controlled bath

After mixing 1 d hydration 2 d hydration

Subtraction: 1 d – after mixing

Aqua indicates dryingRed indicates wetting

All images are 13 mm by 13 mm

Three-dimensional subtracted imageof 1 d hydration – initial microstructureshowing water-filled pores that have

emptied during internal curing (4.6 mm on a side)

2-D image with water evacuatedregions (pores) overlaid on

original microstructure(4.6 mm by 4.6 mm)

Four-Dimensional X-ray Microtomography

Quantitative Analysis

• Four-dimensional image sets analyzed to estimate volume of water moving from LWA to cement paste during first 2 d of hydration

• Analysis based on changes in greylevel histogram with time

• Results compared to conventional measures of hydration including chemical shrinkage, non-evaporable water content, and heat release

Preprocessing of 3-D Image Data

PoresLWA

Sand

Paste

Empty

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3000 3500 4000 4500 5000 5500 6000

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Greylevel value

Pix

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no filter

median3

median5

Median filter applied to remove noise and sharpen greylevel histogram

Temporal Analysis of Greylevel Histograms

PoresLWA

Sand

Paste

Empty

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3000 3500 4000 4500 5000 5500 6000

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Greylevel value

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1 h

26.5 h

47 h

Change in “empty” pores with time quantified

Tomography Water Movement vs. Hydration Measures

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0.01

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0 12 24 36 48

Time (h)

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hea

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<= 3500 <=3900Chemical shrinkage Heat releaseLOI (paste) LOI (IC mortar)

Good ”quantitative” agreement between estimated water movement volume and other measures of hydration

Four-Dimensional X-ray Microtomography

0.000

0.005

0.010

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0.020

0.025

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0 12 24 36 48

Time (h)

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Scaled <=3900 Chemical shrinkage

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

Mixture Proportioning for Internal CuringQuestions 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://ciks.cbt.nist.gov/lwagg.html

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 concreteCf =cement factor (content) for concrete mixtureCS =(measured via ASTM C 1608-05 or computed) chemical shrinkage of cementαmax =maximum expected degree of hydration of cement, [(w/c)/0.36] or 1S =degree of saturation of LWA (0 to 1] when added to mixtureøLWA = (measured) absorption of lightweight aggregate (use desorption measured

at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498–04a)

)*/()**( max LWAfLWA SCSCM

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

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 LWARed – Normal weight sandBlues – Pastes within various

distances of an LWA

10 mm by 10 mm

Mortar from μCT experiment97 % of paste within 2 mm of LWA

Summary• Internal curing especially critical in high

performance blended cement systems• Too little internal curing can actually

result in a lower internal RH than in a system with no internal curing

• X-ray microtomography can be used to “observe” water movement during internal curing in four dimensions

• Internet tools exist to assist in mixture proportioning for internal curing